Carla Meid

Synchrotron X-ray measurement techniques for thermal barrier coated cylindrical samples under thermal gradients.

Measurement techniques to obtain accurate in situ synchrotron strain measurements of thermal barrier coating systems (TBCs) applied to hollow cylindrical specimens are presented in this work. The Electron Beam Physical Vapor Deposition coated specimens with internal cooling were designed to achieve realistic temperature gradients over the TBC coated material such as that occurring in the turbine blades of aeroengines. Effects of the circular cross section on the x-ray diffraction (XRD) measurements in the various layers, including the thermally grown oxide, are investigated using high-energy synchrotron x-rays. Multiple approaches for beam penetration including collection, tangential, and normal to the layers, along with variations in collection parameters are compared for their ability to attain high-resolution XRD data from the internal layers. This study displays the ability to monitor in situ, the response of the internal layers within the TBC, while implementing a thermal gradient across the thickness of the coated sample. The thermal setup maintained coating surface temperatures in the range of operating conditions, while monitoring the substrate cooling, for a controlled thermal gradient. Through variation in measurement location and beam parameters, sufficient intensities are obtained from the internal layers which can be used for depth resolved strain measurements. Results are used to establish the various techniques for obtaining XRD measurements through multi-layered coating systems and their outcomes will pave the way towards goals in achieving realistic in situ testing of these coatings.

Strain response of thermal barrier coatings captured under extreme engine environments through synchrotron X-ray diffraction

The mechanical behaviour of thermal barrier coatings in operation holds the key to understanding durability of jet engine turbine blades. Here we report the results from experiments that monitor strains in the layers of a coating subjected to thermal gradients and mechanical loads representing extreme engine environments. Hollow cylindrical specimens, with electron beam physical vapour deposited coatings, were tested with internal cooling and external heating under various controlled conditions. High-energy synchrotron X-ray measurements captured the in situ strain response through the depth of each layer, revealing the link between these conditions and the evolution of local strains. Results of this study demonstrate that variations in these conditions create corresponding trends in depth-resolved strains with the largest effects displayed at or near the interface with the bond coat. With larger temperature drops across the coating, significant strain gradients are seen, which can contribute to failure modes occurring within the layer adjacent to the interface.

Comparison of Thermal Barrier Coating Stresses via High Energy X-Rays and Piezospectroscopy

Thermal Barrier Coatings (TBC) have been instrumental in advancing the performance and e�ciency of turbine engines over the last decades. The use of high temperature ce- ramics has allowed increased temperatures by way of protecting the load bearing blade substrate and extending its lifetime. Today there continues to exist the need to under- stand the behavior of the TBC to extend the life and performance of both the TBC and the underlying substrate blades. In this study, the TBC was examined by the use of optical spectroscopy and synchrotron X-Ray di�raction to understand the strain and stress expe- rienced by each of the layers in the coating. Raman and Photoluminescence spectroscopy were employed to examine the thermally grown oxide layer (TGO) and the ceramic top coat and to identify the in uence of variations in temperature distribution. X-Ray di�rac- tion measurements were conducted at the Advanced Photon Source, at Argonne National Laboratory allowing the in-situ investigation of variation in loading conditions including a representative ight cycle. A pre-aged specimen was used for di�raction measurements for a more mature oxide layer. Optical spectroscopy measurements provided high resolution stress maps of the oxide scale. The results from this study provide a more complete un- derstanding as to the behavior of the TBC and its development through the lifetime, and can serve to validate and further the development numerical models.

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

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 specimens 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.

Synchrotron XRD Measurements Mapping Internal Strains of Thermal Barrier Coatings During Thermal Gradient Mechanical Fatigue Loading

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 (EBPVD), 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 °C. Throughout high temperature testing, 2-D 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 towards 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.Copyright

Synchrotron XRD Measurements of Thermal Barrier Coatings Subjected to Loads Representing Operational Conditions of Rotating Gas Turbine Blades

High-energy synchrotron x-rays were used in this work to monitor the internal strain behavior of Thermal Barrier Coatings (TBC) under thermal gradient and mechanical loading. Tubular specimens made from Nickel based super alloy with a TBC-system applied onto the outer surface by Electron Beam Physical Vapor Deposition were used to allow for cooling of the internal surface of the substrate while heating the external surface during thermal mechanical cycling. The coating system consisted of a Yittria Stabilized Zirconia (YSZ) top coat and a MCrAlY bond coat. Through transmission along with a 2D detector allowed for the 2D strain monitoring of each layer during high temperature operation. Monitoring the micro-strain of each phase within the layers provides insight into their high temperature behavior which can be used to further develop predictive models including evolution of elastic strain as well as creep and plasticity. Obtained results have shown a large variation in strains during ramp up of in-phase thermal and/or mechanical load with a significant tensile strain mismatch between the two prominent phases of the bond coat. The YSZ has displayed residual compressive strains at the bond coat/YSZ interface and maintained some compressive residual strain during high temperature holds. These results give valuable insight into the mechanics of these complex systems under various high temperature conditions.