Combustion liner having cooling holes with stepped upstream sidewall

A method for drilling holes in a combustion liner, the method comprising:

• providing a base-alloy substrate having a bottom surface and a top surface;
• depositing a thermal barrier coat (TBC) metallic layer on the top surface of the base-alloy structure;
• depositing a TBC ceramic layer on the TBC metallic layer, the TBC ceramic layer having a top surface configured to be exposed to combustion gases during operation within a gas turbine engine;
• and
• drilling a plurality of cooling holes formed through the combustion liner, each extending from an entrance aperture at the bottom surface of the base-alloy structure and an exit aperture at the top surface of the TBC ceramic layer, each of the plurality of cooling holes formed at oblique angles to the top surface of the TBC ceramic layer, thereby forming an oval-characteristic exit aperture at the top surface of the TBC ceramic layer, each of the oval-characteristic exit apertures defining a long axis and a short axis, the long axis extending between an upstream side and a downstream side and the short axis extending between opposite lateral sides, thereby defining an upstream sidewall and a downstream sidewall in each of the base-alloy substrate, the TBC metallic layer, and the TBC ceramic layer,
• wherein each of the cooling holes has a upstream sidewall profile at the upstream sidewall, wherein a first distance between the upstream sidewall and the downstream sidewall of the TBC ceramic layer is greater than a second distance between the upstream sidewall and the downstream sidewall of the TBC metallic layer.