Oxidation and Mechanism of Single Crystal Carbides with Formation of Carbon.

The oxidation of the carbide single crystals (ZrC, HfC, TiC) was carried out at temperatures of 500 to 1500°C and oxygen pressures of 0.02 to 80kPa. X-ray diffraction showed the preferred {200} or {220} orientation of cubic-, tetragonal-, or monoclinic ZrO2, monoclinic HfO2 and tetragonal TiO2 on the oxidized surface of the crystals. The oxide scale consisted of inner dense dark scale (zone 1) and outer porous white/gray scale (zone 2). Zones 1 and 2 contained as much as 23 to 25 at.% carbon and 5 to 11 at.% carbon, respectively. Zone 1 grew parabolically to finally attain a constant thickness, in contrast to zone 2 which increased linearly. From HRTEM at the ZrC/zone 1 interface, it was observed that c-ZrO2 crystallites of 2 to 10nm sizes are formed with the above preferred orientation. It was found from TEM-EDX analysis that a considerable amount of carbon is concentrated at the interface and that oxygen exhibits the concentration gradient both in the ZrC and zone 1 over a distance of 300nm. Zone 1 was separated as carbon films from the oxidized carbides by HF or HF/H2SO4 treatment and characterized by Raman spectroscopy and TEM, which showed the formation of amorphous carbon. The mechanism on oxidation of the carbides with the formation of carbon was elucidated from the above results as well as from the kinetic and thermodynamic considerations. The perspectives for the oxidation of the carbides with formation of carbon is mentioned.