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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: jwatts@umr.edu (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
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.
J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228 225
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.
226 J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228
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.
J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228 227
228 J. Watts, G. Hilmas / International Journal of Refractory Metals & Hard Materials 24 (2006) 222–228
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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