Improvement of Oxidation Resistance of TC4 Alloy by Microarc Oxidation with Sol-Gel SiO2 Sealing

A composite coating was prepared on titanium alloy (TC4) for improving its oxidation resistance at high temperature. The composite coating was fabricated using a two-step process. First a thick and porous oxide layer with pore size of a few micrometer was formed by microarc oxidation (MAO) technique in a NaAlO2 solution. Secondly, a top SiO2 sealing layer was formed on the porous layer by sol-gel dip-coating method followed by post heating treatment. SEM images and EDS mapping reveal an average overall thickness of about 6 mu m and the depth of sealing layer is about 1 mu m. Finally, high temperature oxidation experiment also shows the oxidation behavior of the coated samples is more stable over time, and the degree of oxidation damage is much reduced compared with those samples without sealing. The porous MAO layer provides anchorage sites for the subsequent SiO2 sol-gel coating, which seals the pores and hence significantly enhances the oxidation resistance.