MICROSTRUCTURE AND FRACTURE-BEHAVIOR OF SIC-PLATELET-REINFORCED SI3N4 MATRIX COMPOSITES

The microstructure and crack propagation behavior of 25vol.%SiC-platelet-reinforced Si3N4 composites, which were prepared by hot isostatic pressing (HIP) accompanied by slow (100-degrees-C h-1) and rapid (650-degrees-C h-1) cooling, were investigated. Most of the SiC platelets were polytype alpha structures (4H and 6H) and they were found to be homogeneously dispersed in the Si2N4 matrix. The crystal structure of Si3N4 in both slowly and rapidly cooled composites was beta type (h.c.p.) and most of the Si3N4 grains had an equiaxed shape. A few large and elongated Si3N4 grain were also observed in the neighbourhood of SiC platelets. Most of the Si3N4 grain boundaries in both the composites contained a thin SiO2 amorphous layer about 1-2 nm in thickness. In the slowly cooled composite, cracks formed by indentation mainly propagated along Si3N4 grain boundaries and Si3N4-SiC interfaces with deflection and microcracking, although transgranular fracture of SiC platelets was also observed along the (001) plane. In contrast, in the rapidly cooled composite, fracture occurred mainly intragranularly both in Si3N4 grains and in SiC platelets.