New heat treatment methods for glass removal from silicon nitride and sialon ceramics

Sialon ceramics were discovered simultaneously (but independently) in late 1971 at Newcastle University and also at the Toyota Research Laboratories i n Japan. During the 30 years since their original discovery, the Newcastle laboratory has made a significant contribution to current understanding of the science and technology of these materials. Sialons are of interest as e ngineering materials for high temperature (> 1000 degreesC) applications be cause they can be pressureless-sintered to high density and be designed to retain good mechanical properties even up to approximate to 1350 degreesC, whereas competing metallic materials are weaker and prone to corrosion. A c haracteristic disadvantage of all nitrogen ceramics is that an oxide additi ve is always included in the starting mix to promote densification, and thi s remains in the final product as a glassy phase distributed throughout the grain boundaries of the final microstructure. Since the glass melts at app roximate to 1000 degreesC, the high temperature properties of the final cer amic are in fact determined by the properties of the grain-boundary glass. The most common method of improving high-temperature performance is to heat -treat the material at temperatures of 1100-1350 degreesC in order to devit rify the glass into a mixture of crystalline phases. More specifically it i s desirable to convert the glass into a sialon phase plus only one other cr ystalline phase, the latter having a high melting point and also displaying a high eutectic temperature (max approximate to 1400 degreesC) in contact with the matrix sialon phase. Previous studies have shown that there are a limited number of possible metal-silicon-aluminium-oxygen-nitrogen compound s which satisfy these requirements. The present paper gives an overall revi ew of this subject area and then summarises recent work at Newcastle aimed at total removal of residual grain boundary glass. This has been achieved b y: (1) a post-preparative vacuum heat treatment process to remove the grain boundary glass from silicon nitride based ceramics in gaseous form, (2) ab ove-eutectic heat-treatment (AET) of sialon-based ceramics to crystallize g rain-boundary liquid into five-component crystalline sialon phases. (C) 200 0 Kluwer Academic Publishers.