Oxidation behaviour of TaxTi1−xC and TaxTi1−xCyN1−y whiskers
Introduction
Tantalum carbide (TaC), titanium carbide (TiC) and titanium nitride (TiN) all have material characteristics such as high melting point, favourable mechanical properties and thermodynamic stability that makes them interesting as engineering materials, i.e. as reinforcement materials in high-performance ceramic composites. Characteristic properties such as the thermal expansion coefficient can then be tailored to match the matrix material by varying the chemical composition as outlined by the formula TaxTi1−xC and TaxTi1−xCyN1−y (0 ≤ x ≤ 1, 0 ≤ y ≤ 1). The oxidation behaviour of those materials is therefore important to study, and oxidation studies have been published for several transition metal carbides and nitrides, but there is only one report on the oxidation behaviour of mixed transition-metal carbides [1].
There are investigations of the oxidation behaviour of TiN in the form of thin films [2], [3], [4] and compacted monoliths [5], and as composites containing TiN [5], [6] as the major component or added as a reinforcing agent. Tampieri et al. [6] have studied the oxidation of Al2O3-TiN composites and found a parabolic rate-law behaviour for temperatures exceeding 900°C, while an almost constant oxidation rate was observed for T < 900°C which in turn was ascribed to grain boundary reactions. Gogotsi et al. [5] have performed corresponding studies of Al2O3-TiN, Si3N4-TiN, AlN-TiN and AlON-TiN composites and found that all the oxidation curves could be fitted to a parabolic rate law, independently of the temperature at which the experiments were performed. Shimada and Kozeki [7], [8] studied the oxidation behaviour of TiC particles and found that the oxidation process was diffusion controlled. Based on isothermal and non-isothermal oxidation experiments, they divided the process into four steps, I–IV. Step I covered the α-range 0–0.20 (α = degree of oxidation), and within this range a Ti-oxocarbide phase is formed; step II and III occur in the range α = 0.20–0.60 and involve the formation of reduced Ti oxides. The transition from step II to step III could only be discerned in isothermal experiments to take place in the α-range 0.30–0.40. During step IV (0.6 < α ≤ 1.00) TiO2 is produced in the form of anatase or, at higher temperatures, as rutile. The differential thermal analysis studies performed by Shimada [8] showed a rather broad exothermal DTA peak around 500°C when TiC was exposed to a low partial pressure of oxygen (pO2); and this peak sharpened and shifted slightly towards lower temperatures with increasing pO2. At pO2 ≥ 15 kPa a second and comparatively sharp DTA peak appeared at a temperature about 30°C higher than the first one. The second peak broadened and became less intense with increasing pO2. Mass spectroscopy measurements showed that CO2 was evolved in the temperature region where the exothermal DTA peaks appeared.
Voitovich et al. [1] studied the oxidation of hot-pressed TiC compacts and found a parabolic rate-law behaviour. The same authors also oxidised compacts of the composition Ti1−x−yZrxHfyC and found them to exhibit a lower oxidation resistance than pure TiC. They suggest that this is due to the fact that no protecting layer of TiO2 is formed on the solid solution samples. The oxidation of hot isostatically pressed TaC is reported to proceed by an interfacial reaction [9].
Tampieri et al. [6] oxidised Al2O3-TiC composites and reported that the oxidation initially is governed by grain-boundary reactions, but when a continuous rutile layer is formed then the oxidation kinetics change from almost linear to parabolic.
As described above, most oxidation studies of TaC, TiC and TiN have been performed under isothermal conditions, using compacted monoliths of the materials or using composites with particles of TaC, TiC and TiN, respectively, added as a reinforcing material. Our study now presents the non-isothermal oxidation behaviour of mixed transition-metal carbide, TaxTi1−xC, and carbonitride, TaxTi1−xCyN1−y, whiskers with 0 ≤ x ≤ 1, and 0 ≤ y ≤ 1, together with some isothermal oxidation studies of TaxTi1−xC whiskers yielding information concerning the phase and morphology evolution with temperature and time.
We have recently synthesised TaxTi1−xC and TaxTi1−xCyN1−y whiskers via a carbothermal vapour–liquid–solid (VLS) growth mechanism [10]. The reactions were carried out at 1250°C in argon and nitrogen, respectively. The starting materials consisted of Ta2O5, TiO2, C, Ni and NaCl. Carbon was added to reduce the oxides, and nickel to catalyse the whisker growth. NaCl was used as source of Cl for vapour-phase transport of Ta as oxochloride and Ti as chloride to the Ni catalyst. Whiskers were obtained in a yield of about 80 vol%, with a length of 10–40 μm and a diameter varying from about 0.25 μm for TaC to about 0.55 μm for TiC. The x-value of the whiskers could be varied by altering the weight in ratio of Ta2O5 and TiO2. The nitrogen content decreased with increasing x-value in TaxTi1−xCyN1−y.
The oxidation experiments were performed in a thermogravimetric (TG) apparatus which allows simultaneous recording of the weight change and the heat evolved by the oxidation (DTA). The oxidised samples were characterised by their X-ray powder diffraction patterns and by scanning and transmission electron microscopy (SEM and TEM) studies.
Section snippets
Experimental
The starting materials consisted of in-house made TaxTi1−xC and TaxTi1−xCyN1−y whiskers, prepared as described briefly above and in detail in [10], [11], [12], [13], [14]. The whisker compositions, determined by chemical analysis, are given in Table 1. The C, N, and O contents were analysed by a standard combustion technique. The Ti and Ta contents were determined from spectrometric data and plasma emission lines, respectively, and found to be equal with the nominal ones within experimental
Results and discussion
It is well known that TaC, TiC and TiN can dissolve oxygen into their structural frameworks, and this is also the case in the whisker phases investigated (see Table 1). In order to minimise the oxygen content in the whiskers, an excess of carbon was used in the starting reaction mixture. This implies that the product, besides whiskers, will also contain some extra carbon. Most of this carbon was removed by shaking the product with a mixture of water and an organic solvent. The carbon particles
Concluding remarks
The oxidation experiments with TiC whiskers revealed that they had the lowest oxidation resistance of all those studied and that the oxidation of TiC whiskers could be divided into four steps in accordance with the findings by Shimada and Kozeki [7], [8]. The oxidation resistance of TiCyN1−y whiskers was found to be higher than for TiC, but the recorded DTA curve suggests that the oxidation takes place in two steps rather than in four as observed for TiC. TaC whiskers had the highest oxidation
Acknowledgements
This work has been financially supported by the Swedish Foundation for Strategic Research (SSF) and by the Swedish Research Council for Engineering Sciences (TFR).
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