Fabrication and characterization of ZrB2-based ceramic using synthesized ZrB2-LaB6 powder

Article abstract of DOI:10.1111/j.1551-2916.2008.02509.x

ZrB₂-LaB₆ powder was obtained by reactive synthesis using ZrO₂, La₂O₃, B₄C, and carbon powders. Then ZrB₂-20 vol% SiC-10 vol% LaB₆ (ZSL) ceramics were prepared from co

Full text of DOI:10.1111/j.1551-2916.2008.02509.x

J. Am. Ceram. Soc., 91 [8] 2763–2765 (2008)
DOI: 10.1111/j.1551-2916.2008.02509.x
r 2008 The American Ceramic Society
ournal
J
Fabrication and Characterization of ZrB -Based Ceramic Using
2
Synthesized ZrB –LaB Powder
2
6
z,y
z
,w,z
z
y
Alex Spring, Wei-Ming Guo, Guo-Jun Zhang,* Pei-Ling Wang, and Vladimir D. Krstic
z
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics,
Shanghai 200050, China
yCenter for Manufacturing of Advanced Ceramics and Nano Materials, Queen’s University, Kingston,
ON K7L 3N6, Canada
ZrB –LaB powder was obtained by reactive synthesis using
6
as an alternative route to providing similar oxidation resistance
results. The advantage of this approach is that it offers the pos-
sibility to obtain ZrB -based ceramics with an improved disper-
2
ZrO , La O , B C, and carbon powders. Then ZrB –20 vol%
2
SiC–10 vol% LaB (ZSL) ceramics were prepared from com-
2
3
4
2
6
2
mercially available SiC and the synthesized ZrB –LaB powder
2
via hot pressing at 20001C. The phase composition, microstruc-
ture, and mechanical properties were characterized. Results
sion of LaB
To supplement and complete the previous research on the
addition of LaB to the ZrB –SiC composite, the current study
focuses on fabrication and characterization of ZrB -based ce-
6
and the use of less expensive raw materials.
6
6
2
showed that both LaB and SiC were uniformly distributed in
6
2
the ZrB matrix. The hardness and bending strength of ZSL were
2
2 6
ramics using synthesized ZrB –LaB powder and commercially
1
7.0670.52 GPa and 505.8717.9 MPa, respectively. Fracture
available SiC. ZrB –SiC–LaB (ZSL) samples were prepared by
6
hot pressing. Microstructure and mechanical properties were
characterized on dense ZSL ceramics.
2
1/2
.
toughness was 5.770.39 MPa m , which is significantly higher
than that reported for ZrB –20 vol% SiC ceramics, due to en-
hanced crack deflection and crack bridging near SiC particles.
2
II. Experimental Procedure
The raw materials used in this study were ZrO (Tosoh, Toyama,
2
Japan), La O (SCRC, Shanghai, China), B C (Jingangzuan
2 3 4
I. Introduction
NOWN as ultra-high-temperature ceramics, transition metal
diborides and carbides show a number of excellent prop-
erties such as high melting temperature, high strength, high
Boron Carbide Co. Ltd., Mudanjiang, China), and carbon and
a-SiC (D50 5 0.45 mm, Changle Xinyuan Carborundum Micro-
powder Co. Ltd., Changle, China), with purities of 99%,
K
thermal and electrical conductivity, and chemical stability.
1
9
9.95%, 96%, 99%, and 98.5%, respectively.
With La , ZrO , B C, and carbon in the desired molar ra-
tio as starting powders, ZrB –LaB powder with a volume ratio
Therefore, these ceramics are potential candidates for a variety
of high-temperature structural applications such as thermal pro-
tection systems for leading edges. Zirconium diboride is espe-
O
2 3
2
4
2
6
2
of 7:1 was obtained by reactive synthesis at temperatures rang-
ing from 13001 to 16001C. SiC powder was then added to the
3ꢀ7
cially promising,
as it has a relatively low density compared
with the other candidates for ultra-high-temperature applica-
tions, a desired property in the aerospace industry.
synthesized ZrB
10 vol% LaB (ZSL) ceramics.
2 6 2
–LaB powder to fabricate ZrB –20 vol% SiC–
6
2
Although ZrB has many excellent properties, one of the hur-
The starting mixture was mixed for 24 h in a polyethylene jar
using ethyl alcohol and zirconia balls, and then dried by rotary
evaporation. After being dried, the mixed powder was ground to
dles to overcome is its poor oxidation resistance at high tem-
8
ꢀ11
peratures.
improvements in oxidation resistance because of the formation
of a protective SiO layer at temperatures above roughly
However, oxidation resistance of ZrB –SiC com-
The addition of SiC to ZrB2 has led to
ꢀ200 mesh and then placed in a graphite die with a BN coating.
Powder compacts were heated to 20001C and held for 1 h under
a pressure of 30 MPa in an argon atmosphere.
2
1
2ꢀ14
12001C.
2
posites markedly degrades beyond 18001C, as the SiC actively
Archimedes’ method and the rule of mixtures were used to
determine the actual and theoretical densities, respectively.
Phase composition and microstructure of the composite were
determined by X-ray diffraction (XRD, D/max 2550 V, Tokyo,
Japan) and an electron probe microanalyzer (JEOL JXA-8100F,
Tokyo, Japan) along with energy-dispersive spectroscopy (EDS,
Oxford INCA energy, Oxon, UK), respectively. The hardness
was measured by the indentation method, using a load of 1 kg
for 10 s on a polished surface (Wilson-Wolpert Tukon 2100B,
Instron, Norwood, MA). For the fracture toughness, a load of
1
5ꢀ16
17
Zhang et al. im-
–SiC ceramics to tem-
oxidizes or the SiO
2
layer volatilizes.
proved the oxidation resistance of ZrB
2
6
peratures up to 24001C by the addition of LaB .
The higher oxidation resistance of ZrB –SiC–LaB was
6
2
mainly attributed to the formation of La Zr
2
2
O
7
, which effec-
1
7
tively limited the inward transport of oxygen. Further, a ho-
mogeneous dispersion of LaB6 in the ZrB2 matrix was
considered to be the key to providing improved oxidation re-
2 6
sistance. Reactive synthesis of ZrB –LaB powder can be used
10 kg was used. Without Young’s modulus, the fracture tough-
ness can be calculated by the following equations:
1
8,19
G. Hilmas—contributing editor
ꢀ
ꢀ
ꢁꢁ
3
2
C1 þ C2
KIC ¼ P p
ðtgbÞꢀ
1
(1)
Manuscript No. 24348. Received February 25, 2008; approved April 24, 2008
This work was financially supported by the Chinese Academy of Sciences under the
Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) and the
National Natural Science Foundation of China (Nos. 50632070 and 50602048).
4
where P is the indentation load (N), C and C is the measured
1
diagonal crack length (m), and b is an angle constant (681).
*
Member, The American Ceramic Society.
2
w
Author to whom correspondence should be addressed. e-mail: gjzhang@mail.sic.ac.cn
2
763

2
764
Communications of the American Ceramic Society
Vol. 91, No. 8
and EDS analysis of the ZSL ceramics are shown in Fig. 3. The
dark, SiC phase was easily identified. From the EDS included in
Fig. 3, zones labeled as 1 are the LaB phase and zones labeled as
6
2
2 are the ZrB phase. The microstructure consisted of three dis-
tinct phases, well distributed throughout the sample. The size of
the ZrB grains is largest in the microstructure (about 4–6 mm),
2
while the LaB grains are smaller (about 1–3 mm). The SiC par-
6
ticles had become elongated, having an aspect ratio of roughly
2
:1 (approximately 1–2 mm wide by 2–4 mm long) after densifi-
20
cation. Zhang et al. indicated that the morphology of SiC
grains in ZrB –SiC ceramics varied from equiaxed to whisker-
2
like, depending on the initial size of raw SiC particles, even if all
SiC particles with different sizes were a-polymorphic. The aspect
ratios of SiC grains were about 1.05, 1.75, and 3.05, with the
20
starting particle sizes of 1.45, 1.05, and 0.45 mm, respectively.
They thought that the finer SiC particles more readily formed an
interconnected network, which provided a higher driving force
20
for sintering. As a result, the finer SiC particles coarsened and
elongated during sintering. Based on the above study of Zhang et
al., elongation of the SiC grains should be mainly due to the
small size of the raw SiC particles (D50 50.45 mm) in our work.
The hardness of ZSL was 17.0670.52 GPa, which was
2 6
Fig. 1. X-ray diffraction patterns of the synthesized ZrB –LaB powder
at (a) raw powder, (b) 13001C, (c) 14001C, (d) 15001C, and (e) 16001C.
2
slightly higher than the value of ZrB –20 vol% SiC reported
1
by our previous research (15.8 GPa) and the work of Zhang
8
Flexural strength was determined via a four-point bending with
sample dimensions of 2 mm ꢁ 1.5 mmꢁ 25 mm. The outer span
was 20 mm and inner span was 10 mm.
21
et al. (15.0 GPa), but lower than the value of ZrB –20 vol%
2
6
SiC reported by Chamberlain et al. (24 GPa). The bending
strength of ZSL was 505.8717.9 MPa, which was lower than
2
the value of ZrB –20 vol% SiC reported by our research
1
582 MPa) and Zhang et al. (523 MPa) as well as Chamber-
8
21
(
III. Results and Discussion
6
22
lain et al. (1003 MPa). Zhu et al. reported that the fracture
strength of ZrB –SiC was a strong function of SiC grain size,
increasing from 400 MPa with a SiC grain size of 6 mm to over
00 MPa with a SiC grain size of 1 mm or less. So the relative
(
1) ZrB –LaB Powder Synthesis
2 6
2
XRD analysis of synthesized ZrB –LaB powder, with a hold
2
6
9
time of 1 h at temperatures of 13001 to 16001C, included as
Fig. 1. By 14001C, the initial compounds had almost fully re-
acted, with only small amounts of C and ZrO
lower strength of ZSL may be due to the larger grain size, es-
pecially due to the growth of SiC grains.
The fracture toughness for ZSL ceramics is 5.770.39
2
remaining.
Above 15001C, only peaks for ZrB and LaB were present, in-
2
6
1/2
MPa ꢂ m , which is significantly higher than those reported
dicating that by this temperature, the reaction appears to have
proceeded to completion. It should be noted that because of the
2
in the literature for ZrB –20 vol% SiC materials (4.0–4.8
1
/2 6,18,21
MPa ꢂ m ).
To better elucidate the toughening mecha-
relatively strong intensity of the ZrB
difficult to observe. Figure 2 contains scanning electron micro-
scopic (SEM) images of the ZrB –LaB powder synthesized at
5001 and 16001C. From Fig. 2, the powder produced at 15001C
2 6
peaks, the LaB peaks are
nisms, the paths of some Vickers-indentation-induced cracks
were examined on polished sections via SEM. Figure 4 is an
SEM image of a crack path on the polished surface of ZSL. The
crack in the ZSL ceramics, containing slightly enlongated SiC
grains was tortuous. SEM inspection of the crack propagation
demonstrated crack deflection and perhaps crack bridging near
the SiC particles. These toughening mechanisms cause energy
dissipation during crack propagation, resulting in higher mea-
sured-toughness values.
2
6
1
appears to contain a more uniform distribution of ZrB and
LaB6 particles compared with the powder synthesized at
6001C. The uniform distribution of ZrB and LaB particles
2 6
2
1
would help in forming a more uniform microstructure during
sintering. Hence, the ZrB –LaB powder synthesized at 15001C
was chosen to complete this study.
2
6
(
2) Hot-Pressed ZSL Ceramics
3
IV. Summary
The bulk density of the ZSL ceramics was 5.32 g/cm , which
corresponds to 99% of the theoretical density, in accordance
with the rule of mixtures. A backscattered electron (BSE) image
In this study, ZrB –20 vol% SiC–10 vol% LaB (ZSL) ceramics
2
were prepared by hot pressing using synthesized ZrB
6
2 6
–LaB
(
a)
(b)
2
µm
2
µm
2 6
Fig. 2. SEM images of the ZrB –LaB powder synthesized at (a) 15001C and (b) 16001C.

August 2008
Communications of the American Ceramic Society
2765
2 6
Fig. 3. Backscattered electron image and energy-dispersive spectroscopy analysis of the ZrB –20 vol% SiC–10 vol% LaB ceramics.
3
4
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5
16
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