Properties of reaction-bonded SiC/Si₃N₄ ceramics

The particle size grade combination of silicon powder of 40 μm, fine Si₃N₄ powder of 0.5 μm, Al₂O₃ and MgO powder of 5 μm was studied and the highest density of both green compact and the final ceramic could be obtained when the silicon, Si₃N₄, Al₂O₃ and MgO powder were mixed in a weight ratio of 75:25:1:1. The SiC powder of 45 nm was introduced into the compacts according to different ratio and nitrided at 1400 °C for 20 h. No obvious influence of the addition of SiC on the relative density of the SiC/Si₃N₄ ceramics was observed. SEM photographs of SiC/Si₃N₄ ceramics showed that needle-like β-Si₃N₄ crystals formed in all the SiC/Si₃N₄ ceramics. More and longer needle-like β-Si₃N₄ crystals were seen in the ceramics with the higher SiC content, and no needle-like β-Si₃N₄ crystals were found in the pure Si₃N₄ ceramic prepared by reaction bonding of silicon. A flexural strength rise with the increase of the SiC content in the SiC/Si₃N₄ was found. The phenomenon was attributed to the existence of more needle-like β-Si₃N₄ crystals with the increase of the SiC content, which could improve the flexural strength of the ceramic. As a source of SiO₂, addition of nanometer SiC is helpful for the formation of needle-like β-Si₃N₄.