A study on failure of double-layer thermal barrier coatings subjected to uniaxial compression tests using acoustic emission analysis and digital image correlation

Abstract

The failure behavior and fracture process of double-layer thermal barrier coatings under uniaxial compressive substrate loading has been investigated. Coating systems containing GZO with low and high porosity (LP, HP) were fabricated to examine the influence of microstructure on failure behavior and strain energy. An YSZ-HP single-layer system serves as a reference. All ceramic coatings were deposited via atmospheric plasma spraying (APS) on cylindrical rods and turbine blade-shaped specimens made from CoNiCrAlY (LCO-22) coated, nickel-based, single crystal superalloy (PWA 1483). Prior to compression tests, isothermal pre-oxidation at 1050 °C and dwell-times of 100, 500 and 1.500 hours, as well as cyclic annealing tests between 50 and 1050 °C up to 500 cycles were performed, to study the effects of thermal ageing on strain energy to failure. In-situ acoustic emission (AE) measurements provides quantitative information about the failure processes under compressive substrate loading. A stereo camera system monitors the three-dimensional displacements and the surface fracture processes. For as-sprayed coatings, strain to failure of the investigated GZO/YSZ systems is comparable to the referenced single-layer TBC. AE analysis indicates coating failure at earlier stages and less substrate loads after thermal ageing with increasing dwell-time. Consequently, the pre-oxidation leads to reduced strain to failure values in all investigated coating systems. Digital image correlations (DIC) suggests that the failure behavior of as-sprayed GZO/YSZ coatings is similar to the referenced YSZ system. However, a different behavior was observed for pre-oxidized coatings, where cracking and spallation of GZO occurs predominantly.