Self-organized nanostructuring of composite coatings at high temperatures for drag reduction and self-cleaning

Coated and nanostructured surfaces gain much importance for improving the efficiency of high temperature applications (e.g. in turbines). By embedding ceramic particles with a negative thermal expansion coefficient (NTEC) into a metallic matrix, a reversible thermal activation of a nanostructured surface can be established.At high temperatures a defined drag reducing surface microstructure (“shark-skin”) is formed in the coating surface, while at low temperatures a self-cleaning effect (in idle periods) is achieved by the reversal of the thermal deformation.A feedstock powder produced by high energy milling and consisting of nanocrystalline yttrium oxide and tungsten oxide particles embedded into a conventional MCrAlY alloy was used for the investigations. By using different thermal spray and cladding techniques the powder is deposited on steel- substrates. In the next step, the coating is implanted with yttrium or xenon in order to induce the formation of Y2W3O12. The latter is a ceramic with a strong negative thermal expansion coefficient and is stable up to temperatures above 1373 K. The effects during annealing and the morphology changes are analyzed in detail. Results of the phase transformation, the surface micro-morphology and the microstructured properties of these high-temperature coatings are presented.