Investigation on surface morphology and phase transition characteristics in EDM for 8YSZ TBC on Inconel 718 superalloy

The hole machining of 8 wt% Y₂O₃-stabilized ZrO₂ (8YSZ) thermal barrier coating (TBC) on Inconel 718 superalloy was carried out by electrical discharge machining (EDM) technology. The experimental phenomena, the element composition, and the phase transitions were analyzed by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The observation of microscopic morphology and EDS analysis for the machined 8YSZ surface showed that there were four kinds of debris particles deposited: molten brass particle, the vaporized brass particle, molten 8YSZ particle, and the vaporized 8YSZ particle. The debris particles are produced by physical deposition and chemical adsorption of molten and vaporized electrode and workpiece materials. In addition, the chemical reactions in the discharge gap were analyzed. The analysis results indicated that not only the adsorption of pyrolytic carbon but also the deposition of brass and the chemistry of 8YSZ promote the formation of the conductive layer on the 8YSZ TBC. Furthermore, the effects of the phase transitions on machining quality were studied. The XRD analysis illustrated that 8YSZ existed on the machined surface in cubic and tetragonal phases. The brittle fracture and delamination occur on the surface of thermal-barrier-coated superalloys (TBCs) by thermoelastic stress and transformation stress of phase. Finally, based on the characteristics of each processing stage and the phase transitions occur at 8YSZ coating, the 8 forming stages of conductive layer during EDM drilling process for TBCs were proposed.