Influence of CaTiO3 impurity in SHS-TiN powder on the mechanical properties of ZrO2–TiN composites

Yttria-stabilised tetragonal polycrystalline ZrO₂-based composites with 40 vol.% TiN were hot pressed at 1450 °C for 1 h using a jet-milled thermally synthesized and a self-propagating high-temperature synthesis (SHS) TiN powder. The ZrO₂ phase of the SHS-TiN powder-based composites was found to be substantially coarser than for the jet-milled TiN powder-based ceramics and prone to spontaneous transformation to m-ZrO₂ and microcracking, due to the CaTiO₃ impurity in the SHS-TiN starting powder. In order to prove this, a set of experiments was performed to investigate the effect of the addition of CaO and TiO₂ on an yttria-stabilised tetragonal ZrO₂ polycrystalline (Y-TZP). The addition of 0.2 mol% of CaO to a Y-TZP ceramic was found to destabilise the t-ZrO₂ phase, whereas the addition of 1 mol% TiO₂ results in significant grain growth and the formation of less transformable t-ZrO₂. The CaTiO₃ impurity could be removed from the SHS-TiN powder by hot hydrochloric acid leaching, allowing to obtain a similar microstructure and mechanical properties as with conventional TiN powder.