Improved oxidation resistance of zirconium carbide at 1500°C by lanthanum hexaboride additions

Article abstract of DOI:10.1111/j.1551-2916.2011.04830.x

Addition of LaB₆ is adopted to improve the oxidation resistance of ZrC at 1500°C. Mixed powder of ZrC-25 vol% LaB₆ is reactively hot pressed at 1900°C for 30 min under vacuum with an applied pressure of 25 MPa. The LaB₆ reacts with ZrC to form ZrB₂ and a layered La-containing phase. ZrB₂ improves the oxidation resistance of ZrC in static air. The La-containing phase is beneficial to increasing the relative density of oxide scale during oxidation and in enhancing the oxide scale stability during exposure to thermal cycles.

Full text of DOI:10.1111/j.1551-2916.2011.04830.x

J. Am. Ceram. Soc., 94 [11] 3648–3650 (2011)
DOI: 10.1111/j.1551-2916.2011.04830.x
© 2011 The American Ceramic Society
ournal
J
Improved Oxidation Resistance of Zirconium Carbide at 1500°C by
Lanthanum Hexaboride Additions
Liyou Zhao, Dechang Jia*,† Xiaoming Duan, Zhihua Yang, and Yu Zhou
Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin, 150001, China
Addition of LaB6 is adopted to improve the oxidation resis-
tance of ZrC at 1500°C. Mixed powder of ZrC-25 vol% LaB6
is reactively hot pressed at 1900°C for 30 min under vacuum
with an applied pressure of 25 MPa. The LaB6 reacts with
ZrC to form ZrB2 and a layered La-containing phase. ZrB2
improves the oxidation resistance of ZrC in static air. The
La-containing phase is beneficial to increasing the relative
density of oxide scale during oxidation and in enhancing the
oxide scale stability during exposure to thermal cycles.
II. Experimental Procedure
The starting powders were ZrC (mean particle size 2.1 lm,
>98% purity, Changsha Wing High High-Tech New Materi-
als Co., Ltd., Changsha, China), ZrB2 (D50 = 2.5 lm, 99%
purity, Northwest Institute for Non-ferrous Metal Research,
Xi’an, China), and LaB6 (À300 mesh, >99% purity, North-
west Institute for Non-ferrous Metal Research). Mixed pow-
der of ZrC-25 vol% LaB6 was reactively hot pressed in boron
nitride-coated graphite die at 1900°C for 30 min under vacuum
with an applied pressure of 25 MPa. The obtained sample is
referred as CBL in this study. Monolithic ZrC and ZrC-40 vol
% ZrB2 composite were also prepared. The sintering tempera-
ture was 2000°C, holding time 60 min, and applied pressure
25 MPa. Cylindrical coupons with dimension of Φ12.7 9 4 mm
were cut from the hot-pressed plates. Static oxidation test was
carried out by exposing coupons to stagnant air in a box furnace
at 1500°C for 2 h or 4 h. The heating rate was 15°C/min. Cyclic
oxidation test was also conducted. The coupons were exposed
to stagnant air at 1500°C for 15 min and then removed quickly
from the furnace, and left for cooling in air for 10 min. The
total time in furnace was 2 h. After oxidation, the oxide scale
was ground off and the remained thickness of coupons was mea-
sured to evaluate the oxidation degree of samples. The phase
composition of oxide scale was identified using X-ray diffrac-
tometer (XRD, CuKa radiation, D/max-2200VPC, Rigaku,
Tokyo, Japan). The surface and cross-section of oxidized sam-
ples were observed by scanning electron microscope (SEM,
FEI Quanta 200F, Eindhoven, the Netherlands), which
is equipped with an energy-dispersive spectroscopy (EDS,
EDAX, NJ) system.
I. Introduction
IRCONIUM carbide (ZrC) shows a number of excellent
Z
properties such as high melting temperature, high ther-
mal and electrical conductivity, and high fracture strength.1–3
It is a potential ultrahigh-temperature ceramic (UHTC) for
aerospace applications.4,5
Oxidation resistance is a major issue in the development
of UHTCs. The ZrC has a poor high-temperature chemical
stability in air, which yields a porous and non-protective
oxide scale during oxidation.6,7 Until now, the reports on
8,9
enhancing oxidation resistance of ZrC are scarce. He et al.
prepared ternary and quaternary carbides by incorporation
of Al or Al with Si into ZrC. The obtained compounds show
better oxidation resistance than ZrC due to the formation of
some protective products, but the oxide scale is still porous
because of the inability to sinter and the formation of high
vapor pressure gaseous products. Recently, Zhang et al.10,11
introduced 4 mol% WC into ZrB2 matrix, which promotes
formation of a liquid phase during oxidation and conse-
quently results in liquid-phase sintering of ZrO2 scale. This
increases the relative density of oxide scale and therefore
decreases the rate of oxygen transport. ZrB2-4 mol% WC
performs much better oxidation resistance than monolithic
ZrB2 at 1500°C and 1600°C. It is a novel and promising
strategy to improve the oxidation resistance of UHTCs by
forming liquid during oxidation and in turn enhancing the
protective effect of oxide scale.
III. Results and Discussion
The obtained composite using ZrC and LaB6 as raw materi-
als is composed of three phases, namely, ZrC, ZrB2, and a
layered phase [Fig. 1(a)]. Element La mainly distributes in
the layered phase [Fig. 1(b)]. The layered phase also contains
elements B and C.12
In a previous work, we improved the fracture toughness of
ZrC by addition of LaB6, which is attributed to the in situ
formation of a layered La-containing compound.12 In this
article, we report the enhancement of oxidation resistance of
ZrC by LaB6 addition. It provides a new method for enhanc-
ing oxidation resistance of ZrC.
After static oxidation at 1500°C for 4 h, CBL forms a light
yellow oxide scale, and no cracking of oxide scale or edge
cracking of sample is observed [Fig. 2(a)]. Some bubbles are
detected on the surface of sample, suggesting formation of
liquid and gaseous phases during oxidation. The remained
thickness of coupon is 2.96 mm. ZrC oxidizes catastrophically
in air at 1500°C. The oxide scale significantly spalls from the
substrate [Fig. 2(b)], and the remained thickness of coupon is
1.67 mm. ZrC-40 vol% ZrB2 sample exhibits severe surface
and edge cracking after static oxidation at 1500°C for 4 h
[Fig. 2(c)], but the remained thickness of coupon is the thick-
est, about 3.28 mm. After 2 h cyclic oxidation, the oxide scale
of CBL appears dense and adherent [Fig. 3(a)], and the
remained thickness of coupon is 3.32 mm; in contrast, the
oxide scale of ZrC-40 vol% ZrB2 is damaged seriously
[Fig. 3(b)], and the remained thickness of coupon is 3.16 mm.
B. Fahrenholtz—contributing editor
Manuscript No. 29625. Received April 23, 2011; approved August 05, 2011.
This work was supported by the Program for Changjiang Scholars and Innovative
Research Team in University.
*Member, The American Ceramic Society.
†Author to whom correspondence should be addressed. e-mail: dechangjia@yahoo.
com.cn
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Rapid Communications of the American Ceramic Society
3649
(a)
(b)
Fig. 1. Microstructure analysis of CBL: (a) BSE image, (b) elemental map for the distribution of La.
(a)
(b)
(c)
Fig. 2. Macrographs of different coupons after static oxidation at 1500°C for 4 h: (a) CBL, (b) monolithic ZrC, (c) ZrC-40 vol% ZrB2.
However, we note that the oxide scale on ZrC-40 vol% ZrB2
(a)
(b)
is cracked macroscopically after cooling while the scale on
CBL is dense and adherent. Microstructure analysis reveals
that CBL has denser oxidized surface than ZrC-40 vol%
ZrB2. After 2 h cyclic oxidation at 1500°C, the remained
coupon of CBL is thicker than that of ZrC-40 vol% ZrB2,
indicating the better cyclic oxidation resistance of CBL. So,
the La-containing phase in CBL is beneficial to increasing
the relative density of oxide scale during oxidation and
enhancing the oxide scale stability during cyclic exposures.
Figure 5 shows the XRD patterns of oxide scale on CBL
after static oxidation at 1500°C for 2 h and 4 h. The oxide
scale formed at 1500°C for 2 h is composed of monoclinic
ZrO2 (JCPDS No. 24-1165), orthorhombic LaBO3 (JCPDS
No. 12-762), and hexagonal La2O3 (JCPDS No. 83-1345).
Besides orthorhombic LaBO3, another crystal form of LaBO3
(JCPDS No. 13-571) is found in the pattern of the oxide
scale formed at 1500°C for 4 h. The high background in
Fig. 5(b) is consistent with the presence of glassy phase in
the oxide scale. By far, it is difficult for us to clarify the oxi-
dation mechanism of this new material. But, we believe that
La2O3–B2O3 liquid formed during oxidation could play an
important role in sintering of ZrO2 scale, and the formed
La2O3–B2O3 compounds are beneficial to lowering the coeffi-
cient of thermal expansion of ZrO2 scale,13–15 thus enhancing
the damage resistance of oxide scale during cyclic exposures.
Fig. 3. Macrographs of CBL (a) and ZrC-40 vol% ZrB2 (b) after
cyclic oxidation at 1500°C for 2 h.
Figure 4 shows the surface SEM micrograph of different
samples after static oxidation at 1500°C for 2 h. The surface
of CBL appears relatively dense. The dark phase in back-
scattered electron (BSE) image is ZrO2. Glassy phase is also
observed, which mainly distributes at the boundaries of ZrO2
grain clusters and seals the cracks at the boundaries. EDS
analysis (not shown) indicates that the glassy phase contains
elements La, O, and possibly B. The oxide scale on ZrC is
cracked and porous while the scale on ZrC-40 vol% ZrB2 is
porous.
IV. Conclusions
Introduced LaB6 reacts with ZrC to form ZrB2 and a lay-
ered La-containing phase. After static oxidation test, the
remained coupon of CBL is thicker than that of ZrC, but
thinner than that of ZrC-40 vol% ZrB2. So, CBL has better
static oxidation resistance than ZrC, but inferior static oxida-
tion resistance compared with ZrC-40 vol% ZrB2. Assuming
that all B in the introduced LaB6 is converted to ZrB2, the
volume ratio of ZrC to ZrB2 in CBL is about 3:2. So,
the enhancement of oxidation resistance of ZrC in static air
after LaB6 addition is attributed to the formation of ZrB2.
A new approach is developed to improve the oxidation resis-
tance of ZrC at 1500°C. A quantity of 25 vol% LaB6 is
added to ZrC before densification. After reactively hot press-
ing at 1900°C, introduced LaB6 reacts with ZrC to form
ZrB2 and a layered La-containing phase. The composite pre-
pared by ZrC with 25 vol% LaB6 has better static oxidation
resistance than ZrC at 1500°C, but shows inferior static oxi-
dation resistance compared with ZrC-40 vol% ZrB2. The
enhancement of oxidation resistance of ZrC in static air after

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Vol. 94, No. 11
(a)
(b)
(c)
(d)
Fig. 4. SEM micrographs of the surface of different samples after static oxidation at 1500°C for 2 h: (a) CBL, (b) corresponding BSE image of
(a), (c) monolithic ZrC, (d) ZrC-40 vol% ZrB2.
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