Study On The Preparation And Properties Of TiN-Ti/Al₂O₃ Composites

Ti/Al₂O₃ composites are considered to be ideal cermet materials,because of their advantages of both metal and ceramic phases.However,the intense interfacial reaction in the sintering process of Ti/Al₂O₃ composite produces intermetallic compounds such as Ti₃Al with brittleness at room temperature,which deteriorating the mechanical properties of composites.The raw materials were subjected to liquid nitrogen ball milling in the early stage of this study.The Ti powders were coated in order to suppress the excessive reaction between Ti and Al₂O₃,and the TiN-Ti/Al₂O₃ composites with good properties were obtained.Got this inspiration,TiN phase was introduced directly to expand the Ti/Al₂O₃ system in preparing TiN-Ti/Al₂O₃composites.Meanwhile,the effects of the phase ratio and holding time on the phase,micro-structure,electrical resistivity and mechanical properties of the composites were studied.Ti:N passivation layer was formed on the surface of Ti powder after liquid nitrogen ball milling of raw materials to inhibit the reaction between Ti and Al₂O₃,and TiN as well as TiN-Ti/Al₂O₃ composites were prepared during sintering.In addition,the raw material powders were finely grained under the alternating action of crushing and cold welding.It greatly reduced the sintering temperature of the composites and made the phase distribution symmetrical.And the performances of the sintered samples were greatly improved.In order to explore the effects of TiN on the properties of the composites,TiN was directly added to prepare TiN-Ti/Al₂O₃ composites.The density and fracture toughness of the composites were not substantially affected by the addition of TiN.But the strength and hardness improved with the addition of TiN,and the resistivity decreased continuously.The prolongation of holding time increased the grain size and reduced the strength and hardness of the sample,which was not conducive to the optimization of the material properties.When sintering temperature was1500°C,holding time was 30 min,pressure was 30 MPa,and material volume ratio TiN:Ti:Al₂O₃ was 20:20:60,the TiN-Ti/Al₂O₃ composite showed the best comprehensive properties.The sample at this time had an optimal relative density of 99.49%,Vickers hardness of 14.94 GPa,flexural strength of 321.55 MPa,fracture toughness of 5.28 MPa·m^1/2,and electrical resistivity of 1474.7??·cm.The research showed that the introduction of TiN effectively hindered the diffusion of O element at the interface.In addition it reduced the formation of TiO and Ti₃O and made some AlTi₃ brittle phase combined with N element to form AlTi₃N phase.To a certain extent,the brittleness of composites was alleviated.Meanwhile,TiN had a"pinning"effect on the matrix particles.It inhibited excessive grain growth and played a role of fine-grain strengthening.At the same time,the increase of grain boundaries further inhibited the dislocation and improved the microhardness of the sample.Due to the thermal expansion mismatch between TiN and the matrix,the presence of TiN caused the deflection of the crack and optimized the sample's fracture toughness.In addition,the electrical conductivity of TiN was superior to that of metal Ti.So TiN formed a conductive network in the matrix quickly,which greatly reduced the resistivity of the composite.Compared with Ti/Al₂O₃ composites,the addition of TiN to TiN-Ti/Al₂O₃ composites required the higher temperature of the sintering because TiN had a higher melting point.When the heat preservation time was 30 min,the grain growth achieved the best effect.The prolongation of holding time made grains grow,which weakened the strength of samples and reduced the hardness of samples at the same time.Therefore,the prolongation of holding time was not conducive to improve the performances of composites.