Thermal barrier coatings (TBCs) are widely used in modern turbine engines to provide thermal protection for metallic components in order to maximise engine efficiency through an increase in operating temperature. However, further improvements in TBC technologies require a better understanding of the correlation between microstructure and functional properties.

In this study, we used non-destructive 3D X-ray Computed Tomography (XCT) with sub-micron resolution to investigate the microstructure of Air Plasma Sprayed (APS) Yttria-Stabilised Zirconia (YSZ) TBCs. Features including coating thickness, YSZ topcoat porosity and interface roughness were quantified in 3D. In particular, detailed information regarding the pore shape, spatial and size distribution and connectivity was obtained from real 3D microstructural images of the coatings. These images were then used to build Finite Element (FE) models in order to quantitatively correlate the microstructural features with the elastic modulus and effective thermal conductivity of the porous YSZ coatings. Modulus and thermal conductivity were also experimentally determined using depth-sensing indentation and laser flash thermal diffusivity.

It was found that the APS TBC porosity is mainly in the form of inter-splat voids perpendicular to the spraying direction and also forms complex pore networks. Indeed, the intrinsic connectivity of pores in the YSZ coating dictates the low elastic modulus and thermal conductivity relative to its bulk counterpart. To gain more insight into these findings, a parametric study on artificial 3D microstructures was used to evaluate the effects of porosity volume fraction, shape and distribution on the coating's modulus and conductivity. It was found that for isolated pores, porosity volume fraction is almost linearly related to the effective modulus and conductivity. In addition, when a pore network is present, as in the case of APS TBCs, the effective properties are dominated by the minimum solid section area of the porous medium. In the case of coating elastic modulus, both modelling and experimental results showed a strong anisotropy. In fact, the FE models indicated that the modulus is a function of all porosity factors such as quantity, shape and distribution while thermal conductivity was found to be much less sensitive to porosity factors other than volume fraction.

Keywords: Thermal barrier coatings, X-Ray Tomography, Thermal Conductivity, Elastic Modulus, Image-based Modelling

In order to assess the role of temperature on the development of residual stresses in Al₂O₃ scales, two bond coating (BC) compositions, a vacuum plasma sprayed (VPS) NiCoCrAlYHf BC and a VPS NiCoCrAlYHfSi BC, deposited on Hf-rich directionally-solidified (DS) 247 substrates were thermally-cycled at temperatures ranging from 1075 to 1150 °C with 1-h cycles in 10% H₂O. Photo-stimulated luminescence spectroscopy (PSLS) was used to map residual stresses in the Al₂O₃ scale at the YSZ/BC interface from the same region at regular cycling intervals. All samples exhibited similar stress distributions and rates of interfacial delamination with the exception of the YHfSi BC cycled at 1150 °C which had a lower average compressive stress with more delaminations. This BC composition also had the shortest TBC lifetime which shows that the PSLS measurements correlate well to interfacial damage accumulation in TBCs. The effect of sample curvature on the interfacial stress was assessed by comparing TBC-coated rod specimens to flat buttons.

Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

Mechanism-based lifetime assessment models of thermal barrier coating (TBC) systems rely on accurate knowledge of the experimentally measured interfacial fracture toughness, G_c, over a range of mode mix and especially at mode-II. Utilizing conventional 4-point bend experiments, inverted 4-point bend tests, and a newly developed compression edge-delamination (CED) methodology, has allowed for direct measurement of coating interfacial toughness as a function of mode mix. Two different material systems are examined, both of which consist of an Electron-Beam Physical Vapor Deposited (EBPVD) 7% Yttria-Stabilized Zirconia (YSZ) top coat, which is deposited on either a 1) Pt-modified diffusion aluminide bond coat on a René N5 substrate or a 2) Low Pressure Plasma Spray (LPPS) NiCoCrAly bond coat on a PWA1484 substrate. Using the CED test, reductions in pure mode-II interfacial toughness during thermal cycling between the as-deposited state and at coating lifetime have been determined to be ΔG_c≈250 J/m². Specimen preparation and the use of starter cracks to assure proper delamination will be discussed. Crack face friction has shown to play a significant role on G_c and details regarding the characterization and implementation into the Finite Element model used to extract the interfacial toughness will be examined. Finally, microstructural observations, including morphological and chemical changes, linked to the degradation of the coating interfaces due to thermal cycling will be analyzed.

Surface engineering plays the essential role in enhancing the performance of high pressure turbines which work under the most severe conditions of temperature and mechanical loading. In this study, an intermediate α-Al₂O₃ layer was deposited by sol-gel on the raw surface of APS-CoNiCrAlY Bond Coat, which was deposited on superalloy substrate. The YSZ ceramic Top Coat was deposited on the top of Al₂O₃ layer by APS. The new TBC system was compared with the standard TBC system concerning the behavior under thermal cycling. The samples were investigated by optical microscopy; Scanning Electron Microscopy (SEM) equipped by Energy Dispersive X-ray analysis (EDX), X-ray Diffraction (XRD) and Raman spectroscopy. The crack length within the ceramic layer and the average thickness of TGO were measured using image analysis tool. It was found that the Al₂O₃ layer has the potential to reduce the crack length and TGO thickness by suppressing the formation of detrimental oxides. The Al₂O₃ layer has a barrier effect on oxygen diffusion .This effect enhances the behavior of metallic Bond Coat while suffering oxidation, and hence increases the TBC life. The information generated from the study can be used to improve the performance of the TBC system under different operating conditions.