Self- growing shark skin pattern on the surface of gamma-TiAl alloys

This work deals with surface structuring of gamma-titanium aluminide engineering alloy samples by using the oxidation behaviour of the material between 800 and 1000°C. Previous experiments showed that if the pattern of the structures is similar to that of a shark skin, aerodynamical losses due to shear strain at the surface of a solid moving in a flow are decreased. Tiny riblets arranged parallel to the flow direction hamper the cross flow which causes the most of the wall shear stress. Such improvement in the flow conditions is foreseen to be applied to compressor and turbine blades of aero engines for example. These components operate at temperatures between 500 and 1000°C and in strong temperature gradients. Therefore the surface structuring process should not prevent the growth of a protective oxide layer, necessary to protect the material against high temperature corrosion. Several structuring methods leading to self-growing of a shark skin pattern via partial microalloying of halogens (e.g. Fluor) in the first 100 nm of the sample surface were investigated. On the halogen- rich domains a thin protective Al-oxide layer grows slowly at temperatures up to 1000°C (so called “halogen effect”). On the domains free of halogen a thick mixed Al- and Ti-oxide layer grows fastly and builds the riblets. The growth of the non- protective mixed oxide layer should be stopped by lateral diffusion of the halogen from the halogen-rich to the halogen-free domains in order to prevent the spallation of the riblets. The principle and results on the development of such structures including the nucleation process depending on the halogen distribution over the sample surface are shown and discussed for isothermal oxidation at temperatures between 800 and 1000°C.