J. Am. Ceram. Soc.

, 99 [3] 808–813 (2016)

DOI: 10.1111/jace.14039
© 2015 The American Ceramic Society

Journal
High-Temperature Oxidation of ZrB2–SiC–AlN Composites at 1600°C
Gaoyuan Ouyang,‡,§ Pratik K Ray,‡,§ Matthew J Kramer,‡,§ and Mufit Akinc‡,§,†
‡
Department of Material Science and Engineering, Iowa State University, Ames, Iowa
§
Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa

The effect of AlN substitution on oxidation of ZrB2–SiC was reported on how AlN affects the oxidation behavior of
evaluated at 1600°C up to 5 h. Replacement of ZrB2 by AlN, ZrB2–SiC composites. Al2O3, which forms when AlN is oxi-
with 30 vol% SiC resulted in improved oxidation resistance dized, is a good oxygen barrier at moderate temperatures
with a thinner scale and reduced oxygen affected area. On the and lowers the viscosity of the borosilicate scale, thereby
other hand, substitution of AlN for SiC resulted in a deteriora- improving its ability to flow into pores and form a continu-
tion of the oxidation resistance with an abnormal scale and sig- ous protective oxide layer.15 Taking these into account, AlN
nificant recession. The effect of SiC content was also studied, substitution is expected to alter the oxidation behavior of
and was found to be consistent with the literature for the com- ZrB2–SiC composites. The present study attempts to address
posites without AlN additions. A similar effect was observed the oxidation behavior of ZrB2–SiC composites with AlN
when AlN was added, with the higher SiC content materials substitutions at 1600°C.
showing improved oxidation resistance. X-ray photoelectron
spectroscopy showed the presence of Al2O3 and SiO2 on the
II. Experimental Procedure
surface, which could possibly lead to a modification in the vis-
cosity of the glassy oxide scale. Possibly, the oxidation behav- ZrB2 (Grade B, ~2 lm particle size, H. C. Starck, Karlsruhe,
ior of ZrB2–SiC composites can be improved with controlled Germany), SiC (Grade UF-10, ~1 lm particle size, H. C.
AlN additions by adjusting the Al:Si ratios. Starck), and AlN (Grade C, ~1 lm particle size, H. C. Star-
ck) were used as raw materials in this study. Four composi-
tions were synthesized: ZrB2–30 vol%SiC (ZS73, no AlN
I. Introduction
addition), ZrB2–30 vol%SiC–10 vol%AlN (ZSA631,
ZrB2–SiC composites are promising candidates for ultra- AlN substitution for ZrB2), ZrB2–20 vol%SiC–10 vol% AlN
high-temperature ceramics for hypersonic applications (ZSA721, AlN substitution for SiC), and ZrB2–20 vol% SiC
because of their unique thermal, chemical, and physical prop- (ZS82, no AlN addition). These composites were sintered
erties. These composites have high melting temperatures, from dense compact and subsequently oxidized at 1600°C
ranging from 2270°C (ZrB2–SiC eutectic) to 3050°C1 (pure and their oxidation behavior was studied.
ZrB2), low densities ranging from 3.21 g/cm3 for SiC to Samples were sintered by following the procedure
6.08 g/cm3 for ZrB2, and high thermal conductivity (c.a. described by Zhang et al.5 Ceramic powders were wet milled
60 W
(m
K) 1 at room temperature2). In order to fully den- in a plastic jar using WC as milling media using roller mill
sify ZrB2–SiC composites at ambient pressure, several addi- (Cole-Parmer Lab mill 8000, Vernon Hills, IL) and methyl
tives such as WC, C, and/or B4C have been proposed.3–8 ethyl ketone (MEK) as solvent for 24 h. The powders were
Several alloying additions have also been considered and contaminated by a small amount (1%–2%) of WC from
tested with the goal of improving the oxidation resistance of milling media which is believed to be beneficial as a sintering
ZrB2–SiC. Most of these studies attempted to modify the aid.5 The mixture was milled for another 24 h after binder
microstructure and composition. The common approaches (QPAC40, polypropylene carbonate) addition. Following wet
toward this problem were summarized by Eakins et al.9 as: milling, the solvent was evaporated at 40°C under vacuum.
(1) increase the viscosity of the borosilicate glass (W,9 The sample was ground and sieved through a 300-lm sieve.
TaSi210); (2) inhibit ZrO2 polymorphic transformation (Ta9); Cylindrical samples were prepared by pressing powders, first
(3) substitute SiC with another silicon-containing compound uniaxially, followed by cold isostatic pressing at 310 MPa.
(Ta5Si311); (4) introduce high-temperature protective refrac- The binder was burned out at 600°C for 1 h in flowing argon
tory phase (LaB612); (5) decrease the porosity of ZrO2 by liq- atmosphere before sintering. Finally, samples were placed in
uid phase sintering of ZrO2 scale (WC9). a BN-coated graphite crucible and sintered in an electrical
Recently, AlN has been proposed as a sintering aid for resistance furnace equipped with graphite heating elements
hot pressing of ZrB2–SiC composites.13,14 Addition of AlN (3060-FP20, Thermal Technology Inc., Santa Rosa, CA).
leads to the partial removal of B2O3 from the surface of The sintering profile included two 1-h isothermal holds at
ZrB2 particles, which in turn help with the densification pro- 1250°C and 1450°C under vacuum to remove the surface oxi-
cess. The reaction between AlN and B2O3 results in the for- des.5 Samples were then sintered at 2000°C in flowing helium
mation of Al2O3 and BN, which possibly contributed to a atmosphere for 2 h.
lower degree of grain growth compared to the unmodified Sintered densities were measured using the Archimedes
ZrB2 samples.13,14 However, despite the availability of litera- method according to the ASTM standard B962-13 and con-
ture on the role of AlN as a sintering aid, not much has been verted to relative theoretical densities. Theoretical densities
were estimated using the rule of mixtures under the assump-
tion that there was no change in composition during sinter-
ing. Microstructures and phase assemblages of the sintered
T. Parthasarathy—contributing editor samples were studied using a JEOL 5910Lv (Tokyo, Japan)
scanning electron microscope (SEM) and a Philips PANalyti-
cal (Almelo, Netherlands) X-ray diffractometer (XRD). Com-
position mapping was done using a FEI Quanta-250
Manuscript No. 36608. Received March 23, 2015; approved October 19, 2015.
†
Author to whom correspondence should be addressed. e-mail: makinc@iastate.edu (Hillsboro, OR) SEM equipped with Oxford Aztec energy-
808
March 2016 Oxidation of ZrB2–SiC–AlN 809

dispersive X-ray analysis system. Cross-sections of the presence of AlN and SiC, therefore, was confirmed using X-
samples were polished before analysis. The samples were ray diffraction. Presence of the constituent elements and
etched with molten NaOH/KOH (1:1 molar ratio) at ~200°C phases was further confirmed with EDS elemental mapping
to reveal the ZrB2 grain boundaries for estimation of the as shown in Figs. 2(a)–(d). Substitution of AlN for ZrB2
grain sizes. Measurements were done from three representa- [Figs. 1(b) and 2(b), ZSA631] leads to a finer microstructure
tive areas of each sample, and the grain sizes estimated using (ZrB2 grain size 6.2  0.3 lm in ZS73 and 3.9  0.4 lm in
an Image Analysis program (ImageJ). ZSA631) which is in agreement with the literature.13,14
Samples were placed on ZrO2 crucibles and oxidation tests ZSA721 shows a larger mass gain during oxidation relative
were carried out in a box furnace at 1600°C in ambient air for to ZS82, ZS73, and ZSA631, as shown in Fig. 3. It appears
up to 5 h. They were introduced directly to the preheated fur- that ZS73 and ZSA631 exhibit similar mass gain and seem to
nace for isothermal oxidation and removed after desired time level off around 5 h, whereas ZSA721 shows higher mass gain
intervals. Three samples were tested for each time period and initially and continues to gain weight almost at a linear rate.
their specific mass changes were averaged. Samples were Similarly, the ZS82 sample, despite a lower initial mass change,
weighed before and after the test. The oxidized surface was exhibits a significant mass gain with time. The oxidation of
studied using SEM, XRD, and XPS, after which the oxidized these composites is likely to involve multiple reactions.16 Possi-
cross-section microstructures were studied. For surface XPS, ble chemical reactions are tabulated in Table I. Reactions (1),
the measurement was done using PHITM Physical Electronics (2), and (4) would result in a mass gain, while reactions (3) and
5500 Multitechnique ESCA system (Physical Electronics Inc., (5) would result in mass loss. Reaction (3) also represents
Chanhassen, MN) with monochromatic AlKa radiation active oxidation of SiC, leading to a Si-depleted subsurface.16
(1486.6 eV). The peak positions were determined with refer- Figure 3 reflects the net effect of these reactions. Hence, a
ence to the adventitious carbon peak at 284.6 eV. The atomic microstructural study of the oxidized cross-sections was car-
concentration was calculated by using the sensitivity factors ried out to analyze the oxidation behavior of these samples.
provided with the PHITM acquisition software. The thickness Figure 4 shows the 5-h oxidized cross-section microstruc-
of the oxide scale was determined by measuring the thickness tures of these composites. ZS73, ZSA631, and ZSA721 show
at 10 areas or more, at 100 lm intervals, along the scale. a degree of similarity in their oxidation behavior, with the
scale comprising three layers. While the thickness of these
layers differed for each sample, the general nature remained
III. Results and Discussion
the same. The top layer exhibiting a dark contrast corre-
Figure 1 shows the microstructure of the sintered samples. sponds to the silica scale. ZS73 [Fig. 4(a)] and ZSA631
The relative densities of the composites were found to be [Fig. 4(b)] exhibit a thinner continuous oxide scale in com-
99.4%, 98.4%, 93.5%, and 95.5% for ZS73 [Fig.1(a)], parison to ZSA721 [Fig. 4(c)]. ZSA721 has numerous pores
ZSA631 [Fig.1(b)], ZSA721 [Fig.1(c)], and ZS82 [Fig. 1(d)], and discontinuities within the oxide scale. Additionally,
respectively. The bright phase in these micrographs corre- ZSA721 sample also shows the presence of ZrO2 channels
sponds to ZrB2, while the dark phase corresponds to SiC perpendicular to the surface, extending well into the oxide
and/or AlN. SiC and AlN could not be differentiated in the subscale. Multiple ZrO2 clusters could also be seen through-
backscattered images due to almost identical Z contrast. The out the microstructure. It has been shown in the literature

(a) (b)

(c) (d)

Fig. 1. Backscattered images of ZrB2–SiC–AlN composites, after sintering. (a) ZS73, (b) ZSA631, (c) ZSA721, and (d) ZS82.
810 Journal of the American Ceramic Society—Ouyang et al. Vol. 99, No. 3

(a) (b)

(c) (d)

Fig. 2. Layered EDS elemental maps of ZrB2–SiC–AlN composites, after sintering. (a) ZS73, (b) ZSA631, (c) ZSA721, and (d) ZS82. The
bright phase has high concentration in Zr, B; the gray phase has high concentration in Si, C; the dark phase has high concentration in Al, N.

Table I. Mass Change Associated with Various Oxidation

Reactions17
Number Reactions Mass per mole of Dm, g

1 ZrB2(s) + 52O2(g)? ZrB2 80

ZrO2(s) + B2O3(l)
2 SiC(s) + 32O2(g) ? SiO2(l) + SiC 20
CO(g)
3 SiC(s) + O2(g) ? SiO(g) + SiC 40
CO(g)
4 AlN(s) + 34 O2(g) ? AlN 10
2Al2O3(s) + 2N2 (g)
1 1 17

5 B2O3(l) ?B2O3(g) B2O3 69.6

penetration of oxygen is also possible through pores and

cracks in the ZrO2 channel. Hence, the presence of ZrO2
channels and clusters instead of an uninterrupted silica scale
Fig. 3. Mass change of samples on oxidation in air at 1600°C. exacerbates the oxidation of the composite. The oxidized
Each data point represents average of three samples with error bars microstructures of ZS82 shows the similar layered
indicated for the range of mass change. microstructures, but followed by a large and irregular
Si-depleted region, as shown in Fig. 4(d).
It can be seen from the composition maps in Fig. 5 that
that SiO2 affords improved oxidation resistance in compar- the various layers in ZSA631, ZS73, and ZSA721 are rather
ison to ZrO2 in ZrB2–SiC systems.18,19 According to Opeka regular and relatively planar, whereas ZS82 shows significant
et al.,19 parabolic rate constant for ZrO2 is much higher than irregularities, which is in agreement with the results reported
SiO2. Thus, oxygen transport in ZrO2 channels is expected to by Williams et al.22 The Si map shown in Fig. 5(h) shows
be much higher than the silica scale. Although volume diffu- these irregularities clearly, especially in case of the Si-
sion of oxygen through ZrO2 grain is limited resulted from depleted region. The oxidation behavior in this composite is
its poor electronic conductivity,20 interfacial diffusion of oxy- therefore expected to be relatively stochastic.
gen through ZrO2 interface is three to four orders magnitude The microstructures show the presence of an intermediate
faster.21 Furthermore, the zirconia formed on the scale is not layer of oxide underneath the silica scale for ZS73, ZSA631,
dense—pores are present in the zirconia scale, and rapid and ZS721. The elemental distributions mapped using EDS
March 2016 Oxidation of ZrB2–SiC–AlN 811

(a) (b)

(c) (d)

Fig. 4. Cross-section microstructures of the 1600°C, 5-h oxidized coupons—(a) ZS73, (b) ZSA631, (c) ZSA721, and (d) ZS82. The inset shows
the structure of the oxide scale at a higher magnification (3009).

indicate the presence of Zr, Si, and O, while the backscat- a higher atomic number than Si (40 vis- a-vis 14), has a higher
tered images suggest the presence of two phases. Hence, pre- atomic scattering factor which translates to stronger X-ray
sumably, this layer comprised ZrO2 + SiO2, which would be intensities24 and (ii) the relative amount of ZrO2 at the sur-
in accordance with the results reported in the literature.16,23 face is higher in ZSA721 or ZS82, compared to ZS73 and
The layer underneath this mixed oxide scale exhibits signifi- ZSA631. Crystalline ZrO2 will result in a higher intensity
cant silicon depletion. This Si-depleted region forms the final compared to SiO2, and this effect will be amplified as the
layer of the oxygen affected area. The mechanism of silicon amount of ZrO2 increases relative to SiO2. The SEM studies
depletion has been discussed elsewhere.16 While all these indicated the presence of significant amount of ZrO2 chan-
three compositions show an intermediate layer of nels and clusters near the surface, embedded in the surround-
ZrO2 + SiO2 of comparable thickness, they exhibit significant ing silica in ZSA721, in comparison to ZS73 and ZSA631.
differences in the top layer and the subscale region. ZSA631 The SEM results are, therefore, further corroborated by the
[Fig. 4(b)] has the thinnest top layer of ~30  10 lm, while X-ray evidence.
ZSA721 [Fig. 4(c)] has the thickest silica layer, The presence of ZrO2 channels would provide an easy
~120  10 lm. ZS73 has a silica top layer thickness of pathway for oxygen in ZSA721. This results in higher Si
~50  5 lm. The thickness of the silica scale in ZS82 shows recession in the subscale. However, the presence of a signifi-
a significant variation, but the relatively uniform regions of cant amount of ZrO2 is indicative of inadequate surface cov-
the scale are comparable to ZSA631, showing a thickness of erage by the oxide scale in this particular case. Since the
35  10 lm. The subscale region is comparable for ZS73 primary protection against oxidation in these materials is due
and ZSA631, with the latter having a marginally thinner sub- to the formation of continuous SiO2 layer, it stands to reason
scale (65 lm vis- a-vis 50 lm). However, ZSA721 shows a that the oxidation resistance would be significantly dependent
very thick silicon-depleted subscale, ~150 lm. The ZS82 sam- on the amount of Si available to form SiO2. Furthermore, a
ple also exhibited the layered structure. The presence of SiO2 lower SiO2 content in the scale is likely to be balanced by
in the SiO2 + ZrO2 layer is quite low that the layer boundary higher ZrO2 content, which has an adverse effect on the oxi-
is not easily recognizable to eye. In this case, there is a signif- dation resistance since the presence of ZrO2 in the scale
icant variation in the Si-depleted region ranging from 80 to results in a discontinuous SiO2 layer. The effect of SiO2 con-
150 lm. Based on the microstructure and EDS analysis, the tent is also evident when the oxidation behavior of ZS73 and
net oxygen affected region for ZS73, ZSA631, ZSA721, and ZS82 is compared. ZS82 has a larger (and non-uniform) Si-
ZS82 were 160, 130, 300, and 110–180 lm, respectively depleted region underneath the silica scale. This deterioration
(shown schematically in Fig. 6). in oxidation resistance caused by reduced SiO2 content is
X-ray data were collected from the oxidized surfaces of consistent with the literature.22 The lower degree of mass
these samples (Fig. 7). The diffractograms from ZSA721 and change in ZS82 is likely due to the presence of higher ZrB2
ZS82 were dominated by the peaks of monoclinic ZrO2. content, which results in larger amounts of the volatile B2O3.
However, at the lower diffraction angles 19° < 2h < 22°, This contributes to larger mass loss and lower net mass
extremely small peaks corresponding to SiO2 can be gains. As discussed in the text, mass change is not the best
observed. Peaks corresponding to zircon or mullite were indication of oxidation resistance, especially since it repre-
absent. On the other hand, both ZS73 and ZSA631 exhibit sents the net results of gain (oxidations remaining on the
high background in 2h = 20°–30° region which is the charac- scale) and loss (volatile oxides). ZS73 and ZSA631 have the
teristic of an amorphous phase. The nature of these diffrac- same volume fraction of SiC. However, ZSA631 contains
tion patterns can be attributed to two factors—(i) Zr, having 10% AlN and 60% ZrB2 (as opposed to 70% ZrB2 in ZS73).
812 Journal of the American Ceramic Society—Ouyang et al. Vol. 99, No. 3

(a) (b)

(c) (d)

(e) (f) (g) (h)

Fig. 5. EDS maps of corresponding elements for four 5-h oxidized coupons. (a) ZS73, (b) ZSA631, (c) ZSA721, and (d) ZS82; Their
corresponding Si maps are shown in e, f, g, and h.

(a) (b) (c) (d)

Fig. 6. Schematic representation of the oxygen affected area after

5 h oxidation, based on EDS mapping ((a) ZS73, (b) ZSA631, (c)
ZSA721, and (d) ZS82). The light gray channels/cluster in the top
layer are ZrO2 channels.

The oxide scale that initially forms at high temperatures in Fig. 7. X-ray diffractogram of the surface of ZS73, ZSA631,
these composites is a borosilicate scale. The SiO2–B2O3 sys- ZSA721, and ZS82 coupons after oxidation in air at 1600°C for 5 h.
tem shows a low melting liquid around 440°C on the B2O3
rich side.25 As the SiO2 content increases, the liquidus tem- the oxide scale, which in turn, will result in poorer surface
perature increases correspondingly. B2O3 is known to volati- coverage. Accurate quantification of boron is difficult. Mini-
lize above 1200°C. Therefore, during the oxidation process, mum amount of boron is expected in the scale surface based
as the temperature increases above 1200°C, B2O3 starts to on the XPS results (Table II). This result is in agreement
evaporate from the surface, which drives the liquidus temper- with the results of Shugart et al.,26 Rezaie et al.,27 Levine
atures higher. This is likely to result in increased viscosity of et al.,28 and Karlsdottir et al.29 that the surface B content in
March 2016 Oxidation of ZrB2–SiC–AlN 813

Table II. XPS Data from the Oxidized Surface (Units in References
Atomic Percentage) 1
A. McHale, H. McMurdie, and H. Ondik, Phase Equilibria Diagrams. Vol-
ume X, Borides, Carbides, and Nitrides : Phase Diagrams for Ceramists. Vol.
Sample B O Al Si Zr Al:Si B:Si
X. American Ceramic Society, Westerville, Ohio, 1994.
2
J. W. Zimmermann, G. E. Hilmas, W. G. Fahrenholtz, R. B. Dinwiddie,
ZS73 2 71 — 26 <1 — 0.1 W. D. Porter, and H. Wang, “Thermophysical Properties of ZrB2 and ZrB2–
ZSA631 2 72 4 22 <1 0.2 0.1 SiC Ceramics,” J. Am. Ceram. Soc., 91 [5] 1405–11 (2008).
<1
3
ZSA721 3 72 4 20 0.2 0.15 A. Chamberlain, W. Fahrenholtz, and G. Hilmas, “Pressureless Sintering
of Zirconium Diboride,” J. Am. Ceram. Soc., 89 [2] 450–6 (2006).
ZS82 2 71 — 23 5 — 0.1 4
W. Fahrenholtz, G. Hilmas, S. Zhang, and S. Zhu, “Pressureless Sintering
of Zirconium Diboride: Particle Size and Additive Effects,” J. Am. Ceram.
Soc., 91 [5] 1398–404 (2008).
the glassy scale is low due to the high vapor pressure of 5
S. C. Zhang, G. E. Hilmas, and W. G. Fahrenholtz, “Pressureless Sintering
boria,30 although its amount could be higher deeper in the of ZrB2–SiC Ceramics,” J. Am. Ceram. Soc., 91 [1] 26–32 (2008).
glassy layer. 6
H. Zhang, Y. Yan, Z. Huang, X. Liu, and D. Jiang, “Pressureless Sintering
Urbain et al. reported the viscosities in the Al2O3–SiO2 of ZrB2–SiC Ceramics: The Effect of B4C Content,” Scripta Mater., 60 [7]
559–62 (2009a).
system over a range of compositions and temperatures.15 7
H. Zhang, Y. Yan, Z. Huang, and X. Liu, “Pressureless Sintering of ZrB2–
They showed that as the Al2O3 content increased, the viscos- SiC Composites Strengthening by Ultra-Fine Ceramic Powders,” Rare Metal
ity of the Al2O3–SiO2 system decreased at 1600°C. The pres- Mater. Eng., 38, 916–9 (2009b).
8
ence of Al2O3 from the oxidation of AlN in ZSA631 is S. Zhu, W. Fahrenholtz, G. Hilmas, and S. Zhang, “Pressureless Sintering
of Carbon-Coated Zirconium Diboride Powders,” Mater. Sci. Eng. a-Struc.
therefore likely to lower the viscosity of the oxide scale in Mater. Properties Microstruc. Process., 459[1-2], 167–71 (2007).
comparison to the ZS73 sample and hence result in a contin- 9
E. Eakins, D. Jayaseelan, and W. Lee, “Toward Oxidation-Resistant ZrB2–
uous surface coverage. This, in fact, is reflected in the cross- SiC Ultra High Temperature Ceramics,” Metall. Mater. Transac. a-Phys.
section micrographs [Figs. 4(a) and (b)] for ZS73 and Metall. Mater. Sci., 42A [4] 878–87 (2011).
10
E. Opila, S. Levine, and J. Lorincz, “Oxidation of ZrB2- and HfB2-Based
ZSA631, respectively, where it can be clearly seen that the Ultra-High Temperature Ceramics: Effect of Ta Additions,” J. Mater. Sci., 39
top layer in oxidized ZS73 has a higher ZrO2 content com- [19] 5969–77 (2004).
pared to ZSA631, which would have formed before complete 11
I. Talmy, J. Zaykoski, M. Opeka, and A. Smith, “Properties of Ceramics
scale coverage was obtained. The presence of ZrO2 is not in the System ZrB2–Ta5Si3,” J. Mater. Res., 21 [10] 2593–9 (2006).
12
X. Zhang, P. Hu, J. Han, L. Xu, and S. Meng, “The Addition of Lan-
detected primarily because the depth probed by XPS is typi- thanum Hexaboride to Zirconium Diboride for Improved Oxidation Resis-
cally of the order of tens of nanometers, whereas the ZrO2 tance,” Scripta Mater., 57 [11] 1036–9 (2007).
islands in the micrographs show up at a depth of few 13
F. Monteverde and A. Bellosi, “Beneficial Effects of AlN as Sintering Aid
microns. As expected, given the similar AlN content, the on Microstructure and Mechanical Properties of Hot-Pressed ZrB2,” Adv. Eng.
Mater., 5 [7] 508–12 (2003).
XPS results (Table II) from the oxidized ZSA721 showed a 14
W. Han, G. Li, X. Zhang, and J. Han, “Effect of AlN as Sintering Aid on
similar surface Al content. However, a greater volume frac- Hot-Pressed ZrB2–SiC Ceramic Composite,” J. Alloy. Compd., 471[1-2], 488–
tion of ZrB2 leads to greater availability of Zr to form ZrO2, 91 (2009).
15
in comparison to ZSA631. The porous and permeable ZrO2, G. Urbain, Y. Bottinga, and P. Richet, “Viscosity of Liquid Silica, Sili-
cates and Alumino-Silicates,” Geochim Cosmochim Ac, 46 [6] 1061–72 (1982).
once formed in significantly larger quantities, compared to 16
W. G. Fahrenholtz, “Thermodynamic Analysis of ZrB2–SiC Oxidation:
ZSA631, provides oxygen pathway deeper into the material, Formation of a SiC-Depleted Region,” J. Am. Ceram. Soc., 90 [1] 143–8
which accounts for the difference in oxidation resistance (2007).
17
between these two composites. P. Boch, J. C. Glandus, J. Jarrige, J. P. Lecompte, and J. Mexmain, “Sin-
tering, Oxidation and Mechanical Properties of Hot Pressed Aluminium
Nitride,” Ceram. Int., 8 [1] 34–40 (1982).
18
IV. Conclusions W. C. Tripp, H. H. Davis, and H. C. Graham, “Effect of an SiC Addition
on Oxidation of ZrB2,” Am. Ceram. Soc. Bull., 52 [8] 612–6 (1973).
ZrB2–SiC–AlN compacts with and without AlN were synthe-
19
M. M. Opeka, I. G. Talmy, and J. A. Zaykoski, “Oxidation-Based Materi-
als Selection for 2000°C Plus Hypersonic Aerosurfaces: Theoretical Considera-
sized by pressureless sintering. AlN additions resulted in finer tions and Historical Experience,” J. Mater. Sci., 39 [19] 5887–904 (2004).
microstructures, when substituted for ZrB2. The oxygen 20
T. A. Parthasarathy, R. A. Rapp, M. Opeka, and R. J. KeranS, “A Model
affected region showed a degree of similarity with a SiO2-rich for the Oxidation of ZrB2, HfB2 and TiB2,” Acta Mater., 55 [17] 5999–6010
layer forming on the surface, followed by a ZrO2 + SiO2 (2007).
21
U. Brossmann, R. Wurschum, U. Sodervall, and H. E. Schaefer, “Oxygen
interlayer, and finally a Si-depleted subscale. Chemical analy- Diffusion in Ultrafine Grained Monoclinic ZrO2,” J. Appl. Phys., 85 [11]
sis showed the presence of Al (possibly as Al2O3) in minor 7646–54 (1999).
quantities on the oxidized surface. Based on the work of 22
P. A. Williams, R. Sakidja, J. H. Perepezko, and P. Ritt, “Oxidation of
Urbain et al.,15 this is likely to reduce the viscosity of the ZrB2–SiC Ultra-High Temperature Composites over a Wide Range of SiC
Content,” J. Eur. Ceram. Soc., 32 [14] 3875–83 (2012).
protective SiO2-rich layer, which can flow and cover the sur- 23
A. Rezaie, W. Fahrenholtz, and G. Hilmas, “Evolution of Structure Dur-
face, thereby protecting the composite from further oxida- ing the Oxidation of Zirconium Diboride-Silicon Carbide in Air up to
tion. The ZrB2–30 vol%SiC–10 vol%AlN (ZSA631) showed 1500°C,” J. Eur. Ceram. Soc., 27 [6] 2495–501 (2007).
the best oxidation resistance while the ZrB2–20 vol%SiC–
24
B. Cullity and S. Stock, Elements of X-Ray Diffraction. Prentice-Hall,
Upper Saddle River, New Jersey, 2001.
10 vol%AlN (ZSA721) showed the worst oxidation resis- 25
T. J. Rockett and W. R. Foster, “Phase Relations in the System Boron
tance due to the copious formation of oxygen permeable Oxide–Silica,” J. Am. Ceram. Soc., 48 [2] 75–80 (1965).
ZrO2 channels and clusters, excessive silica viscosity reduc- 26
K. Shugart, S. Y. Liu, F. Craven, and E. Opila, “Determination of
tion, limited Si supply, and higher initial porosity. Therefore, Retained B2O3 Content in ZrB2–30 vol% SiC Oxide Scales,” J. Am. Ceram.
Soc., 98 [1] 287–95 (2015).
we conclude that a more oxidation resistant material can be 27
A. Rezaie, W. Fahrenholtz, and G. Hilmas, “Oxidation of Zirconium
made with a proper Al to Si ratio, tailoring the glass phase Diboride-Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen,” J.
viscosity while still providing sufficient Si content to form a Am. Ceram. Soc., 89 [10] 3240–5 (2006).
28
protective oxide scale. S. Levine, E. Opila, M. Halbig, J. Kiser, M. Singh, and J. Salem, “Evalua-
tion of Ultra-High Temperature Ceramics for Aeropropulsion use,” J. Eur.
Ceram. Soc., 22 [14-15] 2757–67 (2002).
29
Acknowledgments S. N. Karlsdottir and J. W. Halloran, “Rapid Oxidation Characterization
of Ultra-High Temperature Ceramics,” J. Am. Ceram. Soc., 90 [10] 3233–8
This work was supported by the AFOSRHTAM under contract # FA9550-11- (2007).
30
1-201. The authors greatly acknowledge the support from this project. The R. H. Lamoreaux, D. L. Hildenbrand, and L. Brewer, “High-Temperature
authors thank Eric Neuman and Prof. William G. Fahrenholtz for their assis- Vaporization Behavior of Oxides .2. Oxides of Be, Mg, Ca, Sr, Ba, B, Al, Ga,
tance regarding the pressureless sintering of ZrB2–SiC. The authors also thank in, Tl, Si, Ge, Sn, Pb, Zn, Cd, and Hg,” J. Phys. Chem. Ref. Data, 16 [3] 419–
Warren Straszheim for his help in the EDS mapping analyses and Jim Ander- 43 (1987). h
egg for the XPS analyses.