Micro-drilling in ceramic-coated Ni-superalloy by electrochemical discharge machining

Thermal barrier coating has been applied in turbine blades and vanes to improve the combustion temperature and thus the thermal efficiency. However, the fabrication of film cooling hole in the components with thermal barrier coating is challenging because the physical and chemical properties of each layer are quite different. Electrochemical discharge machining (ECDM) is a promising machining method for the fabrication of film cooling hole in these multi-layer materials. The gas film formed around the electrode, which acts as the dielectric during the machining, makes it possible for the ECDM to machine both electrical conductive and non-conductive materials. In this study, the current waveforms for the machining of both the coating and the superalloy substrate were investigated. It showed that only the electrochemical discharge exists during the machining of the coating. However, when machining the superalloy substrate, both the electrochemical discharge and the electrical discharge take place. Analysis of the experimental results revealed that the electrical discharge is the main factor to make the material on the superalloy substrate remove, while the electrochemical discharge results in a small secondary material removal on the machined surface. The surface roughness on the hole sidewall for the coating and the superalloy substrate were measured to be Ra 10.2 μm and Ra 5.6 μm, respectively. The thickness of recast layer on the machined surface of the superalloy substrate is 15 μm.