Study of Delamination Induced by Laser-Drilling of Thermally-Sprayed TBC Interfaces

For the aero-engines combustion chambers with thermal barrier coating (TBC), the performance of the part is improved by laser drilling of highly numerous cooling holes with inclined angles. In case of plasma-sprayed TBC, fine holes with specific and reproducible conditions could be achieved throughout the YSZ/MCrAlY/superalloy TBC. During the laser drilling process, especially in the percussion mode with acute angle, a large crack at the interface between the bond coat and the ceramic can occur. Thus, each hole could be edged with a delaminated area underneath the ceramic top-coat. Furthermore, in case of multi-holes patterns, it could lead to the detrimental spallation of a large area of the ceramic. As the crack path seems to follow the rough profile of the MCrAlY bond coat, the present work is based on the study of the modification of the morphology of the ceramic/metal interface and the influence of this latter on the cracks initiation and propagation. To that end, laser drilling experiments have been performed with a millisecond YAG laser with several interrupted conditions (1 to 4 pulses) at a 20° incidence angle involving various laser powers. Samples with smooth and rough MCrAlY bond coats were achieved through the selection of different MCrAlY powders and various thicknesses of both bond coat and ceramic layers. The various TBC’s were characterized using SEM (Scanning Electron Microscope ) with back-scattering mode on cross-sectionned holes after laser-drilling. EDS microprobe analyses and mapping were also done to investigate the distribution of the oxides near the interface. Preliminary work was carried out to compare the 3D morpphology of the interface before cracking (3D white light profilometer) and after laser-drilling (3D X-ray laminography). The results were analyzed to discuss the influence of the “design” of TBC’s interface properties on the delamination for various laser drilling strategies.