Pulsed laser processing of plasma sprayed thermal barrier coatings

The requirements for thermal barrier coatings on turbine blades in jet engines and gas turbines are stringent due to the extreme temperatures and constant thermal cycling that demand a material with excellent thermal shock resistant and thermal insulation properties. Plasma sprayed zirconia alloys have been found to fit this role with some degrees of success. However, the presence of surface pores and a network of inter-connecting micropores within the coatings accentuates the oxidation of the bond coat, often leading to spallation of the ceramic coat. A pulsed mode Nd:YAG laser was used in the present study to process plasma sprayed zirconia alloy coatings. The results indicated the effective melting of the ceramic layer that yielded a shiny surface finish. Scanning electron microscopy observation of the fractured surfaces revealed distinct zones comprised of columnar grains, granular structures similar to that of sintered ceramics and the typical lamellar structure of plasma sprayed ceramic coatings. The crack area and depressions formed were quantified using an image and analyser. The results show that the average crack area in the treated surface increases with increasing laser energy density. However, the average area of depressions in the treated surface appears to be independent of the laser energy density input.