Effect of NiTi content on the mechanical properties and microstructure of TiB2-TiN-based composite ceramic material prepared using spark plasma sintering

In this study, TiB2-TiN-based composite ceramic materials modified with NiTi were prepared by a two-step spark plasma sintering process (900 degrees C/3 min + 1650 degrees C/5 min). The effects of adding different volume fractions of NiTi (0-12 vol%) on the mechanical properties and microstructure of the ceramic material were investigated. The results showed that the optimal mechanical properties of the composite ceramic were obtained when the NiTi content was 9 vol%. Its flexural strength, fracture toughness and hardness were 942.5 MPa, 5.96 MPa m1/2 and 19.8 GPa, respectively. The addition of NiTi promoted densification during the sintering process, which improved the sinterability and refined the grain size of the materials. The toughening mechanisms were attributed to branching, deflection and bridging of cracks. Furthermore, a new toughening mechanism, phase change toughening was observed due to the addition of NiTi. Its specific manifestation in TiB2-TiN-NiTi cermet materials was the limitation of crack extension, which caused NiTi to transform from austenite to martensite when the crack extended into the region containing NiTi, and this consumed most of the fracture energy. The fracture toughness of the TiB2-TiN-NiTi composite ceramic materials was improved by the ability of NiTi to hinder crack growth.