Microstructure and Mechanical Properties of Si_3N_4/hBN Composites Prepared by Hot-pressing Using Nitrided Si_3N_4 Powders

摘要

In order to produce Si_3N_4/hBN composite with low cost, it seems necessary to use nitrided Si_3N_4 powders since the cost of Si powder is much cheaper than that of Si_3N_4 powder. The purpose of the present work is to investigate the nitride conditions, and in particular, we focused on the relationship between microstructures and mechanical properties of hot-pressed Si_3N_4/hBN ceramic composite using nitrided Si_3N_4 powders. The mixed powders of Si_3N_4 and hBN were prepared by nitriding Si powders at 1380℃ for 24 h, and subsequently sintered by hot-pressing at 1800℃ for 2 h in N_2 atmosphere. The microstructure and mechanical properties of the Si_3N_4/hBN composites were investigated. Flexural strength, Young's modulus, and hardness decreased by the addition of 20 vol% hBN. The addition of BN resulted in a decrease in the modulus as well as an increase in the size of fracture source, both contribute to the observed decrease in mechanical properties. The Si_3N_4/BN based ceramic composites revealed enhanced crack deflection. The Vickers indentation crack paths in specimens are sinusoidal due to pull-out of grains during crack propagation.