Oxidation of porous nanocrystalline titanium nitride. III. Oxidation mechanism

The air oxidation mechanism of nanocrystaline TiN at 500 to 900 CC is examined. It is shown that at t <= 800 degrees C the oxidation of titanium nitride is controlled by the diffusion of oxygen and at t > 800 degrees C the interdiffusion of titanium ions is observed. The oxidation properties of porous TiN are determined by the chemical interaction of oxygen and the reaction surface, which includes the external surface of samples and the internal surface of the pores into which oxygen penetrates. The time and temperature dependence of the weight increment complies with the porous material oxidation model. Active initial oxidation is due to the interaction of oxygen and large internal surface. Short-term self-heating of porous samples is also possible. At t > 800 degrees C, the pores are obliterated with oxides with time, the internal reaction surface reduces, an external oxide film is formed, the oxygen diffusion and weight increment slow down, and the process stabilizes. With temperature increase, these processes are activated and lead to a smaller weight increment at the final stage (2 to 4 h) at 800 degrees C as compared with 600 degrees C. At t > 800 degrees C the pore obliteration rate increases, but due to the interaction of oxygen and titanium ions that diffuse into the external scale surface, weight increment continuously increases with both time and oxidation temperature. The phase composition of the scale also affects the oxidation mechanism of porous TiN. Oxynitride of terminal composition plays a protective role; the transformation of anatase into rutile is accompanied by a decrease in the oxygen diffusion rate; Ti2O3 formed in pores accelerates their obliteration.