Study On Surface Nitriding And Magnetron Sputtering Modification Of Titanium Alloy

The advantages of TC4 titanium alloy exhibit high specific strength,good low temperature performance and excellent corrosion resistance,which is widely used in various fields,such as aeronautical and space field.However,TC4 titanium alloy also shows its disadvantages of low hardness,poor wear resistance,easy seizure and low thermal conductivity,which severely limits its service life.Low temperature plasma nitriding and magnetron sputtering coating technique may provide a chance to change composition and structure of TC4 titanium alloy surface,and significantly strengthen hardness and wear resistance as well as keeping its advantages.In this paper,a hot-wire enhanced plasma magnetron sputtering system is used to achieve low-temperature plasma nitriding and Cr-based hard coatings on the surface of TC4 titanium alloy.The phase structure,microstructure,element content,hardness,elastic modulus,bonding strength,wear resistance and thermal stability of TC4titanium alloy after surface modification were studied.Conclusion can be drawn as following:(1)In the process of low-temperature plasma nitriding of TC4 titanium alloy,surface acid cleaning and plasma cleaning treatment can effectively remove dense oxide layer of the TC4 titanium alloy surface,which promoted the nitriding progress.At low temperature,the morphology of nitrided TC4 titanium alloy remains unchanged,which overcomes the crystal coarse and nitride precipitation occurring at high temperature for long time.(2)The Cr-rich Cr-N coatings showed their coordination deformation ability with TC4 titanium alloy,but with poor wear resistance.Ceramic Cr₂N and/or Cr N phase showed their excellent wear resistance,but with relatively poor toughness.The Cr/Cr₂N multilayer coatings,due to the inhibition of the growth of through columns,exhibited their good toughness and high strength,i.e.,low E as well as high H.On the one hand,the interface of the multilayer structure hindered the propagation of applied force;on the other hand,the low modulus of the multilayer benefited of the releases of external force.As a result,the adhesion between the multilayer coatings and TC4titanium alloy were improved.(3)Compared with the binary Cr-N and Al-N coatings,the ternary Cr-Al-N coatings showed their small grain sizes and the lattice distortion due to Al solid solution,and therefore,the H and E of Cr-Al-N coatings increased.The highest H and E,30 and 320.5 GPa,appeared in the Cr-Al-N coating with 6.82 at.%Al,respectively.The best tribological properties,the wear rate W is only 1.05×10^-16 m³/(N·m),appeared in the Cr-Al-N coating with the highest 13.29 at.%Al.Under the oxidation temperature of 750?,the thickness of Cr₂O₃ oxide layer formed on top of binary Cr-N coating was thicker than that of Cr-Al(13.29 at.%)-N coating,which confirmed that the oxidation resistance of the Cr-Al(13.29 at%)-N coating was improved due to the doping of Al element.