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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. [

The isothermal oxidation of ZrC single crystals with (100) orientation was carried out at temperatures of 500°, 550°, and 600°C at an oxygen pressure of 2.6 kPa for times up to 240 h. A polished cross section of the oxidized crystal was observed by backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Zr, O, and C and their elemental profiles by the linescan method in the ZrC and oxide scale were performed by wavelength dispersive X‐ray microanalysis. A thin foil of a crystal oxidized at 600°C was examined by transmission electron microscopy. It was found that the oxide scale was divided into two regions, zones 1 and 2, which contained 14 to 23 and 7 to 10 at. % carbon, respectively. Zone 1 exhibited an almost compact, pore‐free matrix of c‐ZrO2. In zone 2, some growth and aggregation of the c‐ZrO2 occurred, producing 5‐ to 20‐nm‐sized particles between which carbon should have been present. The thickness of zone 1 increased parabolically up to 240 h at 500°C and probably in an early period at 550° and 600°C, reaching a constant (about 2 (μm), in contrast to the thickness of zone 2, which increased linearly with time.

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The isothermal oxidation of HfC powders was carried out at temperatures of 480" to 600°C at oxygen pressures of 4, 8, and 16 kPa, using an electromicrobalance. The oxidized product was identified by X-ray analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, and electron diffraction, and the morphology of the oxidized grains was observed by scanning electron microscopy. Oxidation proceeds by two processes: a diffusion-controlled process operates up to about 50% oxidation and a phase-boundarycontrolled process operates above about 50 % oxidation. The activation energies for both processes are the same (197 k 15 k.J.mol-'1. The change in the oxidation process is associated with the generation of cracks on the grains, resulting from the growth or expansion stress due to the formation of monoclinic HfO, microcrystallites less than 3 nm in size. In the latter process, the thickness of the diffusion layer is kept constant, being time-independent, which allows the process to apparently obey the phase-boundarycontrolled reaction.