Reactive hot pressing of ZrB2-SiC-ZrC ultra high-temperature ceramics at 1800°C

Article abstract of DOI:10.1111/j.1551-2916.2006.01145.x

A ZrB₂-SiC-ZrC composite was prepared from a mixture of zirconium, silicon, and B₄C via reactive hot pressing at a relatively low temperature (1800°C) for 60 min under 20 MPa in an argon atmosphere. The relative density was 96.8%, the micro-hardness (Hv10) was 16.7 GPa, and the fracture toughness was 5.1 MPa-m^1/2. The presence of ZrC was helpful for the densification process and improved the mechanical properties of the composite. A model of the microstructure development of the composite was proposed to explain the phase distribution.

Full text of DOI:10.1111/j.1551-2916.2006.01145.x

J. Am. Ceram. Soc., 89 [9] 2967–2969 (2006)
DOI: 10.1111/j.1551-2916.2006.01145.x
r 2006 The American Ceramic Society
ournal
J
Reactive Hot Pressing of ZrB2–SiC–ZrC Ultra High-Temperature
Ceramics at 18001C
Wen-Wen Wuz, Guo-Jun Zhang,*,w Yan-Mei Kan, and Pei-Ling Wang
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics,
Shanghai 200050, China
A ZrB2–SiC–ZrC composite was prepared from a mixture of
zirconium, silicon, and B4C via reactive hot pressing at a
relatively low temperature (18001C) for 60 min under 20 MPa
in an argon atmosphere. The relative density was 96.8%, the
micro-hardness (Hv10) was 16.7 GPa, and the fracture tough-
The addition of ZrC to ZrB2–SiC to form a ternary composite
of ZrB2–SiC–ZrC can tailor the microstructure and properties
of ZrB2–SiC, especially the superior resistance to oblation at a
high temperature.10 In the current work, a composite in the
ZrB2–SiC–ZrC system was prepared by RHP, using Zr, Si, and
B4C as starting powders according to reaction (2):
ness was 5.1 MPa m1/2. The presence of ZrC was helpful for
.
the densification process and improved the mechanical proper-
ties of the composite. A model of the microstructure develop-
ment of the composite was proposed to explain the phase
distribution.
ð2 þ xÞZr þ ð1 ꢁ xÞSi þ B4C
¼ 2ZrB2 þ ð1 ꢁ xÞSiC þ xZrC
(2)
When x 5 0, reaction (2) reduces to reaction (1). In the
present work, we took x 5 0.20 based on calculations for
maintaining the content of SiC at 20 vol%. The calculated
volumetric composition is (vol%) 73.6ZrB2120SiC16.4ZrC.
The theoretical density of the composite calculated according
to the rule of mixtures is 5.55 g/cm3, based on the densities of
6.09, 3.21, and 6.44 g/cm3 for ZrB2, SiC, and ZrC, respectively.
The true density should be lower than the calculated one due to
the impurities (mainly Ti). For comparison, a ZrB2–SiC com-
posite without ZrC (when x 5 0) was also prepared. Reactions
(1) and (2) are thermodynamically favorable and exothermic. In
this communication, the samples were produced under relatively
mild conditions (18001C), and the manufacturing process, me-
chanical properties, and microstructure of the composites are
reported, and the effect of ZrC on sintering is discussed.
I. Introduction
IRCONIUM diboride (ZrB2) and hafnium diboride (HfB2) are
ultra high-temperature ceramics (UHTCs) that have a
Z
number of unique properties, including extremely high melting
temperature and hardness, low volatility, and high thermal and
electrical conductivity.1 Studies have shown that composites of
diborides with silicon carbide (ZrB2/SiC and HfB2/SiC) are
candidates for ultra high-temperature applications, because the
addition of SiC can improve the oxidation resistance.2,3 The
presence of SiC can also increase the strength of the materials
by acting as a grain-growth inhibitor.4
For most of the reported studies, ceramic composites have
been fabricated simply by hot pressing from commercially
available powders. Reactive hot pressing (RHP) is an alternative
route. It can produce materials with novel and controlled
microstructures, with high chemical compatibility of the in
situ-formed individual phases, and phase distribution unifor-
mity.5 In boride-containing composites, for example, TiB2–SiC
binary composites and TiB2–Ti(C,N)–SiC ternary composites
have been prepared from a mixture of TiH2, Si, and B4C.6–8
For zirconium boride-containing UHTCs, a high-strength
ZrB2–SiC composite has been prepared by RHP at 19001C for
60 min under 30 MPa pressure from a mixture of Zr, Si, and
B4C according to reaction (1).9 The relative density of this
composite is 97.67% with a Vickers hardness of 21.0 GPa and
II. Experimental Procedure
The starting powders were Zr (purity 95.82%, impurities include
Ti 2.34, Hf 0.52, Fe 0.24, W 0.08, Cr 0.06, Ni 0.03, O 1.09,
particle size o25 mm, Guoyao Chemicals Co. Ltd., Shanghai,
China), Si (purity499%, particle size o50 mm, Yinfeng Silcon
Co. Ltd., Jinan, China), and B4C (purity 99%, particle size
about 2 mm, Jingangzuan Boron Carbide Co., Ltd, Mudanjiang,
China). The stoichiometric powders were mixed and ground in
ethanol in an agate mortar until all of the ethanol was evapo-
rated. The grinding process was repeated once for a total
grinding time of about 1 h. The mixed powder was then dried
and placed in a graphite die with a BN coating. The composite
was then RHPed at a temperature of 18001C for 60 min under a
pressure of 20 MPa in an argon atmosphere. A slow heating rate
(101C/min) was adopted to prevent the reaction from becoming
self-sustaining. The application of pressure was initiated at
15501C. The reacted disk had dimensions of 20 mm (diameter)
and 5 mm (thickness).
a facture toughness of 4.0 MPa ꢀ m1/2
.
2Zr þ Si þ B4C ¼ 2ZrB2 þ SiC
(1)
W. Fahrenholtz—contributing editor
After removing the surface layer from the hot-pressed disk by
grinding, the bulk density was measured using the Archimedes
method. Phase composition was determined by X-ray diffracto-
metry (XRD) using CuKa radiation. The disk was ground with
SiC abrasives and then polished using a diamond pastor of
1 mm. The hardness and fracture toughness were measured by
the indentation method, using a load of 10 kg for 10 s on a
Manuscript No. 21572. Received March 10, 2006; approved April 26, 2006.
This work was financially supported from the Chinese Academy of Sciences under the
Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the
National Natural Science Foundation of China, and the State Key Laboratory of High
Performance Ceramics and Superfine Microstructures of Shanghai Institute of Ceramics.
*Member, American Ceramic Society.
zwAuthor to whom correspondence should be addressed. e-mail: gjzhang@mail.sic.ac.cn
Graduate School of the Chinese Academy of Sciences, Shanghai, China.
2967

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Vol. 89, No. 9
ZrB2
ZrB2
ZrC
Zr
Si
SiC
ZrC
(a)
(b)
20
30
40
50
60
70
2 theta (degree)
20
30
40
50
60
70
Fig. 2. X-ray diffractometry pattern of the mixed powder for preparing
a ZrB2–SiC–ZrC composite after heat treatment at 8001C for 60 min in
an argon atmosphere.
2 theta (degree)
Fig. 1. X-ray diffractometry patterns of the reactive hot-pressed com-
posites: (a) ZrB2–SiC–ZrC; (b) ZrB2–SiC.
process,6,11 reactions (1) and (2) may take place in steps as
follows:
polished surface; the reported value was an average of five
measurements. Scanning electron microscopy (SEM) was
performed to observe the microstructure of the composite.
3Zr þ B4C ¼ 2ZrB2 þ ZrC
(3)
(4)
2ZrC þ B4C þ 3Si ¼ 2ZrB2 þ 3SiC
III. Results and Discussion
The above reactions have large negative Gibbs free energies
(DG298 (3) 5 ꢁ767.641, DG1800 (3) 5 ꢁ703.078, DG298 (4) 5
ꢁ395.212, DG1800 (4) 5 ꢁ320.257 kJ/mol), showing that the
reactions are all thermodynamically favorable.
According to the XRD patterns shown in Fig. 1, phases present
in the composites agree with those predicted from reactions (1)
and (2). This observation means that either ZrB2–SiC or ZrB2–
SiC–ZrC can be produced by in situ reaction.
The experimental values of some properties of both compo-
sites are compared in Table I. The relative density (96.8%) of the
ZrB2–SiC–ZrC composite was higher than that of the ZrC-free
composite. This indicates that the presence of ZrC improves the
densification of the material, even though the reactants of the
two systems are similar.
Compared with the hardness of 21.0 GPa of the composite we
mentioned above,9 the micro-hardness (Hv10) of the composite
is not high as listed in Table I. The main reasons for this may be
the inhomogeneous microstructure and the porosity in the
composites. The regions that are rich in SiC will have a higher
hardness than the other regions according to a similar system of
TiB2–SiC–TiC.12 The fracture toughness of the composite ZrB2–
SiC–ZrC is 5.1 MPaꢀ m1/2, which was higher than that measured
for ZrB2–SiC (4.5 MPa ꢀ m1/2); the formation of ZrC as the third
phase improved the fracture toughness of the composite. It is the
same case in the TiB2–SiC–TiC system.12 The authors of this
study concluded that toughness was lower for ZrB2–SiC because
of little interaction between the phases, but the presence of ZrC
induces strong bonds with both ZrB2 and SiC.
The microstructure of ZrB2–SiC–ZrC composite is shown in
Fig. 3. This picture indicates that the distributions of the in situ-
formed ZrB2, SiC, and ZrC phases in the composite are not
homogenous. The particle size of SiC and ZrC is similar and
generally small, about 1–3 mm, whereas that of ZrB2 is large,
which is about 3–10 mm. SiC and ZrC are mainly located at the
ZrB2/ZrB2 grain boundaries, but the phases do not appear in the
same region. Namely, regions rich in SiC are poor in ZrC. This
supports the conclusion that the SiC phase is formed from ZrC.
Figure 4 shows a schematic of a possible formation sequence
during RHP. For the model, it is assumed that B and C atoms
from B4C diffuse faster than Zr and Si.9,13 Also, Si has a lower
reactivity than Zr, so ZrB2 and ZrC formed before any Si
compounds. At higher temperatures, SiC formed in situ as a
result of the reaction between Si, ZrC, and the residual B4C.
Owing to the large particle sizes of the Zr starting powder and
its agglomeration, the size of the resulting ZrB2 grains are large
Figure 2 shows the XRD pattern of the intermediate products
in the ZrB2–ZrC–SiC system heat treated at 8001C for 60 min
in an argon atmosphere. It can be found that ZrC formed at a
relatively low temperature of 8001C. As the temperature
increased, ZrB2 became the main phase; in addition, SiC
appeared and the amount of ZrC decreased, based on the
peak intensity in the XRD patterns. By analyzing the reaction
Table I. Characteristics of the Obtained Composites
Relative
density
(%TD)
Fracture
Density
(g/cm3)
Hardness
(GPa)
toughness
(MPa ꢀ m1/2
Property
)
Fig. 3. Backscattered electron images of a polished surface of the
reactive hot-pressed composite. The gray phase is ZrB2, the dark phase
is SiC, and the white phase is ZrC.
ZrB2–SiC
ZrB2–SiC–ZrC
5.09
5.37
94.8
96.8
13.8
16.7
4.5
5.1

September 2006
Communications of the American Ceramic Society
2969
The microstructure of the composite was not homogeneous
owning to the coarse particle size of the starting powders. The
reaction took place in steps. First, B4C reacted with Zr to form
ZrB2 and ZrC at a very low temperature. At a higher tempera-
ture, SiC was formed in situ by the reaction of Si with ZrC and
the residual B4C.
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Fig. 4. Microstructure formation mechanism of the ZrB2–ZrC–SiC
composite in the reaction-synthesis process, depicting the conversion
from (a) the powder compact to (b) the intermediate state, and (c) the
final microstructure of the composite.
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IV. Summary
A ZrB2–SiC–ZrC composite was prepared by the RHP of a
mixture of zirconium, silicon, and B4C powders. The hardness
and the fracture toughness of the product measured were 16.7
GPa and 5.1 MPa ꢀ m1/2. The presence of ZrC improves the
densification process and fracture toughness of the composite.
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&