Laser-drilling through plasma-sprayed thermal barrier coatings on nickel based superalloys

Aero-engine manufacturers are continuously faced with the demand for greater engine efficiency and as a result, ever-increasing combustion temperatures. To allow the components to operate inside the harsh, high temperature environments, a combination of small cooling hole technology and thermal barrier coatings (TBCs) are often used. Laser drilling has long been an established method for producing the cooling holes, offering benefits over other non-conventional drilling methods in terms of drilling speed and consumable costs. However, the presence of the TBC, plasma sprayed onto Ni based components will have an effect on the laser drilling of the component. Plasma-sprayed zirconia coatings are also prone to delamination when exposed to thermal gradients and subsequently, using lasers to drill through the coatings may induce spallation of the coating. In the current work laser drilling, with a Nd-YAG laser of Nickel based substrates, coated with a CoNiCrAlY metallic bondcoat and an 8% Yttria-Stabilised Zirconia ceramic TBC thermal barrier coating has been conducted. Key variables investigated are incident angle of the holes and the thickness of bondcoat. It has been found that both variables have an influence on the amount of coating delamination observed at each interface within the coating.