A Study On The Plasma Electrolytic Oxidation Characteristics And The Formation Mechanism Of Black Coatings On Pure Titanium And Ti-6Al-4V Alloy

Titanium and its alloys are excellent structural materials,they are widely used in aerospace,biological engineering,automotive and other fields.The wear and corrosion resistances of Titanium and its alloys can be improved by Plasma electrolytic oxidation?PEO?,and hence expanding the application of titanium and its alloys.This paper studied the characteristics of plasma electrolytic oxidation on pure Ti and Ti-6Al-4V in the concentrated sodium silicate electrolyte.White and black coatings were prepared by changing the treatment time of plasma electrolytic oxidation.The phase composition,microstructure morphologies and formation mechanism of the coatings were explored by SEM,XRD,XPS and other techniques.Black ceramic coating on pure titanium was obtained by PEO treatment on pure titanium with the same condition on the Ti-6Al-4V,it means that the formation of the black coating is irrelevant to the vanadium element.Ti²⁺and Ti3+ ions were detected in the black ceramic coating by XPS while only Ti4+ ions were detected in the white coating.EDS analysis showed that there was a large amount of Na+ in the white film.Therefore,the formation of black film is related to the presence of low-valent titanium ions.White film were prepared for longer duration and its thickness was higher.The formation of the white film was related to the complete oxidation of titanium and the heavy deposition of the silicates.Hydrophilicity,surface roughness and bonding strength of the black and white film obtained by PEO treatment on Ti-6Al-4V alloy were tested.The surface roughness of white coating is much higher than the black coating.The coatings were all hydrophilic,and the white coating is super hydrophilic.The hydorphilicity of the white and black coatings were both worse after polished.This study shows that the super hydorphilicity of the white coating is related to the surface roughness.The bonding strength between the black coating and the substrate was strong,and the value reached 14 Mpa.The white coating has a lower binding strength,while the white coating was pulled off from the black coating in the bonding strength test.The result of friction performance test showed that the coating can improve the wear resistance of the substrate.The white coatings formed on the pure Ti and Ti-6Al-4V alloy were much thicker than the black coatings,their corrosion resistance was poorer than that of black coatings,but both black and white coatings had better corrosion resistance than the bare substrate.