Protective coatings for non-oxide ceramics

Abstract

Protective coatings for non-oxide ceramics have a great potential for reducing oxidation under hot oxidising conditions. In the present paper a technique to coat silicon nitride and silicon carbide ceramics with a glass layer is proposed. The composition of the coating is in the Al₂O₃–SiO₂–Y₂O₃ 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 Si₃ N₄ and SiC and promoted strong adhesion with the ceramic without the formation of any new weak compounds. The coatings are different for Si₃ N₄ and SiC based systems, especially across the original ceramic/glass interface. In 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 Si₃ N₄ 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 that they formed. In silicon carbide based 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.