Synthesis and Mechanical Properties of Silicon Nitride–Tungsten Carbide Composite Ceramics

Abstract

Si₃N₄–WC composite ceramics were sintered with Al₂O₃–Y₂O₃ or AlN–Y₂O₃ at 1500°C and 1600°C in a resistance-heated hot-pressing machine. These composite ceramics were densified, except for the Si₃N₄–60 mol% WC case containing Al₂O₃–Y₂O₃. The added WC did not decompose during sintering, and no WC-derived reaction products were formed. The α-Si₃N₄ did not transform to β-SiAlON in the ceramics sintered with Al₂O₃–Y₂O₃ at 1500°C or AlN–Y₂O₃ at 1600°C, which resulted in a higher α–phase ratio in the sintered bodies. The addition of WC to Si₃N₄ ceramics sintered with Al₂O₃–Y₂O₃ at 1600°C inhibited the phase transformation from α-Si₃N₄ to β-SiAlON. The Young’s modulus for the ceramics increased with increasing WC up to 512 GPa. The Vickers hardness of the ceramics sintered with Al₂O₃–Y₂O₃ increased by increasing WC from 14.4 to 19.3 GPa. When AlN–Y₂O₃ was used as the sintering aid, the hardness increased with increasing WC, reaching a maximum of 21 GPa at 70 mol%. The Si₃N₄–70-80 mol% WC composite ceramics sintered with AlN–Y₂O₃ exhibited a high fracture toughness of 8.9 MPa m^0.5. The addition of WC to Si₃N₄ ceramics improved both hardness and fracture toughness.