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The isothermal oxidation of the 200 face of HfC and TiC single crystals was performed at temperatures of 700°-1500°C and at oxygen pressures of 0.08 -80 kPa for 4 h. The weight gain by oxidation of the two crystals was followed using an electromicrobalance. A polished cross section of the oxidized crystals was observed using backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Hf, Ti, O, and C was performed by wavelength-dispersive X-ray microanalysis. The early-stage oxidation kinetics of HfC crystals were described by the contracting volume equation, followed by slowed reaction in the latter stage, whereas the same equation was applied to the oxidation of TiC over the entire oxidation time. The preferred {200} orientation of monoclinic HfO 2 occurred on the oxidized surface of the HfC crystal. The oxide scale on the HfC crystal consisted of a compact and pore-free black inner scale (zone 1) and a white/gray outer scale that contained many pores (zone 2). Zone 1 contained ϳ25 at.% unoxidized carbon, and zone 2 contained 6 -11 at.% carbon. The oxide scale of TiC was composed of an inner dense lamella subscale (zone 1) with a carbon content of 7-23 at.% and an outer region with laminations that was separated by pores and cracks (zone 2). The Ti 3 O 5 phase, which exhibits a strong 020 line, was formed at depths of >40 m in the scale obtained at 1500°C. Treatment with a concentrated HF solution allowed zone 1 to be separated from the HfC crystal in the form of carbon-containing films, which were characterized using Raman spectroscopy and transmission electron microscopy.

The isothermal oxidation of ZrC powders was carried out at relatively low temperatures of 380" to 550°C at oxygen pressures of 1.3,2.6, and 7.9 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electromicrobalance. The oxidation kinetics are described by the diffusion-controlled Jander's equation, following rapid oxidation in the early stage. Two activation energies were obtained: 138 kJ-mol-' below about 470°C and 180 kJ*mol-' above that temperature. The high-and low-temperature oxidation mechanisms are discussed in connection with the crystallization of cubic ZrOz, accompanied by the generation of cracks, as well as the formation of carbon in the hexagonal diamond form in the product phase. [