Abrasive water jet polishing of TBC blocked inclination holes on superalloys using an optimized nozzle

The inclined cooling holes on turbine blades needs to be polished to remove thermal barrier coating (TBC) blockage before service. So far, no effective methods have been developed to remove this blockage except manual polishing with grinding needles. In this study, polishing of inclined small holes with TBC blockage on GH4169 superalloys was investigated using a low-pressure abrasive water jet with an optimized nozzle. The results show that the abrasive distribution inside the jet resulting from the five types of commonly used nozzles presented Gaussian-like distribution. The nozzle with 14° convergence angle and 1.5 mm straight section was selected for the subsequent polishing experiment for the consideration of the abrasives divergence and boundary. The hole entrance was shaped as a cone after polishing, and the entrance diameter increased with processing time, standoff distance (SOD) and abrasive concentration. The hole-blocked material at the entrance was cleaned rapidly, while those at the trailing edge was removed slowly because of the greater length of blockage. Increasing the abrasive concentration achieved higher blockage removal rate than increasing the SOD. For the 30° inclined holes with a 200 μm thick TBC, approximate 60% of the hole-blocked material can be removed after polishing time of 90 s. The recast layer on the hole wall can be completely cleaned simultaneously owing to electric discharge drilling before coating.