Interfacial reaction on oxidation of carbides with formation of carbon

The oxidation of carbides (ZrC, HfC. TiC) using single crystals with the 200 or 220 face was carried out at temperatures of 500-1500 degreesC and oxygen partial pressures (P-O2) of 0.02-80 kPa. The oxidation proceeded linearly with time. The preferred (200) or (220) orientation of c/t- or m-ZrO2, m-HfO2 and Ti3O5 was recognized on the oxidized surfaces of the crystals or near the reaction interface. The oxide scale consisted of inner dense dark scale (zone 1) and outer porous white/gray scale (zone 2). Zone 1 contained much carbon at around 23-25 at.%. Simultaneous TG-DTA-MS analysis of HfC powders showed that the oxidation of Hf component in HfC occurs to produce carbon at 380-600 degreesC above which remaining carbon is oxidized with evolution of CO2. From HRTEM observation at the ZrC/zone 1 interface, it was found that c-ZrO2 crystallites are formed directly on the ZrC lattice while maintaining the above preferred orientation. The interfacial reaction with the formation of carbon on oxidation of the carbides was discussed from the above results. (C) 2001 Elsevier Science B.V. All rights reserved.