Structure and mechanical properties of MAO coatings formed on aluminum-coated titanium alloy

Application of titanium alloys is restricted due to their low hardness and poor wear resistance. Duplex surface treatment consisting of depositing of Al coating by arc ion plating (AIP) plus micro-arc oxidation (MAO) was carried out on Ti6Al4V alloy to improve its tribological performance. Ceramic coating mainly composed of α-Al2O3, γ-Al2O3 and mullite was fabricated on the aluminum-coated titanium alloy after the MAO treatment, and its thickness, compactness and hardness increased with the MAO time before the titanium alloy was oxidized. After the substrate titanium alloy was locally oxidized due to oxygen anions inward migration, a thin transition layer containing titanium oxides and aluminum titanate formed and incorporation of Ti in the alumina coating occurred. Occurrence of the strong eruptible discharges at these positions was facilitated probably due to the large difference in permittivity of the alumina and the titanium oxides, which led to the formation of penetrating cracks in the MAO coating. The hardness and compactness of the ceramic coating decreased. Tensile test results showed bonding strength of MAO ceramic coating/aluminum layer/titanium alloy substrate system was more than 40 MPa, and the maximum value could reach around 68 MPa. The bonding strength of the ceramic coating/titanium alloy substrate system decreased to around 34 MPa due to the weak bonding between the alumina ceramic coating and the titanium alloy substrate. Tribological test results show wear resistance of titanium alloy was greatly improved due to the fabrication of hard alumina ceramic coating on titanium alloy.