Capturing the Competing Influence of Thermal and Mechanical Loads on the Strain of Turbine Blade Coatings via High Energy X-rays

This paper presents findings of synchrotron diffraction measurements on tubular specimens with a thermal barrier coating (TBC) system applied by electron beam physical vapor deposition (EB-PVD), having a thermally grown oxide (TGO) layer due to aging in hot air. The diffraction measurements were in situ while applying a thermal cycle with high temperature holds at 1000 °C and varying internal air cooling mass flow and mechanical load. It was observed that, during high temperature holds at 1000 °C, the TGO strain approached zero if no mechanical load or internal cooling was applied. When applying a mechanical load, the TGO in-plane strain (e22) changed to tensile and the out of plane TGO strain (e11) became compressive. The addition of internal cooling induced a thermal gradient, yielding a competing effect, driving the e22 strain to compressive and e11 strain to tensile. Quantifying TGO strain variations in response to competing factors will provide a path to controlling the TGO strain, and further improving the lifetime assessment and durability design strategies for TBC systems

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 under-standing 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

Anisotropic lattice expansion determined during flash sintering of BiFeO3 by in-situ energy-dispersive X-ray diffraction

BiFeO3 has a Curie temperature (TC) of 825 °C, making it difficult to sinter using conventional methods while maintaining the purity of the material, as unavoidably secondary phases appear at temperatures above Tc. Flash sintering is a relatively new technique that saves time and energy compared to other sintering methods. BiFeO3 was flash sintered at 500 °C to achieve 90% densification. In-situ energy dispersive X-ray diffraction (EDXRD) revealed that the material did not undergo any phase transformation, having been sintered well below the TC. Interestingly, anisotropic lattice expansion in the material was observed when the sample was exposed to the electric field.U.S. Office of Naval Research (ONR) N00014-10-1- 042, N00014-17-1-2087, Sub 4104-78982U.S. Department of Energy DE-AC02-06CH1135

Capturing The Competing Influence Of Thermal And Mechanical Loads On The Strain Of Turbine Blade Coatings Via High Energy X-Rays

This paper presents findings of synchrotron diffraction measurements on tubular specimens with a thermal barrier coating (TBC) system applied by electron beam physical vapor deposition (EB-PVD), having a thermally grown oxide (TGO) layer due to aging in hot air. The diffraction measurements were in situ while applying a thermal cycle with high temperature holds at 1000 °C and varying internal air cooling mass flow and mechanical load. It was observed that, during high temperature holds at 1000 °C, the TGO strain approached zero if no mechanical load or internal cooling was applied. When applying a mechanical load, the TGO in-plane strain (e22) changed to tensile and the out of plane TGO strain (e11) became compressive. The addition of internal cooling induced a thermal gradient, yielding a competing effect, driving the e22 strain to compressive and e11 strain to tensile. Quantifying TGO strain variations in response to competing factors will provide a path to controlling the TGO strain, and further improving the lifetime assessment and durability design strategies for TBC systems

Real-Time Phase Evolution Of Selective Laser Melted (Slm) Inconel 718 With Temperature Through Synchrotron X-Rays

Selective laser melting (SLM) is an additive manufacturing process that uses laser scanning to achieve melting and solidification of a metal powder bed. This process, when applied to develop high temperature material systems, holds great promise for more efficient manufacturing of turbine components that withstand extreme temperatures, heat fluxes, and high mechanical stresses associated with engine environments. These extreme operational conditions demand stringent tolerances and an understanding of the material evolution under thermal loading. This work presents a real-time approach to elucidating the evolution of precipitate phases in SLM Inconel 718 (IN718) under high temperatures using high-energy synchrotron x-ray diffraction. Four representative samples (taken along variable build height) were studied in room temperature conditions. Two samples were studied as-processed (samples 1 and 4) and two samples after different thermal treatments (samples 2 and 3). The as-processed samples were found to contain greater amounts of weakening phase, δ. Precipitation hardening of Sample 2 reduced the detectable volume of δ, while also promoting growth of γ″ in the γ matrix. Inversely, solution treatment of Sample 3 produced an overall decrease in precipitate phases. High-temperature, in-situ synchrotron scans during ramp-up, hold, and cool down of two different thermal cycles show the development of precipitate phases. Sample 1 was held at 870°C and subsequently ramped up to 1100°C, during which the high temperature instability of strengthening precipitate, γ″, was seen. γ″ dissolution occurred after 15 minutes at 870°C and was followed by an increase of δ-phase. Sample 4 was held at 800°C and exhibited growth of γ″ after 20 minutes at this temperature. These experiments use in-situ observations to understand the intrinsic thermal effect of the SLM process and the use of heat treatment to manipulate the phase composition of SLM IN718

Simulations Mapping Stress Evolution In High Temperature Ceramic Coatings Under Thermal-Mechanical Conditions

Finite element simulations representing thermal barrier coatings on turbine blades enabled mapping of the stress evolution within the multi-layer configuration under thermalmechanical conditions. The study aims to accurately model the transient strain behavior throughout a load cycle due to plasticity, creep, and oxide growth. The results were compared with in-situ experimental quantitative measurements performed previously using synchrotron X-ray diffraction. The studies verify the stress within the thermally grown oxide for critical combinations of temperature and load. These numerical models can be used to predict in-cycle stresses that lead to eventual failure of the coatings. © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

In-situ synchrotron X-ray strain measurements in TBC systems during thermal mechanical cycling

Thermal barrier coatings (TBC) are applied on cooled gas turbine components. Between the heated and cooled surfaces a thermal gradient develops, resulting for constraint components in high multiaxial stresses, which may exceed stresses due to mechanical loading. In order to investigate the damage behaviour of TBC systems under close to reality conditions, thermal mechanical fatigue tests with controlled thermal gradients (TGMF-tests) were performed on coated tubular specimens. The specimen substrate was made from directionally solidified nickel base superalloy IN100 DS, and the coating system comprised a NiCoCrAlY bond coat and a ceramic top coat from partially stabilized zirconia. In TGMF testing specific damages occurred underneath the adherent ceramic top coat, evolving into fatigue cracks, which propagated primarily in the metallic bond coat parallel to the surface. To explain the initiation and evolution of these fatigue cracks, the thermo-mechanical response of the bond coat and the thermally grown oxide (TGO) between bond coat and ceramic top coat is examined and quantified through finite element analyses. The models include non-linear and time-dependent behaviour such as creep, TGO growth stress, and thermo-mechanical cyclic loading. The simulations suggest that stress redistribution due to creep can lead to tensile stresses in the TGO during TGMF testing that are large enough to initiate the cracks investigated. In order to get experimental data of the strain in the different layers of the coating system during TGMF-loading a test device has been assembled allowing in-situ strain measurements in the entire TBC system by means of synchrotron X-rays. Data analyses so far have shown that all layers could be identified, for all layers it is possible to measure strain qualitatively, and the TGO showed a texture. Goal of the ongoing work is the calculation of the full strain tensor in each layer for the entire thermal mechanical load cycle

Synchrotron X-Rays Monitoring Nano-Aluminum Grain Growth Of A Metal Matrix Composite Under Thermo-Mechanical Conditions

The experimental in situ synchrotron study presented here examines the micro structural features of trimodal Aluminum Metal Matrix Composites (MMCs) that influence their mechanical properties. The thermomechanical environment under which these MMCs, consisting of coarse-grained and nanocrystalline aluminum as well as boron carbide reinforcement particles, are manufactured is captured through in situ highenergy X-ray characterization. Results of peak identification indicate the presence of dispersoids, Al4C3, FeAl6, and AlN, formed after cryomilling and analysis of the peaks provide quantitative volume fractions of these compounds in the range of 2.6%, 1.9% and 1.2% respectively. The full width half maximum (FWHM) values of the X-ray diffraction data, collected over time from the sample, were analyzed to establish the change in nano-Aluminum grain size as a function of temperature and applied compressive load. Results monitor the rate of growth in nano-aluminum grain size with an overall increase of 101nm under a compressive stress of 50 MPa with a temperature ramp of 5°C/min for 25-315°C. The results will enable mechanical properties of these MMCs to be correlated with quantitative values of dispersoids to maximize their high strength potential. In addition, the findings on dispersoid content and rate of grain growth provide significant information on processing parameters towards optimizing the manufacturing of these materials. © 2012 AIAA