The Kinetics of the oxidation of tantalum carbide

Powdered samples of TaC (-200 + 270 mesh) were oxidized at various -2 partial pressures of oxygen from 1.3 x 10 to .86 atmospheres in the temperature range 640 - 825°C and in pure oxygen in the range 600 - 630°C. Above 670°C the observed kinetics involved linear oxidation associated with the formation of a non􁪽protective Ta205 layer. Between 670° and 650°C initial oxidation was non-linear but later changed to linear oxidation. From 640 - 600°c oxidation was extremely slow and parabolic oxidation was observed. Parabolic oxidation changed to linear oxidation before more than 25 per cent reaction had occurred, depending on the reaction temperature. Below 725°C a change in linear kinetics was observed. Oxidation has been explained quantitatively in terms of equilibrium chemisorption of oxygen on TaC, in which oxygen splitting occurs on the surface, followed by dissolution of the oxygen atoms in the surface prior to the rate-controlling reactions. Two alternate mechanisms are discussed for the rate controlling reactions. The more plausible mechanism enbodies two parallel rate :controlling processes, a surface reaction with an activati6n energy of 108 Kcal per mole and a diffusion reaction with an activation energy of 55 Kcal per mole. The diffusion reaction is believed to, be diffusion of oxygen, into the TaC lattice partially replacing carbon.