Surface Nitriding Strengthened Ti13Nb13Zr Alloy And Its Tribocorrosion Properties

Ti13Nb13Zr alloy has received increasing attention in the field of artificial hip joints depending on its excellent biocompatibility,good corrosion resistance,and low elastic modulus close to human bone.However,the poor tribological properties severely limit its application in hip prosthesis bearing.Therefore,the surface nitriding-strengthened treatment was conducted on Ti13Nb13Zr alloy through high-temperature gas nitriding process to improve the wear resistance,and an optimized processing parameter was obtained by adjusting the nitriding temperature and holding time.Meanwhile,the nitrogen diffusion behaviour,nitriding layer growth kinetics and growth mechanism were investigated.In addition,the mechanical properties,electrochemical corrosion properties,tribocorrosion properties,and biocompatibility of the nitriding layer were comprehensively evaluated.In the nitriding process,nitrogen could diffuse into the Ti13Nb13Zr alloy substrate and formα-Ti（N）supersaturated solid solution.With the increase in nitriding temperature and holding time,α-Ti（N）gradually transformed into nitrides,and the nitriding layer eventually evolved into an external nitride layer mainly composed of Ti N,an internal nitride layer composed of Ti N,Ti₂N andα-Ti（N）and an N diffusion region composed ofα-Ti（N）.The growth kinetics analysis exhibited that the thickness of the nitride layer was directly proportional to the square root of the holding time,and the reaction rate constant K was calculated to be 16.46.In addition,it was found that alloy elements and their redistribution played a critical role in nitrogen diffusion.During the formation process of theα-Ti（N）and nitrides,the Nb element was squeezed from the N-enriched region owing to itsβ-stabilizing action and limited solubility,and the local Nb segregation along the boundary between the external and internal nitride layers would block the N diffusion channels,making it difficult for theα-Ti（N）below the segregation to convert to Ti N,finally leading to an inhomogeneous phase composition in the internal nitride layer.The surface hardness of Ti13Nb13Zr alloy was significantly improved after surface nitriding-strengthened treatment,and it increased with the nitriding temperature and holding time.When the sample was nitrided for 10 hours at 1200℃,the surface hardness could reach 1300 HV,which was about 5.3 times that of the untreated Ti13Nb13Zr alloy.Moreover,the combination of the hard nitride layer and the N diffusion zone with solid solution strengthening effect allowed the nitriding layer to have excellent hardness and fracture toughness,and they exhibited a gradient decreasing trend with the decreasing of Ti N content in the depth direction.In the electrochemical corrosion test,due to the superior chemical stability and thickness compared to the spontaneously formed Ti O₂ oxide film on titanium alloy,the nitriding layer exhibited higher corrosion potential,lower corrosion current density,and lower corrosion rate in PBS solution,which could well meet the severe requirements of metal implant corrosion rate lower than 0.25μm/year.In the tribocorrosion test,since the nitriding layer had sufficient hardness and strength to resist plastic deformation and destruction,its wear mechanism was dominated by the corrosive wear caused by the synergistic effect of mechanical removal of Al₂O₃ ceramic ball and corrosion of PBS solution.As the nitriding temperature and holding time increased,the Ti N content in the nitride layer gradually increased,leading to a gradual decrease in the degree of corrosive wear.However,prolonged nitriding would cause an increase in surface defects and a decrease in surface strength,thereby reducing wear resistance.Notably,the Ti13Nb13Zr alloy obtained the optimal tribological properties after nitriding for 6 hours at 1200℃.Under the same testing condition,its wear volume was 0.005±0.002 mm³,which decreased by 99.79%compared to the untreated Ti13Nb13Zr alloy,and even when the load was increased to30 N,and the sliding was increased to 12 hours,the wear volume was only 0.040±0.011mm³.In the biocompatibility test,the nitriding layer exhibited nontoxicity of class 0 and had no adverse effects on the growth and propagation of MC3T3E1 osteoblasts.In addition,the nitriding layer did not lead to the release of macrophage inflammatory factors at excessive levels,which showed excellent biocompatibility as the untreated Ti13Nb13Zr alloy.In summary,a wear-resistant nitriding layer with sufficient thickness could be generated on Ti13Nb13Zr alloy through surface nitriding-strengthened treatment with suitable process parameters,which possessed excellent fracture toughness,corrosion resistance,tribocorrosion performance and biocompatibility as well.This work realized the preparation of titanium alloy artificial joint materials with wear-resistant surface and tough core,and it is of great theoretical and practical significance for developing new artificial hip prostheses with low friction,low wear,and long life.