Durable Abrasive Tip Design for Single Crystal Turbine Blades

In order to create and maintain peak efficiency in turbine stages, it is useful to minimize hot air leakage over blade tips.  For unshrouded blades, this means creating minimal clearance over the airfoil tip with rub-tolerate zero-gap materials systems.  In this study, a refractory abrasive tip material was developed and applied to a plain turbine blade tip to work in conjunction with an abradable coating on the tip-shoe.  Process development efforts resulted in abrasive material that could be as much as 2mm thick.  Further, the abrasive media included imbedded grits that would remain in back-up until such time as they were exposed and were pressed into service.  Application methods of both furnace brazing and induction brazing were explored as were pre and post consolidation net shaping.  Evaluation testing included compatibility for the application process onto single-crystal blades.  Performance testing included rub-rig testing and long-term oxidation testing.  Service testing included commercial engine operation for 14,000 hours was followed by metallurgical re-evaluation.  Service performance was mechanically successful, although some material transfer from the abradable tip-shoe was observed which decreased the abrasive nature of the tip.  Metallurgically, some intergranular oxidation was observed, but the grits themselves were well retained and were sufficiently refractory to avoid microstructural or oxidation degradation.