TABLE OF CONTENTS Sialons (Oyama and Kamigaito, 1971; Jack and Wilson, 1972; Hampshire et al., 1978) are a family of advanced structural ceramics that exhibit a good combination of properties such as high strength at elevated temperatures, dimensional stability, good corrosion and erosion behaviour, high elastic modulus, low mass density, good thermal shock resistance and high hardness. These combined properties make them useful as structural materials. However, as with other structural ceramics, they suffer from the disadvantage of low fracture toughness (3.0 8.0 MPa m ) with no R-curve behaviour (Cao and Metselaar, 1991; Shen et al., 1996a, b, c). Although they can be used for engineering components, their reliability is still not adequate in many cases for industry to commit to production. The most attractive and effective way of increasing toughness of sialons is through the use of a second-phase reinforcement, thus generating a sialon matrix composite. In composite form, significant improvements can be achieved such as increased fracture toughness, less strength variability, less flaw sensitivity, reduced crack propagation and better reliability; and even more significantly, the failure manner of sialon composites can be changed and controlled. Recently, several new techniques have been developed, such as ? ? ? sialon transformation (Mandal et al., 1993; Thompson, 1994; Yu et al., 1998b) and the retention of elongated ?-or ?-sialon grains in dense ?/ ?-sialon composites (Shen et al.
