Effect of the oxidization of Si3N4 powder on the microstructural and mechanical properties of hot isostatic pressed silicon nitride

The effect of nanosized oxidized silicon nitride powder particles on the microstructural and mechanical properties of hot isostatic pressed silicon nitride was studied. The starting < alpha > Si3N4 powders were oxidized for 10 and 20 hat 1000 degrees C in ambient air environment. An amorphous oxide layer was observed on the surface of Si3N4 powder particles using high resolution transmission electron microscopy (HRTEM) whose thickness increased with the oxidation time. The powders were hot isostatic pressed at 1500 degrees C and 1700 degrees C in nitrogen gas environment under 20 MPa for 3 h. Complete < alpha > to < beta > transformation was observed in Si3N4 samples sintered at 1700 C under 20 MPa for 3 h compared to Si3N4 samples sintered at 1500 degrees C. Silicon oxynitride (Si2N2O) was found in sintered samples whose amount increased with the oxidation time. The < beta > phase decreased with the increase of Si2N2O phase in the sintered samples. The flexural strength of samples sintered at 1700 degrees C is higher due to the higher amount of < beta >-Si(3)N(4 )phase than for samples sintered at 1500 degrees C. Decreasing flexural strength was observed with the oxidation time. Higher amount of oxygen in the base powders resulted in a higher Si2N2O content in the sintered Si3N4.