Materials Science and Engineering A 464 (2007) 216–224

Improved processing and oxidation-resistance of ZrB2 ultra-high

temperature ceramics containing SiC nanodispersoids
Sung S. Hwang, Alexander L. Vasiliev, Nitin P. Padture ∗
Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA
Received 21 September 2006; received in revised form 28 January 2007; accepted 1 March 2007

Abstract
We have studied the hot-pressing behavior of ZrB2 /SiC ultra-high temperature ceramics (UHTCs) as a function of: (i) SiC starting-powder size,
(ii) SiC vol%, (iii) ZrO2 doping, and (iv) colloidal dispersion of ZrB2 /SiC powder mixtures. It has been found that the addition of SiC promotes
densification of ZrB2 at a moderate hot-pressing temperature of 1650 ◦ C. It has also been found that ball-milling of the ZrB2 /SiC starting-powder
mixtures using ZrO2 balls media results in the doping of the powder mixture with ZrO2 , which promotes hot-pressing densification. Reduction in the
SiC starting-powder size, and colloidal dispersion of the powders, both have been found to promote hot-pressing densification of ZrB2 /SiC materials;
the highest density achieved in such ZrB2 /SiC ceramics is 99.9%. Detailed microstructural characterization of the ZrB2 /SiC ceramics using electron
microscopy shows that some of these materials contain a Zr(O,B)2 phase, and amorphous films at interphase interfaces. Oxidation studies reveal
that SiC grain-size reduction results in improved oxidation-resistance in ZrB2 /SiC materials. The ZrB2 /SiC ceramics produced here possess modest
hardness and toughness properties. The results presented here point to a new strategy for improving processing and oxidation-resistance of ZrB2 /SiC
materials: dispersion and reduction of SiC grains.
© 2007 Elsevier B.V. All rights reserved.

Keywords: Ceramics; Composites; Oxidation; Processing; Microstructure

1. Introduction 1 and 5 ␮m, and the SiC content varies from 5 to 25 vol%
SiC. Although these processing conditions result in fully dense
Zirconium diboride (ZrB2 ) is a leading candidate material ZrB2 /SiC materials, systematic studies on the effects of SiC
for use in critical external surfaces of future aerospace re-entry starting-powder reduction and ZrO2 doping on the hot-pressing
crafts, such as hypersonic aircraft and reusable launch vehi- behavior of ZrB2 /SiC materials are lacking. Also lacking is
cles [1,2]. This is primarily because ZrB2 , a so-called ultra-high detailed characterization of microstructures of ZrB2 /SiC mate-
temperature ceramic (UHTC), has an excellent combination of rials.
mechanical, physical, thermal-shock, and oxidation-resistance Recent oxidation studies have shown that the simultaneous
properties. It has been shown that addition of SiC dispersoids to oxidation of ZrB2 and SiC in ZrB2 /SiC materials results in a less
ZrB2 ceramics results in significant improvements in oxidation- volatile silica-rich surface scale, in place of the more volatile
resistance and mechanical properties compared to ZrB2 alone boria surface scale in the case of pure ZrB2 [1,3,4,6,7,9]. The
[1,3–9]. silica-rich surface scale is also more refractory and resistant
ZrB2 /SiC materials are generally fabricated using hot- to oxygen diffusion, making ZrB2 /SiC materials more oxi-
pressing at high temperatures, in the range 1900 ◦ C dation resistant than pure ZrB2 . Furthermore, the silica-rich
[7,8] to 2100 ◦ C [3]. Hot-pressing at lower temperatures scale is anchored by the oxidized sub-layer consisting of an
(1600–1750 ◦ C) has been shown to result in dense ceramics, but interpenetrating composite of ZrO2 /silica-rich glass. Although
with additions of TaSi2 [6] or Si3 N4 [5]. The average particle effects of dopants on the oxidation behavior of ZrB2 /SiC
sizes of the ZrB2 and SiC starting powders used are between materials have been studied, microstructural effects on the oxi-
dation behavior of ZrB2 /SiC materials have not been studied
systematically.
∗ Corresponding author. Tel.: +1 614 247 8114; fax: +1 614 292 1537. In this work, we have studied the hot-pressing behavior of
E-mail address: padture.1@osu.edu (N.P. Padture). ZrB2 /SiC materials as a function of: (i) SiC starting-powder

0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2007.03.002
 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224 217

respective powder mixtures (∼15 g individual batches) were

ball-milled in methanol using Y2 O3 -stabilized tetragonal ZrO2
(YSZ) balls (10 mm diameter) media (Tosoh Corp., Tokyo,
Japan) for 24 h. All ball-milling was performed in polyethylene
bottles, with powder mixture:media:methanol volume ratio of
1:2:3.2. In an experiment to study effect of powder dispersion
(material Z/S-6), the 40-nm SiC powder was first dispersed in an
aqueous solution of pH 12, with 2 h of ultrasonication. The pH
was adjusted using NH4 OH (Mallinckrodt Baker, Phillipsburg,
NJ). The ZrB2 powder was then added to the dispersion. The
mixture was then ball-milled for 24 h using YSZ balls media.
The pH of 12 was chosen based on results from dispersion studies
(not shown here), where best dispersion was observed.
During a typical ball-milling run, it was found that the YSZ
balls lost about 0.3 g of weight, which is assumed to be incorpo-
rated in the powder mixture. In an effort to isolate the effect of
ball-milling, the powder mixture for material Z/S-7 was mixed
Fig. 1. Bright-field TEM micrograph of the 40-nm SiC starting powder. in methanol, but without the YSZ balls media. In an additional
experiment, powder mixture for material Z/S-8 was mixed in
size, (ii) SiC vol%, (iii) ZrO2 doping, and (iv) colloidal disper- ethanol without the YSZ balls media, but 0.3 g (∼2 wt%) of
sion of ZrB2 /SiC powder mixtures. We have also characterized ZrO2 powder (Alfa Aesar, Ward Hill, MA) was added to the
in detail the microstructures of the resulting ZrB2 /SiC materials powder mixture.
using electron microscopy, in an attempt to elucidate densi- All powder mixtures were dried on hot-plates while being
fication mechanisms. Finally, microstructural effects on the stirred, and the resulting dried powder mixtures were crushed.
oxidation behavior and mechanical properties of the resulting Individual batches of powder mixtures (7.5 g each) were placed
ZrB2 /SiC materials have been studied, in an effort to understand in BN-coated graphite dies (25.4 mm diameter), and hot-pressed
structure–property relations. (GCA Vacuum Industries, Somerville, MA) under vacuum at
1650 ◦ C for 2 h at an applied pressure of 60 MPa. All surfaces
2. Experimental procedure of the hot-pressed specimens were ground and cleaned.
The densities of the consolidated specimens were measured
2.1. Processing using the Archimedes principle, with distilled water as the
immersion medium. The measured densities are reported in
The ZrB2 starting powder used in this study was obtained Table 1, where the theoretical densities of the composites were
from a commercial source (Grade B, H.C. Starck Corp., New- calculated using rule-of-mixture and following density values
ton, MA), with an average particle size of ∼2 ␮m. Three types of the pure phases: ZrB2 6.085 g cm−3 and SiC 3.217 g cm−3 .
of commercial SiC starting powders with different average par-
ticles sizes were used: (i) 1.7 ␮m (Grade B-hp SiC, H.C. Starck 2.2. Oxidation
Corp., Newton, MA), (ii) 0.6 ␮m (UF-15 SiC, H.C. Starck,
Newton, MA), and (iii) 40 nm (experimental grade CVD SiC, Cubes 2 mm × 2 mm × 2 mm were cut out from the hot-
Sumitomo Chemical Company, Tokyo, Japan). Fig. 1 shows pressed materials, and all surfaces were diamond-polished to
a transmission electron micrograph (TEM) of the 40-nm SiC a 1 ␮m finish using routine metallographic methods. Oxidation
powder. Table 1 summarizes the nine powder batches that were tests on these cubes were performed in a box furnace (Ther-
prepared. In the case of materials Z-1, and Z/S-1 to Z/S-5 the molyne, Dubuque, IA) at 1500 ◦ C for 10 min in air.

Table 1
Nomenclature, processing details, and microstructural parameters of ZrB2 /SiC materials hot-pressed at 1650 ◦ C under 60 MPa pressure for 2 h
Material # SiC vol% SiC powder size Relative density (%) ZrB2 grain size (␮m) SiC grain size Remarks

Z-1 0 – 71.6 3.4 – Ball-milled

Z/S-1 5.7 1.7 ␮m 81.7 3.6 1.8 ␮m Ball-milled
Z/S-2 11.4 1.7 ␮m 91.5 3.6 1.9 ␮m Ball-milled
Z/S-3 22.4 1.7 ␮m 97.9 3.9 1.8 ␮m Ball-milled
Z/S-4 22.4 0.6 ␮m 98.6 3.7 0.8 ␮m Ball-milled
Z/S-5 22.4 40 nm 99.6 3.4 80 nm Ball-milled
Z/S-6 22.4 40 nm 99.9 3.4 80 nm Colloidal + ball-milled
Z/S-7 22.4 1.7 ␮m 90.9 3.4 1.8 ␮m Not ball-milled
Z/S-8 22.4 1.7 ␮m 96.4 3.6 1.9 ␮m ZrO2 + not ball-milled
218 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224

Table 2
Hardness, toughness, and oxidation properties of dense ZrB2 /22.4 vol% SiC materials

Material # SiC grain size Hardness, H (GPa) Toughness, KIC (MPa m0.5 ) Average silica layer Average depleted-layer
thickness I* (␮m) thickness II* (␮m)

Z/S-3 1.8 ␮m 15.2 3.8 13 32

Z/S-4 0.8 ␮m 16.7 3.8 4 9
Z/S-5 ∼80 nm 19.9 3.1 3 8
Z/S-6 ∼80 nm (colloidal) 21.3 2.4 2 7
* See Fig. 10 for explanation.

2.3. Characterization polishing, dimpling, and ion-beam milling (DuoMill, Gatan,

Pleasanton, CA). The resulting specimens were observed in a
Cross-sections of the hot-pressed materials and oxidized TEM (Tecnai F20, FEI, Hillsboro, OR), equipped with a X-
specimens were diamond-polished to 1 ␮m finish using twin objective lens pole-piece (spherical aberration coefficient
routine metallographic methods. Polished cross-sections of Cs ∼ 1.3 mm), an atmospheric-thin-window EDS (Phoenix Sys-
only the hot-pressed specimens were then etched using a tem, EDAX, Mahwah, NJ), and an imaging filter (GIF 2000,
HF:HNO3 :H2 O::1:1:1 (by volume) solution for ∼5 s at room
temperature. All cross-sections were then observed in a scan-
ning electron microscope (SEM) (XL 30 ESEM-FEG, Philips,
Eindhoven, The Netherlands). Energy dispersive spectroscopy
(EDS) (EDAX, Mahwah, NJ) in the SEM was used to obtain
compositional maps. In the case of the oxidized specimens, the
thicknesses of the surface oxidation layers on the cross-section
SEM micrographs were measured. The average values from
at least 10 measurements from each material are reported in
Table 2.
The ZrB2 and SiC grains sizes in the hot-pressed materials
were estimated from the SEM micrographs using an image-
analysis software (Clemex Vision, Clemex Technology Inc.,
Longueil, Canada). Approximately 100 ZrB2 and 60 SiC grains
per material were used.
The hot-pressed materials were analyzed by X-ray diffrac-
tion (XRD) using Cu K␣ radiation (XDS 2000, Scintag Inc.,
Sunnyvale, CA), to confirm the phases present.
The 40-nm SiC powder was dispersed on a holey carbon
grid for TEM observation. TEM samples were also pre-
pared out of hot-pressed materials Z/S-3, Z/S-6, and Z/S-7,
using routine methods involving successive steps of grinding,

Fig. 3. Plots of relative density (%) of ZrB2 /SiC materials hot-pressed at 1650 ◦ C
under identical conditions as a function of: (A) vol% SiC and (B) SiC starting-
powder size. In (A), %relative densities of material Z/S-7 made from non-ball-
Fig. 2. XRD pattern of material Z/S-3 showing the presence of ZrB2 and SiC. milled powder mixture and material Z/S-8 made from non-ball-milled powder
Joint Commission on Powder Diffraction Standards (JCPDS) powder diffraction mixture but with ZrO2 additions are also shown. In (B), %relative density of
file numbers 75-1050 and 73-1665 were used to identify the hexagonal ZrB2 and material Z/S-6 made from colloidally dispersed, ball-milled powder mixture is
the cubic SiC phases, respectively. shown.
 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224 219

Gatan, Pleasanton, CA) with electron energy loss spectrometer indentation cracks with the microstructure were also examined
(EELS). The TEM was operated at an accelerating voltage of in the SEM.
200 kV.
3. Results
2.4. Mechanical testing
3.1. Microstructures
Polished cross-sections of only the dense, hot-pressed mate-
rials (Z/S-3, Z/S-4, Z/S-5, and Z/S-6) were indented using a Fig. 2 is an example of a typical XRD pattern, in this case for
Vickers diamond pyramid at a contact load of 1 kg (9.8 N) material Z/S-3, showing the presence of only two phases, ZrB2
(Micromet II, Buehler, Lake Bluff, IL). The indentation sites (hexagonal) and ␤-SiC (cubic).
were examined using an optical microscope, and the inden- A comparison of density data in Table 1 and Fig. 3A for
tation impression diagonals were measured for hardness (H) materials Z-1, Z/S-1, Z/S-2, Z/S-3 clearly shows that, for the
calculations. same ZrB2 and SiC starting powders and processing conditions,
The same specimens were also Vickers-indented using a the density increases with increasing vol% SiC. This is reflected
higher load (P) of 10 kg (98 N) (Zwick, Ulm, Germany) for in the SEM micrographs presented in Fig. 4A–D. The grain sizes
toughness measurements. Radial cracks emanating from the (of both ZrB2 and SiC) are similar between these four materials
Vickers indentation sites (2c) were measured in the optical (Table 1), which is also evident from the SEM micrographs in
microscope. The indentation toughness values of the spec- Fig. 4A–D. There appears to be some coarsening of ZrB2 , where
imens were calculated using the following relation [10]: the starting-powder size is ∼2 ␮m, and the measured grain sizes
KIC = 0.016(E/H)0.5 Pc−1.5 , where E was estimated using the are between 3.4 and 3.9 ␮m. Little coarsening is observed in
Knoop indentation method (indentation load 9.8 N) [11]. The SiC, which is to be expected considering the low hot-pressing
average H and KIC toughness values, from at least five inden- temperature of 1650 ◦ C and the discontinuous nature of the SiC
tation per material, are reported in Table 2. Interactions of the phase.

Fig. 4. SEM micrographs of materials: (A) Z-1, (B) Z/S-1, and (C) Z/S-2 showing the effect of increasing SiC content (same SiC starting-powder size of 1.7 ␮m)
on the microstructures, and (D) Z/S-3, (E) Z/S-4, and (F) Z/S-5, showing the effect of decreasing SiC starting-powder size (same SiC content of 22.4 vol%) on the
microstructures. The gray phase is ZrB2 and the dark phase is SiC.
220 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224

made from 40-nm SiC starting powder, showing the distribution

of SiC grains. For material Z/S-6, where the 40-nm SiC pow-
der was colloidally dispersed, the highest density of 99.9% was
obtained (Table 1; Fig. 3B), and the SEM micrograph in Fig. 5B
shows smaller SiC grain size and better dispersion. Although
the SiC grain sizes in materials Z/S-5 and Z/S-6 appear to be
∼0.6 ␮m in the SEM, TEM studies (Fig. 6A) reveal that the SiC
exists as aggregates of many nanograins in those materials. The
average size of these SiC nanograins is estimated at ∼80 nm.
In contrast, the material Z/S-3 (made from 1.7-␮m SiC starting
powder) contains single-crystal SiC grains, as seen in the TEM
micrograph in Fig. 6B. In both cases the ZrB2 (hexagonal) and
SiC (cubic) were confirmed by selected area diffraction patterns
(SAEDP) in the TEM.
Comparison of densities of materials Z/S-3 and Z/S-7 in
Table 1 and Fig. 3A shows that, much higher densities are
achieved in materials Z/S-3 (97.9%) made from ball-milled
powders, in contrast to material Z/S-7 (90.9%) made with non-
ball-milled powders. The density improves to 96.4% in material
Z/S-8 made with non-ball-milled powders but with ZrO2 addi-
tion.
Fig. 7A is a bright-field TEM micrograph of material Z/S-
Fig. 5. SEM micrographs of materials: (A) Z/S-5 and (B) Z/S-6 showing the
effect of colloidal processing on the microstructure. The gray phase is ZrB2 and 3, showing, in addition to ZrB2 and SiC, a Zr(O,B)2 phase. The
the dark phase is SiC. amount of this phase is quite small, which is perhaps why it is not
detected in the XRD pattern (Fig. 2). The qualitative elemental
composition of that phase was confirmed in the TEM using EDS
A comparison of density data in Table 1 and Fig. 3B for (Fig. 8B) and EELS (not shown here). The Cu and the Y in the
materials Z/S-3, Z/S-4, and Z/S-5 shows that, for the same EDS spectrum in Fig. 7B is from the TEM grid and the YTZ ball-
vol% SiC (22.4 vol%) and processing conditions, the density milling media, respectively. It appears that the Zr(O,B)2 phase
increases slightly with decreasing size of SiC starting powders. is most likely a ZrO2 -rich substitutional solid-solution of ZrO2
While the ZrB2 grain size in these three materials is similar and ZrB2 , with a distorted tetragonal structure. This structure
(3.4–3.9 ␮m), the SiC grain size is seen to decrease with decreas- was revealed by SAEDPs collected from the Zr(O,B)2 region
ing SiC starting-powder size (Table 1 and Fig. 4D–F). Fig. 5A is during careful tilting experiments in the TEM. Fig. 7C is an
a higher magnification SEM micrograph of the material Z/S-5, example of such a SAEDP using [110] zone axis, showing bro-

Fig. 6. Bright-field TEM micrographs of materials: (A) Z/S-6 and (B) Z/S-3. In (A), the SiC aggregates of ∼0.6 ␮m in size consist of ∼80-nm SiC grains, while in
(B) SiC is not aggregated. The inset in (B) is SAEDP from a SiC grain using zone axis B = [1 1 0].
 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224 221

Fig. 7. (A) Bright-field TEM image of materials Z/S-3 showing, in addition to ZrB2 and SiC grains, a Zr(O,B)2 phase. (B) EDS spectrum from the Zr(O,B)2 phase.
(C) SAEDP from the Zr(O,B)2 phase showing tetragonal distortion.

ken symmetry; the orthogonality between horizontal and vertical TEM images of typical interphase interfaces between ZrB2 and
spot-rows is off by ∼2◦ . Zr(O,B)2 substitutional solid-solution SiC in materials Z/S-3 and Z/S-7, respectively. While the Z/S-3
phase has been observed before, but that phase was ZrB2 -rich, material made from ball-milled powders has amorphous films
with a monoclinic structure [12]. at most of the interphase interfaces (Fig. 9A), the Z/S-7 mate-
TEM studies of the material Z/S-7 made from non-ball-milled rial made from non-ball-milled powders appears to have less
powders did not show any evidence of the Zr(O,B)2 phase. Fig. 8 incidence of amorphous films at interphase interfaces (Fig. 9B).
is a typical TEM micrograph from material Z/S-7 showing only This indicates that the ball-milling process introduces condi-
the ZrB2 and SiC phases. Fig. 9A and B are high-resolution tions favorable for the formation of the Zr(O,B)2 phase and
amorphous interphase interfaces.

3.2. Oxidation

Fig. 10A–D are cross-sectional SEM micrographs of oxi-

dized, dense ZrB2 /22.4 vol% SiC materials (Z/S-3, Z/S-4, Z/S-5,
and Z/S-6). The oxidation behaviors of all these materials appear
to be similar, and consistent with what has been observed by
others for ZrB2 /SiC ceramics [4,6], except for the thicknesses
of the oxide layers. It has been shown that the outermost layer
(top) is silica glass, designated as layer I. The compositional
maps in Fig. 11A and B confirm the layer I to be rich in Si
and O. The layer below that is a composite of ZrO2 /silica,
which is somewhat depleted in Si, as seen in the compositional
map in Fig. 11A. This depleted layer is designated as layer II.
Below layer II is the unoxidized ZrB2 /SiC base material. The
micrographs in Fig. 10A–D, together with the quantitative data
reported in Table 2, show clearly that SiC grain reduction results
in a dramatic reduction in the thicknesses of the oxide layers in
these materials. The largest decrease in the thicknesses of layers I
and II occurs by going from a SiC grain size of 1.8–0.8 ␮m. With
Fig. 8. Bright-field TEM micrograph of material Z/S-7, where the Zr(O,B)2 further decrease in the SiC grain size, the decrease in the layer
phase could not be found. thicknesses is marginal, with the colloidally processed material
222 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224

Fig. 9. High-resolution TEM micrographs of typical interphase interfaces (arrows) in materials: (A) Z/S-3 and (B) Z/S-7.

Fig. 10. Cross-sectional SEM micrographs of oxidized materials (1500 ◦ C, 10 min, in air): (A) Z/S-3, (B) Z/S-4, (C) Z/S-5, and (D) Z/S-6. The oxidized surface is
at the top. All main images were taken at the same magnification. Insets are SEM micrographs of near-surface regions at higher magnification (same for all insets).
The silica layer I and the depleted sub-layer II are defined in (A).

Fig. 11. Elemental composition maps of cross-section of oxidized material Z/S-4 in SEM/EDS: (A) Si and (B) O. The oxidized surface is at the top. Both images
were taken at the same magnification.
 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224 223

4. Discussion

The results presented in Table 1 and Fig. 3A show clearly

that the addition of SiC particles promotes densification of ZrB2
ceramics at a moderate hot-pressing temperature of 1650 ◦ C. The
earlier published studies appear to have overlooked this fact, rea-
son for which is not entirely clear. It is worth noting that in most
of those studies dense, pure ZrB2 is used as a reference material,
which requires high temperature hot-pressing (1900–2100 ◦ C).
It is possible that, in an effort to maintain consistent processing
conditions, the same high hot-pressing temperatures were used
to densify ZrB2 with SiC additions. Also, while some studies
report the use of WC-Co balls media [7–9], other do not report
Fig. 12. SEM micrograph of oxidized top surface of material Z/S-4. if ZrO2 ball-milling media was used.
The explanation for the beneficial effect of SiC on densifi-
(Z/S-6) showing highest oxidation-resistance of all materials cation may lie in the fact that most SiC powder particles are
evaluated in this study. known to have an oxidized surface layer. That layer may pro-
Fig. 12 is a SEM image of the top surface of the oxidized mote formation of liquid phases during hot-pressing, assisting
material Z/S-4 showing smooth silica-rich glaze. in densification at lower temperatures. Improvement in den-
sification with increasing SiC content (Fig. 3A), and hence
increasing oxide content, supports this hypothesis. Evidence
3.3. Mechanical properties
of amorphous films at interphase grain boundaries (Fig. 9A)
also lends support to this hypothesis. The observed increase in
The mechanical properties results for the dense ZrB2 /
density of ZrB2 /22.4 vol% SiC materials with decreasing SiC
22.4 vol% SiC materials (Z/S-3, Z/S-4, Z/S-5, and Z/S-6) are
starting-powder size (Fig. 3B) further supports this hypothesis:
reported in Table 2. Both the hardness and toughness are found
the higher surface oxide content in smaller SiC particles (high
to increase with decreasing SiC grain size and better dispersion.
specific surface area) is expected to promote densification.
Fig. 13A and B shows microstructural interactions of indenta-
The Z/S-7 material made from non-ball-milled powders has
tion cracks in materials Z/S-3 and Z/S-6, respectively. The crack
lower density (90.9%) compared to the material Z/S-3 made
in material Z/S-3 with the coarser SiC grains (Fig. 13A) appears
from ball-milled powders (97.9%). It may be argued that ball-
to be wavy and heavily bridged by the SiC grains, while the
milling results in better mixing and particle-size reduction,
crack in material Z/S-6 with finer SiC grains (Fig. 13B) appears
which in turn results in better densification. However, that
relatively straight with little or no bridging.
is not a complete explanation because, as mentioned earlier,
some ZrO2 from the YSZ ball-milling media gets incorporated
indirectly in the ZrB2 /SiC powders during ball-milling. Direct
addition of ZrO2 powder to the non-ball-milled ZrB2 /SiC pow-
der mixture results in a density of 96.4% (material Z/S-8),
which is in-between densities of materials Z/S-7 and Z/S-
3. This shows that ball-milling, together with the attendant
ZrO2 addition, results in high densities in ZrB2 /SiC materials.
Once again, the added oxide appears to promote densifica-
tion during hot-pressing via introduction of liquid phases. It
is not clear what role of the Zr(O,B)2 phase plays in the
densification.
There is clear evidence that decreasing the SiC grain size
in ZrB2 /SiC materials results in an increase in the oxidation-
resistance. A qualitative explanation for this beneficial effect of
SiC grain reduction lies in the consideration of the beneficial
effect of any SiC additions to ZrB2 . It is generally accepted that
the oxidation of pure ZrB2 proceeds with the formation of ZrO2
and boria at the exposed-surface. Boria is highly volatile, and
it does not provide adequate protection against further oxida-
tion of ZrB2 . In the case of ZrB2 /SiC materials, the oxidation
of SiC grains provides a steady supply of silica glass. The silica
Fig. 13. SEM micrographs showing interaction of indentation cracks with
glass combines with the boria formed on the neighboring ZrB2
microstructure in materials: (A) Z/S-3 and (B) Z/S-6. The gray phase is ZrB2 grains, to result in a more refractory borosilicate glass. With fur-
and the dark phase is SiC. Arrows in (A) point to SiC-grain crack-bridging sites. ther evaporative loss of boron, the borosilicate glass eventually
224 S.S. Hwang et al. / Materials Science and Engineering A 464 (2007) 216–224

becomes silica-rich glass, which essentially glazes the surface • Ball-milling of the ZrB2 /SiC starting-powder mixtures using
and provides oxidation protection. YSZ balls media results in the doping of the powder mixture
Now consider a decrease in the size of the SiC grains with ZrO2 , which promotes hot-pressing densification. The
from 1.8 to 0.8 ␮m, while maintaining the SiC content of resulting ZrB2 /SiC materials contain a Zr(O,B)2 phase, and
22.4 vol%. Assuming spherical SiC grains and uniform distribu- amorphous films at interphase interfaces.
tion, this results in an increase in the ZrB2 /SiC interface-length • SiC grain-size reduction results in improved oxidation-
per unit area of exposed-surface and a decrease in the spac- resistance in ZrB2 /SiC materials.
ing between SiC grains, both by about a factor of 2. This is • The ZrB2 /SiC materials produced here possess modest hard-
expected to make SiC more effective in providing the silica- ness and toughness properties.
rich glass, leading to the formation of the protective silica-rich • The results presented point to a new strategy for improving
layer on the material early on during oxidation. Using this argu- processing and oxidation-resistance of ZrB2 /SiC materials:
ment, a further decrease in the SiC grain size to ∼80 nm is dispersion and reduction of SiC grains.
expected to result in a dramatic improvements in the oxidation-
resistance in materials Z/S-5 and Z/S-6. However, only marginal Acknowledgements
improvements in the oxidation-resistance in materials Z/S-5
and Z/S-6 have been observed (Fig. 10 and Table 2). This The authors thank Profs. J. Li and R.A. Rapp for fruitful dis-
is because, as seen in Fig. 6A, the individual ∼80-nm SiC cussions. Funding for SSH was provided by a Korean Research
grains in these materials form aggregates of ∼0.6 ␮m size, Foundation post-doctoral fellowship (KRF-2005-D00218).
which is the effective grain size in the context of oxida-
tion. These results suggest that further improvements in the References
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