Degradation Mechanisms in Ceramic Abradable Coatings for Hot Section Turbine Components
Hot section abradable coatings play an integral role in reaching optimal efficiency of gas turbine engines by preventing unnecessary gas leakage through clearance control. As turbine material technologies advance, there is a push for the development of abradable coatings that can withstand more severe operating conditions and retain the optimum balance of abradability and durability. However, current understanding of abradable coating properties that promote a controlled removal of these highly brittle materials is missing. In this work, a theoretical analysis is first presented for expected abradable mechanisms detailing key properties and microstructural features that enable each mechanism. Next, experimental studies are completed to evaluate coating evolution in representative turbine operating conditions and to determine key factors affecting abradable behavior. Two current technology abradable coatings processed by air plasma spraying, dysprosia-stabilized zirconia (DySZ) with hexagonal boron nitride (hBN) and yttria-stabilized zirconia (YSZ) with nanozone features, have been studied using scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and optical vibrometry. A challenge in evaluating hot section abradable materials is that of testing in engine relevant conditions but in a significantly scaled-down, more controlled lab test. Appropriate testing methodologies were first established. Both as-processed and aged coatings were then tested in a representative macroscratch test to investigate the influence of different defects, their evolution with aging, and observed damage behavior. Results for YSZ-based coatings show highly brittle fracture that penetrates deep within the coating and also changes with aging. The DySZ, hBN coating showed a change in phase of the weakening phase, hexagonal BN, after processing and a significant increase in hardness with aging. Yet, these coatings showed the ability to maintain a shallow damage zone even after aging with little damage propagation further into the remaining coating material. This suggests that the presence of a secondary phase plays a significant role in maintaining a more controlled and consistent damage mechanism. The results of this study, in combination with a study of deformation seen in engine hardware, can be used to build further predictive models of abradable damage accommodation behavior.