Crack deflection in tungsten carbide based laminates

International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228

www.elsevier.com/locate/ijrmhm

Crack deflection in tungsten carbide based laminates

Jeremy Watts *, Greg Hilmas

University of Missouri-Rolla, 222 McNutt Hall, Rolla, MO 65409, United States

Received 27 December 2004; accepted 6 April 2005

Abstract

Laminate materials were manufactured by alternating WC–6%Co layers with equal thickness layers of one of the following com-

positions: WC–16%Co, WC–16%Ni, Co, a W–Ni–Fe alloy, and Ni. One composition used WC–6%Ni in place of WC–6%Co, lay-

ered with the W–Ni–Fe alloy. Flexure specimens, consisting of nine alternating layers, were fabricated with either WC–6%Co or

WC–6%Ni on the tensile surface of the specimen. The ability of these composites to deflect or arrest cracks was investigated in four

point bending. Two of the compositions, WC–6%Co/W–Ni–Fe and WC–6%Co/Ni, exhibited the ability to fail non-catastrophically,

showing inelastic work of fracture as high as 34,400 J/m2.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Tungsten carbide; Laminate; Crack deflection; Fibrous monolith

1. Introduction

Cemented carbides, in particular WC–Co, are hard,

wear resistant materials. These properties make carbidespopular choices for use in highly abrasive environments,

such as drilling [1]. Both the petroleum and mining

industries use roller cone bits for drilling. Cemented car-

bides are the primary materials used for the cutting teeth

on these bits. While their high hardness and good wear

resistance make them common choices for this applica-

tion, there are drawbacks. Cemented carbides are also

brittle, which can lead to decreases in drilling efficiency,or total failure of the bit insert [2]. A material which

could maintain the wear properties of cemented carbides

and yet possess a higher fracture toughness to avoid pre-

mature failure, would represent a leap forward over cur-

rent technology.

Previous research has shown that the use of lami-

nates, or more complex fibrous monolithic architectures

can provide marked increases in fracture toughness as

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

doi:10.1016/j.ijrmhm.2005.04.005

* Corresponding author.

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

well as work of fracture over their similar monolithic

counterparts [3–7]. Laminates and fibrous monoliths

have been shown to behave in much the same way when

tested in flexure due to the utilization of an engineeredarchitecture that contains both strong and weak features

which act to mitigate crack damage and control crack

propagation behavior. The goal of the current study is

to develop new cemented carbide cell and cell boundary

compositions to be used in a fibrous monolith architec-

ture that will exhibit crack deflection. Laminate samples

have been used in this study due to their ease and speed

of production, compared to fibrous monolithic samples,while exhibiting similar failure mechanisms. This allows

the testing of a greater number of compositions. The

laminate samples were produced by alternating layers

of what would be the cell and cell boundary composi-

tions in a fibrous monolithic architecture.

It has been shown that laminate architectures can de-

flect cracks through a number of mechanisms, for exam-

ple using a weak layer or interfacial material [3], using aporous interlayer [8], or using some form of stressed

interface [9,10]. All of these mechanisms rely on a differ-

ence in properties between the two layers; be it elastic

mailto:[email protected]

J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228 223

modulus, thermal expansion, a density gradient, or some

combination thereof. All but one of the compositions in

this study used WC–6%Co as the cell material. Lami-

nates were produced using this composition along with

one of the following compositions as the alternating

layer: WC–16%Co, WC–16%Ni, cobalt, nickel, and aW–Ni–Fe alloy. The goal of this work was to determine

whether the difference in properties between WC–6%Co

and the latter materials would be sufficient to deflect

cracks in laminated cemented carbides.

2. Experimental procedure

The laminates were produced though a sheet lamina-

tion process. Sheets of each individual material were

produced by blending a mixture of the powder, a ther-

moplastic binder (Dow Chemicals Ethylene Ethyl Acry-

late melt index 1.5, Midland, MI), and a plasticizer

(Aldrich Heavy mineral oil, Milwaukee, WI) using a

high shear mixer (C.W. Brabender, South Hackensack,

NJ). All of the raw materials used in this study are pro-vided in Table 1 including their manufacturer, the man-

ufacturer�s designation for the material, and the starting

particle size. Unless otherwise noted, all percentages are

weight percentages.

The rheology of these mixtures must be carefully con-

trolled in order to maintain consistent layer thicknesses

during the lamination process. Powder polymer blends

were produced with �60 vol% solids loading. The mate-rials were blended at 130 �C at a mixer speed of 35 rpm.

Once the mixtures were homogenously blended, they

were pressed into sheets using a heated hydraulic press.

The blended material was placed between Mylar sheets,

heated to a temperature of 150 �C, and pressed at a load

of 31,750 kg to obtain the desired thickness of 0.5 mm

which was controlled by the use of steel shims. After

the material had been pressed, a pressure of 4500 kgwas maintained while the material was allowed to cool

to room temperature. Sheets were produced with a con-

sistent thicknesses, ±25 lm, using this method. The

sheets were then cut into 50 · 50 mm squares and

stacked in alternating layers to obtain a nine layer stack.

The stacks were then placed back in the hydraulic press

between sheets of Mylar and pressed at 135 �C and a

Table 1

Powders used in the production of laminate structures

Powder Manufacturer Designation Particle size

WC–6%Co Kennametal 379 2–5 lmWC–16%Co Kennametal 368 3–9 lmWC Cerac T-1173 <1 lmCo Cerac C-1111 �325 mesh

W Alfa Aesar 10400 1–5 lmNi Alfa Aesar 10255 2.2–3 lmFe Alfa Aesar 00170 <10 lm

force of 4500 kg to obtain a final thickness of 3.8 mm,

as controlled by steel shims. During the final lamination

process the layers were reduced from 0.5 mm to approxi-

mately 0.42 mm. At this stage, if the rheologies of the

individual materials were well controlled, the layer

reduction would be uniform over both compositions.Following lamination, bars were sectioned from the final

billets to a size of 4.95 mm wide by 3.81 mm. thick by

50 mm long.

After the bars were sectioned they underwent a bin-

der burnout process that reached a maximum tempera-

ture of 600 �C over a period of four days. Burnout

was carried out under an atmosphere of 90%Ar–

10%H2 in order to aid in binder removal and preventany oxidation of the carbide and metal powders. Fol-

lowing burnout the bars were placed between alumina

setters in a graphite crucible along with a small crucible

of metal powders; cobalt powder for the carbide samples

and nickel powder for the nickel containing materials.

This was done in order to create an atmosphere of the

associated metals to aid in sintering. The bars were then

sintered in a graphite furnace (Thermal Technologiesmodel 1000-3060-FP20, Santa Rosa, CA) at 1300 �Cfor 30 min under vacuum, and cooled at a rate of

25 �C/min to room temperature. Upon sintering the bars

had nominal dimensions of 3 mm by 4 mm by 45 mm.

The bars were tested in four point bending using an

instrumented load frame (Instron 4204, Canton, MA)

and a fully articulated four point bend fixture with a

lower span of 40 mm and an upper span of 20 mm. Testswere performed under displacement control using a

cross head rate of 0.1 mm per minute. Data from the

tests was collected using Instron Series IX software.

Inelastic work of fracture was calculated using the area

under the curve following the first failure event, and

twice the cross-sectional area of each individual

specimen.

Scanning electron microscope (SEM) analysis wasperformed using a Hitachi S-570 (Tokyo, Japan) with

an LaB6 filament. SEM micrographs were taken using

an accelerating voltage of 15 kV and a working distance

of 15 mm. Energy dispersive spectroscopy (EDS) analy-

sis was performed using an accelerating voltage of 20 kV

and a working distance of 18 mm. Optical images were

obtained using a Nikon 709051 lens with CCD and

Scion image capturing software.Table 2 shows all of the materials used to produce the

laminate structures. Materials in the column marked as

Material 1 were the primary materials and comprised

both the tensile and compressive surfaces of the flexure

specimens. All but one set of samples was produced

using a WC–6%Co material as the primary material.

The reason for using a Nickel bonded WC for one set

of samples will be discussed later. The W–Ni–Fe alloycontains 63.9% W, 24.3% Ni, and 11.8% Fe by volume.

This composition was chosen as a balance between

Table 2

Materials used to produce laminate structures

Sample set Material 1 Material 2

1 WC–6% Co WC–16% Co

2 WC–6% Co WC–16% Ni

3 WC–6% Co Co

4 WC–6% Co W–Ni–Fe

5 WC–6% Ni W–Ni–Fe

6 WC–6% Co Ni

224 J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228

hardness (imparted by the W) and toughness (imparted

by Ni and Fe).

3. Results and discussion

Table 3 gives the overall strength values for each

composition as well as work of fracture (WOF) where

applicable. Figs. 1 and 2 show typical load vs. displace-

ment curves obtained from sample sets 1 and 2 as well as

an inset optical micrograph of their corresponding bend

specimen, respectively. The bottom of the bend speci-

men is the tensile surface in all of the figures. The optical

micrographs of the sides of the bend specimens showthat no significant crack deflection has occurred at either

the WC–16%Co or WC–16%Ni interfaces. In addition,

the linearity of the load deflection curves shows that

there were no fracture events prior to catastrophic fail-

ure. While the materials from sample set 2 did exhibit

Table 3

Strength data for each composition and WOF where applicable

Sample set Composition Strength

(MPa)

Avg WOF

(J/m2)

1 WC–6%Co/WC–16%Co 960 ± 150 NA

2 WC–6%Co/WC–16%Ni 1800 ± 250 NA

3 WC–6%Co/Co 820 ± 130 NA

4 WC–6%Co/W–Ni–Fe 700 ± 140 2500

5 WC–6%Ni/W–Ni–Fe 650 ± 120 NA

6 WC–6%Co/Ni 950 ± 80 20000

Fig. 1. Load vs. displacement curve for a WC–6%Co/WC–16%Co lam

crack bifurcation, it was not due to any interaction with

the layer interfaces. The latter sample sets were devel-

oped with the expectation that there would be a signifi-

cant enough difference in toughness between the two

materials to cause the crack to deflect when traversing

from the 6%Co layer (�10 MPa m1/2) [1] into either

the 16%Co or 16%Ni layers (�18 MPa m1/2) [1]. When

looking at how monolithic cemented carbide materialsfracture, it has been shown that for compositions with

metal binder contents in the range being discussed here

(<30 vol%), that the crack propagates along the metal/

WC interface and across the metal ligaments between

particles [11]. Given this, as the crack propagated from

one layer to the next, in the latter laminated composi-

tions, there was not a significant enough difference in

the environment surrounding the crack tip to cause thecrack to deflect out of plane.

Fig. 3 shows a typical load displacement curve for the

material produced from sample set 3, which contained

pure cobalt as the secondary layer. In this case, the side

of the bend specimen shows that some minimal amount

of crack deflection has occurred. Additionally, the load

deflection curve does exhibit small peaks followed by

sudden load drops prior to the final failure load beingachieved, which has also been observed in alumina/nickel

fibrous monoliths [17]. This indicates that while there

was only minimal delamination between the WC–6%Co

and the Co layers, the Co layers were arresting the crack

and allowing the specimen to continue to support a

significant load well after the initial fracture event. This

result can be explained by the increased toughness of the

Co layers compared to the WC–6%Co layers, however,Co has a hexagonal structure with limited slip planes,

and therefore does not possess sufficient ductility to

force the crack out of plane and create large delamina-

tion fracture events [12].

The load vs. deflection behavior for sample set 4, with

W–Ni–Fe layers, is shown in Fig. 4. The side of the test

specimen shows that large scale crack deflection did

occur in this system. After crack initiation at the tensile

inate architecture with inset optical image of the bend specimen.

Fig. 2. Load vs. displacement curve for a WC–6%Co/WC–16%Ni laminate architecture and inset optical image of the bend specimen.

Fig. 3. Load vs. displacement curve for a WC–6%Co/Co laminate architecture and inset optical image of the bend specimen.

Fig. 4. Load vs. displacement curve for a WC–6%Co/W–Ni–Fe laminate architecture and inset optical image of the bend specimen.


surface of the bars, the crack follows a layer boundary

laterally over half the length of the bar before contin-

uing to propagate through the W–Ni–Fe layer. The load

displacement curve shows that 75% of the load was

maintained following the initial fracture event, and 30%

of the peak load was maintained following subsequent

Fig. 5. SEM micrograph of WC–6%Co interface with W–Ni–Fe as well as EDS spectra from each layer and the depletion region showing the

decrease in Ni and Fe content in the depletion region and the presence of Ni and Fe in the WC–6%Co layer.


fracture events. Specimens from this set typically dem-

onstrated an inelastic work of fracture of �3000 J/m2.

An SEM investigation of these samples showed thatthe interface between the WC–6%Co layers and the

W–Ni–Fe layers had formed a porous interlayer (Fig.

5). The darker phase present in the upper half of the

SEM micrograph is a nickel–iron solid solution while

the lighter phase is tungsten. Energy dispersive spectros-

copy (EDS) revealed that the nickel and iron content in

the porous interlayer was depleted from the bulk W–Ni–

Fe alloy, as shown in Fig. 5 by the decrease in Ni and Fepeak heights relative to the tungsten peak. EDS analysis

also revealed the presence of nickel and iron in the WC–

6%Co layer (Fig. 5), indicating that as these samples

were sintered the Ni–Fe solid solution diffused out of

the W–Ni–Fe alloy and into the WC–6%Co layers leav-

ing behind voids. It has been shown in the past that por-

ous interlayers in an alumina laminate can be used to

create crack deflection in laminar composites [8,13].Further research has been performed in the alumina sys-

tem, as well as in the SiC and B4C systems, in order to

correlate the amount of porosity to its ability to deflect

cracks [14,15]. The porous interlayer in the WC–6%Co/W–Ni–Fe laminates was measured using areal analysis

on SEM micrographs and determined to contain 33%

porosity. Other researchers have shown that porosity

levels greater than 30% should lead to crack deflection

in laminate systems [13,15] as we have also observed in

the current study.

In order to determine if the porous interlayer created

in the WC–6%Co/W–Ni–Fe system was necessary forcrack deflection to occur, a second set of W–Ni–Fe sam-

ples was fabricated containing WC–6%Ni and W–Ni–Fe

layers (Sample set 5). In this case, nickel bonded WC

was used in order to prevent the migration of the Ni–

Fe solid solution. Fig. 6 illustrates that these samples

did not create a porous interlayer, and did not exhibit

crack deflection. The Ni–Fe solid solution is ductile

[16], however with 64% of that layer being comprisedof brittle tungsten the layers still fail in a brittle manner.

Fig. 6. Load vs. displacement curve for a WC–6%Ni/W–Ni–Fe laminate architecture and inset optical image of the bend specimen.

Fig. 7. Load vs. displacement curve of a WC–6%Co/Ni laminate architecture and inset optical image of the flexure specimen.

Fig. 8. SEM micrograph of the WC–6%Co/Ni laminate bend specimen showing the Ni layers bridging the crack as well as delaminations.



While the side of the bend specimen does indicate a

small amount of crack deflection, the load vs. displace-

ment curve (Fig. 6) contains no load drops indicating

that the crack was never arrested, thus not allowing fur-

ther load carrying capacity.

Fig. 7 shows a load vs. displacement curve for thematerials produced from sample set 6 which contained

alternating layers of WC–6%Co and Ni. While it is

not visible in the bend specimen, the load displacement

curve clearly shows that the crack is arrested several

times prior to and after the peak load, allowing contin-

ued load carrying capacity throughout the test. As evi-

denced by the deformation of the test bar, this sample

survived approximately 1 mm of additional deflectionfollowing the initial fracture event, which is over four

times what any of the other laminate systems in this

study had survived. This particular sample�s strength

(1110 MPa), combined with its load carrying ability be-

yond the initial failure event, led to an inelastic work of

fracture of 34,400 J/m2. Nickel, being an FCC metal,

exhibits more ductility than cobalt [12]. Due to this

added ductility the Ni layers were able to bridge the gapsbetween failed WC–6%Co layers as has been observed in

alumina–nickel composites [17] and cause multiple

cracks to form along with large delaminations. The Ni

layer closest to the tensile surface remained bonded to

the WC–6%Co layers, tearing internally and creating

large voids. The second Ni layer above the tensile sur-

face of the bar clearly shows a large degree of necking

prior to failure. The interfaces between WC–6%Coand Ni layer closer to the neutral axis appear to have

failed in shear resulting in delaminations. The crac-

king and delaminations are more readily observed in

Fig. 8.

4. Conclusions

Metal bonded tungsten carbide laminates were pro-

duced by alternating layers of WC–Co with other

WC–Co compositions, metals, or metal alloys. This

work has shown that a wide variety of materials can

be formed into laminar composites using this processing

method. It has also been shown that large increases in

inelastic work of fracture can be obtained; 3000 J/m2

for WC–Co/W–Ni–Fe laminates and over 30,000 J/m2

for WC–Co/Ni laminates. The load vs. displacement

curves of both WC–Co/W–Ni–Fe and WC–Co/Ni lami-

nates indicate that following the initial fracture of the

specimen, the crack was arrested allowing the samples

to continue bearing load. This indicates an increase in

inelastic work of fracture over monolithic WC–6%Co

which does not exhibit any load retention following

the initial fracture event. The WC–Co/Ni laminates also

exhibited the ability to sustain approximately 50% of the

failure load up to 1 mm of deflection beyond the initial

fracture event.

From the results gathered in this study, both WC–

Co/W–Ni–Fe and WC–Co/Ni have shown great promise

as materials to be used in the fibrous monolithicarchitecture. These two compositions will be incorpo-

rated into a fibrous monolithic architecture tested for

their mechanical behavior, and discussed in a later

publication.

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Crack deflection in tungsten carbide based laminates