Investigation on Conductive Layer, Delamination, and Recast Layer Characteristics of Electro-discharge Machined Holes in TBCs

Abstract

In this study, electrical discharge machining (EDM) is used to directly drill normal effusion cooling holes in thermal-barrier-coated nickel-based superalloys (TBCs) via the assisting electrode method. The formation of the conductive layer was studied using scanning electron microscopy and energy-dispersive spectroscopy. The effects of the EDM process parameters including peak current, pulse duration, and duty cycle on delamination and recast layer characteristics were investigated. The analysis results indicate that the conductive layer possesses a feature of bilayer structure for the EDM of TBCs. The bottom layer is generated first due to the deposition of carbon-based products and molten brass debris, and its composition primarily contains C, Cu, Zr, and Zn; the surface layer is the result of the overlying of subsequently molten superalloy debris and carbon-based products, and its composition primarily consists of Ni, C, Cr, Nb, Co, Al, Fe, Cu, and Zr. The microcracks of the superalloy substrate only reside in the recast layer during the EDM of TBCs. The thickness of recast layer sharply increases with increasing peak current, pulse duration, and duty cycle, respectively. The delamination occurs at the ceramic coating/bond coating interface for the EDM of drilling normal holes in TBCs, and it can be eliminated by the selection of low discharge energy and appropriate duty cycle. Additionally, the length of delamination increases with increasing peak current, pulse duration, and duty cycle, respectively. The spalling of ceramic coating appears at the entrance of the hole due to the thermal-shock brittle fracture if excessive peak current, pulse duration, or duty cycle is selected.