Effect Of Nb And Si₃N₄ On Toughness Of Boride Coating On Steel Substrate And Its Mechanism

Boride cermets have many excellent properties such as high temperature resistance,high hardness,wear resistance,corrosion resistance and light relative density.However,the shortcoming of boride cermets is high hardness and poor toughness,which seriously affect the application range of ceramic materials.Therefore,it is of great significance to improve the fracture toughness of ceramics,strengthen and toughen materials,and improve their reliability and service life.In this paper,Mo-Fe-B-Cr flux-cored wire was designed and prepared,and three kinds of coatings were fused on the surface of GCr15 steel by plasma cladding technology.The microstructure and properties of Mo₂FeB₂ coating,Mo₂FeB₂+Nb coating and Mo₂FeB₂+Si₃N₄ whisker coating were studied,and the effects of different heat treatment temperatures on the microstructure and properties of Mo₂FeB₂ coating were also studied.（1）Different contents of Nb element were added to cermet,and the strengthening and toughening effect of Nb element on boride coating was analyzed.The results show that the addition of appropriate Nb element can inhibit the gro wth of hard phase particles in the coating,refine the grain,and make the microstructure distribution of the coating more uniform.When Nb content is 3 wt%,the mechanical properties of cermet coating reach the best.Compared with Mo₂FeB₂ coating,the fracture toughness,impact toughness and hardness of cermet coating are increased by 2.5,3 and 1.12 times,respectively.The strengthening and toughening mechanisms are mainly self-toughening and dispersion strengthening.（2）Different contents of Si₃N₄ whiskers were added to cermet,and the strengthening and toughening effect of Si₃N₄ whiskers on boride ceramic coating was analyzed.The results show that the addition of appropriate amount of Si₃N₄ whiskers can improve the fracture toughness and hardness of the coating,and make the microstructure of the coating more uniform.When Si₃N₄whisker content is 0.5 wt%,the mechanical properties of boride ceramic coating reach the best.Compared with Mo₂FeB₂ coating,the fracture toughness,impact toughness and microhardness of boride ceramic coating are 2.7 times,3.7 times and 1.43 times,respectively.The strengthening and toughening of Mo₂FeB₂ coating by Si₃N₄ whisker is the result of crack deflection and crack bridging.（3）The Mo₂FeB₂ coating was heat treated,and the effect of heat treatment temperature on the microstructure and properties of the coating was analyzed.The results show that after heat treatment,the defects of the coating,such as holes,disappear,promote the microstructure of the coating more uniform,and play a role of grain refinement.When the heat treatment temperature is 950℃,the mechanical properties of boride coating reach the best.Compared with Mo₂FeB₂ coating,the fracture toughness,impact toughness and microhardness of boride coating increase by 2.3 times,3.5 times and 1.15 times respectively.The strengthening and toughening mechanism of Mo₂FeB₂ coating by heat treatment is mainly fine grain strengthening.