Processing and indentation behavior of functionally designed cellular cemented carbides produced by...

International Journal of Refractory Metals & Hard Materials 24 (2006) 229–235

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Processing and indentation behavior of functionally designedcellular cemented carbides produced by coextrusion

Jeremy Watts *, Greg Hilmas

University of Missouri-Rolla, 222 McNutt Hall, Rolla, MO 65409, USA

Received 27 December 2004; accepted 6 April 2005

Abstract

Cemented tungsten carbide composites were manufactured via a coextrusion process to produce a functionally designed cellular

(FDC) architecture. The FDC architecture contained 100–150 lmWC–6%Co cells and an interpenetrating cell boundary phase con-

sisting of either a W–Ni–Fe alloy or Ni. Sample billets containing 17.5%, 30%, and 50% by volume of the cell boundary phase were

engineered and pressurelessly sintered. Flexure specimens were fabricated consisting of cell/cell boundary coextruded filaments

uniaxially aligned in the long axis of the specimen. Indentation behavior as well as flexural response was investigated in order to

determine the ability of these composites to deflect cracks. While samples did not exhibit graceful failure under flexure testing, mean-

ing they could not sustain any load beyond the initial failure event, compositions with 17.5%W–Ni–Fe, 30%W–Ni–Fe, 30% Ni, and

50% Ni displayed the ability to deflect cracks generated by Vickers indents produced using a load of 50 kg.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Fibrous monolith; Functionally designed cellular architecture; Indentation; Crack deflection

1. Introduction

Cemented carbides, particularly cobalt bonded tung-

sten carbide, have been used extensively in abrasive dril-ling operations, such as petroleum drilling and mining

[1]. While carbides have performed well in these applica-

tions, their propensity for brittle fracture remains their

main drawback. If a carbide insert in a drill head fails

catastrophically during drilling it will at best limit con-

tinued performance of the drill bit, reducing its use

cycle, and could result in total failure of the drill. If ce-

mented carbides could be formed into an architecturedesigned to control the propagation of cracks while

maintaining current levels of wear resistance, it would

result in marked improvements over today�s technology.

0263-4368/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmhm.2005.04.019

* Corresponding author.

E-mail address: [email protected] (J. Watts).

Previous research has shown that by using an FDC

architecture, often referred to as a fibrous monolithic

architecture in the ceramic community, measurable

increases in fracture toughness as well as work of frac-ture can be obtained over their monolithic counterparts

[2–6]. FDCs are produced by fabricating a periodic ar-

ray of strong ‘‘cells’’, each of which is surrounded by

an interpenetrating ‘‘cell boundary’’. The material for

the cell boundary is chosen to be a phase designed to

deflect cracks and mitigate crack propagation. Previous

studies have focused mainly on the use of weak cell

boundaries in ceramic systems in the form of BN orgraphite such as the Si3N4/BN [4,6] and SiC/Graphite

systems [5]. Other approaches, however, have also been

successful. Al2O3/Ni composites have displayed an abil-

ity to sustain load beyond the initial fracture event using

a ductile cell boundary phase [7]. Other studies have

been performed to investigate the viability of using a

transforming cell boundary phase to produce controlled

microcracks in a fibrous monolith architecture such as

mailto:[email protected]

230 J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 229–235

the Al2O3/Al2O3–TiO2 system [4], or utilizing a sponta-

neous transformation process as the crack approaches

the cell boundary, such as in the mullite/cordierite sys-

tem [8]. In addition to the architecturally more complex

fibrous monoliths, several 2D laminate systems have

been successful at deflecting cracks by alternating be-tween porous and dense layers of a common ceramic

composition. A dense/porous alumina system has been

shown to deflect cracks [9,10], as well as similarly pro-

duced SiC and B4C composites [11].

The goal of the current study was to evaluate the abil-

ity of the WC–6%Co/W–Ni–Fe and the WC–6%Co/Ni

systems to deflect cracks at the cell–cell boundary inter-

face of FDC architectures. Previous research has shownthat both WC–6%Co/W–Ni–Fe and WC–6%Co/Ni lami-

nate systems exhibit the ability to sustain load beyond

the initial fracture event, also known as graceful failure

[12]. Each composition was produced as FDC architec-

tures consisting of 17.5%, 30%, and 50% by volume of

metal or metal alloy cell boundaries. Samples of each

composition and architecture were then tested by Vick-

ers diamond indentation to observe the behavior ofhighly loaded radial median cracks, if formed. Four

point flexure testing was also performed in order to

determine the interaction of cracks with the cell bound-

ary phase.

2. Experimental procedure

The functionally designed cellular samples were pro-

duced through a coextrusion process. The material sup-

plier, grade, and starting particle size for the powders

used to produce the samples are provided in Table 1.

The W–Ni–Fe alloy used in this study was composed

of 63.9% W, 24.3% Ni, and 11.8% Fe by volume in order

to replicate the composition used in a previous laminate

study [12]. Both cell and cell boundary materials wereproduced by mixing the powders with a thermoplastic

binder (Dow Chemicals Ethylene Ethyl Acrylate melt in-

dex 1.5, Midland, MI) and a plasticizer (Aldrich Heavy

mineral oil, Milwaukee, WI) using a high shear mixer

(C.W. Brabender, South Hackensack, NJ).

At this stage the rheologies of the two materials must

be carefully controlled in order to successfully coextrude

a consistent FDC architecture [13]. After the mixtures

Table 1

Powders used in the production of FM samples

Powder Manufacturer Designation Particle size

(lm)

WC–6 wt%Co Rogers Tool Works 379 2–5

W Alfa Aesar 10,400 1–5

Ni Alfa Aesar 10,255 2.2–3

Fe Alfa Aesar 170 <10

were homogeneously blended the WC–6%Co cell mate-

rial was pressed into a cylindrical rod or ‘‘core’’ between

15 and 21 mm in diameter and �125 mm in length while

the W–Ni–Fe alloy and Ni cell boundary materials were

pressed into crescent shaped ‘‘shells’’ between 1 and

5 mm thick and �125 mm long. By controlling thediameter of the rod and the thickness of the shells, the

final ratio of cell to cell boundary size was established

to be 82.5%/17.5%, 70%/30%, or 50%/50% by volume.

The shells were then laminated onto the rod creating a

feedrod with a distinct core shell architecture and a

diameter of �22.4 mm. The feedrod was then extruded

through a tapered die containing a 2.4 mm spinnerette

heated to 115 �C under a pressure of �23 MPa. Thefeedrod undergoes an �10 fold reduction in size while

maintaining the original core/shell architecture i.e.,

coextrusion. It is at this point in the process that if the

rheologies of the materials have not been properly con-

trolled, the core and shell materials will mix, resulting in

poorly defined cell–cell boundary interfaces. After coex-

trusion the filaments were cut into 125 mm lengths, bun-

dled together in a cylindrical die, and pressed to form asecond feedrod which contained �90 individual fila-

ments. This rod was then extruded using the same con-

ditions as above producing filaments which contained

�90 individual cells/cell boundaries �250 lm in diame-

ter. These filaments were then cut into 100 mm lengths

and aligned uniaxially in a 100 mm by 25 mm rectangu-

lar die. The filaments were consolidated under a pressure

of �17 MPa applied normal to their long axis at a tem-perature of 130 �C. Test specimens 3.8 mm by 5 mm by

50 mm were then sectioned from the billet with filaments

aligned down the long axis of the bar.

After the specimens were sectioned, all of the edges

were chamfered and they underwent a binder burnout

process to remove the organic content, reaching a max-

imum temperature of 600 �C over a period of four days.

Burnout was carried out under an atmosphere of90%Ar–10%H2 in order to aid in binder removal as well

as prevent any oxidation of the carbide or metal pow-

ders. Following burnout the bars were placed between

alumina setters inside a graphite crucible and sintered

in a graphite furnace (Thermal Technologies model

1000-3060-FP20, Santa Rosa, CA) ramped at 20 �C/min up to 1300 �C, held for 30 min, and cooled at a rate

of 25 �C/min to room temperature, all under vacuum. Asmall crucible with a mixture of Co and Ni powders was

placed inside the graphite crucible along with the speci-

mens in order to produce an atmosphere of the metals to

aid in sintering. Upon sintering the bars had nominal

dimensions of 3 mm by 4 mm by 45 mm.

Samples were prepared for indentation testing as well

as scanning electron microscope (SEM) analysis by sec-

tioning 1 cm thick samples from sintered billets and pol-ishing them normal to the long axis of the samples,

allowing the faces of the cells to be analyzed. All

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4Displacement (mm)

Load

(kN

)

17.5% Shell 30% Shell 50% Shell

700 MPa 670 MPa

130 MPa

0.2


Fig. 1. Load vs. displacement curves for WC–6%Co FDCs manufac-

tured with 17.5%, 30%, and 50% W–Ni–Fe cell boundaries.

J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 229–235 231

samples were polished to a 0.25 lm surface finish using

an automatic polisher (Buehler Vector Power Head,

Lake Bluff, IL). High load indentations for examining

radial median crack propagation were made using a

Vickers diamond indenter and a 50 kg load (Leco

V-100-A2 Hardness Tester, St. Joseph, MI). Low loadindentations for examining the hardness of individual

cell and cell boundary materials were made with a

Vickers diamond indenter and loads between 50 and

500 g (Model 300 Tukon Microhardness Tester). SEM

analysis was performed using a Hitachi S-570 (Tokyo,

Japan) with a LaB6 filament.

The bars were also tested in four point flexure using

an instrumented load frame (Instron 4204, Canton,MA) with a screw driven crosshead. The flexure bars

were placed in a fully articulated four point bend fixture

with a lower span of 40 mm and an upper span of

20 mm. All bars were tested with as sintered surfaces.

Tests were performed under displacement control using

a cross head rate of 0.1 mm/min. All load vs. deflection

data from the tests was collected using Instron Series IX

software.

3. Results and discussion

Table 2 summarizes the overall strength data gath-

ered for each composition with their respective architec-

tures. Fig. 1 is an example of a typical load vs.

displacement curve for the WC–6%Co/W–Ni–Fe sys-tem, containing 17.5, 30, and 50 vol% W–Ni–Fe cell

boundaries. The linearity of the load vs. displacement

curves shows that in all three cases there were no frac-

ture events prior to catastrophic failure. A small de-

crease in strength, 700 MPa to 670 MPa, was observed

when moving from the 17.5 vol% cell boundary FDC

architecture to the 30 vol% architecture, and a much

greater loss of strength, 670 MPa to 130 MPa, with anFDC architecture consisting of 50 vol% W–Ni–Fe cell

boundaries. The samples containing 17.5 vol% cell

boundaries were composed of dense WC–6%Co cells

�150 lm in diameter surrounded by a W–Ni–Fe cell

boundary, �20 lm thick, containing large pores result-

ing from the migration of Ni and Fe into the cell as

shown in Fig. 2. This migration is an expected result

Table 2

Strength data for each composition and architecture

Composition Cell boundary (%) Strength (MPa)

WC–6%Co/W–Ni–Fe 17.5 760 ± 60

30 680 ± 50

50 140 ± 15

WC–6%Co/Ni 17.5 1660 ± 60

30 1940 ± 70

50 550 ± 40

and has been discussed in a previous study [12]. The

30 vol% cell boundary architecture has a macrostructure

that was similar to that of the 17.5 vol% architecture;

dense cells surrounded by a porous W–Ni–Fe cell

boundary (Fig. 2). The 50 vol% cell boundary architec-ture was porous throughout, making the interface be-

tween the cells and the cell boundary ill-defined as can

be seen in Fig. 2. This explains why strength was

degraded so rapidly from 30 vol% cell boundaries to

50 vol% cell boundaries. Fig. 3 is an SEM image of

the fracture surface from a 17.5 vol% cell boundary sam-

ple and shows that the crack did not propagate straight

through the samples but instead deflected around cells,propagating preferentially through the cell boundary.

Fig. 4, similarly, is an SEM image of the fracture surface

of a 30 vol% cell boundary sample and shows that the

crack path was largely unaffected by the FDC

architecture.

Figs. 5 and 6 show diamond pyramidal indents

placed in the surface of samples containing 17.5 vol%

W–Ni–Fe and 30 vol% W–Ni–Fe, respectively. Both fig-ures illustrate that cracks generated at or near the cor-

ners of the indents were confined to the W–Ni–Fe cell

boundary phase. Cracks generated within the cell

boundary remained in the cell boundary and did not

propagate into the cells. The latter effect is more readily

observed at a higher magnification (Fig. 7) revealing

how the crack propagated through the cell boundary

from the right corner of the indent shown in Fig. 5. Thissame type of behavior has been observed in the Al2O3/

Ni fibrous monolith system [7] as well as the Si3N4/BN

fibrous monoliths [3]. This indicates that while not suffi-

cient to produce graceful failure, both the 17.5 vol% W–

Ni–Fe and 30 vol% W–Ni–Fe compositions do deflect

cracks into the cell boundary where they remain under

this type of loading condition.

Indentations were also used to measure the hardnessof the cell and cell boundary phases in each of the three

latter architectures. In the 17.5 vol% cell boundary

architecture, hardness values of 1800 kg/mm2 and

Fig. 2. Structure of 17.5% (top left), 30% (top right), and 50% (bottom) W–Ni–Fe FDC architectures showing the WC–6%Co cells surrounded by a

W–Ni–Fe cell boundary as well as the increase in cell boundary thickness and increase in overall porosity as the cell boundary content is increased.

Fig. 3. Fracture surface of WC–6%Co/W–Ni–Fe 17.5% cell boundary

FDC showing crack deflection as evidenced by the group of cells in the

center standing in relief to the surrounding cells.

Fig. 4. Fracture surface of WC–6%Co/W–Ni–Fe 30% cell boundary

FCD showing little evidence of crack deflection.


800 kg/mm2 were measured for the cell and cell bound-

ary, respectively. This is consistent with the Vickershardness values other researchers have measured for

monolithic WC–6%Co materials [14]. Increasing to the

30 vol% cell boundary architecture, hardness decreased

to 1300 kg/mm2 and 450 kg/mm2 for the cell and cell

boundary respectively. These decreases are due to in-

creased Ni and Fe content in the cells and increased

porosity in the cell boundary, both of which are a direct

result of the migration of Ni and Fe into the cell mate-

rial. In the 50 vol% cell boundary composition, the cell

boundary hardness stayed constant at 450 kg/mm2,

however, the hardness of the cell composition decreasedto 860 kg/mm2 due to increases in porosity in the cells of

this architecture.

Fig. 8 shows typical load vs. displacement curves for

the WC–6%Co/Ni system, containing 17.5, 30, and

50 vol% Ni cell boundaries. All three of these materials

Fig. 5. Indentation in WC–6%Co/W–Ni–Fe 17.5% cell boundary

FDC system with arrows indicating cracks generated in the cell

boundary.

Fig. 6. Indentation in WC–6%Co/W–Ni–Fe 30% cell boundary FDC

showing damage in the form of cracking limited to the cell boundary

phase as indicated by arrows.

Fig. 7. Cracks propagating from the corner of an indent in the WC–

6%Co/W–Ni–Fe 17.5% cell boundary FDC system showing the crack

propagating only through the cell boundary.

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1 1.2Displacement (mm)

Load

(kN

)


1580 MPa 1440 MPa

520 MPa


Fig. 8. Load vs. displacement curves for WC–6%Co FDCs manufac-

tured with 17.5%, 30%, and 50% Ni cell boundaries.

Fig. 9. Fracture surface of WC–6%Co/Ni 17.5% cell boundary FDC

material showing voids formed by the Ni cell boundary migration.


exhibited linear behavior up to their peak load, followed

by catastrophic failure. While there was a decrease in

strength from 1580 MPa to 1440 MPa from the17.5 vol% to the 30 vol% cell boundary architecture,

there was also an increase in the amount of displacement

tolerated prior to failure, �0.5 mm as compared to

�0.9 mm, respectively. In studying the structure of each

composition, it was discovered that the cell to cell

boundary ratio defined during green processing, i.e.,

17.5 vol%, 30 vol%, and 50 vol% cell boundaries, was

not maintained during sintering. The fracture surfaceof a 17.5 vol% cell boundary architecture (Fig. 9) shows

that relatively little Ni remained in the cell boundary,

and instead diffused into the carbide cells leaving behind

Fig. 10. Indentation on the surface of a WC–6%Co/Ni 30% cell

boundary FDC composition showing that only those cells contacted

by the indenter were damaged, and those cells adjacent remained

unaffected.


voids during the sintering process. While Ni also dif-

fused into the carbide cells for the 30 vol% and

50 vol% cell boundary compositions, enough Ni re-

mained in place to form a continuous cell boundary

phase.

Fig. 10 shows an indent on the surface of a WC–6%Co/Ni FDC architecture containing 30 vol% cell

boundaries. The irregular outline of the indent shows

that the WC–Co cells were displaced during indentation

and then sprang back once the load had been removed.

This is further evidenced by the Ni cell boundary being

bulged outward around the displaced cells, as it was ex-

truded out from between the cells during indentation

testing. Fig. 11 shows that the 50 vol% cell boundary

Fig. 11. Indentation of the surface of a WC–6%Co/Ni 50% cell

boundary FDC sample with indentation damage limited to cells

contacted by the indenter.

architecture behaved in much the same manner. The

irregular outline of the indent again indicates that cells

in contact with the indenter were displaced and then

sprang back after indentation. The outline of the cells

on the perimeter of the indent clearly shows that as

the WC–Co cells were displaced the nickel surroundingthem was also displaced. Similar behavior has been ob-

served in the SiC/Graphite fibrous monolith system [4],

where indentations resulted in graphite being extruded

out of the cell boundaries. While cracks were not formed

at the cell boundaries under indentation in the WC–

6%Co/Ni system, as they were in the WC–6%Co/W–

Ni–Fe system, the damage from the indent was limited

to only those cells which were contacted by the indenter.As with the WC–6%Co/W–Ni–Fe system, indenta-

tions were used to measure the hardness of the cell

and cell boundary materials in each of the three cell/cell

boundary architectures where applicable. In this case,

only the cell boundary of the 50 vol% architecture was

measured, and found to be 80 kg/mm2 which is consis-

tent with pure Ni metal [15]. The other cell boundary

architectures were not measured due to the lack of a cellboundary phase in the 17.5 vol% architecture and the

proximity of the cells to the indent in the 30 vol% archi-

tecture. The hardness of the cell material was measured

to be 1500 kg/mm2 for the 17.5% cell boundary architec-

ture; below what the monolithic cell material should be

[14], and presumably due to the incorporation of Ni into

the cell phase. A simple rule of mixtures calculation,

assuming that all of the Ni cell boundary was incorpo-rated into the cell, predicts a hardness of �1500 kg/

mm2. The hardness of the cell in the 30% shell architec-

ture was 1400 kg/mm2; decreased by additional incorpo-

ration of Ni. Again using a rule of mixtures calculation

this hardness value indicates that approximately

60 vol% of the Ni cell boundaries was incorporated into

the cells. The hardness of the cell in the 50 vol% cell

boundary architecture was measured to be the same asthat of the 30 vol% architecture, indicating that no addi-

tional Ni was incorporated into the cell phase.

4. Conclusions

Tungsten carbide based functionally designed cellular

(FDC) architectures have been prepared using WC–6%Co as the cell material and both W–Ni–Fe and Ni

as cell boundary materials in varying quantities. These

compositions were chosen based on their ability to fail

gracefully when produced as laminates in a prior study.

While non-catastrophic flexure behavior was not exhib-

ited by the FDC samples in this study, fracture surfaces

of FDC flexure samples clearly show the presence of

crack deflecting behavior. Owing to the increased com-plexity of the FDC architectures, more work is needed

to accurately predict their behavior.


Indentation testing revealed that the WC–6%Co/W–

Ni–Fe FDC system, consisting of either 17.5 vol% or

30 vol% cell boundaries, contained propagating cracks

within the cell boundary phase and not within the cells.

While cracks were not formed in the WC–6%Co/Ni

FDC system, the 30 vol% and 50 vol% cell boundaryarchitectures limited indentation damage to cells which

were contacted by the indenter. The nickel cell boundary

was plastically deformed allowing the WC–Co cells to be

displaced. Indentation testing on these materials shows

promise that FDC architectures can be used to limit

the propagation of cracks through cemented carbide

materials.

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