Improved oxidation resistance of zirconium diboride by tungsten carbide additions

Article abstract of DOI:10.1111/j.1551-2916.2008.02713.x

The effect of tungsten carbide (WC) additions on the oxidation resistance of zirconium diboride (ZrB₂) was studied. Zirconium diboride ceramics, nominally pure and with the addition of 4 mol% WC, were densified by pressureless sintering. During densification, the WC that was added to the ceramic dissolved into the ZrB₂ matrix, forming a solid solution. Oxidation behavior was evaluated using thermal gravimetric analysis up to 1500°C and isothermal furnace oxidation at 1500° and 1600°C. The WC additions reduced both the weight gain and the oxide scale thickness. After oxidation at 1600°C for 3 h, the mass gain decreased from 22 mg/cm ² for nominally pure ZrB₂ to 8 mg/cm² for the WC-containing ceramic. Analysis showed that the WC additions improved the oxidation resistance of ZrB₂ by promoting densification of the zirconia scale formed during oxidation through liquid-phase sintering.

Full text of DOI:10.1111/j.1551-2916.2008.02713.x

J. Am. Ceram. Soc., 91 [11] 3530–3535 (2008)
DOI: 10.1111/j.1551-2916.2008.02713.x
r 2008 The American Ceramic Society
ournal
J
Improved Oxidation Resistance of Zirconium Diboride by Tungsten
Carbide Additions
,w
Shi C. Zhang,* Greg E. Hilmas,* and William G. Fahrenholtz**
Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409
The effect of tungsten carbide (WC) additions on the oxidation
resistance of zirconium diboride (ZrB ) was studied. Zirconium
proposed a model to describe the oxidation behavior and final
oxidized microstructure for ZrB , TiB , and HfB . The model,
2
2
2
,8,12
2
5
diboride ceramics, nominally pure and with the addition of
mol% WC, were densified by pressureless sintering. During
and previous experimental results,
oxide scale was not protective at elevated temperatures because it
revealed that the ZrO
2
4
12
densification, the WC that was added to the ceramic dissolved
into the ZrB2 matrix, forming a solid solution. Oxidation
behavior was evaluated using thermal gravimetric analysis
up to 15001C and isothermal furnace oxidation at 15001 and
was porous. The porosity, combined with the columnar micro-
structure of the ZrO scale, offers channels for oxygen transport
2
15
directly and rapidly to the reaction interface. Oxygen transport
is much faster through the resulting pores as compared with
diffusion through the ZrO crystal structure.
1
6001C. The WC additions reduced both the weight gain and the
2
oxide scale thickness. After oxidation at 16001C for 3 h, the
The strategy most often used to improve the oxidation resis-
2
mass gain decreased from 22 mg/cm for nominally pure ZrB
tance of ZrB
Si-containing compounds
the late 1960s, Kaufman and colleagues reported that addi-
tions of 5–30 vol% SiC to ZrB resulted in a significant decrease
in the oxidation rate of ZrB at intermediate (B12001 to
2
-based materials above 14001C is the addition of
2
2
to 8 mg/cm for the WC-containing ceramic. Analysis showed
8,16–26
2 2
such as SiC, MoSi , or TaSi . In
2
7
that the WC additions improved the oxidation resistance of
ZrB by promoting densification of the zirconia scale formed
during oxidation through liquid-phase sintering.
2
2
2
B14001C) and elevated (above B14001C) temperatures due to
the formation of an adherent, protective layer of borosilicate
1
8–23,27
I. Introduction
glass.
that island-like, porous ZrO
However, the experimental results indicated
23
2
deposits grew during oxidation,
2
IRCONIUM diboride (ZrB ) is attractive for a variety of aero-
offering a rapid transport path for oxygen in some cases. At
temperatures above B16001C, or under the flowing gas envi-
ronments encountered in proposed applications, the protective
Z
theoretical density (6.09 g/cm ) among ultra-high temperature
space applications. ZrB has a unique combination of prop-
erties, including a very high melting point (430001C), the lowest
2
3
1
,2
2
SiO -based glass layer can be removed, leaving behind a porous
ceramics (UHTCs), high strength, and high-elastic modulus.
As a result, ZrB -based ceramics are candidates for applications
2
ZrO skeleton. Formation of a dense ZrO layer during oxida-
2
2
2
tion of ZrB
2
materials, with or without SiC additions, may
such as leading edges and thermal protection systems for reus-
able atmospheric re-entry vehicles, hypersonic flight vehicles,
improve their oxidation resistance.
This research focuses on a method to improve the oxidation
resistance of refractory diborides without the addition of com-
3
–5
and rocket propulsion systems.
been used in other high-temperature structural applications.
As with other nonoxide ceramics, ZrB oxidizes when exposed
to air at elevated temperatures. The oxidation behavior of ZrB
These materials have also
6
,7
pounds that form SiO
improve the oxidation resistance of ZrB
2
when oxidized. Specifically, the goal is to
at elevated tempera-
2
2
2
8–12
tures by promoting the formation of a dense ZrO scale on the
2
surface of ZrB2.
has been studied since the 1960s.
of ZrB
Reaction (1), which leads to measurable mass gain above about
In general, the oxidation
2
can be considered to be stoichiometric according to
1
3
8001C
II. Experimental Procedures
(
1) Powder Preparation and Characterization
ZrB2ðcrÞ þ 5=2 O2ðgÞ ! ZrO2ðcrÞ þ B2O3ðlÞ
(1)
2
ZrB (Grade B, H.C. Starck, Newton, MA) was used to prepare
the materials for this study. Tungsten additions were accom-
plished by adding tungsten carbide (WC) powder (CERAC,
Milwaukee, WI) to some batches. For some samples, the ZrB
powders were attrition milled (Model 01-HD, Union Process,
Akron, OH) using WC milling media to reduce the particle size.
After milling, the average particle sizes were calculated from sur-
face area values determined by nitrogen adsorption (NOVA 1000,
Quantachrome, Boynton Beach, FL). ZrO (Alfa Aesar, Ward
2
Hill, MA) and tungsten oxide (WO ) (Aldrich Chemical, Mil-
3
waukee, WI) powders were used to study the sintering behavior of
the possible oxidation products. The purity, particle size, and
ZrB oxidation can be divided into three stages as the temper-
2
ature increases, based on the behavior of the B
2
O
3
that is
12
2
formed. Below B12001C, molten B O forms a continuous
2 3
film, which results in parabolic mass gain kinetics because one of
the reactants (oxygen) must diffuse through the product layer
(B O ) to react with ZrB . At intermediate temperatures (B12001
2 3 2
to B14001C), paralinear kinetics are observed due to partial
evaporation of the B O . Above B14001C, evaporation of B O
2
3
2 3
is rapid, leaving behind a porous ZrO layer that does not protect
2
14
2
ZrB from further oxidation. Recently, Parthasarathy et al.
2 2 3
suppliers of ZrB , WC, ZrO , and WO are reported in Table I.
N. Jacobson—contributing editor
(
2) Sample Preparation
To enhance the densification of ZrB
H.C. Starck) was added to all of the batches. The as-received
ZrB , 2 wt% B C, and WC (for W-containing batches)
were dispersed in methyl ethyl ketone by ball milling for 24 h
with a dispersant (DISPERBYK-110, BYK-Chemie Co., Wesel,
Germany). Next, 1 wt% binder (Qpac-40, Empower Materials,
2
4
, 2 wt% B C (Grade HS,
2
8
Manuscript No. 24439. Received March 19, 2008; approved August 14, 2008.
This work was funded by the Air Force Research Laboratory on contract number
FA8650-04-C-5704 through the Center for Aerospace Manufacturing Technologies at the
Missouri University of Science and Technology.
2
4
*Member, The American Ceramic Society.
*
*Fellow, The American Ceramic Society.
Author to whom correspondence should be addressed. e-mail: scz@mst.edu
w
3
530

November 2008
Oxidation Resistance of Zirconium Diboride
3531
Table I. Raw Material Characteristics
Surface
area
2
(m /g)
Oxygen
Grade
Particle
content
(%)
Materials
(purity%)
size (mm)
Supplier
ZrB2
B4C
WC
ZrO2
WO3
B499
HS
499.5
991
99.995
2
0.8
o1
0.3
1
15.8
0.9
1.3
H.C. Starck
H.C. Starck
CERAC
Alfa
3.2
—
—
Aldrich
Newark, DE) was added and the mixture was milled for another
4 h. After ball milling, the slurry was dried and ground to form
2
granules for dry pressing. Cylindrical disks 19 mm in diameter
were formed by uniaxial pressing at 18.6 MPa (2.7 ksi), followed
by cold isostatic pressing at 315 MPa (45 ksi).
Compacted pellets were densified by pressureless sintering.
Pellets were heated at 101C/min in a graphite crucible to temper-
atures ranging from 18501 to 20501C using a graphite element
furnace (3060-FP20, Thermal Technology, Santa Rosa, CA).
The furnace atmosphere was a mild vacuum (B20 Pa) for tem-
Fig. 1. Thermal gravimetric analysis results for nominally pure zirco-
nium diboride (ZrB ) and ZrB containing 4 mol% tungsten carbide
2 2
(WC) heated at 101C/min to 15001C in flowing air.
peratures of 16501C or below, but was switched to flowing argon
5
(
2
B10 Pa) for temperatures above 16501C. Hold times were from
to 3 h for sintering. The phases present after heating were
the temperature reached 15001C (Fig. 1). In addition, the mass
gain increased more rapidly as the temperature approached
15001C, indicating that the oxidation rate increased as the
temperature increased. Both the Parthasarathy model, and the
previous experimental studies, have concluded that oxidation of
analyzed by X-ray diffraction (XRD; XDS 2000, Scintag Inc.,
Cupertino, CA). The bulk densities of sintered specimens were
determined using the Archimedes method. Relative densities
were calculated by dividing the measured bulk densities by the
theoretical densities, which were calculated for each composition
2
nominally pure ZrB in air at an elevated temperature produced
based on the amounts of B C and WC in the batch compositions.
4
a porous scale. Figure 2 shows a typical cross-section micro-
structure of the ZrB prepared from as-received ZrB powder
Densified pellets with a diameter of B18 mm and a thickness of
B5 mm were produced. Specimens with dimensions of 5 mm ꢀ
2
2
oxidized at 12001C for 2 h in air. The porous scale consists of
columnar ZrO grains extending to the surface. The channels in
the ZrO outer scale offer a rapid path for oxygen transport to
4
mmꢀ 3 mm were diced from the pellets for oxidation tests.
3) Oxidation Testing
2
2
(
2
the interface between ZrO and the underlying, unoxidized
The mass gain was characterized as a function of temperature for
ZrB -based ceramics with and without WC additions by thermal
ZrB . Hence, increasing the density of the ZrO scale may im-
2
prove the oxidation resistance of ZrB in this temperature range.
2
2
2
gravimetric analysis (TGA; Netzsch Sta 409C/CD, Selb, Ger-
many). Mass gain was also characterized as a function of time at
(
2) Liquid-Phase Sintering
2
15001 or 16001C using static testing in a MoSi resistance heated
Liquid-phase sintering is one approach that may increase the
relative density of the ZrO scale that forms during oxidation of
ZrB
show that ZrO
above 12601C. The minimum amount of WO needed to form a
liquid phase with ZrO
WO can promote densification of ZrO by liquid-phase sinte-
ring, a series of experiments were conducted using mixtures of
horizontal mullite tube furnace (Model 0000543 Rapid Temper-
ature Furnace, CM Inc., Bloomfield, NJ) equipped with gas-tight
end caps. Before oxidation, specimens were cleaned in an ultra-
sonic bath with acetone. For TGA testing, the rectangular spec-
imens were placed in an Al O crucible that had been notched in
2
29
2
at elevated temperatures. Phase equilibrium diagrams
2
forms a liquid phase with WO at temperatures
3
3
2
3
2
at 12601C is B4 mol%. To confirm that
two places near the bottom of the crucible to allow air to flow
freely around the specimens. For isothermal oxidation studies,
similar rectangular specimens were placed on a zirconia setter
with ridges to minimize the contact area between the specimens
and the setter. Specimens were heated at B51C/min to 15001 or
3
2
16001C and held for 1, 2, or 3 h in a flowing air atmosphere.
After cooling to room temperature, the mass was measured
again. The normalized weight gains (mg/cm ) were calculated
from mass gain and calculated surface area of the specimens.
2
(4) Sample Characterization
The thicknesses of the resulting oxidation layers were measured
from polished cross sections. The microstructures of the oxide scale
of some oxidized specimens were observed using scanning elec-
tron microscopy (SEM; S-570, Hitachi, Tokyo, Japan), and el-
emental distribution maps were collected in the SEM using
energy-dispersive spectroscopy (EDS; E2V, Scientific Instru-
ments, Buckinghamshire, U.K.).
III. Results and Discussion
(
1) Oxidation of Nominally Pure ZrB2
The TGA results for nominally pure ZrB
theoretical density) indicated a weight gain of 6.7 mg/cm when
2
materials (498% of
Fig. 2. Fracture surface of the scale on nominally pure zirconium dibo-
ride (ZrB ) with 2 wt% B C that was oxidized at 12001C in air for 2 h.
2 4
2

3532
Journal of the American Ceramic Society—Zhang et al.
Vol. 91, No. 11
2 2
Fig. 3. Fracture surface of ZrO containing 4 mol% tungsten oxide heated to 14501C showing evidence of liquid-phase sintering of the ZrO by a
tungsten-rich phase.
ZrO
2
and WO
3
. Pellets of ZrO
2
(B0.3 mm starting particle size)
with WO additions of 4, 6, and 10 mol% were sintered at
to add the W source in a form that can be uniformly distributed
in the ZrB before sintering and without inhibiting the densifi-
3
2
1
4501C for 3 h in air. All of the pellets reached full density. For
ZrO containing 4 mol% WO , SEM showed that the ZrO
particles grew and developed a round morphology. The ZrO
cation of ZrB
during ZrB oxidation. In a previous study of pressureless sinte-
ring of ZrB
reduction using co-bonded WC grinding media, dissolved into
2 3
. In addition, the precursor should produce WO
2
3
2
2
2
8
2
2
,
WC, present as an impurity from particle size
particles, which were initially B0.3 mm in diameter, grew to B20
mm in diameter (Fig. 3) at 14501C, roughly 70 times their initial
size. Chemical analysis by EDS confirmed that a W-rich phase
2
8,31–33
the ZrB
2
lattice.
patterns of ZrB
Figures 4(a) and (b) show the XRD
containing 4 mol% WC before and after pres-
2
surrounded the round ZrO grains. Microstructure and chemical
2
analyses indicated that a WO addition of 4 mol% led to liquid-
3
sureless sintering at 20501C for 3 h in Ar. The WC phase
that was detected in the green pellets before sintering (Fig. 4(a))
phase formation in ZrO . Further, the liquid phase appeared to
2
2
disappeared after sintering and only ZrB was observed by XRD
promote densification of the mixture by liquid-phase sintering.
The volatilization of WO was also considered. If, like B O ,
(Fig. 4(b)). The peaks in Fig. 4(b) can be indexed to the XRD
powder diffraction file card (PDF card number 34-0423) for
3
2
3
the WO
may not be an effective liquid-phase former for sintering ZrO
The vapor pressures of various WO species (e.g., WO , (WO
etc.) were calculated using thermodynamic software and the
results are summarized in Table II. From the calculation,
3
formed during oxidation evaporated rapidly, then it
hexagonal ZrB
2
. However, the peaks are shifted to slightly
. Because W
2
.
,
higher values of 2y compared with pure ZrB
2
˚
(1.4 A) has a smaller covalent radius than zirconium (Zr)
3
3
3
)
2
3
0
34
(1.6 A), substitution of W into the ZrB lattice is expected to
˚
2
decrease the average unit-cell size, and shift the diffraction peaks
to higher 2y values in Fig. 4.
Thermodynamic analysis indicated that the standard Gibbs
(
1
WO
12 Pa (1.1ꢀ 10 atm.). Based on observations of the sintered
mixture of ZrO and 4 mol% WO shown in Fig. 3, the W-rich
3 3
) had the highest vapor pressure at 15001C, which was
ꢁ
3
free energy of reaction for the oxidation of W to WO was
3
2
3
oxide phase did not appear to undergo significant evaporation
despite the relatively high vapor pressure. This may be due to
the low W concentration in the system, which would reduce
favorable over the entire temperature range studied, from room
temperature to 20001C. XRD phase analysis of an oxidized
mixture of 10 mol% WC and 90 mol% ZrB powders showed
2
the activity of W in the ZrB
outer surface composed of WO
2
or the relatively low fraction of the
, which would reduce the evap-
that monoclinic ZrO
during oxidation at 15001C in air. Because the WC and ZrB
2 3
and hexagonal WO were produced
3
2
oration rate. Thus, the actual vapor pressure of the (WO
species may be lower than the values shown in Table II.
3
)n
form a solid solution, it is reasonable to predict that an inti-
mately mixed oxide scale containing both ZrO and WO would
2
3
form during oxidation. Therefore, WC seems to be an accept-
able precursor for WO because WC forms a solid solution with
ZrB and it oxidizes to form WO when ZrB is oxidized. As
discussed above, the condensed WO appeared to be stable over
3
the temperature range reported in this study. One final consid-
eration was the volume change during oxidation. When W is
3
2
3
2
(
3) Solid Solution Formation
Previous research has shown that oxide impurities inhibit dens-
ification of ZrB by promoting coarsening of ZrB at tempera-
tures below which it can be densified by solid-state sintering.
Therefore, the addition of WO directly to the ZrB would likely
inhibit densification and perhaps have other adverse effects on
the properties of the resulting ceramics. Therefore, it is desirable
2
2
2
8
oxidized to WO , the volume increases by B230%, which means
3
3
2
the oxidation of a small amount of W produces a large volume
of WO . Materials that form protective oxide scales, such as
3
Al, often show a substantial volume increase upon oxidation.
For Al, the volume increases by B25% when it oxidizes to
form a-Al
W-containing ZrB
2
O
3
. Tailoring the overall volume increase when
is oxidized may also be beneficial for the
Table II. Vapor Pressure of Tungsten Oxide (WO ) Species
3
2
n
in Equilibrium with WO Solid
3
2
formation of a dense ZrO -based protective scale.
Vapor species
P at 15001C (Pa)
1.08 ꢀ 10ꢁ3
P at 16001C (Pa)
ꢁ3
(4) Oxidation Behavior
WO3
8.25 ꢀ 10
TGA results for ZrB -containing 4 mol% WC are compared
2
(
(
(
WO3)2
WO3)3
12.2
112
75.2
565
2
with the oxidation of nominally pure ZrB in Fig. 1. Analysis
indicates that the weight gain for the ZrB ceramic with 4 mol%
2
WO3)
4
35.3
20.4
2
2
WC was 4.4 mg/cm compared with 6.7 mg/cm for nominally

November 2008
Oxidation Resistance of Zirconium Diboride
3533
Fig. 5. Weight gain for nominally pure zirconium diboride (ZrB
tungsten-containing ZrB after isothermal oxidation at 15001 and
6001C for 60, 120, or 180 min.
2
) and
2
1
pure ZrB ceramics increased from B50 mm after a 1-h hold to
2
B500 mm after 3 h. Under the same oxidation conditions, the
thickness of the ZrO scale for the WC-containing ZrB was
only B25 mm for 1 h, with an increase to B100 mm after 3 h.
Hence, the WC-containing ceramics exhibited better oxidation
2
2
2
resistance than nominally pure ZrB based on both mass gain
and scale thickness. The reduced weight gain can be attributed
2
to the higher density of the ZrO outer scale at the higher
temperature.
Remarkably, at 16001C, the weight gains for ZrB ceramics
2
containing 4 mol% WC were substantially lower for the same
isothermal hold times as compared with the results at 15001C
(
ZrB gained only 8 mg/cm , compared with 11 mg/cm for the
Fig. 5). For example, after 3 h at 16001C, the WC-containing
2
2
2
same W-containing ZrB
this analysis, WC is more effective at improving the oxidation
resistance of ZrB at higher temperatures, probably due to more
effective liquid-phase sintering of the ZrO scale at 16001C.
2
oxidized at 15001C for 3 h. Based on
2
2
(
5) Microstructure Characterization
To confirm the beneficial role of the W additions, cross sections
of the ZrO scales of nominally pure ZrB and the ZrB con-
2
Fig. 4. X-Ray diffraction pattern of zirconium diboride (ZrB ) con-
taining 4 mol% tungsten carbide (WC) (a) before and (b) after sintering
and (c) after oxidation.
2
2
2
taining WC that were oxidized at 16001C in air for 2 h were
characterized using SEM and compared (Fig. 6). For the nom-
inally pure ZrB , the ZrO oxidation scale was B1.15 mm thick
pure ZrB
the TGA curves for nominally pure ZrB
ZrB both showed the same trend of increasing mass gain as the
temperature approached 15001C. The full benefit of WC addi-
tions is not apparent in this experiment in which the specimens
were heated and cooled with no isothermal treatment. In this
2
, a reduction of about 35% in the mass gain. However,
2
2
(
Fig. 6(a)) and consisted of columnar ZrO
pore channels between them (Fig. 6(b)). The open channels offer
a path for rapid oxygen transport to the ZrO /ZrB interface
where oxidation occurs. In contrast, the ZrO oxidation scale
2
grains with open
2
and WC-containing
2
2
2
2
on the WC-containing ZrB
with a dense microstructure that consisted of equiaxed ZrO
2
was o0.3 mm thick (Fig. 6(c))
case, lower weight gains for the WC-containing ZrB
attributed to the formation of a stable oxide that filled the voids
between the ZrO grains. The volume increase associated with
2
was
2
grains (Fig. 6(d)). This microstructure appears to be a barrier
to oxygen transport. Therefore, the weight gain after oxidation
2
2
at 16001C for 2 h in air was lower, only B7 mg/cm , compared
W oxidation should produce an oxide that fills the voids
between the ZrO grains, much as B O would provide oxida-
2
with the nominally pure ZrB , which had a weight gain of
2
2
3
2
B14 mg/cm under the same conditions. W and Zr elemental
maps of the area shown in Figs. 6(e) and (f) confirmed the
presence of W in the dense surface layer. Thus, the microstruc-
tion protection at lower temperatures.
To fully understand the benefits of WC additions to ZrB
isothermal oxidation studies were conducted at 15001 and
6001C. Figure 5 compares the weight gains of nominally pure
ZrB and ZrB containing 4 mol% WC at 15001 and 16001C for
hold times up to 3 h. The WC-containing ZrB ceramics exhib-
2
,
1
2
2
2
Table III. Comparison of the Thicknesses of the Outer
Scale Produced by Oxidation of Nominally Pure Zirconium
Diboride (ZrB ) and ZrB -Containing 4 mol% Tungsten
ited a lower weight gain at both temperatures for all hold times
compared with the nominally pure ZrB . For example, after 3 h
2
2
2
2
gained 11 mg/cm com-
pared with 15 mg/cm for the nominally pure ZrB . Cross
2
at 15001C, the WC-containing ZrB
2
Carbide (WC) at 15001C
2
Materials
1 h
2 h
3 h
sections of both materials isothermally oxidized at 15001C
were observed using SEM. The thicknesses of the oxidized outer
scales were measured from the SEM images and compared
Nominally pure ZrB2
ZrB 14 mol% WC
50 mm
25 mm
150 mm
50 mm
500 mm
100 mm
2
2
in Table III. The thickness of the ZrO scale for nominally

3
534
Journal of the American Ceramic Society—Zhang et al.
Vol. 91, No. 11
(a)
(b)
(c)
(d)
(e)
(f)
2 2 2
Fig. 6. Scanning electron micrographs of ZrO outer scale for nominally pure zirconium diboride (ZrB ) (a, b) and tungsten carbide-containing ZrB (c,
d) and energy-dispersive spectroscopy compositional maps (e, f) corresponding to the areas in (c) and (d).
ture and composition of the oxide were consistent with the
formation of a W-containing oxide phase, which apparently
sidering all of these factors, it is not surprising that the WO3
phase was not detected using XRD.
2
promoted liquid-phase densification of the ZrO scale.
The phase composition of the outer scale was also investi-
gated further using XRD analysis. After oxidation at 15001C for
IV. Conclusion
3
2 2
h, only monoclinic ZrO was detected on the surface of ZrB
containing 4 mol% WC (Fig. 4(b)). However, additional SEM
analysis, including W element mapping by EDS, of a specimen
oxidized at 16001C (Fig. 6e), indicated that some W was present
A new approach was developed to improve the oxidation resis-
tance of ZrB . A W compound, WC in this case, was added
to ZrB2 before densification by pressureless sintering. During
2
in the ZrO
2
outer scale. Because crystalline WO
3
was not ob-
oxidation, the presence of WO
liquid-phase sintering of the ZrO
microstructure, increased its density, and decreased the rate of
oxygen transport. As a result, WC-containing ZrB had improved
oxidation resistance compared to nominally pure ZrB as mea-
3
in the oxide scale resulted in
served by XRD, its detection by SEM/EDS can be attributed to
2
scale, which modified its
2
9
several factors: (1) according to the phase diagram, the solu-
bility of WO in ZrO is about 2 mol% at room temperature; (2)
if no WO were lost during oxidation testing, only B2 mol%
WO would remain and so the relative intensities of the WO
3
2
2
3
2
3
3
sured by mass gain and oxide scale thickness. The theoretical
considerations and experimental results point to three main
criteria that could be used to select other additives that might
similarly enhance the oxidation resistance of Zr-based UHTCs:
(1) WC formed a solid solution with ZrB and was uni-
2
formly distributed in the sintered ZrB ceramic.
2
peaks would be quite low compared with ZrO , perhaps below
2
the XRD detection limit; (3) the major peaks of monoclinic
WO
y values of 23.2511 and 24.2251, very close to the (110) and
011) planes of monoclinic ZrO , which has peaks at 24.0681 and
3
, which are from the (002) and (110) planes, are located at
2
(
2
2
4.4611; and (4) some of the WO
3
may have evaporated during
content to o2 mol%. Con-
(2) During oxidation, WC-containing ZrB
2
simultaneously
3
formed a liquid
oxidation, reducing the final WO
3
oxidized to form both WO and ZrO . WO
3
2

November 2008
phase with ZrO
form a dense outer scale with an equiaxed microstructure. In
contrast, the ZrO scale that formed on nominally pure ZrB
was porous and consisted of columnar grains.
Oxidation Resistance of Zirconium Diboride
3535
13
A. Rezaie, W. G. Fahrenholtz, and G. E. Hilmas, ‘‘Evolution of Structure
During the Oxidation of Zirconium Diboride-Silicon Carbide in Air up to
2
2
, which enhanced the densification of ZrO to
1
5001C,’’ J. Eur. Ceram. Soc., 27 [6] 2495–501 (2007).
T. A. Parthasarathy, R. A. Rapp, M. Opeka, and R. J. Kerans, ‘‘A Model for
14
2
2
the Oxidation of ZrB
(2007).
2 2 2
, HfB , and TiB ,’’ Acta Mater., 55 [17] 5999–6010
(
3) The volume increase (230%) associated with oxidation
of W to form WO enhanced the increase in total volume from
15
J. Li and T. J. Lenosky, ‘‘Thermochemical and Mechanical Stabilities of the
Oxide Scale of ZrB 1SiC and Oxygen Transport Mechanisms,’’ J. Am. Ceram.
Soc., 91 [5] 1475–80 (2008).
3
2
13% for formation of ZrO during oxidation of nominally pure
2
16
ZrB
of ZrB
4) The criteria for selecting other additives that may pro-
duce similar improvements in oxidation resistance of diboride
ceramics are as follows: (a) forms a liquid phase with ZrO at
2
to 23% for formation of WO
3
and ZrO
2
during oxidation
E. V. Clougherty, R. L. Pober, and L. Kaufman, ‘‘Synthesis of Oxidation
Resistant Metal Diboride Composites,’’ Trans. Metall. Soc. AIME, 242 [6] 1077–
2 (1968).
2
containing 4 mol% WC.
8
(
17
W. C. Tripp, H. H. Davis, and H. C. Graham, ‘‘Effect of an SiC Addition on
the Oxidation of ZrB ,’’ Am. Ceram. Bull., 52 [8] 612–6 (1973).
A. Bongiorno, C. J. Forst, R. K. Kalia, J. Li, J. Marschall, A. Nakano, M. M.
2
18
2
¨
Opeka, I. G. Talmy, P. Vashishta, and S. Yip, ‘‘A Perspective on Modeling Ma-
terials in Extreme Environments: Oxidation of Ultrahigh-Temperature Ceramics,’’
MRS Bull., 31 [5] 410–8 (2006).
moderate temperatures (below 14001C) and at low amounts; (b)
dissolves into the diboride matrix; and (c) has a large volume
increase upon conversion of the metal to the oxide.
19
2
W. G. Fahrenholtz, ‘‘Thermodynamic Analysis of ZrB –SiC Oxidation: For-
mation of a SiC-Depleted Region,’’ J. Am. Ceram. Soc., 90 [1] 143–8 (2007).
M. M. Opeka, I. G. Talmy, and J. A. Zaykoski, ‘‘Oxidation-Based Materials
20
Selection for 20001C1Hypersonic Aerosurfaces: Theoretical Considerations and
Historical Experience,’’ J. Mater. Sci., 39 [19] 5887–904 (2004).
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&