Zirconium carbide-tungsten cermets prepared by in situ reaction sintering

Article abstract of DOI:10.1111/j.1551-2916.2007.01642.x

Zirconium carbide-tungsten (ZrC-W) cermets were prepared by a novel in situ reaction sintering process. Compacted stoichiometric zirconium oxide (ZrO2) and tungsten carbide (WC) powders were heated to 2100°C, which produced cermets with 35 vol% ZrC and 65 vol% W consisting of an interpenetrating-type microstructure with a relative density of ～95%. The cermets had an elastic modulus of 274 GPa, a fracture toughness of 8.3 MPa·m1/2, and a flexural strength of 402 MPa. The ZrC content could be increased by adding excess ZrC or ZrO2 and carbon to the precursors, which increased the density to >98%. The solid-state reaction between WC and ZrO2 and W-ZrC solid solution were also studied thermodynamically and experimentally.

Full text of DOI:10.1111/j.1551-2916.2007.01642.x

J. Am. Ceram. Soc., 90 [6] 1930–1933 (2007)
DOI: 10.1111/j.1551-2916.2007.01642.x
r 2007 The American Ceramic Society
ournal
J
Zirconium Carbide–Tungsten Cermets Prepared by
In Situ Reaction Sintering
S. C. Zhang,*,w G. E. Hilmas,* and W. G. Fahrenholtz*
Department of Materials Science and Engineering, University of Missouri-Rolla, Rolla, Missouri 65409
Zirconium carbide–tungsten (ZrC–W) cermets were prepared
by a novel in situ reaction sintering process. Compacted stoi-
chiometric zirconium oxide (ZrO2) and tungsten carbide (WC)
powders were heated to 21001C, which produced cermets with 35
vol% ZrC and 65 vol% W consisting of an interpenetrating-
type microstructure with a relative density of B95%. The cer-
mets had an elastic modulus of 274 GPa, a fracture toughness of
ture (35401C) and the similarity of its thermal and mechanical
properties to W. For ceramic contents of 20 vol% or higher, W-
based cermets are typically fabricated by hot pressing,4,9–11
which is limited to the formation of simple geometries and mod-
erate sizes.
Recently, Breslin12 and Sandhage and colleagues13–16 fabri-
cated ZrC/W cermets using displacive compensation of porosity.
In this process, a tungsten carbide (WC) preform was sintered to
B50% density at 14501C and then infiltrated with a Zr2Cu alloy
at 13001C. The resulting cermet contained W and ZrC as well as
unreacted WC (430 vol%) and a residual low melting point Cu-
rich phase (45 vol%).15
8.3 MPa m1/2, and a flexural strength of 402 MPa. The ZrC
.
content could be increased by adding excess ZrC or ZrO2 and
carbon to the precursors, which increased the density to 498%.
The solid-state reaction between WC and ZrO2 and W–ZrC
solid solution were also studied thermodynamically and exper-
imentally.
The purpose of this paper is to describe an in situ reaction
sintering process for preparing ZrC/W cermets.
I. Introduction
II. Experimental Procedure
UNGSTEN (W) has the highest melting point (34221C) of the
T
refractory metals.1 In addition, W has high strength (B800
WC (o1 mm, 99% purity, Cerac Inc. Milwaukee , WI) and ZrO2
(ꢁ325 mesh, 499% purity, Alfa Aesar Chemical Co., Ward
Hill, MA) were the starting materials. In addition, ZrC (o2 mm
Grade B, H.C. Starck, Karlsruhe, Germany) or ZrO2 and car-
bon were added to some batches to increase the ZrC fraction in
the final cermets. To reduce the particle size of the ZrO2 and
ZrC, the powders were attrition milled for 2 h using ZrO2 or WC
media, respectively. After milling, the average particle sizes were
calculated from surface areas measured by nitrogen adsorption
(NOVA 1000, Quantachrome, Boynton Beach, FL).
MPa), high thermal conductivity (105710 W ꢀ (mꢀ K)ꢁ1), and a
low thermal expansion coefficient (4.5ꢂ 10ꢁ6
K
ꢁ1) at room
temperature.2,3 This combination of properties makes W attrac-
tive for aerospace applications, such as rocket nozzles, heat
shields, combustion chamber liners, and re-entry components.4,5
However, the mechanical strength of pure W decreases rapidly
as the temperature increases.4,6 Both alloying and particle re-
inforcement have been used to improve the strength of W at
elevated temperatures.
Tungsten forms solid solutions with many metals. Rhenium
(Re) is the most important alloying element for W in aerospace
applications as it has a high melting temperature (31861C) and
reacts with W to form stable intermetallic phases.6 A W alloy
containing 3.6 wt% Re has almost twice the tensile strength (230
MPa) of pure W (130 MPa) at 14001C. Even so, the tensile
strength of this alloy decreases as the temperature increases,
reaching the same strength (50 MPa) as W at 21001C and above,
which are typical temperatures encountered in many of the
aerospace applications. In addition, rhenium is expensive and
markedly increases the cost of W–Re components.6
Fine ceramic particles including La2O3, ZrO2, ThO2, ZrC,
TiN, TiC, and HfC have been used to enhance the elevated
temperature strength of W3,5–9 by reducing grain growth and
limit dislocation motion of W. Ceramic additions also improved
the ablation resistance of the cermets compared with W alloys.5
Therefore, ceramic particle additions improve the high-tempera-
ture properties of W more effectively than alloying.6,5 Among
ceramics, ZrC is attractive because of its high melting tempera-
The precursors, WC, milled ZrO2, and ZrC were dispersed in
methyl ethyl ketone by ball milling for 24 h with a dispersant
(DISPERBYK-110, BYK-Chemie Co., Wesel, Germany). Next,
1 wt% binder (Qpac-40, Empower Materials, Newark, DE) was
added and the mixture was milled for another 24 h. In batches
with extra carbon, a soluble phenolic resin (GP 2074, Georgia
Pacific Co., Atlanta, GA) was added at the same time as the
binder. Granules dried from the slurries were used to form cy-
lindrical pellets 1.9 cm (0.75 in.) in diameter by uniaxial pressing
at 18.6 MPa (2.7 ksi), followed by cold iso-static pressing at 310
MPa (45 ksi). The compositions used in this study are summar-
ized in Table I.
Compacted pellets were heated at 101C/min in a graphite
crucible to temperatures ranging from 14501 to 21501C in a
Table I. Batch Compositions Used to Study Reaction and
Densification
Compositions (moles)
Highest sintered
D. Butt—contributing editor
Sample code
WC
ZrO2
ZrC
Carbon
density (%)
ZrC/W-1
ZrC/W-4
ZrC/W-6
3
3
3
1
1
1.8
0
0.8
0
0
0
2.4
94.5
94
498
Manuscript No. 22604. Received January 5, 2007; approved January 22, 2007.
*Member, American Ceramics Society.
wAuthor to whom correspondence should be addressed. e-mail: scz@umr.edu
This work was funded by the Air Force Research Laboratory on contract number
FA8650-04-C-5704 through the Center For Aerospace Manufacturing Technologies at the
University of Missouri-Rolla.
WC, tungsten carbide; ZrC, zirconium carbide; ZrC/W, zirconium carbide–
tungsten; ZrO2, zirconium oxide.
1930

June 2007
Communications of the American Ceramic Society
1931
graphite element furnace (3060-FP20, Thermal Technology,
Santa Rosa, CA). The furnace atmosphere was a mild vacuum
(B20 Pa) for temperatures of 18501C or below, but was switched
to flowing argon (B105 Pa) for temperatures above 18501C.
Hold times were 1 h for powder reactions and 4 h for sintering.
The phases present after heating were analyzed by X-ray dif-
fraction (XRD) (XDS 2000, Scintag Inc., Cupertino, CA). The
bulk densities of sintered specimens were determined using
Archimedes method. Relative densities were calculated by div-
iding the measured bulk densities by the theoretical densities,
which were calculated for each composition based on the
amounts of W and ZrC predicted in the final composites.
Some sintered specimens were surface ground, polished, and
thermally etched for scanning electron microscopy (SEM, S-570,
Hitachi, Tokyo, Japan) analysis. The flexure strength was meas-
ured according to ASTM C1161-02a for type A bars using a
screw-driven test frame (Model 5881, Instron, Norwood, MA).
The elastic constants were measured according to ASTM Stand-
ard C1259-01 for impulse excitation of cylindrical disks
(Grindosonic, J.W. Lemmens, St. Louis, MO). Vickers hard-
ness was determined from a minimum of 10 indents formed us-
ing a load of 200 g and a dwell time of 15 s (Duramin 5, Struers,
Westlake, OH). The fracture toughness was determined by dir-
ect crack measurements17 using indents produced with a load of
75 kg and a dwell time of 15 s.
Fig. 2. X-ray diffraction patterns showing a shift in the (311) peak of
zirconium carbide (ZrC), suggesting the formation of a ZrC-based solid
solution.
heated to 18501C for 1 h had a uniform distribution of ZrC in a
W matrix.
To characterize densification behavior, pellets (ZrC/W-1)
were sintered at 19001, 21001, and 21501C for 4 h. Pellets had
a relative density of 94.5% when sintered at 21001C for 4 h.
Both the XRD analysis and microstructure suggested that Re-
action 1 can be used to fabricate ZrC/W cermets.
III. Results and Discussion
(1) The WC–ZrO2 Reaction
18
Previous research on the pressureless sintering of ZrB2 re-
vealed that ZrO2 present as an impurity on the particle surfaces
of ZrB2 particles reacted with WC according to Reaction 1:
(2) ZrC/W Solid Solution Formation
ZrC peaks in the XRD patterns of fully reacted ZrC/W (ZrC/W-
1) were shifted compared with the reported positions (JCPDS
card 35-0784 for ZrC) and the peak positions observed for com-
mercial ZrC powder. For example, Fig. 2 shows that the (311)
peak for ZrC was shifted from 65.971 (commercial powder and
JCPDS card) to 66.4751 for ZrC/W, which is also higher than
the corresponding (622) peak position (66.0231) reported for
carbon-deficient ZrC0.7. The increase in angle indicates a de-
crease in the corresponding d-spacing, and, therefore, the lattice
ZrO2 þ 3WC ! ZrC þ 3W þ 2COðgÞ
(1)
In the standard state, the reaction is favorable above
B19501C (i.e., negative DG1) as shown by the solid line in
Fig. 1. However, the reaction could be favorable at temperatures
as low as B13001C under vacuum due to the lower partial pres-
sure of CO, which was assumed to be the nominal furnace pres-
sure (20 Pa). To verify the predictions, pellets batched with the
stoichiometry of Reaction 1 (ZrC/W-1 from Table I) were heat-
ed to 12501, 14501, 16501, 18501, and 19501C and held for 1 h
under mild vacuum (B20 Pa). Analysis of XRD patterns in a
previous study18 indicated that the reaction initiated at 14501C
under vacuum, which was B1501C higher than predicted. Re-
action at 18501C and higher resulted in the formation of ZrC
and W as the major phases (not shown). In addition, pellets
˚
parameter of ZrC. The covalent radius of W (1.38 A) is B12%
smaller than that of Zr (1.57 A), which indicates that W is
19
˚
likely to have some solid solubility in Zr compounds as has been
demonstrated by Song et al.10 and as shown in the Zr–W–C
phase diagram.20 As the position of the (311) peak is shifted to
higher 2y values than the reported position of the (311) peak in
ZrC or the (622) peak in ZrC0.7, it seems likely that the ZrC was
sub-stoichiometric in addition to containing some W. The for-
mation of a (W,Zr)Cx solid solution may be beneficial for den-
sification of the ZrC/W cermet.
(3) Microstructure and Control of Composition
The in situ reaction process used to fabricate ZrC/W cermets is
based on the reaction between WC and ZrO2. From the stoi-
chiometry of Reaction 1, the ZrC–W cermet should contain 65
vol% W and 35 vol% ZrC. Figure 3(a) shows the microstructure
of ZrC/W-1 sintered at 21001C for 4 h in argon. From the image
and EDS analysis (not shown), the W grains (light gray) form an
interconnected network with embedded ZrC grains (dark). The
composition of the starting powders was modified by adding
ZrC (Reaction 2), or ZrO2 and C (Reaction 3), to increase the
ZrC content of the cermet.
ZrO2 þ 3WC þ xZrC ! ð1 þ xÞZrC þ 3W þ 2COðgÞ (2)
ð1 þ xÞZrO2 þ 3WC þ 2xC
! ð1 þ xÞZrC þ 3W þ 2ð1 þ xÞCOðgÞ
(3)
Attrition-milled ZrC with an average particle size of 0.16 mm
was used to form cermets with additional ZrC (x 5 0.8 moles in
Fig. 1. Change in Gibbs’ free energy as a function of temperature for
Reactions 1 and 3. Data for the calculations come from Chase.21

1932
Communications of the American Ceramic Society
Vol. 90, No. 6
Fig. 3. Microstructures of zirconium carbide–tungsten (ZrC/W) cermets produced by (a) Reaction (1), (b) Reaction (2), and (c) Reaction (3), sintered at
21001C for 4 h in Ar.
Reaction 2), which resulted in 50 vol% ZrC. Pellets sintered at
21001C for 4 h had a relative density of 494% and the ZrC
inclusions were smaller (Fig. 3(b)) than in pellets produced by
Reaction 1.
Carbon and excess ZrO2 were also used to increase the
ZrC content (Reaction 3). A soluble phenolic resin was used
as the carbon source. Pellets containing the resin were charred
at 6001C for 4 h in a flowing 90% Ar110% H2 atmosphere
to convert the resin to carbon (yield 40% by weight). The
addition of carbon and ZrO2 increased the ZrC content of the
cermet to B50 vol% and resulted in a high density (498%)
and a finer microstructure (Fig. 3(c)) with features on the order
of 1–3 mm.
(4) Properties
Sintered ZrC/W-1 disks, having relative densities of B95%,
were used to investigate mechanical properties. The elastic
modulus was 274 GPa, the hardness was 6.2 GPa, the flexure
strength was 402 MPa, and the fracture toughness was 8.3
MPa ꢀ m1/2. The elastic modulus, hardness, and fracture tough-
ness were comparable with values of 383, 5.77 GPa, and 9.23
MPa ꢀ m1/2 reported for 30 vol% ZrC/W material produced by
hot pressing.6 Analysis of the indentation crack path by SEM
(Fig. 4) indicates that both crack deflection and crack bridging
contributed to toughening. The strength measured for the sin-
tered cermets was lower than the value of 705 MPa reported for
a hot-pressed W cermet containing 30 vol% ZrC.3 The lower
strength of the sintered material (ZrC/W-1) can be attributed to
the small fraction of porosity (B5 vol%) present and the rela-
tively large grain size in this material. Higher strengths are ex-
pected with a higher density and a smaller grain size, such as
sample ZrC/W-6.
IV. Conclusions
A solid-state reaction between ZrO2 and WC was used to fab-
ricate ZrC/W cermets using an in situ reaction sintering process.
The reaction was thermodynamically favorable at temperatures
as low as B13001C in vacuum. The reaction between 3 moles of
WC and 1 mole of ZrO2 (Reaction 1) could be driven to com-
pletion at 18501C or higher under vacuum. The resulting cer-
mets with 495% relative density and 35 vol% ZrC and 65 vol%
W had an elastic modulus of 274 GPa, a fracture toughness of
8.3 MPa ꢀ m1/2, and a flexural strength of 402 MPa. The sintered
density and ZrC content of the cermet were increased to 498%
and 50 vol% by adding ZrO2 and carbon, respectively. The
(W,Zr)Cx solid solution formation in the cermets was observed
by X-ray analysis.
Fig. 4. Crack deflection (white arrow) and crack bridging (black arrow)
in zirconium carbide–tungsten (ZrC/W) cermets produced by Reaction
(1) under a load of 75 kg and a dwell time of 15 s.

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Communications of the American Ceramic Society
1933
12M. C. Breslin, ‘‘Process for Preparing of Ceramic-Metal Composite Bodies’’;
US Patent 5,214,011, May 25, 1993.
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