Fabrication of TiC–ZrC–Co composites with refined microstructure using ultrafine TiC–ZrC mixture powders

Ultrafine TiC–ZrC mixture powders with various Ti/Zr molar ratios (9:1, 8:2, 7:3, and 6:4) were synthesized by carbothermal reduction of high–energy milled TiO₂–ZrO₂–C. ZrC was found to be insoluble in the TiC lattice at 1400–1500 °C; as a result, carbothermal reduction of TiO₂–ZrO₂–C mixtures led to the formation of the TiC–ZrC mixture powders rather than the (Ti,Zr)C solid solution. TiC–ZrC–Co composites prepared using the TiC–ZrC mixture powders exhibited a refined microstructure, due to the inhibition of the coalescence of TiC grains by ZrC grains distributed in the TiC–ZrC mixture powders. In other words, the ZrC particles distributed in the mixture powders inhibited the coalescence of TiC grains during liquid–phase sintering, resulting in the formation of TiC–ZrC–Co composites with refined microstructure. The refined microstructure of these composites favorably influenced their properties: in particular, the experimental measurements highlighted a significant improvement in the mechanical properties (H_V = 14.8–16.0 GPa, K_IC = 7.2–7.6 MPa m^1/2) of the composites prepared from the ultrafine TiC–ZrC mixture powders, compared with those (H_V = 11.1–13.3 GPa, K_IC = 5.7–6.3 MPa m^1/2) of conventional TiC–ZrC–Co composites.