EXPERIMENTAL CHARACTERIZATION OF THERMAL BARRIER COATINGS USING MICRO-SCALE BENDING TECHNIQUES
Johns Hopkins University
Layered thermal barrier coating (TBC) systems incorporate disparate materials, including a yittra-stablized zirconia (YSZ) top coat, a thermally grown oxide (TGO), and a bond coat, all of which shield the superalloy substrate from high temperature, corrosive and oxidizing environments. There are numerous ways to spall a TBC, but in general top coat spallation is driven by the release of strain energy, which is related to the elastic modulus and the stresses that arise from the difference in thermal expansion coefficients in different layers, TGO growth and top coat sintering. The goal of this study was to measure the top coat modulus to facilitate evaluate of the strain energy in the TBC systems. Micro-beam bending, resonance frequency and curvature techniques were developed and employed to experimentally measure the Young’s modulus of YSZ top coat manufactured via electron physical beam vapor deposition (EBPVD) and air plasma spray (APS), in both attached and freestanding conditions. The EBPVD top coats were obtained from burner rig bars and a commercial turbine vane with a more complex shape and the top coat modulus was determined as a function of substrate geometry, coating thickness, thermal exposure, and calcium-magnesium-alumina-silicate (CMAS) infiltration. The moduli of top coats deposited on a burner rig bar were measured to be approximately 30 GPa when loaded in tension and 50 GPa in compression. The modulus of freestanding samples was shown to vary as a function of position; the modulus of the whole top coat was measured to be 55-60 GPa, while the inner third of the coating that is closest to the bond coat was measured to be 87 GPa. The modulus of top coats infiltrated by CMAS were much higher (~190 GPa) and thermally exposed top coats had moduli that were much lower (7-10 GPa in tension and 15 to 18 GPa in compression). The TBC on the turbine vane had a modulus that varied with position and ranged from 13-20 GPa in tension and 20-60 GPa in compression. In summary, the modulus of EBPVD 7YSZ top coats has been measured and shown to depend on its unique columnar microstructure. The tension and compression asymmetry originates from the gaps between columns that result from the EBPVD process. The variation of the top coat modulus on the commercial turbine vane is caused by the fact that convex surfaces lead to a more open microstructure, whereas the concave surfaces close the intercolumnar spacing. The local sintering that occurs during thermal exposure further enlarges these gaps and reduces the global modulus. The infiltration of CMAS filled the gaps, leading to a denser and more rigid top coat, whereas thermal cycling accentuated vertical cracking and separation.
Thermal barrier coating, Micro-bending