The effect of gas jet and melt flow on laser drilling of thermal barrier coated nickel alloys

Thermal Barrier Coating (TBC) is widely used in aircraft engine hot-ended components to protect the engine components from direct exposure to corrosive high temperatures. Cooling holes of less than 1.5mm diameter in tough heat resistant aerospace alloys, coated with high heat resistant ceramics is normally laser drilled for effusion cooling. A large proportion of these holes are drilled at acute angles from the surface in order to provide essential cooling effects. When low-angle laser drilling these TBC components, laser drilling-induced micro structural damage of the thermal barrier coating occurs. This is undesirable as the propagation of the delamination may occur in service, leading to failure of the coating. In this paper the role of melt ejection and coaxial assist gas jet on laser drilling of TBC substrate is studied by using a numerical analysis based on finite volume simulation. The work has identified a significant role of meltejection on the formation of cracks and delamination at the coating/substrate interface on the leading edge. Findings with the model showed that the melt particles were accelerated towards the leading edge by the coaxial assist gas jet. As a result of molten metal flowing across the leading edge, the damage through mechanical stresses and subsequent erosion was found more serious on the leading edge than the trailing edge. Higher shear stress implies higher viscous forces acting parallel to the free stream direction and may overcome the bonding strength of the coating interface.