Shark-skin inspired surface engineering on intermetallic titanium aluminides for high temperature applications using the
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Shark-skin inspired surface engineering on intermetallic titanium aluminides for high temperature applications using the fluorine effect Raluca Pflumm and Michael Schütze Karl-Winnacker-Institut, Dechema e.V., Theodor-Heuss-Allee 25, D-60316, Frankfurt am Main, Germany ABSTRACT Increasing demands on technical components for high-temperature applications (e.g. turbine blades) promote new developments not only in the field of alloy design, but also in surface engineering. This paper shows that it is possible to structure the surface of intermetallic titanium aluminides in-situ by locally controlled oxidation of the material due to selective doping with fluorine. The aim is to reproduce a shark-skin pattern (parallel riblets with valleys in between) in order to improve the surface aerodynamics. Riblets with widths in the single digit μm range have been generated. The nucleation process, the aspect ratio and the stability of the generated microstructures are discussed as a function of the substrate composition and the oxidation conditions. INTRODUCTION Passive drag reduction by surface modification has been intensively studied and applied in the low temperature range. Applications include swim suits or adhesive structured foil for airplanes or ship hulls [1,2]. Riblet structures which mimic a shark skin lead to drag reduction if they are oriented parallel to the main flow around the moving body or component. The adequate aspect ratio (height/spacing ratio) can be calculated according to the flow condition, which is specific for each application. Surfaces with different riblet geometries have been tested in the laboratory. An overview concerning the correlation between the riblet shape and the measured drag reduction is available in the literature [3,4]. The beneficial effect of the riblets has remained unexploited in the high temperature range. One particularity of the high temperature applications is that the involved materials undergo bulk transformations. As a consequence, changes in the chemical/physical properties or surface topology modifications by oxidation are induced. Considering this, the in-situ structuring of intermetallic titanium aluminides (J-TiAl) inspired by the shark skin is discussed here. J-TiAl-alloys are suitable for the replacement of the heavy Ni-based alloys in turbocharger rotors for the automotive industry, turbine blades or vanes in aero engines or land based turbines due their good strength and creep resistance at high temperatures. A major drawback is their poor oxidation resistance, so that the protection of the substrate against oxidation is an important issue. This paper reports on the in-situ structuring of J-TiAl. Therefore the locally controlled oxidation of J-TiAl via the halogen effect is used in this study. The mechanism of the halogen effect is well understood and has been intensively discussed in the literature [5,6]. Very small amounts of halogen microalloyed near the surface of titanium aluminides prevent the formation of a thick mixed oxide (TiO2/Al2O3) layer and transform th
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