Ultra high-pressure spark plasma sintered ZrC-Mo and ZrC-TiC composites

Ultra-high-pressure spark plasma sintering was applied to ZrC-20 wt%Mo and ZrC-20 wt%TiC composites with a pressure up to 7.8 GPa and temperatures of 1550 °C and 1950 °C. Mechanical performance of the composites was benchmarked against a plain ZrC produced by the same method. Both composites outperformed the pure ZrC with superior hardness and indentation fracture toughness of 2239 HV1 and 5.4 MPa m1/2, and 1896 HV1 and 5.9 MPa m1/2, respectively, for ZrC-Mo and ZrC-TiC composites. It was shown that ultra-high compaction pressure affected the ZrC-20 wt%TiC miscibility gap by lowering the temperature threshold from the usually applied 1800 °C down to 1550 °C resulting in formation of the solid state solution of (Zr,Ti)C. In contrast, the high pressure does not inhibit the carburisation of Mo with ZrC to form MoC, even when experiments were performed in a graphite free environment. The equiaxed morphology of ZrC grains along with a right-shift in XRD peaks for ZrC indicates dissolution of Mo in ZrC resulting in formation of the solid solution of (Zr,Mo)C. High-temperature X-ray diffraction analysis under oxidation conditions was performed on the samples showing degradation of ZrC-20 wt%Mo due to the oxidation of Mo at high-temperature leading to MoO3 vaporisation. Conversely, the oxidation of ZrC-20 wt%TiC composites was characterised by formation of ZrO2 and TiO2 remaining stable up to 1500 °C.