Shark Skin for High Temperature Applications - A Flow Optimized Protective Coating

Abstract

The high speed of sharks is caused by a reduced friction between water and skin due to a riblet structure on the skin surface. This effect has already been used for airfoil wings that were laminated with polymer films structured with riblets. A reduced flow resistance results in less fuel consumption. The challenge is to produce such a riblet structure on turbine blades to noticeably reduce the friction of flowing gas, leading to higher efficiencies of turbine engines. Theoretical considerations show that riblets on turbine blades need to be in the dimension of a few micrometers. This is smaller by an order of magnitude than structures on airfoil wings. The occurring Reynolds’ numbers, which determine the gas flow resistance and therefore the required structure dimensions on the surface, are much higher for turbine blades due to a higher velocity and pressure of the gas. In this study the suitability of some structuring methods on high temperature materials have been investigated. First experiments were performed by depositing titanium on a nickel base alloy through particular metal grids using magnetron sputtering. Furthermore, riblets in the micrometer range were successfully produced on a coating system by picosecond laser treatment. The ideal structured NiCoCrAlY coating deposited on nickel alloys substrate combines high oxidation resistance with low friction in a hot gas flow. This system was tested at 1000°C in air with regard to the stability of the formed structure. After oxidation for 100h the riblets are still intact. The structures were analysed prior and after exposure by using scanning electron microscopy.