20.1 Introduction

Despite the excellent combination of thermal mechanical properties of Si ₃N ₄ and SiC ceramics, the wide application of these materials is still hampered by their relatively low fracture toughness (Lange, 1973; Jack, 1976; Swain, 1994; Kleebe et al., 1999). Improvements in fracture toughness of Si ₃N ₄ and SiC ceramics are mainly achieved by crack deflection and bridging mechanisms (Fairbanks et al., 1987; Bennison and Lawn, 1989; Kelly et al., 1991; Chen and Engineer, 1999). To enhance crack deflection toughening, high aspect ratio acicular grains and a weak interface between the grains and grain boundary phases are desirable; to enhance crack bridging toughening, high aspect ratio grains with large diameters are required. However, weak interfaces and subsequently the debonding of large elongated grains result in the development of fracture origins and loss of strength of the material. Therefore, to develop a superior material with both high toughness and strength, optimization of the microstructure is essential. Accordingly, toughness and strength have been areas of intensive research over the past two decades. The outcomes have been quite impressive: materials with much improved fracture toughness and strength, exceeding 12 MPa m and 1 GPa respectively, have been reported (Hirao et al., 1995; Ohji et al., 1995; Becher et al., 1998).

Although it has long been recognized that microstructure has a significant effect on the erosion performance of ceramic materials,...