Oxidation of Powdered and Sintered Titanium Nitride

Oxidation of three TiN powders prepared by plasma vapour phase synthesis and having the specific surface area of 33, 46 and 75 m 2 g-1, as well as oxidation of pressureless-sintered bodies produced from these powders, was studied. The experiments were performed in flowing air under programmed heating up to 1500°C and under isothermal conditions. Oxidation of ultrafine powders starts above ∽250°C and results in the formation of TiO2 as anatase and brookite at lower temperatures, and as rutile at higher temperatures. The grain size can considerably affect the oxidation rate of TiN powders. The surface of the oxidised sintered TiN samples in most cases was covered by a layer of highly textured rutile crystals. The formation of multi-layered oxide films was observed above 1000°C. The growth of such rutile scales may be based on the increased grain boundary or volume diffusion of titanium ions, recrystallisation and sintering processes in the outer scale at high temperatures, as well as on the gas (N2, NOx) evolution at the oxide/substrate interface or exfoliation of the oxide layers due to internal stresses.