Development of tough α-SiAlON

The development of tough α-SiAlON with elongated grains in the last five years is summarized. This progress has been guided by the improved understanding of phase relations and nucleation/growth kinetics in SiAlON ceramics. Although most α-SiAlON compositions can be processed to contain some elongated grains, their microstructure, fracture toughness and R-curve behavior vary greatly. Such variability is due to the different phase stability of α-SiAlONs and the varying physical chemistry of the competing phases, including the transient/residual liquid. For this reason, microstructure control of α-SiAlON must pay close attention to the composition, starting powder and heating schedule. Seeding with single crystals of an appropriate α-SiAlON composition provides an attractive alternative that simplifies the task of microstructure control, since such seeds are thermodynamically stable and they completely dominate the nucleation statistics. Tough and hard α-SiAlON ceramics containing Ca, Y, Nd, and Yb stabilizers have been obtained using this method, some with toughness exceeding 10 MPam1/2. The ability of maintaining a uniform microstructure of highly elongated grains is the key to high toughness material. (orig.)