Laser surface modification of plasma sprayed yttria stabilised zirconia coatings on type 316L stainless steel

Abstract

Plasma sprayed yttria stabilised zirconia/NiCrAlY thermal barrier coatings proposed for molten LiCl–KCl salt environments contain pores and microcracks that may cause corrosion of the substrate on prolonged exposure. In the present work the as sprayed samples were laser remelted with various scan speeds of 1, 2·5 and 5 mm s⁻¹ to reduce the segmented cracks. Microstructural inhomogeneities like pores and voids have been eliminated on laser remelting, however, segmented cracks were formed irrespective of scan speed. Distinct interface separating fine and coarse grains were observed at all scan speeds. The microhardness of the glazed surface improved and surface roughness was reduced. The beneficial non-transformable tetragonal phase was formed after laser remelting while the as sprayed coating consisted of insignificant monoclinic phase. An attempt was made to seal the pores of the zirconia coating by applying ZrO₂ and ZrO₂ + SiO₂ powder.