Removing hole blockages from thermal barrier coatings using low-pressure abrasive water jet

The micro- and small-sized cooling holes of turbine blades must be polished to remove the thermal barrier coating (TBC) blockage before service. To date, no effective methods have been developed to remove this blockage, except for manual polishing using grinding needles. In this study, the removal of hole blockages from a superalloy material is investigated using a low-pressure abrasive water jet (AWJ) process for the first time. Based on an analysis of the processing mechanism, the kinetic energy threshold for a particle-to-crack TBC is approximate 3.55 nJ. Garnet particles with size of 10 μm and speed of ≥ 60 m/s can theoretically damage the TBC material. For vertical holes, the hole-blocked material near the entrance can be eroded rapidly. For inclined holes, the hole-blocked material at the trailing edge has a much greater volume and length and requires a longer time to remove. Using relatively larger jet size increases the material removal rate of the blockage, however, resulting in a broader hole entrance. When applying a relatively larger standoff distance, the capability of the AWJ in removing the blockage was enhanced owing to the more dispersed distribution of abrasives in the jet. The blockage ratio can be reduced from 60% to approximately 25% after 90 s of AWJ processing. Furthermore, the impingement of abrasives can remove the recast layer from the hole wall caused by electrical discharge drilling prior to coating.