1users.telenet.be/erasmus-wolverhampton/Thesis.doc · Web viewA die to produce cylindrical samples...

PREPARATION AND CHARACTERISATION OF

SUBMICRON/NANO STRUCTURED POWDERS FROM TUNGSTEN

CARBIDE –COBALT/ALTERNATIVE BINDERS HARDMETALS

Peter ADRIAENSEN en Raf MOORS

Afstudeerwerk ingediend tot het behalen van het diploma vanindustrieel ingenieur in elektromechanica optie automatisering

master in de industriële wetenschappen: elektromechanica

Promotoren: dr. ir. T. Laoui (University of Wolverhampton) dr. ir. A. Van Bael (XIOS Hogeschool Limburg)

XIOS HOGESCHOOL LIMBURGDEPARTEMENT INDUSTRIELE WETENSCHAPPEN EN TECHNOLOGIE

A. Academiejaar 2004 - 2005

-Abstract- III

Abstract

Cobalt has been the most suitable and most commonly used binder for tungsten carbide

based hardmetals. The most important factor in favour of cobalt (Co) is its excellent

wetting behaviour for tungsten-carbide (WC).

Due to the poor corrosion resistance of Co, its high cost and environmental toxicity,

substantial research has been devoted to find suitable alternative binders for WC systems.

The aim is to reduce the amount of Co, or possibly, to completely replace Co binder. Two

promising alternatives are described and utilised in this project, the first one is a mixture

of iron (Fe), nickel (Ni) and cobalt (Co) and the second alternative is composed of iron

(Fe) and manganese (Mn). Compared to cobalt, Fe and Mn are very cheap and non toxic.

A literature review was performed on different relevant aspects covering the field of

hardmetals, powder preparation methods, powder metallurgy and nanomaterials. The

submicron/nano-structured composite powders were prepared by the mechanical alloying

method using both planetary ball and high-energy ball milling processes.

A series of experiments were performed with the planetary ball mill by varying milling

time (2.5, 5, 10 hrs) and rotation speed (250, 400rpm) parameters to process WC-10wt

%Co, WC-10%FeNiCo and WC-10%FeMn. It was noticed that as the milling time

increased (above 2.5 hours for 150rpm) the amount of elements (Fe, Cr) picked up from

the stainless steel vial inner wall increased. The contamination level increased further at a

rotation speed of 400rpm. This indicates that both speed and time should be kept low to

minimise contamination or a hard steel vial should be utilised. For that, additional powders

were prepared using the high-energy ball mill.

The grain size of WC phase was calculated using the Scherer equation and the

corresponding X-ray diffraction peaks while the WC particle size was evaluated using

Raf Moors – Peter Adriaensen

-Abstract- IV

scanning electron microscopy images. Composite powders were successfully made in

which fine WC particles (submicron down to about 200nm size) were distributed within

the matrix (Co, FeNiCo or FeMn).

The next step would be to compact such powder for a subsequent sintering process. For

that appropriate compaction dies were designed using Inventor CAD software. A die to

produce cylindrical samples for microstructural and hardness analyses was designed as

well as another die to produce samples for 3-point bending tests. Both dies were designed

according to ASTM standards.


-Acknowledgements- V

-

Acknowledgements

The aim of our final thesis project was to prepare submicron/nano-structured powders from

WC-Co system and replace Co with suitable alternatives. This project was accomplished at

the University of Wolverhampton (UK) in line with our Master Degree Industrial Sciences.

First of all, we would like to thank everybody who helped to bring our final thesis to a

good end. A special word of thanks goes to our supervisor Dr. ir. T. Laoui and to S.

Hewitt, of the University of Wolverhampton, for enriching us with the knowledge they

have and the daily good care for us.

Further, we would like to thank Dr. ir. A. Van Bael, of the XIOS Hogeschool Limburg, for

allowing us the opportunity to accomplish our training in Wolverhampton and for reading

our final thesis project and Ms. Bauwens for helping us arrange the paperwork involving

our stay in Wolverhampton.

We would also like to thank our parents, for giving us the opportunity to do our thesis

project abroad.

Last word of thanks to everybody, especially our parents and girlfriends, for supporting us

in the difficult times we sometimes had.

Our stay at the UK was part of a project in the Erasmus framework.


-Table of contents- 1

Table of contents

Abstract................................................................................................................................III

Acknowledgements...............................................................................................................V

Table of contents....................................................................................................................1

List of figures.........................................................................................................................6

List of tables...........................................................................................................................9

List of symbols.......................................................................................................................9

1 Introduction and project objectives..............................................................................10

1.1 Introduction..........................................................................................................10

1.2 Objectives of the thesis........................................................................................12

1.2.1 Looking for alternative binders to Co for WC particles..............................12

1.2.2 Powder Processing by Mechanical Alloying (MA).....................................12

1.2.2.1 Searching for best fit parameters for planetary ball milling....................13

1.2.2.2 Horizontal high energy simoloyer............................................................14

1.2.3 Powder characterization...............................................................................14

2 Literature review..........................................................................................................15

2.1 Hard metals..........................................................................................................15

2.1.1 Introduction..................................................................................................15

2.1.2 Powder production.......................................................................................15

2.1.3 Powder production techniques.....................................................................16

2.1.3.1 Atomization..............................................................................................16



2.1.3.2 Gas- and water atomization......................................................................16

2.1.3.3 Centrifugal process...................................................................................18

2.1.3.4 Chemical processes..................................................................................18

2.1.3.5 Electrolysis...............................................................................................19

2.1.4 WC-Co.........................................................................................................19

2.1.5 Alternative binders to Co for WC................................................................20

2.1.5.1 Fe-Mn as alternative binder to Co for WC...............................................20

2.1.5.2 Fe/Ni/Co as alternative binder to Co for WC...........................................21

2.1.6 Grain growth................................................................................................21

2.1.6.1 Grain growth inhibitor..............................................................................22

2.1.6.1.1 The effect of V8C7 and Cr2C2 additives on the sintering of WC-Co. .23

2.1.6.1.2 Effect of V8C7 and Cr3C2 additions on WC-Co grain growth and

mechanical properties...........................................................................................23

2.2 Powder metallurgy...............................................................................................25

2.2.1 The process...................................................................................................25

2.2.1.1 Mix the powder with a suitable lubricant.................................................25

2.2.1.2 Powder compaction..................................................................................25

2.2.1.3 Sintering...................................................................................................25

2.2.2 Reasons for using PM..................................................................................26

2.2.3 Applications of PM......................................................................................27

2.2.3.1 Self-lubricating bearings..........................................................................27

2.2.3.2 Hard metals..............................................................................................28

2.2.3.3 Friction materials.....................................................................................28

2.2.4 The future of PM..........................................................................................29

2.3 Nanostructural materials......................................................................................29

2.3.1 What are nanostructured materials...............................................................29

2.3.2 Synthesis......................................................................................................30

2.3.2.1 Mechanical alloying.................................................................................31

2.3.2.1.1 Mechanism of alloying.......................................................................32

2.3.2.1.2 Types of mills.....................................................................................36



2.3.2.1.2.1 Planetary ball mills......................................................................37

2.3.2.1.2.2 High energy ball milling.............................................................39

2.3.2.1.2.3 Other types of mills.....................................................................41

2.3.2.1.3 Process variables................................................................................43

2.3.2.1.3.1 Milling container.........................................................................43

2.3.2.1.3.2 Milling speed...............................................................................44

2.3.2.1.3.3 Milling time.................................................................................45

2.3.2.1.3.4 Grinding medium........................................................................45

2.3.2.1.3.5 Ball-to-powder weight ratio........................................................46

2.3.2.1.3.6 Extent of filling the vial..............................................................47

2.3.2.2 Liquid phase techniques...........................................................................47

2.3.2.3 Vapour phase techniques..........................................................................47

2.3.2.4 Plasma heating.........................................................................................48

2.3.2.5 Solid phase techniques.............................................................................49

2.3.2.6 Equal channel Angular Extrusion............................................................49

2.3.2.6.1 Simple shear concept..........................................................................51

2.3.2.6.2 Inhomogeneous deformation..............................................................52

2.3.3 Properties......................................................................................................53

2.3.4 WC-Co particles...........................................................................................53

2.4 Crystal structures and Point Defects....................................................................54

2.4.1 The Body-Centered-Cubic (BCC) structure.................................................54

2.4.2 The Hexagonal-Close-Packed (HCP) structure...........................................55

2.4.3 Miller indices – Cubic Crystals....................................................................56

2.4.4 Close Packed planes.....................................................................................57

2.5 Grain measurement of WC...................................................................................58

2.5.1 BET Surface Area........................................................................................59

2.5.2 X-ray sedigraph............................................................................................59

2.5.3 Laser Diffraction..........................................................................................60

2.5.4 Ultracentrifuge.............................................................................................60

2.5.5 Photon correlation spectrography................................................................61



2.5.6 Microscopical image analysis, SEM, TEM..................................................61

2.5.7 X-ray line broadening..................................................................................62

2.5.8 Chemical reaction.........................................................................................62

3 Experimental procedure...............................................................................................63

3.1 Description of the powders..................................................................................63

3.1.1 Tungsten carbide (WC)................................................................................63

3.1.1.1 Tungsten carbide < 20µm........................................................................63

3.1.1.2 Tungsten carbide < 4.3µm.......................................................................64

3.1.2 Cobalt (Co)...................................................................................................64

3.1.2.1 Cobalt < 20µm.........................................................................................64

3.1.2.2 Cobalt < 4,3µm........................................................................................65

3.1.3 Iron (Fe).......................................................................................................65

3.1.4 Nickel (Ni)...................................................................................................65

3.1.5 Manganese (Mn)..........................................................................................65

3.1.6 Vanadium Carbide (VC)..............................................................................65

3.2 Preparation of the powders...................................................................................66

3.3 Milling process.....................................................................................................66

3.3.1 Planetary ball mill........................................................................................66

3.3.2 Horizontally high energy mill......................................................................67

3.3.3 Development of dies for compaction...........................................................68

3.3.3.1 Compaction die........................................................................................68

3.3.3.1.1 The die for compaction......................................................................68

3.3.3.1.2 The upper punch.................................................................................69

3.3.3.1.3 The lower punch.................................................................................69

3.3.3.1.4 The die for the Charpy test.................................................................70

3.3.3.1.5 Die for 3 point bending test................................................................70

3.4 Analysis methods.................................................................................................71

3.4.1 Sieving..........................................................................................................71

3.4.2 X-ray diffraction...........................................................................................71



3.4.2.1 Formulae used in the X-ray diffraction....................................................72

3.4.2.2 Example: unit cell size from Diffraction data..........................................73

3.4.2.3 Instrumentation........................................................................................74

3.4.3 XRF..............................................................................................................75

3.4.3.1 Description of the machine......................................................................75

3.4.3.2 Preparation of the samples.......................................................................76

3.4.4 Scanning electron microscopy.....................................................................76

3.4.4.1 Description of the machine......................................................................76

3.4.4.2 Preparation of the samples.......................................................................77

3.4.5 Optical microscopy......................................................................................77

4 Results and discussion..................................................................................................78

4.1 Results from planetary ball mill...........................................................................78

4.1.1 Getting started..............................................................................................78

4.1.2 Reference sample.........................................................................................78

4.1.2.1 XRD.........................................................................................................79

4.1.2.2 XRF..........................................................................................................80

4.1.2.3 Calculated grain size of the starting WC particles...................................81

4.1.3 XRD results of the planetary ball milled samples at 250 rpm.....................82

4.1.3.1 The 2,5 hours milled sample....................................................................82

4.1.3.2 The 5 hours milled sample.......................................................................84

4.1.3.3 The 10 hours milled sample.....................................................................85

4.1.4 The SEM pictures of the 250 rpm samples..................................................86

4.1.4.1.1 The 2,5h milled sample......................................................................86

4.1.5 XRD results of the planetary ball milled samples at 400 rpm.....................87

4.1.5.1 The 5 hours milled sample.......................................................................87

4.1.5.2 The 10 hours milled sample.....................................................................88

4.1.6 The SEM pictures of the 400 rpm samples..................................................89

4.1.6.1.1 The 10h milled sample.......................................................................89

4.1.7 Contamination level.....................................................................................89



4.1.8 The grain size calculation.............................................................................91

4.1.8.1 The Scherer equation................................................................................91

4.1.8.1.1 The planetary ball milled samples at 250 rpm...................................91

4.1.8.1.2 The planetary ball milled samples at 400 rpm...................................92

4.1.8.1.3 Conclusions of the Scherer equation..................................................92

4.1.9 Strain evaluation from peak shift.................................................................95

4.2 Results from the high energy mill........................................................................95

4.2.1 The estimation of the contamination level of the milled powder.................96

4.2.2 SEM pictures from samples milled with the high energy mill.....................97

4.2.2.1 SEM pictures from Fe/Mn as alternative binder......................................97

4.2.2.2 SEM pictures from Fe/Ni/Co as alternative binder..................................98

5 Conclusions and suggestions for further work...........................................................100

5.1 Introduction........................................................................................................100

5.2 Conclusions........................................................................................................101

5.2.1 The planetary ball mill...............................................................................101

5.2.2 The horizontally high energy mill..............................................................101

5.2.3 The alternative binders...............................................................................102

5.3 Further work.......................................................................................................102

6 References..................................................................................................................104

Appendix A (Website for the project)...............................................................................A-1

Appendix B (Videoconferencing facilities at UoW)..........................................................B-1

Appendix C (Technical drawings).....................................................................................C-1

Appendix D (Dutch Summary).......................................................................................D-1


-List of figures- 7

List of figures

Figure 1: Gas atomization, EPMA, 2004..........................................................................................................18

Figure 2: Water atomization, EPMA, 2004.......................................................................................................18

Figure 3: Particle shape with gas atomization, EPMA, 2004...........................................................................18

Figure 4: Particle shape with water atomization, EPMA, 2004.......................................................................19

Figure 5: Comparison of the effect of grain refiner content on WC grain size, LUYCKX, S., 2001................25

Figure 6: Venn-diagram, GERMAN, R.M.........................................................................................................27

Figure 7: Self -lubricating bearing, EPMA, 2004.............................................................................................29

Figure 8: Friction materials, EPMA, 2004.......................................................................................................29

Figure 9: Ball-powder-ball collision of powder mixture during MA, SURYANARAYANA, C., 2001..............33

Figure 10: Narrow particle size distribution caused by tendency of small particles to weld together and

large particles to weld together and large particles to fracture, SURYANARAYANA, C., 2001......................35

Figure 11: Refinement of particle size and grain sizes with milling time, SURYANARANA,C., 2001.............36

Figure 12: Schematics of microstructural evolution during milling of a ductile-brittle combination of

powders, SURYANARAYANA, C., 2001............................................................................................................37

Figure 13: Fritch Pulverisette 5 planetary ball mill.........................................................................................38

Figure 14: Movement of balls in the planetary ball mill, Zoz...........................................................................39

Figure 15: Movement of balls in the planetary ball mill during milling, Zoz...................................................39

Figure 16: Schematic depicting the ball motion inside the ball mill, Courtesy of Gilson Company................40

Figure 17: High-energy mill, Metal-Powder....................................................................................................41

Figure 18: Running high-energy mill, Zoz........................................................................................................41

Figure 19: Inside a high-energy mill, Zoz.........................................................................................................42

Figure 20: SPEX 8000 mills in the assembled condition, SURYANARAYANA,C., 2001..................................42

Figure 21: Attritor, IMP....................................................................................................................................43

Figure 22: Working principle of the attritor, SAAR-HARDMETAL.................................................................43

Figure 23: Grinding medium, NARANG ENTERPRICES................................................................................47

Figure 24: Principle of plasma heating, Max Planck Institute.........................................................................50

Figure 25: Principle scheme of the ECAE........................................................................................................51

Figure 26: Sketch of a BCC structure, ALCAN.................................................................................................55

Figure 27: The atoms in a BCC structure, ALCAN..........................................................................................55

Figure 28: Sketch of a HCP structure, ALCAN................................................................................................56

Figure 29: Atoms in a HCP structure, ALCAN.................................................................................................56

Figure 30: Miller indices – Cubic crystals, ALCAN.........................................................................................57


-List of figures- 8

Figure 31: Negative Miller index, ALCAN........................................................................................................57

Figure 32: Cubic lattices only a direction will be perpendicular to the plane, ALCAN...................................58

Figure 33: First plane of FCC..........................................................................................................................58

Figure 34: The second plane of FCC................................................................................................................59

Figure 35: The third layer of FCC....................................................................................................................59

Figure 36: BET surface area.............................................................................................................................60

Figure 37: X-ray sedigraph...............................................................................................................................60

Figure 38: Laser diffraction..............................................................................................................................61

Figure 39: Ultracentrifuge................................................................................................................................61

Figure 40: Photon correlation spectography....................................................................................................62

Figure 41: The beam of the photon correlation spectography..........................................................................62

Figure 42: The range were the different techniques are valid..........................................................................63

Figure 43: SEM picture of WC particles < 20 micron......................................................................................65

Figure 44: Fritsh Pulverisette 5........................................................................................................................68

Figure 45: The Maltoz operation software for the high energy mill.................................................................68

Figure 46: Assembly of the compaction die......................................................................................................69

Figure 47: Assembly of the upper punch...........................................................................................................70

Figure 48: Assembly of the lower punch...........................................................................................................70

Figure 49: Assembly of the Charpy test die......................................................................................................71

Figure 50: Assembly of the 3 point bending die................................................................................................71

Figure 51: Fritsh Sieve......................................................................................................................................72

Figure 52: Philips XRD.....................................................................................................................................73

Figure 53: Reflection of X-rays from 2 planes of atoms in a solid...................................................................74

Figure 54: X-ray diffraction pattern.................................................................................................................75

Figure 55: Schematic of an X-ray powder diffractimeter.................................................................................75

Figure 56: Spectro Xepo XRF machine.............................................................................................................76

Figure 57: ZEIS SEM........................................................................................................................................77

Figure 58: The optical microscope...................................................................................................................78

Figure 59: XRD pattern of the unmilled WC-Co reference powder sample.....................................................80

Figure 60: Graph of the contributing elements in the WC-Co reference sample.............................................81

Figure 61: SEM picture of the starting WC material < 20 micron...................................................................83

Figure 62: XRD pattern of the 2.5h ball-milled WC-Co sample @ 250 rot/min..............................................84

Figure 63: XRD pattern of the 5h ball milled WC-Co sample @ 250 rot/min.................................................85

Figure 64: XRD pattern of the 10 h ball milled WC-Co sample @ 250 rot/min..............................................86

Figure 65: SEM picture of the 2,5h planetary ball milled sample....................................................................87

Figure 66: XRD pattern of the 5h ball milled WC-Co sample @ 400 rot/min.................................................88


-List of figures- 9

Figure 67: XRD pattern of the 10h ball milled WC-Co sample @ 400 rot/min...............................................89

Figure 68: SEM picture of the planetary ball milled sample at 400 rpm.........................................................90

Figure 69: Estimation of the contributing elements in milled powder..............................................................91

Figure 70: Contamination level in function of the milling time........................................................................91

Figure 71: The excel counting sheet for the average grain size calculation with the Scherer equation (250

rpm milled sample)............................................................................................................................................92


rpm milled sample)............................................................................................................................................93

Figure 73: Fourier analysis of the un-milled WC powder < 20 micron...........................................................94

Figure 74: Results of the Winfit! V1.2 on the <20 micron WC particles........................................................95

Figure 75: Grain size from the <20 micron WC particles with the Winfit! V1.2 software...............................95

Figure 76: Peak shift of WC with 250 rpm milling speed.................................................................................96

Figure 77: Estimation of the contributing elements in milled powder..............................................................97

Figure 78: contamination after milling.............................................................................................................98

Figure 79: SEM picture of the 1h horizontal energy milled sample WC-Fe/Mn..............................................99

Figure 80: SEM picture of the 1h horizontal energy milled sample WC-Fe/Ni/Co........................................100

Figure 81: Homepage of the website...............................................................................................................A-4

Figure 82: Personal data page........................................................................................................................A-4

Figure 83: Navigation page of the website......................................................................................................A-5

Figure 84: Movies page...................................................................................................................................A-6

Figure 85: The link page.................................................................................................................................A-6

Figure 86: Comparison in cost between communication possibilities............................................................B-4

Figure 87: View over the Access Grid video conference at RIATec office Wolverhampton...........................B-5

Figure 88: The micros used for the video conference.....................................................................................B-7

Figure 89: Audio component with the high-end echo canceller......................................................................B-7


-List of tables- 10

List of tables

Table 1: The composition of the powders..........................................................................................................67

Table 2: Used compositions for the binders......................................................................................................67

List of symbols

BCC Body-Centered-Cubic

BETBrunauer, Emmett, and Teller (the three scientists that optimized the theory for measuring surface area)

Co CobaltFe Iron

H2O WaterHCl HydrochlorateHCP Hexagonal-Close-Packed

HNO3 Nitryl hydroxideMA Mechanical AlloyingMn ManganeseNi NickelPM Powder Metallurgy

SEM Scanning Elektron microscopy

TEMTransmission Electron Microscopy

UoW University of WolverhamptonVC Vanadium CarbideWC Tungsten CarbideXRD X-ray diffractionXRF X- Ray Fluorescence


-Introduction and project objectives- 11

1 Introduction and project objectives

1.1 Introduction

Metal powders are used in industry for a diverse range of products. Some of these products

include welding electrodes, paints, printing inks and explosives. In all these products the

particles retain their identities. Traditional powder metallurgy is a process whereby a solid

metal, alloy or ceramic in a form of a mass of dry particles, normally less than 150 µm in

maximum diameter, is converted into an enginering component of a predetermined shape

possessing properties which allow it to be used in most cases without further processing.

The basic steps in traditional powder production are:

Powder production

Compaction of the powder

Sintering which involves heating the preform to a

temperature below the melting point of the major

constituent, at which point the powder particles lose their

identities through inter-diffusion processes and required

properties are developed.

Since early civilisations, there can be no doubt that certain metals in the form of powders,

and consolidated by sintering, have been forced to meet man’s needs. The iron pins keying

the marble blocks of the Parthanon in Athens are believed to have been made in this way

whilst the Delhi Pillar, dating from 300 A.D., is made of more than 6 tons of sponge iron.

With the advent of melting practices from Iron and Copper, powder metallurgy remained

unchanged for many centuries.

By the mid-19th century, a new process was developed based on the principle of fusion.

Further major industrial development in powder metallurgy had to await the advent of the



electric lamp and the search for a filament material of high melting point, low evaporation

rate and adequate conductivity. Tungsten met these requirements but the sintered material

was brittle. The difficulty was resolved in 1910 when it was discovered that tungsten bars,

produced by sintering at 3000°C, developed enough ductility to permit continued working

at progressively lower temperatures. The required filament diameter could be cold drawn.

The process is still the standard method used all over the world for the production of

incandescent lamp filaments.

The mid-1920’s saw the emergence of two powder metallurgy products. That set the

patterns for the future industrial development of the technology. The first of these was a

hard, wear-resistant product known as cemented carbide. Cemented carbide is produced by

compacting and sintering a mixture of tungsten carbide and cobalt. Developed originally

for wire-drawing dies, cemented carbides have found extensive application in metal

cutting, rock drilling and hot working dies. Later on, this principle was applied to the

heavy metals.

In the late 1920’s, a second significant development was invented. It was the porous

bronze self-lubricating bearing. The powder mixture is pressed and compact sintered in a

sizing die after which it is impregnated with oil.

The first sintered iron structural parts were produced towards the end of the 1930’s. At this

point, it was recognized that the process for producing the porous bearing was a basis for

other applications. It was an important development because up until then the powder

metallurgy process had only been used to make products which could not be made by any

other technique. Now it was possible to produce new components at a relatively low cost.

This was the main reason for the success. During the past twenty years; the growth in this

industrial sector of the industry has been substantial. One of the reasons was the

contribution from the design engineer who has responded to an increasingly effective

dialogue with the powder metallurgy technologist.



The powder metallurgy process is neither energy nor labour intensive; it conserves

material, is ecologically clean, and it produces components of high quality with

homogeneous and reproducible properties. In recent years, these attributes have attracted

studies into the application of the technique to the development of materials for high

technology uses. Until now, the scientific input into the process and its products had been

confined largely to the specialist areas of the refractory metals and nuclear materials.

Recently, there has been a lot of scientific research in the traditional powder metallurgy

industry, especially in the process and product developments.

A process designated “mechanical alloying”(MA) has been developed in which metal

powders are subjected to high energy milling. Carbides and other non-metallic additives

become coated with the softer metal, allowing the particles to become redistributed as a

very fine dispersion through repeated fracture and re-welding of the composite powder

particles. Originally this process was developed for alloys used in high temperature service

but lately the process has been extended to light alloys for aerospace applications and other

engineering materials [Jenkins 1993, pp. 1-5].

1.2 Objectives of the thesis

1.2.1 Looking for alternative binders to Co for WC particles

For the production of hard metals, Tungsten Carbide is the most commonly used hard

compound. During processing there has to be one compound that will turn into a liquid

phase to form a solid alloy. This compound is called the binder. The most effective binder

for most applications is cobalt. WC-Co cermets or hard metals are composites with more

than 60 vol% WC bound by a metallic phase (Co), which exhibit attractive mechanical

properties making them indispensable in a variety of industrial applications [Santhanam et

al. 1990]. To overcome the shortcomings of cobalt, namely poor corrosion resistance, high

cost and environmental toxicity [Gonzalez et al. 1995], substantial research has been

carried out to find other materials that can replace it. Investigations of binder substitutes



that can replace or reduce the Co phase will be based on Fe/Ni/Co alloys. Our first mission

is to find the appropriate amounts of each element in the alloy. Other alternative binders

will be investigated, including Fe/Cu/Co which were studied by Laoui, Froyen and Kruth

[1999].

1.2.2 Powder Processing by Mechanical Alloying (MA)

The hard metal powder particles as well as the binder particles are captured in a vial

together with a number of metal balls. As the vial starts rotating, the balls collide and some

amount of powder is trapped in between them. During this process, the powder particles

are repeatedly flattened, cold welded, fractured and rewelded.

The powder particles will be plastically deformed by the force of the impact. This leads to

work hardening and fracture. The new surfaces created enable the particles to weld

together, leading to an increase in particle size in an early stage of the process. During

deformation, the particles become work hardened. These particles will fracture by a fatigue

failure mechanism. It is also possible that they fracture by the fragmentation of fragile

flakes. In the absence of strong agglomerating forces, fragments generated by this

mechanism may continue to reduce in size. At this stage, the inclination to fracture prevails

over cold welding. Due to the continued impact of grinding balls, the structure of the

particles will steadily refine.

Alloy powder compositions will be based on WC-10wt.% (Fe/Ni/Co) and WC-10wt.%

(Fe/Mn) with the addition of a grain growth inhibitor, VC at a level of 1wt.%. A reference

sample of WC-10wt% Co will be utilized.

For mechanical alloying, two different techniques are used. The first one was the planetary

ball milling whereas the second involved milling with a horizontal high energy simoloyer.



1.2.2.1 Searching for best fit parameters for planetary ball

milling

The various powder combinations will be mixed with ethanol and sealed in a stainless steel

vial with stainless steel balls. Powder milling will take place in a planetary ball mill. The

influence of rotation speed and milling time will be investigated. After milling, the mixture

will be dried in an oven.

Structural analysis of the powders will involve by X-ray diffraction and optical or

Scanning Electron Microscope (SEM).

1.2.2.2 Horizontal high energy simoloyer

Because of the long milling times to achieve nanostructured grains with the planetary ball

mill, an other type of mechanical alloying is preferred. Therefore the horizontal high

energy simoloyer is recommended. With this type, nanostructures are obtained after a few

hours milling. The influence on longer milling times is investigated as well. The process is

carried out under an inert argon atmosphere.

1.2.3 Powder characterization

A very important factor for hard metals is the grain size of the hard phase. There are many

possible techniques to determine this grain size. It was our aim to find the most suitable

technique for grain size determination in our application. The first technique we have used

was X-ray diffraction (XRD) combined with several analytical analysis techniques, for

example the Sherer equation. After we have discovered that XRD is only useful for grain

size determination in a range between 0,1 and 10µm we had to search for other techniques.

These techniques are described in chapter 4.5. In our further investigation, for grains in

between that range, Scanning Elektron Microscopy (SEM) was the easiest solution.


-Literature review- 16

2 Literature review

Before the practical experiments could take place, a thorough literature review was needed.

This chapter must be seen as a guide to place our study in the field of hard metals.

2.1 Hard metals

Hard metals are an important subject in our daily live. Therefore is interesting to get a wide

spectrum to place our study well.

2.1.1 Introduction

In many areas of engineering especially aerospace and car manufacture, demands for

higher functionality and reliability are leading to the increasing use of materials with

specially tailored properties. Examples of these properties include low wear under

exposure to corrosive media, high heat resistance, increased mechanical strength and low

specific gravity. Materials which can be mentioned in this context are sintered, hard, wear-

resisting materials based on the carbides of one or more of the elements tungsten, tantalum,

titanium, molybdenum, niobium and vanadium, bonded with a metal of lower melting

point usually cobalt. Tungsten carbide is the most widely used.

2.1.2 Powder production

The first step in powder technology is the production of powders. For example, if there is a

need for an alloy of tungsten and carbide, tungsten powder and carbon (graphite) have to

be mixed. This mixture is tungsten-carbide (WC).

In mechanical alloying a suitable binder in powder form is necessary. The most common

used binder for WC is cobalt (Co).



2.1.3 Powder production techniques

Many different techniques are used to produce powders. A brief description of some

common used production techniques will follow below. For further reading we recommend

specific literature.

2.1.3.1 Atomization

In this process molten metal is broken up into small droplets and rapidly frozen before the

drops come into contact with each other or with a solid surface.

The principal method is to disintegrate a thin stream of molten metal by using the impact

of high energy jets of gas or liquid.

Air, nitrogen and argon are commonly used gases, and water is the most widely used

liquid.

Atomization is particularly useful for the production of alloys in powder form, since the

metals are fully alloyed in the molten state. That way each powder particle has the same

chemical composition [EPMA 2004].

2.1.3.2 Gas- and water atomization

The particle size can be controlled by changing several parameters:

design and configurations of the jets,

pressure and volume of the atomizing fluid,

thickness of the stream of metal etc.



A schematic of the vertical gas atomization and water atomization technique are shown in

figures 1 and 2.

Figure 1: Gas atomization, EPMA, 2004

Figure 2: Water atomization, EPMA, 2004

The rate of solidification mainly determines the particle shape. This varies from spherical,

if a low heat capacity gas is employed, to highly irregular if water is used. In principle the

technique is applicable to all metals that can be melted. Results of gas and water

atomization are displayed in figures 3 and 4 [EPMA 2004].



Figure 3: Particle shape with gas atomization, EPMA, 2004

Figure 4: Particle shape with water atomization, EPMA, 2004

2.1.3.3 Centrifugal process

Rotation is used to accelerate and disintegrate the melt by centrifugal atomization. There

are several different variants based on this principle. The most important difference

between them is the choice of vacuum or protective atmosphere. A vacuum atmosphere

limits the methods of heating to electron-beam melting, whilst using argon or helium

permits heating by arc or plasma. Further on, the flight path required to complete

solidification is much longer in vacuum than in an inert atmosphere, which requires special

equipment considerations [EPMA 2004].



2.1.3.4 Chemical processes

Thermal decomposition of a chemical compound is used in some cases, mainly in nickel

carbonyl.

In the beginning, it was a process developed for refining nickel and raw metals which

caused a selective reaction between carbon monoxide and nickel or another raw material

under pressure, resulting in the formation of carbonyl. Carbonyl is a gaseous substance

produced at the reaction temperature. This gas decomposes at a higher temperature and a

lower pressure.

The same process is used for iron, and carbonyl iron powder finds small scale application

where its very high purity is useful.

Typically the particle size of carbonyl iron powder is 1 - 5 µm, but, as in the case of nickel,

it can be tailored to suit particular requirements.

In the Sherritt-Gordon process, nickel powder is made by hydrogen reduction of a solution

of a nickel salt under pressure [EPMA 2004].

2.1.3.5 Electrolysis

By choosing suitable conditions, composition, strength of the electrolyte, temperature,

current density, etc., many metals can be deposited in a spongy or powdery state. However

it is possible that some extensive washing, drying, reducing, annealing and crushing may

be required.

Copper is the main metal to be produced in this way but chromium and manganese

powders can also be produced by electrolysis with these powders. However, a dense and

normally brittle deposit is formed and requires crushing from powders.



Electrolytic iron was at one time produced on a substantial scale but it has been largely

superseeded by powders made by less costly processes. Very high purity and high density

are two distinguishing features of this process [EPMA 2004].

2.1.4 WC-Co

For the production of hard metals, tungsten carbide (WC) is the basic and most widely

used hard compound, whereas cobalt was found to be the best binder material for most

applications. Tungsten alloys and hard metals are essentially composite materials

consisting of a refractory or hard phase bound by a more ductile matrix [Nishiyama 1977].

The reason for selecting the WC-Co system is twofold: it is a classic system, exhibiting

excellent wetting properties between the two phases and it has attractive features for

applications (prototyping of cutting tools, mould inserts and cavities) [Honeycombe 1981,

pp.35-42]. However there are also disadvantages with cobalt as the binder, namely the high

cost, poor corrosion resistance and environmental toxicity [Gonzalez et al., 1995].

Therefore significant effort has been done to replace cobalt as a binder in the system.

2.1.5 Alternative binders to Co for WC

There are different reasons (cost, toxicity and corrosion resistance) to search for alternative

binders to Co for WC and in this part a few alternative binders are given.

2.1.5.1 Fe-Mn as alternative binder to Co for WC

The properties of WC-hard metals are determined by the carbides, as well as the binder

metals and can be widely varied through WC-content, WC-grain size and alloying

additions. The carbides are responsible for properties such as hardness and wear resistance,

but need to be bonded by metals or alloys or provide toughness and strength in the

composite material.



Iron–manganese alloys show characteristics similar to Co with regard to melting

temperature, crystal structure, and γ(fcc) → ε(hcp) phase transformation on cooling and

because of the high wear resistance of Fe-Mn steel, it was felt that Fe-Mn alloys could

provide a high wear resistance when used as a binder for WC-based hard metals

[Hanayaloglu et al. 2001, pp. 315-322].

Mn, like nickel (Ni), is an austenite stabilizer, but when alloyed with Fe, it is

approximately twice as effective as Ni in stabilizing the austenite phase down to room

temperature. Fully or partially austenitic structures can be retained at room temperature

with Fe–Mn alloys containing 13.5 wt % Mn, whereas a Fe-Ni alloy would require 30 wt

% Ni [Shimizu and Tanaka 1978, pp. 685-693 – Schumann 1967, pp. 275-283].

Within the intermediate (10–15 wt.%) Mn range, the Fe-Mn alloy system also exhibits

transformation characteristics of pure Co. That is, the alloys undergo a γ(fcc) → ε(hcp)

martensitic phase transformation on cooling that may well create stacking faults by a

deformation-induced transformation similar to that which is claimed to occur in Co

[Nishiyama 1977].

An additional attraction of Fe-Mn alloys is the possibility of creating a binder phase with

its own inherent wear resistance due to carbon being dissolved from the WC into the

binder phase during the sintering [Hanayaloglu et al. 2001, pp. 315-322].

2.1.5.2 Fe/Ni/Co as alternative binder to Co for WC

In the early days of hard metal history, especially in the German hard metal industry during

and after the Second World War some efforts were made to produce hard metals with Fe-

based binders.



These hard metals never entered the market successfully and disappeared quickly due to

detrimental properties or difficulties in producing stable qualities. In the late 1970’s,

Prakash made the first, well-founded investigations on a broad range of (Fe/Ni/Co)-alloys

and showed that hard metals with such Fe-rich binders have improved properties such as

higher hardness, abrasive wear resistance, toughness and strength compared to Co bonded

hardmetals [Prakash 1993, pp. 80-109 – Prakash 1979].Binders based on Fe-Ni/Ni-Co and

Fe-Cu-Co alloys are reported to be good Co substitutes for cemented carbides and diamond

tools [Gonzalez et al., 1995] [Gonzalez et al., 1998].

From previous investigations, the binder composition 9 wt% (75wt%Fe//15wt%Ni/10wt

%Co) gives results similar to the Co binder [Gille et al. 1999]. However the porosity

increases a little bit when a mixed binder is used instead of a pre-alloyed binder and the

Vickers hardness increases in comparison of the Co binder while the density stays the

same.

Another composition that has been investigated by Laoui, Froyen and Kruth [1999] for the

effect on selective laser sintering process, was the binder composition of 9,5 wt%(54wt

%Fe/28wt%Ni/18wt%Co).

2.1.6 Grain growth

One of the most important factors is grain growth in the hardmetal powders and can be

explained as the phenomenon of increasing grain size of the hard metal particles (WC in

this case) after sintering. This has a negative effect because the purpose of mechanical

alloying is to achieve a homogeneous structure. That’s why inhibitors are usually added to

decrease the size of the binder grains.

The purity of the starting materials is also a very important factor that has to be known

before any experiments can commence.



2.1.6.1 Grain growth inhibitor

There are different kinds of grain inhibitors and the efficiency of the inhibitor depends on

the binder that is used. Therefore, grain inhibitors are discussed for a range of possible

binders and different preparations of the powders, starting with cobalt.

There has to be a difference between high-carbon alloys and low-carbon alloys because the

grain growth inhibitor has a different efficiency in these systems [Brookes K.J.A. 1998, pp.

76-78].

After the WC powder has been compounded, the total composition of the powder has to be

made. The powder that is used in this thesis follows the formula (90 – x) wt% WC, 10%

Co and x wt% grain inhibitor. From previous works, it is already known that 1 wt% grain

inhibitor gives a good effect so x equals 1 in the following experiments [Brookes K.J.A.

1998, pp. 76-78].

Before high energy milling can begin, a decision has to be made whether dry milling or

wet milling should be used. The pros and cons will be explained briefly in the following

points [Brookes K.J.A. 1998, pp. 76-78].

There is a spectrum of possibilities in the sort of grain refiner but the most effective grain

growth inhibitors for the WC-Co system have been found to be V8C7 and Cr2C2 [Brookes

K.J.A. 1998, pp. 76-78].



2.1.6.1.1 The effect of V8C7 and Cr2C2 additives on the sintering of

WC-Co

There is still a debate on whether it is more effective to add V8C7 and or Cr3C2 powders to

WC and Co powders at the milling stage or to add the oxides of vanadium or chromium to

W before carburization. The later option has the advantage that oxides are cheaper than

carbides and that the V8C7 and Cr3C2 produced during carburization together with WC are

better dispersed [Brookes K.J.A. 1998, pp. 76-78]. However, nowadays most producers do

not produce their own WC and their only option is to add V8C7 and/or Cr3C2 at the milling

stage.

Both carbides are added to WC-Co in very small amounts, typically not exceeding 1 wt%.

These small amounts dissolve totally in the cobalt during sintering, but it has been found

that during cooling V8C7 precipitates as nanoparticles of (V,W)C [Gille et al. 1999] while

Cr3C2 remains in solution in the cobalt, although it tends to diffuse towards the WC-Co

bounderies [Okada et al. n.d.]

Both V8C7 and Cr2C3 have been found to lower the onset temperature of the eutectic

reaction in WC-Co. In the case of V8C7 it was found to decrease by 25-30°C [Lucyckx and

Alli 2000, pp. 507-510] and in the case of Cr3C2 by approximately 40°C [Luyckx et al.

1996, pp. 39-41].

2.1.6.1.2 Effect of V8C7 and Cr3C2 additions on WC-Co grain growth

and mechanical properties

It is generally agreed that V8C7 is the most effective grain growth inhibitor, since smaller

amounts are required to achieve equal inhibiting effects. Figure 5 shows a comparison

between the grain size obtained in sintered WC-10 wt% Co using equal amounts of V8C7

and Cr3C2.



However, there does not appear to be an agreement on the effect of the two inhibitors on

the properties of the sintered material. Some authors claim that additions of V8C7 harden

the material more than additions of Cr3C2 , while other authors claim the opposite

[Lucyckx and Alli 2000, pp. 507-510].

Figure 5: Comparison of the effect of grain refiner content on WC grain size, LUYCKX, S., 2001

As far as resistance to fracture is concerned, the general belief among hardmetal producers

is that V8C7 embrittles WC-Co more than Cr3C2 does, and that the embrittlement is due to

the intrinsic brittleness of V8C7. The parameter controlling crack resistance is the binder

mean free path, which, at constant Co content, is proportional to the WC grain size. Thus

for equal inhibitor additions V8C7 lowers the crack resistance more than Cr3C2 because it

produces finer grain sizes and narrower mean-free paths [Lucyckx and Alli 2000, pp. 507-

510].

Since Cr3C2 remains in solution while V8C7 precipitates during cooling, Cr3C2 increases the

total binder content and hardens the binder. On account of this, alloys containing Cr3C2



reach the same hardness at larger WC grain sizes and for equal hardness have higher crack

resistance than alloys containing V8C7 [Lucyckx and Alli 2000, pp. 507-510].

The main advantage of adding V8C7 as a grain refiner, however, is that it can aid the

development of extremely fine grain sizes, which can be obtained only by adding much

larger amounts of Cr3C2 [Luyckx and Alli 2000, pp. 507-510].

2.2 Powder metallurgy

Among the various metalworking technologies, powder metallurgy is the most diverse

manufacturing approach. One attraction to powder metallurgy (PM) is its ability to

fabricate high quality, complex parts to close tolerances in an economical manner [German

n.d., pp. 6-7].

2.2.1 The process

The process can be seen as three different steps and they will be explained in the following

points.

2.2.1.1 Mix the powder with a suitable lubricant

In this step, the powder will be mixed with a suitable lubricant, e.g. zinc stearate. The only

reason why a lubricant is added is to reduce friction during the compaction process.

The powders could be produced by the processes described in 4.1.2 earlier in this thesis.

2.2.1.2 Powder compaction



After applying pressure on the milled powder, a compacted part is produced. This part

requires only sufficient cohesion to enable it to be handled safely and transferred to the

next stage. Such compacts are referred to as green which means unsintered. The terms

green density and green strength are used to describe the green state.

2.2.1.3 Sintering

Sintering takes place in a protective atmosphere under vacuum. The temperature will be

controlled below the melting point of the main constituent so that the powder particles

weld together and confer sufficient strength to the object for its intended use.

This process is called sintering. In certain cases a minor constituent becomes molten at the

sintering temperature in which case the process is referred to as liquid phase sintering. The

constituent that becomes molten is referred to as the binder.

The amount of liquid phase must be limited so that the part (green state) retains its shape.

In certain special cases compaction and sintering are combined at an elevated temperature

such that sintering occurs during the process. This is referred to as hot pressing or pressure

sintering [EPMA 2004].

2.2.2 Reasons for using PM

Three overlapping categories provide an introductory concept for the reasons for using

PM. Figure 6 is a Venn diagram showing how the applications for PM can be categorized.

First are the many applications which rely on the low cost production of complex parts.

Components for the automotive industry represent good examples of this area and their

production is a large PM activity. Within the area of economical parts production come

concerns with productivity, tolerances and automation.



Figure 6: Venn-diagram, GERMAN, R.M.

Contrasting the powder route with fusion metallurgy (casting), both the precision and cost

are very attractive. Furthermore, with casting there are problems and costs associated with

segregation, machining and final tolerances which can be avoided with metal powder-

based approaches.

As figure 6 shows, there are also unique properties or unique microstructure justification

for using PM approaches. Some examples include porous metals, oxide dispersion

strengthened alloys, cermets and cemented carbides.

The inability to fabricate these unique microstructures by alternative techniques has

contributed a large part to the growth of PM.

The final circle shown on the Venn-diagram corresponds to captive applications. These are

the materials which are quite difficult to process by any other techniques. Ideal examples

are the reactive and refractory metals where fusion techniques are not practical. Another

rapidly emerging group of materials are the amorphous or glassy metals. In many cases, it



is desirable to form a powder and develop low temperature processing to avoid the

microstructural damage accompanying elevated temperatures. PM techniques are attractive

since all of the processing can be performed in the solid state.

Usually elements from all three categories shown in figure 6 exists in most practical PM

applications. Indeed the major growth and expansion will most likely com from further

combinations of these three elements in forming unique, low cost, high quality products.

2.2.3 Applications of PM

In this section, a few applications of PM are given.

2.2.3.1 Self-lubricating bearings

Self-lubricating bearings are typically made of bronze. The starting materials may be

mixed from elemental powders of copper and tin, fully pre-alloyed bronze powder, or

mixtures of the three [EPMA 2004].

These kinds of bearings are largely used in the automotive industry. Figure 7 gives a

picture from self-lubricating bearings.

Figure 7: Self -lubricating bearing, EPMA, 2004



2.2.3.2 Hard metals

Hard metals are widely used as high precision cutting tools because of their good wear-

resistance.

2.2.3.3 Friction materials

Sintered metal friction components are particularly useful for heavy-duty applications, e.g.

aircraft breaks, heavy machinery clutch and brake linings etc.

They consist essentially of a continuous metal matrix, into which varying amounts of non-

metallic friction generators, such as silica and emery are bonded [EPMA 2004].

A picture of friction materials is given in figure 8.

Figure 8: Friction materials, EPMA, 2004



2.2.4 The future of PM

The past successes of PM have derived from the economic benefits. In the more recent

times, the unique/difficult to process materials have contributed tot the expansion in the

technological base. These same attributes are expected to continue to come together to

form new applications for PM. Five areas appear to hold the necessary ingredients for

continued growth:

1. High volume production of precise, high quality structural parts, typically from

ferrous based alloys.

2. Difficult-to-process materials, where fully dense high performance alloys can be

fabricated with uniform microstructures.

3. Specially alloys, typically composites containing mixed phases.

4. The nonequilibrium materials such as amorphous, microcrystalline or metastable

alloys.

5. The complex parts possessing unique and uncommon shapes or ingredients.

The future promises more challenges with the combination of cost savings and factors such

as reliability, quality, strength, dimensional control and unique shaping capabilities. The

appreciation of these advantages will provide both economic and technological growth

opportunities [German n.d., p. 7].

2.3 Nanostructural materials

2.3.1 What are nanostructured materials

o a broad class of materials, with microstructures modulated in zero to three

dimensions on length scales less than 100 nm.



o materials with atoms arranged in nanosized clusters, which become the

constituent grains or building blocks of the material

o any material with at least one dimension in the 1-100nm range

Conventional materials have grains sizes ranging from microns to several millimeters and

contain several billion atoms each. Nanometer sized grains contain only about 900 atoms

each. As the grain size decreases, there is a significant increase in the volume fraction of

grain boundaries or interfaces. This characteristic strongly influences the chemical and

physical properties of the material. For example, nanostructured ceramics are tougher and

stronger than the coarser grained ceramics. Nanophase metals exhibit significant increases

in yield strength and elastic modulus.

The growing demand for nanopowders arises from the change in physical, chemical and

electrical properties exhibited by particles when their size falls below about 100 nm. The

laws of quantum physics, rather than the laws of classical physics, come into play at these

small particle sizes and the behaviour of the surfaces start to dominate the bulk behaviour

of the material. For example, materials that would normally be conductors of electricity

can become insulators at the nanoscale, or vice versa [Pritchard 2004].

Using a variety of synthesis methods, it is possible to produce nanostructured materials in

the following forms: thin films, coatings, powders and as a bulk material [Nanostructured

Materials 2004].

2.3.2 Synthesis

There are two basic approaches to the production of nanomaterials [Nanostructured

Materials 2004]. The first is the ‘top-down’ approach, which involves the breaking down

of the bulk material into nanosized structures or particles. These techniques are an

extension of those that have been used for producing micron-sized particles. An example



of this technique is mechanical alloying. The alternative approach, which has the potential

of creating less waste and is more economical, is the ‘bottom-up’ approach. In this

approach individual atoms or molecules are built up to form the require nanostructure or

nanoparticles. Many of these techniques are still under development or are just beginning

to be used for commercial production of nanopowders. The major technical difficulties to

overcome in developing a successful bottom-up approach is controlling the growth of the

particles and then stopping the newly formed particles from agglomerating. Other technical

issues are ensuring the reactions are complete so that no unwanted reactant is left on the

product and completely removing any growth aids that may have been used in the process.

Bottom-up approaches, in current use or at an advanced stage of development, can be

classified into liquid, vapour or solid phase techniques [AP Materials Inc n.d.].

2.3.2.1 Mechanical alloying

Pure powders with particle sizes in the range of 1 to 200 μm, the so called raw materials,

are widely commercially available. The only consideration that has to be made is that the

powder particle size is smaller than the grinding ball size, so the starting size of the powder

isn’t important. The explanation therefore is that the powder particle size decreases

exponentially with time, so it will reach a small value of a few microns after only a few

minutes of milling.

In the early days of MA, the powder charge consisted of at least 15 vol% of a ductile,

compressible and deformable metal powder to act as a binder. However mixtures of fully

brittle materials have been milled successfully to form an alloy [Koch 1991, pp. 193-245].

Therefore, it is no longer necessary to have a ductile metal powder during milling [Ivanov

1992, pp. 475-480].

As a result, for the production of novel alloys ductile-ductile, ductile-brittle, and brittle-

brittle powder mixtures are milled. Mixtures of solid powder particles and liquids have also



been milled in recent times. Milling metal powders with a liquid medium is termed wet

grinding. When there is no liquid involved in the milling process it is referred to as dry

grinding [Okada et al. 1992, pp. 862-864 - Bellosi et al. 1997, pp. 255-260].

Wet grinding has been reported as a more suitable method to obtain finer-ground products

than dry grinding because the solvent molecules are adsorbed on the newly formed

surfaces of the particles and lower their surface energy. Another advantage is the less-

agglomerated condition of the powder particles in the wet condition. It has been reported

that the rate of amorphization is faster during wet grinding than during dry grinding

[Dolgin et al. 1986, pp. 281-289]. A disadvantage of the wet grinding, however, is the

increased contamination of the powder.



2.3.2.1.1 Mechanism of alloying

During high-energy milling, the powder particles are repeatedly flattened, cold welded,

fractured and rewelded. Whenever two steel balls collide, some amount of powder is

trapped in between them. During each collision, there are around 1000 particles with an

aggregate weight of 0,2 mg trapped [Suryanarayana 2001d, p.32] (figure 9).

Figure 9: Ball-powder-ball collision of powder mixture during MA, SURYANARAYANA, C., 2001

The powder particles will be plastically deformed by the force of the impact. This leads to

work hardening and fracture. The new surfaces created enable the particles to weld

together, leading to an increase in particle size. In the early stages of milling, the particles

are soft and as a result, their tendency to weld together and form large particles is high.

This statement is only valid when using either a ductile-ductile or ductile-brittle material

combination. A broad range of particle sizes develops. Some of them are as large as three

times the starting particles. The composite particles at this stage have a characteristic

layered structure. This structure consists of various combinations from the starting

constituents. During deformation, the particles become work hardened. These particles will

fracture by a fatigue failure mechanism. It is also possible that they fracture by the

fragmentation of fragile flakes. In the absence of strong agglomerating forces, fragments

generated by this mechanism may continue to reduce in size. At this stage, the inclination



to fracture prevails over cold welding. Due to the continued impact of grinding balls, the

structure of the particles will steadily refine. However, the particle size remains to be the

same. As a consequence, the inter-layer spacing decreases and the number of layers in a

particle increase.

Nevertheless, it should be remembered that the efficiency of particle size reduction is very

low. The efficiency in a conventional ball mill is about 0.1%. It is possible that in high-

energy ball milling processes the efficiency may be somewhat higher, but it is still less

than 1% [Suryanarayana 2001c, pp. 32-33].

The remaining energy is lost, mostly in the form of heat. However, a small amount is also

utilized in the elastic and plastic deformation of the powder particles.

Steady-state equilibrium is attained when a balance is achieved between the rate of

welding and the rate of fracturing after milling for a certain length of time. The rate of

welding tends to increase the average particle size [Benjamin 1990, pp. 122-127] while the

rate of fracturing tends to decrease the average composite particle size. Smaller particles

are able to resist deformation without fracturing. They also tend to be welded into larger

pieces. The overall tendency is to drive both, the very fine and the very large particles,

towards an intermediate size. At this stage, each particle contains essentially all of the

starting ingredients together. Due to the accumulation of strain energy, the particles reach

their saturation hardness. The particle size distribution at this stage is narrow. The reason is

that particles larger than average are reduced in size at the same rate. These particles split

smaller than the average grow trough agglomeration of smaller particles [Lee et al. 1998,

pp. 235-239]. From the forgoing, it is clear that during MA heavy deformation is

introduced into the particles.

This is shown by the attendance of a variety of crystal defects such as:

dislocations,



vacancies,

stacking faults,

and increased number of grain boundaries.

Figure 10: Narrow particle size distribution caused by tendency of small particles to weld together and large

particles to weld together and large particles to fracture, SURYANARAYANA, C., 2001

The presence of this defect structure enhances the diffusivity of solute elements into the

matrix. Further, the refined microstructural features decrease the diffusion distances. The

slight rise in temperature during milling further aids the diffusion behaviour.

Consequently, true alloying takes place amongst the constituent elements. While this type

of alloying generally takes place nominally at room temperature, it may sometimes be

necessary to anneal the mechanically alloyed powder at an elevated temperature for

alloying to be achieved. This is particularly true when formation of intermetallics is

desired.

To develop a given structure in any system, you require specific times. These times would

be a function of the initial particle size and characteristics of the ingredients as well as the

specific equipment used for conducting the MA operation and the operating parameters of

the equipment. In most cases, the rate of refinement of the internal structure (particle size,



crystallite size, lamellar spacing, etc.) is roughly logarithmic with processing time and

therefore the size of the starting particles is relatively unimportant. Lamellar spacing

usually becomes small and the crystallite (or grain) size is refined to nanometer

dimensions. These dimensions can be acquired in a few minutes to an hour. That way, the

production of nanostructured materials is easy. This is one reason why MA has been

extensively employed to produce nanocrystalline materials [Koch 1993, pp. 109-129 –

Suryanarayna 1995b, pp. 41-64].

As mentioned above, it is possible to conduct MA of three different combinations of

metals and alloys:

ductile-ductile,

ductile-brittle,

brittle-brittle systems.

The ductile-brittle combinations are the most important ones because WC-Co and the

possible alternative binders for Co belongs to this combination. This combination is going

to be investigated.

More information for the other two combinations can be found in:

BENJAMIN, J.S. (1990) Metal Powder Rep. 45, pp. 122-127.

GILMAN, P.S. and BENJAMIN, J.S. (1983) Annu Rev Mater Sci. 13, pp.279-300.

DAVIS, R.M. and KOCH, C.C. (1987) Scripta Metall. 21, pp. 305-310.

DAVIS, R.M., McDERMOTT, B. and KOCH, C.C. (1988) Metall Trans. A19, pp.

2867-2874.



Figure 11: Refinement of particle size and grain sizes with milling time, SURYANARANA,C., 2001

The traditional ODS (oxide dispersion strengthened) alloys belong to this category because

the brittle oxide particles are dispersed in a ductile matrix. The microstructural evolution in

this type of system was also described by Benjamin and others [1983, pp. 279-300]. In the

initial stages of milling, the ductile metal powder particles get flattened by the ball-

powder-ball collisions, while the brittle oxide or intermetallic particles get

fragmented/comminuted (Fig. 12a). These fragmented brittle particles tend to become

occluded by the ductile constituents and trapped in the ductile particles. The brittle

constituent is closely spaced along the interlamellar spacings (Fig. 12a).

With further milling, the ductile powder particles become work hardened and the lamellae

get convoluted and refined (Fig.12b). The composition of the individual particles

converges toward the overall composition of the starting powder blend. With continued

milling, the lamellae get further refined, the interlamellar spacing decreases, and the brittle

particles get uniformly dispersed, if they are insoluble as in an ODS alloy (Fig. 12c).

On the other hand, if the brittle phase is soluble, alloying also occurs between the ductile

and brittle components also. That way chemical homogeneity is achieved. Whether

alloying occurs or not in a ductile-brittle system also depends on the solid solubility of the

brittle component in the ductile matrix. If a component has a negligible solid solubility



then alloying is unlikely to occur, e.g., boron in iron. Thus, alloying of ductile-brittle

components during MA requires not only fragmentation of brittle particles to facilitate

short-range diffusion, but also reasonable solid solubility in the ductile matrix component.

Figure 12: Schematics of microstructural evolution during milling of a ductile-brittle combination of

powders, SURYANARAYANA, C., 2001

2.3.2.1.2 Types of mills

Different types of milling equipment are used to produce mechanically alloyed powders

[Suryanarayama 2001a, pp. 1-184].There is a big difference between laboratory equipment

and milling devices for commercial use.



They differ in their:

capacity,

efficiency of milling

additional arrangements for cooling,

heating

2.3.2.1.2.1 Planetary ball mills

A popular mill for MA is the planetary ball mill as shown in figure 13.

Figure 13: Fritch Pulverisette 5 planetary ball mill

Its name is derived from the movement of the vials that describe a planet-like movement.

While the support disk performs a circular movement, a special drive mechanism causes

the vials to rotate around their own axes. The centrifugal forces caused by the rotating

movements will act on the contents of the vials. Since the vials and the supporting disk



rotate in opposite directions, the centrifugal forces alternately act in like and opposite

directions as shown in figure 13. This causes the grinding balls to run down the inside wall

of the vial which is called the friction effect. Hereby the material will be ground; the

grinding balls will lift off and travel freely through the inner chamber of the vial. Their

flight will be ended as they bump against the opposing inside wall, termed as the impact

effect.

The Fritch Pulverisette has the possibility of changing the movement direction during the

milling process.

Figure 14: Movement of balls in the planetary ball mill, Zoz

Figure 15: Movement of balls in the planetary ball mill during milling, Zoz



Figure 14 and 15 displays the movement of the balls inside of the vials when the planetary

ball mill is working.

Grinding vials and balls are available in different materials, including tungsten-carbide,

agate, silicon-nitride, sintered corundum, zirconia, chrome steel, Cr-Ni steel, and plastic

polyamide [Suryanarayama 2001a, pp. 1-184].

Figure 16: Schematic depicting the ball motion inside the ball mill, Courtesy of Gilson Company

2.3.2.1.2.2 High energy ball milling

An example of the top-down technique is high-energy ball milling. Often an abrasive is

added to the process to aid the milling process and milling times can vary from several

hours up to many days. In general, increased energies and milling times result in decreased

particle sizes. An alternative technique is cryogenic milling, where the material is first

cooled to a low temperature to make it more brittle and easier to break down by milling.

The difficulty with top-down approaches is ensuring all the particles are broken down to

the required particle size. Milling typically results in a Gaussian size distribution with a

long ‘tail’ representing the un-milled product. Furthermore, longer milling times will result

in more milling impurities, which together with any milling aids, if used, can be difficult to

remove. The Simoloyer is a horizontal high-energy ball mill and is known from academic



as well as industrial applications in mechanical alloying, high-energy milling and reactive

milling.

These devices supply the highest relative velocity of grinding media, a high level of kinetic

energy transfer, an intensive grinding effect and short processing times. The contamination

of the processed powders by the milling tools is naturally lower since the process is based

on the collision of grinding media.

Since a horizontally arranged rotor inside the grinding vessel accelerates the grinding

media, these devices do not have to move unnecessarily any large masses like the entire

chamber/mill in case of vibration or planetary ballmills.

Large-scale systems of several hundred litres volume and are economically and

ecologically favourable.

Figure 17: High-energy mill, Metal-Powder

Atmosphere and cooling seems non-problematic as these mills can be operated, loaded and

unloaded under vacuum or inert gas and are equipped with efficient cooling or cooling and

heating systems. Figure 17 shows a real image of a horizontal high-energy ball mill in

operation.



The effect of collision in MM is visualised in figures 18 and 19 and the working principle

can be imagined where the rotor is the tool to transfer the kinetic energy into the grinding

media and the grinding media transfers it into the powder material.

The system is operated with water cooling or heating at rotation frequencies up to 1800

rpm.

Figure 18: Running high-energy mill, Zoz

Figure 19: Inside a high-energy mill, Zoz

2.3.2.1.2.3 Other types of mills

Many other types of milling devices are available. Shaker mills such as SPEX mills (figure

20), which mill about 10 to 20 g of powder at a time, are most commonly used for

laboratory investigations and for alloy screening purposes.



Figure 20: SPEX 8000 mills in the assembled condition, SURYANARAYANA,C., 2001

The most common type of this mill contains only one vial. The sample and the grinding

balls are put together in the vial, secured in the clamp and swung energetically backwords

and forwards several thousand times each minute. This shaking motion is combined with

lateral movements of the ends of the vial. This causes the appearance of a figure of eight

movement of the vial. The milling and mixing of the sample is caused by the impact of the

balls against the sample with each swing of the vial. As a result, of the amplitude (about 5

cm) and speed (about 1200 rpm) of the clamp motion, the ball velocities are high (on the

order of 5 m/s). This results in an unusually large force of the ball’s impact. Therefore,

these mills can be classified as a high-energy type [Suryanarayama 2001a, pp. 1-184].

An attritor mill (figure 21) consists of a vertical drum with a series of impellers inside it.

Powder size reduction is caused by the impact between balls, between balls and container

wall, a gitator shaft, and impellers. A powerful motor rotates the impellers, which in turn

agitate the steel balls in the drum [Suryanarayama 2001a, pp. 1-184].



Figure 21: Attritor, IMP

Attritor mills allow milling of large amounts of powder at a time (from about 0,5 to 40

kg). The velocity of the grinding medium is much lower (about 0,5 m/s) than in Fritsch or

SPEX mills and consequently the energy of the attritors is low. The working principle is

shown in figure 22.

Figure 22: Working principle of the attritor, SAAR-HARDMETAL

Commercial mills for MA are much larger in size than the mills described above and can

process over hundred pounds at a time. Mechanical alloying for commercial production is



carried out in ball mills of up to about 1250 kg capacity [Suryanarayama 2001a, pp. 1-

184].

2.3.2.1.3 Process variables

Due to the complexity of the MA process optimization involves a number of parameters to

achieve the desired product phase and/or microstructure. The most important variables that

have an effect on the final product are:

type of mill,

milling container,*

milling speed,*

milling time,*

type, size, and size distribution of the grinding medium,*

ball-to-powder weight ratio,*

extent of filling the vial,*

milling atmosphere,

process control agent,

temperature of milling.

(* these are the parameters which will be explored and explained. An explanation of the

other terms can be found in C. Suryanarayana [2001b, pp. 21-29]

Some of these parameters are linked with each other. For example, the optimum milling

time depends on the type of mill, size of the grinding medium, temperature of milling, ball-

to-powder ratio, etc.

One important conclusion from the above parameters is that the material of the vial, the

balls and one of the powders has to be the same to decrease the contamination.

2.3.2.1.3.1 Milling container



Due to impact of the grinding medium on the inner walls of the container, some material

will be dislodged and get incorporated into the powder. This leads to contamination of the

powder or alterations in the chemistry of the powder. The powder may be contaminated

with the grinding vial material if the material of the grinding vessel is different from that of

the powder. If the two materials are the same, the chemistry can be changed. Therefore

there have to be taken precautions against the additional amount of the element

incorporated into the powder.

Hardened steel, tool steel, hardened chromium steel, tempered steel, stainless steel, WC-

Co, WC-lined steel [Ivanov et al. 1999, pp. 377-383], and bearing steel are the most

common types of materials used for the grinding vessels.

2.3.2.1.3.2 Milling speed

The faster the mill rotates, the higher the energy input into the powder. However there are

limitations to the increase in speed. For example, increasing the speed of rotation will

increase the speed with which the balls move. Above a critical speed, the balls will be

pinned to the inner walls of the vial and will not fall down to exert any impact force, so it

is important that the maximum speed stays below this critical value.

The high amount of energy will cause an increase in temperature, so temperature will be

another limitation for the maximum speed. In some cases, higher temperatures can be an

advantage, especially when diffusion is required to promote homogenization and alloying

in the powders. There is also a disadvantage; the increase in temperature accelerates the

transformation process and results in the decomposition of supersaturated solid solutions or

other metastable phases formed during milling [Kaloshkin et al. 1997, pp. 565-570].

Summary: the maximum temperature is influenced by many parameters and these values

vary widely (also dependent on the type of mill).



Research has been carried out looking at the difference of the final powder constitution

when vanadium and carbon powders were milled together at different energy levels [Calka

A. et al. 1993, pp. 189-195]. For example, at very low milling energy (or speed), the

powder consisted of nano-sized grains of vanadium and amorphous carbon, which on

annealing formed either V2C or a mixture of V and VC. At an intermediate energy level,

the milled powder contained a nanostructure. This powder will be transferred to VC when

it’s annealed. At the highest energy level, the VC formed directly on milling.

2.3.2.1.3.3 Milling time

A very important parameter is the milling time. The milling time should be chosen to

achieve a steady state between fracturing and cold welding of the powder. The type of mill

has an influence on the times required.

By choosing the appropriate time, one should also consider the type of mill used. Also the

combination of powders is a parameter which should be considered. As the milling time

increases, the level of contamination will also increase. Therefore it is desirable that the

powder is milled just for the required time and no longer [Suryanarayana 2001a, pp. 1-

184].

2.3.2.1.3.4 Grinding medium

The most common steels used for the grinding medium (milling-balls) are: hardened steel,

tool steel, hardened chromium steel, tempered steel, stainless steel, WC-Co and bearing

steel. The impact force on the powder created by the balls is determined by the density of

the grinding medium. Therefore the density must be high enough.

The milling efficiency is also dependant on the size of the grinding medium, a larger

medium has higher impact on the powder particles. Therefore efficiency will increase with

a larger medium.



Also the final constitution is dependent upon the size of the grinding medium. For

example, it has been reported that, when balls of 15 mm diameter were used to mill the

blended elemental Ti-Al powder mixture, a solid solution of aluminium in titanium was

formed. On the other hand, use of 20 and 25 mm diameter balls resulted in a mixture of

only the titanium and aluminium phases, even after a long milling duration [Lai and Lu

1998]. Figure 23 shows a figure of the grinding medium.

Generally the sizes that are used in an investigation are limited to one ball diameter.

Although there have been studies where different sized balls were used [Atzom 1990, pp.

487-490 and Gavrilov et al. 1995, p. 11].

Figure 23: Grinding medium, NARANG ENTERPRICES

It has been predicted that the highest collision energy can be obtained if balls with different

diameters are used.

During the first time period of the milling process, the powder becomes coated onto the

surface of the milling balls. Hereby high wear of the grinding medium is prevented. Also

contamination of the powder due to wear of the milling balls will be avoided.

However, to become a homogeneous final product, the thickness of this layer must be kept

to a minimum. This powder coating on the grinding balls is also difficult to remove.

The amount of cold welding and the amount of powder coated onto the surface of the balls

will be reduced by using a combination of large and small sized balls during milling

[Takacs et al. 1994, pp. 5864-5866]. Using balls of the same size in diameter causes the



balls to follow certain tracks and the balls roll along a well defined trajectory. Therefore it

is useful to combine small and larger balls in order to obtain a randomized motion [Takacs

1996, pp. 453-464].

2.3.2.1.3.5 Ball-to-powder weight ratio

The ratio of the weight of the balls to the powder (BPR), sometimes referred to as charge

ratio (CR), is an important variable in the milling process. Investigations have been done

with BPR varied from 1:1 [Chin and Perng 1997, pp. 235-238 and pp. 121-126] to 220:1

[Kis and Beke 1996, pp. 465-470]. The most common ratio used in a small capacity mill is

10:1. When milling is preformed in a large capacity mill, a higher BPR of up to 50:1 or

even 100:1 is used.

The higher the BPR, the greater the number of collisions per minute, due to higher weight

proportion of the balls, leading to more energy being transferred to the powder particles

and so alloying takes place faster.

There is a possibility that due to the higher energy, more heat is generated. This could have

an effect on the final constitution of the powder. The BPR may also effect the phase

formed after the milling process.

2.3.2.1.3.6 Extent of filling the vial

In order to allow the balls to move so they can produce impact forces on the particles, it is

necessary that there is enough space in the vial. Therefore, the extent of filling the vial with

the powder and the balls is important. If the quantity of the balls and the powder is very

small, then the production rate is also very small. On the other hand, if the quantity is

large, then there is not enough space for the balls to move around and so the energy of the

impact is less. Therefore it is important not to overfill the vial. Generally 50% of the vial

space is left empty.



2.3.2.2 Liquid phase techniques

Wet chemistry techniques that have been used for producing larger particles are being

adapted to produce nanosized material, the product crystallising or precipitating out of the

solution. In order to achieve the reduction in particle size the reaction chemistry must be

chosen to provide a fast spontaneous reaction and also be able to limit subsequent growth

of the particles after nucleation. The particle size can be controlled by the use of various

polymers, gels or micro-emulsions to constrain the growth of the particles [AP Materials

Inc. n.d.].

2.3.2.3 Vapour phase techniques

There are a number of techniques that can be classed under this category, but they all

involve two basic steps. The first step is the vaporisation of the material followed by a

rapid controlled condensation to produce the required size of particle. In some techniques

the powder formed is the same composition as the starting material, while others rely on

decomposition occurring during the vaporisation step to produce the desired product.

Production of carbon black, obtained by collecting the soot produced by the burning of

natural gas or other hydrocarbons, is probably the earliest example of a vapour phase

technique. Silica (silicon dioxide) and titanium dioxide are also produced in bulk by a

combustion (flame synthesis) process. A modification of the flame synthesis, known as

sodium flame and encapsulation (SFE) technology [Argonide 2004], is currently under

development, which should also allow the bulk production of non-oxide powders of

ceramics, metals and composites. In SFE the particles are encapsulated as they formed by a

protective layer, which prevents oxidation and inhibits agglomeration.

Other energy sources besides combustion are now being used to vaporise the material. For

example metal nanopowders can be produced by an exploding wire technique. This

technique is applicable to any metal that is available as a continuous ductile wire[Shenzen

n.d.]. Diamond nanopowders are being produced by a detonation process [Tetronics ltd.



n.d.]. Metal, metal oxide and ceramic nanopowders are being produced using a plasma arc

to generate the vapour [Qinetiq Nanomaterials Inc. 2003 – Office of technology Transfer

n.d]. An electromagnetic vaporisation process (EVP) [Epma 2004a], using an alternating

magnetic field to generate fine droplets of molten metal, is being used to produce metal

nanopowders. Yet another variant is laser ablation [Okada et al. 1992, pp. 862-864]. In this

technique a high-energy laser beam is used to evaporate a compressed precursor powder.

The evaporated material is then condensed into nanoparticles by a process of collision and

growth. Development work continues on all of these techniques to increase their yields and

reduce the production costs.

2.3.2.4 Plasma heating

Plasma heating provides a clean, directional, controllable, high intensity, localised source

of contaminant free heat. Due to the extreme high temperatures in the plasma zone (up to

20 000 degrees Kelvin) a range of novel materials can be produced. These often take the

form of spherical nanometric powders with diameters in the range 15 - 300 nm, with

exceptionally high specific surface area/mass ratios, up to >200m²/g. These materials have

generated a considerable level of commercial interest in the electronic, ceramic,explosive

enhancement and catalytic sectors [AP Materials Inc n.d.]. Figure 24 shows the principle of

plasma heating.



Figure 24: Principle of plasma heating, Max Planck Institute

2.3.2.5 Solid phase techniques

An example of this technique is mechanochemical processing (MCP) that is being used to

produce metal oxide nanopowders. Dry milling is used to induce chemical reactions

through ball-powder collisions that result in nanoparticles being formed within a salt

matrix. The particle size produced depends on the chemistry of the reactant mixture,

milling and heat treatment conditions. Particle agglomeration is minimised by the salt

matrix, which is then removed by a simple washing procedure.

2.3.2.6 Equal channel Angular Extrusion

Equal Channel Angular Extrusion (ECAE) was invented in the former Soviet Union by

Vladimir Segal in 1977 [Tamu n.d.].

ECAE is an innovative process capable of producing uniform plastic deformation in a

variety of materials, without causing significant change in geometric shape or cross



section. Multiple extrusions of billets by ECAE permits severe plastic deformation in bulk

materials. By changing the orientation of the billet between successive extrusions, complex

microstructures and textures can be developed. Changing the chosen billet orientation after

each pass, five fundamental equal-channel angular extrusion routes are defined and utilized

to obtain different textures and microstructures.

Research has concentrated in two areas:

development of theoretical and practical knowledge of the mechanics of ECAE (mathematically modelling, etc.)

investigation of the various technological advantages of ECAE. Many advantages were found with ECAE. ECAE has also been found to be an excellent method for powder consolidation.

The process involves extruding a metal billet through a die consisting of two equal

channels intersecting at an angle (see figure 25). After the billet has passed through the exit

channel, it has experienced severe plastic deformation, primarily in the form of shear

strain, yet no change in cross section. Therefore, one can re-insert the billet into the entry

die and impose more severe plastic strain. It is not uncommon for the ECAE processing

procedure to involve up to ~ 16 – 20 passes through the die. Different processing routes are

specified by the sequences of twists about the billet long axis imposed after each pass and

prior to re-insertion.



Figure 25: Principle scheme of the ECAE

ECAE is a process involving both SPD and strain path changes. With each pass through

the die, one can impose from approximately 83% up to 100% shear strain as the die angle

decreases from 135° to 90° [Segal 1995, pp. 57-164 – Iwahashi et al. 1996, pp. 143-146].

The severe plastic deformation generated in one pass alone is sufficient to generate

subgrain boundaries, the preliminary stages of eventual grain refinement. Reinsertion of

the billet changes the strain path for all routes, including route A, which involves no twists

about the billet axis. Thus, by altering the sequences of twists from pass to pass, one

effectively alters the strain path imposed, which leads to changes in microstructural

development. The combined influence of SPD and strain path changes can potentially lead

to the unique microstructures responsible for their superior strength and ductility.

Numerous ECAE studies in recent years have shown that the relationships between the

resulting microstructure (e.g. grain size, boundary misorientation, texture), material (e.g.

crystal structure, alloying, initial microstructure), and deformation history (die geometry,

route, number of passes, friction) are indeed complex ones. As one can imagine, there is a

great amount of flexibility in the design of the ECAE process. Material specifications for

these advanced nanomaterials will likely depend on application, some wanting a specific

ultrafine grain size, maximum strength and ductility, or a minimum degree of anisotropy



and inhomogeneity in properties, to name a few. The challenge in advancing ECAE to the

marketplace will be in developing predictive capabilities for designing nanomaterials to

specification and optimizing the process for fewer passes and larger specimens. Achieving

such a task will involve multiscale modeling from the nanoscale level up to the continuum

level (at least 7 orders of magnitude difference).

2.3.2.6.1 Simple shear concept

Naturally some of the first ECAE modeling efforts focused on the largest scale; that is, the

continuum level, modeling the plastic flow of the billet during one pass. The simplest way

is to classify it as a homogeneous deformation of simple shear localized along the

intersection plane of the two channels (figure 25) The applied shear γ as a function of Φ was determined by Segal [1995, pp. 57-164].

Once a strain state is defined, it is common to reduce it to a scalar measure, typically the

von Mises equivalent strain εvM. This scalar measure of strain is considered to be additive

from pass to pass; for N passes the accumulated strain is NεvM. Due to the strain path

changes, no point in the material experiences continuous monotonic simple shear in

multipass ECAE [Beyerlain et al. 2003, pp. 122-138 –Vogel et al. 2003, p. 2661].

2.3.2.6.2 Inhomogeneous deformation

In reality, ECAE deformation can neither be generally described as a simple shear along a

single plane nor as homogeneous. Finite element models (FEMs), both 2-D and 3-D [Park

and Suh 2001, pp. 3007-3014 – Kim et al. 2000, pp. 86-90 – Li et al. 2000 – Bowen et al.

2000, pp. 87-99 – Kim 2002, pp. 172-179 – Kim et al. 2001, pp. 856-864 – Budilov et al.



2004, pp. 189-198 – Beyerlein et al. 2004, pp. 185- 192] and slip line theory [Segal 2003,

pp. 36-46 – Stoica and Liaw 2003, pp. 119-133], have exposed much more regarding the

internal plastic flow characteristics and the conditions leading to non-uniform plastic flow

in a single pass [Park and Suh 2001, pp. 3007-3014 – Kim et al. 2000, pp. 86-90 – Li et al.

2000 – Bowen et al. 2000, pp. 87-99 – Kim 2002, pp. 172-179 – Kim et al. 2001, pp. 856-

864 – Budilov et al. 2004, pp. 189-198 – Beyerlein et al. 2004, pp. 185- 192 – Semiatin et

al. 2000, pp.1841-1851 – Segal 2003, pp. 36-46 – Stoica and Liaw 2003, pp. 119-133]. In

most cases, plastic deformation takes place in a broad zone whose shape depends

sensitively on factors such as contact friction, material flow response, backpressure,

pressing rate, and die design. Upon passing through such broad plastic deformation zones

(PDZs), the sample is left in an inhomogeneous deformation state, which can vary

significantly from top to bottom. It appears that the ideal simple shear viewpoint is

approached when the inner and outer corners are sharp, frictionless interfacial conditions

exist, backpressure is applied, and the material is rigid plastic [Park and Suh 2001, pp.

3007-3014 - Stoica and Liaw 2003, pp. 119-133].

Inhomogeneous deformation during each ECAE pass will most certainly lead to

heterogeneity in the final microstructure, texture, and mechanical properties across the

sample of the material [Beyerlain et al. 2003, pp. 122-138 and Li et al. n.d.].



2.3.3 Properties

The material properties of the nanostructured materials show remarkable improvement or

deviation from the properties exhibited by the coarser grained material. These unique

properties are attributed to the significant increase in grain boundary area due to the small

grain size. In terms of the mechanical properties, nanostructured metals have shown

increases in hardness values (ultimate and yield strengths). Specifically, pure nanophase

metals have shown a clear increase in hardness with decreasing grain size, following the

well known Hall-Petch equation. Nanostructured ceramics have exhibited superplastic

properties at low temperatures. This is significant as ceramics are conventionally brittle

materials [Nanostructured Materials 2004].

2.3.4 WC-Co particles

WC-Co hardmetals finds wide range of industrial applications. They are for example being

widely used in metal cutting tools because of their high hardness, good wear-resistance,

good fracture resistance and high temperature strength [Zhang et al. 2003 pp. 1123-1128].

Mechanical properties of hardmetals are strongly dependent on the microstructure of the

WC-Co hardmetal, and additionally affected by the microstructure of WC powders before

sintering. An important feature is that the toughness and hardness increase simultaneously

with the refining of WC. Therefore significant effort in research for development of

nanostructured WC-Co hard metals is done.

Nanometre materials, characterized by an ultrafine grain size, were prepared by vapor

condensation [Gleiter and Maquardt 1984, p. 263] until 1988 when Shingu et al. [1998, p.

29] found that nanocrystalline Al-Fe alloy can also be obtained by mechanical alloying.

Due to its simplicity and relatively inexpensive equipment high energy ball milling is a

simple and efficient way of manufacturing the fine powder with nanostructure. Another

advantage is its easy way to upscale the process. The problem of contamination



[Suryanarayana 2001, p. 1] can be eliminated by using a milling media that is made of

WC-Co hardmetal.

After a certain period of high energy milling the grain size of WC can be reduced to

nanoscale and become embedded in the cobalt particles together with a considerable

internal strain [Zhang et al. 2003, pp. 1123-1128].

2.4 Crystal structures and Point Defects

2.4.1 The Body-Centered-Cubic (BCC) structure

Figure 26: Sketch of a BCC structure, ALCAN

The BCC structure has one atom located at each of the cube corners and one atom at the

cube centre. Each atom has 8 neighbours i.e. coordination no. = 8. Examples of metals with

BCC structures include Cr, Fe, Mo [Cottrel 1995].



Figure 27: The atoms in a BCC structure, ALCAN

2.4.2 The Hexagonal-Close-Packed (HCP) structure

The HCP structure also has a coordination no. of 12. In this case two lattice parameters are

required, denoted a and c. The ideal HCP crystal has the closest packing of all and it is

possible to calculate the ideal c/a ratio = . Real metals deviate from this ratio

e.g., Mg has c/a = 1,623 and Zn has c/a = 1,856 [Cottrel 1995].

Figure 28: Sketch of a HCP structure, ALCAN



Figure 29: Atoms in a HCP structure, ALCAN

2.4.3 Miller indices – Cubic Crystals

It is often necessary to refer to crystal planes and directions for example to describe the

orientation of a particular precipitate type. The Miller indexing system provides a method

of achieving this using the three axis x, y, z system.

In the Miller system, however, the indices are used are inverse indices. Thus the direction

from the origin through the atom position x = 1, y = ½, z = 0 has the

Miller indices [1, 2, 0] [Cottrel 1995].

Directions are always denoted using [] square brackets.



Figure 30: Miller indices – Cubic crystals, ALCAN

Where the intercept on any axis is negative the corresponding Miller index will also be

negative. This negative index is represented using a bar over the integer, e.g. ( 00).

Figure 31: Negative Miller index, ALCAN

It is sometimes necessary to refer to all similar planes or directions in a crystal. This is

done by using different brackets, to describe a family of planes <uvw> to describe a

family of directions [Cottrel 1995].

For example, the cube faces have the indeces (100), (010), (001), ( 00), (0 0), (00 ). The

notation to describe cube faces would be . Similarly the notation to describe the cube

edges would be <100>.



For cubic lattices only a direction will be perpendicular to the plane which has the same

indices.

Figure 32: Cubic lattices only a direction will be perpendicular to the plane, ALCAN

2.4.4 Close Packed planes

Arrangement of atoms in a close packed plane. This is the arrangement of atoms in the

basal plane of an HCP structure or the (111) plane of FCC

Figure 33: First plane of FCC

The next plane is positioned over the spaces in the first plane to form the stacking sequence

A-B.



Figure 34: The second plane of FCC

The third layer can then be positioned over the A atoms. This forms an HCP sequence

ABABAB. If the third layer is positioned over the remaining un-occupied spaces this

forms the FCC lattice with the sequence ABCABC [Cottrel 1995].

Figure 35: The third layer of FCC

2.5 Grain measurement of WC

To get a good characterisation of the powder, it is needed to have a descent grain

measurement. There are different possibilities to measure them and every possibility has its

own range.

2.5.1 BET Surface Area



This is generally a research laboratory method for the powders that only gives a value for

the mean grain size; no information on grain size distribution is obtained. The surface

roughness of grains may affect the result and it is a slow method. For WC in the range 0.2

– 1000 m²g-1, d (diameter) is given by 6/ρSp where ρ = density and Sp = surface area/unit

mass [1970, pp. 429-448 – Friederich and Exner 1984, pp. 334-341 – Fischmeister et al.

1966, pp. 106-124 – Le Roux and King 1987, pp.243-248]. Figure 36 shows a picture of

this apparatus.

Figure 36: BET surface area

2.5.2 X-ray sedigraph

It gives information on grain size distribution as well as mean value, presented as wt

fraction cumulative probabilities. It is used for powders with mean grain sizes between the

1 – 8 μm grain size ranges. The measurement time is about 1h. There is a need to

deagglomerate and disperse the powders [1970, pp. 429-448 – Friederich and Exner 1984,

pp. 334-341 – Fischmeister et al. 1966, pp. 106-124 – Le Roux and King 1987, pp.243-

248]. Figure 37 shows a X-ray sedigraph apparatus.



Figure 37: X-ray sedigraph

2.5.3 Laser Diffraction

It is intended only for WC-Co agglomerated ready-to-press powders. It can give

information on grain size distribution in the range 2 – 1000 μm [1970, pp. 429-448 –

Friederich and Exner 1984, pp. 334-341 – Fischmeister et al. 1966, pp. 106-124 – Le Roux

and King 1987, pp.243-248]. The measurement time is about 10 min. Figure 38 shows a

laser diffraction machine.

Figure 38: Laser diffraction

2.5.4 Ultracentrifuge

The method can give grain size distributions, especially in the range < 0.5 μm where the X-

ray sedigraph and laser diffraction methods have limitations [1970, pp. 429-448 –


and King 1987, pp.243-248]. Figure 39 shows the machine.



Figure 39: Ultracentrifuge

2.5.5 Photon correlation spectrography

This is a relatively expensive method, used for correlation with the ultracentrifuge for grain

sizes in the range 2 nm – 3 μm. One measurement takes 2h [Exner 1970, pp. 429-448 –


and King 1987, pp.243-248]. Figure 40 and figure 41 are examples from photon correlation

spectography.

Figure 40: Photon correlation spectography



Figure 41: The beam of the photon correlation spectography

2.5.6 Microscopical image analysis, SEM, TEM

These are very direct methods: they are widely used in research. SEM in particular is used

to look at heterogeneity, i.e. large grains in a fine matrix. For mean grain sizes less then

about 0.5 μm a good resolution SEM is required, typically, with a field emission electron

source. SEM is better than TEM for morphological studies. However, polycrystallinity of

grains is easier to see in TEM [Anders 1992, pp. 195-204]. TEM suffers from poor

statistical accuracy for size measurements but is useful for very small grains less then 50

nm in size.

2.5.7 X-ray line broadening

A research method for ultrafine or nanograin powders. It only gives a mean grain size

value, not grain size distribution. Careful analysis and interpretation of data is required,

since grains may have internal structure. X-ray diffraction is insensitive in the range

between 0,1 and 10 µm [WHISTON C. 1987, p. 92]

2.5.8 Chemical reaction



Dissolution rates of WC in an iodide solution can be used to measure grain size (Krupp –

Widia) or alternatively in 10 ml HCl (32%), 10 ml HNO3 (conc), 50 ml H2O. Interrupted

experiments to measure the amount of unreacted WC gives a measure of the WC grain

size, since coarser grains dissolve more slowly than finer grains [Roebuck et al. 1999, pp.

47-54].

Summary of the most frequently used grain size measurement methods

In figure 42, a view over the different ranges of the discussed grain size measurement

methods is given.

Figure 42: The range were the different techniques are valid


-Experimental procedure- 73

3 Experimental procedure

The different steps in the procedure are been spoken here.

3.1 Description of the powders

To become familiar with the powders and the equipment, starting experiments included the

use of spare WC/Co powder samples with unknown sizes, purity and composition.

Therefore the powders were sieved with a 20 µm sieve so the ball milling could take place

with more accuracy. After sieving, an X-ray analysis took place to investigate the sizes,

purity and composition of the powders.

These powders are used to investigate the parameters for planetary ball milling, powder

compression and sintering temperatures parameters. Different samples were prepared with

the same composition (90 wt% WC a 10 wt% Co) and were milled at different conditions.

For research on the influence of alternative binders and inhibitors we’ve used new powders

with known compositions and sizes.

3.1.1 Tungsten carbide (WC)

Experiments took place with two kinds of tungsten carbide powders: tungsten carbide

powder with measurements less then 20 micron and measurements less then 4.3 micron.

3.1.1.1 Tungsten carbide < 20µm

Company: unknown

Purity: XRF results:



Sizes: sieved with 20µm sieve

An SEM picture of pure WC <20µm is given in fig 43:

Figure 43: SEM picture of WC particles < 20 micron

3.1.1.2 Tungsten carbide < 4.3µm

Company: William Rowland Limited

Purity: > 99.999%

Sizes: 3.48µm

3.1.2 Cobalt (Co)

Experiments took place with two kinds of cobalt powders: cobalt powder with measures

less then 20 micron and measures less then 4.3 micron.



3.1.2.1 Cobalt < 20µm

Company: unknown

Purity: XRF results:

Sizes: sieved with 20µm sieve

3.1.2.2 Cobalt < 4,3µm


Purity: 99.87%

Sizes: 1.5µm

3.1.3 Iron (Fe)


Purity: > 99.8%

Sizes: 5.2 – 6.4µm

3.1.4 Nickel (Ni)


Purity: > 99.8%

Sizes: 2.9µm

3.1.5 Manganese (Mn)


Purity: > 99.10%

Sizes: 90.40% < 325 mesh



3.1.6 Vanadium Carbide (VC)

Company: NewMet Koch

Purity: 99.8%

Sizes: 10µm



3.2 Preparation of the powders

Following powder compositions were prepared:

Name Wt% WC Wt% Binder Wt% inhibitor

A WC-Co 90 wt% WC 10 wt% Co

B WC-Co with inhibitor 89 wt% WC 10 wt% Co 1 wt% VC

C WC- Fe/Mn 90 wt% WC 10 wt% Fe/Mn

D WC- Fe/Mn with inhibitor 89 wt% WC 10 wt% Fe/Mn 1 wt% VC

E WC - Fe/Ni/Co 90 wt% WC 10 wt% Fe/Ni/Co

Table 1: The composition of the powders

Compositions for the binders:

Binder Wt% Fe Wt% Ni Wt% Co Wt% Mn

1 Fe/Ni/Co 75 15 10 /

2 Fe/Mn 86.5 / / 13.5

Table 2: Used compositions for the binders

The powders are weighted with a Fischerbrand PS- 200 balance, with an accuracy of

0.1mg.

3.3 Milling process

There were two kinds of milling processes; first the planetary ball mill and secondary the

horizontally ball mill. Both of them are discussed in this chapter.

3.3.1 Planetary ball mill

The powders are milled in a Fritch pulveristette 5 planetary ball mill. To find the optimal

milling parameters, the reference sample has been milled for 2.5 ; 5 and 10 hours at

velocities of 250 and 400 rpm. We have used stainless steel vials and grinding balls. The



used ball to powder ratio was 15:1. The vial was filled with 125 ml ethanol. After milling

the vials were placed in a fumeboard until the ethanol was vaporized (figure 44).

Figure 44: Fritsh Pulverisette 5

3.3.2 Horizontally high energy mill

A ZoZ high energy simoloyer (figure 45) has been used for milling. The milling process

was programmed with the software Maltoz. The first step was mixing of the powders,

involving 5 minutes rotating with a speed of 200 rpm. After this a 30 minutes programmed

routine was executed. This routine consisted of 12 repetitions of 1minute milling at 600

rpm and 4 minutes at 1000 rpm. The emptying procedure was 20 minutes milling at a

velocity of 1200 rpm. We used a ball to powder ratio of 1:10 with stainless steel balls.

Because the small particles were hard to remove, this high speed was necessary.



Figure 45: The Maltoz operation software for the high energy mill

The milling proceeded under an inert argon atmosphere. Therefore it was necessary to

create a vacuum. With the combination of an airlock system and an Edwards vacuum

pomp, this was easy to achieve.

3.3.3 Development of dies for compaction

To further research the characteristics of the used powder combination, various dies have

been made to enable tests such as compaction and Charpy impact to be carried out.

The dies that are used in these experiments follow the ASTM standards. The 3D pictures of

the different dies are displayed on the following pages and they are all designed with the

Autodesk Inventor 5.0. The standards can be seen on www.astm.org.. The specific

drawings can be found in appendix C.

3.3.3.1 Compaction die


http://www.astm.org/


First of all, there is a die in which powder compaction takes place, incorporating two

punches, an upper punch and a lower punch. These punches apply pressure on to the

powder mixture. At the end of the compaction phase, the lower punch will push the green

powder compact out of the die.

3.3.3.1.1 The die for compaction

The die consists of an inner die and an outer die. The inner die gives the green its specific

form. An assembly of the die is shown in figure 46.

Figure 46: Assembly of the compaction die

3.3.3.1.2 The upper punch

In the assembly shown in figure 47, the upper punch is secured to the test machine via 2

M16 screws. The upper punch is mounted on an additional mounting plate and 1 M8

screw.



Figure 47: Assembly of the upper punch

3.3.3.1.3 The lower punch

The lower punch (figure 48) has the same geometrical form as the upper punch with an

overall length of 90 mm compared to 60 mm for the upper punch.

Figure 48: Assembly of the lower punch

3.3.3.1.4 The die for the Charpy test



The die for the Charpy test is designed in accordance with the ASTM-standards. The die

also consists of an inner die and an outer die. Figure 49 shows an assembly of the die.

Figure 49: Assembly of the Charpy test die

3.3.3.1.5 Die for 3 point bending test

The die for 3 point bending test is designed in accordance with the ASTM-standards. The

die also consists of an inner die and an outer die. Figure 50 shows an assembly of the die

Figure 50: Assembly of the 3 point bending die



3.4 Analysis methods

During the thesis there were different kinds of analysis methods that took place. They are

discussed in this chapter.

3.4.1 Sieving

For a rough determination of particle sizes we’ve used a Fritch analysette 3 pro sieve with

20µ mesh sieves. The sieve was programmed for a cycle of 20 minutes whereby the

amplitude alternated between 0.5 and 1 mm (figure 51).

Figure 51: Fritsh Sieve

3.4.2 X-ray diffraction

X-rays are electromagnetic radiation of wavelength about 1 Å (10-10 m), which is about the

same size as an atom. They occur in that portion of the electromagnetic spectrum between

gamma-rays and the ultraviolet. The discovery of X-rays enabled scientists to probe

crystalline structure at the atomic level. X-ray diffraction has been in use in two main

areas, for the fingerprint characterization of crystalline materials and the determination of



their structure. Each crystalline solid has its unique characteristic X-ray powder pattern

which may be used as a "fingerprint" for its identification. Once the material has been

identified, X-ray crystallography may be used to determine its structure, i.e. how the atoms

pack together in the crystalline state and what the interatomic distance and angle are etc.

X-ray diffraction is one of the most important characterization tools used in solid state

chemistry and materials science. The technique is the only one available for phase

identification, and the associated mini-computer holds 60.000 inorganic and 20.000

organic reference patterns. This allows fast automatic identification of several phases.

The size and the shape of the unit cell for any compound can be determined easily using

the diffraction of X-rays.

A Philips PW 1729 X-ray generator with a Cu, shown in figure 52, anode has been used to

measure the X-ray diffraction patterns. The machine was operated at 40 kV and 10 mA.

The software used with the machine is X’pert plus.

Figure 52: Philips XRD

3.4.2.1 Formulae used in the X-ray diffraction



The path difference between two waves:

2 x wavelength= 2dsin(θ)

For constructive interference between these waves, the path difference must be an integral

number of wavelengths:

n x wavelength= 2x

This leads to the Bragg equation:

n x wavelength = 2dsin(θ)

Figure 53 shows the x-ray diffraction pattern from a single crystal of layered clay. Strong

intensities can be seen for a number of values of n; from each of these lines the value of d

can be calculated meaning the interplanar spacing between the atoms in the crystal

[Institute for materials research 2004].

Figure 53: Reflection of X-rays from 2 planes of atoms in a solid



3.4.2.2 Example: unit cell size from Diffraction data

The diffraction pattern of copper metal was measured with x-ray radiation of wavelength

1.315 Å. The first order Bragg diffraction peak was found at an angle 2θ of 50.5 degrees.

Calculate the spacing between the diffracting planes in the copper metal [Institute for

materials research 2004].

The Bragg equation is:

n x wavelength = 2dsin(θ)

Rearrange this equation for the unknown spacing d:

d = n x wavelength/2sin(θ).

=> theta is 25,25 degrees, n =1, and wavelength = 1.315Å, and therefore

d= 1 x 1,315/(2 x 0,4266) = 1,541 Å

Figure 54: X-ray diffraction pattern

3.4.2.3 Instrumentation

The X-ray diffraction experiment requires an X-ray source, the sample under investigation

and a detector to pick up the diffracted X-rays. Fig 55 is a schematic diagram of a powder

X-ray diffractometer [Institute for materials research 2004].



Figure 55: Schematic of an X-ray powder diffractimeter

The X-ray radiation most commonly used is that emitted by copper, whose characteristic

wavelength for the K radiation is =1.542Å. When the incident beam strikes a powder

sample, diffraction occurs in every possible orientation of 2θ. The diffracted beam may be

detected by using a moveable detector such as a Geiger counter, which is connected to a

chart recorder. In normal use, the counter is set to scan over a range of 2θ values at a

constant angular velocity. Routinely, a 2θ range of 5 to 70 degrees is sufficient to cover the

most useful part of the powder pattern. The scanning speed of the counter is usually 2θ of 2

degrees min-1 and therefore, about 30 minutes are needed to obtain a trace [Institute for

materials research 2004].

3.4.3 XRF

3.4.3.1 Description of the machine

For making an estimation of the contamination level, we’ve used a Spectro Xepo XRF

machine shown in figure 56. We can only speak of an estimation because the sample used

for this analyze has a diameter of 25 mm. A small sample as this isn’t representative for a

large amount of powder.



Figure 56: Spectro Xepo XRF machine

By using this machine the composition off the investigated powder will be given.

Unfortunately this machine only detects elements heavier than sodium. This means that the

level of carbon can’t be measured.

3.4.3.2 Preparation of the samples

Preparation of the samples involved mixing 85 wt% powder with 15 wt% wax. After the

powder and wax are mixed, the mixture is compressed with a pressure of 3 bar.

3.4.4 Scanning electron microscopy

The SEM was used to take magnified pictures so we could see if the WC was embedded in

the Co. Another reason to use the SEM was to measure the size of the grains.



3.4.4.1 Description of the machine

To get a high magnification of the samples, so the WC embedded in Co became visible,

we’ve used a ZEIS Scanning Electron Microscope (SEM), equipped with a backscattered

detector at 20 kV (figure 57).

Figure 57: ZEIS SEM

3.4.4.2 Preparation of the samples

Powders are mixed with non-conductive bakelite. Hot mounting was applied at a

temperature of 170°C for 7 minutes. The mixture is compressed is compressed with a

pressure of 3.2bar. After a 3 minutes cooling time, the samples were ready for grinding and

polishing. The surfaces were then ground 600 and 1200 grinding papers and a final polish

was applied using first a 6µm cloth followed by a 1µm cloth.



After polishing, optical microscopy was applied to see if the surface was good enough for

SEM.

3.4.5 Optical microscopy

The samples for SEM were first inspected with a regular optical microscope, shown in

figure 58, to make sure that the surfaces were well polished.

Figure 58: The optical microscope


-Results and discussion- 91

4 Results and discussion

4.1 Results from planetary ball mill

4.1.1 Getting started

To become familiar with the powders and the equipment, starting experiments included the

use of WC/Co powder samples with unknown sizes, purity and composition. Therefore the

powders were sieved with a 20 µm sieve so the ball milling could take place with more

accuracy. After sieving, an X-ray analysis took place to investigate the sizes, purity and

composition of the powders.

As a starting sample, a WC-Co sample was made with a composition of 90 wt% WC and

10 wt% Co; no inhibitors where added and this sample will be referred

The parameters that were changed for the ball milling were the velocity (rpm) of the

milling machine and the milling time. To investigate those parameters, different samples

were prepared with the same composition (90 wt% WC a 10 wt% Co) and were milled at

different conditions so the made composition was milled at a rotational speed of 250 rpm

and 400 rpm and this for 2.5h; 5h and 10h. After milling, the contamination level was

estimated with an XRF test and the average particle sizes and the contributing elements

were determined with the help of an XRD print and the equation of Scherer.

4.1.2 Reference sample

As a reference sample, a mixture of 90 wt% WC and 10 wt% Co (no inhibitors were

added) was made. This is the reference sample and it won’t be milled.



4.1.2.1 XRD

Figure 59 is giving the XRD pattern of the not milled reference sample. The particles used

for the XRD are the composite particles because in those particles the WC is embedded

best into the Co. The composite particles are used for every experiment following.

With this graph, the starting elements and their average size could be measured. That way,

it was easier to search for the links between the velocity, the milling time and their effect

on the grain size of the particles.

Figure 59: XRD pattern of the unmilled WC-Co reference powder sample

The WC peaks at 2θ = 31.6° and 2θ = 35.9° together with the Scherer equation were used

to determine the average grain size of the WC particles. During the milling process, the



particles need to be broken down into smaller particles and need to be embedded into the

Co (MA).

These 2 peaks are chosen, and will be used throughout the whole research, because the

possible contamination elements, during milling, coming from the used stainless steel vials

and balls will not interfere these peaks. That way, these peaks are 2 representative peaks

for the WC.

4.1.2.2 XRF

To determine the contributing elements in the powder and to make a quick estimation of

the quantities, an XRF was used. In the figure 60, the most important elements were named

in function of their estimated quantities.

0

10

20

30

40

50

60

wt%

W Ni Co Fe Mn Cr

Elements

Estimation of the quantities of the contributing elements the not-milled powder

Figure 60: Graph of the contributing elements in the WC-Co reference sample



The level of W is only about 60 wt% because the XRF machine can not detect elements

with an atomic number below 11 and C has an atomic number of 6. The quantity of Co is

estimated at 10 wt%. There are smaller quantities of Ni, Fe, Mn and Cr and they are

ascribed to the powder production process and the purity of the starting elements used to

make the powders.

4.1.2.3 Calculated grain size of the starting WC particles

The Scherer equation was used to calculate the grain size from the starting WC particles:

κ = 1

λ = X-ray wavelength, λCu = 1,5418

β = FWHM of the diffraction line

θ = diffraction angle

The values of β and θ can be found in the XRD data because β is the value, in radians, of

the WC peak situated at 2θ and θ (in radians) is half the value of 2θ, the position of the

peak. The formula was used on both of the WC peaks (2θ = 31.6° and 2θ = 35.6°), and

afterwards there was made an average of the 2 sizes. That way, a more accurate result for

the average grain size was achieved.

In the starting WC particles, the FWHM of the two WC peaks are both 0.00635 radians.

The diffraction angles are respectively for the 2 WC peaks 0.275 and 0.310 radians. When

these values are put into the Scherer formula, the result is respectively for the 2 WC peaks:



From these results, the grain size is calculated as the average of the results of the Scherer

equation:

This was an unexpected result because the size of the starting WC material was sieved with

a 20 micron mesh and couldn’t be in the nanoscale. Therefore, a SEM picture of this

material was shot, so that a quick estimation of the sizes of the starting WC particles could

be made. Figure 61 shows this SEM picture.

Figure 61: SEM picture of the starting WC material < 20 micron

Out of the SEM picture, the grain size of the starting WC material was estimated

approximately 10 μm. The reason for this difference is because X-ray diffraction is

insensitive in the range between 0.1 and 10 µm [WHISTON C. 1987, p. 92]. Because



we’ve spend a lot of time using the XRD machine for grainsize determination and to find

out why the results didn’t fulfil our expectations, the results from XRD are included in this

work

4.1.3 XRD results of the planetary ball milled samples at 250 rpm

4.1.3.1 The 2,5 hours milled sample

The 2.5 hours ball milled sample was wet-grinded with stainless steel vials and balls with a

diameter of 5 mm while the used liquid was ethanol. Every half an hour, there was a pause

of 10 min, to let the vials, balls and powder cool down.

After milling, an XRD test was done on the sample. Figure 62 gives the XRD pattern of

the sample.



Figure 62: XRD pattern of the 2.5h ball-milled WC-Co sample @ 250 rot/min

The first thing what was special on this graph, is that the peaks are lightly shifted to the

right because the values of the same WC peaks have now a 2θ value of 31.8° and 35.9°

what can be seen visually. This means that there are tensions in the powder so they will not

be broken down, but will be pressed together. Another option to see that there is strain in

the particles, is calculating the lattice parameters and the result of these parameters will be

different then those of the not milled reference sample.

The FWHM (= Full Width Half Maximum) of the WC peaks at the given 2θ angles are

respectively 0.00714 and 0.00714 radians. The FWHM is bigger then with the reference

sample what means that there is a reduction in grain size of WC.



4.1.3.2 The 5 hours milled sample

To make a good representative calculation for the best fit parameters of the ball-milling

machine, the same quantities of WC and Co, the same milling cycle, material of the vials

and bowls as well as the ethanol liquid was used. There was made an XRD test on the

sample too with pattern given in figure 63.

Figure 63: XRD pattern of the 5h ball milled WC-Co sample @ 250 rot/min

Another thing that was conspicuous was the fact that there was no clear line broadening

what means that the particles of WC stay more or less the same. The FWHM (= Full Width

Half Maximum) of the WC peaks at the given 2θ angles are respectively 0.00555 and

0.000555 radians. The FWHM of these peaks are smaller as those of the reference sample

what means that the particles are bigger as those of the starting material.



Again, the peaks moved lightly to 2θ = 31.7° and 2θ = 35.9°. That means that there are

tensions in the powder particles what means that they are not broken down but pressed

together.


The same parameters as described for the 5 hours milled sample were used. There was

made a XRD on the sample to given in figure 64.

Figure 64: XRD pattern of the 10 h ball milled WC-Co sample @ 250 rot/min

The 2θ angles of the same WC peaks are now approximately 31,818° and 35,773°. Again

there are tensions and the peaks are shifted to the right.

Another thing that was conspicuous was the fact that there was no clear line broadening

what means that the particles of WC stay more or less the same. The FWHM (= Full Width

Half Maximum) of the WC peaks at the given 2θ angles are respectively 0.00748 and



0.00834 radians. The FWHM of these peaks are smaller as those of the reference sample

what means that the particles are bigger as those of the starting material.



together.

4.1.4 The SEM pictures of the 250 rpm samples

The purpose of the SEM pictures was twofold. The first reason was to look if the Co is

embedded with WC. The second reason is to get a view on the size of the WC particles so

that these sizes can be compared to these of the Scherer equation, discussed later. It is also

important to say that for these experiments the composite particles are used.

In the pictures, the little light spots are WC and the grey matrix represents Co phase. The

black areas are bakelite used to hold the powders.

4.1.4.1.1 The 2,5h milled sample

Figure 65 shows the SEM view over the 2.5h planetary ball milled sample.



Figure 65: SEM picture of the 2,5h planetary ball milled sample

It is clear that WC particles are embedded within the Co matrix and the size of the largest

WC particles is 0.8 micron.


WCCo


4.1.5 XRD results of the planetary ball milled samples at 400 rpm


As well as the 250 rot/min, the 5 hours ball milled sample was wet-grinded with stainless

steel vials and balls with a diameter of 5 mm while the used liquid was ethanol. Every half

an hour, there was a pause of 20 min, to let the vials, balls and powder cool down.

After milling, an XRD test was done on the sample. Figure 66 gives the XRD pattern of

the sample.


The first thing what was special on this graph, is that the peaks are lightly shifted to the

right because the values of the same WC peaks have now a 2θ value of 31.8° and 35.8°

what can be seen visually. This means that there are tensions in the powder so they will not



be broken down, but will be pressed together. Another option to see that there is strain in

the particles, is calculating the lattice parameters and the result of these parameters will be

different then those of the not milled reference sample.


respectively 0.00748 and 0.00831 radians. The FWHM is bigger then with the reference

sample what means that there is a reduction in grain size of WC.


The 2θ angles of the same WC peaks are now approximately 31.7° and 35.9°. Again there

are tensions and the peaks are shifted to the right. Figure 67 gives the XRD pattern of the

10 hours milled sample.


respectively 0.00807 and 0.00879 radians. The FWHM of these peaks are bigger as those

of the reference sample what means that the particles are bigger as those of the starting

material. This is different then the 250 rot/min samples and follows our expectations.



together.




4.1.6 The SEM pictures of the 400 rpm samples

4.1.6.1.1 The 10h milled sample

Figure 68 shows the SEM picture of the 10h planetary ball milled sample at 400 rpm. The

cursor bars shows that the sizes of the embedded WC particles are between 2.273 μm and

239.2 nm. This is quite a large range, but on such pictures it is not clear how deep the WC

particles are embedded, so the grains size will be short to the average of these sizes.

Another thing that is clear, is that the WC particles are embedded in the Co.



Figure 68: SEM picture of the planetary ball milled sample at 400 rpm

4.1.7 Contamination level

Because the vials of the planetary ball mill are stainless steel, and the main content of the

powder is WC, which is very hard, there is a natural wear of the vials. This causes other

elements to contaminate the powder. The main contaminating elements are Fe, Cr, Ni and

Mn.

The level of contamination is investigated by XRF. This is not a very accurate way to find

the precise amount of elements in weight percent. The beam of the XRF covers only the

surface of the prepared sample and the infiltrating depth is unknown and depends on the

material. Therefore different samples of the same powder may give different results, but

nevertheless, it is a good estimation.

Figure 69 shows how the contaminating elements increase in weight percent with

increasing milling times and speed. It is obviously that with stainless steel vials a milling

time of 10h at 400 rpm is to long. Both time and speed must be high enough to reduce the

particle size and get the WC embedded in the Co particles.


Co

WC


0

10

20

30

40

50

60

wt%

W Ni Co Fe Mn Cr

Elements

Estimation of the contributing elements in milled powder

not milled5h@250rpm10h@250rpm5h@400rpm10h@400rpm

Figure 69: Estimation of the contributing elements in milled powder

Contamination after milling

0,3545

34,737

47,026

0,3545

60,353

75,071

010203040

50607080

0 2 4 6 8 10

milling time (h)

Cont

amin

atio

n (w

t%)

250rpm

400rpm

Figure 70: Contamination level in function of the milling time

In figure 70 shows the contamination in function of the time. It is clear that the longer is

milled, the more contamination appears. Another thing what is important, is that the

contamination level, with the same milling time, of the 400 rotational speeds is higher then

the contamination of the 250 rotational speeds.

4.1.8 The grain size calculation



To determine the best fit parameters and to use the Taguchi matrix, it is necessary to

determine the grain sizes of the WC particles after milling. The first method is the Scherer

equation.

4.1.8.1 The Scherer equation

4.1.8.1.1 The planetary ball milled samples at 250 rpm

The grain sizes of the milled samples are calculated with the help of an excel counting

sheet and the Scherer formula, discussed in 6.1.2.3 Figure 71 gives an overview of this

sheet and the out coming results for the 250 rpm milled samples.

Grain size of WC for the 2,5h milled powder at 250rpm (90 wt% WC and 10 wt%

λ β θ dXRD dXRD

1.542 0.00714 0.277 224.509 0.02241.542 0.00714 0.313 226.989 0.0227 average 225.749 Angström 0.0226 μm

Grain size of WC for the 5h milled powder at 250rpm (90 wt% WC and 10 wt%

λ β θ dXRD dXRD



λ β θ dXRD dXRD



rpm milled sample)



4.1.8.1.2 The planetary ball milled samples at 400 rpm

The grain sizes of the milled samples are calculated with the help of an excel counting

sheet and the Scherer formula, discussed in 6.1.2.3. Figure 72 gives an overview of this

sheet and the out coming results for the 400 rpm milled samples.


λ β θ dXRD dXRD

1.542 0.00748 0.277 214.280 0.0214

1.542 0.00831 0.312 194.940 0.0194

average 204.610 Angström 0.0204 μm


λ β θ dXRD dXRD

1.542 0.00807 0.277 198.516 0.0198

1.542 0.00879 0.313 184.453 0.0184



rpm milled sample)

4.1.8.1.3 Conclusions of the Scherer equation

Because these results are not compatible with the results of the SEM, another method for

calculating grain size of the WC was needed. A possible reason why the Scherer equation

was not effective for the grain size calculation, was that there where lattice strains in the

WC grains coming from the ball milling [Segmüller et al. 1989, pp. 21-66]. Thus, a

formula that included a parameter for these strains was searched. The first person’s dealing

with these strains was Stokes and Wilson. Because this was still a single line method (less



accurate), another (multi line) method was needed, coming from the Fourier convolution

method to obtain the purely physical broadened line profiles and the Warren-Averbach

formula is such a formula.

The main disadvantages of these multi-line analysis are:

They are not useful for large line overlapping

Not useful for weak structural broadening

Stokes can not be applied without severe errors

The main reason why integral breadth fittings became more attractive, was because of the

mathematical process involved. Nevertheless, a full powder pattern fitting was used. This

is the moment that Voigt and pseudo Voigt were introduced into the field of x-ray powder

diffraction.

There are a lot of software packets dealing with line-broadening analysis and for our

purpose we used the Winfit! V1.2 program from S. Krumm.

Figure 73 shows the Fourier analysis of the XRD pattern from the starting WC particles <

20 μm for the two discussed WC peaks.



Figure 73: Fourier analysis of the un-milled WC powder < 20 micron

Figure 74 shows the results of the program.



Figure 74: Results of the Winfit! V1.2 on the <20 micron WC particles

Figure 75 shows the grain size calculation for the WC grains. Again, there is a big

difference between the results from Winfit! V1.2 software and the SEM pictures.

Figure 75: Grain size from the <20 micron WC particles with the Winfit! V1.2 software



4.1.9 Strain evaluation from peak shift

As already discussed, there are strains in the grains due to the milling. In this part, a clear

view is given over the peak shifts. The calculations of the lattice strains are not given in

this thesis.

In figure 76 a graph is given of the unmilled reference powder and the 10 h ball-milled

powder at 250 rpm.

Figure 76: Peak shift of WC with 250 rpm milling speed

It is clear that there is a peak shift and it can be seen by the space between the 2 red

vertical markers. Grain size determination from XRD results is not possible because XRD

line broadening analysis is insensitive in the range between 0,1 and 10µm (WHISTON C.

1987, p. 92).

4.2 Results from the high energy mill



In this part, a good powder characterisation will be made for the two considered alternative

binders. First, an estimation of the contamination level was made on a powder composition

of WC-10 Co. This powder was milled for half an hour, 1 hour and 3 hours.

Because XRD is not usable in our situation, we didn’t continue with this method and used

SEM to determine the grain sizes.

The high energy mill was programmed for 10 minutes of mixing the powders at a low

rotational speed of 500 rpm followed by a cycle of 15 minutes milling at a speed of 1000

rpm and 5 minutes break. This cycle was repeated 3 times thus in total there was 1h of

milling at high rotational speed. Thereafter, the powder was discharged for approximately

15 minutes.

4.2.1 The estimation of the contamination level of the milled powder

With the horizontal high energy mill the same problem occurs as with the planetary ball

mill namely contamination. This contamination in the milled powder comes from the wear

from the stainless steel grinding unit and balls. The main contaminating elements are Fe,

Cr, Ni and Mn.

It has to be considered that XRF gives only an estimation of the weight percentages of the

contributing elements of the powder but XRF is reliable enough to determine the trend of

contamination level with higher milling times.

In figure 77 an estimation of the most important contamination elements is shown. It is

clear that the longer the milling times are, the more contaminating elements are in the

powder. As with the planetary ball milled powder, the level of contamination increases

with the milling time.



0

10

20

30

40

50

60

wt%

W Ni Co Fe Mn Cr

elements


unmilled30min high energy1hour high energy

Figure 77: Estimation of the contributing elements in milled powder

Figure 78 shows that the contamination in weight percent doubles when the milling time

doubles.

contamination after milling

0,3545

1,4105

3,683

0

0,5

1

1,52

2,5

3

3,5

4

0 0,5 1

milling time (h)

cont

amin

atio

n le

vel (

wt%

)

estimation of thecontamination level

Figure 78: contamination after milling



4.2.2 SEM pictures from samples milled with the high energy mill

The purpose of the SEM pictures was twofold. The first reason was to look if the

alternative binder is embedded with WC. The second reason is to get a view on the size of

the WC particles. It is also important to say that for these experiments the composite

particles are used.

In the pictures, the little light spots are WC and the grey spots are coming from the

alternative binder. The black spots are coming from the bakelite used to make the samples.

4.2.2.1 SEM pictures from Fe/Mn as alternative binder

Figure 79 shows the SEM picture of the 1h milled sample. The cursor bars shows that the

sizes of the embedded WC particles are between 1,273 μm and 39,2 nm. This is quite a

large range, but on such pictures it is not clear how deep the WC particles are embedded,

so the grains size will be short to the average of these sizes.

Another thing that is clear, is that the WC particles are embedded in the Fe/Mn binder. Our

first conclusions are that this is a good alternative to Co but further investigations

involving the mechanical properties are necessary to know if it is a good alternative.



Figure 79: SEM picture of the 1h horizontal energy milled sample WC-Fe/Mn

4.2.2.2 SEM pictures from Fe/Ni/Co as alternative binder

Figure 80 shows the SEM picture of the 1h milled sample with Fe/Ni/Co as alternative

binder to Co. The cursor bar below the picture shows that the sizes of the embedded WC

particles are between 1,273 μm and 39,2 nm. This is quite a large range, but on such

pictures it is not clear how deep the WC particles are embedded, so the grains size will be

short to the average of these sizes.

Another thing that is clear, is that the WC particles are embedded in the Fe/Ni/Co binder.

Our first conclusions are that this is a good alternative to Co but further investigations

involving the mechanical properties are necessary to know if it is a good alternative.


Fe/MnWC


Figure 80: SEM picture of the 1h horizontal energy milled sample WC-Fe/Ni/Co


WC

Fe/Ni/Co

-Conclusions and suggestions for further work- 118

5 Conclusions and suggestions for further work

5.1 Introduction

Metal powders are used in industry for a diverse range of products. Some of these products

include welding electrodes, paints, printing inks and explosives. In all these products the

particles retain their identities. Traditional powder metallurgy is a process whereby a solid

metal, alloy or ceramic in a form of a mass of dry particles, normally less than 150 µm in

maximum diameter, is converted into an enginering component of a predetermined shape

possessing properties which allow it to be used in most cases without further processing.

The basic steps in traditional powder production are:

Powder production

Compaction of the powder

Sintering which involves heating the preform to a

temperature below the melting point of the major

constituent, at which point the powder particles lose their

identities through inter-diffusion processes and required

properties are developed.

These 3 phases are described in the literature review and the powder production phase has

been carried out practical.

Due to the poor corrosion resistance of Co, its high cost and environmental toxicity,

substantial research has been devoted to find suitable alternative binders for WC systems.

The aim is to reduce the amount of Co, or possibly, to completely replace Co binder. Two

promising alternatives are described and utilised in this project, the first one is a mixture

of iron (Fe), nickel (Ni) and cobalt (Co) and the second alternative is composed of iron

(Fe) and manganese (Mn). Compared to cobalt, Fe and Mn are very cheap and non toxic



The first step in powder metallurgy is the powder production. The next step would be to

compact such powder for a subsequent sintering process. For that appropriate compaction

dies were designed using Inventor CAD software, a die to produce cylindrical samples for

microstructural and hardness analyses and another die to produce samples for 3-point

bending tests. Both dies were designed according to ASTM standards.

5.2 Conclusions

5.2.1 The planetary ball mill

A series of experiments were performed with the planetary ball mill by varying milling

time (2.5, 5, 10 hrs) and rotation speed (250, 400rpm) parameters to process WC-10wt

%Co, WC-10%FeNiCo and WC-10%FeMn. It was noticed that as the milling time

increased (above 2.5 hours for 150rpm) the amount of elements (Fe, Cr) picked up from

the stainless steel vial inner wall increased. The contamination level increased further at a

rotation speed of 400rpm. This indicates that both speed and time should be kept low to

minimise contamination or a hard steel vial should be utilised.

The results from the XRF tests show that the contamination level, using the 400 rpm

rotational speed, almost doubles when the milling time is kept constant. SEM micrographs

indicate that further particle size reduction was not achieved with increasing the milling

time at this rotation speed. It can be concluded that using a milling speed higher than 200

rpm is not beneficial with the stainless steel vial.

XRD technique was utilised to determine the grain size of WC phase (average of 24.1 nm).

The technique is also useful to determine the lattice strains exerted on the WC phase

(which can be determined from of the XRD peak shifts) due to the mechanical milling

process. SEM was used to evaluate the particle size of WC (average of 1.103 μm).



5.2.2 The horizontally high energy mill

To eliminate the problem of a high contamination level found in the mechanically milled

powder, a hard steel vial was utilised. For that, additional powders were prepared using the

horizontal high-energy ball mill. Furthermore, this equipment allowed achieving a fine

structure with shorter milling time due its much higher rotation/milling speeds capabilities.

From the results of the first test, it was obvious that this method allowed a much quicker

way to reach the nanosize structure of the powders. This is due to the high kinetic energy

of this process, offering milling speeds over 1000rpm, a serious milling time reduction was

possible. Furthermore, compared to the planetary ball milling equipment, the

contamination level was fairly low (1 wt% Fe by with the horizontally high energy mill to

25.1 wt% Fe with the planetary ball milled sample, milled for 5h at 250 rot/min) as shown

by the XRF analysis. An other advantage of the horizontally high energy mill is the large

capacity of the container (2 litters). Hereby it is possible to produce a relatively larger

quantity of powder in one single cycle.

Considering these points we can conclude that the horizontally high energy mill has a

higher efficiency than the planetary ball mill.

5.2.3 The alternative binders

Two alternative binders for Co have been investigated and mixed with WC. The first one

was Fe/Mn as alternative. Out of the SEM picture, it was clear that WC particles were

nicely embedded in Fe/Mn binder and the size of WC particles was reduced by the

mechanical alloying process. That was a promising result. The compaction phase and the

sintering phase will be carried out in a later phase.



Similarly for the second alternative binder Fe/Ni/Co, SEM pictures revealed that WC

particles were reduced in size and distributed within the binder matrix. Again, the

compaction and sintering phase will be carried out in a later phase.

5.3 Further work

Further work includes powder compaction of the different powder compositions prepared

by mechanical alloying process followed by sintering the compacted samples. Parameters

related to both the compaction and sintering experiments should be optimised for each

powder system (WC-Co, WC-FeNiCo, WC-FeMn) in order to prevent excessive grain

growth of the WC submicron/nano-structure achieved in the powders.

For the sintered samples, the physical and mechanical properties, such as density;

hardness/microhardness and 3-point bending fracture strength, will be measured as well as

microstructural analysis, evaluation of WC grain size and distribution, will be performed.


-References- 122

6 References

ADVANCED POWDER TECHNOLOGY website n.d. Manufacturers of the worlds

finest powder. < http://www.apt-powders.com/ > (last accessed on 1octobre 2004).

AGNEW, S.R. et al. (1998) Proceedings of the 19th Risø International Symposium on

Materials Science, Roskilde, p. 201.

ALMOND, E.A. and ROEBUCK, B. (1988) Mat. Sci. and Engng, A105/106, p.237.

ANDERS NORDGREN (1992) Microstructural characterisation of cemented carbides

containing tailored structure variations using SEM-based automatic image analysis, J.

Hard Materials, 3(2), pp. 195 – 204.

AP Materials Inc website. Missouri n.d. < http://www.apmaterials.com > (accessed on

25september 2004).

ARGONIDE (2004) Nanomaterial Technologies. <http://www.argonide.com>(last

updated 2004 accessed on 24 September 2004).

ASTM: B 312 -96 (1996) Standard Test Method for Green strength for Compacted

Metals Powder Specimens.

ASTM: B331-95 (2002) Standard Test Method for Compressibility of Metal Powders

in Uniaxial Compaction.

ASTM: B925-03 (2003) Standard Practices for Production and Preparation of Powder

Metallurgy (P/M) Test Specimens.


-References- 123

ATZOM, M. (1990) Phys Rev Lett., 64, pp. 487-490.

BELLOSI, A. et al. (1997) Mater Sci Forum, 238, pp. 255-260.

BENJAMIN, J.S. (1990) Metal Powder, 45, pp.122-127.

BENJAMIN, J.S. et al. (1983) Metal Powder, 34, pp.279-300.

BENJAMIN, J.S.(1976) Sci Amer 234(5),pp.40-48.

BEYERLAIN, I.J., TOME, C.N. and LEBENSOHN, R.A (2003) Modeling texture and

microstructural evolution in the equal channel angular extrusion process, Mater. Sci.

Eng., pp. 122–138.

BEYERLEIN, I.J. et al. (2004) Heterogeneity in texture development in single pass

equal channel angular extrusion, in: ZHU, Y.T. et al. (Eds.), Ultrafine Grained

Materials III. TMS (The Minerals, Metals & Materials Society), pp. 185–192.

BOWEN, J.R. et al. (2000) Mater. Sci. Eng, pp. 87–99.

BROOKES, K.J.A.(1998) Hard metals and other materials. International Carbide, pp.

76-78.

BUDILOV, I.V. et al. (2004) Three dimensional modeling of plastic deformation flow

during ECAP, in: ZHU, Y.T. et al. (Eds.), Ultrafine Grained Materials III. TMS (The

Minerals, Metals & Materials Society), pp. 193–198.

CALKA, A. et al. (1993) Mechanical alloying for structural applications in CALKA,

A. (eds.) Materials Park. Ohio: ASM International, pp. 189-195.

CHIN, Z.H. and PERNG, T.P. (1997) Mater Sci Forum. pp. 235-238 and pp. 121-126.


-References- 124

COTTREL, A. (1995) An introduction to metallurgy. London: Edward Arnold.

DOLGIN, B.P. et al. (1986) Non-Cryst Solids p. 87 and pp. 281-289.

ECKHOFF, R F (2003) Dust Explosions in the Process industries (3rd Edition., Gulf

Professional Publishing, ISBN 0-7506-7602-7.

EGAMI, A. et al. (1993), 13th Intern. Plansee Seminar, Reutte, 3, p. 639

EGAMI,A. et al.(1993),13th Intern. Plansee Seminar, Reutte, 3, p. 641.

EPMA (2004a) EPMA Non-Members Area, http://www.epma.ch.

EPMA (2004b) EPMA Non-Members Area

< http://www.epma.com/non_members_area/non_members.htm > (accessed on 23

September 2004).

EPMA (2004c) EPMA Non-Members

Area<http://www.epma.com/non_members_area/non_members.htm> (accessed on 28

July 2004).

EXNER, H.E.(1970) Methods and Significance of Particle- and Grain Size Control in

Cemented Carbide Technology. Powder Metallurgy, 13(26), pp. 429-448.

FISCHMEISTER, H.F., EXNER, H.E. and LINDELÖF, G. (1966) Particle-size

Analysis in Cemented Carbide Technology. Modern developments in Powder

Metallurgy. New York: Plenum Press, pp. 106-124.


-References- 125

FRIEDERICH, K.M. and EXNER, H.E.(1984) Metallographical Investigations on

Tungsten Carbide Powders. Prakt. Met., 21, pp. 334 – 341.

FURUKAWA, M., HORITA, Z. and LANGDON, T.G. (2002) Factors influencing the

shearingpatterns in equal-channel angular pressing, Mater. Sci. Eng., p. 97.

GAVRILOV, D., VINOGRADOV, O., SHAW, W.J.D. (1995) in POUSARTIP, A.and

STREET, K. (eds) Proc. Inter. Conf.on Composite Materials, ICCM-10, vol. 3.London:

Woodhead Publishing, p. 11.

GERMAN, R.M. n.d. Powder Metallurgy Science. p. 7.

GERMAN, R.M. n.d. Powder Metallurgy Science. pp.6-7.

GILLE, G. et al. (1999) Advanced Grades of WC and Binder Powder – their properties

and applications.

GILMAN, P.S.and BENJAMIN, J.S.(1983) Annu Rev, Mater Sci, 13, pp. 279-300.

GLEITER,H and MAQUARDT (1984) Z.Metal p. 263.

GONZALEZ et al. (1995) WC-(Fe-Ni-Co) Hardmetals with improved toughness

through isometral heat treatments. Journal of Materials Science, 30, pp. 3435-3439.

GONZALEZ, R. et al. (1995) Mat. Sci., 30, p. 3435.

HANAYALOGLU, C., AKASAKAL,B. and BOLTOM, J.D. (2001) Production and

identification analysis of WC/Fe-Mn as an alternative binder to Cobalt bonded

hardmetals. Material characterization, 47, pp. 315-322.

HONEYCOMBE, R. (1981) Steels—microstructures and properties. London: Edward


-References- 126

Arnold, p25.

HONEYCOMBE, R.W.K.(1981) Steels, microstructures and properties. London:

Edward Arnold, pp35-42.

INSITUTE FOR MATERIALS RESEARCH (2004) X-ray Powder Diffraction

<http://materials.binghamton.edu/labs/xray/xray.html> (accessed on 16 August 2004).

IVANOV, E. (1992) Mater Sci Forum., (88), pp. 475-480.

IVANOV, E., et al.(1999) Mater Res, (14) , pp. 377-383.

IWAHASHI, Y. et al. (1996) Principle of equal-channel angular pressing for the

processing of ultra-fine grained materials. Scr. Mater., 35 (2), pp. 143–146.

JENKINS, I.(1993) Introduction. in THÜMMLER, F. and OBERACKER, R.

(eds.)Introduction to Powder Metallargy. London: The institute of Materials, pp. 1-5.

KALOSHKIN, S.D. et al. (1997) Mater Sci Forum, 235, pp. 565-570.

KIM, H.S. (2002) J. Mater. Res. 17 pp.172–179.

KIM, H.S., HONG, S.I. and SEO, M.H. (2001) J. Mater. Res., pp. 856–864.

KIM, H.S., SEO, H.H. and HONG S.I. (2000) On the die corner gap formation in equal

channel angular pressing. Mater. Sci. Eng., pp. 86–90.

KIS, V & BEKE,D.L.(1996) Mater Sci Forum, pp. 225-227, pp. 465-470.

KOCH, C. C. (1991) Processing of metals and alloys. in CAHN, R.W. (ed.) Materials


-References- 127

science and technology - a comprehensive treatment. Weinheim: VCH

Verlagsgesellschaft GmbH, pp. 193-245.

KOCH, C.C.(1993) Nanostructured Mater, 2, pp. 109-129.

KOCKS, U.F., TOME, C.N. and WENK, H.R. Wenk (2000) Texture and Anisotropy.

second ed., Cambridge: Cambridge University Press.

LAI, M.O. and LU, L. (1998) Mechanical alloying. Boston: Kluwer Academic

Publishers.

LAOUI, T., FROYEN, L., KRUTH, J.P. (1999) Alternative Binders to Co for

WCparticles for SLS process, Proceedings of the 8th European Conference on

RapidPrototyping and Manufacturing, Nottingham, U.K., July 6-8, pp. 299-311.

LE ROUX, H. and KING, R.P. (1987) The exponential distribution of linear intercepts

of Cobalt in Tungsten Carbide – Cobalt compacts using Automatic image analysis,

Acta Stereol., 6(3), pp. 243 – 248.

LEE, P.Y., YANG, J.L., Lin, H.M.J.(1998) Mater Sci 33, pp. 235-239.

LI, S. et al. (2000) Finite element analysis of plastic deformation and deformation

zones in equal channel angular extrusion. Mater. Sci. Eng. A.

LI, S. et al. (n.d.) Heterogeneity of deformation texture in equal channel angular

Extrusion of copper. Acta Mater.

LUYCKX, S. & ALLI, M.S.(2000) comparison between V8C7 and Cr3C2 as grain

refiners for WC-Co. Materials & Design , 22, pp. 507-510.


-References- 128

LUYCKX, S. et al.(1996) Powder Metall. pp.39-41.

Nanostructured Materials (2004) n.d.

<http://www.rpi.edu/dept/materials/COURSES/NANO/crawford/> (updated on 11-24-

97,accessed on 23 September 2004).

NICOARA, G. et al. (1997) Mater Sci Forum pp. 235-238 and pp. 145-150.

NISHIYAMA, Z. (1977) Martensitic transformations, London: Academic Press.

OFFICE OF TECHNOLOGY TRANSFER website n.d. < http://ott.rice.edu> (last

accessed on 27september 2004).

OKADA, K. et al. (1992) Mater Sci Lett., p. 11 and pp. 862-864.

OKADA,K. et al. n.d. Proceed. advances in powder metal & particulate materials.

PARK, J. W. and J.-Y. Suh (2001) Effect of die shape on the deformation behaviour in

equal-channel angular pressing. Metal. Mater. Trans., pp. 3007–3014.

PRAKASH, L. (1993) Proc. 12th Int. Plansee Seminar. Reutte, 2, pp.80-109.

PRAKASH, L.(1979) Doctoral Thesis, University Karltuhe.

PRITCHARD, D.K. (2004) Literature review – explosion hazards associate with

nanopowders. Health and safety laboratory.

<http://www.nanotec.org.uk/evidence/nanopowdersReviewI.pdf> Buxton (last updated

2004, accessed on 23 September 2004).


-References- 129

QINETIQ NANOMATERIALS Ltd website(2003) < http://www.nano.qinetiq.com>

(last accessed on 25 september 2004).

RÖDIGER et al. (1998) Microwave sintering of hardmetals. International Journal of

Refractory Metals & Hard Materials, 16, pp. 409-416.

ROEBUCK, B. et al. (1999) Comparison of new and conventional grain size

Measurement methods for WC powders and hardmetals. Hard material powders, pp. 47

– 54.

SANTHAMAN, A.T., TIERNAY, P. HUNT, J.L. (1990) in Metals Handbook Vol.

2,ASM International, p. 950.

SCHUMANN, H. (1965) The form of hexagonal epsilon– martensite in austenitic

steels. Rostock: Wiss Z Univ pp. 423-429.

SCHUMANN, H. (1967) Metallography of the gamma–epsilon– alpha transformation

in high-alloy steels. Prakt Metallogr, pp.275-283.

SCHUMANN, H.(1967) Metallography of the gamma–epsilon–alpha

transformationing high-alloy steels. Prakt Metallogr, 4, pp. 275-283.

SEGAL, V.M. (1995) Materials processing by simple shear, Mater. Sci. Eng., pp. 57–

164.

SEGAL, V.M. (2003) Slip line solutions, deformation mode and loading history during

equal channel angular extrusion, Mater. Sci. Eng., pp. 36–46.

SEGMULLER, A. NOYAN, I.C. and SPERIOSU, V.S. (1989) X-ray Diffraction

Studies of Thin Films and Multilayer Structures, Prog. Crystal Growth and Charact.

18 (5), pp. 21-66.


-References- 130

SEMIATIN, S.L., DELO, D.P. and SHELL, E.B. (2000) Acta Mater. 48 pp. 1841–

1851.

SHENZHEN J. New Materials Development Co Ltd. N.d.

<http://newnanomaterial.com> (last accessed on 23 september 2004).

SHIMIZU, K. and TANAKA, Y. (1978) The γ →ε→ α martensitic transformations in

an Fe–Mn–C alloy. Trans Jpn Inst Met, 19, pp. 685–693.

SHIMIZU, K. and TANAKA, Y. (1978) The γ→ε→α’ martensitic transformationsin

an Fe–Mn–C alloy. Japan:Inst Met, pp. 685-693.

SHINGU, P.H. et al. (1998) Met Alloys. Suppm. Trans, Jpn. Inst Met. (3) p. 29.

STOICA, G.M. and LIAW, P.K. (2003) Evaluating the induced strain during equal

channel angular processing, in: LIAW, P.K. et al. (eds.), Materials Lifetime Science

and Engineering, TMS, pp. 119–133.

SUBRAMANIAN, R. et al.(1996) Scripta. Metall., 35(5), p. 583.

SUBRAMANIAN, R. and SCHNEIBEL, J.H. (1997) Intermetallic bonded WC-based

cermets by melt infiltration. Intermetallics, 5, pp. 401-408.

SURYANARAYANA, C. (2001a) Mechanical alloying and milling. Progress in

Materials Science, 46, pp. 1-184.

SURYANARAYANA, C. (2001b) Progress in Materials Science (46) pp. 21-29.

SURYANARAYANA, C. (2001c) Progress in Materials Science, (46), pp. 32-33.


-References- 131

SURYANARAYANA, C. (2001d) Progress in Materials Science, (46), p. 32

SURYANARAYANA, C. (1995a) Intermetallics. 3, pp.153-160.

SURYANARAYNA, C. (1995b) International Materials, 40, pp. 41-64.

TAKACS, L. (1996) in SURYANARAYANA, C. et al. (eds) Processing and properties

of nanocrystalline materials. Warrendale: TMS, pp. 453-464.

TAKACS, L., PARDAVI – HORVATH, M.J. (1994) Appl Phys., 75, pp. 5864-5866.

TAMU (n.d.) The history of Equal Channel Angular Extrusion.

< http://www.mengr.tamu.edu/Research/ecae/history.htm > (accessed on 17 October

2004).

TETRONICS Ltd website n.d. <http://www.tetronics.com> ( updated 2004, last

accessed on 27 september 2004).

TETRONICS n.d. < http://www.tetronics.com/pdffiles/nano2.pdf > (last accessed on

29 september 2004).

VISWANADHAM, R.K., LINDQUIST, P.G. and PECK, J.A. (1983) in ed. R. K.

Viswanadham, D. J. Rowcliffe and J. Gurland (eds.) Science of Hard Materials.

NewYork: Plenum Press, p. 873.

VISWANDHAM, R.K. and LINDQUIST, P.G. (1987) Mett. Trans., 18 A, p. 2175.

VOGEL, S. et al. (2002) Mater. Sci. Forum. pp. 408–412 and pp.673–678.

VOGEL, S.C. et al. (2003) Investigation of texture in ECAE materials using neutron


-References- 132

diffraction. Mater. Sci. Forum p. 2661.

WHISTON, C. (1987) PRICHARD, F. E. (eds.) X-ray methods. Chichester: ACOL,

p.92

WITTMANN, B., SCHUBERT, W.D., LUX, B. (2002) WC grain growth inhibition in

nickel an iron binder hardmetals. International Journal of Refractory Metals & Hard

Materials, 20, pp. 51-60

YAMAZAKI, T. et al. (1997) Mater Sci Lett., 16, pp. 1357-1359.

ZHANG, F.L a,d WANG, C.Y. and ZHU,M. (2003) Nanostructured WC/Co composite

powder prepared by high energy ball milling, Scripta Materiala,49, pp. 125-142.


-Appendix A-

-

Appendix A (Website for the project)


-Appendix A- A-2

Table of Contents

Table of Contents...............................................................................................................A-2

A. Website for the project...............................................................................................A-3

A. i. The intropage.....................................................................................................A-3

A. ii. Personal file...................................................................................................A-4

A. iii. Navigation page.............................................................................................A-5

A. iv. Photo and movie page....................................................................................A-5

A. v. Links..................................................................................................................A-6

A. vi. Thesis.............................................................................................................A-6


-Appendix A- A-3

A. Website for the project

The easiest way to keep in touch with Belgium, was communication through a website.

The website can be accessed on the following adres: http://users.pandora.be/erasmus-

wolverhampton.

Because it was our first experience with web design, more information involving html

programming and software for building websites had to be searched on the internet. In the

end of this effort, the program Dreamweaver MX 2004 was chosen to commence with the

web design because of the good help-index and the functionality of the program and

everything would be programmed in html.

Before the website could be built, there had to be made a plan view of the different pages

and a decision had to be made about the topics that would be handled. Afterwards, the

programming was a lot easier and the website could be developed.

It was very important that the opening page of the website was saved as index.html

otherwise the website couldn’t get accessed.

Because we wanted to give our website a personal tune, we searched the internet for some

applets, i.e. the clock following the mouse-cursor in the navigation page. Those applets

could be copied easily into the Dreamweaver MX 2004 program.

A. i. The intropage

Figure 81 shows the intropage of the website. By clicking in the picture, information of

person in question will be given while clicking on one of the logo’s of the schools or firm

connect you to their personal website. On the bottom of the page is a counter programme,

so the number of visitors is reported, and a song is inserted.


http://users.pandora.be/erasmus-wolverhampton

http://users.pandora.be/erasmus-wolverhampton

-Appendix A- A-4

Figure 81: Homepage of the website

A. ii. Personal file

A window with some personal data is shown while clicking on a person in the picture of

the opening page. Figure 82 gives you a view of this page.


-Appendix A- A-5

Figure 82: Personal data page


-Appendix A- A-6

A. iii. Navigation page

The navigation page will open by clicking on the red word “FURTHER”. Figure 83 shows

an example of this page in which navigation to every page is possible. The mouse cursor

gives you the date and the hour of the day while in the middle of the page a movie is

shown with a representation of the city and the university. There’s also the opportunity to

write something in the guestbook. The brown buttons are all Flash buttons that could be

inserted easily in Dreamweaver MX 2004. By clicking on one of them, a certain movement

will be made with a matching sound.

Figure 83: Navigation page of the website

A. iv. Photo and movie page

In these pages, different kinds of pictures and movies of us can be seen. They are all

classified by items or dates as shown in figure 84.


-Appendix A- A-7

Figure 84: Movies page

A. v. Links

On this page, shown in figure 85, different links involving our thesis project and our home

are given. By clicking on one of them the website of the involved firm will open.

Figure 85: The link page


-Appendix A- A-8

A. vi. Thesis

By clicking on the “thesis”-button, an up-to-date PDF-file will be shown.


-Appendix B- B-1

Appendix B (Videoconferencing facilities at UoW)


-Appendix B- B-2

Table of contents

Table of contents................................................................................................................B-2

B. i. Introduction............................................................................................................B-3

B. ii. Access Grid............................................................................................................B-5


-Appendix B- B-3

B. i. Introduction

Video conferencing has in the past been relatively expensive, but prices are now coming

down considerably, as it is possible for anyone with a fast enough internet connection to

operate a video conference. Sometimes the conferencing takes place over a private network

or VPN, which guarantees better performance, but there will be a trend towards running

video conferences over the public internet as technology improves.

A Virtual Private Network, or VPN, is a private communications network usually used

within a company, or by several different companies or organisations, communicating over

a public network. VPN message traffic is carried on public networking infrastructure (ie,

the Internet) using standard (possibly unsecure) protocols.

VPNs use cryptographic tunneling protocols (the transmission of one data protocol

encapsulated in another) to provide the necessary confidentiality, sender authentication and

message integrity to achieve the privacy intended. When properly chosen, implemented,

and used, such techniques can indeed provide secure communications over unsecure

networks.

Note that such choice, implementation, and use are not trivial and there are many unsecure

VPN schemes on the market. Users are cautioned to investigate products they propose to

use very carefully. 'VPN' is a label which, by itself, provides little except a marketing tag.

Video conferencing can be used for:

conducting interviews

holding meetings

setting up meetings

giving lectures

and has the advantage that it can reduce the need for travel (figure 86). Because

participants in a video conference may be working in different time zones, care must be


http://en.wikipedia.org/wiki/Time_zone

http://en.wikipedia.org/wiki/Protocol

http://en.wikipedia.org/wiki/Tunneling

http://en.wikipedia.org/wiki/Cryptographic

http://en.wikipedia.org/wiki/Communications_network

http://en.wikipedia.org/wiki/VPN

-Appendix B- B-4

taken with the organisation. Video conferencing can also be used within organisations to

provide immediate telepresence, using internal LAN’s as the communications

infrastructure.

Figure 86: Comparison in cost between communication possibilities

Telepresence means a human/machine system in which the human uses of (head-mounted)

displays and body-operated remote actuators and sensors to control distant machinery.

Provides a virtual environment for humans to control devices, robots, etc., in a hostile or

remote real environment.

Transparent telepresence is the experience of being fully present at a live real world

location remote from one's own physical location. Someone experiencing transparent

telepresence would therefore be able to behave, and receive stimuli, as though at the

remote site.

The resulting vicarious interactive participation in activities, and the carrying out of

physical work, brings benefits to a wide range of users. Examples include the emergency

and security services, entertainment and education industries, and those of restricted

mobility such as the disabled or elderly.


http://en.wikipedia.org/w/wiki.phtml?title=Vicarious&action=edit

http://en.wikipedia.org/wiki/Stimuli

http://en.wikipedia.org/wiki/Remote

http://en.wikipedia.org/wiki/Transparent

http://en.wikipedia.org/wiki/Robot

http://en.wikipedia.org/wiki/Sensor

http://en.wikipedia.org/wiki/Actuator

http://en.wikipedia.org/wiki/LAN

http://en.wikipedia.org/wiki/Telepresence

-Appendix B- B-5

For any telepresence system there are three essential sub-systems, i.e. the home site

technology which interfaces to the user and the communication link, the communication

link itself which interfaces to the home site and the remote site, and the remote site

technology which interfaces with the communication link and possibly a remote site

human.


-Appendix B- B-6

B. ii. Access Grid

The aim of Access Grid is to provide an effective environment for remote group-to-group

collaboration. Whilst this includes various audio and video conferencing components, they

are used within a peer-to-peer model that is coordinated via a separate peer-to-peer

services layer presented as virtual spaces.

Figure 87: View over the Access Grid video conference at RIATec office Wolverhampton

Access Grid is a research project that attempts to provide a sense of presence that

approaches that experienced in face-to-face meetings. The project is largely developed and

supported by a worldwide research community that consists primarily of academic

institutions.


-Appendix B- B-7

In order to build a wide scale test bed for building collaboration tools, the Access Grid

project specifically defines the minimum set of requirements necessary to be considered an

Access Grid Node. These requirements are specified in a manner that avoids specific

hardware and software implementations, but rather encourages the use of standards based

digital media tools. This provides a level playing field for research into alternative node

devices and the integration of other hardware and software components.

1. Costs

It is possible to buy an Access Grid node piecemeal and install software, etc. 'inhouse'.

This may be appropriate if Access Grid development is to take place. Another route is to

use an established commercial organisation that is fully involved in and accepted by the

Access Grid community. The product supplied is fully integrated with the Access Grid

system and the virtual spaces model and is based upon software in use by the community.

2. Display quality

An Access Grid node provides a large-scale high-resolution display either by projecting

onto a wall or using a rear projection system. The minimum specifications require

3072x768 pixels, at a distance between 2 and 8 times the height of the projected image.

This allows 18 QCIF and 6 CIF video streams to be displayed; additionally an entire XGA

screen is available for collaborative applications to be used simultaneously.

3. Visual Quality

Each Access Grid node transmits four video streams. These can be used for a variety of

purposes, including multiple camera angles of few participants, whole room shots, close-up

views or to transmit video for other media such as VCRs or document viewers. One


-Appendix B- B-8

frequent usage is to use one outgoing feed for a presenter, two for the local audience and

one feed to show remote participants the local display.

4. Audio Quality

The audio quality in an Access Grid node is of a very high quality. The audio stream is

sent uncompressed and sampled at 16 bits at 16 KHz. A single audio stream provides mono

audio, but the system is capable of sending multiple audio streams if necessary.

Figure 88: The micros used for the video conference

The audio component (figure 88) utilises a high-end echo canceller (Gentner XAP400 or

XAP800), level balancer and good quality microphones. Participants have hands-free, full-

duplex audio (i.e. many people can speak simultaneously). The resulting sound is as good

as that experienced when the participants are co-located.


-Appendix B- B-9

Figure 89: Audio component with the high-end echo canceller

5. Networking issues

An Access Grid node requires connectivity to an IP-based, multicast-enabled network. It is

possible to interact with the Access Grid via a multicast-unicast bridge (or reflector, or

tunnel) even where the Local Area Network is not multicast-enabled, but this is usually

used as a temporary, stopgap solution. However, the quality of experience using a bridge is

indistinguishable from the experience when using full multicast.

The Local Network connection requires 100Mbps connectivity. The Wide Area Network

requires 10Mbps.

6. Security

The Access Grid Toolkit v1.0 has implemented a trivial proof of concept for security

where an Access Control List is used to allow access to a virtual venue. If a particular user

is allowed into the venue, the venue provides keys with which to encrypt the audio and

video streams shared among the participants. Another approach is to share audio using the

public telephone system.


-Appendix B- B-10

Securing shared data is tackled on an ad hoc basis. Most collaborative tools have their own

security features that are utilised. For example when PowerPoint is used to share a

presentation, then the presentation itself is separately held at each node. Only control

events, such as 'Page Down', are broadcast and not the data within the presentation itself.

These control events, whilst not encrypted, are of little use in themselves.

7. Appropriate usage

The Access Grid is well suited for 3-6 participants at each of 2-12 sites. It can be used in

very formal meetings, web casts, classroom style interactions, or for unstructured

interactions where the desire is simply to provide a continual sense of presence with

remote collaborators.

The Access Grid does not provide any formal floor control mechanisms, since that would

be in conflict with the premise that if participants feel more presence then standard social

norms can and will govern interactions. Similarly, formal voting mechanisms are not

supported.


-Appendix C- C-1

Appendix C (Technical drawings)


-Appendix C- C-2

List of contents

List of contents...................................................................................................................C-2

C. i. Assembly die..........................................................................................................C-3

C. ii. Outer die.................................................................................................................C-4

C. iii. Inner die..............................................................................................................C-5

C. iv. Assembly lower punch.......................................................................................C-6

C. v. Bottom plate...........................................................................................................C-7

C. vi. Lower punch.......................................................................................................C-8

C. vii. Assembly upper punch.......................................................................................C-9

C. viii. Top plate...........................................................................................................C-10

C. ix. Upper punch.....................................................................................................C-11

C. x. Assembly die Charpy test.....................................................................................C-12

C. xi. Outer die...........................................................................................................C-13

C. xii. Inner die............................................................................................................C-14


-Appendix C- C-3

C. i. Assembly die


-Appendix C- C-4


-Appendix C- C-5

C. ii. Outer die


-Appendix C- C-6


-Appendix C- C-7

C. iii. Inner die


-Appendix C- C-8


-Appendix C- C-9

C. iv. Assembly lower punch


-Appendix C- C-10


-Appendix C- C-11

C. v. Bottom plate


-Appendix C- C-12


-Appendix C- C-13

C. vi. Lower punch


-Appendix C- C-14


-Appendix C- C-15

C. vii. Assembly upper punch


-Appendix C- C-16


-Appendix C- C-17

C. viii. Top plate


-Appendix C- C-18


-Appendix C- C-19

C. ix. Upper punch


-Appendix C- C-20


-Appendix C- C-21

C. x. Assembly die Charpy test


-Appendix C- C-22


-Appendix C- C-23

C. xi. Outer die


-Appendix C- C-24


-Appendix C- C-25

C. xii. Inner die


-Appendix C- C-26


-Appendix C- C-27


-Appendix D- D-1

Appendix D

(Dutch Summary)


-Appendix D- D-2

Voorwoord

Dit document is bedoeld als bijlage van onze thesis “bereiding en karakterisering van

submicron / nanogestructureerde poeders van wolfraamcarbide- kobalt / alternatieve

bindmiddelen hardmetalen” . Hierin wordt een Nederlandse samenvatting gegeven van de

thesis. Voor diepgaande uitleg rond bepaalde onderwerpen is het daarom ook aangeraden

de volledige thesis te raadplegen.

We hebben ervoor gekozen om onze thesis in het Engels te schrijven omdat we dit

onderzoek hebben gedaan aan “The University of Wolverhampton” in Engeland. Het

spreekt voor zich dat communicatie met onze promotor ter plaatse op deze manier vlotter

verliep.

We hebben voor dit project drie maanden doorgebracht in Engeland in het kader van een

Erasmus project.

Onze promotor van de Xios Hogeschool Limburg was Dr. Ir. A. Van Bael

De promotor van de University of Wolverhampton was Dr. Ir. T. Laoui

Adriaensen Peter

Moors Raf

- Samenvatting Thesis -

-Appendix D- D-3

Abstract

Tot op de dag van vandaag is kobalt (Co) een van de meest geschikte en meest gebruikte

bindmiddelen voor hardmetalen die gebaseerd zijn op wolfraam-carbide (WC). Een

belangrijke reden waarom Co zoveel gebruikt wordt in deze groep van hardmetalen is het

uitgesproken goede bevochtiginggedrag van Co voor WC.

Door onder andere de slechte corrosievastheid, de hoge kost en giftige eigenschappen, is er

uitgebreid onderzoek besteed aan het vinden van geschikte alternatieve bindmiddelen voor

WC systemen. Dit onderzoek heeft als doel de hoeveelheid Co te reduceren of indien

mogelijk, kobalt volledig te vervangen. Er zijn twee veelbelovende vervangers opgenomen

en gebruikt in dit project. De eerste mogelijkheid is een samenstelling van ijzer (Fe), nikkel

(Ni) en kobalt (Co), en de tweede een samenstelling van ijzer (Fe) en mangaan (Mn). In

vergelijking met Co zijn dit goedkope, niet giftige materialen.

Eerst werd er een grondige literatuurstudie uitgevoerd over hardmetalen, poeder-

preparatiemethoden, poedermetallurgie en nanomaterialen. De submicro/nano-

gestructureerde poeders werden vervolgens klaar gemaakt met behulp van het mechanisch

legeringproces. Hiervoor werd gebruik gemaakt van zowel de planetary ball mill als de

high-energy ball mill.

Een serie van experimenten werd uitgevoerd met de planetary ball mill. Er werden

verschillende bewerkingstijden (2.5, 5, 10 uren) en verschillende rotatiesnelheden (250,

400 rpm) gebruikt om WC-10wt%Co, WC-10wt%FeNiCo en WC-10wt%FeMn te

bereiden. Opvallend was dat bij langere bewerkingstijden (langer dan 2.5 uren voor 150

tr/min) grotere concentraties elementen (Fe, Cr) werden opgenomen van de binnenwanden

en de roestvaste stalen ballen. De hoeveelheid contaminatie nam meer toe naarmate de

rotatiesnelheid opgedreven werd naar 400 tr/min. Dat geeft aan dat zowel de snelheid als

de tijd zo laag mogelijk gehouden dient te worden om de contaminatie te minimaliseren

ofwel dient een hardmetalen container gebruikt te worden. Om deze reden werden er extra

poeders klaargemaakt met behulp van de high energy ball mill.


-Appendix D- D-4

De korrelgrootte van de WC fase werd uitgerekend door gebruik te maken van de Scherer

vergelijking. Deze vergelijking werd toegepast op de overeenstemmende pieken van het X-

stralen diffractie patroon (XRD). De grootte van de WC deeltjes werd geëvalueerd met een

elektronenmicroscoop (SEM). Er werden succesvol composiet poeders gemaakt waarin

fijne WC deeltjes (submicron tot ongeveer 200 nm in grootte) verdeeld waren in de

matrix (Co, FeNiCo of FeMn).

De volgende stap is het samenpersen van de poeders alvorens aan het sinterproces te

beginnen. Voor de compactiefase werden de geschikte matrijzen ontworpen. Het ontwerp

gebeurde met het CAD softwarepakket Inventor. Er werden twee matrijzen ontworpen: een

eerste voor het maken van cilindrische stukken, gebruikt voor microstructurele- en

hardheidonderzoeken, en een tweede matrijs voor het maken van stukken voor de 3-punts

buigtest. Bij dit ontwerp werd rekening gehouden met ASTM normen.


-Appendix D- D-5

Inhoudsopgave

Voorwoord.........................................................................................................................D-2

Abstract..............................................................................................................................D-3

Inhoudsopgave...................................................................................................................D-5

Lijst van figuren.................................................................................................................D-8

Lijst van tabellen................................................................................................................D-9

Lijst van symbolen.............................................................................................................D-9

1 Inleiding en doelstellingen.......................................................................................D-10

1.1 Inleiding...........................................................................................................D-10

1.2 Doelstellingen..................................................................................................D-11

2 Literatuuronderzoek.................................................................................................D-13

2.1 Alternatieve bindmiddelen...............................................................................D-13

2.1.1 Fe-Mn als alternatief bindmiddel.............................................................D-13

2.1.2 Fe/Ni/Co als alternatief bindmiddel.........................................................D-13

2.2 Korrelgroei.......................................................................................................D-14

2.3 Poedermetallurgie............................................................................................D-14

2.4 Nanogestructureerde materialen......................................................................D-15

2.4.1 Wat zijn nanogestructureerde materialen.................................................D-15

2.4.2 Mechanisch legeringproces......................................................................D-15

2.4.2.1 Mechanisme van mechanisch legeren..................................................D-16

2.5 Korrelgrootte bepaling van WC.......................................................................D-20

2.5.1 Laser Diffractie........................................................................................D-20

2.5.2 X- stralen diffractie (XRD)......................................................................D-21


-Appendix D- D-6

2.5.3 Microscoop beelden, SEM, TEM............................................................D-21

3 Experimentele procedure.........................................................................................D-23

3.1 Geteste poedersamenstellingen........................................................................D-23

3.2 Mechanisch legeringsproces............................................................................D-24

3.2.1 Planetary ball mill....................................................................................D-24

3.2.2 High energy mill......................................................................................D-24

3.3 Analyse............................................................................................................D-25

3.3.1 X- stralen diffractie..................................................................................D-25

3.3.2 Scanning elektronen microscoop (SEM).................................................D-26

4 Resultaten.................................................................................................................D-27

4.1 Planetary ball mill............................................................................................D-27

4.1.1 Referentie materiaal.................................................................................D-27

4.1.1.1 XRD patroon van het niet verwerkte referentie materiaal...................D-27

4.1.1.2 Berekende korrelgrootte van de start WC deeltjes..............................D-28

4.1.1.3 Onderzoek naar de vervuilingsgraad (XRF)........................................D-30

4.1.2 XRD resultaten van planetary ball milling aan 250 tr/min......................D-31

4.1.2.1 2,5 uur verwerkte staal met planetary ball milling aan 250 tr/min......D-31

4.1.2.2 Het 5 uur verwerkte staal met planetary ball milling aan 250 tr/min. .D-32

4.1.2.3 Het 10h verwerkte staal met planetary ball milling aan 250 tr/min.....D-33

4.1.3 De SEM foto’s van 250 tr/min stalen met planetary ball milling............D-34

4.1.3.1 Het 2,5u verwerkte staal met planetary ball milling aan 250 tr/min....D-34

4.1.4 XRD resultaten van planetary ball milling met 400tr/min......................D-35

4.1.5 Het 5 uur verwerkte staal met planetary ball milling aan 400tr/min.......D-35

4.1.6 Het 10u verwerkte staal met planetary ball milling aan 400tr/min..........D-36

4.1.7 SEM foto’s van 400 tr/min verwerkte stalen met planetary ball milling.D-36

4.1.7.1 Het 10u verwerkte staal met planetary ball milling aan 400tr/min......D-37

4.1.8 Vervuilingsgraad bij planetary ball milling.............................................D-38

4.1.9 Korrelgrootte berekening.........................................................................D-39

4.1.9.1 Poeder stalen van planetary ball mill aan 250tr/min............................D-40

4.1.9.2 Poeder stalen van planetary ball mill aan 400 tr/min...........................D-40


-Appendix D- D-7

4.1.9.3 Besluiten van de Scherer vergelijking.................................................D-41

4.1.9.4 Weergave van spanningen door piekverschuivingen...........................D-43

4.1.9.5 Besluiten van de Warren Averbach methode.......................................D-44

4.2 High energy horizontal mill.............................................................................D-44

4.2.1 Schatting van de vervuilingsgraad van het verwerkte poeder.................D-44

4.2.2 SEM foto’s van de stalen gelegeerd met de high energy mill.................D-45

4.2.3 SEM foto’s met Fe/Mn als alternatieve binder........................................D-46

4.2.4 SEM foto’s met Fe/Ni/Co als alternatieve binder....................................D-47

5 Besluiten..................................................................................................................D-48

5.1 De planetary ball mill.......................................................................................D-48

5.2 De horizontal high energy mill........................................................................D-49

5.3 Alternatieve bindmiddelen...............................................................................D-50

5.4 Verderzetting van het onderzoek.....................................................................D-50


-Appendix D- D-8

Lijst van figuren

Figuur 1: Voorstelling van poeder gevangen tussen 2 ballen MA, SURYANARAYANA, C., 2001________D-17

Figuur 2: Fritch Pulverisette 5 planetary ball mill____________________________________________D-18

Figuur 3: Voorstelling van de ballen in de planetary ball mill, Zoz_______________________________D-19

Figuur 4: Bewegingsbaan van de ballen in de container, Courtesy of Gilson Company, p.17___________D-19

Figuur 5: Opstelling van de High energy mill, Metal-Powder___________________________________D-20

Figuur 6: Voorstelling van de ballen in high energy mill_______________________________________D-21

Figuur 7: Laser diffraction analyse________________________________________________________D-22

Figuur 8: XRD- patroon van niet verwerkt WC-Co referentiestaal________________________________D-29

Figuur 9: SEM foto van start WC < 20 micron_______________________________________________D-30

Figuur 10: grafiek van de verschillende elementen in het referentiestaal___________________________D-31

Figuur 11: XRD patroon van 2,5 uur verwerkt materiaal aan 250tr/min___________________________D-32

Figuur 12: XRD- patroon van 5uur verwerkt materiaal aan 250 tr/min____________________________D-34

Figuur 13: XRD- patroon van 10uur verwerkt materiaal aan 250 tr/min___________________________D-35

Figuur 14: SEM foto van 2,5uur verwerkte staal met de planetary ball mill________________________D-36

Figuur 15: XRD patroon van het 5uur verwerkte materiaal aan 400 tr/min_________________________D-37

Figuur 16: XRD patroon van 10uur verwerkt materiaal aan 400tr/min____________________________D-38

Figuur 17: SEM foto van 10u verwerkt materiaal aan 400 tr/min________________________________D-39

Figuur 18: Schatting van de hoeveelheid vervuilende elementen_________________________________D-40

Figuur 19: Vervuilingsgraad in functie van verwerkingstijd_____________________________________D-41

Figuur 20: Excel rekenblad om de gemiddelde korrelgrootte te bepalen m.b.v. de Scherer vergelijking (250

tr/min verwerkt staal)___________________________________________________________________D-42


tr/min verwerkt staal)___________________________________________________________________D-43

Figuur 22: Fourier analyse van niet verwerkte poeder < 20micron_______________________________D-44

Figuur 23: Resultaten van Winfit!v1.2 op <20µ deeltjes________________________________________D-44

Figuur 24: Korrelgrootte van <20µ WC deeltjes met Winfit!v1.2_________________________________D-45

Figuur 25: Piekverschuiving van WC bij 250 tr/min___________________________________________D-45

Figuur 26: Schatting van de hoeveelheid vervuilende elementen in het poeder______________________D-47

Figuur 27: vervuiling na de verwerking____________________________________________________D-47

Figuur 28: SEM foto van het staal WC-Fe/Mn dat gedurende 1h gelegeerd is in de Horizontally high energy

mill_________________________________________________________________________________D-49


mill_________________________________________________________________________________D-50


-Appendix D- D-9

Lijst van tabellen

Tabel 1 Samenstellingen poeders............................................................................................................D-24

Tabel 2: Samenstellingen binders............................................................................................................D-25

Lijst van symbolen

Co KobaltFe IJzer

H2O WaterHCl Waterstofchloride

HNO3 SalpeterzuurMA Mechanisch legerenMn MangaanNi NikkelPM Poeder Metallurgie

SEM Scanning Elektron microscopieTEM Transmissie Elektronen MicroscoopUoW Universiteit van WolverhamptonVC Vanadium CarbideWC Wolfraam CarbideXRD X- stralen diffractieXRF X- straal fluorescentie


-Appendix D- D-10

1 Inleiding en doelstellingen

1.1 Inleiding

Metaalpoeders vinden hun toepassing in tal van producten. Telkens bewaren de

poederdeeltjes hun oorspronkelijke eigenschappen. Poedermetallurgie is een proces

waarbij doorgaands een metaal, een legering of een composiet in vaste deeltjes van

maximum 150µm wordt omgezet in een bepaalde vorm. De bekomen vorm wordt meestal

niet meer verder bewerkt.

De basisstappen in poedermetallurgie zijn:

- poederproductie

- poedercompactie

- sintering

Hoewel dit proces reeds eeuwen gebruikt wordt, wordt er de laatste jaren toch meer en

meer de aandacht op gevestigd. Er is een proces ontwikkeld, zogenaamd mechanisch

legeren, waarbij de poeders fijn gemalen worden. Hierbij worden hardmetalen deeltjes

verfijnd en verdeeld in een bindend materiaal. Oorspronkelijk was dit proces enkel bedoeld

voor materialen die hoge temperaturen moesten weerstaan, maar vindt nu toepassing in

verschillende ingenieurstoepassingen. Zo worden de bekomen hardmetalen vaak gebruikt

in de lucht- en ruimtevaart of als snijmateriaal. [Jenkins 1993, pp. 1-5].

Tot op de dag van vandaag is kobalt (Co) een van de meest geschikte en meest gebruikte

binders voor hardmetalen die gebaseerd zijn op wolfraam-carbide (WC). Een belangrijke

reden waarom Co zoveel gebruikt wordt in deze groep van hardmetalen is het

uitgesproken goede bevochtiginggedrag van Co.

De reden om voor het WC-Co systeem te kiezen is tweevoudig; het is een klassiek systeem

dat zeer goede bevochtigingeigenschappen vertoont tussen de twee fases, en dit systeem


-Appendix D- D-11

heeft aantrekkelijke eigenschappen voor tal van toepassingen [Honeycombe 1981, pp.35-

42].

Er zijn nochtans ook negatieve eigenschappen aan dit systeem met kobalt als bindend

metaal. Onder andere: de hoge kost, de giftige eigenschappen en de slechte

corrosieweerstand van kobalt (Gonzalez et al.,1995). Om deze redenen wordt er heel wat

onderzoek gedaan naar mogelijke vervangmiddelen voor kobalt in dit systeem.

1.2 Doelstellingen

Bij aanvang van deze studie werd eerst een uitgebreid literatuur onderzoek verricht. Dit

was nodig om een zekere basiskennis rond het onderwerp te verkrijgen. De resultaten van

deze studie worden dan ook samengevat weergegeven in het hoofdstuk

literatuuronderzoek.

De tweede fase van deze thesis was het vertrouwd geraken met de verschillende machines

en de aanmaak van een aantal poederstalen. Hierbij werd voornamelijk de aandacht

gevestigd op de procesparameters. De voornaamste parameters zijn verwerkingstijd en

snelheid. Verschillende combinaties van deze parameters werden onderzocht en zijn

beschreven in dit werk.

Het doel van dit werk is het bereiden van submicron/ nanogestructureerde poeders met

behulp van het mechanisch legeringproces. Bij dit proces wordt de grootte van de WC-

hardmetaaldeeltjes gereduceerd .

Een ander aspect in deze thesis was de zoektocht naar alternatieve bindmiddelen. Tot op

heden gebruikt men voornamelijk kobalt als bindmiddel voor wolfraam carbide (WC)

hardmetalen. De voornaamste reden hiervoor is de uitstekende bevochtigingeigenschap van

dit systeem.


-Appendix D- D-12

Jammer genoeg heeft kobalt ook een aantal negatieve eigenschappen, zijnde: de hoge prijs,

de giftigheid en de corrosiviteit [Gonzalez et al.,1995]. Daarom worden twee mogelijke

vervangingsmiddelen, ijzer/mangaan en ijzer/nikkel/kobalt in deze thesis onderzocht.

Om de eigenschappen van de poederstalen te kennen is het belangrijk een zicht te krijgen

op de afmetingen van de poederdeeltjes. Dit onderzoek werd in eerste instantie verricht

met behulp van X- stralen. Uiteindelijk is gebleken dat deze techniek niet geschikt was

omdat de bekomen poederdeeltjes te klein waren voor deze analyse (WHISTON C. 1987,

p. 92). Na deze vaststelling werd er gekozen voor onderzoek met elektronen microscoop

SEM.

De geproduceerde stalen worden later gecomprimeerd en gesinterd. Na deze stappen

moeten dan de mechanische eigenschappen worden gemeten. Voorbeelden van deze

metingen zijn: hardheidsmetingen, buigproef, kerfslagproef enz. De matrijzen die nodig

zijn voor de compactie en de metingen moesten ontworpen worden volgens de ASTM

standaards. Het tekenen van deze matrijzen gebeurde met het softwarepakket Inventor.


-Appendix D- D-13

2 Literatuuronderzoek

2.1 Alternatieve bindmiddelen

Een onderdeel van onze thesis was het zoeken naar alternatieve bindmiddelen voor kobalt.

Er werd getracht om de negatieve eigenschappen van kobalt weg te werken door andere

alternatieve binders te zoeken voor kobalt. Eventueel het verminderen van de benodigde

hoeveelheden kobalt heeft ook positieve gevolgen, zowel economisch, technisch als

ecologisch.

2.1.1 Fe-Mn als alternatief bindmiddel

Ijzer- Mangaan legeringen vertonen gelijkaardige karakteristieken als kobalt. Onder andere

smeltpunt, kristalstructuur en fasetransformaties bij koelen zijn gelijk.

Omwille van de hoge slijtvastheid van Fe-Mn staal, werd er van uit gegaan dat deze

legering ook een hoge slijtvastheid voor WC hardmetalen kon leveren [Hanayaloglu et al.

2001, pp. 315-322].

2.1.2 Fe/Ni/Co als alternatief bindmiddel

Prakash was de eerste die (Fe/Ni/Co)-legeringen onderzocht en toonde aan dat harde

metalen met een ijzerrijk bindmiddel een aantal verbeterde eigenschappen had. Hogere

hardheid, slijtvastheid en sterkte zijn een aantal voorbeelden van deze eigenschappen

[Prakash 1993, pp. 80-109 – Prakash 1979].

Bindmiddelen gebaseerd op Fe-Ni/Ni-Co en Fe-Cu-Co worden beschreven als goede

vervangmiddelen voor kobalt (Gonzalez et al., 1995) (Gonzalez et al., 1998).


-Appendix D- D-14

2.2 Korrelgroei

Deze zeer belangrijke eigenschap kan beschreven worden als het verschijnsel waarbij de

hardmetaal deeltjes in grootte toenemen na sinteren. Dit heeft een negatief effect omdat

men met mechanisch legeren een zo fijn en homogeen mogelijke structuur tracht te

bekomen. Om dit verschijnsel tegen te gaan worden korrelgroeiremmers toegevoegd aan

de poeders. In het onderzoek wordt Vanadium Carbide (VC) gebruikt als korrelgroei

remmer. De invloed van VC wordt bestudeerd door de korrelgrootte na sintering te bepalen

bij een staal met, en een staal zonder VC.

2.3 Poedermetallurgie

Poedermetallurgie is een proces waarbij doorgaands een metaal, een legering of een

composiet in vaste deeltjes van maximum 150µm wordt omgezet in een bepaalde vorm. De

bekomen vorm wordt meestal niet meer verder bewerkt.

Naast de verschillende metaalbewerkingtechnieken, vergt poedermetallurgie een totaal

andere aanpak. Een groot voordeel bij poedermetallurgie is de mogelijkheid om met hoge

kwaliteit, complexe onderdelen met nauwe toleranties, te fabriceren op een economisch

verantwoorde wijze [German n.d., pp. 6-7]. De techniek bestaat uit een aantal

verschillende stappen:

Stap 1: Het poeder wordt gemengd met een geschikt smeermiddel (zinc stearate). Deze

stap heeft enkel als doel het verminderen van de wrijving tijdens de compactiefase.

Stap 2: Het poeder wordt gecomprimeerd. Wanneer de poeders onder een bepaalde druk

komen te staan, wordt een vaste vorm verkregen. Door de cohesiekrachten tussen

de poederdeeltjes is het mogelijk deze compacte vorm te bewerken. De termen

dichtheid en sterkte beschrijven de eigenschappen van deze compacte vorm.


-Appendix D- D-15

Stap 3: De vaste vorm wordt gesinterd. Het sinteren, gebeurt in een inerte atmosfeer of

onder vacuüm. De temperatuur zal onder het smeltpunt van de hardmetaaldeeltjes

gehouden worden maar boven de smelttemperatuur van het bindend metaal.

Hierdoor zullen de hardmetaaldeeltjes in het vloeibaar geworden bindmiddel

opgenomen worden. De hoeveelheid vloeistoffase mag niet te groot worden zodat

de oorspronkelijke vorm behouden blijft.

2.4 Nanogestructureerde materialen

Waar poeders, met een doorsnede van bijvoorbeeld 150µm, hun eigenschappen danken aan

de wetten van de gewone fysica, spelen de wetten van de kwantum fysica hun rol bij de

nanogestructureerde materialen. Hierdoor veranderen zowel de chemische als fysische

eigenschappen van materialen met een doorsnede van 100nm. Zo zullen bijvoorbeeld

nanogestructureerde keramieken harder en sterker zijn dan hun grovere soortgenoten

[Pritchard 2004]. In dit opzicht is het dus belangrijk kennis te hebben over de mogelijke

methoden om nanogestructureerde materialen te vervaardigen en te onderzoeken.

2.4.1 Wat zijn nanogestructureerde materialen

Waar conventionele materialen korrelgroottes hebben van micrometers tot millimeters,

hebben nanogestructureerde materialen slechts korrelgroottes kleiner dan 100 nanometer.

Hierdoor zit ook een enorm verschil in het aantal atomen dat elke korrel bevat. Zo heeft

een conventionele korrel verscheidene biljoenen atomen waar een gemiddelde

nanogestructureerde korrel er slechts minder dan 900 bevat [Nanostructured Materials

2004].

2.4.2 Mechanisch legeringproces

Deze techniek werd in het onderzoek gebruikt om de wolfraam-kobalt hardmetaaldeeltjes

te verkleinen tot een nano-structuur.


-Appendix D- D-16

De grondstof voor deze techniek zijn zuivere poederdeeltjes met groottes tussen 1 en

200µm. De startafmetingen zijn niet erg belangrijk, de enige voorwaarde is dat de

poederdeeltjes kleiner zijn dan de ballen. De reden waarom de begingrootte niet erg

belangrijk is, is dat de deeltjesgrootte in verhouding tot de verwerkingstijd exponentieel

afneemt [Koch 1991, pp. 193-245].

De onderzochte legeringen zijn gebaseerd op WC-10wt.% (Fe/Ni/Co) en WC-10wt.%

(Fe/Mn) met toevoeging van 1wt% korrelgroei remmers (VC). Ter vergelijking gebruiken

we een referentiestaal gebaseerd op WC-10wt% Co.

Er zijn twee verschillende technieken gebruikt voor het mechanisch legeringproces. De

eerst gebruikte techniek is de planetary ball mill. Als tweede techniek is er de horizontal

high energie mill. Deze principes worden kort uitgelegd in het hoofdstuk

“literatuuronderzoek” van deze samenvatting.

Het probleem van deze techniek is het verzekeren dat alle deeltjes verkleind worden. Deze

techniek resulteert dan ook in een typische Gauss verdeling met een lange start, welke de

niet verkleinde deeltjes afbeeldt.

2.4.2.1 Mechanisme van mechanisch legeren

Tijdens het verwerken worden de deeltjes herhaaldelijk platgedrukt, koud gelast, gebroken

en opnieuw gelast. Telkens 2 ballen botsen, bevind zich er een hoeveelheid poeder

tussenin. Dit wordt schematisch voorgesteld in figuur 1.

Figuur 1: Voorstelling van poeder gevangen tussen 2 ballen MA, SURYANARAYANA, C., 2001


-Appendix D- D-17

De deeltjes worden plastisch vervormd door de kracht van de botsingen. Dit leidt tot

verharden en breken van de deeltjes. De nieuwe oppervlakken worden opnieuw koud aan

elkaar gelast waardoor de deeltjes terug groter worden. In het begin van het proces zijn de

deeltjes nog zacht waardoor ze makkelijk opnieuw aan elkaar gelast kunnen worden (in

geval van een combinatie van brosse en taaie materialen). Door het verdere verwerken

worden de deeltjes harder en zullen breken onder invloed van het vermoeiingsmechanisme.

Aangezien de ballen blijven inwerken op de deeltjes zal de structuur ervan blijven

verfijnen.

Hoewel de grootte van de deeltjes niet meer verandert, zullen de tussenlagen verkleinen en

het aantal lagen in het deeltje toenemen [Suryanarayana 2001c, pp. 32-33].

2.4.2.1.1 Planetary ball milling

Een vaak gebruikte machine voor mechanisch legeren is de planetary ball mill; afgebeeld

in figuur 2.

Figuur 2: Fritch Pulverisette 5 planetary ball mill

De naam is afgeleidt van de bewegingen die de containers maken. De basisplaat voert een

draaibeweging uit, terwijl de containers afzonderlijk rond hun as draaien. De draaizin van

de containers afzonderlijk is tegengesteld aan de draaizin van de basisplaat.


-Appendix D- D-18

De centrifugale krachten veroorzaakt door de rotaties weken in op de inhoud van de

containers. Hierdoor bewegen de ballen in de containers zich langs de wanden van de

containers waardoor het poeder geplet wordt.

Figuur 3: Voorstelling van de ballen in de planetary ball mill, Zoz

Figuren 3, 4 laten zien hoe de ballen zich in de containers bewegen voor en tijdens de

werking van de planetary ball mill.

De containers en ballen zijn verkrijgbaar in verschillende materialen waaronder

wolfraamcarbide, siliciumnitride, chroom staal, Cr-Ni staal, en plastic polyamide

[Suryanarayama 2001a, pp. 1-184].

Figuur 4: Bewegingsbaan van de ballen in de container, Courtesy of Gilson Company, p.17


-Appendix D- D-19

2.4.2.1.2 High energy ball milling

Met deze techniek kunnen al nanogestructureerde deeltjes bekomen worden in enkele

minuten. Over het algemeen zal bij langere verwerkingstijden en hogere energie, de

deeltjesgrootte verminderen.

De horizontale high energy mill wordt gebruikt voor zowel academische als industriële

toepassingen.

De hoge kinetische energie geleverd door deze machines resulteert in lage

verwerkingstijden. De kortere verwerkingstijden zorgen er ook voor dat er minder

vervuiling optreedt.

Met dit principe is het enkel nodig de inwendige rotor te laten versnellen. In tegenstelling

tot de planetary ball mill waarbij alle containers moeten versneld worden, wordt bij deze

techniek de energie beter benut.

Figuur 5: Opstelling van de High energy mill, Metal-Powder


-Appendix D- D-20

Het verwerken kan moeiteloos onder inerte argon atmosfeer gebeuren aangezien de

machine is uitgerust met een airlock systeem.

Figuur 5 toont een afbeelding van de machine in werking, volledig met airlock systeem.

De rotor wordt, zoals getoond in figuur 6, gebruikt om de kinetische energie over te

brengen op de ballen en het poeder.

Figuur 6: Voorstelling van de ballen in high energy mill

2.5 Korrelgrootte bepaling van WC

Om te weten of we te maken hebben met nanogestuctureerde poeders is het natuurlijk

belangrijk om te weten hoe groot de korrels van onze poeders zijn. Ook om de geschikte

instelparameters te zoeken van de machines is dit een belangrijk onderdeel. Er zijn

verschillende technieken beschikbaar om korrelgroottes te onderzoeken. Het probleem dat

zich bij het onderzoek stelt is dat de belangrijke hardmetalen deeltjes opgesloten zitten in

de binder. We zijn echter enkel geïnteresseerd in de afmetingen van de hardmetaaldeeltjes.

Een aantal technieken houden hiermee geen rekening en zijn dus niet bruikbaar. De

bruikbare technieken zullen hier kort besproken worden.

2.5.1 Laser Diffractie

Deze techniek is enkel bedoeld voor WC-Co poeders die klaar zijn voor compactie. Er

wordt informatie gegeven over de spreiding van de deeltjes tussen 2 en 1000µm. De

nodige tijd voor een meting is ongeveer 10min. [1970, pp. 429-448 – Friederich and Exner


-Appendix D- D-21

1984, pp. 334-341 – Fischmeister et al. 1966, pp. 106-124 – Le Roux and King 1987,

pp.243-248] Een voorstelling van het toestel wordt gegeven in figuur 7. Dit toestel is

bruikbaar voor ons onderzoek maar was echter niet beschikbaar in het labo.

Figuur 7: Laser diffraction analyse

2.5.2 X- stralen diffractie (XRD)

Aanvankelijk werd er van uitgegaan dat deze techniek geschikt was voor ons onderzoek.

Er werd dan ook veel tijd in dit onderzoek geïnvesteerd. Toen de analyse van de resultaten

onmogelijk uitkomsten opleverden volgde een zoektocht naar de mogelijke foutoorzaken.

Uiteindelijk is er gevonden dat deze methode niet geschikt is in het gebied tussen 0,1 en 10

µm (WHISTON C. 1987, p. 92)..

2.5.3 Microscoop beelden, SEM, TEM

Dit zijn zeer directe methodes, die vaak worden toegepast in onderzoek. Vooral SEM is

interessant om naar heterogeniteit te kijken. Voor korrels kleiner dan 0,5µm is een hoge

resolutie nodig. TEM is een techniek die beter toelaat polykristallinniteit te zien. Het

nadeel met TEM is de lage statistische nauwkeurigheid, maar de techniek is bruikbaar voor


-Appendix D- D-22

heel kleine korrels minder dan 50nm. [Anders 1992, pp. 195-204]. Deze techniek werd ook

gebruikt om de deeltjesgrootte te bepalen nadat er werd vastgesteld dat X –stralen

diffractie niet geschikt was voor de deeltjes die wij moesten onderzoeken (tussen 0,1 en 10

µm) (WHISTON C. 1987, p. 92).


-Appendix D- D-23

3 Experimentele procedure

Het onderzoek bestaat uit een aantal verschillende fasen. Eerst is er het aanmaken van een

aantal testsamenstellingen. Deze zijn gebaseerd op 90 gewichtsprocent (wt%)

wolfraamcarbide plus korrelgroeiremmer en 10 gewichtsprocent (wt%) binder. Er worden

verschillende samenstellingen als binder getest.

Na afwegen van de samenstellingen worden de poeders gemalen in een van beide

vermalingmachines. Hierin vindt het mechanisch legeringsproces plaats.

Na dit proces worden de poeders verzameld en wordt de grootte van de hardmetaaldeeltjes

onderzocht.

3.1 Geteste poedersamenstellingen

Volgende poeders werden klaargemaakt voor onderzoek:

Naam Wt% WC Wt% Binder Wt%

korrelgroeiremmer

A WC-Co 90 wt%

WC

10 wt% Co

B WC-Co met korrelgroeiremmer 89 wt%

WC

10 wt% Co 1 wt% VC

C WC- Fe/Mn 90 wt%

WC

10 wt% Fe/Mn

D WC- Fe/Mn met

korrelgroeiremmer

89 wt%

WC

10 wt% Fe/Mn 1 wt% VC

E WC - Fe/Ni/Co 90 wt%

WC

10 wt%

Fe/Ni/Co

Tabel 1 Samenstellingen poeders


-Appendix D- D-24

Samenstellingen van de bindmiddelen:

Binder Wt% Fe Wt% Ni Wt% Co Wt% Mn

1 Fe/Ni/Co 75 15 10 /

2 Fe/Mn 86.5 / / 13.5

Tabel 2: Samenstellingen binders

3.2 Mechanisch legeringsproces

Het legeren gebeurde met twee verschillende machines. Voor de planetary ball mill wordt

de invloed van snelheid en verwerkingstijd onderzocht. Voor de high energy mill worden

de parameters ingesteld zoals ze door de leverancier van de machine [ZoZ.] zijn

meegedeeld.

3.2.1 Planetary ball mill

De poeders zijn verwerkt met een Fritch pulveristette 5 planetary ball mill. Om de optimale

parameters te vinden werd er een referentiestaal verwerkt gedurende 2,5; 5 en 10 uur aan

snelheden van 250 en 400 tr/min. We hebben roestvaste stalen containers en ballen

gebruikt. De gebruikte massaverhouding van bal tot poeder was 15/1. De container werd

gevuld met 125ml ethanol.

3.2.2 High energy mill

Het proces werd geprogrammeerd met het programma Matoz. De eerste stap was het

mengen van de poeders. Dit gebeurde gedurende 5minuten op een snelheid van 200tr/min.

Hierna werd een 60 minuten durend programma uitgevoerd. Dit programma bestond uit 12


-Appendix D- D-25

herhalingen van 4 minuten werken aan 1000 tr/min en 1 minuut werken aan 600 tr/min.

Het leegmaken van de machine gebeurde gedurende 20 min aan een snelheid van 1200

tr/min. De gebruikte massaverhouding van bal tot poeder was 1:10. De ballen en de

verwerkingseenheid zijn vervaardigd uit roestvast staal. Het proces vond plaats onder een

inerte argon atmosfeer

3.3 Analyse

Zoals al aangegeven is het onderzoek naar de deeltjesgrootte van de hardmetalen zeer

belangrijk. Het gebruik van X- straal analyse kon niet worden toegepast in ons onderzoek.

Omdat dit niet onmiddellijk duidelijk was, werd er aan deze techniek in ons onderzoek

toch veel tijd gespendeerd. Daarom hebben we de resultaten hiervan toch weergegeven.

3.3.1 X- stralen diffractie

X- straal zijn elektromagnetische stralen met een golflengte van 10-10 m, dit is ongeveer de

grootte van een atoom. Deze stralen verschijnen in het elektromagnetische spectrum in

vormen tussen gamma stralen en ultraviolet.

X- stralen diffractie wordt gebruikt ter herkenning van kristallijne materialen en bepaling

van de structuur.

Elke kristallijne vaste stof heeft een unieke X- straal karakteristiek welke kan gebruikt

worden als een “vingerafdruk” ter identificatie. Eenmaal een materiaal geïdentificeerd is,

kan X- stralen diffractie gebruikt worden om de structuur te bepalen, m.a.w. hoe de atomen

op elkaar gestapeld zijn in de kristalstructuur, de interatomaire afstandshoeken enz. Deze

techniek heeft de beperking niet bruikbaar te zijn in het gebied tussen 0,1 en 10 µm

(WHISTON C. 1987, p. 92).


-Appendix D- D-26

3.3.2 Scanning elektronen microscoop (SEM)

Na de vaststelling dat X- stralen diffractie niet geschikt was om de deeltjesgrootte van de

hardmetalen te bepalen was de enige beschikbare oplossing SEM.

Om een sterke vergroting van de stalen te krijgen, zodat de WC korrels in Co goed

zichtbaar zijn, hebben we een ZEIS scanning elektronen microscoop (SEM) gebruikt. Deze

microscoop was uitgerust met een backscatterd detector. Dit zorgt ervoor dat de elementen

met de zwaarste atomaire massa het lichtst verschijnen op de foto. Een spanning tussen

20kV en 25kV werd gebruikt om deze machine te bedienen.


-Appendix D- D-27

4 Resultaten

De bekomen resultaten worden onderverdeeld in 2 groepen. Enerzijds zijn er resultaten van

de planetary ball mill, waarvoor de geschikte procesparameters worden gezocht. En

anderzijds zijn er de resultaten van de High energy horizontal mill.

Voor beide machines worden de resultaten van X - straal diffractie (XRD) en onderzoek

naar vervuiling (XRF) weergegeven.

4.1 Planetary ball mill

Het eerste onderzoek werd gevoerd naar de instelling van de juiste parameters voor het ball

milling proces. Om de gegevens te bekomen werd een referentie staal verwerkt op

verschillende snelheden en gedurende verschillende verwerkingstijden. De resultaten van

dit onderzoek worden hieronder weergegeven.

4.1.1 Referentie materiaal

Als referentie staal werd een samenstelling van 90 wt% WC en 10 wt% Co (geen

korrelgroei remmers) gebruikt.

4.1.1.1 XRD patroon van het niet verwerkte referentie materiaal

Figuur 8 geeft het XRD- patroon van het niet verwerkte referentie materiaal

Met behulp van deze grafiek kunnen alle elementen en de gemiddelde korrelgrootte

bepaald worden. Op die manier is het eenvoudiger het verband te bepalen tussen

verwerkingstijd, snelheid en de korrelgrootte.

-Samenvatting Thesis-

-Appendix D- D-28

Figuur 8: XRD- patroon van niet verwerkt WC-Co referentiestaal

De WC pieken op 2θ = 31.6° en 2θ = 35.9° samen met de Scherer vergelijking (zie 4.1.1.2)

worden gebruikt om de gemiddelde WC korrelgrootte te bepalen

Deze pieken zijn gekozen, en zullen in ons volledige werk gebruikt worden, omdat

eventuele vervuiling afkomstig van de containers en ballen niet interfereert met deze

pieken. Op deze manier zijn deze twee pieken representatief voor WC.

4.1.1.2 Berekende korrelgrootte van de start WC deeltjes

De Scherer vergelijking werd gebruikt om de grootte van de WC korrels te berekenen.

κ = 1

λ = X-ray golflengte, λCu = 1,5418

β = FWHM van diffractie lijn

θ = diffractie hoek


-Appendix D- D-29

De waardes van β en θ kunnen in de XRD data gevonden worden. β is de breedte, in

radialen, van de WC piek die zich op 2θ bevindt. En θ (in radialen) is de halve waarde van

2θ, de positie van de piek. De formule wordt gebruikt op de twee pieken(2θ = 31.6° en 2θ

= 35,6°) waarna een gemiddelde bepaald wordt. Op die manier wordt een grotere

nauwkeurigheid bekomen.

In de start WC korrels zijn de FWHM van beide pieken 0,00635 radialen. The diffractie

hoeken zijn respectievelijk 0,275 en 0,310 radialen. Wanneer deze resultaten in de

vergelijking worden gevoegd levert dit voor de twee pieken:

Het gemiddelde van deze resultaten wordt berekend:

Dit is een onverwacht resultaat omdat de startpoeders gezeefd zijn in een 20µm zeef. Ter

verduidelijking is een SEM foto gemaakt waarmee op een snelle manier de afmetingen

bepaald kunnen worden. Figuur 9 toont de SEM foto.

Figuur 9: SEM foto van start WC < 20 micron


-Appendix D- D-30

Op deze foto zien we dat de gemiddelde korrelgrootte van de startdeeltjes ongeveer 10µm

is. Uit de literatuur is gebleken dat deze afwijking veroorzaakt is omdat XRD niet geschikt

is voor deeltjes met een grootte tussen 0.1 en 10µm (WHISTON C. 1987, p. 92).

4.1.1.3 Onderzoek naar de vervuilingsgraad (XRF)

Om alle elementen in het poeder te identificeren en de hoeveelheid in massapercenten te

schatten, werd gebruik gemaakt van XRF analyse. In figuur 10, zijn de meest belangrijke

elementen en de geschatte hoeveelheden weergegeven.

0

10

20

30

40

50

60

wt%

W Ni Co Fe Mn Cr

Elements

Estimation of the quantities of the contributing elements the not-milled powder

Figuur 10: grafiek van de verschillende elementen in het referentiestaal

De hoeveelheid W is slechts 60 wt% omdat XRF slechts elementen zwaarder dan natrium

detecteert. Hierdoor wordt de hoeveelheid koolstof niet gedetecteerd. De hoeveelheid

kobalt wordt geschat op 10 wt% De onzuiverheden zijn in kleine hoeveelheden aanwezig

en afkomstig van de poederproductie.


-Appendix D- D-31

4.1.2 XRD resultaten van planetary ball milling aan 250 tr/min

Hier worden de XRD resultaten bekeken die behaald zijn met de planetary ball mill aan

een rotatiesnelheid van 250 tr/min.

4.1.2.1 2,5 uur verwerkte staal met planetary ball milling aan 250 tr/min

Na verwerking is een XRD test gedaan. Het patroon hier van is weergegeven in figuur 11.

Figuur 11: XRD patroon van 2,5 uur verwerkt materiaal aan 250tr/min

Het eerste opvallende aan dit patroon is dat beide pieken lichtjes naar rechts verschoven

zijn. De nieuwe 2θ waardes van de pieken zijn 31.8° en 35.9° .Dit betekent dat er

spanningen in het poeder zijn. De deeltjes zijn dus niet gebroken maar in elkaar gedrukt.

Een andere manier om deze spanningen waar te nemen is het uitrekenen van de lattice

parameters. Het resultaat van deze berekeningen zal verschillend zijn van de lattice

parameters van het niet verwerkte referentiepoeder. Voor uitleg rond de lattice parameters

verwijzen we u naar de volledige thesis onder hoofdstuk literature review.

De FWHM (= Full Width Half Maximum) van de WC pieken op de 2θ hoeken zijn

respectievelijk 0.00714 en 0.00714 radialen. De FWHM is groter dan deze bij het

referentie poeder. Dit betekent dat er een korrelverkleining is opgetreden.


-Appendix D- D-32

4.1.2.2 Het 5 uur verwerkte staal met planetary ball milling aan 250

tr/min

Het opvallendste aan dit patroon is de zeer kleine verbreding van de piek. Hierdoor zouden

de korrels in principe even groot zijn.

Zoals men kan zien in figuur 12, zijn beide pieken lichtjes naar rechts verschoven. De

nieuwe 2θ waardes van de pieken zijn 31.8° en 35.9° .Dit betekent dat er spanningen in het

poeder zijn. De deeltjes zijn dus niet gebroken maar in elkaar gedrukt. Ook hier kunnen

weer de lattice parameters uitgerekend worden om de spanningen vast te stellen. Het

resultaat van deze berekeningen zal verschillend zijn van de lattice parameters van het niet

verwerkte referentie poeder.


respectievelijk 0.00555 en 0.00555 radialen. De FWHM is kleiner dan deze bij het

referentiepoeder. Dit betekent dat er een korrelvergroting is opgetreden.

Figuur 12: XRD- patroon van 5uur verwerkt materiaal aan 250 tr/min


-Appendix D- D-33

Ook hier zijn de pieken lichtjes verschoven naar respectievelijk 2θ = 31.7° en 2θ = 35.9°.

Dit is opnieuw te wijten aan het ontstaan van spanningen door samendrukken van de

deeltjes.

4.1.2.3 Het 10h verwerkte staal met planetary ball milling aan 250 tr/min

Het XRD patroon van dit poeder wordt weergegeven in figuur 13.

Figuur 13: XRD- patroon van 10uur verwerkt materiaal aan 250 tr/min

De twee hoeken van de pieken zijn nu respectievelijk 31.8° en 35.8°. Opnieuw zijn de

spanningen weer zichtbaar aan de hand van de verschuivingen.


respectievelijk 0.00748 en 0.00834 radialen. De FWHM is kleiner dan deze bij het

referentiepoeder. Dit betekent dat er een korrelvergroting is opgetreden.


-Appendix D- D-34

4.1.3 De SEM foto’s van 250 tr/min stalen met planetary ball milling

Hoofdzakelijk wordt de SEM gebruikt om vast te stellen of de WC deeltjes goed omgeven

zijn door kobalt. Een bijkomende reden om deze analyse techniek te gebruiken is een

duidelijk zicht op de afmetingen van de WC korrels. Op die manier kunnen we deze

resultaten vergelijken met de resultaten van XRD. De lichtere vlekken in op de foto stellen

WC voor, de grijze delen zijn Co. Aangezien X- stralen diffractie onbruikbaar is voor het

onderzoek, is SEM de enige optie om de deeltjesgrootte vast te stellen. Jammer genoeg zijn

er maar een klein aantal SEM foto’s gemaakt omdat tijdens het onderzoek de apparatuur

beschadigd raakte. De oorzaak van dit technisch defect is niet achterhaald. Mogelijk is dit

te wijten aan een installatiefout. De machine werd in ons onderzoek voor de eerste maal

gebruikt. Dezelfde bemerking geldt voor het onderzoek naar stalen die aan 500tr/min

verwerkt werden.

4.1.3.1 Het 2,5u verwerkte staal met planetary ball milling aan 250

tr/min

Figuur 14 toont de SEM foto van het 2.5 uur verwerkte materiaal.

Figuur 14: SEM foto van 2,5uur verwerkte staal met de planetary ball mill


-Appendix D- D-35

Het is duidelijk dat Wc zich in de Co bevindt. De grootte van de WC korrel in het kobalt is

800,4 nm.

4.1.4 XRD resultaten van planetary ball milling met 400tr/min

Hier worden de XRD resultaten bekeken die behaald zijn met de planetary ball mill aan


4.1.5 Het 5 uur verwerkte staal met planetary ball milling aan 400tr/min

Figuur 15 toont het XRD patroon van het materiaal na verwerking.

Figuur 15: XRD patroon van het 5uur verwerkte materiaal aan 400 tr/min

Het valt onmiddellijk op dat beide pieken lichtjes naar rechts verschoven zijn ten opzichte

van het referentiestaal. De nieuwe 2θ waardes van de pieken zijn 31,7177° en

35,7926° .Dit betekent dat er spanningen in het poeder zijn. De deeltjes zijn dus niet

gebroken maar in elkaar gedrukt. Een andere manier om deze spanningen weer te geven is

het uitrekenen van de lattice parameters. Het resultaat van deze berekeningen zal

verschillend zijn van de lattice parameters van het niet verwerkte referentiepoeder.


-Appendix D- D-36


respectievelijk 0,00748 en 0,008311 radialen. De FWHM is groter dan deze bij het

referentiepoeder. Dit betekent dat er een korrelverkleining is opgetreden.

4.1.6 Het 10u verwerkte staal met planetary ball milling aan 400tr/min

Het XRD- patroon wordt weergegeven in figuur 16. Beide pieken zijn nu gelegen op de 2θ

waardes 31,818° en 35,773°. Dit betekent dat er spanningen in het poeder zijn.


respectievelijk 0,008074 en 0,008786 radialen. De FWHM is groter dan deze bij het

referentiepoeder. Dit betekent dat er een korrelverkleining is opgetreden.

Figuur 16: XRD patroon van 10uur verwerkt materiaal aan 400tr/min

4.1.7 SEM foto’s van 400 tr/min verwerkte stalen met planetary ball milling


-Appendix D- D-37

Hier worden de SEM resultaten bekeken die behaald zijn met de planetary ball mill aan


4.1.7.1 Het 10u verwerkte staal met planetary ball milling aan 400tr/min

Figuur 17 toont de SEM foto van een 10 uur verwerkt staal in de planetary ball mill aan

400 tr/min. De meetlijnen in de foto geven weer dat de afmetingen van WC korrels in Co

tussen 2,273 μm en 239,2 nm liggen. Dit is nogal een groot gebied. Maar op een SEM foto

is slechts een dwarsdoorsnede zichtbaar, en niet de volledige korrel in het Co. De

gemiddelde korrelgrootte zal dus dicht bij het gemiddelde van deze metingen liggen.

Het is duidelijk zichtbaar op de foto dat de WC korrels in het Co zitten.

Figuur 17: SEM foto van 10u verwerkt materiaal aan 400 tr/min


-Appendix D- D-38

4.1.8 Vervuilingsgraad bij planetary ball milling

Omdat de containers en de ballen van roestvast staal zijn, en de inhoud gebaseerd is op

hard WC, is het vanzelfsprekend dat er slijtage is in de containers. Dit veroorzaakt het

verschijnen van andere elementen in de poederstalen. De belangrijkste elementen zijn Fe,

Cr Ni en Mn. De graad van vervuiling wordt onderzocht met behulp van XRF analyse.

Omdat slechts een klein gebied onderzocht wordt, is dit niet de meest nauwkeurige manier

om de exacte gewichtspercenten van de elementen te onderzoeken. Ondanks dit gegeven is

het een goede manier om een schatting te maken van de graad van vervuiling in het poeder.

Figuur 18 toont hoe de vreemde elementen toenemen bij stijgende snelheid en

verwerkingstijd. Het is duidelijk dat 10u aan 400tr/min een te hoge vervuilingsgraad

oplevert.

0

10

20

30

40

50

60

wt%

W Ni Co Fe Mn Cr

Elements


not milled5h@250rpm10h@250rpm5h@400rpm10h@400rpm

Figuur 18: Schatting van de hoeveelheid vervuilende elementen


-Appendix D- D-39

Contamination after milling

0,3545

34,737

47,026

0,3545

60,353

75,071

010203040

50607080

0 2 4 6 8 10

milling time (h)

Cont

amin

atio

n (w

t%)

250rpm

400rpm

Figuur 19: Vervuilingsgraad in functie van verwerkingstijd

Figuur 19 geeft de vervuilingsgraad in functie van de tijd weer. Het is duidelijk dat hoe

langer we verwerken, hoe hoger de vervuilingsgraad wordt. Er wordt ook duidelijk

weergegeven dat de vervuilingsgraad bij verwerking aan 400tr/min veel hoger is als bij

250tr/min.

4.1.9 Korrelgrootte berekening

Om de meest optimale parameters in te stellen moeten we de invloed op de korrelgrootte

kennen. Een eerste methode om deze groottes te bepalen is met behulp van de Sherer

vergelijking. Deze methode bleek niet geschikt te zijn voor de materialen met

korrelgroottes tussen 0,1 en 10 µm (WHISTON C. 1987, p. 92) Omdat onderzoek naar de

afwijking tussen de verwachtte waardes en de berekende waardes m.b.v. XRD veel tijd in

beslag heeft genomen, geven we toch de resultaten van deze analyse.


-Appendix D- D-40

4.1.9.1 Poeder stalen van planetary ball mill aan 250tr/min

De korrelgrootte wordt berekend met behulp van een Excel rekenblad en de Sherer

vergelijking. Rekenblad en vergelijking voor de stalen verwerkt aan 250tr/min worden

weergegeven in figuur 20.

Grain size of WC for the 2,5h milled powder at 250rpm (90 wt% WC and 10 wt%

λ β θ dXRD dXRD



λ β θ dXRD dXRD



λ β θ dXRD dXRD



tr/min verwerkt staal)

4.1.9.2 Poeder stalen van planetary ball mill aan 400 tr/min

De korrelgrootte wordt berekend met behulp van een Excel rekenblad en de Sherer

vergelijking. Rekenblad en vergelijking voor de stalen verwerkt aan 400tr/min worden

weergegeven in figuur 21.


-Appendix D- D-41


λ β θ dXRD dXRD

1.542 0.00748 0.277 214.280 0.0214

1.542 0.00831 0.312 194.940 0.0194



λ β θ dXRD dXRD

1.542 0.00807 0.277 198.516 0.0198

1.542 0.00879 0.313 184.453 0.0184



tr/min verwerkt staal)

4.1.9.3 Besluiten van de Scherer vergelijking

Omdat deze resultaten niet overeenstemmen met de resultaten van SEM moest er een

andere methode gezocht worden om de korrelgroottes te bepalen. Een mogelijke reden

hiervoor was omdat er spanningen in de korrels zaten [Segmüller et al. 1989, pp. 21-66].

Daarom zijn we op zoek gegaan naar een vergelijking die rekening houdt met de

spanningen in de korrels. Een methode die hiervoor geschikt is, is de Warren Averbach

formule. Om deze te gebruiken is er eerst een Fourier analyse nodig van het XRD patroon.

Het programma da wij gebruikten voor deze analyse was Winfit! V1.2 geschreven door S.

Krumm. Figuur 22 toont de fourier analyse van het XRD patroon van de WC pieken van

de niet verwerkte poederdeeltjes met een grootte kleiner dan 20µm..


-Appendix D- D-42

Figuur 22: Fourier analyse van niet verwerkte poeder < 20micron

Figuur 23 toont de resultaten van het programma

Figuur 23: Resultaten van Winfit!v1.2 op <20µ deeltjes

Figuur 24 toont de berekeningen van de WC korrels. Opnieuw is er een groot verschil

tussen de resultaten van SEM en de resultaten met de software.


-Appendix D- D-43

Figuur 24: Korrelgrootte van <20µ WC deeltjes met Winfit!v1.2

4.1.9.4 Weergave van spanningen door piekverschuivingen.

Zoals al besproken zijn er spanningen in de deeltjes afkomstig van het verwerken. Hier

wordt een duidelijk beeld gegeven van de verschuivingen van deze pieken.

Figuur 25 geeft de grafieken van niet verwerkt poeder, aan 10u verwerkt poeder aan 250

tr/min.

Figuur 25: Piekverschuiving van WC bij 250 tr/min


-Appendix D- D-44

De verschuivingen van de pieken zijn zichtbaar tussen de 2 rode verticale lijnen.

4.1.9.5 Besluiten van de Warren Averbach methode

Ook de resultaten van deze analyse komen niet overeen met de verwachtte resultaten. De

verschuivingen van de pieken geven aan dat er spanningen ontstaan tijdens het mechanisch

legeringsproces. Uiteindelijk is gebleken dat deeltjes tussen 0,1 en 10 µm niet kunnen

worden opgemeten met XRD apparatuur (WHISTON C. 1987, p. 92). Hierdoor moeten we

overschakelen op een andere techniek.

4.2 High energy horizontal mill

In dit deel wordt een analyse van het poeder met alternatieve bindmiddelen gemaakt. Een

eerste onderzoek gaat uit naar de schatting van de graad van vervuiling van het poederstaal

WC-10 Co. Dit poeder is met de high energy mill verwerkt gedurende een half uur, 1 uur

en 3 uren.

In het deel planetary ball mill werd al aangegeven dat het onderzoek met XRD geen

bruikbare resultaten opleverde. Daarom werden deze resultaten hier niet nog eens vermeld.

4.2.1 Schatting van de vervuilingsgraad van het verwerkte poeder.

Ook met dit type machine treedt het probleem van vervuiling op. Ook hier zijn de

voornaamste elementen afkomstig van de verwerkingseenheid en ballen Fe, Cr, Ni en Mn.

Er wordt opnieuw de nadruk gelegd op het feit dat XRF slechts een schatting is van de

hoeveelheid vreemde elementen in de samenstelling. Maar de techniek is geschikt voor de

trend van vervuiling in functie van de verwerkingstijd uit te zetten.Figuur 26 toont de

schatting van de meest belangrijke vreemde elementen. Het is duidelijk dat naarmate de

verwerkingstijd toeneemt, de vervuilingsgraad stijgt.


-Appendix D- D-45

0

10

20

30

40

50

60

wt%

W Ni Co Fe Mn Cr

elements


unmilled30min high energy1hour high energy

Figuur 26: Schatting van de hoeveelheid vervuilende elementen in het poeder

Figuur 27 toont dat de vervuilingsgraad in gewichtspercent ongeveer verdubbelt wanneer

de verwerkingstijd verdubbelt.

contamination after milling

0,3545

1,4105

3,683

0

0,5

1

1,52

2,5

3

3,5

4

0 0,5 1

milling time (h)

cont

amin

atio

n le

vel (

wt%

)

estimation of thecontamination level

Figuur 27: vervuiling na de verwerking

4.2.2 SEM foto’s van de stalen gelegeerd met de high energy mill

Het nut van de SEM foto’s was tweeledig. De eerste reden was om te kijken of de binder

omringt was met WC. De tweede reden was om een zicht te krijgen over de grootte van de


-Appendix D- D-46

WC deeltjes. Het is ook belangrijk om te vermelden dat voor deze experimenten de

composiet deeltjes gebruikt werden.

In de foto’s stellen de kleine lichte puntjes WC voor en de donkere vlekken de alternatieve

binder. De zwarte vlekken komen van het bakeliet dat gebruikt werd om de samples te

maken.

4.2.3 SEM foto’s met Fe/Mn als alternatieve binder

Figuur 28 toont de SEM foto van het sample dat gedurende 1 yur gelegeerd is. De maatstaf

onderaan de foto laat zien dat de WC deeltjes een grootte hebben tussen 1,273 μm en 39,2

nm. Dit is een redelijk brede variatie maar op de foto’s is het niet zichtbaat hoe diep deze

particulen omringt zijn met de binder dus zal de effectieve grootte ongeveer overeen

komen met het gemiddelde van de kleine en grootste deeltjes.

Wat ook meteen duidelijk was, is het feit dat de WC deeltjes omringd waren met de Fe/Mn

binder. De eerste conclusies zijn dan ook dat deze binder een goede alternatieve binder is

voor Co maar dat verdere onderzoeken, aangaande mechanische eigenschapen,

noodzakelijk zijn om te weten of dit werkelijk zo is.


mill


Fe/MnWC

-Appendix D- D-47

4.2.4 SEM foto’s met Fe/Ni/Co als alternatieve binder

Figuur 28 toont de SEM foto van het sample dat gedurende 1 uur gelegeerd is. De maatstaf

onderaan de foto laat zien dat de WC deeltjes een grootte hebben tussen 1,273 μm en 39,2

nm. Dit is een redelijk brede variatie maar op de foto’s is het niet zichtbaat hoe diep deze

deeltjes omringt zijn met de binder dus zal de effectieve grootte ongeveer overeen komen

met het gemiddelde van de kleine en grootste deeltjes.

Wat ook meteen duidelijk was, is dat de WC deeltjes omringd waren met de Fe/Mn binder.

De eerste conclusies zijn dan ook dat deze binder een goede alternatieve binder is voor Co

maar dat verdere onderzoeken, aangaande mechanische eigenschapen, noodzakelijk zijn

om te weten of dit werkelijk zo is.


mill


Fe/MnWC

-Appendix D- D-48

5 Besluiten

De literatuurstudie in deze thesis levert bruikbaar materiaal voor onze begeleidende

doctoraatstudent aan de universiteit van Wolverhampton. De poederstalen die wij hebben

klaargemaakt zullen zeker worden gecomprimeerd en gesinterd. Daarna zullen de nodige

metingen erop worden verricht. Om dit uit te voeren zullen de matrijzen die wij ontworpen

hebben worden gebruikt. Wat ons praktisch onderzoek betreft, kunnen we de besluiten

verdelen in enerzijds planetary ball mill en horizontal high energy mill.

Het eindwerk kadert in een doctoraatsonderzoek met betrekking tot alternatieven voor

kobalt als bindmiddel in hardmetalen op basis van wolfraam-carbide deeltjes. Het praktisch

onderzoek in het eindwerk heeft voornamelijk betrekking op het mechanisch legeren met

behulp van twee uitvoeringswijzen, met name “planetary ball mill” en “horizontal energy

mill”.

5.1 De planetary ball mill

Een serie van experimenten werd uitgevoerd met de planetary ball mill. Er werden

verschillende bewerkingstijden (2.5, 5, 10 uren) en verschillende rotatiesnelheden (250,

400 tr/min) gebruikt om WC-10wt%Co, WC-10wt%FeNiCo en WC-10wt%FeMn te

bereiden. Opvallend was dat bij langere bewerkingstijden (langer dan 2.5 uren voor 150

tr/min) grotere concentraties elementen (Fe, Cr) werden opgenomen van de binnenwanden

en de roestvaste stalen ballen. De hoeveelheid contaminatie nam meer toe naarmate de

rotatiesnelheid opgedreven werd naar 400 tr/min. Dat geeft aan dat zowel de snelheid als

de tijd zo laag mogelijk gehouden dient te worden om de contaminatie te minimaliseren

ofwel dient een hardmetalen container gebruikt te worden.

De resultaten van het vervuilingonderzoek (XRF) geven aan dat bij een snelheid van

400tr/min, de vervuiling bijna verdubbelt t.o.v. 250 tr/min (bij constante

verwerkingstijden). Beelden van de elektronenmicroscoop (SEM) tonen aan dat er geen

verdere deeltjes verkleining gebeurt bij stijgende verwerkingstijden (bij 400tr/min). Hieruit


-Appendix D- D-49

kunnen we besluiten dat snelheden hoger dan 200tr/min, bij gebruik van roestvaste

containers en ballen, geen voordeel opleveren.

X- stralen diffractie (XRD) werd toegepast om de korrelgrootte van de WC fase te bepalen

(gemiddelde grootte van 24.1 nm). Deze techniek geeft ook aan waar er spanningen

optreden in de korrels van de WC fase ( zichtbaar door de verschuivingen van de pieken in

het XRD patroon). Deze spanningen zijn afkomstig van het mechanisch legeringproces.

De grootte van de WC deeltjes werd geëvalueerd met een elektronenmicroscoop (SEM)

(gemiddelde grootte van 1.103 μm). Er werden succesvol composiet poeders gemaakt

waarin fijne WC deeltjes (submicron tot ongeveer 200 nm in grootte) verdeeld waren in

de matrix (Co, FeNiCo of FeMn).

5.2 De horizontal high energy mill

Om het probleem van de grote vervuiling te vermijden wordt gebruik gemaakt van een

hardmetalen container. Daarom worden extra poeders klaargemaakt in de horizontal high

energy mill. Omwille van de grotere verwerkingscapaciteit en hogere snelheden laat dit

toestel toe poeder met een zeer fijne structuur te bereiden, in kortere verwerkingstijden.

Na de eerste test werd onmiddellijk duidelijk dat met deze techniek veel sneller

nanogestructureerde poeders konden gemaakt worden. Dit is te wijten aan de hogere

kinetische energie van het proces (snelheden boven 1000tr/min), waardoor de

verwerkingstijden verminderd konden worden. Verder leverde deze techniek in

vergelijking met de planetary ball mill veel minder vervuiling op. Dit geven de resultaten

van vervuilinganalyse (XRF) duidelijk weer.

Een bijkomend voordeel van deze techniek is de grote inhoud van de container (2liter).

Hierdoor kunnen relatief grote hoeveelheden poeder in een cyclus verwerkt worden.

Uit voorgaande punten kunnen we besluiten dat de horizontally high energy mill een

hogere efficiëntie heeft dan de planetary ball mill.


-Appendix D- D-50

5.3 Alternatieve bindmiddelen

Twee alternatieve bindmiddelen ter vervanging van kobalt werden onderzocht en verwerkt

met WC. Het eerste alternatief was Fe/Mn. De SEM foto’s laten duidelijk zien dat de WC

deeltjes goed in de Fe/Mn binder gedrongen zijn. Ook werd hierop duidelijk dat de grootte

van de WC deeltjes verkleind werden door het mechanisch legeringproces. Dit is een

veelbelovend resultaat. De compactiefase en het sinterproces zullen in een latere fase

worden uitgevoerd.

De zelfde resultaten werden opgetekend voor het tweede alternatieve bindmiddel

Fe/Ni/Co. SEM foto’s geven weer dat de WC deeltjes goed verdeeld werden over het

bindmiddel. Ook hier worden De compactiefase en het sinterproces zullen in een later fase

worden uitgevoerd. Ook hier worden de compactiefase en het sinterproces een latere fase

uitgevoerd.

5.4 Verderzetting van het onderzoek

Verdere stappen in dit onderzoek zijn de compactiefase en sintering van de verschillende

poederstalen. Parameters die in invloed hebben op de compactiefase en sintering moeten

onderzocht en geoptimaliseerd worden voor elke poedersamenstelling (WC-Co, WC-

FeNiCo, WC-FeMn). Door juiste instelling kan vermeden worden dat de WC korrels met

submicron/ nano- structuur, terug te groot gaan worden.

De fysische en mechanische eigenschappen van gesinterde stalen moeten worden

onderzocht. Dit kan gebeuren door; hardheidstesten, 3punts buigtesten en

dichtheidsmetingen. Ook moet na sintering de microstructuur, waaronder de verdeling en

grootte van WC korrels, opnieuw gecontroleerd worden.


1users.telenet.be/erasmus-wolverhampton/Thesis.doc · Web viewA die to produce cylindrical samples...

Documents

Transcript of 1users.telenet.be/erasmus-wolverhampton/Thesis.doc · Web viewA die to produce cylindrical samples...