Kevin Knipe

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.

Particle Size Effect on Load Transfer in Single Particle Composite Samples via X-Ray Diffraction

Particulate composites are widely used in many aerospace applications including protective coatings, adhesives, or structures, and their mechanical properties and behavior have gained increasing significance. The addition of modifiers such as alumina generally leads to improved mechanical properties. In this work, samples with an isolated alumina particle embedded in an epoxy matrix were created to replicate the ideal assumptions for many particulate mechanics models. The effect of particle size on load transfer is determined here using a unique X-Ray Diffraction experimental set-up at the Canadian Light Source. At the Very Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron (VESPERS) beamline, a custom miniature mechanical load frame was used to apply compressive loads to each sample. At three different compressive loads, the alumina within each sample was exposed to a hard X-ray beam which created a diffraction pattern that was collected by a 2-D detector. A trend of increasing load transfer with increasing particle size was observed during the analysis of the diffraction rings. Results from this work provide experimental insight into the effect of particle size on load transfer in single particle composites and can serve to experimentally validate the theoretical load transfer models that currently exist.

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.

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.

Finite element modelling and vibration control of a tetrahedral space frame applied to micromachining

This paper presents the modelling and vibration control of the tetrahedral space frame. The tetrahedral frame is a structure that is used in precision machining applications. When machining at a high precision requirement, structural vibration is of the utmost concern. This research develops finite element models using ANSYS and Matlab that can be used to implement a positive position feedback controller. In order to control and reduce the vibration magnitude, collocated piezoelectric actuators and sensors are placed at the optimal positions on the tetrahedral space frame. ANSYS is used to conduct a modal analysis on the structure to obtain the mode shapes, which determines the weakest positions and critical vibration modes under certain machining conditions. In Matlab, a finite element model is created which uses Timoshenkos beam elements and it is further converted to state-space allowing the model to be controlled using programme simulations. A positive position feedback controller is chosen due to its non-sensitivity to spillover effects.