Oxidation resistance of hafnium diboride ceramics with additions of silicon carbide and tungsten boride or tungsten carbide

Article abstract of DOI:10.1111/j.1551-2916.2011.04462.x

Dense samples of HfB₂-SiC, HfB₂-SiC-WC, and HfB ₂-SiC-WB were prepared by field-assisted sintering. The WB and WC additives were incorporated by solid solution into the HfB₂ and the HfC that formed during sintering. Oxidation of the samples was studied using isothermal furnace oxidation between 1600° and 2000°C. Sample microstructure and chemistry before and after oxidation were analyzed by scanning electron microscopy and X-ray diffraction. The addition of WC and WB did not alter oxidation kinetics of the baseline HfB₂-SiC composition below 1800°C; however, at 2000°C, HfB₂-SiC-WC and HfB ₂-SiC-WB had oxide scales that were 30% thinner than the oxide scale of HfB₂-SiC. It is believed that WC and WB promoted liquid-phase densification of the HfO₂ scale, thereby reducing the path of oxygen ingress, during oxidation.

Full text of DOI:10.1111/j.1551-2916.2011.04462.x

J. Am. Ceram. Soc., 94 [8] 2600–2607 (2011)
DOI: 10.1111/j.1551-2916.2011.04462.x
r 2011 The American Ceramic Society
ournal
J
Oxidation Resistance of Hafnium Diboride Ceramics with Additions of
Silicon Carbide and Tungsten Boride or Tungsten Carbide
,
w,z,y
,z,y
,z
Carmen M. Carney,*
z
Triplicane A. Parthasarathy,* and Michael K. Cinibulk*
Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7817
yUES Inc., Dayton, Ohio 45432
Dense samples of HfB –SiC, HfB –SiC–WC, and HfB –SiC–
However, as these materials have been tested in environments
that more closely simulate atmospheric reentry and hypersonic
cruise applications that include higher temperatures (417231C)
2
WB were prepared by field-assisted sintering. The WB and WC
2
2
additives were incorporated by solid solution into the HfB and
2
8
–13
the HfC that formed during sintering. Oxidation of the samples
was studied using isothermal furnace oxidation between 16001
and 20001C. Sample microstructure and chemistry before and
after oxidation were analyzed by scanning electron microscopy
and X-ray diffraction. The addition of WC and WB did not alter
and flowing air
at which the protective SiO
the melting point of SiO (17231C) it is observed that the less
it has become apparent that there is a point
2
-based scale begins to fail. Above
2
2 2
viscous SiO will flow into the pores of the HfO and even flow
from the sample resulting in a less protective porous outer
scale. Recent research has focused on extending the oxidation
resistance of HfB –SiC to these temperatures by adding addi-
1
2
oxidation kinetics of the baseline HfB –SiC composition below
2
1
8001C; however, at 20001C, HfB –SiC–WC and HfB –SiC–
2
2
WB had oxide scales that were 30% thinner than the oxide
2
,13–17
7
tional phases.
appropriate additive that will accomplish one or more goals of
There are three strategies for selecting the
scale of HfB –SiC. It is believed that WC and WB promoted
2
liquid-phase densification of the HfO scale, thereby reducing
2
the path of oxygen ingress, during oxidation.
(1) controlling the phase transformations of the HfO scale, (2)
2
increasing the viscosity of the SiO scale, or (3) promoting dens-
2
2
ification of the HfO scale.
Mixed results have been obtained with the addition of rare-
earth elements (Ta, La and Nd) and some transition metals (Ti,
Cr, and Cr) with reduced oxidation being observed for oxidation
I. Introduction
RANSITION metal borides and carbides with melting temper-
7,15–21
temperatures below 17001C
and increased oxidation above
T
atures exceeding 27001C are commonly referred to as
ultra-high-temperature ceramics (UHTCs). Of these materials,
hafnium diboride (HfB ) has shown the highest oxidation
resistance. Although its density is higher than that of zirco-
12,15
17,21
17001C.
Most recently, Zhang et al.
2
have prepared ZrB -
based UHTCs with additions of WC to promote liquid-phase
sintering in the oxide scale. It was shown that the sample con-
taining WC had a denser ZrO scale and the weight gain during
2
oxidation was reduced at temperatures of 16001C and below.
The present paper focuses on the addition of tungsten (W) to
2
1
–5
1
–4
3
nium diboride (ZrB2)
(10.5 vs 6.09 g/cm ) its high melting
point (431001C) and high thermal conductivity (B100 W/MK
at room temperature) make it a leading candidate for extreme
environment thermal protection systems such as those found at
an HfB
2
-based UHTC to promote improved oxidation resis-
–HfO phase diagram
6,7
tance at temperatures 416001C. The WO
3
2
the sharp leading edges of hypersonic vehicles.
The oxidation of HfB proceeds according to reaction (1)
where the B liquid formed is volatile and expected to be
removed through evaporation above 11001C. Although HfO is
suggests a solid solution between the phases of at least 5 mol%
2
22
WO at temperatures up to and beyond 20001C. Similar to
3
2
O
3
2 3 2 3
ZrO –WO , HfO will form a liquid phase with WO at tem-
2
22
peratures above 12801C. In the present study, W was added in
two phases: (1) WC and (2) WB. Samples with W-based addi-
a stable refractory with a melting point of 27501C the oxide
formed on the surface of the diboride is porous and nonprotec-
tive from further oxygen penetration. As such, development
3
2
tives are compared with a baseline composition of HfB –15
vol% SiC. SiC content was held constant in the samples to allow
a direct comparison. Fifteen volume percent SiC was chosen
based on internal experiments that showed it to provide oxida-
tion scales of similar total lengths and comparable weight gains
as 20 vol% SiC at the tested temperatures, but with a thinner
SiO2 scale. The samples were oxidized between 16001 and
of these materials in the 1960s led to the addition of 5–30 vol%
of silicon carbide (SiC) to form a borosilicate glass upon
1
,3–5
oxidation.
Available B
borosilicate glass that covers or fills the pores of the HfO scale.
Above 11001C, SiC will oxidize by reaction (2).
2
O
3
can dissolve in SiO to form a protective
2
2
The SiO
2
glass has shown to improve oxidation resistance
2
0001C. Because of the differences in weight between the for-
at moderate temperatures (o17001C) when compared with
3
,6,7
mation of HfO
2
and WO
3
and the volatility of WO , scale
3
thickness was used as a measure of oxidation resistance.
2
the oxidation of pure HfB .
HfB2ðcrÞ þ 5=2 O2ðgÞ ! HfO2ðcrÞ þ B2O3ðlÞ
SiCðcrÞ þ 3=2 O2ðgÞ ! SiO2 þ COðgÞ
(1)
(2)
II. Experimental Procedure
(
1) Powder Processing
Commercially available HfB
2
, b-SiC, WC, and WB powders
N. Jacobson—contributing editor
were used to prepare the samples. Table I is a list of the sup-
pliers, purity, and starting particle sizes. Powders were used as
received except for the HfB which was premilled using Si N
2
3
4
grinding media in isopropanol for 60 h to reduce the grain size.
The weight change of the grinding media was 0.07 g, which was
0.01 wt% of the total batch weight. Representative batches of
Manuscript No. 28917. Received November 19, 2010; approved January 21, 2011.
This work was financially supported under Contract No. FA8650-10-D-5226.
*
Member, The American Ceramic Society.
w
Author to whom correspondence should be addressed. e-mail: carmen.carney@
wpafb.af.mil
milled HfB were measured using a laser diffraction particle size
2
2
600

August 2011
Oxidation Resistance of Hafnium Diboride Ceramics with SiC, WB, or WC
2601
Table I. Starting Powder Size and Purity
Particle sizew
Purity (%)
z
Impurities (wt%)
Powder
Supplier
HfB2
SiC
Cerac (Milwaukee, WI)
Alfa Aesar (Ward Hill, MA)
ꢁ325 mesh (44 mm)
o1 mm
99.5
99.8
Zr (0.2), Fe (0.02)
C (1.39), O (0.5),
N (0.13), Fe (0.0151),
Al (0.013), Mo (0.0134)
Cr (0.01), Fe (0.01)
Mg (0.08)
WC
WB
w
Cerac
Cerac
o1 mm
99.5
99.5
ꢁ325 mesh (44 mm)
z
As received. As reported for any impurity over 0.01%.
analyzer (LS230, Beckman Coulter, Brea, CA) and had an av-
erage particle size of 1.3 mm (D90 5 2.2 mm). The oxygen content
using a threshold and area measurement routine in Adobe
Photoshop (Fovea Pro 4, Reindeer Graphics).
of the premilled HfB was 0.86 wt% compared with 0.58 wt% in
2
the as received powder as measured by Leco Corporation (St.
Joseph, MI). Compositions were chosen based on the HfO –
2
III. Results
WO phase diagram. The prepared samples were HfB –15 vol%
3
(1) Processing and Sample Characteristics
2
2 2
SiC (HS), HfB –15 vol% SiC–3 vol% WC (HSWC) and HfB –
Samples were heated to 21001C and held for 5–9 min. allowing
at least 4 min past the last observed piston movement change to
ensure complete densification. Figure 1 is a plot of the relative
piston motion for the heating portion of the program for each of
the samples. The maximum piston motion was normalized so
that the final piston motion values were equal for ease of com-
parison. The first sample to densify was HSWC followed by
HSWB and finally HS. HSWC reached its maximum piston
motion at 20001C, while HSWB and HS did not reach full den-
sity until 21001C. The more rapid densification of the samples
containing W is as expected from previous data concerning
15 vol% SiC–3 vol% WB (HSWB). The powder mixtures were
ball milled in isopropanol for 24 h with SiC grinding media,
dried at room temperature, and subsequently dry milled for
1
2 h. Typical weight loss of the SiC grinding media after milling
the additives with HfB was 0.2 mg (0.1 wt% of the total batch).
The powders were sieved through an 80-mesh (177 mm) screen.
2
(2) Sintering and Sample Preparation
The milled powders were loaded into a 25-mm-diameter graphite
die. A layer of BN and graphite foil separated the powder from
the die with the powder in contact with the graphite foil. The
powder-filled dies were cold pressed at approximately 20 MPa
and loaded into the field-assisted sintering (FCT Systeme GmbH,
Model HPD 25-1, Rauenstein, Germany) unit. The graphite die
was wrapped in graphite felt to limit heat radiation from the die.
Heating and cooling rates were 501C/min. and 32 MPa was ap-
plied while heating up to 16001C. The 32 MPa was held for the
remainder of the sintering schedule and rereleased during the free
cool (below 10001C). The samples were held at 21001C for times
between 5 and 9 min. The temperature was measured by a py-
rometer focused on the bottom of a bore hole in the upper punch
B5 mm from the surface of the powder. Densification was mon-
itored by tracking the movement of the pistons.
1
7,24
densification of diborides containing WC additives. The
densities of the HS, HSWC, and HSWB samples were measured
3
to be 9.93, 10.14, and 10.30 g/cm , while the theoretical values
3
are 9.92, 9.99, and 9.91 g/cm , respectively. The discrepancy be-
tween the measured densities and theoretical values for the W-
containing samples is due to the formation of high-density
phases such as HfC and offset by some lower density oxides
as discussed below.
Figure 2 is a series of micrographs of the as-sintered HS
(Fig. 2(a)), HSWC (Fig. 2(b)), and HSWB (Figs. 2(c) and (d))
samples. The micrographs in Figs. 2(a)–(c) were obtained after
electrochemical polishing. The SiC is distinct and uniformly dis-
tributed throughout each of the samples. The SiC phase was de-
fined by the presence of only Si and C in the EDS spectrum and is
identified as the darkest phase in the images (labeled A in
Fig. 2(d)). The backscattered electron images revealed the grain
Sintered samples were cut with a wire electron discharge
machine into 2.5 mm ꢀ 2.0 mm ꢀ 9.0 mm rectangles and
polished using diamond slurry to a 1 mm finish on all six sides
using an autopolisher to ensure samples with parallel sides and
uniform sizes.
2
structure of the HfB indicated by the presence of Hf-B in the
EDS spectra (labeled B in Fig. 2(d)). The average grain size of
HfB in the HS, HSWC, and HSWB samples was 2.6, 1.8, and 1.7
2
mm, respectively. It was observed that the SiC morphology was
changed upon addition of WC or WB so that 20%–30% of the
SiC grains became rod-shaped with average aspect ratios of 2.9.
(
3) Oxidation Exposure and Sample Characterization
Polished samples were heated in a zirconia element furnace
ZrF-25: Shinagawa Refractories Co., Tokyo, Japan) to 16001,
8001, or 20001C and held at temperature for 30 min. The heat-
ing and cooling rates were 51C/min., which were limited by the
furnace. Samples were placed on a concave ZrO crucible to
limit contact between the sample and ZrO . Oxidized samples
were polished in cross section perpendicular to the bottom
2.5 mm ꢀ 9.0 mm side facing the crucible) of the sample to a
mm finish using diamond slurry. Sintered samples were pol-
ished to 6 mm using diamond slurry and then polished to 1 mm
(
1
2
2
(
1
2
3
using electrochemical–mechanical polishing to reveal grain
structure and reduce SiC pullout. The microstructures were
characterized using scanning electron microcopy (SEM: Quanta,
FEI, Hillsborough, OR) along with energy-dispersive spectros-
copy (EDS: Pegasus 4000, EDAX, Mahwah, NJ) for elemental
analysis. Samples were also prepared for transmission electron
microscopy (TEM: Phillips CM200, FEI) using a focused ion
beam microscope (FIB: FEI Dual Beam 235, FEI). The crys-
tallographic analysis of the oxide was performed on an X-ray
diffractometer with CuKa radiation (XRD: Rigaku 2500,
Tokyo, Japan). Densities of the oxide scale and phase analysis
were measured from SEM micrographs at ꢀ 1000 magnification
Fig. 1. Plot of relative piston motion versus time for the HS, HSWC,
and HSWB samples. The onset of densification was more rapid for
samples containing W. The vertical line indicates the time at which
21001C was attained.

2
602
Journal of the American Ceramic Society—Carney et al.
Vol. 94, No. 8
Fig. 2. SEM micrographs of the sintered (a) HS, (b) HSWC, and (c) HSWB samples. (d) Backscattered electron image of a region of HSWB highlighting
the five phases in the sample: (A) SiC, (B) HfB , (C) W–C–O–B, (D) Hf–C–W, and (E) HfC.
2
Figure 2(d) is a backscattered electron SEM micrograph
highlighting the minor phases found in both HSWB and
HSWC. Grain pullout is evident in this sample as electrochem-
ical–mechanical polishing was not used to prepare the specimen.
Grains containing W were identified by EDS mapping con-
ducted with a 20 keV beam. The grains highlighted in the
W map (labeled C in Fig. 2(d)) were subsequently analyzed by
point EDS analysis using a 10 keV beam. The 10 keV analyses of
at least 15 distinct areas revealed the presence of B, C, and O in
different concentrations with W. An additional phase composed
of Hf and C was identified through point EDS analysis. In some
instances (labeled D in Fig. 2(d)) the Hf-C phase contained W,
while in others (labeled E in Fig. 2(d)) no W was found. Image
analysis of five EDS maps taken at ꢀ 2000 combined with back-
scattered electron images revealed that on average 0.5% of the
sample was composed of the unidentified W-containing phase in
both the HSWC and HSWB samples. Because HfC has a higher
evidence of a WC or WB phase remaining in the samples.
At least one additional phase is suggested by small peaks un-
identifiable by any combination of W, C, O, Si, Hf, or B in the
database. The weak peaks near 2y values of 301 and 381 may be
monoclinic HfO
2
or peaks of the unknown phase(s). A ternary
rhombohedral HfW
4
B
5
or hexagonal (Hf,W)12
B
2ꢁx phase is
possible, but not conclusively identified due to the low intensity
of the peaks and variability in the lattice parameters reported for
these phases.
2
5,26
3
3
density (12.2 g/cm ) than HfB (11.1 g/cm ) and WB (10.77 g/
2
3
cm ) its formation could contribute to the increase in the mea-
sured density of the HSWC and HSWB samples.
XRD patterns were taken of the sintered samples to deter-
mine the phases present (Fig. 3). Hexagonal HfB and cubic SiC
2
are readily identified and are the only phases present in the HS
sample. In the HSWC and HSWB samples there is a shift in the
2
HfB peak attributable to a replacement of W on the Hf lattice.
˚
As expected, a replacement of the smaller W atom (1.41 A) for
˚
the Hf atom (1.58 A) on the hexagonal lattice shifts the peaks to
higher 2y values with the (002) peak experiencing the greatest
shift of all peaks within the observed angular range. Peaks
matching the HfC pattern are found that are also shifted to
higher 2y values confirming the EDS findings of a Hf–W–C
phase. The peak shifts are shown in Table II. There is no
Fig. 3. XRD of the sintered HS, HSWC, and HSWB samples showing
peaks of SiC (3C), HfB , and HfC. The HfB and HfC peaks are both
shifted to higher 2y values due to solid solutions formed with WC, WB,
or W. The peaks marked with a plus symbol are unknown phase(s)
which may include monoclinic HfO2.
2
2

August 2011
Oxidation Resistance of Hafnium Diboride Ceramics with SiC, WB, or WC
2603
Table II. Peak Shifts Observed for HfB and HfC in Samples
2
HSWC and HSWB
Peak shift in 2y
Peak
HSWC
HSWB
HfB (001)
2
HfB (100)
2
HfB (101)
2
HfB (002)
2
0.01
0.01
0.03
0.05
0.04
0.5
0.04
0.03
0.06
0.09
0.05
0.3
HfB (110)
2
HfC (111)
HfC (200)
HfC (220)
0.5
0.7
0.3
0.5
(2) Oxidation
Oxidation was carried out at 16001, 18001, and 20001C in a
stagnant air atmosphere. The heating and cooling rates were
51C/min, so that the samples experienced oxidizing temperatures
before and after the 30-min hold at maximum temperature.
Table III lists the average total oxide scale thickness as measured
from at least 10 points on each sample. The average total scale
thickness was measured by first performing EDS to determine
the boundary between oxidized grains and the unaltered bulk.
A measurement was then taken using the ruler tool in the xT
microscope control software of the SEM. The oxide thickness
values at 16001 and 18001C are within one standard deviation of
each other for all samples suggesting no improvement or detri-
ment to oxidation resistance by adding W-containing phases at
these temperatures. While at 20001C both HSWC and HSWB
had oxide scale thicknesses that were 430% thinner than the
Fig. 4. SEM micrograph of the W-containing phases that are found in
the oxide scale after samples HSWC and HSWB are oxidized to 16001C.
Black arrows indicate the intergranular phase found between HfO that
2
is composed of W–O and possibly Hf, while the white arrow indicates
spherical grains that are found throughout the oxide scale that also con-
tain W–O and possibly Hf.
(
B) Oxidation at 18001C: Representative images of the
oxide scales formed on the samples after exposure to 18001C are
found in Fig. 5. Point EDS analysis showed four layers that are
labeled in Fig. 5: (I) dense SiO outer layer with distributed
2
HfO
HfO
2
grains, (II) porous HfO
with inclusions of varying concentrations of Si, O, and C,
2 2
penetrated by SiO , (III) porous
2
HS sample. As a comparison, samples of HfB –20 vol% SiC–3
vol% WC heated to 16001 and 18001C had average total scale
thicknesses of 47 and 73 mm, respectively.
2
and (IV) porous HfB with inclusions of varying concentrations
2
of Si, O, and C. The average length of each layer was calculated
using the same method as the total scale length to find that lay-
ers (I)–(III) of all samples were all within 3 mm. The porosity of
all the layers was also similar; in particular, layer (III) was cal-
culated to be 90%71% dense for all samples. In measuring
porosity by the thresholding method, the inside of the pores
were taken as part of the matrix, so the actual density would be
somewhat lower, but the same method was used for all samples.
Layer (IV) is commonly known as the depleted layer and has
(
A) Oxidation at 16001C: HSWC and HSWB samples
heated to 16001C produced oxide scales with the same morphol-
ogy and chemistry of the scale observed on the HS samples
heated to 16001C. The oxide scales were composed of an outer
layer of SiO and a porous HfO layer underneath. EDS analysis
of the SiO
whose concentration varied throughout the scale, but was never
1 mol% assuming Al as the only impurity in SiO . W-con-
taining phases were observed as submicrometer spherical grains
scattered throughout the HfO layer in the oxidized HSWC and
HSWB samples. Additionally, a W-containing phase was found
between the HfO grains (Fig. 4) in areas where porosity or
grain pullout due to polishing allowed its observation. Point
EDS analysis of the features is limited by their size, but exam-
ination of at least 10 regions with the same features always
showed the presence of W and O and sometimes Hf. The Hf
signal may be from the background, while any potential small Si
2
2
2
scale in each of the samples revealed Al impurities
4
2
8,13–17,21
been reported for ZrB
average thickness of the depleted layer in the HS samples was
mm; however, samples HSWC and HSWB did not have a layer
IV); the entire porous interface between the bulk and the oxide
is comprised of HfO . As in the 16001C oxidized samples, the
2 2
–SiC and HfB –SiC systems.
The
2
6
(
2
2
SiO scale in each of the samples had Al impurities whose con-
2
centration varied throughout the scale, but was never 41 mol%
assuming Al as the only impurity in SiO .
2
W-containing grains are present throughout all the layers in
both HSWC and HSWB. Examples of the W-containing grains
are indicated by arrows in Fig. 5(d). EDS of the larger (1–2 mm)
grains showed the presence of W and O. A third phase com-
(
K, 1.74 kV) peak may be obscured by the Hf (M, 1.645 kV) and
W (M, 1.775 kV) peaks that bookend it in the energy spectra.
Monoclinic HfO and hafnon (HfSiO ) are found by XRD
analysis for each of the samples. HfSiO is the reaction prod-
uct from the combination of HfO and SiO . Calculated HfO
SiO2 phase diagrams show that HfSiO is stable up to
2
4
4
prised of Hf, Si, and O is recognizable between HfO grains and
2
2
2
2
–
the amorphous SiO . The phase is likely HfSiO (labeled in
2
2
4
2
7,28
Fig. 5(d)) as evidenced by its presence at 16001C and the increase
in the peak intensity observed in the XRD patterns of all the
B17261C.
samples after oxidation to 18001C. Besides HfSiO , only mono-
4
2
clinic and tetragonal HfO are present in the XRD patterns of
all samples heated to 18001C.
(C) Oxidation at 20001C: Figure 6 is a series of com-
bined micrographs taken of the oxide scale resulting from ex-
Table III. Oxide Scale Thickness
Oxide scale thickness (mm)
posure to 20001C. The layers of oxide scale for the HS sample
Temperature (1C)
HS
HSWC
HSWB
2
heated to 20001C are: (I) a dense SiO outer layer with distrib-
uted HfO grains; (II) porous HfO penetrated by SiO (III)
2
porous HfO with inclusions of varying concentrations of Si, O,
2
2
2
1600
1800
2000
3573.2
7875.5
826756.1
3775.3
6775.5
537728.1
3472.3
7677.2
565723.8
2
and C; and (IV) porous HfB with inclusions of different con-
centrations of Si, O, and C. The total scale thickness (Table I)

2
604
Journal of the American Ceramic Society—Carney et al.
Vol. 94, No. 8
Fig. 5. SEM micrographs of the (a) HS, (b) HSWC, and (c) HSWB samples after oxidation at 18001C. The white arrows indicate the location of W
containing grains in HSWC (d) while the HfSiO , HfO , and SiO phases are labeled. HfSiO was found in all three samples oxidized to 18001C. The
oxide scale layers are indicated in (a) as (I) dense SiO outer layer with distributed HfO grains, (II) porous HfO penetrated by SiO , (III) porous HfO
with inclusions of varying concentrations of Si, O, and C, and (IV) porous HfB with inclusions of varying concentrations of Si, O, and C.
4
2
2
4
2
2
2
2
2
2
was reduced by the addition of WC and WB while the SiO
penetration into the scale and the density increased. In the
HSWC and HSWB samples the distinction between layer (I)
2
and layer (II) is blurred through the extensive formation of
HfSiO . Layer (III) is reduced from an average thickness of 357
mm in HS to 170 mm in HSWC and 93 mm in HSWB. In the
4
Fig. 6. SEM micrographs of the (a) HSWB, (b) HSWC, and (c) HS samples after oxidation at 20001C. The layers: (I) a dense SiO
distributed HfO grains; (II) porous HfO penetrated by SiO and HfSiO ; (III) porous HfO with inclusions of varying concentrations of Si, O, and C;
and (IV) porous HfB with inclusions of varying concentrations of Si, O, and C are indicated in the images. The magnified image in (a) shows the SiO
and HfSiO /HfO in layer (I) and (II), while the magnified image in (b) shows W-rich phases found in layer (III) as indicated by the arrows.
2
outer layer with
2
2
2
4
2
2
2
4
2

August 2011
Oxidation Resistance of Hafnium Diboride Ceramics with SiC, WB, or WC
2605
Fig. 7. (a) SEM micrograph showing three distinct glass phases of the HSWB sample oxidized to 20001C: (A) W-rich, (B) SiO
C) SiO -rich, and (D) HfO
surface glass had been removed. The inset is the SEM micrograph of the surface of the sample showing the Pt cap applied for FIB cutting. The HfO
2
with Al, Fe, Ca, and Ba,
(
2
2
. (b) A TEM section from layer (I). The dark phase is W-rich. (c) TEM micrograph of a section that was prepared after the
(the
2
light phase in the SEM and the dark phase in TEM) was monoclinic as determined by selected area electron diffraction (not shown). (d) TEM-EDS of
the glass phase showing the composition of the light and dark glasses in (c).
HSWC (Fig. 6(b)) and HSWB (Fig. 6(a)) the average thickness
of the depleted later (layer (IV)) is 22 mm in HSWC and 32 mm in
HSWB compared with 90 mm in HS. Image analysis calculated a
layer (III) density of 87%, 96%, and 97% in HS, HSWC, and
HSWB, respectively.
taken of the glass phase. Fitting the EDS pattern under the as-
sumption that all the impurities are oxides, the light glass con-
2
tains o0.6 mol% of any impurity with HfO having the highest
concentration. The dark glass contained Si, O, Hf, W, Ca, Al,
Ba, Fe, and possibly Mg. The low intensity of the MgKa peak
and its overlap with an Hf M peak make Mg difficult to discern
in this spectra. Selected area electron diffraction analysis re-
vealed all phases in Fig. 7(b) and those in 7(c), exclusive of the
2
The SiO in layer (I) of the HS sample had impurities of Al,
Ca, Ba, and Fe whose concentration varied spatially. Addition-
ally, the SiO in the oxide scale contains the same impurities (Al,
2
Ca, Fe, Ba) as were found in the HS sample but were also shown
to form phases with W. Figure 6(a) is a magnified view of layers
HfO
The XRD patterns of the sample surfaces after oxidation at
20001C reveal prominent HfSiO peaks in the HSWB sample,
2
, to be amorphous.
(
ing the lighter HfO grains. The HfSiO does not contain W, but
I) and (II). Extensive HfSiO
4
formation is observed surround-
4
which correlates well with the observed microstructure. Approx-
2
4
small pockets (indicated by an arrow) of Hf–Si–W–O can be
found in layer (II) of both the HSWC and HSWB samples.
Figure 6(b) is a magnified image of layer (III) showing the W-
containing spheres that were found scattered throughout in both
the HSWC and HSWB samples.
imately 60% of the top surface of the 20001C oxidized HSWB is
4
comprised of the dense HfSiO /Hf–Si–W–O phase while local-
ized regions of dense HfSiO /HfO comprise approximately
4
2
30% of the top surface observed in 20001C oxidized HSWC.
Although HfO would be expected to transform to tetragonal at
2
SEM and TEM images of the W-containing phases in the
glass are shown in Fig. 7. In Fig. 7(a) SEM-EDS suggests three
20001C, the slow cooling rate (51C/min) would allow conversion
to the monoclinic phase at lower temperatures and account for
the small tetragonal peak (o15% of the height of the monoclinic
(ꢁ111) peak) observed for all the samples.
distinct glass phases: (1) W-rich (labeled A), (2) SiO
2
with Al,
Fe, Ca, and Ba (labeled B), and (3) SiO -rich (labeled C). A
2
TEM section from layer (I) shows a similar microstructure for
the multiphase glass with the black phase being W-rich. Figure
7
(c) is an overview of a section that was prepared after the sur-
IV. Discussion
face glass had been removed. The inset is the SEM micrograph
of the surface of the sample showing the Pt cap applied for FIB
cutting. The HfO (the light phase in the SEM and the dark
phase in TEM) was monoclinic as determined by selected area
electron diffraction (not shown). TEM-EDS (Fig. 7(d)) were
After sintering, no pure WB or WC phases were observed
in the HSWC and HSWB samples. The W was incorporated
in the HfB and HfC matrix as evidenced by their peak shifts in
2
2
the XRD spectra. HfC can be formed by a reaction of HfO2

2
606
Journal of the American Ceramic Society—Carney et al.
Vol. 94, No. 8
and excess C. HfO
HfB powders and oxidation during milling. Amorphous
borides (B O ) also exist at the surface. C can be introduced
from wear of the high-density polyethylene bottles during
milling or from the graphite dies used to sinter the powders.
The oxides may be consumed at elevated temperatures through
a reaction with free carbon by reactions (3) or (4) or with WC
2
is present as oxygen contamination on the
IV–VI transition metal (Hf, Zr, Ti, Ta, W) oxides tend to exhibit
microphase separation and crystallization upon cooling.
1
5,37,38
2
Glass compositions that exhibit phase separation posses an
increased viscosity in the two-phase region compared with a
2
3
3
8
single-phase liquid.
Only a few phase diagrams for W and Hf in B O or SiO are
reported. The most studied diagram, HfO –SiO , reveals that at
2
3
2
2
2
(
reaction [5]).
20001C a single liquid phase exists for compositions with
approximately 60 mol% (or greater) of SiO and that at lower
SiO concentrations the Si–Hf–O liquid phase is found in equi-
librium with HfO
liquid is formed at all compositions above 14301C. No phase
diagram exists for HfO –B but a study of glass melts found
that o1 mol% HfO could be dissolved in B O . However,
2
HfO2ðsÞ þ B2O3ðlÞ þ 5CðsÞ ! HfB2ðsÞ þ 5COðgÞ
2
2
.
7,37
(3)
(4)
(5)
ꢂ
2
3 2 3
In the WO –B O system a single-phase
DGrð2026:85 CÞ ¼ ꢁ8:6 KJ=mol
39
2
2 3
O
4
0
HfO2ðsÞ þ 3CðsÞ ! HfCðsÞ þ 2COðgÞ
DGrð2026:85 CÞ ¼ ꢁ111:5 KJ=mol
2
2
3
ꢂ
the same study found that sodium borosilicate or aluminum
borosilicate glasses can dissolve up to 17 mol% HfO with the
allowable HfO concentration dependent on the concentration
2
2
HfO2ðsÞ þ 3WCðsÞ ! HfCðsÞ þ 3WðsÞ þ 2COðgÞ
DGrð2026:85 CÞ ¼ ꢁ489:3 KJ=mol
39
of the other glass species. As evidenced by Fig. 7 the glass
chemistry found in the 20001C sample has spatial variances in
chemistry. The presence of Al, Na, Ca, Ba, and Fe in the glass
could originate from the impurities in the starting powders or
ꢂ
The presence of HfC after sintering diborides has been shown
be incorporated from the Ca-stabilized ZrO crucible that holds
2
2
9,30
to be dependent on sintering temperature and additives.
The phase diagram of HfB and WB suggests a maximum
the sample. Group I and Group II impurities have been
observed in our previous experiments and also reported in the
2
B2
1
6
W concentration of 23 mol% at B23651C and 10 mol%
literature, but have not received much attention to date. How-
ever, as the development of new UHTC compositions seek
to add elements in order to improve glass properties such as
viscosity the presence of these impurities may play a crucial role
in the outcome of these efforts.
3
1
or above at temperatures above 12001C, while melting exper-
iments suggest that approximately 4 mol% W can be incorpo-
3
rated into the HfB lattice. The same crystalline lattice and
2
2
similar size allow 40 mol% of WC to be dissolved in HfC at
2
33
0271C. Limited literature exists for the HfC–WB system, but
The phase diagrams for WO
unavailable at the temperatures of interest, but inspection of
other transition metal oxides with SiO such as Nb –SiO
and Cr –SiO suggest two-phase glasses above 16951 and
22001C, respectively.
two-phase region in the WO
3 2 2 3
with SiO or B O are
the similar XRD peak shift values of HSWC and HSWB suggest
solubility of W in HfC in the HSWB sample. The W that is
not incorporated into either HfC or HfB can be found in grains
O
2
2
5
2
O
2
2
3
2
4
1,42
shown to contain W, O, C, and B. The addition of W-containing
phases increases the sinterability of the samples, results
Therefore the observed existence of a
–SiO system seems logical.
3
2
in smaller HfB
2
grains, and promotes evolution of acicular
SiC grains. A similar SiC microstructure was shown by Zhang
The two-phase glass may improve oxidation resistance by
increasing the viscosity of the glass at 20001C and/or during
its slow cool down.
3
4
2 4
et al. for pressureless sintered ZrB –SiC with B C additions
2
and WC impurities. However, Chamberlain et al. do not show
4
The impact of densification in the HfO
examined in the HSWC and HSWB samples. The HfO
contained W both as individual grains and as a phase between
HfO grains. Reactions (6)–(8) describe the possible oxidation of
WC and WB.
2
-based layers was
the same SiC microstructure for WC impurities in hot pressed
2
2
ZrB –SiC. It is possible that excess B or C (from WB, WC,
3
4
or B C ) in the system can promote the b (cubic) to a (hexag-
2
4
4
3,44
onal) transformation at temperatures below 18001C, with the
3
5
a-SiC having acicular grain morphology. Unfortunately the
SiC XRD peak has a very low intensity and the main b and
a-SiC peaks are within 0.071, so phase identification is imprac-
tical. Engineering the microstructure may prove beneficial as it
has been shown that the size and morphology of the SiC and
WCðcrÞ þ 5=2 O2ðgÞ ! WO3ðcrÞ þ CO2ðgÞ
2 WBðcrÞ þ 9=2 O2ðgÞ ! 2 WO3ðcrÞ þ B2O3ðlÞ
WO3ðcrÞ ! WO3ðgÞ
(6)
(7)
(8)
2
9,35,36
2
HfB grains can affect strength.
The oxidation of HS, HSWC, and HSWB samples resulted
in oxide scales with comparable total thicknesses and scale
morphologies at 16001 and 18001C. The difference in the aver-
age thickness of the total oxide scale for each sample was within
the spread measured for an individual sample. The oxide scales
formed on the HSWC and HSWB samples at 20001C were sig-
nificantly different than the scale formed on HS. The HSWC
and HSWB samples had less porous scales that were 30% thin-
ner than the scale formed on the HS sample. The three methods
by which W-additions may impact oxidation resistance are (1)
2 3 3
The HfO –WO phase diagram shows a solubility of WO of
about 5 mol% at 16001–20001C, while above 5 mol% the HfO
2
2
2
solid solution will be accompanied by a liquid. The solubility
of W in HfB and HfC is between 4 and 40 mol% suggesting
2
localized regions of W concentrations necessary for phase sep-
aration could exist. The resulting liquid has a melting point of
14731C. The presence of this liquid can promote liquid-phase
2
sintering of the HfO resulting in the observed denser oxide
controlling the phase transformations of the HfO
2
scale, (2) in-
creasing the viscosity of the SiO scale, or (3) promoting dens-
scales for the HSWC and HSWB samples. The reduction in the
length of the total SiC-depleted layer at 18001C in the samples
containing W can be explained by a reduction in oxygen pen-
etration through the denser oxide scale. Upon cooling, the
2
ification of the HfO
nor delayed the monoclinic to tetragonal phase transformation,
thus not affecting the density of the HfO by this method.
2
scale. The addition of W neither promoted
2
2
resulting crystalline phases are a HfO solid solution with
Because no W was found in the SiO at 16001 and 18001C
by SEM-EDS the addition of W-containing phases would not be
expected to alter the viscosity or melting temperature of the
o5 mol% WO and WO with no solid solubility for HfO .
2
3
3
2
SEM-EDS confirmed the existence of W-rich phases between
the HfO grains in the postoxidation analysis.
In addition to the denser HfO layer (layer (III)), layer (II) in
2
SiO2 glass and the SiO -rich layer would be expected to be
2
2
3
,4,18,21
protective at 16001 and 18001C.
observed until above 18001C which is above the melting
temperature of pure SiO (17231C), so the two phase may
form as the glass cools. Borate and silicate glasses with Group
A two-phase glass is not
the samples with W additions is composed of HfSiO and HfO .
4
2
Although a dense HfSiO
oxygen diffusion coefficient than the porous SiO
found in the outermost regions of HS, the decomposition
4
would be expected to have a lower
2
2
-filled HfO
2

August 2011
temperature of HfSiO
formed on cooling and would not be responsible for the decrease
Oxidation Resistance of Hafnium Diboride Ceramics with SiC, WB, or WC
2607
2
7
16
E. Opila, S. Levine, and J. Lorincz, ‘‘Oxidation of ZrB - and HfB
2
2
-Based
4
4
is 17261C so that the HfSiO is likely
Ultra-High Temperature Ceramics,’’ J. Mater. Sci., 39 [19] 5969–77 (2004).
S. C. Zhang, G. E. Hilmas, and W. G. Fahrenholtz, ‘‘Improved Oxidation
17
in oxygen penetration at 20001C.
Resistance of Zirconium Diboride by Tungsten Carbide Additives,’’ J. Am. Ceram.
Soc., 11 [91] 3530–5 (2008).
The combined effect of the more viscous outer layer and
denser inner layer promote oxidation resistance and provide for
an overall thinner scale at exposure temperatures of 20001C in
HSWC and HSWB.
18
F. Peng and R. F. Speyer, ‘‘Oxidation Resistance of Fully Dense ZrB
with SiC, TaB and TaSi Additives,’’ J. Am. Ceram. Soc., 91 [5] 1489–94
2008).
2
2
2
(
19
I. G. Talmy, J. A. Zaykoski, and M. Opeka, ‘‘High-Temperature Chemistry
and Oxidation of ZrB Ceramics Containing SiC, Si , Ta Si , and TaSi ,’’
J. Am. Ceram. Soc., 91 [7] 2250–7 (2008).
2
N
3 4
5
3
2
20
M. Gasch, ‘‘Physical Characterization and Arcjet Oxidation of Hafnium-
Based Ultra High Temperature Ceramics Fabricated by Hot Pressing and Field
V. Conclusion
Dense HfB –SiC samples were prepared with additives of WC
2
Assisted Sintering,’’ J. Eur. Ceram. Soc., 30 [11] 2337–44 (2010).
S. C. Zhang, W. G. Fahrenholtz, and G. E. Hilmas, ‘‘Oxidation of ZrB
21
2
and
and WB. Both the WC and WB additives promoted sintering
while reducing the grain size of HfB . Solid solutions of W–Hf–
B and W–Hf–C were formed in both samples. The WC- and
ZrB -SiC Ceramics with Tungsten Additions,’’ ECS Trans., 16 [44] 137–45 (2009).
2
22
2
L. L. Y. Chang, M. G. Scroger, and B. Phillips, ‘‘Condensed Phase Relations
in the Systems ZrO –WO –WO and HfO –WO –WO ,’’ J. Am. Ceram. Soc., 50
2
2
3
2
2
3
WB-containing HfB
2
–SiC samples oxidized to 16001 and
–SiC sample.
[4] 211–5 (1967).
23
J. Tiley, K. Shiveley II, G. B. Viswanathanb, C. A. Crouse, and A. Shiveley,
‘Novel Automatic Electrochemical–Mechanical Polishing (ECMP) of Metals for
Scanning Electron Microscopy,’’ Micron, 41 [6] 615–21 (2010).
18001C exhibited similar oxide scales as the HfB
2
‘
However, the samples with WC and WB showed a 30% reduc-
tion in scale thickness when the samples were oxidized at 20001C
due to a more viscous phase separated glass found in the out-
ermost regions of the scale and a denser inner HfO2 that
restricted oxygen penetration to the sample.
24
A. L. Chamberlain, W. G. Fahrenholtz, and G. E. Hilmas, ‘‘High Strength
Zirconium Diboride-Based Ceramics,’’ J. Am. Ceram. Soc., 87 [6] 1170–72 (2004).
A. Rundqvist and S. Harsta, ‘‘The Crystal Chemistry of Kappa-Phases,’’
25
J. Solid State Chem., 70, 210–8 (1987).
V. I. Kuz’ma, B. I. Lakh, D. A. Stadnyk, and Y. B. Kovalyk, ‘‘Systems Ha-
26
fnium–Tungsten–Boron, Hafnium–Rhenium–Boron, and Niobium–Rhenium–
Boron,’’ Powd. Metall. Metal. Ceram., 9 [12] 1003–6 (1971).
27
Acknowledgment
D. Shin, R. Arroyave, and Z.-K. Liu, ‘‘Thermodynamic Modeling of the Hf–
Si–O System,’’ CALPHAD, 30, 375–86 (2006).
S. V. Ushakov, A. Navrotsky, Y. Yang, S. Stemmer, K. Kukli, M. Ritali,
28
The authors would like to thank Pavel Mogilevsky for his help in preparing the
TEM-FIB sample and useful discussions regarding TEM-EDS.
M. A. Leskel, P. Fejes, A. Demkov, C. Wang, B.-Y. Nguyen, D. Triyoso, and
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29
W.-M. Guo, J. Vleugels, G.-J. Zhang, P. L. Wang, and O. Van der Biest,
‘Effects of Re (Re5 La, Nd, Y, Yb) Addition in Hot-Pressed ZrB –SiC
Ceramics,’’ J. Eur. Ceram. Soc., 29, 3063–8 (2009).
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