Amorphous sol–gel SiO2 film for protection of Ti6Al4V alloy against high temperature oxidation

Amorphous SiO2 thin films were deposited on Ti6Al4V alloy by sol–gel processing. Isothermal and cyclic oxidation tests of the coated and uncoated specimens were performed at 700 and 800 °C. The SiO2 film exhibited beneficial effects on the oxidation resistance of the alloy. Titania scales formed on the uncoated specimens, and severe spallation and stratification of the scales were observed. The oxidation rates of the silica coated specimens were decreased significantly. The silica film shrunk to about a quarter in thickness, probably by mechanism of crystallization of silica and evaporation of the organic additments. The oxide scales formed on the coated specimens were multilayered. Beneath the silica film, formation of a thick rutile titania layer followed by a thin alumina layer occurred. Above the silica film, alumina plus minor titania layer formed. It is deduced therefore that the growth of the multilayered and mixed oxide scales was dominated by both outward diffusion of metal and inward diffusion of oxygen.