Strengthening and prevention of oxidation of aluminum nitride by formationof a silica layer on the surface

A silica (SiO2) layer was deposited on the surface of an AIN ceramic in ord er to increase the strength and to prevent the high-temperature oxidation o f the material. The layer was formed on the surface by exposing coupons to the atmosphere downstream of a bed of SiC powder in a flowing H-2-0.1% H2O atmosphere at 1450 degrees C. A reaction between the SiC powder and H2O in the H-2 gas resulted in the generation of SiO2 "smoke" in the product gas s tream. Part of the SiO2 smoke was subsequently deposited on the surface of the AIN specimen to form a dense and uniform SiO2 layer. The strength of AI N was improved by about 20% apparently because of blunting of surface defec ts by SiO2. More importantly, the layer was very effective in protecting th e AIN from the oxidation at elevated temperatures, through tbe inhibition o f transport of oxidants to the sample surface.