Surface Investigation of Laser Glazed Mullite Thin Films on Yttria-Stabilized Zirconia Coatings

Abstract

Thermal barrier coatings (TBCs) are used in the cooled parts of gas turbines to reduce the temperature of blades and hot engine components, allowing higher operating temperatures and yielding increased efficiency. The corrosive gases which come from combustion of low grade fuels can penetrate into the TBCs and reach the metallic components and bond coat and cause hot corrosion and erosion damage. Mullite thin films have been applied onto the top layer of yttria-stabilized zirconia (YSZ) coatings by sol-gel dipping process in order to decrease the porosity as well as improve thermal stability of the coatings. Glazing the top layer by laser beam is an advanced approach to seal TBCs surface. The laser beam has the advantage of forming a dense thin layer composed of micrograins. CO2 laser beam assisted in the densification of the surface by remelting a thin layer of the exposed surface. The laser glazing converted the rough surface of TBCs into smooth micron-sized grains with size of 1-4 microns. Mullite compounds have remained on the surface after laser treatment since alumina and silica are still present, as indicated in EDX spectra. The results revealed that the roughness increases as the grain size decreases.