Processing, phase evaluation and mechanical properties of MoSi2 doped 4TaC–HfC based UHTCs consolidated by spark plasma sintering

• Consolidation of nearly full dense, mono phase 4TaC–HfC based composites
• Liquid phase assisted sintering using MoSi2 during intermediate stage of sintering
• TaC and HfC diffusion and formation of binary solid solution at 2000 °C
• Solid solution formation enhanced densification at the final stage of sintering
• Activated crack deflection as the main toughening mechanism

In this research, binary 4TaC–HfC based composites were consolidated using carbide materials and addition of 0–15 vol.% MoSi2 by means of spark plasma sintering at 2000 °C. The nearly full dense and monophase specimens were fabricated with a relative density value higher than 99%. Mechanical tests revealed values of 18–19 GPa and 4–4.3 MPa·m1/2, for average Vickers hardness and fracture toughness of the composites, respectively. Analysis of linear shrinkage during densification revealed that MoSi2 addition increased densification rate and decreased the time required to reach full density at 2000 °C. It is proposed that at the intermediate stage of sintering, mass transfer can be accelerated by formation of a silicide based liquid phase and viscous flow mechanisms. The formation of binary 4TaC–HfC solid solution phase enhanced the densification process at the final stage of sintering.