HIGH-TEMPERATURE ACTIVE OXIDATION AND ACTIVE-TO-PASSIVE TRANSITION OFCHEMICALLY VAPOR-DEPOSITED SILICON-NITRIDE IN N-2 O-2 AND AR O-2 ATMOSPHERES

The oxidation behavior of chemically vapor-deposited silicon nitride i n N-2-O-2 acid Ar-O-2 atmospheres was studied using a thermogravimetri c technique at temperatures 1823 to 1923 K. Active oxidation was obser ved at low oxygen partial pressures. The active oxidation rates increa sed with increasing oxygen partial pressure (P-O2) up to a certain P-O 2 and then passive oxidation occurred. The transition oxygen partial p ressures from active to passive oxidation were determined. The rate-co ntrolling step for the active oxidation could be oxygen diffusion thro ugh a gaseous boundary layer near the S3N4 surface. Decomposition of S I3N4 does not seem to be associated with the mass loss behavior. The W agner model was employed to explain the oxidation behavior.