Abstract

SiAlON ceramics are important materials for applications under high mechanical and thermal stress such as cutting tools or engine components. These solid solutions offer a wide variety of microstructures and properties depending on composition and sintering parameters. As observed in most ceramics, also for SiAlON, the effect of subcritical crack growth has to be expected. However, preliminary tests on this material have shown very little subcritical crack growth, whereby the determination of the crack growth exponent n causes difficulties. This work presents a new method to obtain statistically more significant results. SiAlON ceramics can be derived from both alpha- and beta-silicon nitride modifications by substituting silicon by aluminium and nitrogen by oxygen. The possibility to stabilize two crystal structures offers the opportunity to design materials ranging from pure alpha-, mixed alpha/beta-, up to pure beta-SiAlONs. In general, alpha-SiAlONs are in the form of equiaxed grains with high hardness and good wear resistance, but low fracture toughness, whereas beta-SiAlONs have elongated grains with high fracture toughness, but relatively low hardness. To combine the advantages of both SiAlONs, alpha/beta-SiAlON composites have been developed over the past years by different mechanisms including the choice of starting powders, sintering parameters and the amount of additives. Sintering additives are necessary because SiAlONs are densified by a liquid phase sintering process like silicon nitride materials.