Si-Al-O-N Ceramics, Structure and Properties

SiAlON ceramics are receiving a great deal of attention for both room and elevated temperature applications thanks to their unique properties: high values of strength, hardness, fracture toughness, corrosion and oxidation resistance and thermal shock resistance. Cutting tool inserts and milling tools, bearings, refractories, thermocouple tubes, grinding balls are some examples of commercial SiAlON products. Widespread utilization of SiAlON ceramics in critical applications strongly depends on the manufacturing of parts with superior properties by the application of advanced shaping and sintering techniques. Since two major phases of SiAlON ceramics, alpha and beta, are formed by the substitution of silicon and nitrogen in the crystal structure of silicon nitride by aluminum, oxygen and rare-earth elements (they can also be alkaline earth and alkaline elements) in the ionic form, easier densification provided by these additives without sacrificing high temperature properties can be simultaneously obtained. The properties of SiAlON ceramics are controlled by structural parameters such as alpha/beta-SiAlON phase ratio, solubility of Al and O ions in the β-Si 3 N 4 structure ( z value in the chemical formula of β-Si 6−z Al z O z N 8−z ), type of rare-earth element used and crystallization of grain boundary phase(s). In this article, the aim is to explain and correlate the mechanical, thermal, and optical/electrical behavior of SiAlON ceramics with these corresponding structural parameters. Moreover, a brief section on applications is included at the end of the article to make clear structure-property-performance relationships in SiAlON ceramics.