Hard and tough alpha-SiAlON ceramics

Abstract

Yb, Nd/Ca and Ce/Ca containing a-sialon starting compositions have been densified by either pressureless sintering or capsul free sinter/HIP'ing using "conventional" alpha-Si3N4 powder. X-ray diffraction studies of both compositions after sintering revealed that only alpha-sialon was observed as a matrix phase. Microstructural characterization of the relatively high nitrogen rich starting composition resulted in a typical equiaxed grain morphology, as expected. But the relatively less nitrogen rich starting composition exhibited surprisingly an elongated grain morphology with typical grain length of 5-20 mu m and aspect ratios of 4-15. The mechanical property measurements on the material with equiaxed grains showed a hardness value of 19.76 GPa and a toughness value of 3.96 MPam(1/2) as expected. The alpha-sialon material with elongated grains proved to have a hardness value of 19.30 GPa (HV10) but fracture toughness value of 6.31 MPam(1/2). Crack deflection, crack bridging and grain pull-out mechanisms were accounted for the significant improvement in fracture toughness.