Research On The Wear And Corrosion Properties And Its Principle Of NiTi Alloy Coating Prepared By Laser Cladding On Bearing Surface

Based on the bearing working conditions,it’s the outermost layer with a depth between 50 to 200μm of the bearing surface,which usually bears loads,wear and corrosion.Therefore,it is proposed in this paper to prepare Ni Ti alloy coatings on the stainless-steel bearing surface to improve its wear and corrosion resistance instead of manufacturing the whole bearing with the expensive Ni Ti alloy.However,the conventional equiatomic Ni Ti alloy（55Ni Ti,55 wt.%Ni）is insufficient for applications such as precision machining and under load conditions due to its super-elasticity.According to the Ni-Ti binary phase diagram,55Ni Ti can solve more Ni atoms with increasing temperature over 630℃.When the temperature is further increased up to1050℃,a solutionized single phase 60Ni Ti（60 wt.%Ni）is expected,which has even higher hardness,elastic modulus and yield strength than 55Ni Ti alloy.Therefore,it has been considered as the ideal bearing material.Meanwhile,as 60Ni Ti alloy powder is a kind of customized material,the production of which is time and labor consuming,a blend powder solution is used in this paper to reduce the cost.Two groups of mixed powder,namely,55Ni Ti alloy powder plus single Ni powder（55Ni Ti+5Ni）and single Ni powder plus single Ti powder（60Ni+40Ti）,are prepared with the Ni to Ti weight ratio of 60%:40%for laser cladding.The microstructure,phase composition and element distribution of the coatings were analyzed using a scanning electron microscope（SEM）with configured energy-dispersive spectrometer（EDS）,X-ray diffractometer（XRD）and X-ray photoelectron spectroscopy（XPS）.The influence of Ni content and composition of the powder blend on the microstructure and properties of the coating were discussed.The formation energy and lattice parameter of the crystal after substitution by Fe and Cr atoms,which diffused from substrate,was calculated using First-Principles.In addition,annealing treatment was conducted on the coatings as post-treatment to study the effect of annealing temperature and holding time on the improvement of the coating properties.Firstly,according to the results of contrast test,the optimal parameters for laser cladding of the Ni Ti alloy powders were summarized as followings:2.0 k W power,4.0mm/s laser scanning speed,50 r/min powder feeding rate and 45%overlap ratio.With the addition of Ni,defects in 55Ni Ti coating such as pores and cracks were significantly reduced,the coating became more denser with a reduction in average grain size of 20%.The coatings were mainly consisted of Ni Ti and Ni₃Ti and with minor phases such as Ni₄Ti₃,Fe₂Ti and Ti O₂.The content of Ni₃Ti in 55Ni Ti+5Ni coating reached 34%,which was 10 percentages higher than that of 55Ni Ti coating,because the Ni-rich environment caused by the addition of Ni was beneficial for the precipitation of Ni₃Ti phase.Meanwhile,the wear and corrosion resistance of the coating has also been improved due to the addition of Ni.The surface hardness of 55Ni Ti+5Ni coating reached 831.1 HV,which was 7.2%higher than 55Ni Ti coating,and the wear mass loss reduced 10%with a 5%reduction in friction coefficient compared with the 55Ni Ti coating.The improvement of wear resistance was attributed to the higher content of second phases and smaller grain size in the coating.The smaller grain size was also favorable for the formation of the passive film mainly consisting of Ti O₂,which could improve the corrosion resistance of the coatings.The results showed that the initiation of the corrosion in the Ni Ti coatings was selective,which occurred preferentially at the interfaces of dendrites and inter-dendritic areas,then spread first to dendrites rather than the inter-dendritic areas,because the content of Ti was higher in the inter-dendritic area.Beyond that,the contrast test between 55Ni Ti+5Ni coating and 60Ni+40Ti coating was designed to study the influence of different powder blend on the microstructure and properties of the coating.The XRD results described similar phase composition in both coatings,but the content of Ni₃Ti phase in 60Ni+40Ti coating was higher,which was even over 50%.However,the surface hardness of 60Ni+40Ti coating was 14%lower compared with 55Ni Ti+5Ni coating.In the meantime,plenty of second phases in the coating would form many“primary batteries”,which has disadvantageous influence on the coating anti-corrosion performance.Due to the increasing content of Ni and the diffusion of Fe element from substrate in the coating,the phase transformation temperature of the Ni Ti coatings decreased,which suppressed the martensite transformation in the coating as well.Furthermore,during the preparation of Ni Ti alloy coatings on stainless steel,lots of Fe and Cr atoms diffused into the cladding layer and formed solutionized microstructure.The formation energy and lattice parameter after substitution of Fe and Cr were calculated according to the First-Principles and the results showed that,both Fe and Cr atoms prefer to substitute Ni atom in the Ni Ti（B2）crystal structure.However,in the Ni₃Ti crystal structure,Ni has 2 different crystal positions,Fe atom prefers to substitute Ni-1 and Cr atom prefer to substitute Ni-2 atom instead.Subsequently,coatings were annealed under 450℃,650℃and 850℃.XRD results displayed that the phase composition and content of 60Ni+40Ti coating remained almost unchanged.The content of Ni₃Ti and Ni Ti₂ phase in 55Ni Ti+5Ni coating increased after annealing.The microstructure observed in 55Ni Ti+5Ni coating after annealing under 850℃changed from dendrites to single grains,which was the symbol of the completion of recrystallization,and the grains kept the trend of growing with longer holding time.In addition,the density and wear resistance of the coatings have been improved obviously after annealing treatment.Results show that,55Ni Ti+5Ni coating annealed under 650℃for 2 hours exhibited the best wear performance.