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

A silica (SiO₂) layer was deposited on the surface of an AlN ceramic in order to increase the strength and to prevent the high-temperature oxidation of 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 H₂-0.1% H₂O atmosphere at 1450°C. A reaction between the SiC powder and H₂O in the H₂ gas resulted in the generation of SiO₂ 'smoke' in the product gas stream. Part of the SiO₂ smoke was subsequently deposited on the surface of the AlN specimen to form a dense and uniform SiO₂ layer. The strength of AlN was improved by about 20% apparently because of blunting of surface defects by SiO₂. More importantly, the layer was very effective in protecting the AlN from the oxidation at elevated temperatures, through the inhibition of transport of oxidants to the sample surface.