Effect Of NbC In Mechanical Properties Of Ni-based Amorphous Composite Coating

Amorphous coating by laser cladding through adding a certain components of alloy on the surface of the conventional material have great hardness,corrosion resistance and wear resistance,et al.The nickel based amorphous composite coating was deposited on the middle carbon steel by using the high power fiber laser.One-step method is used to obtain the amorphous composite coating on the middle carbon steel by coaxial powder feeding,to improve the hardness and wear resistance of the middle carbon steel surface.The main research contents are included as following: the analysis of phase morphology and microstructure of nickel based amorphous composite coating,the contribution of NbC particles to the mechanical properties of the coating,the effect of heat input on the structure and mechanical properties of the coating and the thermal cycling and residual stress of laser cladding process.The results of the study are as follows:Amorphous composite coatings composed of amorphous phase,NbC phase and small dendrite phase were successfully prepared by one-step laser cladding.When the heat input is 56.3 J/mm,generate planar crystal,cellular crystal and columnar dendrites can be found near the interface,followed by columnar grain refinement and the fine equiaxed grain growth.The middle of the coating is mainly composed of amorphous phase and NbC particles and fine equiaxed crystal phase composition,NbC particles dispersed uniformly in the coating.With the increase of laser heat input,the width of the coating is stable and the height of the coating increases obviously,and the amorphous phase content of the coating decreases.The fine dendrite gradually replaced the amorphous and fine equiaxed grains.The mechanical properties were tested systematically to investigate the effect of NbC.The mean value of coating fabricated with 52.5 J/mm was about 1049.3 HV.NbC particles contributed to the microhardness values according to the SEM image of the indentation.The nanohardness and elastic modulus of NbC-free matrix under 5 mN were 1500.8 HV and 290 GPa,which were higher than the microhardness of coating.An indentation hardness of 952.6 HV was given as the microhardness by omitting the value of the NbC particles through the Nix–Gao model.The contribution to hardness of NbC is 96.7 HV,which agreed well with the calculated data 92.5 HV by Orowan-ashby and Tabor equations.A large load of Vickers hardness indenter was used in the cross-sectional coating to produced micro-cracks,and the direction of crack propagation was discussed.The results show that the inhibition and proliferation to the cracks by NbC particles.In addition,with the increase of heat input,the amorphous content of coating decreases,resulting in a decline in the value of the hardness.The friction and wear test results show that the coating with high hardness at low heat input,the NbC particles can prevent Al2O3 ball cutting on the surface of the coating and improve the wear resistance of the coating;Nb C particles and coating materials are easy to shed in the friction process in the low hardness coating at high heat input,NbC particles would have a collaborative work with Al2O3 ball to wear the coating surface,reduce the wear resistance of the coating.By using the ANSYS finite element software and the method of “birth and death of element” to simulate the process of the laser cladding nickel based amorphous composite what can be got the temperature field distribution and the stress field of cladding coating was simulated by indirect method.The simulation results were evaluated by X-ray stress measurement.Results indicate that simulation and test results are basically consistent with the values and trend in the transverse tensile stress on the path which was perpendicular to the scanning direction,and the longitudinal stress test without stress because of the cracks released the stress in the cladding coating.Thus,the simulation results can be referenced.There was residual tensile stress in the coating,and the longitudinal tensile stress was much larger than the transverse tensile stress.The longitudinal residual tensile stress is the main reason for cladding cracking.The relative stress concentration exists between the cladding layer and substrate,which increases the possibility of crack formation.