High-performance processing for film cooling holes on EB-PVD TBC-coated superalloys utilizing assisted electrode electrochemical discharge machining

Abstract

With the escalating operating temperatures of turbine blades, single film cooling technique is inadequate, and high-temperature-resistant thermal barriers coatings (TBCs) prepared through electron beam physical vapor deposition (EB-PVD) are extensively adopted. However, due to the distinct physical attributes of each individual layer and the rigorous requirements for no recast layer and high productivity, the fabrication of film cooling holes on EB-PVD TBC-coated superalloys remains challenging. In this study, a novel assisted electrode electrochemical discharge machining approach has been proposed, during which the 8 wt% yttria stabilized zirconia (8YSZ) ceramic topcoat is processed within deionized water rather than hydrocarbon-based dielectric, while the NiCrAlY bond coat and Ni-based superalloy substrate are treated within a low conductivity solution. The 8YSZ ceramic topcoat removal mechanisms of evaporation, melting, thermal spalling, and mechanical erosion were evaluated with morphology analysis. The negligible damage to the 8YSZ ceramic topcoat during processing was revealed through subsurface structure investigation. Then, the voltage/current waveforms and processing phenomena were recorded to examine the processing characteristics of distinct layers. In addition, comparative experiments were conducted to assess the processing performance. The results suggest that compared to assisted electrode electrical discharge machining, assisted electrode electrochemical discharge machining achieves a ten-fold enhancement in material removal rate, but a 58.4 % reduction in tool wear ratio. The processed surface also exhibits superior integrity, evident from the absence of conductive layer on the 8YSZ ceramic topcoat or recast layer on the Ni-based superalloy. Hence, assisted electrode electrochemical discharge machining shows promising application prospects for high-performance processing of film cooling holes on EB-PVD TBC-coated superalloys.