OXIDATION BEHAVIOR OF CVD TIN-TI5SI3 COMPOSITE COATINGS

Titanium silicide Ti5Si3 has been introduced in TiN coatings by in sit u chemical vapor deposition (CVD) in order to improve their oxidation resistance. The behaviors of these TiNSi composites and of pure TiN in air have been compared in the range 800-1150 degrees C. Oxidation kin etics have been deduced from in situ thickness measurements of the oxi dation layers as a function of time, by means of high-temperature X-ra y diffractometry. Parabolic time dependence for the TiN oxidation with an activation energy of 187 kJ mol(-1) and a pre-exponential factor o f 1.4 10(-3) cm(2) s(-1) has been found in agreement with results prev iously reported. Concerning the TiNSi, two regimes of oxidation have b een evidenced. Above 950 degrees C, the oxidation layers grow paraboli cally with time according to a.diffusion-limited process. Below 950 de grees C, a fast oxidation occurs in a first stage. Then, the higher th e temperature, the faster a threshold is reached corresponding to a co mplete passivation. Taking into account SIMS and XPS analyses in addit ion with TEM and EDS information, some assumptions have been proposed to explain the TiNSi oxidation mechanism, (C) 1998 Elsevier Science S. A.