Effects Of W And Nb Doping On The Microstructure And Properties Of Mo₂NiB₂ Based Cermets

Mo₂NiB₂ based cermets have excellent comprehensive mechanical properties and service properties,such as high hardness,high strength,excellent wear resistance and corrosion resistance,which are mainly used in industrial molds,cutting tools and other fields.However,the hardness and other mechanical properties of Mo₂NiB₂ based cermets still have a certain gap compared with traditional cemented carbides.Therefore,it is necessary to further improve its sintering process and improve its comprehensive mechanical properties.In addition,Mo₂NiB₂ based cermets can be used as coating materials in the field of ship,so the corrosion resistance of Mo₂NiB₂ based cermets to Cl^-was systematically studied in this paper.In this paper,Mo₂NiB₂ based cermets doped with different contents of W and Nb were successfully prepared by spark plasma hot pressing sintering with pure Mo,Ni and B powders as raw materials.The phase composition and microstructure of the cermet were observed and analyzed,and the mechanical properties such as flexural strength,fracture toughness,hardness and compressive strength,and corrosion resistance were measured.The experimental results show that the crystal structure of Mo₂NiB₂ based cermets does not change after doping W and Nb,and its crystal structure still belongs to orthorhombic system.After doping W and Nb,W and Nb enter into Mo₂NiB₂ hard phase,and(Mo,W)₂NiB₂solid solution phase and(Mo,Nb)₂NiB₂ solid solution phase are formed.After doping with W,the flexural strength value and fracture toughness value of Mo₂NiB₂ based cermets show a typical negative correlation.The flexural strength value increases first and then decreases.When the material is doped with 0.1 mol W,the maximum flexural strength is 823.36 MPa.When the doping amount of W is 0.15 mol,the hardness and compressive strength of the material reach the maximum values of 1026.03 HV₁ and 1928.24MPa,respectively.The reason is that W and part of Mo undergo a replacement solid solution reaction,which in turn has a solid solution strengthening effect on the matrix.In addition,doping with W can significantly improve the corrosion resistance of the material,due to the formation of a protective passive film on the corrosion surface.Among them,the internal dense layer of passive film plays an important role in the corrosion resistance of materials,which can effectively prevent the electrochemical corrosion process of materials and improve its corrosion resistance.It is found that the material has the most excellent corrosion resistance when doped with 0.15 mol W.After doping with Nb,the fracture toughness of the material can not be significantly improved,and the fracture morphology of the material shows a typical brittle fracture morphology.When the doping amount of Nb is 0.05 mol,the flexural strength of the material reach the maximum value of 854.93 MPa.The hardness value and compressive strength value of the material show the same change trend,that is,it increases first and then decreases.When the doping amount of Nb is 0.15 mol,the hardness value and compressive strength value of the material reach the maximum value at the same time,which are 1203.4 HV₁ and 2769.43MPa,respectively.With the doping of Nb,the corrosion resistance of the material is significantly improved,because a layer of passive film is formed on the corrosion surface,which prevents the corrosion process of the internal material,thus protecting the material and improving the corrosion resistance of the material.The experimental results show that the corrosion resistance of Mo₂NiB₂ based cermets is the best when doped with 0.15 mol Nb.