Mechanical properties of shark-skin like structured surfaces for high-temperature applications

Structuring surfaces with so-called riblets improve the drag reduction on surfaces of airplane wings or blades for aero-engines. Here the result of collaboration between four research groups which focused on the transferability of the riblet approach to high temperature material systems. With respect to the applications excellent mechanical properties and high temperature oxidation resistance of the riblets are necessary. Applicable processes via halogenation, coatings fabrication, and laser ablation plus oxidation to process small riblets on relevant material systems for aero engines (IN718, PWA1483, TNMB1) are presented. The riblets have different compositions such as metallic, half- and completely oxidized. The riblet morphology is characterized and studies of the mechanical stability outlined promising results. All tested riblets exhibit good bonding to the substrate. Results of quasi cyclic four point bending tests and low cycle fatigue are discussed taking into account the differences between the riblet and the substrate composition. Applicable processes via halogenation, laser fabrication, and laser ablation followed by oxidation to process riblets on material systems for aero-engines (Ni-based alloys, gamma TiAl) are presented. Several micrometer-sized riblets with optimum dimensions exhibiting high stability are fabricated. The riblet morphology is characterized and a study of the mechanical stability (low-cycle fatigue and quasi cyclic four-point bending tests) outlined promising results.