Pure waterjet controlled depth machining for stripping ceramic thermal barrier coatings on turbine blades

Gas turbine blades are high-performance composite parts made of nickel-based superalloys coated with a bilayer coating system for thermal protection. This coating system consists of a metallic bond coat and a ceramic thermal barrier coating (TBC), typically made of yttria-stabilized zirconia. During gas turbine operation the coating system undergoes degeneration and needs to be renewed periodically. Conventional overhaul processes remove the TBC and bond coat completely in several machining steps consisting of mechanical grid blasting and chemical stripping processes. Currently selective removal of TBC without influencing the bond coat is not applicable. Pure waterjet (PWJ) controlled depth machining promises to be one of the most flexible non-conventional structuring techniques for TBC materials. Selective TBC stripping promises the flexibility of either complete or locally restricted TBC removal without damaging or contaminating the bond coat. For economic industrial use of this environmental-friendly process a complete Computer-aided (CAx) toolchain comprising Computer-aided drawing (CAD), Computer-aided manufacturing (CAM) and complementary process analysis is needed. Presently no suitable CAx toolchain supports this innovative PWJ process for defined layer machining of composite parts. This paper shows the integration of PWJ controlled depth machining process within CAx toolchain in Siemens NX for selective stripping TBC on complex free-form surfaces. The strong dependence of the machining results on energy input per unit length leads to challenges for synchronization of parametrization and machine dynamics. The evaluation of different machining strategies and machining head configurations resulted in an increasing process performance and economic efficiency of this developed PWJ process.