Applications on High-Energy X-Rays to Stress Measurements of Thermal Barrier Coatings

Abstract

High-energy X-rays from a synchrotron radiation source, SPring-8, were applied to the stress measurements of thermal barrier coating (TBC). The specimen had a zirconia top coat on a bond coat of NiCoCrAlY sprayed on the substrate of Ni-base super alloys. The stress in the bond coat was measured through the top coat using the diffraction of Ni3Al 311 by high-energy X-rays with an energy of 72 keV. The sin2ψ method was used to determine the stress value. A specially designed furnace with a wide beryllium window was developed to conduct in-situ measurements of the internal stress in the bond coating at the room temperature, 773, 1073, and 1373 K. The internal stress was tensile at the room temperature, and decreased with increasing temperature. At 1073K or higher, the internal stress in the bond coat was released due to softening of the bond coat. The normal stress perpendicular to the coating surface of TBC was evaluated by a new hybrid method. Since the penetration depth of low-energy X-ray is very small around a few micrometers for zirconia, the stress value measured by the sin2ψ method is the in-plane stress, σ1, and the stress perpendicular to the surface was zero. On the other hand, the penetration depth of high-energy X-rays is very deep, so the measured stress value will be the in-plane stress minus the out-of-plane stress, i.e. σ1−σ3. The normal stress perpendicular to the surface, σ3, i.e. the spalling stress, was estimated from these two measurements. The specimens were exposed in air atmosphere at 1373 K for 500, 1000, and 2000 h. The distribution of the spalling stress in the top coat was estimated by the hybrid method. The spalling stress near the interface to the bond coat changed to a large tension after long-time exposure.