Wear,Corrosion And Oxidation Behavior Of Ti-6Al-4V Based Coating Materials Modified By Si,C,SiC And Al

Due to its high specific strength,good corrosion resistance and good heat resistance,titanium alloy has been widely used in aerospace,chemical industry,shipbuilding,biomedical and other fields.Among them,Ti-6A1-4V alloy is the most typical titanium alloy with the largest consumption.However,Ti-6A1-4V alloy has poor wear resistance,poor oxidation resistance above 350℃ and low corrosion resistance in halogen ion solution,which limits its application.Surface modification is one of the most effective ways to improve these properties.In this study,Ti-6A1-4V alloy was directly used as the substrate material to ensure a good interfacial adhesion between the surface coating material and the matrix.The addition of Si,C,SiC and A1 can induce the formation of Ti₅Si₃ and TiC reinforcements and enhance the solution strengthening effect of Al,the Ti-6A1-4V surface coating material was formed on Ti-6A1-4V alloy by powder metallurgy technology.The microstructure,wear,corrosion and oxidation behavior of Ti-6A1-4V alloy with Si,C,SiC and A1 additions were systematically studied in order to develop Ti-6A1-4V based surface coating materials with good properties.The main conclusions are as follows:1.Densification behavior and microstructure evolution of Ti-6Al-4V based coatings modified by Si,C,SiC and A1By controlling the content of the second phase（Ti₅Si₃ and TiC）≤4vol.%and the content of added A1≤4wt.%,it is found that there is a transition layer of about 50～80μm between the Ti-6A1-4V based coatings and the Ti-6A1-4V matrix after sintering at 1300℃ for 2h.The main phases of the coating material,the transition layer and the Ti-6A1-4V substrate are consistent,which are α+β lamellar structure,indicating that the interface between the coating and the substrate alloy is well bonded.Besides,the size of Ti₅Si₃ and TiC particles induced in Ti-6A1-4V based coating is about 100nm.With the increase of Si,C,SiC and A1 content,the porosity and average pore size of Ti-6A1-4V based coating materials continue to increase.Moreover,the increase of C or A1 content will lead to the sharp increase of porosity of Ti-6A1-4V coating materials,and the effect of C element on porosity is more significant,and the porosity of the coating material is increased from 0.46%to 3.48%with the addition of 4vol.%TiC.In addition,the average pore diameter of the coating material can reach 100.20μm when C（4.0vol.%TiC）is added alone,but the average pore diameter is only about 34.40μm when SiC（4.0vol.%（Ti₅Si₃+TiC））is added,which is equivalent to the effect of Si and A1 addition.The results show that there is a synergistic effect in the precipitation of Ti₅Si₃ and TiC,which can inhibit the formation of large-scale pores in Ti-6A1-4V based coating.The effect of the above elements on the porosity of Ti-6A1-4V based coating is SiC<Si<Al<C.Si has the tendency to refine the α lamellar thickness of Ti-6A1-4V based coating material,but the addition of C,SiC and A1 will lead to the increase of αlamellar thickness,and the effect of Al is more significant.The average α lamellar thickness increases from 5.50μm to 9.54μm with the addition of 4wt.%Al,which is related to the fact that Si is a β stable element,while Al and C are α stable elements.The thickening effect of the above elements on α plate of Ti-6A1-4V based coating material is SiC<C<Al.2.Wear behavior of Ti-6A1-4V based coatings modified by Si,C,SiC and A1The sliding wear tests was carried out with GCr15 friction pairs（64-66HRC）under the contact load of 10N.The results show that the microhardness of Ti-6A1-4V based coating increases,and the friction coefficient and specific wear rate(mm3·N^-1·m^-1)decrease with the increase of Si,C,SiC and A1 content.When the content of the second phase Ti₅Si₃,TiC,Ti₅Si₃+TiC is 4vol.%,or the content of A1 is 4wt.%,the microhardness of the coating material increased by 35.90,48.82,42.44 and 45.24%（from 331.8Hv0.5）,the friction coefficient decreased by 11.64,14.77,12.26 and 14.46%（from 0.418）,and the specific wear rate decreased by 28.43,41.61,29.08 and 37.88%(from 1.692×10^-4),respectively.The results show that the wear resistance of the coating material is better by adding C and Al,and the strengthening effect of the above elements on the wear resistance of Ti-6A1-4V coating material is Si<SiC<Al<C.The microstructure analysis of the worn surface shows that the wear mechanisms of Ti-6A1-4V based coatings with Si,C,SiC and A1 addition are adhesive wear and abrasive wear.Compared with the carbide（Fe,Cr）3C（about 1000-1230HV）and acicular martensite（about 700HV）in GCr15 friction pair,the precipitation of high hardness second phase TiC（about 3200HV）and Ti₅Si₃（about 1150HV）can improve the cutting resistance,adhesion resistance and crack propagation resistance of the coating material,and the solid solution strengthening of A1 can also improve the cutting resistance and adhesion resistance of the matrix,thus improving the wear resistance of Ti-6A1-4V based coatings.3.Corrosion behavior of Ti-6A1-4V based coatings modified by Si,C,SiC and A1It is found that the corrosion resistance of Ti-6A1-4V based coatings deteriorates with the addition of Si,or the content of TiC and Ti₅Si₃+TiC more than 1vol.%and the content of added A1 more than lwt.%in 3.5%NaCl solution,but the corrosion resistance of the coatings was improved by controlling the content of TiC and Ti₅Si₃+TiC at lvol.%or the content of added A1 at lwt.%,where the corrosion potential（mV）vs.SCE of the latter shifted from-327.07 to-270.54,-263.1 and-258.35,respectively,and the corrosion current(μA cm^-2)decreased from 0.15 to 0.13,0.14 and 0.08,respectively.The reason for the improvement of corrosion resistance is related to the increase of α lamellar thickness and the pinning effect of second phases on passive film.The reason for the deterioration of corrosion resistance is related to the increase of porosity,and the galvanic corrosion of the second phase cathode Ti₅Si₃（-117 mV vs SCE）and TiC（-45 mV vs SCE）relative to the anode Ti-6A1-4V（-327 mV vs SCE）matrix.The continuous deterioration of Si addition is also related to the refinement of α lamellar thickness.4.Oxidation behavior of Ti-6A1-4V based coatings modified by Si,C,SiC and A1The results of cyclic oxidation test show that the oxidation resistance of Ti-6A1-4V based coating is improved by adding Si,C,SiC and Al.The reaction rate constant kp(×10^-3 mg·cm^-2·h^-1)of the coating with the content of 4vol.%Ti₅Si₃ decreased from 2.96,62.56,574.74 to 1.88,31.86,298.73 when oxidized at 873,973 and 1073K,respectively,with a decrease of 36.49,49.07 and 48.02%.From the microstructure analysis of the oxide layer,the main reasons for the improvement of oxidation resistance of Ti-6A1-4V based coating materials are as follows:the formation of fine and dense oxide is induced by the precipitation of Ti₅Si₃ and TiC nano second phases and the increase content of Al₂O₃,which inhibits the diffusion of O to the inner layer.Especially,no obvious O is observed in the inner layer when the content of Ti₅Si₃ or Ti₅Si₃+TiC is 4vol.%;the thermal expansion coefficient of Ti₅Si₃ is close to that of Rutile-TiO₂ and Al₂O₃,which reduces the internal stress of oxide film during cyclic oxidation.The results show that the oxidation resistance of Ti-6A1-4V based coating is enhanced by the above elements in the order of C<Al<SiC<Si.The above research results on the microstructure,wear,corrosion and oxidation behavior of Ti-6A1-4V based coating materials added with Si,C,SiC and Al have reference value for different surface coating treatment technologies.