Study On The Development Of TiB₂-Based Ceramic Tool Toughened And Strengthened By NiTi Alloy And Its Cutting Performance

Titanium boride based composite ceramic material is one of the commonly used materials for manufacturing ceramic tools due to its excellent properties such as high hardness,high oxidation resistance,low density,excellent chemical stability and wear resistance.However,the application of titanium boride ceramic materials is limited by their poor sintering performance,low strength and poor toughness.To solve this problem,this thesis adopts NiTi alloy as the metal binder to toughen and reinforce the TiB₂ceramic material by using its superelasticity,and uses a combination of discharge plasma sintering and two-step sintering process to improve its sintering properties and microstructure.In this thesis,TiB₂-Ti N-NiTi composite ceramic tool material is prepared,its optimal sintering process parameters and the optimal content of NiTi alloy are determined,and the cutting performance of the tool and its wear mechanism are investigated by cutting experiments.TiB₂-Ti N-6NiTi composite ceramic tool material is prepared by spark plasma sintering technology,and the optimal two-step sintering process is obtained through the study as follows:the sintering process is carried out under vacuum sintering environment with a temperature rise rate of 100℃/min to 900℃and thermal pretreatment for 3 min,and then the sintering process is continued with a temperature rise rate of 100℃/min to1650℃for 5 min.The sintering pressure is kept constant at 30 MPa during the sintering stage.It is found that the two-step sintering process can significantly improve the microstructure of the composite ceramic tool material compared with the TiB₂-Ti N-6NiTi composite ceramic tool material prepared by the conventional sintering process,and its flexural strength,fracture toughness and hardness reach 931.3 MPa,5.8 MPa·m^1/2and 19.15 GPa,respectively.On this basis,it is found that the optimum content of NiTi alloy is 9 vol.%,and its comprehensive mechanical properties reach the maximum value of 942.5 MPa,5.96MPa·m^1/2 and 19.8 GPa,respectively.While when the NiTi alloy is added at 12 vol.%,though its flexural strength and hardness decrease to 800.5 MPa and 18.5 GPa,respectively,but its fracture toughness appears to be significantly increased to 7.44MPa·m^1/2.Through analysis,it is found that the toughening mechanism of TiB₂-Ti N-NiTi composite ceramic tool material is mainly crack deflection,crack bridging,crack branching and transformation toughening of NiTi alloy.In addition,the presence of NiTi alloy can also hinder crack propagation and induce crack bridging,thereby playing a toughening role.Therefore,the more NiTi alloy content,the more obvious the toughening effect.The cutting performance of TiB₂-Ti N-NiTi composite ceramic tools is investigated by selecting hardened 40Cr as the cutting workpiece.The optimal cutting process is 300m/min cutting speed,0.2 mm back draft and 0.102 mm/r feed.The cutting performance of TiB₂-Ti N-NiTi composite ceramic tools is improved continuously with the increase of NiTi alloy content,and the composite ceramic tools have the best cutting performance when the NiTi alloy content is 12 vol.%.Due to the addition of NiTi alloy,the strength and toughness of TiB₂-Ti N-NiTi composite ceramic tools are significantly improved,and the ceramic tools are given the ability to resist cutting forces.As a result,the main cutting force is significantly reduced by 18.3%compared with TiB₂-Ti N composite ceramic tools,and the cutting temperature during cutting is only 304℃,which is 53%lower compared with TiB₂-Ti N composite ceramic tools.After analysis,it is found that the higher the content of intermetallic compound NiTi,the stronger the impact resistance obtained through superelasticity,and the effective cutting distance up to 4500 m.The main wear mechanisms of TiB₂-Ti N-NiTi composite ceramic tools are bonded wear and slight chipping of cutting edge on the rake face,and bonded wear and abrasive wear on the flank face.