Microstructure and mechanical properties of pressureless sintered and HIPed silicon nitride ceramics

Monolithic silicon nitride ceramics with yttria and nanometric alumina as sintering aids were developed with the aim to improve mechanical strength and toughness. Yttria and alumina amounts were varied in the different compositions. The pressureless sintering was set at 1760°C for 1 h to reach closed porosity in all samples. Than, post-HIPing treatments with a N2 applied pressure of 180 MPa were performed at 1760°C or 1850°C. The room temperature hardness, toughness and flexural strength reached values up to 16 GPa, 9 MPa.m1/2 and 975 MPa, respectively. Different improvements in strength up to about 30% were revealed after post hipping tests in comparison to the as sintered materials. The correlation between starting compositions, thermal treatments, microstructures and mechanical properties were investigated and discussed