The influence of surface roughness of sprayed zirconia coatings on laser treatment

The influence of surface roughness of plasma sprayed zirconia coatings on post-laser treatment was investigated. To minimize the influence from the non-uniform laser beams obtained through the usual optics on the laser treatment of coatings, a conventional YAG laser gun was modified, and laser beams with a uniform intensity were obtained by installing the kaleidoscope. The melted depth of the polished coatings was approximately half that of as-received coatings even when treated at the same power density (38.4 W/mm2) and this difference of melted depth between the as-received coatings and polished coatings increased with the power density (47.9 W/mm2). In the case of as-sprayed coatings with a rough surface, uneven and roughly treated areas were sharply increased in accordance with the power density. As-received coatings with surface roughness (Ra, 4.7 μm) showed an increased diffuse reflectance compared with polished coatings (Ra, 0.27 μm), and the difference in reflectance between the as-sprayed and polished coatings was approximately 10% near the wavelength of the Nd/YAG laser (1016 nm). In this experiment, the appearance of the monoclinic phase, one of the important indicators determining the stability of TBC coating, was not observed and non-transformable t′ phase was still predominant after laser treatment.