Mechanisms and kinetics of the hydrothermal oxidation of bulk titanium silicon carbide

Article abstract of DOI:10.1111/j.1551-2916.2009.03570.x

Hydrothermal oxidation of bulk Ti₃SiC₂ in continuous water flow was studied at 500°-700°C under a hydrostatic pressure of 35 MPa. The oxidation was weak at 500°-600°C and accelerated at 700°C due to the formation of cracks in oxides. The kinetics obeyed a linear time-law. Due to the high solubility of silica in hydrothermal water, the resulting oxide layers only consisted of titanium oxides and carbon. Besides general oxidation, two special modes are very likely present in current experiments: (1) preferential hydrothermal oxidation of lattice planes perpendicular to the c-axis inducing cleavage of grains and (2) uneven hydrothermal oxidation related to the occurrence of TiC and SiC impurity inclusions. Nonetheless the resistance against hydrothermal oxidation is remarkably high up to 700°C.

Full text of DOI:10.1111/j.1551-2916.2009.03570.x

J. Am. Ceram. Soc., 93 [4] 1148–1155 (2010)
DOI: 10.1111/j.1551-2916.2009.03570.x
r 2010 The American Ceramic Society
ournal
J
Mechanisms and Kinetics of the Hydrothermal Oxidation of Bulk
Titanium Silicon Carbide
Haibin Zhang,*,w Volker Presser, Christoph Berthold, Klaus Georg Nickel,* and Xin Wang
Applied Mineralogy, Institute for Geosciences, Eberhard-Karls-Universitat Tubingen, WilhelmstraXe 56,
¨
¨
D-72074 Tubingen, Germany
¨
Christoph Raisch and Thomas Chasse
´
Institute for Physics and Theoretical Chemistry, Eberhard-Karls-Universitat Tubingen, Auf der Morgenstelle 8,
¨
¨
D-72076 Tubingen, Germany
¨
Lingfeng He and Yanchun Zhou*
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
Hydrothermal oxidation of bulk Ti3SiC2 in continuous water
flow was studied at 5001–7001C under a hydrostatic pressure of
35 MPa. The oxidation was weak at 5001–6001C and acceler-
ated at 7001C due to the formation of cracks in oxides. The
kinetics obeyed a linear time-law. Due to the high solubility of
silica in hydrothermal water, the resulting oxide layers only
consisted of titanium oxides and carbon. Besides general oxida-
tion, two special modes are very likely present in current exper-
iments: (1) preferential hydrothermal oxidation of lattice planes
perpendicular to the c-axis inducing cleavage of grains and (2)
uneven hydrothermal oxidation related to the occurrence of TiC
and SiC impurity inclusions. Nonetheless the resistance against
hydrothermal oxidation is remarkably high up to 7001C.
fast reactor. Nappe
´
et al.10 investigated the behavior of Ti3SiC2
under nuclear and electronic irradiation for a cladding material
for future fuels.
In addition to those environments, hydrothermal oxidation is
a common category of oxidation involved in energy-related sys-
tems because high-temperature and high-pressure water is fre-
quently handled. In contrast to the excellent high-temperature
oxidation resistance, advanced ceramics, such as non-oxide ma-
terials like silicon carbide11–16 or silicon nitride17–19 and even
alumina as an oxide,20 are vulnerable to corrosion at relatively
low temperatures during hydrothermal conditions.
Our recent work explored the hydrothermal oxidation behav-
21
ior of Ti3SiC2 and Ti3Si0.9Al0.1C2 powders.22 The primary re-
sults showed that titanium, silicon, and aluminum were
selectively extracted from the Ti3SiC2 and Ti3Si0.9Al0.1C2 sub-
strates during hydrothermal oxidation, resulting in the forma-
tion of the corresponding oxides and carbon. The presence of
carbon was attributed to the unique bonding and structural
characteristics of Ti3SiC2 and Ti3Si0.9Al0.1C2.21,22
In this work, for the first time, we extended the investigations
to bulk Ti3SiC2 in steady-state conditions (flowing water) to
study the hydrothermal oxidation kinetics, the composition and
structure of the resulting layers. The systematic characterization
of the hydrothermal oxidation of bulk Ti3SiC2 will greatly ben-
efit its application in energy related systems, such as light water
reactor.
I. Introduction
AX phases exhibit a striking combination of the prominent
M
properties of both ceramics and metals.1–3 They possess
the general formula of MN11AXN (N 5 1, 2 or 3), where M is a
transition metal, A is an A-group (mostly IIIA and IVA) ele-
ment and X is either C or N. As an important member of this
family, titanium silicon carbide (Ti3SiC2) displays numerous sa-
lient properties, such as good oxidation resistance, low density,
high elastic modulus and strength, chemical stability, excellent
thermal shock resistance, good machinability and high ratio of
fracture toughness to strength.1–4
Ti3SiC2 has obvious advantages over brittle ceramics such as
silicon carbide because of good machinability,1,2,5 high damage
tolerance,6,7 and reliability.8 Therefore, Ti3SiC2 has lately at-
tracted great attention as a prospective material for energy-
related systems. In the literature, some experiments evaluating
the damage resistance of this ceramic under related conditions
can be found. For example, Barnes et al.9 studied the corrosion
behavior of Ti3SiC2 in circulating molten lead at 6501 and 8001C
for a possible cladding or structural material in a lead-cooled
II. Experimental Procedure
The bulk Ti3SiC2 was fabricated by the solid-liquid reaction
synthesis and simultaneous in situ hot pressing process.23 The
grains size of the as-prepared bulk specimen was approximately
20 mm. Impurities including 5 wt% SiC, 5 wt% TiC, and minor
amounts of graphite were macroscopically evenly distributed,
but locally aggregated, in some regions, up to 4100 mm.
3 ꢀ 4 ꢀ 8 mm3 rectangular bars were used for the hydrother-
mal oxidation experiments. These bars were ground on 1000 grit
SiC papers, polished using diamond paste (grain size: 1 mm),
chamfered and degreased in acetone.
R. Naslain—contributing editor
Specimen bars were inserted into the gold-coated autoclave
with a continuous water flow (SITEC Industrietechnologie
GmbH, Chemnitz, Germany). A boxer type pump was used to
maintain a controllable and constant flow-rate during the time
of the experiments. At a flow rate of 15 mL/h, distilled water in
the autoclave (30 mL) was replaced approximately every 2 h.
The experiments were carried out at 5001 to 7001C under a
Manuscript No. 26677. Received August 9, 2009; approved November 27, 2009.
This work was supported by DFG (Grants Ni 299/9 and Ni 299/13-1), the National
Outstanding Young Scientist Foundation for Y. C. Zhou (Grant 59925208), Natural
Science Foundation of China (Grants 50232040, 50302011, 50832008 and 90403027),
‘‘863’’ project and High-tech Bureau of the Chinese Academy of Sciences.
*Member, The American Ceramic Society
wAuthor to whom correspondence should be addressed. e-mail: hbzhang1978@
gmail.com
1148

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Mechanisms and Kinetics of the Hydrothermal Oxidation of Bulk Titanium Silicon Carbide
1149
hydrostatic pressure of 35 MPa. Hydrothermal oxidation kinet-
ics was evaluated by the mass change of the specimens, which
was measured using an electric microbalance (Sartorius AG,
(2) Phase Analysis
Figure 2 shows XRD patterns of the Ti3SiC2 samples after
hydrothermal oxidation. Anatase (TiO2) is distinguished as the
only reaction product at 5001 to 6001C while anatase and rutile
(both TiO2 modifications) occur at 7001C. The diffraction peaks
associated with rutile become much more intensive with oxida-
tion time, owing to the phase transformation from anatase to
rutile. This phase transformation was previously discovered in
the hydrothermal oxidation of Ti3SiC2 and Ti3Si0.9Al0.1C2 pow-
ders,21,22 as well as the oxidation of Ti3SiC2 in air.25 Silica (either
crystalline or amorphous) was not detected in this work. In
contrast to this, a-cristobalite (tetragonal SiO2 modification) is
present after hydrothermal oxidation of Ti3SiC2 and Ti3
Si0.9Al0.1C2 powders at 7001C.21,22 It is noteworthy that the
diffraction peaks from the Ti3SiC2 substrate are visible even af-
ter 20 h at 7001C, indicating the formed oxide scales are quite
thin (i.e., a few micrometers) at or below these conditions.
Raman spectroscopy is highly sensitive to carbon and, there-
fore, a valuable tool to identify carbon allotropes26,27 and in-
terpret structural characteristics.28 This is why structural
characterizations of carbon were carried out using Raman spec-
troscopy (Fig. 3). The Raman spectra clearly verify strong bands
of the graphite band (G-band) and the disordered/nanocrystal-
line carbon band (D-band),11–15 which provides the evidence for
the presence of carbon. Due to carbon consumption by reaction
with water,15,21,22 the amounts of as-formed carbon decrease
with reaction time at 7001C (Fig. 3(j)–(l)). It can be seen from
Fig. 4 that the positions of G-bands are constant and indepen-
dent of experimental conditions, while D-bands display a sig-
nificant down-shift with oxidation temperatures and time. In
Fig. 3, the D-bands are asymmetric and another two peaks at
1179 and 1287 cmꢁ1 are assigned by fitting using two Gaussian–
Lorenz mixed curves. Previous studies about hydrothermal ox-
idation of SiC suggested these broad peaks in the D-band region
originated from the contribution of sp3 carbon.11–14 This anal-
ysis cannot unambiguously be confirmed from Raman spectros-
copy alone, because the Raman scattering cross section of sp3
carbon is much lower than sp2 carbon unless the laser wave-
length lies in the ultraviolet range.29,30
Gottingen, Germany) with the precision of 0.1 mg.
¨
After hydrothermal oxidation, the reaction products were
analyzed by X-ray diffraction (XRD), Raman spectroscopy, and
X-ray photoelectron spectroscopy (XPS). XRD was performed
using a Bruker D8 Advance Microdiffractometer (Bruker AXS
GmbH, Karlsruhe, Germany) with a Co-sealed tube, an HOPG
graphite primary monochromator and a 500 mm monocapillary
with 300 mm pinhole. Raman spectroscopy was recorded using a
Dilor Labrams 2 (Horiba Jobin Yvon GmbH, Bensheim, Ger-
many) and Ar1-ion laser with the wavelength of 488.0 nm. Us-
ing a grating with 1800 lines/mm, a spectral resolution of ca. 1.7
cmꢁ1 was obtained. XPS were measured using a SPECS Phoi-
bos100 electron energy analyzer and MgKa radiation (1253.6
eV, nonmonochromatized, 0.7 eV intrinsic line width). Typical
pass energies were 50 eV for overview spectra and 10–20 eV for
detail spectra, resulting in an overall energy resolution of 1 eV.
The thermodynamic reactions were calculated using the FactS-
age 5.4 Gibbs free energy minimization program.24 The calcula-
tions were performed assuming a constant total pressure of 35
MPa and the gas phases were assumed to show ideal behavior.
The surfaces and cross-sections of the specimens were exam-
ined using a scanning electron microscope (LEO VP 1450, Carl
Zeiss AG, Oberkochen, Germany, 15 kV acceleration current)
equipped with an energy-dispersive spectroscopy (EDS) from
Oxford (INCA energy 300, Oxford Instruments, Abingdon,
U.K.). Before the examination, a thin platinum layer (ca. 35
nm) was sputtered onto the surfaces of the samples. SE-mode
images were recorded using an Everhart-Thornley SE-detector
(Carl Zeiss AG).
III. Results
(1) Kinetics
Figure 1 presents the weight gain per unit area as a function of
exposure time for the Ti3SiC2 specimens hydrothermal oxidized
in flowing water at 5001 to 7001C under a hydrostatic pressure of
35 MPa. The kinetics obey a linear time-law, which differs from
the parabolic kinetics shown in air oxidation of Ti3SiC2 at 5001
to 9001C.25 The weight gain is slight at 5001 to 6001C (linear
rate-constants: 1.2ꢀ 10ꢁ2 and 2.5 ꢀ 10ꢁ2 mg/mm2/h, respec-
tively) and is considerably accelerated at 7001C (linear rate-
constant: 24.2 ꢀ 10ꢁ2 mg/mm2/h).
Fig. 2. X-ray diffraction patterns of the Ti3SiC2 samples after hydro-
thermal oxidation at 5001–7001C at 35 MPa. CoKa radiation was
employed. PDF No.: anatase/021-1272, rutile/021-1276, and Ti3SiC2/
040-1132.
Fig. 1. Weight gain per unit area as a function of exposure time for the
Ti3SiC2 specimens hydrothermal oxidized in flowing water at 500–7001C
under a hydrostatic pressure of 35 MPa.

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Journal of the American Ceramic Society—Zhang et al.
Vol. 93, No. 4
Fig. 5. Typical X-ray photoelectron spectroscopy C1s spectrum for the
Ti3SiC2 sample hydrothermal oxidized at 5001C for 100 h. The fitted
curve is the red solid line; the measured C1s peak is the black dot line.
Fig. 3. Raman spectra of bulk Ti3SiC2 after hydrothermal treatment at
5001C for (a) 6, (b) 20, (c) 42, and (d) 100 h; 6001C for (e) 6, (f) 20, (g) 42,
and (h) 100 h; and 7001C for (i) 6, (g) 20, (k) 42, and (l) 95 h under the
pressure of 35 MPa.
(3) Scale Morphology and Composition
Figure 6(a) to (c) depict typical surface morphologies of the
Ti3SiC2 specimens hydrothermally oxidized at 6001 to 7001C.
Short hydrothermal exposure at/below 6001C yields a crack-free
oxide scale (Fig. 6(a)). However, cracks do appear once tem-
perature is up to 7001C, and become pronounced with longer
exposure time (Fig. 6(b) and (c)). These cracks provide pathways
for water transportation to the carbide interface and facilitate
further hydrothermal oxidation, which is responsible for the ac-
celeration oxidation at 7001C. Similar cracks were previously
observed in intermediate-temperature oxidation of Ti3SiC2,25
Ti3AlC2,36 and Ti2AlC37 in air.
Figure 7(a)–(c) display the cross-sections of the specimens
hydrothermally oxidized at 7001C and the corresponding EDS
line scan. The images at 5001 to 6001C are similar to Fig. 7(a), so
they are not shown here for briefness. It can be seen from
Fig. 7(a) that thin oxide layer is formed after 20 h at 7001C,
which consists with the presence of matrix peaks in XRD mea-
surements (Fig. 2). Thick oxide layer can cover the substrate, for
example, after 95 h at 7001C (Fig. 7(b)), but it is not dense, and
therefore does not provide protection. Thus linear kinetics is
expected.
This is why we also applied XPS for closer investigating the
sp2/sp3 ratio. 31–35 Figure 5 shows the typical C1s spectrum for
the Ti3SiC2 sample hydrothermal oxidized at 5001C for 100 h.
The C1s peak appears at 285.6 eV with a full width at half
maximum (FWHM) of 2.0 eV, which considerably exceeds that
of graphite (1.3 eV) and diamond (1.4 eV).31–33 The asymmetric
broadening to higher binding energy and the small shoulder to-
gether with high FWHM suggest that at least two components
contribute to this peak. Considering the actual physical inter-
pretation, three components are employed for the fitting. The
first component at 285.3 eV (FWHM5 1.4 eV) corresponds to
sp2 carbon, and the second at 286.4 eV (FWHM 5 1.3 eV) orig-
inates from sp3 carbon. The third at 288.4 eV comes from C-O
contamination due to air exposure of the sample surface.32–35
This result is quite similar to the work on synthesizing sp2/sp3
hybrid diamond-like carbon,31–35 and confirms the results of
Raman spectroscopy.
As indicated by the line scan in Fig. 7(c), the scale is rich in Ti
and O but deficient in Si. The concentration of C remains con-
stant without significant difference between substrate and scale.
These results are fully consistent with XRD (Fig. 2) and Raman
spectroscopy (Fig. 3).
In addition to the general oxidation, other two modes were
exhibited during hydrothermal oxidation of bulk Ti3SiC2, which
were never found in air oxidation. Figure 8(a) and (b) show the
cross-section morphology of Ti3SiC2 hydrothermal oxidized at
5001C for 42 h and the corresponding elemental mapping.
Phases appearing dark grey in Fig. 8(a) penetrate deep into
the matrix, and they are parallel to each other. Their elemental
distribution is deficient in Si and rich in both O and Ti
(Fig. 8(b)), indicating titanium oxide. Based on above results,
we speculate that hydrothermal oxidation can tear Ti3SiC2
grains apart and attack the weak-spots of this material: the lay-
ered configuration. This means that a single Ti3SiC2 grain is
oxidized firstly along planes perpendicular to the c-axis. Further
experiments like EBSE or TEM are required to fully confirm
this deduction. In addition, it has been observed that the weak
Fig. 4. Wavenumbers of the G- and D-band at various experimental
conditions obtained via Raman spectroscopy (cf. Fig. 3). Note the
down-shift of the D-band at higher temperatures/longer exposure times.

April 2010
Mechanisms and Kinetics of the Hydrothermal Oxidation of Bulk Titanium Silicon Carbide
1151
Fig. 6. Scanning electron microscopic images (SE-mode) showing typical surface morphologies of the Ti3SiC2 specimens hydrothermal oxidized at (a)
6001C for 20 h, (b) 7001C for 6 h, and (c) 7001C for 95 h.
layered configuration can induce delamination of grains after
mechanical deformation.1–3
simply be applied to a general situation that bulk Ti3SiC3 pos-
sess better hydrothermal oxidation resistance than bulk TiC and
bulk SiC. Therefore, we are planning to perform the hydrother-
mal oxidation of bulk TiC and SiC under the same conditions to
further compare, on the basis of quantitative data, the hydro-
thermal oxidation kinetics of Ti3SiC2, SiC and TiC.
No doubt this is an interesting and important phenomenon
although we cannot currently explain it. For TiC, no hydro-
thermal oxidation kinetics data are reported up to now. In case
of the hydrothermal oxidation of SiC, it has been well recog-
nized that the resulted silica is not protective because of its high
solubility in hydrothermal water, and then SiC presents weight
Another weak spot in the Ti3SiC2 matrix was identified as
TiC and SiC impurities. Figure 9 displays a typical morphology
of interfaces between impurities and the Ti3SiC2 substrate. Ox-
idizing species can deeply penetrate along the interfaces con-
suming the TiC or/and SiC inclusions, which suggests that bulk
Ti3SiC2 may exhibit a somewhat better resistance than both TiC
and SiC impurities under hydrothermal oxidation. This oxida-
tion scheme is only observed at 7001C in our experimental con-
ditions. In absence of comparative kinetics data of bulk TiC and
bulk SiC under the same conditions, this phenomenon can not
Fig. 7. Cross-sections (SE-mode scanning electron microscopy) of the specimens hydrothermal oxidized at 7001C for (a) 20 h, (b) 95 h, and (c) energy-
dispersive spectroscopy line scan taken from the white line in Fig. 7(b).

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Journal of the American Ceramic Society—Zhang et al.
Vol. 93, No. 4
Fig. 8. (a) Cross-section morphology of Ti3SiC2 hydrothermal oxidized at 5001C for 42 h; (b) Elemental mapping of the white rectangular region
in Fig. 8(a).
loss.12,13,16 Thus, for SiC impurity, the higher solubility of silica
than titania in hydrothermal water is probably responsible for
this phenomenon; and for TiC impurity, we envisage defect
structures from carbon deficiency as well as low density and
small grains size, etc. to affect the hydrothermal oxidation be-
havior of TiCx impurities in the matrix.
experimental parameters of this work suggests the dissolution
rate of silica is over 500 times faster than that of TiO2.38–41 The
dissolved silica could not precipitate in the final products be-
cause it was exported from the autoclave with the continuous
water flow. In the studies of Ti3SiC2 and Ti3Si0.9Al0.1C2 pow-
ders,21,22 the situation was different because gold capsules used
to contain sample powders/water provided closed environments.
Thus the dissolved silica precipitated in the capsules as cooled to
ambient temperature. For SiC fibers,12 non-dense and spalling
silica layers were observed after hydrothermal oxidation because
of the high solubility of silica.
Gaseous compounds also were produced during the hydro-
thermal oxidation process. The temperature dependent partial
pressures of the volatile compounds are calculated at a H2O:
Ti3SiC2 molar ratio of 900:1 considering that there is sufficient
water in the system (Fig. 10). The dominating volatile species are
H2O, H2, CO2, CO, and CH4. It should be noted that methane
significantly decreases with temperatures.
Kraft et al.12 investigated the hydrothermal oxidation of
CVD SiC fibers under the pressure of 200 MPa, and the linear
rates, if using the unit of weight, were 9.12 ꢀ 10ꢁ2, 1.15 and
13.38 mg/mm2/h at 500, 600 and 700 respectively. Barringer
et al.13 studied the hydrothermal oxidation of high purity CVD
SiC at 5001C and 25 MPa, and the linear rate was calculated as
6.3 ꢀ 10ꢁ3 mg/mm2/h. For sintered SiC under much more mod-
erate conditions (3601C) the linear rate was calculated as
2.8 ꢀ 10ꢁ2 mg/mm2/h.16 Comparing the data obtained here,
Ti3SiC2, as a sintered and impurity containing ceramic, presents
amazing hydrothermal oxidation resistance.
Based on the results of XRD, Raman, XPS and thermo-
dynamic calculations, the hydrothermal oxidation reactions of
bulk Ti3SiC2 was described as follows: Ti3SiC2 was firstly
IV. Discussion
(1) Reaction Products and Hydrothermal Oxidation
Reactions
Crystalline silica was not detected by XRD (Fig. 2) after hydro-
thermal oxidation, and the EDS line scan (Fig. 7(c)) clearly
showed the deficiency of Si in the oxide layers. Also, amorphous
silica was not detected using Raman spectroscopy. The absence
of silica is attributed to its high solubility in hydrothermal
water.38–40 The extrapolation of these literature data to the
Fig. 10. Temperature dependent partial pressures of the volatile com-
pounds at a H2O:Ti3SiC2 molar ratio of 900:1 under the pressure of 35
MPa. The amount of water is estimated using: density of 0.087 g/cm3
(7001C, 35 MPa), volume of 30 mL. The amount of Ti3SiC2 is estimated
using: density of 4.53 g/cm3, volume of 6.8 mm3. Only reacted Ti3SiC2 is
considered, and we used 50 mm as the thickness of reacted Ti3SiC2.
Fig. 9. Cross-section morphology of Ti3SiC2 hydrothermal oxidized at
7001C for 95 h.

April 2010
Mechanisms and Kinetics of the Hydrothermal Oxidation of Bulk Titanium Silicon Carbide
1153
oxidized to TiO2 and SiO2 with the survival of carbon, and a
mixture layer of TiO2, SiO2, and carbon was formed on the
surface. This corresponded to the reactions
Ti3SiC2 þ 8H2O ¼ 3TiO2 þ SiO2 þ 2C þ 8H2
(1)
and
Ti3SiC2 þ 8H2O ¼ 3TiO2 þ SiO2 þ C þ CH4 þ 6H2: (2)
The formed SiO2 dissolved into water and TiO2 remained.
Simultaneously, the resulting carbon was consumed by reaction
with water. Because CO2 possessed higher partial pressures than
CO (Fig. 10), the carbon consuming reactions15 were
C þ 2H2O ¼ CO2 þ 2H2
(3)
(4)
1
1
C þ H2O ¼ CO2 þ CH4
Fig. 11. Wavenumbers of D bands as a function of the intensity ratio of
D/G bands.
2
2
The significant decrease of methane with higher temperatures
indicated that reactions (1) and (3) were preferred above 5501C,
and reactions (2) and (4) were favored below 5501C. Conse-
quently, as the equilibrium state was achieved after the con-
sumption of the entire sample, the predominating reactions
between water and Ti3SiC2 were:
experimental error of these values, the general trend of smaller
domain sizes is unambiguously verifiable by both (1) the D/G
intensity ratio and (2) the D-band downshift.
Figure 12 illustrates the relationship between the D/G band
ratio and experimental conditions. This figure points out that
the domain size of carbon decreases with temperatures and time.
This is owing to more severe reactions between carbon and wa-
ter happens at higher temperatures and longer time, which re-
duce carbon domain sizes.
above 5501C:
The formed carbon showed both sp3 and sp2 hybridization.
Considering the low temperatures and pressures of the reactions,
hydrothermal oxidation of Ti3SiC2 is likely to produce sp3 car-
bon in analogy to studies on SiC.11–14
Currently we are attempting to optimize the experimental
conditions, such as using diamond seeds and metal catalysts, to
increase the amounts of sp3 carbon. Further research about this
topic is necessary, because the formation mechanism of sp3 car-
bon in the course of hydrothermal decomposition of Ti3SiC2 is
not clearly understood.
Ti3SiC2 þ 12H2O ¼ 3TiO2 þ SiO2 þ 2CO2 þ 12H2
below 5501C:
Ti3SiC2 þ 9H2O ¼ 3TiO2 þ SiO2 þ CO2 þ CH4 þ 6H2 ð6Þ
ð5Þ
1
3
2
2
(2) Carbon Formation During Hydrothermal Oxidation
The mechanism for carbon formation has been discussed in de-
tail in previous studies.21,22 Briefly, carbon was produced by ex-
traction of titanium and silicon out of the carbide. Both
thermodynamics and kinetics factors contributed to the forma-
tion of carbon, and kinetics played an essential role on this pro-
cess.21,22 From thermodynamics, the reason was the affinity of
metals to water and oxygen was higher than that of car-
bon.15,42,43 From kinetics, carbon formation was related to
the unique layered structure and weak Ti–Si bonding of
Ti3SiC2.1–3,44–49
The Raman spectra demonstrated the positions of G-band to
be independent of experimental conditions, but the D-band dis-
played down-shift with temperatures and time (Figs. 3 and 4).
This down-shifting behavior is known to result from smaller
domain sizes and was reported, for example, by Cancado et al.50
Their results showed that the domain size of carbon was de-
creased accompanied with the downshift of the D-band and the
constant position of the G-band. Another size-depending pa-
rameter is the D/G band ratio, which is inversely proportional to
the domain size of carbon.50,51 Plotting both (Fig. 11), the shift
of the D-band and the D/G band ratio, we see a linear corre-
lation between these two size-sensitive parameters. A semi-quan-
titative equation for determining domain size of carbon (La) is51
The production of sp3 carbon from SiC was explained similar
to diamond formation at low pressures by CVD.11–14 The for-
mation of hydrogen played an import role because in CVD di-
amond synthesis hydrogen dissociated into atoms to etch
preferentially sp2 bonded carbon, which facilitated retention of
sp3 carbon and nucleation of diamond.11–14
(3) Cracks Formed in Oxide Layers
The formation of cracks in the oxide scales contributed to the
acceleration of hydrothermal oxidation of Ti3SiC2 at 7001C.
ꢀ
ꢁꢁ1
ID
IG
LaðnmÞ ¼ ð2:4 ꢀ 10ꢁ10Þll4aser
;
(7)
where llaser is the laser wavelength in nm unit, I is the intensity
D
IG
ratio between D- and G-bands. Based on this equation, the
domain size of carbon obtained in this work ranges between 17
and 28 nm. Although it is difficult to clearly state the relative
Fig. 12. Relationship between the intensity ratio of D/G bands and the
experimental conditions.

1154
Journal of the American Ceramic Society—Zhang et al.
Vol. 93, No. 4
14T. Kraft and K. G. Nickel, ‘‘Carbon Formed by Hydrothermal Treatment of
a-SiC Crystals,’’ J. Mater. Chem., 10 [3] 671–80 (2000).
Anatase transformed to rutile at high temperatures due to the
higher stability of rutile.52,53 This phase transformation resulted in
tensile stresses in oxides because the density of rutile (4.26 g/cm3)
is larger than that of anatase (3.84 g/cm3). The generation of
tensile stresses could crack the oxides.2,25,36,37 From XRD
(Fig. 2)/SEM (Fig. 6) results, only anatase TiO2 was detected
at 5001 to 6001C and no cracks were present in the oxides. Once
temperature was up to 7001C, however, cracks were formed in
the oxide layers accompanied with this phase transformation.
Also, this phase transformation induced crack formation in the
oxide scale during air oxidation of Ti3SiC2,25 Ti3AlC2,36 and
Ti2AlC.37
15N. S. Jacobson, Y. G. Gogotsi, and M. Yoshimura, ‘‘Thermodynamic and
Experimental Study of Carbon Formation on Carbides Under Hydrothermal
Conditions,’’ J. Mater. Chem., 5 [4] 595–601 (1995).
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Sintered and Chemically Vapor Deposited Silicon Carbide Ceramics in Water at
3601C,’’ J. Mater. Sci. Lett., 22, 581–4 (2003).
17S. Kitaoka, T. Tsuji, T. Katoh, Y. Yamaguchi, and K. Sato, ‘‘Tribological
Characteristics of Si3N4 Ceramic in High-Temperature and High-Pressure Water,’’
J. Am. Ceram. Soc., 77 [2] 580–8 (1994).
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sion of Pure, Hot Isostatically Pressed Silicon Nitride,’’ J. Am. Ceram. Soc., 76 [5]
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V. Conclusions
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Thermodynamic Study of the Hydrothermal Oxidation Behavior of Ti3SiC2 Pow-
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Hydrothermal oxidation of bulk Ti3SiC2 was investigated at
5001 to 7001C under a hydrostatic pressure of 35 MPa in a con-
tinuous water flow. The reaction obeyed a linear law. The
hydrothermal oxidation rate was slow below 7001C and dra-
matically accelerated at 7001C due to the formation of cracks in
oxides. The phase transformation from anatase to rutile (two
TiO2 modifications) was responsible for the formation of cracks.
Titanium and silicon were selectively extracted from Ti3SiC2
during hydrothermal oxidation, resulting in the formation of the
corresponding oxides and sp2/sp3 hybrid carbons. However, due
to the high solubility of silica in hydrothermal water, the formed
oxide layers only contained titanium oxides and carbon. Besides
general oxidation, two special modes very likely demonstrate in
current experiments: (1) preferential hydrothermal oxidation of
lattice planes perpendicular to the c-axis inducing cleavage of
grains, (2) uneven hydrothermal oxidation because of the oc-
currence of TiC and SiC impurity inclusions. Because the
oxidation processes will certainly accelerate further with increas-
ing temperature we expect an application limit in the order of
7001C for this material, which is, for many uses, a quite good
value.
22H. B. Zhang, X. Wang, C. Berthold, K. G. Nickel, and Y. C. Zhou, ‘‘Effect of
Al Dopant on the Hydrothermal Oxidation Behavior of Ti3SiC2 Powders,’’ J. Eur.
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23Y. C. Zhou, Z. M. Sun, S. Q. Chen, and Y. Zhang, ‘‘In-Situ Hot Pressing/
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Acknowledgments
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Haibin Zhang is grateful to the Alexander von Humboldt Foundation. We
appreciate the experimental assistance of Mr. D. Russ and Mr. T. Kiemle of
Tubingen University.
¨
32J. Filik, P. W. May, S. R. J. Pearce, R. K. Wild, and K. R. . Hallam, ‘‘XPS and
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