An Comparative Investigation On Thermal Oxidation Of Ti6Al7Nb And Ti6Al4V Medical Titianium Alloys

This paper studied comparatively the oxide films on Ti6Al7Nb and Ti6Al4V alloys by thermal oxidation.The surface characterization and oxidation kinetics of the alloys were evaluated at different oxidation temperature and time.The thickness, density,phases and surface hardening were characterized with the testing methods such as optical microscopy(OM),XRD and Vickers hardness tester.Using XPS, composition and chemical state of the surface oxide film were investigated.The mechanism of the formation of the surface oxide films and the effect of the adding elements were discussed.The results show that the thickness of oxide films on both alloys increases with temperature and time,while the density of the oxide films decrease.The weight gain curve and cross-sectional OM micrographs reveal that the oxide film on the surface of Ti6Al7Nb is thinner and denser than that on Ti6Al4V alloy and has stronger adhesion with substrate.Thermal oxidation behaviour of Ti6Al7Nb alloy obey parabolic and linear oxidation kinetics at 600～800℃and above 900℃,respectively.At 700℃parabolic oxidation is dominant up to 36h of oxidation,and prolonged oxidation yielded linear oxidation kinetics for themal oxidation behaviour of Ti6Al4V.Vickers hardness tester's results reveal that the surface hardness values of both alloys decrease with increasing indenter loads,which is the typical behaviour of surface hardneing.The surface hardness increases with oxidation temperature or time, while the surface hardness of Ti6Al4V alloy decrease at 900℃.At same oxidation temperature and time,the surface hardness value of Ti6Al4V alloy is slightly larger than that of Ti6Al7Nb alloy.The Maximum surface hardness values of both alloys are obtained at 600℃for 72h.XRD results reveal that,after oxidation of 1h at temperature ranging from 600℃to 900℃,surfaces of both Ti6Al7Nb and Ti6Al4V are mainly covered by rutile TiO₂ (R-TiO₂).The difference is that peaks of anatase TiO₂(A-TiO₂) and Al₂O₃ are detected in the oxide film of the Ti6Al4V at 600℃and 900℃,respectively.When the oxidation temperature is above 800℃and oxidation time is longer than 24h, surfaces of all specimens are covered by R-TiO₂ and Al₂O₃ together.XPS analysis confirmed that the surface oxide film only contain cations with the highest or most stable valences.The preferential oxidation of Al is due to its the minimum standard redox potential in all adding element.In the process of thermal oxidation,the outer most sections of the surface oxide film of both Ti6Al7Nb and Ti6Al4V are allways Al₂O₃.Nb dose not change fundamentally the corrosion mechanism itself,however,it plays,at least,a role on a mechanism active during the initial stages of corrosion,rendering the oxide film more protective.Above 700℃,severe oxidation of V causes a loss of corrosion resisitance of Ti6Al4V alloy.