Research Of TC4 Titanium Alloy Micro-arc Oxidation And Its Composite Coatings Friction And Wear Properties

Titanium and Ti alloys are widely used in aerospace,marine,medical and other fields due to their excellent properties such as a high strength-to-weight ratio,good corrosion resistance,bio-compatibility and many others,but the low surface hardness and poor wear resistance severely limit their further applications.Micro-arc oxidation(MAO)is a method to coat a ceramic oxide film,which have high surface hardness and uniformly porous,on the surface of valve metal under high voltage.In this paper,micro-arc oxidation is applied to form a porous oxide film on the surface of TC4 titanium alloy as the load support bases,and then coated with lubricating PTFE particles,prepared a self-lubricating composite coating layer,which can improve the friction and wear properties of TC4 titanium alloy surface.The main findings are as follows:Though the study of TC4 titanium alloy micro-arc oxidation in different electrolytes,the paper analyzes the influence of the components formation and concentration to the film,SEM,EDS are used to characterize the surface morphology and microscopic composition of the films.The results show that,Na3PO4 and(NaPO3)6 are film-forming agent,which are essential components of electrolytes,and their minimum solubility is 5g/L,when the concentration increases,film surfaces become more uniform porous,the appropriate concentration is lOg/L.Na2SiO3 can effectively reduce the arc-voltage and promote deposition efficiency,the lowest concentration is 5g/L,the arc-voltage is about 185V,but when the concentration increases,the arc-voltage does’t continue to drop significantly.Na2MoO4 can increase electrolyte conductivity,create more micro-porous on the films,but high concentration will cause the film burned for excessive current,and the film is easy to produce the loose coatings on the surface.Adding NaF can improve the surface morphology of the film to make it more uniform porous,the optimum concentration is 1g/L.KOH provide sufficient OH-to electrolyte,which can promote membrane efficiency and film thickness,but high levels potassium hydroxide can cause serious loose coatings on the surface,its optimum amount is 2g/L.By mixing several electrolyte compositions,two optimum electrolyte systems are configured,the first electrolyte is Na3PO4-Na2SiO3-Na2MoO4-NaF,and the second is(NaPO3)6-Na2SiO3-Na2MoO4-KOH.The electrical parameters of both electrolytes are explored.Films surface morphology,microstructure composition and thickness are analyzed by SEM,EDS and optical microscope.The results show that,in the same electrical parameters,the micro-arc oxidation coatings prepared in the first electrolyte have a more uniform pore size and higher porosity surface than the second,but the film hardness and thickness are lower than the latter.The electrical parameters influence of two electrolytes shows a similar rule.When the voltage increases,the film aperture and thickness increase,but surface porosity decreases,the film is easy to destroy if the voltage exceeds 400V.The film thickness increases when frequency decreases.The surface of film turns obviously rougher when duty-factor larger.The film thickness growth as time passed,but the thickening rate will significantly reduce after more than 30min.The friction and wear performance tests of TC4 MAO film and TC4 MAO-PTFE composite coatings show that,MAO films present poor wear resistance under pin-to-disc test conditions.The film prepared in first electrolyte will be worn at a load of 50N,and the film of second electrolyte worn at 60N,but the friction coefficient and wear resistance are improved compared to TC4 alloys,and friction coefficient fluctuate within the range of 0.2 to 0.4.MAO-PTFE composite coatings exhibit excellent friction and wear properties,friction coefficient is about 0.15,and the fluctuation is very small.Heat treatment to the composite coatings wear shows a great impact,after 3h heat treatment at 327 ℃,the wear resistance is significantly improved.