Exfoliation and Fracture Behavior of Oxide Films Formed on Titanium and Its Alloy in High Temperature Environments

It is very difficult to obtain mechanical properties of oxide films formed on a material in high temperature environments despite its importance of estimating material degradation caused by such as thermal stress. Corrosion/oxidation tests were conducted for pure titanium and titanium alloy in high temperature corrosive environments of wet air and water vapor with hydrogen chloride at temperatures from 673 K to 973 K to look into basic behavior of degradation and the growth of titanium oxide films. It was found that oxide films were usually formed on the specimen surface and the growth was accelerated by the corrosiveness of the environment. In order to examine mechanical properties and exfoliation of corrosion products or oxide films formed on titanium and its alloy, tests of single particle impact on the specimen surface with a glass bead were performed in high temperature corrosive environments. The piling-up surfaces around impact craters were formed and plastically strained. The oxide film formed on the metal surface was detached in a wide range of the circumference and fractured a little far from the rim of the crater. Then fracture and exfoliation stress of the oxide film were estimated by the calculation of impact energy and fractured and detached areas. It was found that both the fracture and exfoliation stress of the oxide films were different depending on the corrosive environment and chemical composition of titanium alloy.