Oxidation studies of alpha-sialon ceramics

SiAlON ceramics are the solid solutions of Si3N4 where Si and N are partially substituted by Al and O respectively and dopants and/or sintering aids are added. They are excellent candidate for advanced high-temperature structural applications due to combined strength, toughness, thermal shock resistance, wear resistance, chemical inertness, and easy manufacturing. There has been significant advance in the sciences of synthesis and the improvement of mechanical properties of SiAlONs in recent years, bringing this family of engineering ceramics closer to market utilization. An exciting example is the development of a new generation of alpha-SiAlONs with whisker-like or elongated grain microstructure at the University of Pennsylvania. The combination of hardness, strength, fracture toughness and easy sinterability makes alpha-SiAlON a highly promising low-cost ceramic for many engineering applications such as aircraft bearings, advanced engines, and cutting tools.;Like other silica-forming ceramics; SiAlONs are thermodynamically unstable and will oxidize when exposed to an oxidizing environment. Resistance to oxidation is a critical design and performance criterion for ceramic components and structures, especially under chemically aggressive conditions such as in a combustion engine. Ironically, processing strategies, principally via introduction of sintering additives, that lead to precise control of microstructural evolution and enhancement of mechanical properties, often result in adverse effects on the high-temperature oxidation stability of ceramics. There is a dearth of information on the oxidation behavior of the new class of SiAlONs developed in general and the roles of the various additives in specific.;The main goal of this research is to establish a knowledge base in the oxidation front of alpha-SiAlON so as to facilitate further development of mechanically robust and chemically stable SiAlONs and accelerate the pace of their practical exploitation. The oxidation behavior of alpha-SiAlONs of a large variety of composition has been investigated under variety of temperature and time conditions. It has been found that alpha-SiAlON ceramics exhibit a strong, dopant-dependent oxidation behavior. The oxidation resistance increases in order of Li, Ca, Nd, Y, and Yb. The dopant-dependent oxidation behavior can be interpreted based on a unifying parameter, the field strength of the cations. Si3N4 additive markedly accelerates the oxidation of Y- and Yb-doped alpha-SiAlON ceramics whereas incorporation of AIN results in significantly improved oxidation resistance of Y- and Yb-doped alpha-SiAlONs, even when Si3N4 is co-added. (Abstract shortened by UMI.)...