Tribological Properties And Corrosion Behavior Of Laser Nitride Coating On Ti-6Al-4V Alloy

Titanium and titanium alloys have been widely used in aerospace,marine,petrochemical,biomedical and other engineering fields,owing to comprehensive properties such as high specific strength,good corrosion resistance,high temperature resistance,and excellent biocompatibility.However,in some conditions of higher wear resistance or erosion are limited for its low hardness and poor tribological properties.The laser gas nitriding method is developed by utilizing a concentrated laser heat source to irradiate the surface of titanium and its alloys in a nitrogen-rich environment that makes nitrogen entering into the surface melting zone to form a large thickness and combined with the base material in-situ metallurgy nitrided layer,which was an effective method to modify the low hardness,tribological properties and corrosion resistance in special environments of titanium and its alloys.In this paper,a fiber laser was applied to modify the surface wear resistance and corrosion resistance of the Ti-6Al-4V alloy substrate in a pure nitrogen and nitrogen-argon mixed atmosphere by using a gas nitriding method.The nitrided layer depth,microstructure,phase composition and distribution of element were analyzed by XRD,SEM,and EDS,respectively,and the hardness in the depth direction was tested by the microhardness tester as well as the corrosion behavior was detected by chemical corrosion experiments test.The results show:In condition of a pure nitrogen atmosphere,the laser power was in the range of1200²000 W,and other parameters are certain,Under the 2000 W laser power,an optimal laser nitrided layer with uniformly distributed nitrides,free cracks and the maximum depth of 910?m was in-site synthesized,and with the microstructures of high nitrogen concentration dendritic TiN and low nitrogen concentration fine dendritic Ti₂N on the surface and bottom region respectively.The acicular crystals?'-Ti?N?are formed in the heat affected zone.The hardness near the laser nitrided layer surface was about970 HV_0.5,and reduced to 530-360 HV_0.5 in the heat affected zone.The wear resistance of the laser nitrided layer was 55%higher than that of the Ti-6Al-4V substrate.In3.5%NaCl solution,the current density of the nitrided layer was one order of magnitude lower than that of Ti-6Al-4V substrate.Compared with the Ti-6Al-4V substrate,the corrosion potential of the nitrided layer moved positively by 0.091 V,and the polarization resistance increased 65 times.In condition of a pure nitrogen atmosphere,the laser scanning speed was in the range of 5²5 mm/s,and other parameters are certain.When the scanning speed was in10²5 mm/s,free cracks nitrided layer can be obtained,and TiN with a high nitrogen was formed in the surface layer,what's more,fine and low nitrogen Ti_xN in the bottom,simultaneously?'-Ti?N?needle-shaped crystals in the heat affected zone.Under the 10mm/s,the optimized nitrided layer was achieved,of which the depth was about 880?m,the hardness of the near surface and the heat affected zone was about 970 HV_0.5 and360⁶80 HV0.5,respectively,and the wear rate of the nitrided layer was 1.17�10^-4mm³/Nm,which was 69%lower for than substrate.The polarization resistance was135604?�cm² in 3.5%NaCl solution,which was 40 times higher than that of Ti-6Al-4V substrate.In condition of a mixed atmosphere of nitrogen and argon,the nitrogen argon proportion was in the range of 20:0²:18,and other parameters are certain.When the nitrogen argon proportion was 20:0,the surface of the nitrided layer found the TiN phase,and the cross section developed coarse and dense dendrites resulted in cracking,which depth was 655?m,and the near surface hardness reached 1172 HV_0.5.While the nitrogen argon proportion was in the range of 18:2²:18,TiN_0.9 phase was formed on the surface of the nitrided layer,and there was no crack in the cross section,which depth was in509⁴4?m,and the surface hardness was 997⁶39 HV_0.5.When the nitrogen argon proportion was in 18:2²:18,the wear rate of the nitrided layers were 2.315³.639�10^-4mm³/Nm,which were 40⁵%lower than that of Ti-6Al-4V substrate,and the corresponding polarization resistance were 115791⁴1506?�cm² that higher 34¹2times than that of Ti-6Al-4V substrate.While with the decrease of the nitrogen argon proportion,the wear resistance and corrosion resistance of the nitrided layers gradually decrease.In condition of the nitrogen argon proportion was 18:2,a nitrided layer with deeper thickness,higher hardness,better wear resistance and corrosion resistance,and free cracks can be obtained.