Abstract The fabrication of film cooling holes on components coated with thermal barrier is challenging because the physical and chemical properties of each layer are quite different. Electrochemical discharge machining (ECDM) is a promising machining method for producing the film cooling holes on such multi-layer materials. Observation of the discharges show different characteristics when machining ceramic-coated Ni-superalloys. In this investigation, the discharge current during machining of both the coating and the superalloy substrate were recorded and analyzed. Experimental results indicate that the electrochemical discharge exists only during the machining of the coating, while both electrochemical discharge and electrical discharge take place when machining the superalloy substrate. Further analysis of the experimental results revealed that electrical discharge is the main contributor to material removal on the superalloy substrate, and the electrochemical discharge causes a small secondary material removal on the machined surface, resulting in a reduction of surface roughness.