Research On Microstructure And Nano Indentation And Scratch Tests Of Nbc Reinforced Iron Matrix Surface Composite

A NbC reinforced iron matrix surface composite was produced by in-situ synthesized process through casting and heat treatment technology.The phase composition and microstructure of composite were analyzed.Using the thermodynamics and diffusion theory,the formation process and mechanism of the composite is discussed.The hardness and elastic modulus of composites were measured through nano-indentation test,and the elastic-plastic deformation behavior was also analyzed in the process of indentation.The fracture toughness of NbC dense ceramic layer was measured through micro-indentation method,and the crack growth behavior and toughening mechanism was also studied.The abrasive wear behavior and deformation mechanism of NbC dense ceramic layer which are wearing of different shape of abrasive particles were studied by micro-scratch and nano-scratch.The result shows that:(1)The NbC reinforced iron matrix surface composite was produced by casting at 1400 ? and heat treatment at 1178 ? with niobium board and gray cast iron as raw materials.The volume fraction of NbC ceramic decreases along the surface to matrix,and the grain size increases gradually.According to the characteristics of microstructure,the composite can be divided into NbC dense ceramic layer,gradient distribution layer and particles dispersion layer.The surface of NbC dense ceramic layer are distributed many concentric ring which are closely arranged.With the matching,the cross section of NbC dense ceramic layer present a wavy layered microstructure.The NbC is generated by diffusing of carbon atoms to niobium lattice,belonging to the interstitial diffusion mechanism.The grain of NbC growth in the process of diffusion and migration gradually with Oswald mechanism and grain orientation connection growth mechanism.The relationship between carburized layer thickness(d)and reaction time(t)is as follows:d2 =kt +d02,where d0 is the initial thickness and the k = 1.06 ×10-7 cm2/s.(2)The hardness and modulus of elasticity of NbC dense ceramic layer reach 20.8 GPa and 505.3 GPa,respectively.And they are decline gradually from the NbC dense ceramic layer to matrix measured on the cross section of composite.The fracture toughness is far higher than the general ceramic materials,which the Kc reaches 8.0?14.6 MPa·m1/2 and 4.5?9.7 MPa·m1/2 measured on the surface and cross section of NbC dense ceramic layer,respectively.The strengthening and toughening mechanism of NbC dense ceramic layer are fine-grain strengthening and microcrack toughening.(3)The scratch test results show that,as the load increasing,the NbC dense ceramic layer surface experienced plastic deformation,micro crack initiation,crack propagation and particles flaking when it is in wearing of the large spherical particles.The wear mechanism of the cross section is extrusion-tear abrasive.With the decrease of volume fraction of the NbC particles,the wear form transform from particles peeling off into plastic flow and the ploughing abrasive.The wear mechanism are slight abrading at low load and chisel cutting and particles peeling at high load when NbC dense ceramic layer is in wearing of the sharp-pointed particles.