Dependence of mechanical properties of SiC whisker/Si3N4 composite ceramics on crystal type of Si3N4 powders

The dependence of the mechanical properties of SiC whisker (SiC(w))/Si3N4 composite ceramics on the crystal type (α or β) of Si3N4 starting powders were investigated for two kinds of composite ceramics (WR and WL) in which the longer axes of SiC(w) were random and unidirectionally oriented to the specimen axis, respectively. The results obtained were as follows; (1) The shape of β-Si3N4 grains in WR and WL composite ceramics were nearly the same as those of monolith; acicular grains for α-type powder and almost equiaxed grains for β-type powder. (2) Both the flexural strength (FS) and KIC of the WR and WL composite ceramics from α-type powder were higher than those from β-type powder. The ratios of both FS and KIC of composite ceramics from α-type powder to those from β-type powder were lower than those of FS and KIC of the monolith, which was ascribed to higher load transfer effect of SiC(w) than crack deflection effect by Si3N4 grains. (3) The increases in FS and KIC of composite ceramics by the uni-directional orientation of SiC(w) were higher in the composite from α-type powder than those in the composite from β-type powder. This was attributed to the preferential orientation of acicular β-Si3N4 grains in the composite from α-type powder which was promoted by the uni-directional orientation of SiC(w).