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Faculty of Technology and Science Department of Materials Engineering

Patrik Karlsson

Tribological characterization of

selected hard coatings

Master thesis of 20 credit points

Date/Term: 2009-03-06

Supervisor: Pavel Krakhmalev

Krister Svensson

Examiner: Jens Bergström

Serial Number: 2009: 01

Acknowledgement

Following persons deserve special thanks.

Associated Professor Pavel Krakhmalev for great supervision throughout the thesis project,

interesting discussions of the thesis project and material science, fast response of questions,

and great help with scientific tools.

Senior University lecturer Krister Svensson for great supervision, fast response, and guideline

for AFM issues.

Johan Nordström at the company of Oerlikon Balzers for supply of PVD coatings and

material data.

Odd Sandberg at the company of Uddeholm Tooling AB for supply of substrate for PVD

coatings.

The department of Materials Engineering at the University of Karlstad for letting me use the

scientific tools needed to perform this thesis.

Abstract

Hard coatings are often used for protection of tool surfaces due to coating properties like low

friction and high wear resistance. Even though many of the hard coatings have been tested for

wear, it is important to try new wear test setups to fully understand tribological mechanisms

and the potential of hard coatings. Few experiments have been performed with dual-coated

systems where the sliding contact surfaces are coated with the same, or different, hard coating.

The dual-coated system could be the solution to many new technical devices and perhaps a

further improvement of conventional coated systems.

In this thesis, the wear tests of dual-coated systems were performed in dry reciprocating

sliding mode at room temperature. This, quite off the ordinary, wear test setup was performed

to study selected hard coatings and set focus on wear mechanisms in forthcoming future

surface coating application areas like MEMS and orthopedic implants.

Wear tests of four different PVD hard coatings, CrN, TiAlN, WC/C and diamond-like

coating (DLC) were performed in a slider-on-flat-surface (SOFS) tribo-tester with

reciprocation sliding mode at room temperature and dry sliding with TiAlN coated counter

body. Wear mechanisms and the amount of wear were estimated, by investigation of the wear

scars produced in SOFS, by means of scanning electron microscopy (SEM), atomic force

microscopy (AFM) and optical profilometer (OP).

Typical wear mechanisms found for coated surfaces in reciprocation sliding contact were

crack formation, surface flattening for shorter sliding distance, elongation of surface defects,

debris and thin film formation. Two types of film formation were found: tribo-oxidation film

and formation of a self-lubrication film. The tribo-oxidation was the most evident for CrN and

the formation of a self-lubrication film was revealed for DLC, where smearing of asperities

were the initiation of the process. The DLC coatings showed lowest friction coefficient and

worn volume of all the selected hard coatings.

Adhesion measurements were performed for all coatings by AFM. Both the unworn and

worn surface of each coating were investigated and two coatings, DLC and TiAlN, showed

low adhesion forces, which indicated promising properties for small scale devices like MEMS

and NEMS with coated, non-sticking, surfaces.

Contents

1. Introduction ....................................................................................................................1

2. Surface coatings..............................................................................................................2

2.1 Deposition of hard coatings.....................................................................................3

2.1.1 Properties of hard coatings ..............................................................................4

2.1.2 Hard coating classifications .............................................................................9

2.2 Tribology of coatings ............................................................................................11

2.2.1 Wear simulation ............................................................................................15

2.2.1.1 Hertzian contact pressure...........................................................................15

2.3 Surface characterization methods ..........................................................................17

3. Aims.............................................................................................................................20

4. Experimental part .........................................................................................................20

4.1 Material properties ................................................................................................20

4.2 Material preparation..............................................................................................21

4.2.1 Substrate surface preparation procedure.........................................................21

4.2.2 Preparation of sliding wheel ..........................................................................22

4.2.3 Brushing of coatings......................................................................................22

4.3 Wear tests .............................................................................................................22

4.3.1 SOFS setup ...................................................................................................24

4.4 Surface characterization ........................................................................................24

4.4.1 Atomic Force Microscopy (AFM) .................................................................25

4.4.1.1 Tip calibration ...........................................................................................25

4.4.2 Scanning Electron Microscopy (SEM)...........................................................26

4.4.3 Optical Profilometer (OP)..............................................................................27

5. Results..........................................................................................................................28

5.1 CrN.......................................................................................................................28

5.1.1 Original surface of CrN .................................................................................28

5.1.2 Friction of CrN..............................................................................................29

5.1.3 Optical profilometer measurements of CrN....................................................30

5.1.4 Scanning electron microscope measurements of CrN.....................................32

5.1.5 Atomic force microscopy measurements of CrN............................................37

5.2 DLC......................................................................................................................38

5.2.1 Original surface of DLC ................................................................................38

5.2.2 Friction measurements of DLC......................................................................38

5.2.3 Optical profilometer measurements of DLC ..................................................39

5.2.4 Atomic force microscopy measurements of DLC...........................................44

5.2.5 Scanning electron microscope measurements of DLC....................................46

5.3 TiAlN ...................................................................................................................47

5.3.1 Original surface of TiAlN..............................................................................47

5.3.2 Friction measurements of TiAlN ...................................................................48

5.3.3 Optical profilometer measurements of TiAlN ................................................49

5.3.4 Scanning electron microscope measurements of TiAlN .................................53

5.4 WC/C....................................................................................................................54

5.4.1 Original surface of WC/C..............................................................................54

5.4.2 Friction measurements of WC/C....................................................................55

5.4.3 Optical profilometer measurements of WC/C ................................................56

5.4.4 Scanning electron measurements of WC/C ....................................................60

5.5 Adhesion measurements........................................................................................62

6. Discussion ....................................................................................................................63

6.1 Friction of selected hard coatings ..........................................................................64

6.2 Tribological characterization of selected hard coatings..........................................65

6.2.1 Load condition 1 ...........................................................................................67

6.2.2 Load condition 2 ...........................................................................................72

6.2.3 Load condition 3 ...........................................................................................73

6.3 Adhesion measurement correlation to worn volume..............................................75

7. Conclusions ..................................................................................................................77

8. References ....................................................................................................................78

1

1. Introduction

Longer service life, ability to tolerate greater loads, ease and low cost of maintenance,

environmental gain in conservation of resources, improved response in kinetic systems, lower

energy consumption, resistance to corrosion, low friction, use of low cost base material, etc

are just a few good reasons for coating machine parts. Many industries have understood the

advantages with coated systems, and that is why coated machine parts and tools like gears,

bearings and cutting tools can be coated with DLC, WC/C, CrN and TiAlN.

Even though tribological performance of many hard ceramic coatings is quite well

investigated, there still are many tribological phenomena that can not yet be explained. The

wear mechanisms at macro scale are believed to have reached a level where the understanding

are somewhat established. The problem with further understanding arise when tribological

studies at the micro scale and nano scale show that the macro level laws of friction can not be

applied. Differences in scale proportions enhance some forces at smaller scales that play a

minor part at macro scale tribological performance, for example adhesion forces. Many new

small technical devices like MEMS are hindered in further development because of high

adhesion forces rather than high friction forces. Contact surfaces may stick to each other due

to adhesion forces and the device function breaks down. This problem with adhesion force

might be solved with coated contact surfaces with low surface potential.

In this thesis investigation of selected coatings: CrN, TiAlN, WC/C and DLC was

performed at both macro and micro level by means of scanning electron microscopy (SEM),

atomic force microscopy (AFM) and optical profilometer (OP). Wear tests were performed in

a slider-on-flat-surface (SOFS) tribo-tester with three different load conditions: constant depth,

constant contact pressure, and constant load. The load conditions were based on a model of

Hertz contact pressure theory. The wear test setup was chosen with dual coated system, where

TiAlN was the coating on the tool steel. The TiAlN was chosen since the coating was harder

than the rest of the selected coatings. By creating dual coated systems, the investigations were

presenting a wear test setup which was not as common as tribological research of hard

coatings done by others.

Wear tests were performed at macro/ engineering level and the wear tracks was

investigated all the way down to the nano scale in an atomic force microscope, where

adhesion measurements were performed. The investigations of the different coatings showed

many interesting results, where investigations like adhesion measurements of the wear tracks

turned out to be unique.

2

2. Surface coatings

In order to improve surfaces properties of material, coatings have been used throughout

history and improvements are achieved every year. The beginning of hard coating techniques

can be dated back to 1643 when Evangelista Torricelli did his famous experiment with an

upturned glass tube filled with mercury, in which he established the existence of vacuum.

With the existence of vacuum, Michel Faraday developed the deposition technique in 1838. In

the 1960s the technique started to spread to industrial applications when cutting tools were

coated with TiN for the first time in U.S. industry [1].

Surface coatings are used in a wide area of applications and the deposition techniques are

basically divided into three groups: solid phase, liquid phase and vapor phase [2]. Hard

coatings are one of many surface layer deposition methods, fig 1:

In the crystallization group, see fig 1, two major deposition techniques are used: physical

vapor deposition (PVD) and chemical vapor deposition (CVD). Both methods produce

surface coatings with superior tribological properties and they are used to deposit thin film of

hard coatings on components to extend their performance and life under severe environmental

conditions.

The final coating properties depend on process parameters like temperature, pressure and

deposition technique. By controlling process parameters, it is possible to design coatings for a

Fig. 1. Illustrates the variety of surface coatings divided in manufacturing methods [1].

3

Fig. 2. Subdivided PVD methods are described in picture (a) [6]. Picture (b), (c) shows

evaporation and sputtering deposition respectively [3].

a)

b) c)

wide range of substrate materials and application areas. In this thesis hard surface coatings

produced from the vapor phase, by physical vapor deposition, will be examined.

2.1 Deposition of hard coatings

Physical vapor deposition (PVD) is widely used today as a manufacturing method for coating

of substrate to enhance surface properties with superior resistance to tribological phenomena

like wear, friction, oxidation etc. The deposition of the thin film takes place in a vacuum

chamber. Normally the substrate is cleaned and dried with N2 gas before entering the vacuum

chamber. After insertion of substrate into chamber, vacuum is increased (10-2 – 10-2 Torr) and

the surface is de-gased by a high current density plasma by sputtering at elevated temperature

(150 - 500°C) [3]. During the last step in PVD, a material is evaporated and deposited and

forms a thin film on a substrate material. The temperature during deposition of PVD is low

when compared with other methods like Chemical Vapor Deposition (CVD). One advantage

of deposition at lower temperatures is that unwanted softening and geometrical changes of the

substrate do not occur and subsequent heat treatment steps are not needed [4, 5].

Even though there exist several PVD methods, they can be divided into two major parts [6],

evaporation PVD and sputtering PVD, fig 2.

The sputtering mechanism involves glowing discharge and momentum transfer process to

produce a thin film on a substrate. High-energy particles cause atom or cluster of atoms to be

knocked free from the surface of a target, which contain the coating material. The coated film

on the substrate contains condensed atoms from ejected target material and particles that have

reacted with gas that the PVD vacuum chamber contains.

4

The process starts with a glowing discharge that forms a flux of ions that are pointed to the

target material and the ions start to sputter atoms from the target in dependence on surface.

Via momentum transfer, the atoms are transferred and condensed on the substrate. As the

process continues, a thin film is formed. Sputtering PVD can be subdivided to diode,

magnetron, ion beam and triode sputtering.

In PVD by evaporation, the target coating material is placed in a crucible that is heated to

sublimation point in high vacuum environment. There are four main methods for evaporation:

induction heating, resistance heating, arc and electron beam gun. By heating the crucible, the

coating material is vaporized and condensed on the substrate forming a thin film.

2.1.1 Properties of hard coatings

PVD coatings are popular to use in coating of machine parts and tools. Requirements of the

coated parts are pushed forward as the development of new hard coatings continues. Mainly

the requirements of coating material properties are [7]:

• low heat transfer coefficient

• low friction coefficient

• high wear resistance

• high hardness

• high toughness

• fine-grain, crystalline microstructure

• chemical inertness

• smooth surface morphology

• good adhesion to substrate

Not many bulk materials can fulfill all these requirements and that is why the PVD

coatings have gain high popularity in application areas in severe working environment. The

properties of PVD coatings can often be explained by their prominent microstructure during

growth. During deposition the coating gets high density of non-equilibrium built-in structural

defects during the bombardment of particles against the substrate in the growth process of the

coating. Since the sputtering process takes place with reactive gas like Ar+ or N+ the particles

consists of back-scattered inert gas neutrals or ions accelerated towards the substrate via

negative substrate bias potential [8]. In the arc evaporation the particles consists of metal ions

5

Fig. 4. SEM pictures of CrN deposited by arc evaporation (a,c) and sputtering (b,d) on Si substrate.

Picture (a) shows typical morphology of arc deposited PVD coating with droplets and picture (b) shows

a finer morphology of sputtered PVD with insert picture of 4x magnified view of the same surface.

Picture (c) and (d) shows fracture cross-section of arc and sputtered PVD respectively [10].

Fig. 3. Surface defects in PVD coatings due to ion-bombardment [6].

of multiple ionization states. The defects created during deposition, fig 3, act as obstacles for

dislocation movement and are one of many other strengthening agents of PVD coatings. Other

mechanisms of strengthening of hard thin films during deposition are second phase particles,

solutes, internal boundaries (column, grain and phase boundaries), high density of point and

line defects etc [9].

Typical microstructure for PVD coatings are the columnar structure. This columnar

structure is formed during deposition, when a flow of atoms reaching the substrate with a

limited range of directions. Even though the microstructure often has a columnar structure the

surface morphology can have different appearance depending on deposition method, fig 4,

[10].

6

Fig. 5. Correlation between hardness and grain size (a), hardness and biaxial

residual stress in sputtered hard coatings (b). TiN and TiB1.4N0.65 follow the Hall-

Petch relation [15].

As seen in fig 4, the PVD coatings deposited by arc evaporation are often rougher than that

of PVD coatings deposited by sputtering. Droplets are formed during arc evaporation and

contain material ejected from the source surface. This occur due to the rapidly melt of the

source material by the arc [6], [9]. The droplets can be found both on the coating surface and

in the body of arc deposited coatings [10], [11]. The columnar structure has many advantages

e.g. great tolerance against erosion, stresses, thermoshock etc [12]. Along with many

advantages of columnar structure there are some disadvantages too, which are more or less

connected to error in process parameters. If the density of the columnar structure is not

sufficient, pores can form between columns and form gaps that reach all the way through the

coating down to the substrate. These kinds of pores may lead to bad corrosion resistance,

since corrosive media can reach the substrate-coating interface [13].

PVD coatings have high strength due to its small grain size (or columnar width) in the size

of 10-30 nm for single-phase coatings. Many, but not all, of these coatings follow the Hall-

Petch relationship.

d

kyys += 0σσ (1)

Where ysσ is the yield stress, 0σ is the lattice resistance to dislocation movement, yk is the

Hall-Petch factor which depends on the material and measure the relative hardening

contribution of grain boundaries and d is the grain size [14]. The strengthening effect due to

decreasing grain size can be illustrated by increasing of hardness of the coating, fig 5.

7

Grain size in hard coatings can reach down to the size of 2-3 nm in nanocolumnar coatings

like TiN-TiB2 so that high hardness is achieved (~42 GPa) [9]. Besides the Hall-Petch

relationship, such high hardness value can be explained by the formation of almost perfect

crystals during deposition. In TiN-TiB2 the addition of B improves the cohesion of grain

boundaries by affecting the local bonding at the interface, which in turn results in the high

hardness of TiN-TiB2.

The achievement of small grain size in TiN-TiB2 can be explained by processes that takes

place during deposition, fig 6. By sputtering of the TiN-TiB2 target, B, N and Ti atoms arrive

to the substrate. This results in formation of nuclei, which consist of TiN, TiB2 and TiN-TiB2

in grain-boundary.

Boron segregates to surface and interface due to its low solubility in TiN. This leads to

formation of disordered areas enriched with B. These areas cover TiN surface and inhibit

mobility of boundary. The formation of disordered areas filled with B affects film growth and

hinder grain coarsening. Similar process takes place for N when the B areas promote

nucleation of TiB2, which has low solubility for N. The broken up growth leads to formation

of small grains and grain coarsening can not take place [9].

Another strengthening mechanism influencing properties of hard coatings is alloying. By

replacing atoms in an original crystal structure with other atoms of different size, stresses in

the lattice causes barrier to dislocation movement and the material gets stronger. As an

example of alloying strengthening in ternary carbon nitride coatings, N atoms are replaced by

the bigger C atoms in TiCN and smaller Al atoms partly replace Ti in TiAlN [16, 9].

Atomic bondings also play a key role and determine mechanical properties of coatings. In

hard coatings three types of bonding are typical: metallic, covalent and ionic bonding, fig 7.

Fig. 6. Nucleation during film growth of sputtering deposition of TiN-TiB2 [9].

8

a)

b)

Fig. 7. Atomic bonding in PVD coatings. Metal to nonmetal structure (a)

and atomic bonding (b) [6], [9].

Covalent bonding is found in the hardest coatings and is typical of high-energy bonds e.g.

4.5 eV for H-H. The bond is formed when atoms are sharing electrons, forming electron pairs.

The covalent bond can be found in coatings like diamond, B4C, SiC, AlN and Si3N4, fig 7.

The covalent bond contributes to hardness and thermal stability but decrease the adhesion of

the coating to the substrate.

Metallic bond contributes to toughness and adhesion to the substrate for the coating but

decrease hardness of the coating. The metallic bonding is formed in crystals of metals

containing conducting electrons. Charged ions are positioned in lattice cites. These ions are in

equilibrium with the conducting electrons, which fills the lattice space. The sea of electrons

contains free electrons widespread to all metallic ions. Interactions between external electron

shells form additional bond forces, which occur for transition metals. Metallic bond has high

energy e.g. 4.29 eV for Fe, which belongs to the transient group of elements with

incompletely filled electron shells.

The ion bonding is formed when valence electron transition from a less electronegative

atom to a more electropositive atom occurs. The ion bonding occurs between atoms of metal

and non-metals, mainly oxides, and the bonding energy can be up to 7.9 eV e.g. NaCl. The

ion bonding results in brittle behavior and high thermal expansion but increase the chemical

stability of the coating, fig 7.

9

f)

g)

Fig. 8. Overview of coating structures. Multi-component (a), multi-phase (b), composite

(c), multi-layered (d), gradient (d). (f), (g) shows mono-layered coating and

multilayered coating respectively with increasing mechanical destruction: I, II, III [2].

Table 1. Typical characteristics of coatings with Covalent-, Metallic- and ion-bond [2]

The different atomic bonding in hard coatings is often found as complex combination of

interaction between different bond structures, table 1, [6].

2.1.2 Hard coating classifications

Hard coatings exist in various forms, complex and less complex mixtures. As mentioned

before, the coatings are fabricated of nitrides, carbides and borides of transition metals,

carbon based etc. Often the chemical composition is used when a specific coating is addressed,

for example TiN, CrN, TiAlN, TiC and c-BN. Some of these coatings, with modification,

have gained a name on the global coating market without using the chemical composition, for

example Dymon-iCTM, Graphic-iCTM and MoSTTM [6]. This is though more of an exception

than a rule. To classify coatings, it is common to break down the different coatings in

subcategories: monolayer coatings and complex coatings, where the complex coatings are

further subdivided into multi-component coatings, multi-phase coatings, composite coatings,

multi-layer coatings and gradient coatings [2, 17], fig 8.

Monolayer or single phase coatings consists of a metal e.g. Al, Cr, Mo, Au, Ag and Cu or a

phase like TiN, TiC and CrN. The complex coatings consist of more than one material and the

10

variety of material distribution is high. The multi-component coatings base on carbides and

nitrides with transition metals like TiN, VN, CrN, ZrN, NbN, HfN, TaN, WN, TiC, VC and

ZrC. Solid solution of carbides and nitrides is usually formed in these coatings, by alloying

with a third element. They usually form a substitution solid solution. The multi-component

coatings usually have good tribological properties. Recent modern advanced multi-component

coatings increase these properties further e.g. Ti(C,N), (Ti, Al)N, (Ti, B)N, (Ti, Zr)N, (Ti,

Nb)N, (Ti, Al, V)N and (Ti, Al, V, Cr, Mo,)N [2,6].

Multi-phase coatings consist, as the name proposes, of several phases. Composite coatings

are multi-phase where one phase is dispersed in another phase matrix like Ti/Al2O3. More

advanced composite coatings with three-dimensional nanostructure [9] and extremely small

grain size have reached the open coating market. One of these coatings was mentioned earlier:

TiN-TiB 2. With properties like high hardness, thermal stability and low friction coefficient,

the advanced composite coatings are popular in tool applications like cutting tools.

Multi-layer coatings, also known as micro laminates or sandwich-coatings consist of

several different layers built on top of each other. The sandwich-coatings consist of layers

with different mechanical properties. The area in between the layers consists of transition

layers, which are basic layers partly diffused into each other. With this kind of coating

structure, several different mechanical properties can be included in one coating e.g. good

adhesion to the substrate for the first layer, good corrosion resistance for the second layer and

good tribological properties for the top layer.

The number of layer can vary a lot e.g. three-layer coatings: TiC/ Ti(C,N)/ TiN, TiC/ TiN/

Al 2O3, six-layer coating: TiN/ Ti(C,N)/ ZrN/ (Ti, Al)N/ HfN/ ZrN, eight-layer coating: TiN/

Ti(C,N)/ Al2O3/ TiN/ Al2O3/ TiN/ Al 2O3/ Ti(C, N) [2]. To reach good adhesion between the

layers it is important to get the transition layers to form a coherent interface zone. This is

often achieved by coupling materials with similar atomic bonding structure or materials that

are mutually soluble.

By constructing hard coatings in several layers, the coating gets better resistance to

mechanical destruction compared to mono-layer coatings, fig 8. In monolayer coatings,

propagation of cracks that usually forms in hard coatings, during mechanical destruction, goes

through the whole coating section. In a multi-layer structured coating, the cracks that formed

on the surface are inhibited to continue growing due to the next coating layer and interface.

Instead, the crack stops or turns in another direction, which leads to destruction of small

pieces of the coating instead of dramatic failure of the whole system as in the monolayer

coating. By acting as a crack barrier, the multi-layered coating can withstand much higher

11

Fig. 9. Tribological processes that occur for coatings in mechanical contact [6].

mechanical load compared to a single-layer coating. Gradient coatings e.g. TiN/ Ti(C, N)/ TiC

are similar to multi-layer coatings with the exception that the different layers are not divided

in steps. Instead the gradient coatings show a more continuous structure [6].

The development of coatings has gone from single layer or single phase coatings to layered

structures of various kinds. Superior mechanical properties have been achieved by further

development of hard coatings into nano-structured coatings like nano-composite and nano-

columnar coatings e.g. TiN-TiB2 and TiB2 [9]. These coatings consist of superior mechanical

properties like high hardness and high melting point, compared to ordinary hard coating.

Nano-structural coatings are a hot research object in material science and as the research

trends towards nanoscience, the knowledge of the fundamental construction blocks of

coatings, and the ability to manipulate them at the atomic level are increasing.

2.2 Tribology of coatings

Several parameters are involved in the tribophysical and chemical process during contact of

coatings in relative motion, fig9. That is why the phenomena connected to tribology of hard

coatings are hard to distinguish. It is not often that one specific wear behavior occurs without

involving several other wear phenomena.

One way of describe ordinary tribological scenarios for coating behavior in mechanical

contact is to divide the situations, fig 10, into macromechanical, micromechanical, chemical

and nanomechanical or nanophysical effects [7].

12

Fig. 10. Tribological contact mechanisms [19].

Fig. 11. Macromechanical contact conditions and wear mechanisms for coatings [19].

The friction and wear phenomena’s in macromechanical tribological mechanisms are

described by the stress and strain distribution over the whole contact area, plastic and elastic

deformation that stress and strain result in, and the total wear particle formation. There are

four typical parameters, which control tribological contact behavior: the surface roughness,

the hardness relationship between coating and substrate, the thickness of the coating, the size

and hardness of the debris in the contact [18]. The correlation between the four parameters,

fig 11, results in several different contact conditions characterized by specific tribological

mechanisms [19].

The hardness ratio substrate/ coating are an important parameter. Reduction of friction can

be achieved by using soft films. These films may reduce sliding-originated surface tensile

stresses which lead to subsurface cracking, and eventually to severe wear. Hard coatings can

13

Fig. 12. Film fracture of thin hard coating film due to substrate deformation [18].

also reduce friction and wear by prevent ploughing, if it is applied on a softer substrate. A

hard coating has built in compressive stresses which can prevent the occurrence of tensile

forces. If the substrate is increased in hardness, further improvement of the system can be

achieved e.g. ploughing and deflection due to counterpart is inhibited by better load support

from the substrate.

The thickness of the coating affects the ploughing component of friction in soft coatings.

For rough soft coatings the degree of asperity penetration through the coating into the

substrate is affected by the thickness also. Reduced contact area and lower friction can be

achieved by applying a hard coating, which is thick enough to support a softer substrate in

carrying the load [18]. Thin hard coatings on the other hand are not preferable on a soft

substrate, since coating fracture occurs if substrate is deformed, fig 12.

High surface roughness has negative influence of friction and wear. Scratching of hard

asperities in the counter-face often occurs during sliding and the asperities may lead to

abrasive or fatigue wear. The asperities also lead to reducing of the real-contact area with

extremely high contact stress. The high stress at asperities may be subject to asperity

interlocking and breaking, which contributes to higher friction. Even though loose particles or

debris often are present during sliding contacts they should be avoided if possible. In some

sliding conditions they may contribute very much to friction and wear e.g. by particle

entrapping, embedding, hiding, crushing, etc.

Stress and strain formation, particle formation and material liberation at asperity-asperity

contact are described by micromechanical tribological mechanisms. At micromechanical level,

typical of 1 µm to nanometers in size, the basic mechanisms for nucleation of cracks are shear

and fracture. The cracks nucleate, propagate and lead to material liberation and formation of

wear scar and wear particles. The fundamental understanding of micromechanical tribological

phenomena is poor and more research has to be done in that area in order to gain further

knowledge of tribological phenomena at the smaller scale [19].

The tribochemical reactions that take place on the coated surface during mechanical

contact of sliding change the composition of the surface and, thereby, also its mechanical

14

Fig. 13. Thin film formation of coatings [6].

properties: a new material pair is formed [6]. This affect the friction and wear of the coating

since these tribological phenomena include surface-related mechanism e.g. shear, cracking,

asperity ploughing etc. High local pressures and flash temperatures, which can reach up to

1000°C during sliding, trigger the tribochemical reactions at spots where asperities smash

together. The tribochemical reactions can be divided into two parts: formation of thin film on

coating and oxidation of coating.

The formation of a thin film is believed to form during sliding on coatings like Diamond-

Like coating (DLC) with very low friction behavior [6, 20] down to µ = 0.01-0.15. The

formation of hydrocarbon-rich microfilm, or graphite, on the hard coating can be the

explanation for the low friction coefficient. From a micro-scale viewpoint there is a soft

coating on a hard substrate but now the coating acts as a hard substrate when the film forms,

fig 13. The thin film also inhibits ploughing and thereby the friction is reduced.

Oxide layer is easily formed on metals in environment containing oxygen e.g. air. This

applies to metals like copper, iron, aluminum, nickel, zinc, chromium to mention a few. The

oxides that forms, influence the tribological behavior of the surface in different ways e.g.

copper oxide is sheared more easily then the metal. Aluminum oxide, on the other hand, may

form a very hard thin layer. The small dimension of the oxide particles compared to the

surface roughness does not necessary mean that they contributes to an abrasive tribological

effect. Sometimes the oxide particles assemble up to form layers, strong enough to carry the

load [6].

Nanophysical changes in coatings, fig 10, are still under intensive research. With scientific

tools like atomic force microscope (AFM) the possibility of study friction and wear on a

molecular scale has been achieved. The aim is to find the origin of friction at the atomic scale

and determinate the relationship of the friction laws at the microscale with the nanoscale

friction. The latest research regarding friction suggest that friction arise from lattice vibrations

due to sliding contact of two surfaces where their outer surface atoms moves in opposite

direction. The mechanical energy needed to slide the surfaces onto each other is believed to

15

Fig. 15. Standardized wear test methods [6].

be converted into elastic energy, or phonons, which eventually is converted into heat [19, 21,

22]. Further research will perhaps show other result or confirm and develop the statement, and

new theories will explain the origin of tribological phenomena at the nanoscale [19].

2.2.1 Wear simulation

Hard coatings, as mentioned earlier, are used in many machine applications and technical

devices. The interaction between surfaces in relative motion can be of various kinds, fig 14.

Since the variety of contact situation of real applications is high, it is obvious that the wear

simulation models for real applications also vary a lot. Some of the wear simulation models

are standardized, fig 15. They are pin-on-flat, pin-on-disc and block-on-ring. [6].

The pin-on-flat equipment are used for tribological coating evaluation regarding wear rate,

coefficient of friction etc in both dry and lubricated reciprocating sliding conditions. This is

also true for the pin-on-disc equipment with the exception that larger specimens must be used

and the sliding is continuous, not reciprocating.

The block-on-ring equipment is often used for determination of adhesive wear rate and the

tests are performed under lubricated wear conditions [6].

2.2.1.1 Hertzian contact pressure

When materials are tested, the questions of contact pressure often arise. Materials with

different mechanical properties will experience different contact pressure when applying the

Fig. 14. Interacting surfaces in relative motion [6].

16

same load. In real applications, due to relatively high contact areas, plasticity of the coating

and the substrate is never achieved. Therefore mathematical analysis of stresses is a good

approximation. One way of estimating the contact pressure is to apply the Hertzian contact

theory formulas [23].

In the Hertzian contact theory, the contact pressure distribution, ),( yxp , over the contact

area is expressed as:

22

0 1),(

−

−=b

y

a

xpyxp (2)

Where p0 is the maximum contact pressure, a and b are the major and minor axes in an

elliptical contact. For the elliptical contact situation between a wheel and a flat surface, the

major and minor axes can be expressed as:

3/2

2

1

−

=

R

R

a

b (3)

Where the R1 and R2 are the principal radii of curvature of the wheel. The maximum contact

pressure for elliptical contact then can be expressed as:

==

2

11

3/1

213

2*

0

6

2

3

R

RF

RR

PE

ab

Pp

ππ (4)

Where F is a shape factor and E* is the effective elastic modulus, which can be expressed as:

1

2

22

1

21* 11

−

−+

−=

EEE

νν (5)

Where 1E and 2E are the elastic modulus for the wheel and the sample respectively. 1ν and

2ν are the Poisson number for the wheel and the sample respectively.

The distance between two distance points when compressing the wheel to the flat sample

can be expressed as:

( )

=

2

12

3/1

2*2/121

2

16

9

R

RF

ERR

Pδ (6)

The Hertzian theory of contact pressure is based on some assumptions:

• The surfaces are continuous and non-conforming

• The strains are small

• Each solid can be considered as an elastic half-space

• The surfaces are frictionless

17

2.3 Surface characterization methods

Among many surface characterization methods of hard coatings, three methods are more

common for tribological characterization of coatings. These methods are scanning electron

microscopy (SEM), optical profilometer (OP) and atomic force microscopy (AFM).

The optical profilometer uses phase-shifting/ vertical-sensing interference technology

combined with an optical microscope. This surface characterization method provide a non-

contact 3D method of measuring the roughness of surfaces with sub-nanometer height

resolution capability for smooth surfaces and approximately 3 nm height resolution for

rougher surfaces [7]. The profilometer works with an interchangeable magnification objective,

fig 16. The objective is coupled with a beam splitter and reference mirror, which together acts

as an interferometer unit. White light is split in the interferometer, where part of the light

travels to a spot on the sample of interest and the rest of the light travels to a reference mirror.

When the two parts recombines, interference fringes are produced at the point of focus. By

measuring the resulting interference pattern irradiance from sequential shifting of the phase of

one light beam of the interferometer relative to the other light beam by known amount, the

surface height of the sample of interest are determined. The optical profilometer measure the

sample surface by using a piezoelectric transducer, which moves the objective in the vertical

direction in steps of 50 to 100 nm. The irradiance signal from the profilometer measurements

is detected by using a CCD array, and the recorded data can be converted and displayed as

topography information on a computer screen.

The scanning electron microscopy (SEM) method uses a high-energy electron beam to

emit electrons from the surface of the sample of interest. The electron beam, with energy

ranging from a few hundred eV to 40 keV, is focused by condenser lenses and can be moved

in the x and y axes directions in a raster by scanning coils or deflection plates. The spot size

of the electron beam is typical of 0.4 nm to 5 nm in size, and the resolution of SEM is

approximately 1 nm to 20 nm [17]. Particles with different energy levels are detected after the

a) b)

Fig. 16. The optical profilometer (a) with close up on the microscope objective (b) [7].

18

Fig. 17. Electron bombardment of sample surface in SEM [17].

electron beam has interacted with the sample, which takes place in a vacuum environment.

These particles are secondary electrons, backscattered electrons and x-rays, fig 17.

The secondary electrons have low energies and are typical ejected from within a few

nanometers from the sample surface by inelastic scattering interaction with the beam incident

electrons. The secondary electron detections are very sensitive to the topography of the

sample, and this is also why the secondary detectors are used for topography investigations of

surfaces. Secondary electron detectors collect scattered secondary electrons, which are sent to

a computer for graphical display as the SEM moves in a raster. Planes which are in line with

the detectors collects more secondary electrons and these parts appears more bright on the

computer screen compared to other places on the sample, which are tilted away from the

detectors. The intensity difference, in the secondary electron detector input signal, appears as

a topographic picture of the sample on the computer screen.

Backscattered electrons are collected from elastic collisions with constituent atoms of the

sample. The intensity of the backscattered electron signal gets higher as the atomic number

increases (higher atomic density) for elements belonging to the sample of interest. The

backscattered electrons can, therefore, reveal information of the atomic composition of the

sample of interest.

X-rays are generated by atoms following the emission of a secondary electron. Energy

dispersive spectrometry (EDS) or wavelength dispersive spectrometry (WDS) can be used for

analyzing the x-ray energy emitted, which is characteristic of the atom. The x-ray analysis can

reveal chemical composition of the sample of interest.

The atomic force microscopy (AFM) method belongs to the methods of scanning probe

microscopy (SPM). The AFM uses a small cantilever with a sharp tip (probe), which is used

to scan the surface of interest, fig 18. The tip radius of curvature are typical on the order of

nanometer, which gives a vertical resolution of less than 0.1 nm and lateral resolution of about

0.2 nm [7]. By mounting a sample on a piezoelectric tube, the sample surface can be scanned

by letting the piezoelectric tube keep a constant force against the AFM tip and then

19

continuously move in the x and y direction during scanning. The sample surface differences in

height and friction are recorded during scanning by using a laser beam and detector that

register the cantilever vertical movements (height) and torsion (friction). The laser beam is

pointed to the cantilever and reflected to the detector, which consists of four quadrant sensors

(split-diode photodetector), fig 18. The signals from the detector are sent to a computer, which

calculate the difference between the four quadrants and then present the data as friction map

and height map, which can present surface roughness and topography in 3D.

There are two primary scanning modes of AFM: contact and non-contact mode. The

contact mode was described earlier, when the piezoelectric tube keeps a constant force against

the cantilever tip in the z-direction. In the contact mode, the tip is in contact with the sample

surface throughout the scanning procedure. The non-contact mode, or tapping mode, uses a

piezo mounted over the cantilever, which keeps the cantilever oscillating at the resonant

frequency of the cantilever. The amplitude of the oscillation is kept large enough to avoid the

tip getting stuck to the sample due to adhesion forces. Non-contact mode is often used in

roughness measurements when the sample is soft or effects of lateral forces must be

minimized [7].

The AFM can also measure adhesion force of a sample surface. This is performed by

letting the piezoelectric tube move in the z-direction, moving the tip against the sample until it

finally snaps on to the surface by weak attraction forces like Van der Whaals forces. The

piezo continues to press the tip to the sample surface until the cantilever bends and the

deflection signal reaches a setpoint force value. When the setpoint value is reached, the tip is

slowly moved away from the sample until the tip finally snaps off from the sample surface.

By using the deflection measurements of the cantilever and Hooke’s law, the adhesion force

can be calculated. The adhesion force is then the applied force needed for the tip to snap off

from the sample surface.

b) a)

Fig. 18. Principles of AFM, with close up on the tip and cantilever (a) and laser beam with reflection to

the split-diode photodetector (b). The photodetector detects the AFM tip height movement (AFM-

signal) and friction (FFM-signal) against the sample surface during scanning [7].

20

3. Aims

By coating two surfaces that are going to slide against each other, several positive factors

could occur. The friction could be reduced, which is positive for engineering applications at

macro level. If the adhesion force would be reduced in the dual coated system, the application

at smaller scale would open up new possibilities to new technical devises like MEMS with

non-sticking surfaces.

In this thesis, four different coatings, CrN, TiAlN, WC/C and DLC were selected for

tribological characterization. By creating wear scars on each coating in a SOFS-machine with

reciprocating sliding at room temperature, wear mechanisms were investigated after each

wear test, on each wear scar, by means of scanning electron microscopy, atomic force

microscopy and optical profilometer. By varying load and sliding length, the progress of wear

and wear mechanisms were studied. This was performed to develop knowledge on general

performance of hard coatings, wear mechanisms, and ideas of coating selection for advanced

applications. In general, the wear mechanisms were expected to be different between the

nitrides and the carbon coatings but with the dual-coated system, the effects on friction and

wear mechanisms with the TiAlN coated tool were uncertain due to the poor research of this

type of wear test material setup.

The friction was expected to vary between the selected coatings, and the friction

coefficients were to be studied for comparison after the wear tests.

Adhesion measurements was performed by atomic force microscope on each wear track to

investigate if any of the selected coatings had potentials, with non-sticking surfaces, for

coating of parts used in small devices like MEMS.

4. Experimental part

4.1 Material properties

Mechanical properties for materials that were used in this thesis are following, table2:

Table 2. Mechanical properties of materials.

21

Vancron 40, PM tool steel, alloyed with N was used as a substrate. The substrate was supplied

by Uddeholm Tooling AB. The substrate arrived in the form of heat treated circular pieces

with dimensions of 18 mm diameter and 5 mm thickness and a hardness of 63.3 HRC.

Hardness of the substrate was chosen to be high enough to avoid plastic deformation of the

substrate during sliding test of the coated test samples [7]. PVD-coatings used in this thesis

were supplied by Oerlikon Balzers and the material properties and chemical composition data

was supplied by the manufacturer.

4.2 Material preparation

Before sending substrate materials to Oerlikon Balzers for coating, the substrate surface was

prepared for mirror-like surface roughness. When the coatings process was complete and the

samples arrived the final polishing and brushing was performed.

4.2.1 Substrate surface preparation procedure

The substrate preparation was divided into following steps:

• Mounting of test samples in Bakelite.

• Rough manual grinding on paper, Struers 150, to remove scratches introduced into the

samples when the test samples were cut into specified thickness by Uddeholm

Tooling.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

• Automatic grinding for 6 minutes, 25 N load, Allegro plus –diamond grinding wheel

with 6 µm diamond spray and blue lubricant.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

• Automatic polishing for 7 minutes, 25 N load, polishing wheel with 3 µm diamond

paste and green lubricant.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

• Removal of Bakelite.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

The substrate preparation procedure described above was developed by slowly increase time

in steps for both grinding and polishing. The time for grinding and polishing was increased

until the result of mirror-like surface was achieved when the surface was examined by using

light optical microscope and SEM. By slowly increase time for substrate preparation scratches

was removed and no new surface defects, introduced to the surface by too long polishing time

[24], was found.

22

4.2.2 Preparation of sliding wheel

The sliding wheel used for wear test in SOFS was prepared in a procedure similar to the

substrate for test samples. Grinding and polishing were done manually in a turning machine.

Both grinding and polishing were performed in 2 minutes steps with breaks for surface

investigation in light optical microscope. The steps were divided into:

• Grinding with paper, Struer 500.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

• Grinding with paper, Struer 800.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

• Grinding with paper, Struer 1000.

• Ultrasonic cleaning in alcohol for 20 min followed by drying in warm air for 20 min.

• Polishing with polishing paper, green lubrication and 3 µm diamond paste.

The sliding wheel was polished until a mirror-like surface with no scratches was achieved.

4.2.3 Brushing of coatings

When the coatings arrived they lacked a final common industrial procedure: brushing. This

final stage of the coating process is done to remove most of the droplets that can form during

the PVD process. The process is not like polishing, when a mirror-like surface is sought.

Brushing is often done with small hard particles that are firmly moved over the coating in

order to remove droplets without destroying the coating surface itself. With lack of proper

tools a method was developed to achieve similar brushing result with slightly different tools:

• Ultrasonic cleaning of samples in alcohol for 20 min followed by drying in warm air

for 20 min.

• 15 seconds manually brushing of samples coated surface using a polishing wheel,

0.25 µm diamond paste and water as lubrication. The samples were turned 90° every

third second.

• Ultrasonic cleaning of samples in alcohol for 20 min followed by drying in warm air

for 20 min.

4.3 Wear tests

Wear tracks were produced by SOFS under reciprocating sliding test mode. Three different

conditions for reciprocating sliding in SOFS were used: constant wheel penetration depth,

constant Hertzian contact pressure and constant load. The constant condition values were

23

approximated by using Hertzian contact theory for an elliptical counter body pressed against a

flat surface. With mmR 51 = and mmR 252 = the shape factors was estimated to 07.11 =F

and 96.02 =F [23]. The Hertz theory formulas were simulated in Matlab with load range from

1 N to 300 N and step size of 1 N, fig 19.

In the simulation the material properties described earlier were used. Young’s Modulus of

TiAlN was used for the wheel and the samples elastic modulus was set to the coating of

investigation. This setup was used for all simulations except one where the elastic modulus of

the substrate was used for both the wheel and the sample.

Data from supplier indicated that the thickness range of the coatings was 1-4 µm. In order

to avoid penetration down to the substrate during tests, the penetration depth was set to 2 µm

and corresponding values of load were selected, fig 19. The constant contact pressure was set

Fig. 19. Results from Matlab simulation with max Hertzian contact pressure (a) and depth (b). Broken lines and arrows in (a) and (b) were the corresponding load for the pre-determined load condition e.g. contact pressure of 1865 MPa (a) and depth of 2 µm (b).

0 50 100 150 200 250 3000

500

1000

1500

2000

2500

3000

3500Load vs Max Contact Pressure

Load (N)

Max

con

tact

pre

ssur

e (M

Pa)

Substrate/ Substrate

TiAlN/ TiAlN

TiAlN/ WC-CTiAlN/ CrN

TiAlN/ DLC

0 1 2 3 4 5 6 7 8

x 10-3

0

50

100

150

200

250

300Approach of distance points vs Load pressure

Approach (mm)

Load

(N

)

Substrate/ Substrate

TiAlN/ TiAlN

TiAlN/ WC-CTiAlN/ CrN

TiAlN/ DLC

a)

b)

24

to 1865 MPa which is a value in between service conditions for coated machine elements [25].

For the constant load conditions the load was set to 60 N during sliding in SOFS. The starting

point for sliding with constant load was the set with the highest number of slidings. After

sliding in SOFS the friction curve was analyzed and by pinpointing a change in the friction

curve the next set for number of slidings in SOFS was chosen: labeled x in the table 3.

4.3.1 SOFS setup

The sliding mode during sliding in SOFS was reciprocating sliding with a speed of 0,017 m/s

and sliding length of 15 mm. The sliding wheel (counter body) was coated with TiAlN, fig 20.

Before sliding the samples and the sliding wheel was cleaned in alcohol for 20 min and dried

in warm air for 30 min. After the cleaning process, the samples were attached to a magnetic

fixture and locked in position. During sliding, data was collected continuous by computer

software. After sliding, the samples were cleaned and placed in vacuum chamber and the data

from software were collected, examined and presented in friction graphs by using Matlab.

4.4 Surface characterization

Three different scientific tools used for tribological investigation of the selected coatings were

used before and after the wear simulation: AFM, SEM and OP.

Fig. 20. Coated wheel and sample

Table 3. SOFS sliding setup.

25

Fig. 21. AFM force distance – graphs are shown in (a) and (b), where the two calibration points

mentioned earlier can be seen in (b) as two crosses. The brighter cantilever in (c), (d) and (e) is

the calibrated cantilever used for the calibration of the cantilever in use (the darker one in (c), (d)

and (e)). In (c) the cantilevers are in setup position and in (d), (e) point spectroscopy is done on

hard surface and reference cantilever respectively.

a) b)

c)

d)

e)

4.4.1 Atomic Force Microscopy (AFM)

Veeco Innova atomic force microscope was used to investigate the worn samples.

Before the investigation was performed the samples were cleaned in alcohol and dried in

warm air.

4.4.1.1 Tip calibration

SiN (NFC18) AFM-tip from µ-mash was used throughout the investigations in AFM. Before

every investigation-session the tip was calibrated for stiffness using a pre-calibrated tip made

by µ-mash. The calibration procedure started with voltage-signal calibration of the software.

By doing a point spectroscopy (adhesion measurement) on a hard surface it was possible to

attach two points on the force-distance curve and let the software calibrate the voltage

sensitivity by measuring the slope (V/ µm) between the two attached points, fig 21 (b). When

the point spectroscopy was performed, the tip approached the sample and eventually jumped

to contact. The tip was pushed to the sample until a setpoint was reached and after that, the tip

started to retract from the sample. The last step when the tip snapped off the sample surface

was also the measurement of an adhesion force, fig 21 (a).

When the sensitivity was calibrated, the distance to distance - slope (µm/ µm) was checked

to see that the software calibration was performed right i.e. the slope equals unity.

26

When the sensitivity was calibrated the tip was moved to the end of the pre-calibrated tip and

another point spectroscopy was done. This time the two points were again attached to the

force-distance curve, and the distance to distance – slope was measured. By knowing the

stiffness of the calibrated cantilever, the new cantilever with unknown stiffness could be

calculated by using the following equation:

−= 1

soft

hardref d

dkk (5)

Where k and refk are the calculated stiffness of the cantilever in use and the stiffness of the

reference, i.e. calibrated, cantilever respectively. hardd and softd are the point spectroscopy

(µm/ µm) measured on a hard surface and the reference cantilever respectively, where hardd

equals to 1 if the sensitivity calibration is correct. Several point spectroscopies were

performed on the reference cantilever and an average stiffness value was calculated for use in

the AFM investigations.

Adhesion measurements were performed on every sliding track and original surfaces of the

hard coatings. This was done by first scanning an area of 40 x 40 µm in the middle of a

sliding track. In the selected area, 9 different points were selected for adhesion test. The raw

data from the adhesion tests was collected from the AFM and plotted by using Matlab and

Microsoft Excel. For error estimation of the adhesion test, 9 point spectroscopies were

performed on one spot, and the difference in adhesion force was examined. The raw data from

the scanned areas was collected for examination of surface roughness and topography by

using SPM Lab Analysis.

The tip shape was examined before every new sample was mounted for investigation in the

AFM. By using a tip shape calibration sample, (PA01) from µ-mash, it was possible to see if

the tip shape was sharp enough to register the small patterns that the calibration surface was

made of. The tip was changed to a new one when the pattern indicated blunting of the tip.

When a new tip was mounted, the stiffness calibration of the new tip was performed again.

4.4.2 Scanning Electron Microscopy (SEM)

Surface topography and overview pictures were collected in the scanning electron microscope.

Secondary electron were used for topographic investigations of the surface and back scattered

electrons were used when difference in elements present at the surface was to be distinguished.

27

4.4.3 Optical Profilometer (OP)

The optical profilometer was used for investigation of surface roughness of the original

surfaces and sliding tracks of the samples. The sliding tracks depths were also investigated by

using the profilometer. This was done by selecting points along a track, fig 22.

Depth, width and surface roughness of the track were then measured at the points within the

wear track. Average values were calculated for surface roughness, depth and width of the

sliding tracks produced by SOFS.

Worn volume was estimated by using the optical profilometer software, Vision 32. By

scanning the surface, with the wear track included, it was possible to choose the worn area

only and use a built-in function of worn volume calculation in the software.

Optical profilometer scans were also performed for contact points on the coated wheel. The

surface scans of the wheel were performed for comparison of wear performance with the

worn samples e.g. the optical profilometer scans of the wheel were supposed to indicate if the

wheel was worn or not after the wear tests.

Sample

wear track

1

2

3

Fig. 22. Profilometer investigations of the wear tracks produced in SOFS.

28

5. Results

5.1 CrN

5.1.1 Original surface of CrN

AFM, Optical Profilometer and SEM were used to study the surface morphology of CrN.

SEM revealed a surface morphology typical for arc deposited coatings: droplets and holes, fig

23 (d) – (f). AFM showed a height of ~2 µm for the largest droplets on the CrN surface, fig 23

(a), and ~0.4 µm for smaller droplets, fig 23 (b). The larger holes and droplets diameter was

about 6 – 10 µm. The Optical Profilometer 40X60 µm scans indicated a surface roughness

(Ra) of 54 nm +/- 14 nm.

Fig. 23. Overview pictures of CrN collected by AFM (a), (b) (with one common height

color bar), optical profilometer (b) and scanning electron microscope (d), (e) and (f).

c) d)

f) e)

4035302520151050

40

35

30

25

20

15

10

5

0

4035302520151050

40

35

30

25

20

15

10

5

0

X[µm]

2.00 µm

0.00 µm

a) b)

[µm]

[µm] [µm]

[µm]

29

5.1.2 Friction of CrN

Friction data from SOFS software was collected during sliding of samples. The data was

analyzed and presented in the graphs, fig. 24. According to the model of Hertzian contact

theory for elliptical contact the CrN was tested under contact pressure of 1865 MPa at 83N

load and 1674 MPa at 60N load test conditions. The friction curves showed similar behavior

for both loads: higher initial friction coefficient of 0.7, which decreased and stabilized to 0.6

+/- 0.05 after 5000 mm sliding distance (333 slidings).

( )

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1CrN 83N load, 640 slidings in SOFS

Distance (mm)

Fric

tion

coef

ficie

nt

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9CrN 60N load, 640 slidings in SOFS

Distance mm

Fric

tion

coef

ficie

nt

a)

b)

Fig. 24. Friction graphs of CrN after 640 slidings in SOFS with 83N load (a) and 60N load (b).

30

Fig. 25. Optical profilometer scans showing CrN wear tracks of sample after 40, 160 and 640 slidings (a),

(c), (e) and corresponding wear of wheel (b), (d), (f) at 83N load. Arrows indicate sliding direction.

a)

e) f)

c) d)

b)

5.1.3 Optical profilometer measurements of CrN

Optical profilometer scans of the wear tracks CrN were used for investigation of wear

mechanisms and measurements of the wear tracks revealing wear depths, widths etc. The

investigations showed flattening and some abrasive scratches for short sliding distance, fig 25

(a), where the sample and wheel scan were collected with 10x and 5x magnification lens

respectively. With increasing sliding distance the coating was more worn and surface defects

like holes was elongated. Optical profilometer scans were also done for the wheel to see if the

wheel was worn or not. The wheel scans were filtered for better vision of the wear, which

showed the scans as flat surfaces, fig 25 (b), (d) and (f). The scans revealed that the wheel was

scratched for shorter sliding distance and for longer slidings the wheel was worn and the

coating destroyed after 160 slidings. The surface roughness values (Ra) in the flat parts of the

wear tracks, measured at 3 points along the wear track, decreased as sliding distance increased:

73.71 nm +/- 3.91nm, 68.79 +/- 2.09 nm and 53.69 +/- 2.0 nm after 40, 160 and 640 sliding

respectively.

31

b) a)

d) c)

Fig. 26. Optical profilometer scans where figure (a) and (c) shows wear of sample after 16 and 640

slidings respectively with corresponding wear of wheel (b) and (d) at 60N load. The arrows indicate the

sliding direction.

The surface roughness measurements were also done by scanning an area, which included the

wear track, of 920 µm X 1200 µm, at three points along the wear track. After selecting the

worn area manually in software Vision 32 the surface roughness (Ra) showed an increasing

surface roughness with increasing sliding distance: 172. 23 nm +/- 5.02 nm, 384 nm +/- 31.27

nm and 643.60 nm +/- 115.15 nm after 40, 160 and 640 slidings respectively at 83N load.

For lower load and shorter sliding distance, the CrN showed adhered material, possible

from tribo oxidation, and flattening, fig 26 (a). After longer slidings, fig 23 (c), (d), both the

coating and the wheel were worn. The surface roughness (Ra) of the tracks was 71.36 nm +/-

1.02 and 61.77 nm +/- 7.46 after 16 and 640 slidings respectively. The wheel showed a round

piece of worn coating with a region looking like delamination, fig 26 (d).

.

The wear track profiles were collected by using the profilometer with line scan direction

across the wear track. The data from the line scans was exported to Matlab software where the

data was interpret and presented in graphs, fig 27. The graphs represented an average value of

the width and depth from the middle section of the wear tracks for CrN. Numerical average

values, measured at three points along each track, showed following: with 83N load the

average max wear depth was 0.65 µm +/- 0.33 µm, 1.45 µm +/- 0.66 µm, and 2.07 µm +/-

0.910 µm after 40, 160 and 640 slidings respectively. Along with the increasing average wear

depth the average wear width also increased from 320.89 µm +/- 46.27 µm, 445.89 µm +/-

31.03 µm to 506.78 µm +/- 28.76 µm after 40, 160 and 640 slidings respectively.

32

Fig. 27. Wear track profiles for CrN at a load of 83N (a) and 60N (b).

0 100 200 300 400 500 600-2

-1.5

-1

-0.5

0

0.5CrN 83N load

Scan length (micro meter)

Dep

th (

mic

ro m

eter

)

40 slidings

160 slidings640 slidings

0 100 200 300 400 500 600-2

-1.5

-1

-0.5

0

0.5CrN 60N load

Scan length (micro meter)

Dep

th (

mic

ro m

eter

)

16 slidings

640 slidings

a) b)

Worn Volume for CrN

0,00E+005,00E-041,00E-031,50E-032,00E-032,50E-033,00E-033,50E-034,00E-03

0 5000 10000

Sliding distance (mm)

Wor

n vo

lum

e (m

m^3

) CrN 83NCrN 60N

Fig. 28. Worn volume for CrN

At 60N load the average wear depth were 0.40 µm +/- 0.250 µm, 2.17 µm +/- 1.37 µm and

average wear width were 397.33 µm +/- 15.52 µm, 530.00 µm +/- 16.57 µm after 16 and 640

slidings respectively.

The wear profile graphs, fig 27, together with average worn volume for the whole track, fig

28, shows that the wear was higher with increasing load for CrN. This was true since the wear

profile for higher applied load was wider compared to lower load conditions.

5.1.4 Scanning electron microscope measurements of CrN

Scanning electron microscope was used for investigation the worn surface of CrN after wear

test in SOFS. CrN showed a flatted surface with some abrasive wear after 40 slidings, fig 29

33

Fig. 30. Scanning electron microscope images from investigation of CrN. Picture (a), and (b) present

wear mechanisms found in the middle of the track for CrN with 83N load and 40 slidings. Picture (c) –

(f) summaries wear after 160 slidings. Arrows indicate sliding direction.

d) c)

a) b)

a) b)

Fig. 29. Scanning electron microscope images from investigation of CrN after 40 slidings at 83N

load. Figure (b) shows close up of upper edge from wear track in (a). Arrows indicate sliding

direction.

(a). Cracks were also visible at the edge of the wear track, fig 29 (b). Material defects that

were found at the original surface, were pressed down and pushed out in the sliding direction.

The pressed down droplets, found after short sliding distance, fig 30 (a), (b), were also

observed after longer slidings with the exception for more elongated structure of the surface

defects, fig 30 (c), (d).

34

b) a)

Fig. 31. CrN 83N load and 640 slidings. Figure (b) showed a close up image of middle area of wear

track in (a). Arrows indicate sliding direction.

After the longest sliding distance, the wear track of CrN showed elongated holes with crack

formation in between the holes, fig 31 (b).

Scanning electron investigation indicated a thin film formation in the wear tracks. The

discovery was first detected at the edge of the wear track, fig 31 (a) with a close up (b). Once

the film was detected, scanning electron microscope investigations focused on areas where the

scanning electron microscope detectors could distinguish between the original coating and the

film formed. At the edge of the wear track, it was easier to see the difference between

untouched area and the thin film on the worn surface, fig 31 (a), (b). The pictures in fig 31

were collected by tilting the specimen for better view of the topographical changes. Due to the

tilted specimen the determination of the film thickness was difficult to be made, but the

thickness was roughly less than 1 µm.

a) b)

Fig. 32. SEM investigation of CrN 83N load, 640 slidings. Picture (a) and (b) showed edge of wear track where (b) showed a close up on the thin film formation. The arrows indicate the sliding direction

35

b) a)

c) d)

Fig. 33. SEM investigation of CrN 83N load, 640 slidings. Picture (b) – (d) showed close up

pictures from circular area in (c) with different SEM detectors in use: Inlens (b), Se2 (c) and BSE

(d). Arrows indicate sliding direction.

The thin film was further investigated using different detectors in scanning electron

microscope. This was performed to investigate if the film was formed from elements

belonging to the coated wheel or the coated sample. The detectors used in the investigation

were Inlense detector, fig 33 (b), secondary electron detector, fig 33 (c), and back scattered

electron detector, fig 33 (d). Both secondary electron detector and back scattered electron

detector showed that smeared parts of the formed film had different density, compared to

material defects within the coating and surrounding worn area of the smeared film. The

difference in density indicated that the film was formed from a mixture of elements, where

some of the elements did not originate from the coating.

There were also cracks present in the thin film, which revealed its presence better by using

the back scattered electron detector and the secondary electron detector in scanning electron

microscope. The thin film was smeared out in the sliding direction.

36

a)

g) f) e)

c) d)

b)

Fig. 34. EDX line scan (b) at edge of wear track (a) for CrN after 640 slidings at 83N load. EDX

shows the chemical composition (c) – (g) of scanned path (b).

By using EDX, the chemical composition of the film formation was roughly estimated. A line

scan was performed over an area of deposited film, fig 34 (a), (b). The investigation showed

that the film contained a high concentration of O and smaller amount of Ti and Al. Cr and N

were also detected, but decreased in parts of the line scan where the film was present.

37

Fig. 35. AFM investigation of CrN after 40 slidings at 83N load. Lateral force, deflection and height mode

are shown in (a), (b) and (c) respectively. A line scan over a particle is showed in (d) with corresponding

height graph (e). Force-distance graphs from adhesion tests are shown in (f) and (g) where (f) corresponds

to adhesion of the particle and (g) corresponds to adhesion of the surroundings of the particle.

d)

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Z position (micro meter)

defle

ctio

n (m

icro

New

ton)

Force-Distance curve

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Z position (micro meter)

defle

ctio

n (m

icro

New

ton)

Force-Distance curve

a) b) c)

e)

f) g)

5.1.5 Atomic force microscopy measurements of CrN

One selected debris particle, fig 35 (a) – (e), was investigated by AFM for friction and

adhesion. A setpoint of 0.04 µN and scan rate of 1.326 µm/ s was used. Since the lateral force

scan showed differences in friction and adhesion, a test was performed for investigation of the

connection between friction and adhesion. The investigation showed that, in this case, high

friction corresponded to high adhesion and vice verse. The particles showed a height of ~0.2

µm and were found in wear tracks after a short sliding distance at 83N load.

38

Fig. 36. Overview pictures of DLC from AFM (a), (b), SEM (c) and Optical Profilometer (d).

d)

1086420

10

8

6

4

2

0

4035302520151050

40

35

30

25

20

15

10

5

0

1.00 µm

0.00 µm

[µm]

[µm] [µm]

[µm]

a) b)

c)

5.2 DLC

5.2.1 Original surface of DLC

SEM, AFM and Optical Profilometer were used to study the topography of the DLC surface

before wear tests. SEM investigations, fig 36 (c), showed rounded columnar asperities, placed

mostly in clusters. The diameter of the rounded parts was about 1 µm. The surface beneath the

round structure was dense with a small amount of holes with a diameter of 0.5 – 1 µm. The

AFM verified the morphology seen by SEM and the AFM height scans showed round

asperities with heights reaching close to 1 µm, fig 36 (a), (b), (c). The optical profilometer

with 40 X 60 µm scans indicated a surface roughness (Ra) of 94.84 nm +/- 16.02 nm

5.2.2 Friction measurements of DLC

Friction data from SOFS software was collected during sliding of samples. The data was

analyzed and presented in graphs, fig. 37, by using Matlab. The first contact of wheel to

39

Fig. 37. Friction graphs of DLC after 640 slidings at 52N (a) and 216N (b)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.1

0.2

0.3

0.4

0.5

0.6

0.7DLC 52N load, 640 slidings in SOFS

Distance (mm)

Fric

tion

coef

ficie

nt

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9DLC 216N load, 640 slidings in SOFS

Distance (mm)

Fric

tion

coef

ficie

nt

a)

b)

coating surface showed a friction of 0.6 which decreased quickly and stabilized to 0.09 +/-

0.03 after a few slides, fig 37 (a). The distance to change was 210 mm (14 slidings) for both

52N (1161 MPa) and 60N (1211 MPa) load. For 216N (1865 MPa) load , fig 37 (b), the initial

friction coefficient was 0.7, which decreased down to 0.15 after 700 mm sliding distance (47

slidings) and stabilized at 0.1 +/- 0.015 after 2700 mm sliding distance (180 slidings).

5.2.3 Optical profilometer measurements of DLC

Optical profilometer scans of the wear tracks were used for investigation of wear depth, width,

wear mechanisms etc. The sample and wheel scan were collected with 10x and 5x

magnification lens respectively where the wheel scans were filtered for curvature and the

scans were used to see if the wheel was worn or not after wear tests. The investigations of

surface topography at lower load (52N) showed scratching of wheel contact point, fig 37 (b),

and the sample surface showed adhered material in the wear track, fig 37 (a), with increasing

40

Fig. 38. Scanned pictures from optical profilometer showing DLC wear tracks of sample after 40,

160 and 640 slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 52N load. The

arrows indicate the sliding direction.

a)

f)

d) c)

b)

e)

homogenous structure for longer sliding distance. The surface roughness (Ra) of the wear

tracks decreased as sliding distance increased: 118.33 nm +/- 10.03 nm, 115.49 +/- 3.25 nm

and 111.17 nm +/- 2.46 nm after 40, 160 and 640 slidings respectively.

The surface roughness values (Ra) were collected by first scanning the sample, including the

wear track, with 5x magnification lenses (920 µm X 1200 µm). When the raw data from the

scan was saved, it was possible to manually select the worn area in Vision 32 and collect the

surface roughness from the wear track only, and not including the surrounding surface of

unworn coating surface. The values were collected at three points along the wear track and the

error estimation was calculated by standard deviation of the three measurements. This was

performed for all wear tracks of DLC.

The scratching of the wheel was visible with a very short sliding distance at 60N load, fig

39 (b) and the sample surface showed a wear track with smearing of asperities or adhered

material in the middle section of the wear track. With increasing sliding distance at 60N load

41

c)

b) a)

d)

Fig. 39. Scanned pictures from optical profilometer showing DLC wear tracks of sample after 8 and

640 slidings (a), (c) and corresponding wear of wheel (b), (d), at 60N load. The arrows indicate the

sliding direction.

the wear track showed similar surface topography as for the lower load of 52N, fig 38 (e),

with the difference that material was pushed to the side of the wear track, fig 39 (c) and

evidence of ploughing at the end contact points of the wheel, fig 39 (d). The surface

roughness of the wear tracks, from the profilometer scans, were 127.47 nm +/- 8.23 nm and

96 nm +/- 15.02 nm after 8 and 640 slidings respectively at an applied load of 60N.

At increased load (216N), flattening was observed on the wear track surface and the surface

roughness decreased with increasing sliding distance. The surface roughness measurements

from the optical profilometer scans showed a surface roughness of 35.53 nm +/- 4.86 nm,

27.64 nm +/- 10.78 nm and 18.50nm +/- 9.38 nm after 40, 160 and 640 slidings respectively.

The wheel showed built up material at end points of contact region, fig 40 (b), (d), (f). The

wheel contact point showed a flat area more than a worn surface. The wear tracks also showed

formation of built up material. This built up material was placed at the side of the wear track.

The optical profilometer scans of DLC wear tracks also showed a small amount of abrasive

wear for longer sliding distance, fig 40 (c), (e).

42

a) b)

c) d)

e) f)

Fig. 40. Scanned pictures from optical profilometer showing DLC wear tracks of sample after 40, 160 and 640

slidings (a), (c), (e) and corresponding wear of wheel (b), (d), (f) at 216N load. The arrows indicate the sliding

direction.

Despite the larger applied load, the DLC coating did not show evidence of severe wear, but

much wider tracks were found, compared to track width at lower load. At 52N load the

average wear widths were +257.33 µm +/- 19.36 µm, +276.22 µm +/- 8.91 µm and 290.33

µm +/- 6.14 after 40, 160 and 640 slidings respectively. Similar results were found for 60N

load. For 216N load the average widths were 387.33 µm +/- 21.87 µm, 487.33 µm +/- 39.10

µm and 575. 12 µm +/- 19.10 µm after 40, 160 and 640 slidings respectively.

Wear track profiles were collected, as complement to scanned