Microstructural evolution of Si3N4 ceramics from starting powders with different α-to-β ratios

Microstructural evolution of Si3N4 ceramics and thermal/mechanical properties were investigated with respect to the initial alpha-to-beta ratios of the starting powders. The fast growth of elongated beta-grains was observed in the alpha powder, while the growth of relatively equiaxed grains occurred in the beta powder. In the cases where alpha/beta mixed raw powder was used, exaggerated bimodal microstructural evolution occurred as a result of the abnormal grain growth. These microstructural variations of the Si3N4 had a significant influence on the mechanical and thermal properties, such as strength, fracture toughness and thermal conductivity. The bimodal microstructure, which consisted of large elongated grains surrounded by fine matrix grains, resulted in both high flexural strength and high fracture toughness. Controlling the microstructural evolution, resulted in a high thermal conductivity of 86W/mK, a high flexural strength of 912 MPa, and a high fracture toughness of 7.88 MPa. m(1/2) from alpha/beta mixed raw powder. The achievement of both high thermal and mechanical properties of Si3N4 ceramics was highly essential for thermal management applications. (C) 2016 The Ceramic Society of Japan. All rights reserved.