It was shown that, when a small amount of O was dissolved in TiC1-x monocrystals at 1700 to 2050K, the O mobility and its activation energy had comparable values to those for C vacancies. On the basis of the O solubility, and the O defect diffusion coefficients, it was possible to estimate the values of the O diffusion coefficients in samples which were saturated with O and nearly saturated with C. The activation energy for O solubility had the opposite sign to that of the activation energy for O defect diffusion and, consequently, the activation energy for O diffusion was lower than the activation energy for O mobility alone. Thus, the higher the degree of O-saturation, the higher were the values of the O diffusion coefficients at a given temperature. Moreover, the effective activation energy for C diffusion in monocrystals of constant C-vacancy concentration was equal to the activation energy for C-vacancy defect diffusion. Thus, at a given C-vacancy concentration, the activation energy for C diffusion was higher than the corresponding activation energy for O diffusion in this carbide. However, at temperatures higher than 1700K, only small numbers of C vacancies could be filled by O reaching its maximum saturation, and the O diffusion coefficients were lower than the C self-diffusion coefficients over the entire temperature range. This implied that, at constant temperature and C-vacancy concentrations, O diffused slower than C in the carbide lattice. The results led to the conclusion that TiC oxidation was controlled by the diffusion of O into the carbide and by the formation of oxicarbide, before the growth of various oxide phases.

Oxygen Diffusion in TiC at High Temperatures. E.M.Fryt: Solid State Phenomena, 2000, 72, 63-8