Reactive hot pressing of ZrC - SiC ceramics at low temperature

Article abstract of DOI:10.1111/jace.12090

Composites of ZrC-SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC_1-x, SiC, and ZrSi. Low-tempera

Full text of DOI:10.1111/jace.12090

J. Am. Ceram. Soc., 96 [1] 32–36 (2013)
DOI: 10.1111/jace.12090
© 2012 The American Ceramic Society
Rapid Communication
ournal
J
Reactive Hot Pressing of ZrC–SiC Ceramics at Low Temperature
Xin-Gang Wang, Guo-Jun Zhang,*,† Jia-Xiang Xue, Yun Tang, Xiao Huang,
Chang-Ming Xu, and Pei-Ling Wang
State Key Laboratory of High Performance Ceramics and Superfine Microstructures,
Shanghai Institute of Ceramics, Shanghai 200050, China
Composites of ZrC–SiC with relative densities in excess of
98% were prepared by reactive hot pressing of ZrC and Si at
temperature as low as 1600°C. The reaction between ZrC and
Si resulted in the formation of ZrC1−x, SiC, and ZrSi. Low-
temperature densification of ZrC−SiC ceramics is attributed to
the formed nonstoichiometric ZrC1−x and Zr–Si liquid phase.
Adding 5 wt% Si to ZrC, the three-point bending strength
of formed ZrC0.8–13.4 vol%SiC ceramics reached 819
102 MPa with hardness and toughness being 20.5 GPa and
3.3 MPaÁm1/2, respectively.
ZrC–SiC composites enhances the sinterability of ZrC ceram-
ics and results in ZrC–SiC composites with improved
mechanical properties.15 However, the research studies deal-
ing with reactive hot pressing of ZrC–SiC composites have
been very few. In this work, the investigation of ZrC–SiC
composites fabricated by reactive hot pressing (RHP) using
ZrC and Si as starting materials was carried out. The reac-
tion process and the densification behavior during the RHP
were also discussed.
II. Experimental Procedure
I. Introduction
The starting powders were ZrC (99% purity, 0.5–3 lm,) and
Si (>99% purity, <50 lm; Yinfeng Silcon Co. Ltd., Jinan,
China), in which ZrC was synthesized by carbothermal
reduction as described in the previous work.11 To decrease
the particle size of Si, the powder was milled for 8 h using
Si3N4 balls as the milling medium to obtain a final particle
size of about 1–3 lm. ZrC without and with different content
(2.5, 5.0, 10 wt%) of Si were mixed by mixing in ethanol for
24 h in a plastic bottle and the corresponding samples were
designated as ZS0, ZS2.5, ZS5, and ZS10, respectively.
A rotary evaporator was used to remove the ethanol at 70°C.
The powder mixtures were sieved to 200 mesh. The powders
were then compacted in a graphite die lined with a graphite
foil and coated with BN. The compacts were hot-pressed at
temperatures ranging from 1300°C to 2000°C in 100°C incre-
ments to produce samples with dimensions of 22 mm in
diameter for investigation of the reaction process and densi-
fication behavior. Rectangular samples with dimensions of
37 mm (length) 9 30 mm (width) 9 5 mm (high) were
prepared for mechanical properties measurements. A heating
rate of 20°C/min was used and a pressure of 40 MPa was
applied from 1350°C. The atmosphere was vacuum (<10 Pa)
under 1000°C and then switched to flowing argon. After
holding at soaking temperatures for 60 min, the applied pres-
sure was removed and the furnace was cooled naturally to
room temperature. The bulk densities of sintered ceramics
were measured using the Archimedes method. The final rela-
tive densities were determined as the ratio of experimental
bulk densities to theoretical ones calculated from the rule of
mixtures based on the final phase content according to XRD
analysis results at 1600°C. The theoretical density of ZrC1Àx
from reference,16 ZrSi (5.66 g/cm3) from JCPDS Card
72-2031, and SiC (3.21 g/cm3) from JCPDS Card 75-0254
were used to calculate the theoretical densities of the
obtained ZrC-based ceramics. Phase composition was deter-
mined by X-ray diffraction (XRD; D/max 2550 V, Rigaku Co.,
Tokyo, Japan). The XRD profiles of the products were
recorded through a Huber G670 imaging plate Guinier camera
(CuKa1, Ge monochromator, 40 KV, 30 mA) with internal
standard LaB6 (k = 4.15692 A). The lattice parameters of ZrC
phase were determined by indexing and least-squares refine-
ment with the MDI Jade5.0 software.17 The microstructure
IRCONIUM carbide is an important member in the family
of ultra-high temperature ceramics duo to its high melt-
Z
ing point, high hardness, good thermal shock resistance, and
solid-state phase stability.1,2 Consequently, ZrC could be a
good potential material for structural components used in
next-generation rocket engines and hypersonic spacecraft.3 In
addition, ZrC is being considered as one of the possible
materials for the inert matrix fuels (IMF) in the generation-
IV nuclear reactor systems, due to its excellent neutronic and
high-temperature mechanical properties as well as resistance
to corrosion by fission products.4,5 During the preparation of
IMF, low temperature is generally needed to maintain the
stability of fuels and avoid the reaction between the matrix
and the fuel materials.6 However, ZrC has poor sinterability,
mainly owing to its strong covalent bond and low self-diffusion
coefficient. In general, pressure-assisted techniques and high
sintering temperatures (>2000°C) are applied to obtain dense
ZrC bodies by hot pressing from commercially available pow-
ders.7 Without any sintering aids, ZrC ceramics with relative
density ranging from 94% to 97% can be obtained by hot
pressing under 30–40 MPa pressures at temperatures higher
than 2200°C.8,9 To reduce the sintering temperature required
for densification of ZrC, metal and nonmetal sintering aids
such as Nb, MoSi2, and VC have been chosen.2,10,11
In addition to the additive approach that is commonly
used to obtain dense ZrC-based ceramics, reactive hot press-
ing (RHP) is another way to obtain high-density ceramics,
which has an advantage of producing ceramics at reduced
temperatures compared with non-reactive processes.12,13 ZrC
ceramics prepared by RHP method using Zr and C as raw
powders has been reported.14 The results showed that nearly
fully dense ZrC with fine grains was obtained at tempera-
tures as low as 1200°C–1600°C. The addition of SiC to form
G. Hilmas—contributing editor
Manuscript No. 31633. Received June 25, 2012; approved October 18, 2012.
*Member, The American Ceramic Society
†Author to whom correspondence should be addressed. e-mail: gjzhang@mail.sic.ac.cn
32

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Rapid Communications of the American Ceramic Society
33
of fracture surfaces was observed by scanning electron
microscopy (SEM; Hitachi S-570, Hitachi Ltd., Tokyo,
Japan) in secondary electron mode. The backscattered elec-
tron (BSE) images were obtained by electron probe microan-
alyzer (JXA-8100F, JEOL, Japan). Before the observation,
the samples were polished with a diamond paste to 0.5 lm.
The chemical compositions of polished surfaces were then
analyzed with energy-dispersive spectroscopy (EDS, Oxford
INCA energy) that was linked to the electron probe microan-
alyzer. Quantitative analyses of phases were done using
Image pro Plus 5.0 program. The hardness was measured by
the Vickers indentation method (Instron Wilson-Wolpert
Tukon 2100B, Instron Co., Norwood, MA) using a load of
9.8 N and a dwell time of 15 s on a polished surface. The
indentation fracture toughness was calculated according to
the equation of Evans.18,19 The values of hardness and frac-
ture toughness were based on an average of five measure-
ments for each specimen. Flexural strength was examined by
a three-point bending test on bars with dimensions of
2 mm 9 2.5 mm 9 25 mm. The testing span and crosshead
speed used were 20 mm and 0.5 mm/min, respectively. The
average flexural strength of ZS5 was obtained based on the
measurements of six bars.
tures are increased from 1500°C to 1600°C, which is well
agreed with the shrinkage of samples during hot-pressing
processes presented by displacement curves versus sintering
time under different temperatures, as shown in Fig. 1(b). For
ZS0 and ZS2.5, the increase in sintering temperature does
not make the samples fully dense, as the relative densities of
ZS0 and ZS2.5 are less than 83% and 95% even after sinter-
ing at 1800°C and 2000°C, respectively, as shown in
Fig. 1(a). The relative density changes can also be confirmed
by the microstructure images. The fracture surface of ZS5
and ZS10 sintered at 1500°C and 1600°C are shown in
Fig. 2. A large amount of small pores can be found on the
fracture surface of ZS5 [Fig. 2(a)] and ZS10 [Fig. 2(c)] sin-
tered at 1500°C, while the grains are homogeneously distrib-
uted with average size less than 1 lm for both samples. For
increasing the sintering temperature to 1600°C, almost no
pores can be found from the microstructures of both ZS5
and ZS10, as shown in Figs. 2(b) and (d), respectively, which
is in accordance with the relative density data as mentioned
above.
X-Ray Diffraction patterns of the ZS5 and ZS10 sintered
at different temperatures are shown in Fig. 3. The disappear-
ance of Si diffraction peaks in XRD patterns of ZS5 sintered
at 1300°C, as shown in Fig. 3(a), implies that the reaction
between ZrC and Si occurred starting from below this tem-
perature. On the other hand, ZrSi was detected in ZS5 and
ZS10 sintered at 1300°C, whose intensities of XRD peaks
decreased a little bit with the increase in temperature up to
1600°C, especially for ZS5. The second phase b-SiC (JCPDS
Card 65-0360) was identified clearly in both ZS5 and ZS10
sintered at 1500°C and 1600°C, while the XRD peak inten-
sity of b-SiC increased from 1500°C to 1600°C. According to
the results of phase identification of ZS5 and ZS10, the reac-
tions between ZrC and Si could be assumed as the following
two processes. First, part of ZrC reacted with Si to form
ZrSi and SiC. With the increase in temperature, carbon atom
from the other part of ZrC diffused into ZrSi phase to form
ZrC1Àx and SiC. The assumption could be confirmed by the
determination of lattice parameter and microstructure analy-
ses. The lattice parameter of ZrC in ZS5 and ZS10 sintered
at 1300°C is 4.697(3) and 4.696(2) A, respectively (see
Table I, the values in the parenthesis represent the standard
deviation of the lattice parameter), which is about the same
as that of the raw ZrC powders (4.697(1) A), revealing that
the carbon defect concentration in ZrC1Àx lattice was very
low. With the sintering temperature increase up to 1500°C,
the lattice parameters of ZrC1Àx in ZS5 and ZS10 decease to
4.695(1) and 4.693(1) A, respectively, implying the formation
III. Results and Discussion
The variation of the relative densities for ZS0, ZS2.5, ZS5,
and ZS10 with sintering temperatures is shown in Fig. 1(a).
It is noted that the relative densities of ZS5 and ZS10 sin-
tered at 1600°C and 1700°C are very close (98% and 99%,
respectively) and much higher than that of ZS0 and ZS2.5
at the same temperatures. There is a remarkable jump in
relative density of ZS5 and ZS10 when the sintering tempera-
(a)
(a)
(b)
(b)
(c)
(d)
Fig. 1. The relative density versus sintering temperature of ZrC–
SiC–based ceramics (a) and the displacement curves of the punch of
ZS10 versus sintering time at 1500°C and 1600°C (b).
Fig. 2. The fracture surface of ZS5 and ZS10 sintered at different
temperatures. (a) ZS5°C–1500°C, (b) ZS5°C–1600°C, (c) ZS10°C–
1500°C, (d) ZS10°C–1600°C.

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Vol. 96, No. 1
(a)
(a)
(b)
(b)
Fig. 4. The polished surface of ZS5 (a), ZS10 (b) sintered at 1600°C,
and EDS spectra.
Fig. 3. XRD patterns of ZS5 (a) and ZS10 (b) sintered at the
different temperatures.
particles (average size about less than 500 nm) inside SiC
grains should be the same with the large ones, but probably
include some amount of residual ZrSi. In addition, it is
noticed that the amount of ZrSi in the samples is affected by
the Si content in the starting composition. For Si content of
less than 5 wt%, only a trace of ZrSi could be formed, as
shown in Fig. 4(a), indicating that almost all the ZrSi reacted
with ZrC to form ZrC1Àx and SiC. According to the theoreti-
cal calculation that if all the added Si reacted with ZrC in
ZS2.5 and ZS5, the final composition of the obtained ceram-
ics would be ZrC0.9–7.5 vol%SiC and ZrC0.8–13.4 vol% SiC,
respectively. With an increase in the Si content up to 10 wt
%, the ZrSi content increased to greater than 10 vol%. The
mass fraction of the ZrSi phase in ZS10 was calculated by
using the K value method based on the relative intensity of
the strongest diffraction peak of ZrC and ZrSi and then con-
verted to volume fraction.20 The final composites of ZS10
sintered at 1600°C was ZrC0.8–16.7 vol% SiC–10.9 vol%
ZrSi. The image analysis method was also used to quantify
the phase content of ZS5 and ZS10 and the results were
Table I. The Lattice Parameters of ZS5 and ZS10 at
Different Temperatures
Lattice parameters (A) of ZrC1Àx
Sample
at 1300°C
at 1500°C
at 1600°C
ZS5
ZS10
4.697(3)†
4.696(2)
4.695(1)
4.693(1)
4.688(1)
4.688(1)
†The values in the parentheses represent the standard deviation of the
lattice parameters of ZrC1Àx
.
of ZrC1Àx. For ZS5 and ZS10 sintered at 1600°C, the lattice
parameter of ZrC1Àx both decrease to 4.688(1) A, which is
corresponding to that of ZrC0.8, (4.687 A).9 The carbon
defect concentration changes in ZS5 and ZS10 could confirm
the formation of nonstoichiometric ZrC1Àx in reaction
process.
shown in Table II. The final composition of ZS5 was ZrC0.8
–
The BSE images of polished surfaces for ZS5 and ZS10
and their EDS analysis are shown in Fig. 4. The phase with
light color is ZrC1Àx matrix, while the phase with 2–15 lm
grains and dark color is SiC. The ZrSi phase with gray color
can be seen in the BSE image for sample ZS10 [Fig. 4(b)].
From the polished surface of ZS5 and ZS10, it can be found
that some small grains (particle size less than 1.0 lm) existed
in the SiC grains. According to the image contrast and the
EDS line-scanning analysis (Fig. 5), the larger particles (aver-
age size about 0.5–1.0 lm) with high Zr content and low Si
content inside the SiC grains should be ZrC1Àx. The smaller
10.0 vol%SiC–2.6 vol%ZrSi, while the composition of ZS10
was ZrC0.8–14.5 vol%SiC–13.2 vol%ZrSi. It indicates that
there is still 2.6 vol% ZrSi in ZS5 sample sintered at 1600°C,
which is in accordance with the phase diagram of Zr–Si–C,21
although it is difficult for XRD method to detect such a trace
of ZrSi.
From the phase evolutions, the densification improvement
of ZrC with Si addition was owing to the formation of non-
stoichiometric ZrC1Àx. Based on our previous work,16 the
densification of ZrC was affected by the carbon defects in
the ZrC lattice, which not only promoted mass transfer

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Rapid Communications of the American Ceramic Society
35
Fig. 5. The EDS line-scanning analysis of ZS5 showing the distribution of Zr and Si elements.
(390 MPa) of ZrC–20 vol%SiC composites (ZrC grain
size > 10 lm, SiC grain size < 2 lm) by hot pressing,15 prob-
ably owing to the fine grain size (<2 lm) of the matrix ZrC.
Although the SiC grains were in large size, the strength
behavior of ZS5 would not be poor, especially with the exis-
tence of inside submicrometer-sized ZrC particles. The
strength of the obtained ZrC–SiC composites might be
mainly controlled by the grain size of the ZrC matrix, but
further investigation should be done to verify it.
Table II. The Phase Content by XRD Analysis and Image
Analysis, and the Mechanical Properties of ZS5 and ZS10
XRD
analysis
(vol%)
Image
analysis
(vol%)
Hv1.0
(GPa)
KIC
(MPaÁm1/2
r
(MPa)
Samples SiC
ZrSi
SiC ZrSi
)
ZS5
13.4 trace 10.0
16.7 10.9
2.6 20.5 1.0
3.3 0.1
2.9 0.3
819 102
ZS10
14.5 13.2 19.0 0.5
—
IV. Summary
ZrC–SiC composites were prepared by reactive hot pressing
at temperatures up to 1600°C using ZrC and Si as raw mate-
rial. The lattice parameter and microstructure analyses con-
firmed that carbon from the ZrC reacted with Si to form
ZrSi and SiC above 1300°C, with an increase in temperature
up to 1600°C, carbon atoms from remaining ZrC diffused
into the ZrSi phase to form ZrC1Àx and SiC. The carbon
defects in the final ZrC1Àx matrix, and part of the reaction
product of ZrSi, promoted the densification of ZrC-based
ceramics. The low-temperature sintered ZrC–SiC ceramics
(ZS5) had a Vickers hardness and fracture toughness of
20.5 1.0 GPa and 3.3 0.1 MPaÁm1/2, respectively, which
were comparable to the literature results. The bending
strength of ZS5 was as high as 819 102 MPa.
through solid-state diffusion but also had the advantage of
accelerating plastic flow during the hot-pressing processes.
Meanwhile, the existence of the ZrSi phase also has the
advantage of improving the densification. According to the
phase diagram of Si–Zr,22 a liquid phase containing Zr and
Si could appear at about 1630°C, which was close to the
densification temperature (1600°C) when considering the
infrared measurement error of temperature (20°C–30°C). It
seems that the molten ZrSi phase aggregated together to
form large ZrSi particles first, and then carbon from the
ZrC matrix diffused into these particles to form SiC and
ZrC1Àx. The formed liquid phase should have enhanced the
rearrangement of ZrC particles, improved the densification
behavior, and also promoted the growth of SiC grains. The
detail of the microstructure formation mechanism is an
interesting issue, and it would be investigated in the future
work.
The mechanical properties of ZS5 and ZS10 were prelimi-
narily evaluated and shown in Table II. Due to the lower
hardness of ZrSi (about 10 GPa)23 compared with that of
ZrC and SiC, the hardness of ZS10 (19.0 0.5 GPa) was a
little lower than that of ZS5 (20.5 1.0 GPa). The hardness
of low-temperature (1600°C) sintered ZS5 and ZS10 was
comparable to those of monolithic ZrC sintered at 1900°C–
2000°C (about 20 GPa).2,20 The fracture toughness of ZS5
(3.3 0.1 MPaÁm1/2) and ZS10 (2.9 0.3 MPaÁm1/2) were
higher than those monolithic ZrC (1.9 0.4 MPaÁm1/2) and
close to the ZrC with addition of 9 vol% MoSi2
(3.3 0.4 MPaÁm1/2)2. The three-point bending strength of
ZS5 was 819 102 MPa, which was much higher than that
Acknowledgments
Financial supports from the Chinese Academy of Sciences under the Program
for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the
National Natural Science Foundation of China (No. 91 026 008, 11 175 228
and 11 205 229), and the State Key Laboratory of High Performance Ceram-
ics and Superfine Microstructures are greatly appreciated.
References
1K. Upadhya, J. M. Yang, and W. P. Hoffman, “Materials for Ultra High
Temperature Structural Applications,” Am. Ceram. Soc. Bull., 76 [12] 51–6
(1997).
2D. Sciti, S. Guicciardi, and M. Nygren, “Spark Plasma Sintering and
Mechanical Behaviour of ZrC-Based Composites,” Scripta Mater., 59 [6]
638–41 (2008).
3Y. G. Wang, Q. M. Liu, J. L. Liu, L. T. Zhang, and L. F. Cheng, “Deposi-
tion Mechanism for Chemical Vapor Deposition of Zirconium Carbide Coat-
ings,” J. Am. Ceram. Soc., 91 [4] 1249–52 (2008).

36
Rapid Communications of the American Ceramic Society
Vol. 96, No. 1
4G. Vasudevamurthy, T. W. Knight, E. Roberts, and T. M. Adams, “Labo-
13W. W. Wu, G. J. Zhang, Y. M. Kan, and P. L. Wang, “Reactive Hot
Pressing of ZrB2-SiC-ZrC Composites at 1600 degrees C,” J. Am. Ceram.
Soc., 91 [8] 2501–8 (2008).
ratory Production of Zirconium Carbide Compacts for Use in Inert Matrix
Fuels,” J. Nucl. Mater., 374 [1–2] 241–7 (2008).
5D. Gosset, M. Dolle, D. Simeone, G. Baldinozzi, and L. Thome, “Struc-
tural Evolution of Zirconium Carbide Under Ion Irradiation,” J. Nucl. Mater.,
373 [1–3] 123–9 (2008).
14C. Nachiappan, L. Rangaraj, C. Divakar, and V. Jayaram, “Synthesis and
Densification of Monolithic Zirconium Carbide by Reactive Hot Pressing,”
J. Am. Ceram. Soc., 93 [5] 1341–6 (2010).
6C. Shih, J. S. Tulenko, and R. H. Baney, “Low-Temperature Synthesis of
15B. Ma, X. Zhang, J. Han, and W. Han, “Fabrication of Hot-pressed ZrC-
Based Composites,” Proceedings of the Institution of Mechanical Engineers
Part G J.Aero. Eng, 223 [G8] 1153–7 (2009).
Silicon Carbide Inert Matrix Fuel Through
J. Nucl. Mater., 409 [3] 199–206 (2011).
a Polymer Precursor Route,”
7P. Barnier, C. Brodhag, and F. Thevenot, “Hot-Presing Kinetics of Zirco-
nium Carbide,” J. Mater. Sci., 21, 2547–52 (1986).
16X. Wang, W. Guo, Y. Kan, G. Zhang, and P. Wang, “Densification Behavior
and Properties of Hot-Pressed ZrC Ceramics with Zr and GraphiteAdditives,”
J. Euro. Ceram. Soc, 31 [6] 1103–11 (2011).
8V. P. Bulychev, R. A. Andrievskii, and L. B. Nezhevenko, “The Sintering
of Zirconium Carbide,” Powder Metall. Met. Ceram., 16 [4] 273–6 (1977).
9G. V. Samsonov, M. S. Koval’chenko, R. Y. Petrykina, and V. Y.
Naumenko, “Hot Pressing of the Transition Metals and Their Carbides
in Their Homogeneity Regions,” Powder Metall. Met. Ceram., 9 [9] 713–6
(1970).
17Materials Data JADE Release 5, “XRD Pattern Processing.” Materials
Data Inc.(MDI), 1999.
18A. G. Evans, and E. A. Charles, “Fracture Toughness Determinations by
Indentation,” J. Am. Ceram. Soc., 59 [7–8] 371–2 (1976).
19X. G. Wang, W. M. Guo, Y. M. Kan, and G. J. Zhang, “Hot-Pressed ZrB2 Ceram-
ics With Composite Additives of Zr and B4C,” Adv. Eng. Mater., 12 [9] 893–8 (2010).
20D. W. Ni, G. J. Zhang, Y. M. Kan, and P. L. Wang, “Synthesis of
Monodispersed Fine Hafnium Diboride Powders Using Carbo/borothermal
Reduction of Hafnium Dioxide,” J. Am. Ceram. Soc., 91 [8] 2709–12 (2008).
21ACerS-NIST Phase Equilibria Diagram. CD-ROM Database, Version 3.1,
2005
10W. G. Lidman, and H. J. Hamjian, “Reactions During Sintering of a
Zirconium Carbide-Niobium Cermet,” J. Am. Ceram. Soc., 35 [9] 236–40
(1964).
11X. G. Wang, J. X. Liu, Y. M. Kan, and G. J. Zhang, “Effect of Solid
Solution Formation on Densification of Hot-Pressed ZrC Ceramics with MC
(M=V, Nb, and Ta) Additions,” J. Euro. Ceram. Soc, 32 [8] 1795–802
(2012).
22H. Okamoto, “The Si-Zr (Silicon-Zirconium) System,” J.Phase Equilib, 11
[5] 513–9 (1990).
12A. L. Chamberlain, W. G. Fahrenholtz, and G. E. Hilmas, “Low-Temper-
ature Densification of Zirconium Diboride Ceramics by Reactive Hot Press-
ing,” J. Am. Ceram. Soc., 89 [12] 3638–45 (2006).
23W. Martienssen and H. Warlimont, Springer Handbook of Condensed Mat-
ter and Materials Data. Springer, Berlin Heidelberg, 2005.
h