Experimental measurements of thermal barrier coating interfacial fracture toughness as a function of mode-mix
Lockyer-Bratton, Simon J.
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Mechanism-based lifetime assessment models of thermal barrier coating (TBC) systems for gas turbine engines rely on accurate knowledge of the experimentally measured interfacial fracture toughness over a range of mode mix and especially at mode-II. Previously no reliable test method had been employed to evaluate these properties under pure mode-II conditions, which are most representative of critical TBC spall delamination upon turbine engine cool down. A newly developed compression edge-delamination (CED) test, based off of theoretical considerations by John Hutchinson, has been employed to measure the strain energy release rate (Gc) associated with delamination between the bond coat and top coat layers under a nearly pure mode-II loading condition. Utilizing modified 4-point bend experiments and the CED methodology, has allowed for direct measurement of coating interfacial toughness as a function of mode mix. The material system examined was provided by collaborators at GE and consists of an Electron-Beam Physical Vapor Deposited (EBPVD) 7% Yttria-Stabilized Zirconia (YSZ) top coat, which is deposited on a Pt-modified diffusion aluminide β-(Ni,Pt)Al bond coat on a single crystal René N5 substrate. Using the CED test, a 50% reduction in mode-II interfacial toughness was associated with thermal cycling. Results for as-deposited samples tested using a modified 4-point bend technique matches previously reported data, and a mode-mix dependent toughness function for the as-deposited interfacial toughness has also been discovered using results from this study and from previously reported values. Specimen design and preparation and the use of starter cracks to assure proper delamination are discussed. Crack face friction is shown to play a significant role in calculation of the interfacial toughness and details regarding the experimental characterization of the interfacial friction coefficient and implementation into the finite element model used to extract the interfacial toughness are examined. Details regarding the use of Digital Image Correlation (DIC) to calculate the critical stress for crack growth in the CED test are also be discussed. Results from the both the CED and the modified 4-point bend experiments are examined and analyzed along with microstructural and chemical observations of degradation of the coating interfaces as a result of thermal cycling.