JOURNAL OF MATERIALS SCIENCE LETTERS 21, 2002, 955– 958
Oxidation of Ta
x
Ti
1
−
x
C whiskers under formation of carbon
M. JOHNSSON
Department of Inorganic Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
E-mail: matsj@inorg.su.se
S. SHIMADA
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University,
Sapporo 060-8628, Japan
Tantalum carbide (TaC) and titanium carbide (TiC)
are both promising compounds for use as reinforc-
ing materials in high-temperature structural ceramic
composites. Both compounds have high melting points
(>3000
◦
C) and high strength. By changing the chemi-
cal composition as outlined by the formula Ta
x
Ti
1−x
C
(0 ≤ x ≤ 1), properties such as the thermal expansion
coefficient and the heat conductivity can be tailored.
In order to characterize those potential high tempera-
ture materials, their oxidation behavior is important to
study.
Several carbide phases are known to oxidize under
transient formation of carbon e.g. ZrC, TiC, HfC, and
NbC under certain conditions [1]. The oxidation mech-
anisms have been studied in some detail during recent
years, and the following main reactions can generally
be discerned.
MeC + O
2
(g) = Me(O, C) + C (1)
Me(O, C) + O
2
(g) = MeO
x
+ C (2)
C + O
2
(g) = CO
2
(g) (3)
For simplicity, the reactions are not balanced. Me
denotes a metal and MeO
x
metal oxides formed by the
oxidation process; first lower and finally higher oxides.
Equilibrium calculations using the thermodynamic
database HSC [2] suggest that when both TiC and TaC
oxidize according to the reaction scheme above, the
metal reacts first, and then carbon to form CO
2
, see
Fig. 1a and b. The database does not comprise formation
of solid solutions, and therefore support for reaction (1)
could not be obtained.
In an earlier study of Ta
x
Ti
1−x
C whisker oxidation
[3], the focus was on the oxidation onset temperature,
the phase composition and the morphology of the final
oxide product. It is of interest to investigate how oxi-
dation of the whiskers proceeds with a change of the x
value. No such study has been reported before on the
oxidation of mixed carbide whiskers. The aim of the
present work is to study the non-isothermal oxidation
of Ta
x
Ti
1−x
C whiskers by simultaneous TGA–DTA–
MS analysis in order to reveal how oxidation of the
whiskers occurs with changing x value and whether or
not elemental carbon is formed during the reaction.
The material investigated consisted of Ta
x
Ti
1−x
C
whiskers prepared via a carbothermal vapor–liquid–
solid (VLS) growth mechanism. The whiskers were ob-
tained in a yield of about 80 vol%, with a length of 10–
40 µm and a diameter varying from 0.25 µm for TaC to
about 0.55 µm for TiC. Typical Ta
x
Ti
1−x
C whiskers are
shown in Fig. 2. The synthesis reactions are described
in more detail in [4–6], and the whisker compositions
determined by chemical analysis are given in Table I.
The C and O contents were analyzed by standard com-
bustion techniques. The Ti and Ta contents were de-
termined from spectrometric data and plasma emission
lines, respectively, and found to be equal to the nominal
ones within experimental error.
(a)
(b)
Figure 1 Equilibrium calculations showing that the metal oxidizes be-
fore carbon in TiC and TaC: (a) oxidation of TiC at 600
◦
C, (b) oxidation
of TaC at 600
◦
C.
0261–8028
C
2002 Kluwer Academic Publishers
955