Properties of hot-pressed of SiC/Si₃N₄ nanocomposites

SiC/Si₃N₄ nanocomposites were prepared with carbon black and α-Si₃N₄ powders by hot-pressed method. The microstructure, phase composition, dielectric and mechanical properties of the nanocomposites were investigated. The relative densities of SiC/Si₃N₄ nanocomposites decreased from 99 to 86% when the carbon black content increased from 0 to 10 wt%. The X-ray diffraction (XRD) analysis shows that there are SiC crystals formed during the sintering process in the samples with carbon black. The main crystalline phase in the pure Si₃N₄ ceramic is β-Si₃N₄. The scanning electrical microscope (SEM) photographs reveal that the Si₃N₄ grains in Si₃N₄ ceramic are big and elongated. But the transformation of α-Si₃N₄ to β-Si₃N₄ is impeded in the samples with carbon black. More α-Si₃N₄ were retained in the nanocomposites with the increase of the carbon black content. The Si₃N₄ grains in samples with carbon black are small and particulate. The reason is that the incorporation of carbon black increases the viscosity of sintering liquid and prevents the transformation of α-Si₃N₄ to β-Si₃N₄ and the growth of Si₃N₄ grain. The strength of the nanocomposites is degraded due to the addition of the carbon black. The real and imaginary parts of the permittivity and dielectric loss of the SiC/Si₃N₄ nanocomposites all increase with the increase of carbon black content.