Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si3N4-TaC and Si3N4-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500-1600 °C using a resistance-heated hot-pressing technique. The addition of 20-40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0-40 mol% TaC or ZrC at 1500 °C, 0-80 mol% TaC at 1600 °C, and 0-60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si3N4 was larger than that of β-SiAlON; α-Si3N4 transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si3N4-to-β-SiAlON phase transformation. Young's modulus for the dense Si3N4-TaC and Si3N4-ZrC ceramics increased with the carbide amount, and the hardness of dense Si3N4-ZrC and Si3N4-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si3N4 content. Dense Si3N4-TaC and Si3N4-ZrC ceramics, with larger quantities of α-Si3N4 sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si3N4 proportion. Both the hardness and fracture toughness of the dense Si3N4-TaC and Si3N4-ZrC ceramics were strongly related to the proportion of α-Si3N4 in the sintered body.