Dependence of Mechanical Properties of SiC Whisker/Si₃N₄ Composite Ceramics on Crystal Type of Si₃N₄ Powders

Abstract

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(W_R and W_L) 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 W_R and W_L 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 K_IC of the W_R and W_L composite ceramics from α-type powder were higher than those from β-type powder. The ratios of both FS and K_IC of composite ceramics from α-type powder to those from β-type powder were lower than those of FS and K_IC 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 K_IC of composite ceramics by the unidirectional 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).