Optimization and improvement of sialon ceramics with new heat treatment techniques

Abstract

Sialons are of interest for use as engineering materials for high temperature (>1000°C) applications becuase they retain good mechanical properties at these temperatures, whereas competing metallic materials are weak and prone to corrosion. A characteristic disadvantage of all nitrogen ceramics is that an oxide additive is always included in the starting mix to promote densification, and this remains in the final product as a glassy phase distributed throughout the grain boundaries of the final microstructure. Since this glass softens at ~1000°C, the high temperature properties of the final ceramic are in fact determined by the properties of the grain-boundary glass. The most common method of obtaining better high-temperature performance is to heat-treat the material at temperatures of 1100-1350°C in order to devitrify the glass into a mixture of crystalline phases. More specifically it is desirable to convert the glass into a sialon phase plus only one other crystalline phase, the latter having a high melting point and also displaying a high eutectic temperature (max ?1400°C) with the matrix sialon phase. Previous studies have shown that there are a limited number of possible metal-silicon-aluminium-oxygen-nitrogen compounds which satisfy these requirements. Three new heat treatment techniques have been developed to improve and optimize sialon ceramics; these include: (1) post-preparative vacuum heat treatment to remove the grain boundary glass in gaseous form, (2) above-eutectic heat-treatment to crystallize liquid phase into new five component sialon phases, (3) heat treatment of rare earth oxide densified sialon systems to tailor the microstructure and mechanical properties