Preparation And Tribological Properties Of Nitride Layer On Titanium Alloy Surface Controlled By Femtosecond Laser Energy Density

Titanium and its alloys have excellent properties,such as high strength,good corrosion resistance and good biocompatibility,which make them widely used in aerospace and biomedical fields.But the hardness of titanium alloy is low and the wear resistance is poor,which greatly limits the application of titanium alloy in wear resistance environment.As an important strengthening phase of titanium alloy,titanium nitride has high hardness and excellent wear resistance.Preparation of titanium nitride layer on the surface of titanium alloy can improve the wear resistance of titanium alloy.Laser gas nitriding is one of the most promising surface modification techniques for improving the poor friction properties of titanium alloys.The remelting principle based on thermal accumulation effect under high repetition rate of femtosecond laser has been preliminarily applied to polishing process.Based on the advantages of femtosecond high repetition frequency and the problems of thermal deformation and crack in traditional continuous laser nitriding and nanosecond laser nitriding,in order to obtain a nitriding layer with good surface roughness and no crack,this paper tentatively applies femtosecond laser to nitriding titanium alloy in nitrogen environment,and focuses on the influence of energy density on the depth of nitriding layer,phase structure,tin dendrite microstructure and friction properties.The results show that:(1)Femtosecond laser titanium nitride alloy technology has a strict processing window:when the scanning speed range is 60～100mm/s and the energy density is0.14～0.58J/cm~2,a nitrided layer with relatively good surface quality and sufficient nitrogen content can be prepared in this processing window.(2)The change of energy density can effectively regulate the nitriding layer surface roughness,surface hardness,nitriding layer phase ratio and cooling rate,the specific effects are as follows:When the energy density increases from 0.14J/cm~2 to0.58J/cm~2,the surface roughness Sa increases from 0.59μm to 3.25μm,and the surface hardness increases from 414HV to 1389HV.The Ti N diffraction peak is enhanced,and the volume fraction of Ti N is increased,indicating that the nitrogen content on the surface of the nitriding layer increases gradually,and the atomic percentage of near-surface nitrogen element increases from 18.68%to 33.82%.(3)The energy density can realize the effective control of cracks and other defects.When the scanning speed is 80mm/s and the energy density ranges from 0.35to 0.48J/cm~2,the Sa value of surface roughness is about 0.85μm and the depth of nitriding layer is 53～112μm.Nitride layer with good surface quality and no crack in the surface hardness range of 880～1247HV.(4)Femtosecond laser surface nitriding can effectively improve the friction and wear properties of titanium alloy:when the energy density is 0.58J/cm~2 and the scanning speed is 80mm/s,the average friction coefficient of nitriding layer is 72.5%of the original,and the wear resistance is improved by 89.6%.The wear surface of the original sample showed serious abrasive wear and plastic deformation.The wear performance of the remelting sample in argon gas environment is slightly improved compared with the original sample,mainly abrasive wear,which proves that laser remelting is helpful to improve the wear resistance of titanium alloy.(5)When the energy density is 0.14J/cm~2,the average friction coefficient of the nitriding layer is 96.1%of the original,but the wear resistance of the nitriding layer is lower than that of the original.When the energy density is 0.35J/cm~2,the average friction coefficient of the nitriding layer is 90.2%of the original,and the wear resistance of the nitriding layer is increased by 34.9%.When the energy density is0.58J/cm~2,the average friction coefficient of the nitriding layer is 72.5%of the original,and the wear resistance of the nitriding layer is increased by 89.6%.When the scanning speed is 80mm/s and the energy density is 0.35J/cm~2,the nitriding layer with hardness of 880HV and thickness of 53μm without crack is obtained,and the wear capacity is 34.9%higher than that of the original sample.