Laser drilling of cooling holes through plasma sprayed thermal barrier coatings

Laser drilling is a non-contact process that can be used to form small holes in a wide variety of materials with a high degree of precision and reproducibility. The advantages of being able to drill difficult materials, specifically superalloys and ceramics, are exploited in the laser drilling of cooling holes in thermal barrier coated superalloys. Plasma sprayed thermal barrier coatings (TBCs) consist of a partially stabilised zirconia top coat and a NiCrAlY bond coat, deposited on a Ni-superalloy substrate. Laser drilling was performed using a pulsed Nd:YAG laser (λ=1.06 μm), with a focussed spot power density of approximately 0.2 MW mm−2. Holes were drilled at both normal and inclined angles of incidence. The effect of drilling from the side of the metallic substrate, rather than the ceramic top coat, has been investigated. Laser-induced microstructural features, such as interfacial cracking, are reported. For the conditions used here, assist gas has a negligible effect on the volume of material removed per pulse, but does create additional features in the resolidified ceramic surrounding the hole. A spontaneous debonding technique was used, in conjunction with a finite difference process model, to investigate the effect of laser drilling on the interfacial toughness of plasma sprayed TBCs. Drilling was found to increase slightly the interfacial toughness of the TBC. This is attributed to the crack arresting effect of a resolidified ceramic layer that partially lines the hole. [Copyright &y& Elsevier]