Protective coatings for non-oxide ceramics

Protective coatings for non-oxide ceramics have a great potential for reducing oxidation under hot oxidising conditions. In the present paper a technique to cent silicon nitride ann silicon carbide ceramics of the a glass layer is proposed. The composition of the coating is in the Al(2)O(3)-SiO(2)-Y(2)O(3) system and is close to the composition of the grain boundary phase of the two dense materials. At the processing temperature that was slightly higher than the glass softening temperature, the glass coating wet both Si(3)N(4) and SiC and promoted strong adhesion with the ceramic without the formation of any new weak compounds. The coatings are different for Si(3)N(4) and SiC based systems, especially across the original ceramic/glass interface. hz the case of silicon nitride, the grains at the interface that were in direct contact with the glass dissolved until the glass became saturated with nitrogen. Then the Si(3)N(4) grains moved from their original position towards the free glass surface. The extent of dissolution and consequent grain removal was influenced by the sintering aids used to densify the silicon nitride, and in particular by the refractoriness of the secondary phase thar they formed. In silicon carbide bused systems, the attractive force between the grains is stronger than in silicon nitride. As a consequence, at the processing temperature the liquid glass coating wet the SiC well but did not affect its microstructure because there was no driving force for glass penetration even after long treatments. (C) 2000 IoM Communications Ltd.