Synchrotron X-Ray Diffraction Measurements Mapping Internal Strains of Thermal Barrier Coatings During Thermal Gradient Mechanical Fatigue Loading

Abstract

An understanding of the high temperature mechanics experienced in thermal barrier coatings (TBC) during cycling conditions would be highly beneficial to extending the lifespan of the coatings. This study will present results obtained using synchrotron X-rays to measure depth resolved strains in the various layers of TBCs under thermal mechanical loading and a superposed thermal gradient. Tubular specimens, coated with yttria stabilized zirconia (YSZ) and an aluminum containing nickel alloy as a bond coat both through electron beam-physical vapor deposition (EB-PVD), were subjected to external heating and controlled internal cooling generating a thermal gradient across the specimen's wall. Temperatures at the external surface were in excess of 1000 degrees C. Throughout high temperature testing, 2D high-resolution XRD strain measurements are taken at various locations through the entire depth of the coating layers. Across the YSZ, a strain gradient was observed showing higher compressive strain at the interface to the bond coat than toward the surface. This behavior can be attributed to the specific microstructure of the EB-PVD-coating, which reveals higher porosity at the outer surface than at the interface to the bond coat, resulting in a lower in plane modulus near the surface. This location at the interface displays the most significant variation due to applied load at room temperature with this effect diminishing at elevated uniform temperatures. During thermal cycling with a thermal gradient and mechanical loading, the bond coat strain moves from a highly tensile state at room temperature to an initially compressive state at high temperature before relaxing to zero during the high temperature hold. The results of these experiments give insight into previously unseen material behavior at high temperature, which can be used to develop an increased understanding of various failure modes and their causes.