Marion Bartsch

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.

Interfacial Fracture Toughness Measurement of Thick Ceramic Coatings by Indentation

The basic requirement for the use of a ceramic coating is sufficient adhesion to its substrate. A measure of the adhesive properties of a coating is the interfacial fracture toughness. The test method applicable for interfacial fracture toughness measurements depends on the mechanical properties of the material system and the geometry of the test piece. In this work, indentation methods have been evaluated for the estimation of the fracture toughness of ceramic thermal barrier coatings on metallic substrates. Coatings of 100 to 300 µm thickness were applied by electron beam – physical vapour deposition. The performed test types were Vickers indentation at the interface of polished cross sections of the coating system and Rockwell indentation with a brale C indenter, penetrating the coating perpendicular to the surface. Both tests generate delamination, in which the delamination crack length corresponds to the interfacial fracture toughness. Fracture surfaces and cross sections of the fractured coatings were investigated by optical and scanning electron microscope. Determined fracture toughness values are discussed with respect to the loading conditions in the test and the fracture process – i.e. interaction between indenter and coating system and the crack propagation path.

Testing and Characterization of Ceramic Thermal Barrier Coatings

This paper gives a short overview of tests applied for the investigation of long term behaviour of thermal barrier coating systems. A variety of tests has been conducted on an exemplary material system with the coatings applied by electron beam physical vapour deposition. Damages and damage evolution in different tests are compared. Since the observed damage mechanisms are different, it is proposed to design laboratory tests as realistic as possible, especially if the test data are used for lifetime assessment. In order to get reasonable testing times, the damage accumulation has to be described as a function of loading history, long time before failure. For the case of final failure by spallation of the ceramic top coat, it is proposed to use the apparent interfacial fracture toughness as damage parameter. Several methods for measuring the apparent fracture toughness of brittle coatings are discussed with respect to their application to thermal barrier coating systems.

Developments in Processing of Ceramic Top Coats of EB-PVD Thermal Barrier Coatings

Partially Yttria Stabilized Zirconia (PYSZ) based Thermal Barrier Coatings (TBC) manufactured by EB-PVD process are a crucial part of a system, which protects the turbine blades situated at the high pressure sector of aero engines and stationary gas turbines under severe service conditions. These materials show a high strain tolerance relying on their unique coating morphology, which is represented by weakly bonded columns. The porosity present in ceramic top coats affects the thermal conductivity by reducing the cross sectional area through which the heat flows. The increase in thermal conductivity after heat-treatment relates to the alteration of the shape of the pores rather than the reduction of their surface-area at the cross section. The studies carried out by the authors demonstrate that the variation of the parameters during the EB-PVD processing of PYSZ based top-coats alters the columnar morphology of the coatings. Consequently, these morphological changes affect primarily the thermal conductivity and eventually the Young’ Modulus which are the key physical properties of this material group. New ceramic compositions covering zirconia coatings stabilized with alternative oxides, pyrochlores and hexaluminates are addressed. Failures occurring in ceramic top coats mark the lifetime of TBC system and therefore, it is necessary that their performance should go beyond that of the-state-of-the-art materials. This context summarizes the research and developments devoted to future generation ceramic top coats of EB-PVD TBCs.

R&D Status and Needs for Improved EB-PVD Thermal Barrier Coating Performance

The key issues for thermal barrier coating development are high temperature capability and durability under thermal cyclic conditions as experienced in the hot section of gas turbines. Due to the complexity of the system and the interaction of the constituents, performance improvements require a systems approach. However, there are issues closely related to the ceramic top coating and the bond coat, respectively. Reduced thermal conductivity, sintering, and stresses within the ceramic coating are addressed in the paper as well as factors affecting failure of the TBC by spallation. The latter is primarily governed by the formation and growth of the thermally grown oxide scale and therefore related to the bond coat. A strategy for lifetime assessment of TBCs is discussed.

Effect of HIP Parameters on the Micro-Structural Evolution of a Single Crystal Ni-Based Superalloy

For reducing the porosity of single crystal (SX) nickel-based superalloys, Hot Isostatic Pressing (HIP) is used. High pressures of about 100-170 MPa lead to local deformation, which close the pores. However, since HIP also requires high temperatures (1000-1200°C) it has a pronounced effect on the microstructure and the local distribution of elements. This contribution analyses the effect of different HIP treatments on both the microstructure and the segregation of the SX superalloy LEK94 in the as-precipitation-hardened state. In addition, the effects of rapid or slow cooling are analyzed. To distinguish the effect of pressure from those of temperature, the HIPed samples are compared with specimens annealed at atmospheric pressure.

Determining the Elastic Moduli of the Individual Component Layers of Cylindrical Thermal Barrier Coatings by Means of a Mixed Numerical - Experimental Technique

Cylindrical specimens made of the Ni-based super-alloy Inconel 625 (IN 625) were coated with (a) NiCoCrAlY, or (b) NiCoCrAlY and yttria-stabilised zirconia (YSZ: in this case, zirconia with 7-8 wt% yttria), using the electron beam - physical vapor deposition (EB-PVD) technique. In the bi-layer coatings, the YSZ layer is the thermal barrier coating (TBC) and the NiCoCrAlY layer is the metallic bond coat (BC). The BC improves the bonding between the substrate and the ceramic TBC, while the low thermal conductivity of the TBC oers high-temperature protection to the substrate. This paper focuses on the determination of the elastic moduli of the substrate and the coating layers of the test samples. The elastic moduli of the three dierent materials (IN 625, NiCoCrAlY and YSZ) were determined by means of a mixed numerical - experimental technique (MNET). The employed MNET was based on the comparison of the experimentally measured resonant frequencies of the rst bending mode of the test samples to the numerically calculated ones. The unknown elastic properties were determined by ne-tuning the elastic material parameters of the numerical models so as to enable the reproduction of the experimentally measured resonant frequencies.

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.

Time-Economic Lifetime Assessment for high Performance Thermal Barrier Coating Systems

Strategies for time-economic lifetime assessment of thermal barrier coatings (TBC) in service are described and discussed on the basis of experimental results, achieved on material systems with coatings applied by electron beam physical vapour deposition. Service cycles for gas turbine blades have been simulated on specimens in thermo-mechanical fatigue tests, accelerating the fatigue processes by an increase of load frequency. Time dependent changes in the material system were imposed by a separate ageing, where the samples were pre-oxidized prior to the fatigue test. Results of thermo-mechanical fatigue tests on pre-aged and as-coated specimens gave evidence of interaction between fatigue and ageing processes. An alternative approach is used, which is focused on the evolution of a failure relevant damage parameter in the TBC system. The interfacial fracture toughness was selected as a damage parameter, since one important failure mode of TBCs is the spallation near the interface between the metal and the ceramic. Fracture mechanical experiments based on indentation methods have been evaluated for monitoring the evolution of the interfacial fracture toughness as a function of ageing time. It was found that the test results were influenced by both changes of the interface (which is critical in service) and changes in the surrounding material.