Study On Laser Cladding Ni-based Self-lubricating Composite Coating On Titanium Alloy Surface

Titanium alloys have many outstanding advantages,such as high melting point,low density,high specific strength,good corrosion resistance and biocompatibility.They are widely used in aerospace,biomedicine and other fields.However,due to its low hardness and poor wear resistance,it can not usually be used as an important part of friction kinematic pairs.Laser cladding technology is used to prepare a self-lubricating coating on the surface of titanium alloy,which has both mechanical properties of the matrix and antifriction properties of lubricants.It can effectively improve the surface properties of titanium alloy.Therefore,laser cladding of Ni-based self-lubricating anti-wear and friction-reducing composite coatings on Ti6Al4V alloy surface was carried out to obtain coatings with high hardness and excellent tribological properties.In this paper,Ni Cr BSi（particle size 45-105μm）,Ti N（particle size 1-3μm）,WS₂（powder size 0.5-1μm）powders were used as cladding materials to form laser cladding self-lubricating composite coating precursors by high energy ball milling.Ni-based self-lubricating composite coatings were prepared on Ti6Al4V alloy surface by IPG optical fiber laser processing system based on the good laser cladding formability of Ni Cr BSi powder and the good toughness of Ti Ni,the enhancement of high hardness ceramic phase Ti N and the good lubricity of WS₂ and the characteristics of renewable sulfide lubricating phase.The microstructure of the composite coating was studied by optical microscopy（OM）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,and the hardness and Tribological Properties of the coating were evaluated by microhardness tester and multi-functional friction and wear tester.The results show that the laser cladding Ni Cr BSi coating is mainly composed ofγ-Ni,Ti_xNi_y and Ti B₂ phases due to dilution.The phase composition of Ti N/Ni-based composite coatings is mainly composed ofγ-Ni,Ti_xNi_y,Ti N and Ti B.Different content of Ti N has a great influence on the coating defects.When Ti N content exceeds 30 wt.%,the wettability of mixed powder and matrix is reduced,and the brittleness of the coating increases with the increase of ceramic phase.As a result,defects such as pore and crack appear in the composite coating.The coating prepared with 30%Ti N-70%Ni Cr BSi has more uniform Ti N dendrites.After adding lubricating phase WS₂（10wt.%-30wt.%）into Ti N-reinforcedNi-based composite coatings,some WS₂ decomposes due to laser thermal action,and reacts with Ti element in the substrate to form a new phase.The self-lubricating composite coatings are mainly composed of Ti-Ni,Tix Niy,Ti N,Ti W and Ti S phases.Ti N/Ti S/Ni based composite coatings are formed after WS₂ is added.Compared with Ti N/Ni based composite coatings,white phase Ti W and needle phase Ti S appear in the coatings with WS₂ added.When 30wt.%WS₂ is added,more WS₂ is easier to evaporate and produce gas during laser cladding,which can not escape in the process of rapid solidification,resulting in porosity and defects in the coating.The microhardness of Ti N/Ni based composite coatings prepared from 30%TiN-70%NiCrBSi is 926.51HV_0.2,which is about 2.5 times higher than that of the matrix(370HV_0.2).The microhardness of Ti N/Ni based composite coatings is significantly improved due to the dispersion strengthening effect of hard phase Ti N and Ti_xNi_y in Ti N/Ni based composite coatings.The microhardness of Ti N/Ti S/Ni-based self-lubricating composite coatings prepared from 30wt.%Ti N-50wt.%Ni Cr BSi-20wt.%WS₂ is842.08HV_0.2,which is lower than that of Ti N/Ni-based composite coatings.The existence of lubricating phase Ti S in Ti N/Ti S/Ni-based self-lubricating composite coatings reduces the hardness of the coatings.The higher microhardness of Ti N/Ni composite coating can effectively prevent the grinding ball from penetrating into the coating,but also increase the friction between the composite coating and the grinding ball.Therefore,the wear rate of Ti N/Ni based composite coatings(9.1′10^-6mm³/N×m)is lower than that of matrix(22′10^-6mm³/N×m),and the friction coefficient（0.4304）is higher than that of matrix（0.4134）.The wear rate of Ti N/Ti S/Ni-based self-lubricating composite coatings(9.45′10^-6mm³/N×m)is similar to that of Ti N/Ni based composite coatings,but the friction coefficient（0.36）is much smaller than that of Ti N/Ni based composite coatings.As the lubricating phase Ti S in Ti N/Ti S/Ni-based self-lubricating composite coatings forms a lubricating transfer film on the wear surface during the friction process,it has a certain protective effect on the coatings and improves the wear resistance and friction reduction of the coatings.Ti N/Ti S/Ni Based Self-lubricating Composite Coating with 50%Ni Cr BSi-30%Ti N-20%WS₂ has the best wear resistance and friction reduction.