An investigation into delamination mechanisms in inclined laser drilling of thermal barrier coated aerospace superalloys

Thermal barrier coatings are widely used in aircraft engine hot-end components to protect the component materials from direct exposure to the damaging high temperatures. Effusion cooling techniques are widely used in these situations and laser drilling is normally employed to produce holes of less than 1.5 mm diameter in tough aerospace alloys coated with heat-resistant ceramics. In order to provide the essential cooling effects, a large proportion of these holes must be drilled at acute angles to the surface. During this low-angle laser drilling, microstructural damage of the thermal barrier coating may occur; this is highly undesirable as propagation of the delamination may occur in service, leading to premature failure of the coating. In this article, the role of melt ejection and the coaxial assist gas jet in low angle laser drilling of a thermal barrier coated substrate is studied using a finite volume method. The work identifies the significant role of melt ejection in the formation of cracks and delamination at the coating/substrate interface on the leading edge of a laser-drilled cooling hole. The numerical model shows that the melt particles are accelerated towards the leading edge by the coaxial assist gas jet, and as a result of molten metal flowing across the edge the damage through mechanical stresses and subsequent erosion is found to be more serious here than at the trailing edge. The higher shear stress found at the leading edge also implies higher viscous forces acting parallel to the free stream direction to overcome the bonding strength of the coating interface.