Innovative processing techniques to obtain aerodynamically optimised riblets in NiCoCrAlY bond coats on aero-engine turbine blades

The high speed of sharks is caused by reduced friction between water and skin due to riblet structures on the shark skin. A potential application field for the riblet surfaces is blades and vanes for aeroengines. Those parts require coatings that act as oxidation and corrosion protection and could additionally reduce skin friction drag on the surface when riblets are applied. This reduction in friction of the air flow would lead to higher efficiencies of the engines. The presented work deals with the fabrication, oxidation and characterization of riblet structures with micrometer dimensions on state-of-the-art engine materials for aerodynamically optimized high temperature applications. The necessary riblet sizes for compressor inlet, compressor outlet, turbine inlet, and turbine outlet are calculated on the basis of one example aeroengine. Based on these calculations, the feasibility of various riblet structure fabrication methods is investigated. Direct riblet fabrication methods such as laser material removal and indirect structuring methods such as photolithography and subsequent electrodeposition of nickel have shown the best results in obtaining riblets of desired sizes. The fabricated riblets on EB-PVD deposited NiCoCrAlY bond coats are oxidised at 900°C for simulating the high temperature application in an aeroengine. The structures are found to be intact after oxidation. The effectiveness of the riblet structures to reduce the wall shear stress is measured by scaled-up models in an oil channel. The measured riblet design by means of photolithography and by means of laser removal has shown a significant wall shear stress reduction of 4.9 and 2.41% respectively