Abstract:
Silicon nitride specimens using yttrium oxide, aluminium oxide and magnesium oxide as additives were fabricated via pressureless sintering, and then mechanically tested. The ceramic powders were mixed at the fixed composition of 85wt% Si3N4 and 15wt% additives, but ratio of yttrium oxide:aluminium oxide:magnesium oxide additives were varied with 12 compositions. All mixed powders were die pressed, CIPed, and then sintered at 1800 ํC for 3 hours under 0.1 MPa nitrogen pressure. The bulk density of sintered specimens were ranging from 3.142-3.275 g/cubic cm (95-99% theoretical density) and the additive composition of 1wt%MgO + 2.8wt%aluminium oxide + 11.2wt%yttrium oxide obtained the highest density. The hardness of sintered specimens were ranging from 1105-1576 kg.m-2 and the additive composition of 3wt%MgO + 12wt%Yttrium oxide obtained the highest hardness. The young's modulus of sintered specimens were ranging from 261.3-290.6 GPa and the additive composition of 3wt%MgO + 12wt%Yttrium oxide obtained the highest young's modulus. The fracture toughness of sintered specimens were ranging from 4.66-7.41 MPa.m1/2 and the additive composition of 3wt%MgO + 12wt%Yttrium oxide obtained the highest fracture toughness. The flexural strength of sintered specimens were ranging from 459-816 MPa and the additive composition of 1wt%MgO + 14wt%Yttrium oxide obtained the highest flexural strength. The effects of microstructure to the fracture toughness and flexural strength were discussed