Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

We used the continuous weighing method to study the oxidation kinetics in air for TiN specimens pressed and sintered from nanocrystalline powders with particle size <= 55 nm. Oxidation was carried out at 500-1000 degrees C for 240 min. By comparing with the oxidizability of compact titanium, we estimated the total reaction surface S of the porous specimens as a function of their oxidation conditions. The mass of absorbed oxygen Am was calculated from the mass gain Delta P, taking into account the volatile component N-2. We have shown that the maximum mass gain Delta m at 600 degrees C is due to reaction of oxygen with the largest reaction surface. Within 120 min, external pores close up, S decreases, and then a continuous oxide layer forms in which diffusion of oxygen is slowed down. At 700-800 degrees C, the process of closing up of the-pores is activated, and S decreases by an order of magnitude compared to 600 degrees C. After the first 40-50 min, a continuous oxide film forms and virtually no further mass gain occurs. As the temperature increases, the oxidation rate increases. At 900 degrees C, the reaction surface becomes equal to the external surface of the specimen, but the thickness of the scale increases linearly. We hypothesize that for T > 850 degrees C, counterdiffusion of titanium ions is superimposed on diffusion of oxygen.