Chemical reactions, anisotropic grain growth and sintering mechanisms of self-reinforced ZrB2-SiC doped with WC

Article abstract of DOI:10.1111/j.1551-2916.2010.04278.x

Transmission electron microscopy study of pressureless sintered ZrB ₂-20 vol% SiC composites with WC (5-10 vol%) additions was carried out in this work. Two independent chemical reactions in which the ZrO ₂ impurities were removed from ZrB₂ grain surface were confirmed. Three new formed phases, (W, Zr)_ssB, (Zr, W) _ssC, and (W, Zr)_ssSi₂, apart from the plate-like ZrB₂ and SiC grains, were identified in the samples. Microstructure observations strongly suggest that the liquid phases in terms of (W, Zr)_ssB and (W, Zr)_ssSi₂, appear in this system during the sintering process below 2200°C, and the former could well wet the ZrB₂ grain boundaries. The mass transport process between ZrB₂ and (W, Zr)_ssB was also confirmed by scanning-transmission electron microscope analysis. The above results supports that the densification at 2200°C is assisted by liquid phase and the elongation of ZrB₂ platelets resulted from the Ostwald ripening during the dissolution-diffusion-precipitation process.

Full text of DOI:10.1111/j.1551-2916.2010.04278.x

J. Am. Ceram. Soc., 94 [5] 1575–1583 (2011)
DOI: 10.1111/j.1551-2916.2010.04278.x
r 2011 The American Ceramic Society
ournal
J
Chemical Reactions, Anisotropic Grain Growth and Sintering
Mechanisms of Self-Reinforced ZrB2–SiC Doped with WC
Ji Zou,z,y Shi-Kuan Sun,z,y Guo-Jun Zhang,*,w,z Yan-Mei Kan,z Pei-Ling Wang,z and Tatsuki Ohji**,z
zState Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics,
Shanghai 200050, China
yGraduate School of the Chinese Academy of Sciences, Beijing 100049, China
zNational Institute of Advanced Industrial Science and Technology, Nagoya, Aichi 463-8560, Japan
Transmission electron microscopy study of pressureless sintered
ZrB2–20 vol% SiC composites with WC (5–10 vol%) additions
was carried out in this work. Two independent chemical reactions
in which the ZrO2 impurities were removed from ZrB2 grain
surface were confirmed. Three new formed phases, (W, Zr)ssB,
(Zr, W)ssC, and (W, Zr)ssSi2, apart from the plate-like ZrB2 and
SiC grains, were identified in the samples. Microstructure ob-
servations strongly suggest that the liquid phases in terms of (W,
Zr)ssB and (W, Zr)ssSi2, appear in this system during the sinte-
ring process below 22001C, and the former could well wet the
ZrB2 grain boundaries. The mass transport process between
ZrB2 and (W, Zr)ssB was also confirmed by scanning-transmis-
sion electron microscope analysis. The above results supports
that the densification at 22001C is assisted by liquid phase and
the elongation of ZrB2 platelets resulted from the Ostwald rip-
ening during the dissolution–diffusion–precipitation process.
into two categories: (1) transition metal silicides (MoSi2, ZrSi2,
TaSi2),10–12 whose densification effect was thought to be the
formation of a liquid phase, which can accelerate the mass
transportation process between ZrB2 grains; (2) carbon and
carbides (WC,13 B4C,14 and B4C–C15), which react with and
remove the oxides layer on the surfaces of the starting ZrB2
particles.
Among the second group of the sintering aids, tungsten car-
bide (WC) is of great interest, due to the multiple and beneficial
effects under various aspects, which are summarized as follows.
First, the highest four-point bending strengths (over 1 GPa) in
ultra-high temperature ceramics (UHTCs) at room temperature
were reported in ZrB2–SiC–WC composites.1 Second, WC can
improve the oxidation resistance of ZrB2 by forming WO3–ZrO2
liquid phase on its surface oxide layer during oxidation at
moderately high temperature.16 Third, WC is conducive to the
formation of elongated ZrB2 grains,7 and ZrB2–SiC with this
unique interlocking microstructure possesses high tough-
ness (6.5 MPa ꢀ m1/2
) compared with the common ones
I. Introduction
(3–4 MPa ꢀ m1/2).17
IC-REINFORCED ZrB2 composites (normally with 10–30 vol%
In addition, cobalt-bonded WC milling media is widely uti-
lized to grind ZrB2 powders.13–15Owing to the hardness differ-
ence between WC and ZrB2, WC could spontaneously be added
into ZrB2 powders during this milling process. On the other
hand, oxide impurities, in terms of ZrO2 and B2O3, are always
present on the surface of ZrB2.18 Unlike B2O3, most of which
could be removed from the starting powders through its evap-
oration at temperatures above 13001C in vacuum furnace during
sintering, ZrO2 removal is more difficult. The external ZrO2
made the chemical reactions in ZrB2–SiC–WC systems more
complex. Though WC is considered as an effective sintering aid
for ZrB2, the reactions among ZrB2, SiC, WC, and the effective
role of WC on the densification and the elongation of ZrB2
grains are not fully clarified.
S
SiC in particulates or whiskers forms1–3) have a series of
excellent properties, such as high bending strength (500–
1000 MPa),1,3–4 high oxidation resistance,5,6moderate tough-
ness, (3.5–6 MPa ꢀ m1/2) and high hardness (18–20 GPa).1–4,7
The addition of SiC lowers the melting point of ZrB2-based
system from about 32001 to 23201C,8 ZrB2–SiC system has been
drawing great attention since recent years due the combination
of properties mentioned above and its potential applications in
hypersonic technology. For instance, it could serve as thermal
protection materials for sharp leading edges in reusable launch
vehicles as well as for hypersonic flight.9
Because of the poor sinterability of ZrB2 and SiC, their
condensation was typically accomplished by applying pressures
(20–30 MPa) at high temperature (18001–20001C).4 Hot press
sintering indeed has some advantages, but it poses a number of
problems in the fabrication of complex components, which re-
stricts the applications of ZrB2–SiC under the engineering aspect.
To solve this problem, various sintering aids have been
added into ZrB2 in order to enhance its pressureless densificat-
ion. The ceramics mainly used as sintering aids can be divided
Several investigations have proved that the reaction between
WC and ZrO2 could occur,14 and these results could explain
why WC enhances the densification of ZrB2. The reaction be-
tween WC and ZrB2 was also observed. However, neither spe-
cific product, nor reaction path was identified.1,7 In our previous
report7 based on energy-dispersive spectroscopy (EDS) analysis,
we speculated about the occurrence of a displacement reaction
between WC and ZrB2, but the identification of the phase prod-
ucts was restricted by the experimental conditions.
H.-J. Kleebe—contributing editor
How does WC react with ZrB2? Are the reaction products the
solid solution of Zr in WB as well as W in ZrC as assumed?
What is the morphology of the ZrB2 grains, rod or platelet?
What is its growth direction? Why could WC favor the densifi-
cation and elongation of ZrB2 grains? These issues were
connected with the high toughness of self-reinforced ZrB2–
SiC–WC-based ceramic.7 A thorough investigation on them
could optimize the sintering aids utilized for the acceleration
of the anisotropic growth of ZrB2 grains, which is a new way to
Manuscript No. 28071. Received May 24, 2010; approved October 25, 2010.
This work was financially supported by the Chinese Academy of Sciences under the
Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the
National Natural Science Foundation of China (No. 50632070), International Science and
Technology Cooperation Project of Shanghai (No. 08520707800), and the CAS Special
Grant for Postgraduate Research, Innovation and Practice.
*Member, The American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: gjzhang@mail.sic.ac.cn
**Fellow, The American Ceramic Society.
1575

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Journal of the American Ceramic Society—Zou et al.
Vol. 94, No. 5
Table I. Characteristics of the Raw Materials
Particle size
Material
(D50) (mm)
Impurities (%)
Supplier
Received ZrB2
14
O 0.5, Na 0.01, Al 0.01, Ca 0.06, Fe 0.2,
Si 0.01, Hf 1.5, Ti 1.9, V 0.01
O 1.7, others were not analyzed
B 0.33, O 1, Ca 0.24, Cl 0.1, Fe 0.16, V 0.09 Xinyuan Carborundum Co., Ltd., Changle, China
C6.13, Cr 0.03, Co 0.01, Mo 0.01, O 0.2
NA
Sanxing Ceramics Materials Co., Ltd., Gongyi, China
The same
Milled ZrB2
a-SiC
WC
1.4w
0.45w
o1w
0.3w
Hard alloy Co., Ltd., Zhuzhou, China
CSG Holding Co., Ltd., Shenzhen, China
ZrO2
wFrom the supplier. NA, Not analyzed.
realize high toughness in UHTCs. To this end, observations on a
small length scale are required, which are the preferential ad-
vantages of high-resolution transmission electron microscopy
(HRTEM). However, the studies to use HRTEM to analyze the
microstructure of sintered UHTCs are very few.19,20 So in this
paper, we investigated the chemical reactions, phase formation
and microstructure evolutions of the ZrB2–SiC–WC composites
prepared by pressureless sintering by HRTEM. The mechanisms
for the formation of such a tough interlocking microstructure in
ZrB2–SiC ceramics are thoroughly discussed.
II. Experimental Procedure
Commercially available powders of ZrB2, SiC, and WC were
used as the raw materials. The main impurities, particle size, and
suppliers are listed for the powders in Table I. Five and 10 vol%
WC-doped ZrB2–20 vol SiC composites (ZSW5, ZSW10) were
prepared by pressureless sintering at 20001–22001C for 2 h. The
whole sintering schedule included an external isothermal holds
at 16501C in vacuum, and then argon gas was introduced into
the furnace. The details of the processing and sintering proce-
dures have been shown elsewhere.7
Fig. 1. Scanning electron micrograph of ZSW5 (a) and ZSW10 (b) sintered at 22001C. EDX spectra from selected points A and B in (b) were shown in
(c) and (d), respectively.

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Mechanisms of Self-Reinforced ZrB2-SiC Doped with WC
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Table II. Typical Distance of Planes in (W, Zr)ssB and
(Zr, W)ssC Derived from Electron Diffraction and Refined
X-Ray Diffraction (XRD) Compared with the Corresponding
Values in Pure WB and ZrC
The planes
From electron
From XRD
From JCPDF
card 35-0738
of (W, Zr)ssB Intensity diffraction (nm) refinement (nm)
w
(112)
(103)
(105)
(200)
100
90
87
26
0.216
0.276
0.231
0.158
0.2133
0.2727
0.2291
0.1558
0.2740
0.2302
0.1566
The planes
From electron
From XRD
From JCPDF
card 35-0784
of (Zr, W)ssC Intensity diffraction (nm) refinement (nm)
(111)
(200)
(220)
(311)
100
90
62
50
0.270
0.233
0.164
0.140
0.2675
0.2316
0.1638
0.1397
0.2708
0.2345
0.1659
0.1414
Fig. 2. The XRD pattern of ZrB2—20 vol% SiC—10 vol% WC
sintered at 22001C, the position of ZrB2 peaks (JCPDF 34-0432) was
arrowed.
The error level of the data collected by electron diffraction and XRD refine-
ment were 10ꢁ3 and 10ꢁ4 nm, respectively. wThe peak was overlapped.
Powder mixtures of ZrB2–WC (ZW) and ZrB2–WC–ZrO2
(ZWO) were pressed and used to investigate the chemical reac-
tions as well as the sintering mechanism. Their ratios were set as
1:1 and 3:6:1, according to the possible reactions which are dis-
cussed in the Section III(2). These additional pellets were
heated in vacuum at 14501, 16501, or 18501C for 1 h.
The phase analysis of the as-sintered samples and additional
pellets was conducted by X-ray diffraction (XRD; D/Max-2250
V, Rigaku, Tokyo, Japan) using CuKa radiation. The micro-
structures of the polished surface were examined by SEM (JXA-
8100, JEOL, Tokyo, Japan) with EDS (Oxford Instruments,
Oxford, U.K.), and all the acceleration voltages were set as 10
keV. Commercial software package (HSC Chemistry 6.1) was
used to perform the thermodynamic calculations. The grain size
and the aspect ratio of ZrB2 grains were measured on an average
of 100 grains by the software of Image Pro-plus.
ning. The phase analysis and characterizations were performed
using a 200 kV TEM (JEM2100, JEOL), which is equipped with
an X-ray energy-dispersive detector systems (EDS Model Link-
ISIS, Oxford Instruments) and an appurtenance of scanning-
transmission electron microscope (STEM). The minimum
analyzed spot sizes of EDS were 10 nm and the atomic ratios
of interested elements are calculated according to the Cliff–
Lorimer equation from the acquired EDS spectra.
III. Results and Discussion
(1) Microstructure and Phase Identification
The typical polished surfaces of ZrB2–SiC with different
amounts of WC are shown in Figs. 1(a) and (b). Besides ZrB2
and SiC, the microstructure contains two extra phases. Accord-
ing to the chemical analysis from the EDS in Figs. 1(c) and (d)
and the slight peak shifts in the XRD pattern (Fig. 2), these
The transmission electron microscopy (TEM) samples prep-
aration was performed by standard techniques, which included
mechanical grinding, polishing, dimpling, and finally ion thin-
Fig. 3. (a) Bright field TEM image showing faceted ZrB2 grains surrounded by (Zr, W)ssC and (W, Zr)ssB, (b)–(d) and (e)–(g) are the corresponding
electron diffraction patterns recorded at different tilting angles and indexed as ZrC and WB.

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Journal of the American Ceramic Society—Zou et al.
Vol. 94, No. 5
Fig. 4. TEM image of two typical ZrB2 palates (a) and (c), Inset: their corresponding SAED patterns; The HRTEM (b) and (d) show preferential
growth directions of the elongated ZrB2 palates in (a) and (b), Inset: the preferential growth direction and the crystal morphology of ZrB2 palates.
peaks have been indexed to (W, Zr)ssB and (Zr, W)ssC.7 This
assumption is different from Chamberlain’s work, which were
indexed to WC and unknown phase.1 In order to verify our
hypothesis, detailed phase identifications were conducted by
selected area electron diffraction (SAED) with different tilting
angles from one single (W, Zr)ss B or (Zr, W)ssC grain, as shown
in Figs. 3(b)–(g). Their typical crystal plane spacing was calcu-
lated as shown in Table II. They are consistent with the XRD
Fig. 5. EDS (a) and SAED (b) patterns of (W, Zr)Si2 phase.

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Mechanisms of Self-Reinforced ZrB2-SiC Doped with WC
1579
/1–10S direction. Both of them are consistent with the growth
direction obtained from floating zone growth of ZrB2 or
21
22
HfB2 single crystals. Similar grain growth behavior was also
observed in ZSW5 sintered at 22001C.
Apart from the phase mentioned, a new type of crystalline
phase was also detected in this study. The chemical analysis of
this impurity by EDS (Fig. 5(a)) showed that the average com-
position of the unknown phase was: W: 27 at.%, Zr: 8 at.%, and
Si: 64 at.%.
In the W–Si binary phase diagram, only two thermodynamic
favorable compounds exist. One is W5Si3, and the other is WSi2.
The d-values calculated from the SAED (Fig. 5(b)) show that
the distances of the (002) and (010) planes of the unknown phase
are 0.413 and 0.316 nm, slightly higher than those in WSi2
(JCPDF 11–0195), which are 0.391 and 0.297 nm. The covalent
˚
˚
radius difference between W (1.42 A) and Zr (1.60 A) is quite
small. According to Hume–Rothery rule, we assume that a sub-
stitution of Zr to W occurred within the unit cell. The incorpo-
ration of Zr with a larger covalent radius atom into WSi2 crystal
lattice resulted in the expansion of the unit cell. On the other
hand, the ratio (W1Zr)/Si in the unknown phase was equal to
1.8, which was very close to WSi2 (2), but far deviated from
W5Si3 (0.6).
All the above discussions support that the unknown phase
is a solid solution, (W, Zr)ssSi2, in accordance with the XRD
result of ZrB2–SiC–WC composites obtained at lower sintering
temperature.7
Fig. 6. Change in the Gibbs’ free energy of reaction as a function of
temperature for reactions ((1)–(3), (5)).
refinement, slight higher (in (W, Zr)ssB case) or lower (in
(Zr, W)ssC case) than those from the corresponding values
in JCPDF cards, due to the solid solution effects. Furthermore,
these patterns (Figs. 3(b) and (f)) of low-indices basic zone
axes are substantially similar with that in pure ZrC (Fm3m
)
and WB (I41/amd). Combined with XRD analysis, the above
results showed that the incorporation of Zr did not significantly
change the crystal structure of WB, due to the approximation
of the atomic radii. Similar phenomenon also occurred to
ZrC. No ternary or other phases containing W–Zr–B–C were
identified.
(2) Reaction Path
The only reaction that has been confirmed in the ZrB2–ZrO2–
SiC–WC systems is between WC and ZrO2, with formation of
ZrC and W, reaction (1).13–14
Both the SiC and ZrB2 grains in ZSW5 and ZSW 10 are
elongated. SiC grains are similar in these two samples, while the
sizes of ZrB2 are different. The elongated ZrB2 grains in ZSW10
are nominally 10–30 mm long, with an aspect ratio of 3.8, higher
than those of ZSW5, which are 5–15 mm and 2.9, respectively.
Some typical TEM images of such ZrB2 grains are shown in
Fig. 4. Most of their interfaces are planar with clear lattice
fringes. Two kinds of typical interlayer distances (0.275,
0.159 nm) were measured from 20 randomly selected elon-
gated grains, which are consistent with the interplanar dis-
tance of the (100) and (110) planes of ZrB2, respectively. The
SAED pattern of corresponding grains can be indexed as [010]
and [110] zone spots (inset Figs. 4(a) and (c)). These results
mean that ZrB2 grains have two preferential growth direc-
tion,21 [210] and [110] (as illustrated inset in Fig. 4(b)), which
form a two dimensional network. Hence, the real morphology
of ZrB2 grains are plate rather than rod in the ZSW10 sample
sintered at 22001C.
3WC þ ZrO2 ¼ ZrC þ 3W þ 2CO ðgÞ
(1)
However, according to reaction (1), the amount of ZrO2
(1.9 mol%) in the current work is not sufficient to consume all
the added WC (11.5 mol%). Based on the reaction products of
WB and ZrC as identified, the other possible way to consume the
excess WC by reaction with ZrB2, reactions the following
reactions:
5 ZrB2 þ 4WC þ C ¼ 5ZrC þ 2W2B5
ZrB2 þ 2WC ¼ ZrC þ 2WB þ C
(2)
(3)
The changes of Gibbs free energy (dGm) for the reactions
concerned, as a function of temperature, have been plotted in
Fig. 6. From Fig. 6 it is visible that reactions (2) and (3) could
not occur below 22001C. It is noticed that carbon appears as a
product in reaction (3) and carbothermal reduction of ZrO2 (re-
action (4)) has a very large, negative dGm at lower temperature
and pressure. If reactions (3) and (4) were merged and ZrO2 was
ZrB2 grain tend to grow with c-axis perpendicular to the plate
plane, because the fastest growing planes of the IVa group dibo-
ride (ZrB2, HfB2, and TiB2) are the {010}{110} families and
the low-activation energy diffusion path is along /210S and
Table III. Summary of XRD Results for the Mixtures Containing: (1) 50 Vol% WC and 50 Vol% ZrO214; (2) ZrB2; and WC (Mole
Ratio 1:1); (3) ZrB2, WC, and ZrO2 (Mole Ratio 3:6:1), Sinterd at Different Tempreatures for 1 h in Vacuum
Sintering temperature
System
Reactions
Room temperature
14501C
16501C
18001Cw
WO
3WC1ZrO2 5 ZrC13W12CO
WC/s, ZrO2/s
WC/s, ZrOs/s
W2C/w
ZrB2/s, WC/s
W, ZrC, WC,
W2C, ZrO2
ZrB2/s, WC/s
ZrC/vw, WB/vw
ZrB2/vw, ZrC/s,
WB/s, unknown/w
W, W2C/tr
ZrC, ZrO2
ZrB2/s, WC/s
ZrC/s, WB/s
ZrC/s, WB/s
ZW
ZrB212WC 5 ZrC12WB1C
ZrB2/s, WC/s
ZWO
3ZrB216WC1ZrO2 5 4ZrC1
6WB12CO
ZrB2/s, WC/s
ZrO2/s
ZrB2/w, WC/w
ZrC/w, WB/s
w18501C for the reaction between WC and ZrO2 in Zhang et al.14 s, strong; w, weak; vw, very weak; tr, trace.

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Journal of the American Ceramic Society—Zou et al.
Vol. 94, No. 5
Fig. 7. XRD patterns for the powder mixtures containing (a) ZrB2 and WC (molar ratio 1:1); (b) ZrB2, WC and ZrO2 (molar ratio 3:6:1) sintered at
different temperatures in vacuum for 1 h. The dotted arrows indicated more W was incorporated into ZrC phase when temperature elevated.
added into reaction (4), the favorable reaction temperature be-
tween ZrB2 and WC (reaction (5)) will be decreased remarkably.
(3) Sintering Mechanisms and Formation of Interlocking
Microstructure
Some important conclusions explored in the present and previ-
ous investigations, which are relevant for the development of the
ZrB2 grains and the sintering mechanisms, are summarized as
follows:
(i) Elongated ZrB2 grains formed when WC was added. How-
ever, the same morphology configuration did not appear in the
ZrB2–SiC samples even when all the other processing conditions
were kept the same.
ZrO2 þ 3C ¼ ZrC þ 2CO ðgÞ
(4)
(5)
3ZrB2 þ 6WC þ ZrO2 ¼ 4ZrC þ 6WB þ 2CO ðgÞ
Thermodynamic calculations confirmed the above specula-
tions. Reaction (3) in the ZW system is favorable above 23001C,
while the reaction (5) in ZWO system is favorable above 18071C
in standard state. Furthermore, a mild vacuum (about 5 Pa) was
maintained during the first sintering stage, which would decrease
the favorable temperature of reaction (5), from 18071C at 1 atm
to 12341C at 5 Pa. Because no gaseous phase is released from
reaction (3), the vacuum will not affect the reaction temperature
in ZW systems.
To verify the occurrence of the above mentioned reactions,
powder mixtures (Table III) were heated at lower temperature,
to analyze the crystalline phases formed at 14501, 16501, and
18501C. Stacked XRD patterns of the starting compacts and
those treated at different temperatures are presented in Fig. 7.
The phases formed during 1 h isothermal holds at different
temperatures have been summarized and compared in Table III.
The reaction between ZrB2 and WC is really difficult. In ZW,
besides ZrB2 and WC, no other phases were observed until the
temperature reached 16501C, and the reaction was not com-
pleted even after isothermal holds at 18001C. In ZrO2–WC sys-
tems, WC seems to decompose to W2C and C first. Though WC
appears effective for removing ZrO2, traces of W2C were still
observed after permanence at 21001C. The most effective way
for removing ZrO2 appeared in ZWO, in this system, WB and
ZrC were formed and changed to the major phases at the tem-
perature as low as 14501C. After the holding at 18001C, only
WB and ZrC were left. Furthermore, the peak shift (at 55.3231
for ZrC (220)) in ZWO increased with the temperature (as ar-
rowed in Fig. 7(b)), indicating the formation of (Zr, W)ssC at the
same time.23,24
ZrO2, well known as impurity in borides, served as a catalyst,
which accelerates the reaction between WC and ZrB2. Based on
the above analysis, two different reaction paths of WC in ZrB2
(ZrO2)–SiC can be imagined: (a) when enough ZrO2 exists on
the surface of starting ZrB2 particles, WC will first react with
ZrO2 to form ZrC and W as indicated by reaction (1); (b) when
limited ZrO2 was around the ZrB2 grains, the ternary reaction
between ZrB2, WC and ZrO2 takes place, and thus WB and ZrC
can be found in the sample according to reaction (5).
Fig. 8. The BE-SEM pictures of ZrB2–SiC—10 vol% WC sintered at
20001C (a) and 22001C (b). Note the different morphologies of the
brightest phase (W, Zr)ssB in the two images: rounded at 20001C, and
irregular shaped at 22001C, typical of crystallization from a liquid phase.
The as-formed W from reaction (1) could either react with
SiC to form WSi2 or could be incorporated into the ZrB2 or ZrC
lattice, as confirmed by XRD patterns and EDS results.

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Mechanisms of Self-Reinforced ZrB2-SiC Doped with WC
1581
Fig. 9. Typical microstructures of (W, Zr)ssB (a, d, i) and(W, Zr)ssSi2 (b, c). (e)(g) showed that ZrB2 grains were wetted by (W, Zr)ssB. Example of a
WB/ZrB2 interface showing a clean grain boundary (h). Insets in (a) and (i) are the electron diffraction of ZrC and faceted ZrB2 grain.
(ii) The aspect ratio of the ZrB2 grains could be tailored by
the sintering temperature or the WC addition. The increasing
WC amount accelerates the anisotropic grain growth of ZrB2.
When grains are immersed in a liquid phase for enough time,
which is in the chemical equilibrium with the grains, the shape of
the grain as well as its crystallographic orientations can be de-
termined by Wulffs rules. It can be described by the following
reaction25:
(4) Possible Liquid Phases at 22001C
Besides SiC and ZrB2, the sintered ceramic composite contains
amounts of (W, Zr)ssSi2, (W, Zr)ssB, and (Zr, W)ssC. The melt-
ing point (Tm) of WSi2 is 2150 1C, the Tm of Zr is only 19201C,
and we are not aware of any ternary compound existing in the
W–Si–Zr system was reported. For subternary system WSi2–Si–
Zr (along Si–Zr side and at Si corner in the W–Si–Zr system), it
is possible to form two pseudo-binary systems WSi2–Si4Zr5 and
WSi2–Zr (all with melting points) in case of no intermediate
compounds was formed. According to the principle of phase
diagram topology, addition of Zr into WSi2 will lower its melt-
ing temperature as liquidus line shifting away from Tm 5
21601C. As such, the melting points of corresponding solid
solution of (W, Zr)Si2 will be below 21601C. Thus, it must be
molten at the sintering temperature (22001C).
g1=h1 ¼ g2=h2 ¼ gi=hi
(6)
where, hi represents the distance perpendicular from the crystal
center to the plane and gi is the specific interfacial energy of the
crystallographic plane i.
ZrB2 has a hexagonal crystal structure (P6/mmm), benefiting
from this point, its interface free energy is anisotropic. Based on
Wulffs rules and the above observations, it is natural to consider
that the reasons for the elongation of ZrB2 grains could be that:
(1) liquid phase(s) was formed in ZrB2–SiC–WC and the liquid
phase must be associated with WC additions and temperature;
(2) the liquid phase(s) could well wet the matrix, so the matter
transfer process between the matrix and the liquid can easily
occur. Some evidence concerning the above assumptions was
considered as below.
The measured Tm of (Zr, W)ssC is as high as 29001C.26 Thus,
the task here is to verify the existence of liquid (W, Zr)ssB phase
at or below the sintering temperature, for no literatures men-
tioned its eutectic point. It is interesting that the microstructure
features of ZSW sintered at 20001 and 22001C, especially
the distributions of (Zr, W)ssC and (W, Zr)ssB phases are quite
different.
At 20001C, as supported by reaction (5), (Zr, W)ssC and (W,
Zr)ssB usually appeared in the same region, at ZrB2/ZrB2 or

1582
Journal of the American Ceramic Society—Zou et al.
Vol. 94, No. 5
Fig. 10. The bright field image (a) and high-angle annular dark-field imaging (HADDF, b) of elongated ZrB2 grains. The line scan (dotted in (b)) of W
content and the spot analysis (circled in b) of the molar ratio between W and Zr are shown in (c) and (d), respectively.
ZrB2/SiC grain boundaries, adjacent to each other (as arrowed
in Fig. 8(a)). Nevertheless, the regions rich in (Zr, W)ssC are
poor in (W, Zr)ssB, when the sintering temperature increases to
22001C (Fig. 8(b)). Such grain arrangement and movement were
nearly impossible to be completed in a solid state sintering pro-
cess due to the limited diffusion. A direct evidence for the liquid
phase of (W, Zr)ssB is described in Fig. 9(a). Small (Zr, W)ssC
grain was found to be immersed in (W, Zr)ssB grain, strongly
supporting the melting of (W, Zr)ssB that occurred during this
stage.
reaction (1) and ZrO2 content, 5 vol% WC is enough to con-
sume all the ZrO2. Thus, though the addition of WC increased
from 5 to 10 vol%, the resultant amount of (W, Zr)ssSi2 was
unchanged. It could not explain the anisotropic grain growth of
ZrB2 accelerated at the same time. Second, the observed (W,
Zr)ssSi2 was limited and ZrB2 grains were not well wetted by it
according to the relatively large dihedral angles in the range
from 301 to 601 measured in their solid/liquid grain boundaries.
Compared with (W, Zr)ssSi2, the appearance of (W, Zr)ssB-
related liquid phase was more reasonable to explain the forma-
tion of plate-like ZrB2, as discussed below:
(i) Wetting or dewetting of grain boundary. Some detailed
microstructures of (W, Zr)ssB, obtained from ZSW10 are shown
in Figs. 9(a), (d)–(g). Most of their interfaces are flat sintered at
20001C (Fig. 9(d)), while curved with concaveness toward itself
when the sintering temperature was elevated to 22001C (Figs.
9(a), (e)–(g)). This suggests a tendency of (W, Zr)ssB from
dewetting (Fig. 9(d)) to wetting of grain boundaries (Figs.
9(a), (e)–(g)), which favored in the elongation of ZrB2 grains
at the same time. Furthermore, clean grain boundary between
(W, Zr)ssB and ZrB2 was observed (Fig. 9(h)).
(5) ZrB2 Grains Evolution in the Liquids
The existence of (W, Zr)ssSi2 grains was scarce in all the as-
observed areas. Among the (W, Zr)ssSi2 grains, there are only
two different configurations. One is located near the SiC grains
(Fig. 9(b)), indicating that the formation of (W, Zr)ssSi2 should
be related to SiC and the following diffusion of Zr (from ZrB2
or ZrC) into WSi2. Another configuration has been found at
the triple junction point (Fig. 9(c)). To segregate all the silicides
into the triple points by single solid diffusion is very unlikely
without the help of capillary force induced by means of liquid,
again, and the microstructure observed above strongly suggests
(W, Zr)ssSi2 to be a liquid phase that appeared in the matrix.
However, when such kind of liquid phase was used to explain
the elongation of ZrB2 grains, some arguments against this
hypothesis are raised.
(ii) The solubility and matter transfer process between the
liquid phase and the matrix.
Some W and Zr were incorporated into ZrB2 and WB, as
confirmed by Fig. 1. It demonstrated that ZrB2 and the liquid
phase were mutually soluble. Another direct evidence for such
matter transfer process was the inhomogeneous distribution
of W in elongated ZrB2 grains. Figs. 10(a) and (b) are the
bright field images and high-annular angle dark-field images
(HADDF) at the same region. HADDF is highly sensitive to
variations in the atomic number of atoms in the sample. By us-
First, one of the most probable way for the formation of (W,
Zr)ssSi2 relied on the reaction between SiC and W. Large
amount of SiC exists in the matrix, whereas W from the reac-
tion between ZrO2 and WC is substantially limited. Based on

May 2011
Mechanisms of Self-Reinforced ZrB2-SiC Doped with WC
1583
ing of it, it is easy to distinguish the phases of (W, Zr)ssB and (W,
Zr)ssSi2 (A0 and B0 as arrowed in Fig. 10(b)), while nearly no
contrast difference could be observed from their corresponding
bright field images (A and B as arrowed in Fig. 10(a)). Further-
more, both of the line scan (W) result (Fig. 10(c)) and the spot
analysis (the molar ratio of W and Zr, Fig. 10(d)) show a com-
position change of W in ZrB2 grains was existed, normally lower
in the center and higher near the edge. So, the matter transfer
process indeed existed in these two phases as confirmed by the
process of cations interdiffusion.
identify the sources and the composition of the liquid phases and
it to advance plausible hypotheses on the driving force which led
to the anisotropic grain growth of ZrB2 grains.
Acknowledgment
The authors thank Mrs. Mei-Ling Ruan and Mr Qiang Zheng at Shanghai
Institute of Ceramics for their help in HRTEM measurements and analysis.
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The following conclusions are obtained based on the micro-
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(1) The actual morphology of ZrB2 grains in ZrB2–SiC–WC
sintered at 22001C was platelets with preferential growth direc-
tion normal to c-axis, and the aspect ratios could be further
tailored by the WC amount.
(2) Thermodynamic predictions and XRD analysis confirmed
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ZrO2 by single WC, and another one is the reaction in ZrO2–
WC–ZrB2. Here, ZrO2 plays an important role on the reaction
process, for it can decrease the favorable reaction temperature
between WC and ZrB2, from 23001 to 12341C.
(3) The densification of ZrB2–SiC doped by WC was en-
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be either (W, Zr)ssB or (W, Zr)ssSi2. When (W, Zr)ssB was con-
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&