Shark inspired surfaces for aerodynamically optimized high temperature applications

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 based riblets. It provides a reduced flow resistance resulting in less fuel consumption. A new application for riblet surfaces could be jet engine blades. The reduced friction of the gas flow would lead to higher efficiencies of engines. Calculations show that the riblet size on jet engine blades varies between single-digit micrometers up to several tens of micrometers, depending on local temperature, pressure and gas velocity. In this study structuring surface areas of high temperature nickel alloys, which are used for compressor and turbine blades near the combustion chamber, was investigated. The suitability of several structuring methods on high temperature materials has been examined. Initially, experiments were performed by depositing titanium on a nickel base alloy through particular metal grids using magnetron sputtering. Furthermore, riblets were successfully produced on a NiCoCrAlY coating by picosecond laser treatment. The ideal structured coating combines high oxidation resistance with low friction in a hot gas flow. For single-digit micrometer structures, photolithography with subsequent electrodeposition of nickel was investigated. All fabricated structures were oxidized at 900-1000°C for up to 100h and the resulting shape was characterized using scanning electron microscopy. The most accurate structures were obtained using photolithography. Here, the shape of the riblets persisted also after oxidation.