Interfacial reaction and cracking mechanism on oxidation of metal carbides

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Oxidation of transition metal carbide (MeC, Me=Ti, Zr, Hf, Nb) was studied at an oxygen partial pressure of 4 - 16 kPa and in a temperature range of 400 to 800ﾟC, using the powder and single crystal samples. The oxidation mechanism was explained from a kinetic viewpoint and on SEM and TEM observation of the oxide/carbide interfaces. The summary was as follows ;Oxidatin of TiC proceeded by a diffusion-controlled process in four steps. Cracking occurred by formation of oxycarbide/titanium suboxide. Oxidation of ZrC produced two oxide scale zones 1 and 2. Zone 1 showed relatively pore-free scale containing 14 - 23 at%. Zone 2 contained many pores and large cracks with a carbon content of 7 - 10 at%. Zone 1 scale thickened parabolically in an early period, then is kept constant. In contrast, depth of zone 2 scale increased linearly with time. Oxidation behavior of HfC was quite similar to that of ZrC.Oxidation of NbC occurred by a phase boundary-controlled process which resulted from the porous scales having colmnar grains normal to the crystal surface.