Effect of various additives on the oxidation behavior of ZrB 2-based ultra-high-temperature ceramics at 1800°C

Article abstract of DOI:10.1111/j.1551-2916.2009.03420.x

The effect of various additives such as TiB₂, HfB₂, CrB₂, TaB, TaSi₂, LaB₆, La₂O ₃, and AlN on the oxidation behavior of ZrB₂-based ultra-high-temperature ceramics (UHTCs) at 1800°C was investigated. The additions of CrB₂ and HfB₂ to the ZrB₂-SiC composites have minor effect on the structure evolution of the oxide scale and oxidation resistance. However, all the other additives, especially for Ti and Ta compounds, have changed the oxidation mechanism resulting in a marked detrimental effect on the oxidation resistance of ZrB₂-based UHTCs.

Full text of DOI:10.1111/j.1551-2916.2009.03420.x

J. Am. Ceram. Soc., 93 [2] 345–349 (2010)
DOI: 10.1111/j.1551-2916.2009.03420.x
r 2009 The American Ceramic Society
ournal
J
Effect of Various Additives on the Oxidation Behavior of ZrB -Based
2
Ultra-High-Temperature Ceramics at 18001C
w
Ping Hu, Xing-Hong Zhang, Jie-Cai Han, Xiao-Guang Luo, and Shang-Yi Du
Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, China
1
1,22–24
The effect of various additives such as TiB , HfB , CrB , TaB,
2
17001C.
on the oxidation resistance of ZrB
tures above 17001C is very unclear.
The purpose of this work was to investigate the effect of the
several representative additives (i.e., TiB , HfB , CrB , TaB,
TaSi , LaB , La , and AlN) on the oxidation behavior and
Unfortunately, the influence of these additives
2
2
TaSi , LaB , La O , and AlN on the oxidation behavior
2
-based UHTCs at tempera-
2
of ZrB -based ultra-high-temperature ceramics (UHTCs) at
6
2
3
2
1
the ZrB –SiC composites have minor effect on the structure
8001C was investigated. The additions of CrB and HfB to
2 2
2
2
2
2
evolution of the oxide scale and oxidation resistance. However,
all the other additives, especially for Ti and Ta compounds,
have changed the oxidation mechanism resulting in a marked
2
6
2 3
O
resistance of ZrB -based UHTCs at 18001C. The oxidation
mechanism and regularity were also discussed.
2
detrimental effect on the oxidation resistance of ZrB -based
2
UHTCs.
II. Experimental Procedure
The samples used here for oxidation testing were fabricated from
I. Introduction
2 2 2 2
commercial powders, including ZrB , SiC, TiB , HfB , CrB ,
TaB, TaSi , LaB , La , and AlN, with a purity higher than
2
6
2 3
O
ATERIALS with improved properties are needed for better
thermal protection of hypersonic vehicles proposed for
9
9%. The material compositions (vol%) are as follows: (a) ZrB2;
M
(
b) ZrB –20SiC; (c) ZrB –20TaSi ; (c) ZrB –20SiC–10TaB; (e) ZrB –
2
next generation space and atmospheric reentry vehicles. Sharp
leading edges and nose tips will have to withstand exposure
to high temperatures and severe thermal cycling in both neutral
and oxidizing environments. These extreme conditions require
materials that possess superior oxidation resistance, low creep,
2
2
2
2
2
0SiC–10CrB
2
;
(f) ZrB
–20SiC–5La
2
–20SiC–10TiB
(i) ZrB
The powder mixtures of these materials were ball milled in
2
;
(g) ZrB
2
–20SiC–
10LaB
6
; (h) ZrB
2
O
2 3
2
–20SiC–10AlN.
ethanol for 8 h and dried in a rotating evaporator. Milled pow-
ders were then uniaxially hot pressed in a boron nitride coated
graphite die at 20001C for 60 min and 30 MPa of applied pres-
sure. Bulk density and theoretical density were evaluated using
the Archimedes method and the rule-of-mixture, respectively.
Sample coupons 2.0 cm  1.0 cm  0.35 cm were cut from the
hot-pressed materials, and all surfaces were diamond polished to
a 1 mm finish. Coupons were ultrasonically cleaned successively
in detergent, deionized water, acetone and alcohol before expo-
1
–4
and excellent thermal shock properties. Currently available
thermal protection materials cannot survive under such extreme
2
temperatures. ZrB -based UHTCs are the most promising can-
didates for such applications due to the unique combination of
properties, such as high melting point, high thermal conductiv-
ity, and resistance to erosion/corrosion.
4
–11
Because of strong covalent bonding and low self-diffusion,
high temperatures and external pressures are required to densify
2
sure. Samples were loaded into a slotted ZrO refractory brick
diborides. Generally, nitrides (i.e., AlN, Si
sintering aids for the improved densification of ZrB2.
additives such as MoSi , ZrSi , and B C were also introduced to
However, the influence of these sinte-
3 4
N , and ZrN) were as
1
2–14
and then exposed to 60 min oxidation in stagnant air at 18001C,
using a bottom-loading furnace with zirconia heating elements.
Two samples of each composition were oxidized. X-ray diffrac-
tion (Rigaku, Dmax-rb, Tokyo, Japan) and scanning electron
microscopy (SEM; FEI Sirion, Eindhoven, the Netherlands)
along with energy dispersive spectroscopy (EDS; EDAX, Mahwah,
NJ) were used to characterize the phase composition and micro-
structure of the surface and cross section of the samples after
oxidation experiments.
Other
2
2
4
1
5–17
enhance densification.
ring aids on the ultra-high-temperature performance of UHTCs
is not well understood, especially for the oxidation behavior.
ZrB
2
containing 15–30 vol% SiC additive are prominent
5
–7,9–11,18–21
UHTCs withstanding temperatures 18001C.
Intense
research is ongoing to enhance the oxidation resistance of these
composites. The introduction of the 10 mol% diborides im-
2
proved the oxidation resistance of ZrB –SiC at 13001C and the
weight gain decreased in the sequence of the modifying addi-
tives: CrB , TiB , NbB , VB , and TaB , correlating well with
2
2
2
2
2
III. Results and Discussion
3
1
41
the cation field strength for Cr , Ti , Nb , V , and Ta
51
41
51 11
.
Beside diborides, other compounds such as TaSi , Ta Si , and
Macrographs of ZrB -based UHTCs after oxidation at 18001C
2
2
5
3
are shown in Fig. 1. Extensive oxidation and swelling of the
sample were observed in the pure ZrB after oxidation. More-
2
MoSi were also introduced to further improve the oxidation
2
resistance of the UHTCs. The favorable effect of these additives
on the oxidation resistance of UHTCs was observed below
over, cracking and spalling were also found in the oxide scale as
can be seen in Fig. 1. The formed ZrO are very stable and ad-
2
herent to the unreacted ZrB
However, the formed ZrO would not adherent to the unreacted
ZrB at temperatures above 15001C. The large volume change
upon conversion of ZrB to ZrO during oxidation associated
2
at temperatures below 12001C.
E. Opila—contributing editor
2
2
2
2
2 2 3
with phase transformation of ZrO and the evaporation of B O
Manuscript No. 26265. Received May 11, 2009; approved August 31, 2009.
This work was supported by the National Natural Science Fund for Outstanding
Youths (10725207), Postdoctoral Science Foundation of Heilongjiang province and Post-
doctoral Science Foundation of China.
are responsible for the occurrence of the cracks and spallation.
The color of the samples significantly changed after oxidation as
shown in Fig. 1. This mostly depends on the additive cations. Ta
wAuthor to whom correspondence should be addressed. e-mail: huping@hit.edu.cn
3
45

3
46
Rapid Communications of the American Ceramic Society
Vol. 93, No. 2
containing samples oxidized at 18001C formed a pink oxide,
whereas Al and La containing coupons formed a white oxide.
The additions of CrB and HfB to the ZrB –SiC composites
difference of the oxidation resistance of these diborides in which
ZrB
HfB2.
Oxygen diffusion through the ZrO
2
is more oxidation resistant than CrB
2
while is less than
2
2
2
have minor effect on the oxidation behavior. However, all the
other additives have a detrimental effect on the oxidation be-
havior of the ZrB -based UHTCs. Particularly, the UHTCs
2
skeleton phase, liquid
glass phase, cracks, and pores are the dominant routes of the
oxygen transport in the oxide scale. Therefore, reducing the
amount of the pores and oxygen diffusion rate in condense
phases are effective ways to decrease the inward oxygen diffu-
sion resulting in enhanced oxidation resistance. Apparently, ox-
ygen transport through the skeleton phase can be reduced by
changing the composition of this phase. It is well known that
2
containing TaSi and TiB as shown in Fig. 1 revealed a cata-
2
2
strophic increase in oxidation rate at 18001C. Surprisingly, the
samples with TiB and TaSi additions exposed at 18001C for 1
h were completely consumed, which are also less resistant to
oxidation than monolithic ZrB . It should be noted that the ex-
2
2
2
istence of the silica glass significantly increase the temperature
range of the glass as the primary oxygen barrier leading to en-
hanced oxidation resistance of ZrB . However, these materials
2
ZrO become nonstoichiometric by forming oxygen lattice va-
cancy under low oxygen partial pressure conditions. Oxygen
lattice vacancy allows rapid oxygen ion transport through the
scale. One approach to resolve the oxygen transport issue is to
2
containing silicides exhibited poor oxidation resistance, which
indicated that the additions of Ta and Ti are both quite detri-
mental to the oxidation resistance of ZrB -based UHTCs at ul-
51
modify the scale with higher valence cations (i.e., Nb and
5
1
2
Ta ) into the ZrO
tration and ion diffusion. Recent studies revealed that the ad-
dition of tantalum via refractory compounds to ZrB matrix
2
that can reduce oxygen vacancy concen-
trahigh temperature. The oxidation resistance of ZrB –20TaSi
2
2
and ZrB
ZrB
TiB significantly reduced the oxidation resistance of ZrB –SiC
2
–20SiC–10TiB
2
is far less resistant to oxidation than
2
2
2
2
–20SiC at 18001C. It was also found that the addition of
UHTCs led to higher oxidation resistance. Levine et al. added
tantalum to zirconium diboride for improved oxidation resis-
2
2
25
composite in the previous study. The thickness of oxide scale
for ZrB –20SiC–10TiB is five times higher than that for ZrB
0SiC under the same arc jet testing conditions. Additionally,
tance. The oxidation rate of ZrB –20 vol% SiC was significantly
2
2
3
2
2
2
–
reduced with the addition of TaSi
studied up to 20% additions of TaB
2
at 16271C. Talmy et al.
to ZrB –20 vol% SiC.
2
2
2
the oxide scale with a porous structure was formed in TiB con-
2
Furnace oxidation temperatures up to only 14001C were used.
They reported significant improvements in oxidation resistance
and attributed these improvements to phase separation in the
glass. Another explanation for this improved behavior is that Ta
taining UHTCs after oxidation. Similarly, porous scale was also
observed in this case. The formation of the porous structure was
2
6,27
probably due to the outward diffusion of the cations.
additions of CrB and HfB to the ZrB –SiC composites have
The
additions result in substitution of Ta on the Zr site in ZrO , re-
2
2
2
2
minor effect on both the structure evolution and oxidation re-
sistance. The structure of oxide scale for ZrB –SiC–CrB com-
posites is analogous to that of ZrB –SiC composites as shown in
ducing the concentration of oxygen vacancy in the ZrO
ever, the Ta (i.e., TaSi and TaB ) addition performed poorly at
ultrahigh temperatures since the existence of Ta O would lower
2
. How-
2
2
2
2
2
2
5
Figs. 2(a) and (b). Similar structure was also observed in the
the melting point of oxide scale through forming an intermediate
phase (i.e., Ta ) leading to the melting and degrada-
Á 6ZrO
tion of the ZrO skeleton, which acts mechanical support for
the oxide scale. Apparently, the decrease of melting point
for oxide scale has a detrimental effect on the oxidation resis-
tance at ultrahigh temperature. The integrity of ZrO skeleton
2
was not formed after oxidation at 18001C. Furthermore, the
oxidized ZrB
and HfB have minor effect on the structure evolution of the
oxide scale. Interestingly, the addition of HfB is beneficial to
2
–SiC–HfB
2
(not shown). The additions of CrB
2
2
O
5
2
2
2
2
the oxidation resistance of ZrB
introduction of CrB is detrimental to the oxidation perfor-
mance of the materials. This is most likely attributed to the
2
–SiC composites whereas the
2
(
a)
(b)
(c)
ZrB2
ZrB -20%volSiC
ZrB -20%volTaSi (c)
2 2
2
(
d)
(e)
(f)
ZrB -20%volSiC-10%volTaB
ZrB -20%volSiC-10%volCrB2
ZrB -20%volSiC-10%volTiB2
2
2
2
(
g)
(h)
(i)
ZrB -20%volSiC-10%volLaB6
ZrB -20%volSiC-5%volLa O
ZrB -20%volSiC-10%volAlN
2
2
2
2
3
2
Fig. 1. Macrographs of the oxidized ZrB matrix ultra-high-temperature ceramics with various additives at 18001C for 1 h.

February 2010
Rapid Communications of the American Ceramic Society
347
vporous structure, including spalling and cracking, was also
found in Ta containing UHTCs after exposure to air at 18001C
as shown in Fig. 2(c), which resulted in the accelerated oxida-
tion. It is generally accepted that gas phase diffusion through the
cracks and pores, even in the Knudsen diffusion regime, is much
easier than diffusion in condensed phases. Therefore, improve-
ment of UHTC oxidation resistance by Ta additions is strongly
dependent on the oxidation conditions, especially temperature.
18001C for 1 h. EDX analysis evidenced that the bright zones
contained Zr and La oxide while the dark zones consisted of
silica glass. The formed scale is strongly different from that
without additives. These additives promoted the phase agglom-
eration leading to a reduction of the oxidation resistance. Al-
though La was added to the material in the different
compounds, they have a similar effect on the structure evolu-
tion and oxidation resistance of UHTCs. This means that the
effect of additives to oxidation behavior of UHTCs is signifi-
cantly dependent on the metallic element cations.
6 2 3
Addition of other additives, such as LaB , La O , and AlN,
into UHTCs has an analogous effect on the structure evolution
and oxidation resistance. The introduction of these additives
leads to a double detrimental effect. On the one hand, addition
Remarkable transport of ZrO
the surface layer in La-containing UHTCs was also detected
after oxidation. This feature was not observed in ZrB –SiC
2
-rich oxides from inner scale to
31
31
31
of other additive cations, such as Al , Y , or La , into the
scale would lower the eutectic temperatures of the oxide scale,
leading to an instability of the skeleton and an accelerated
transport of the zirconia to the surface. On the other hand,
the concentration of oxygen vacancy increases due to introduc-
2
composites in which a dense oxide layer was formed. SEM stud-
ies showed the evidence of the phase agglomeration in the oxide
scale of La-modified samples after furnace oxidation tests as can
be seen in Figs. 2(d) and (e). Agglomeration of silica-rich glass
cannot act as an effective barrier to the inward diffusion of ox-
ygen resulted in accelerated oxidation rate. The addition of cat-
ions with lower valance will increase the oxygen transport
tion of the cation with a lower valence to ZrO , which resulted in
2
a reduced oxidation resistance. Figures 2(d) and (e) show SEM
cross-section micrographs of La added ZrB –SiC oxidized at
2
(
a)
(b)
1
00µm
100µm
(
c)
(d)
Silica glass agglomeration
5
00µm
500µm
(
e)
3
00µm
Fig. 2. Scanning electron microscopy micrographs of the cross-section of ZrB
a) ZrB –20 vol% SiC, (b) ZrB –20 vol% SiC–10 vol% CrB , (c) ZrB –20 vol% SiC–10 vol% TaB, (d) ZrB
vol% SiC–5 vol% La
2
-based ultra-high-temperature ceramics after oxidation at 18001C for 1 h.
(
2
2
2
2
–20 vol% SiC–10 vol% LaB , (e)ZrB –20
2 2
6
2 3
O .

3
48
Rapid Communications of the American Ceramic Society
Vol. 93, No. 2
through the ZrO
2
phase resulting in increased oxidation rate of
ZrB , which can be validated by the thickness of the SiC-deple-
higher melting point (27231C). With respect to Ta compounds
(i.e., TaB , TaB, TaSi ), they are beneficial to oxidation resis-
2
2
2
tion layer. SiC depleted layer was not clearly detected in the La
containing UHTCs suggesting that the oxygen transport
through the ZrO was the rate controlling step, which is signifi-
2
tance below about 16001C due to the reduction of oxygen va-
cancy concentration whereas detrimental at higher temperature
because the oxidation performance of remarkably affected by
the decrease of eutectic temperatures for the oxide scale above
approximate 17001C. Therefore, we should take both the ben-
eficial and detrimental aspects into account when we choose
modifiers to improve the oxidation performance. In addition, we
also need know whether beneficial or detrimental aspect is dom-
inant under the certain condition.
cantly different from that without additives in which the dom-
inant route of the oxygen transport is pores in the SiC depleted
1
8,19,28,29
layer.
through ZrO
The increase of the inward oxidation diffusion
-rich oxides and outward cationic diffusion was
2
probably responsible for the enhanced oxidation rate.
Figure 3 shows the weight gains of ZrB -based UHTCs mod-
ified by CrB , HfB , TaB, AlN, and La , which are consistent
2
2
2
2 3
O
with the oxidation performance observed from macrographs
and oxide scales. Only addition of HfB was found to beneficial
IV. Conclusion
2
to the oxidation resistance and all other additions are detrimen-
tal to the oxidation performance. TaB addition exhibited poor-
est oxidation resistance among CrB , HfB , and TaB. The
2 2
addition of AlN significantly decreased the oxidation resistance
although only 5 vol% was added to the material. These addi-
2 2 6 2 3
tives, such as TiB , TaB, TaSi , LaB , La O , and AlN, exert a
2 2 2
The additions of CrB and HfB to the ZrB –SiC composites
have minor effect on the structure evolution and oxidation re-
sistance at 18001C and do not change oxidation mechanism.
2 2
However, all the other additives, such as TiB , TaB, TaSi ,
LaB , La , and AlN, have detrimental effect on the oxidation
6
2 3
O
behavior of the ZrB -based UHTCs. These additives have
2
great influence on oxidation resistance through changing the
structure evolution of the oxide scale and oxygen transport
route during oxidation process. The effect of these additives
on the oxidation resistance of UHTCs is significantly different
from these at lower temperatures. The oxygen diffusion through
changed the scale structure and dominant oxygen transport
route during oxidation process resulting in the change of the
oxidation mechanism. Particularly, the UHTCs containing
TaSi and TiB revealed a catastrophic increase in oxidation
2
2
rate at 18001C and the samples with TiB
exposed at 18001C for 1 h were completely consumed, which are
also less resistant to oxidation than monolithic ZrB . The effect
2 2
and TaSi additions
ZrO
controlling step for ZrB
cies allow rapid ion transport through the scale.
the addition of lower valance cations to ZrB -based UHTCs will
2
skeleton, silica-rich liquid phase and voids are the rate
2
-based UHTCs. Oxygen lattice vacan-
2
2,30
2
Obviously,
of additives to oxidation behavior of UHTCs is significantly
dependent on the metallic element cations. The additives with
lower cations (i.e., LaB , La O , and AlN) would lower the eu-
6 2 3
tectic point of the oxide scale leading to an accelerated transport
of the zirconia to the surface and increase the inward oxygen
2
diffusion through ZrO -rich oxides as a result of the increase of
the oxygen vacancy concentration and thus reduced oxidation
resistance. The effect of addition of higher cations (i.e., TaB and
2
TaSi ) could be either beneficial or detrimental, which is strongly
dependent on the temperature. Ta additions performed poorly
under ultrahigh temperature due to liquid phase formation lead-
ing to the degradation of the mechanical stability of the scale
and scale adherence.
2
2
increase the oxygen vacancy concentration of ZrO , resulting in
increased oxygen diffusion. Therefore, the oxygen diffusion
rate through ZrO skeleton significantly enhanced, leading to
the change of oxygen transport routes and increased oxidation
2
rate of ZrB
clearly detected in La containing (i.e., LaB
2
. This can be explains why no SiC depleted layer was
and La ) ZrB
6
O
2 3
2
–
SiC composites and the oxidation resistance decreased. It has
been reported that AlN significantly improved the densification
1
2
of ZrB
2
-based UHTCs. However, the addition of AlN should
be avoided in the viewpoint of oxidation resistance. Apparently,
31
31
21
other lower cations, such as Y , La , and Mg , also should
be excluded for the improved oxidation resistance. The intro-
duction of higher cations (i.e., Ta ) may decrease the oxygen
51
vacancy concentration of ZrO leading to the decreased oxygen
2
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