Preparation And Mechanical Properties Of Enhanced Toughened Mo₂NiB₂ Foundation Cermet

Ternary boride based cermets has excellent high temperature resistance and corrosion resistance,and is widely used in aviation,automobile and tool industries.However,the fracture toughness and strength of ternary boride based cermets are poor,which limits its further application in the industrial field.Therefore,it is of great application value to develop ternary boride based cermets with high strength,high toughness and high wear resistance.In this thesis,the toughening and strengthening mechanism of ternary boride based cermets was studied.By optimizing sintering process,adding toughening phase silicon carbide whisker（SiCw）and reinforcing phase element V,ternary boride based cermets with high toughness,hardness and wear resistance was successfully prepared.Firstly,the preparation process of Mo₂Ni B₂ based cermets was optimized,including ball milling time,sintering temperature and holding time.Observation of the ball milled powder by scanning electron microscope found that the particle size of the powder after ball milling for 23 hours was the smallest and the distribution was uniform.By analyzing the microstructure and mechanical properties of the cermet,it was found that when the sintering temperature was 1250°C and the holding time was 40 minutes,the two phase structure of the cermet was uniformly distributed without large pores,and the mechanical properties were the best.On the basis of the optimized sintering process,the Mo₂Ni B₂ based cermets toughened by SiCw was successfully prepared by reactive boronizing sintering technology.According to the microscopic observation and analysis,the grain size of cermets with 0.5wt%SiCw can be refined,and the grain size of cermet with 0.5wt%SiCw reaches the minimum value of 3.57μm,which is 6.7%lower than that without SiCw.According to the physical and mechanical properties test,the Vickers hardness,bending strength and fracture toughness of cermet with 0.5wt%SiCw are 1113 HV,1620 MPa and 27.97 MPa·m^1/2,which are increased by 16.8%,22.7%and 25%respectively compared with cermets without SiCw.It is proved that SiCw can significantly improve the mechanical properties of Mo₂Ni B₂ based cermets,because the addition of SiCw inhibits the growth of hard phase grains,refines the grains,and does not cause phase changes.Scanning electron microscope（SEM）was used to observe the crack path and fracture morphology,and the toughening mechanism of SiCw mainly included whisker bridging,crack deflection,whisker pulling out and microcrack toughening.In addition,the addition of SiCw increases the transgranular fracture of cermets,which consumes more energy than intergranular fracture,and also increases the fracture toughness of cermet.Through friction and wear experiments,it was found that the friction coefficient and wear surface area of cermet decreased after adding SiCw,which indicated that adding SiCw could improve the wear resistance of cermet.On the basis of adding 0.5wt%SiCw,the Mo₂Ni B₂ based cermets with V was prepared,and its microstructure,physical and mechanical properties and wear resistance were tested and analyzed.It was found that the Mo₂Ni B₂ based cermets with 2wt%V was evenly distributed,and the hard phase grains were fine,but the density decreased slightly.The mechanical properties of Mo₂Ni B₂ based cermets with 2wt%V are the best,with Vickers hardness of 1233 HV,bending strength of 1680 MPa,which are increased by 10.8%and 3.7%respectively compared with those without V.According to the experiment,the Vickers hardness of Mo₂Ni B₂ based cermets with 2wt%V is obviously improved,and the addition of V mainly strengthens cermets by three ways:changing the crystal structure of hard phase,solution strengthening and grain refining.Through friction and wear experiments,it was found that the cermet with 2wt%V had the best wear resistance,and the wear rate and friction coefficient reached the minimum values of 0.51 and 1.08×10^-5 mm³/N·m,respectively,which decreased by 5.5%and 59%compared with those without V,indicating that V can effectively improve the wear resistance of cermets.This study has solved the problems of low strength,poor toughness and poor wear resistance of Mo₂Ni B₂ based cermets,and revealed the toughening mechanism of SiCw and the strengthening mechanism of V,which provided theoretical basis and technical guidance for preparing high performance Mo₂Ni B₂ based cermets,and had important research value for improving wear resistance and expanding its application range.