| Due to its unique combination of high strength to weight ratio,superior corrosion resistance and good biocompatibility,titanium alloys has been applied in aeronautical,chemical and biomedical industries widely.However,it has been limited used in key tribological components due to its low hardness,poor wear resistance and insufficient high temperature oxidation resistance.As a surface modification technique,laser cladding can produce a coating bonding well with the titanium alloys without pores and crack.Preparing composite coatings with excellent tribology and oxidation resistance is a economical and reliable approach.Three kinds of composite coatings with different ratio of Ni-Mo-Si powders were synthesized on titanium alloy by laser cladding to improve the high-temperature oxidation resistance and tribology.The micro-hardness,microstructure,phase and chemical composition were analysed by Vickers micro-hardness tester,scanning electron microscope?SEM?,energy dispersive spectroscopy?EDS?,X-ray diffraction?XRD?.The samples of composite coatings and Ti6Al4 V alloy underwent tests of isothermal oxidation at the temperature of 900?during 100 h in static air atmosphere,the corresponding oxidation mechanism was analyzed by thermodynamics and kinetics.The tribological properties test of Ti6Al4 V alloy and three kinds of coatings were carried out using high temperature friction and wear testing machine,and the wear mechanism was analyzed.The result shows that the anti-wear and oxidation resistant composite coatings have a fine bonding to titanium alloy and are free of cracks and holes.Laser cladding Ni-Mo-Si composite coatings are mainly consists of primary Ti5Si3,Mo5Si3,MoSi2 reinforcement particles and the inter-primary?-Ti,NiTi inter-metallic compounds matrix.With the increase of Ni content in composite coatings,the inter-metallic compounds matrix increased and the hard phase Ti5Si3 was refined.The average micro-hardness of the coating N1,N2 and N3 was 917.5HV0.5,850.8 HV0.5,and 795.6 HV0.5,respectively.which are2.62,2.43 and 2.27 times than that of Ti6Al4 V alloy,respectively.This indicates that the hardness decreased with the reduced content of Mo-Si.After 100 hour oxidation at 800?,the mass gain per unit area of Ti6Al4 V alloy is about 23.38 mg/cm2.The thickness of surface oxide film is about 100?m,which contains loose and porous TiO2.There are layers and cracks in the oxide film,and the separation and peeling between the coating and the coating indicates that the oxide film have no antioxidant properties.The mass change of coating N1 showed a minus tendency:the mass gain after 60 hours is-0.31 mg/cm2,0.21 mg/cm2 after 80 hours and 1.31 mg/cm2 after 100 hours which accounts for about 5.6%than than of Ti6Al4 V alloy.As well As coating N1,coating N2 shows good oxidation resistance with a mass gain about 2.81 mg/cm2,which is12%of the titanium alloy substrate.Unlike the two coatings before,coating showed a increase changing of mass,about 13.54 mg/cm2 after oxidizing for 100 h,which was 57.9%of the titanium alloy.All three kind of coating have better oxidation resistant than that of titanium alloy.At room temperature,the friction coefficient of Ti6Al4 V alloy is 0.52,the wear rate is22.6×10-5mm3/Nm,the wear mechanism is oxidation wear,massive spalling and furrow.Friction coefficient of coating N1/N2/N3 is 0.44/0.36/0.43,the wear rate is 16.2×10-5/11.2×10-5/17.5×10-5mm3/Nm,respectively.At high temperature of 800?,the friction coefficient of Ti6Al4 V alloy is 0.46,the wear rate is 13.9×10-5mm3/Nm;friction coefficient of N2 coating is 0.28,the wear rate is 0.65×10-5mm3/Nm.The coating N2 shows better high temperature oxidation and wear resistant.In this test conditions,laser cladding Ni-42%Mo-28%Si composite coating has the best high temperature oxidation resistance and wear resistance.Which can provide theoretical reference of coating preparation to extend the application of titanium alloy in engine and other high temperature and high wear working environment. |
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