Residual Stresses In Thermal Barrier Coatings Research Articles

Numerical Simulation of Initial Residual Stress in Thermal Barrier Coatings: Planar geometry model

Thermal barrier coatings (TBCs) are one of the key materials of turbines for propulsion and power generation. A typical TBCs system generally contains four layers, i.e. top ceramic coat (TC), thermally grown oxide (TGO), bond coat (BC) and substrate. The material parameters （such as Young’s modulus, Poisson’s ratio and thermal expansion coefficient） of each layer will result in the different cooling velocity of thermal barrier coatings (TBCs) system during the deposition process. Initial residual stresses in TBCs occur after cooling to the ambient, which play an important role for evaluating the failure and reliability of TBCs. In this paper, the initial residual stresses of a planar TBCs model were obtained by finite element method, the effects of cooling kinds and the thickness of TC and TGO on the fields of initial residual stresses were also considered. The results may be useful for the optimization of TBCs system design and preparation.

Optimal Data Processing Method for the Application of Eu3+ Photoluminescence Piezospectroscopy in Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are widely used to protect gas turbine blades but internal stress near the interface in TBCs is one of the main causes of thermal barrier failure under thermal cycling. A non-destructive inspection technique based on Eu3+ photoluminescence piezospectroscopy has been successfully used to analyze the residual stress in TBCs, but systematic and quantitative evaluation of data processing is still needed, especially with respect to the identification of peak positions. In this work, processing methods for Eu3+ photoluminescence spectroscopy data were studied to characterize TBC internal stress. Both physical and numerical experiments were carried out where Eu3+ luminescence spectra were obtained from a sample of europium-doped yttria-stabilized zirconia (YSZ:Eu3+) under step-by-step uniaxial loading, and the simulated spectra were numerically deduced from the measured spectra. The peak shifts were then obtained by processing the spectral data in different ways (Gaussian, Lorentzian, pseudo-Voigt fitting, and the barycenter method), and comparing the results. We found that the Gaussian function, rather than the commonly used Lorentzian function, is the most appropriate method for the application of Eu3+ photoluminescence piezospectroscopy in TBCs because it provides sufficient sensitivity, stability and confidence for quantitative stress analysis.

Diffraction grating strain gauge method: error analysis and its application for the residual stress measurement in thermal barrier coatings

Diffraction grating strain gauge (DGSG) is an optical strain measurement method. Based on this method, a six-spot diffraction grating strain gauge (S-DGSG) system has been developed with the advantages of high and adjustable sensitivity, compact structure, and non-contact measurement. In this study, this system is applied for the residual stress measurement in thermal barrier coatings (TBCs) combining the hole-drilling method. During the experiment, the specimen’s location is supposed to be reset accurately before and after the hole-drilling, however, it is found that the rigid body displacements from the resetting process could seriously influence the measurement accuracy. In order to understand and eliminate the effects from the rigid body displacements, such as the three-dimensional (3D) rotations and the out-of-plane displacement of the grating, the measurement error of this system is systematically analyzed, and an optimized method is proposed. Moreover, a numerical experiment and a verified tensile test are conducted, and the results verify the applicability of this optimized method successfully. Finally, combining this optimized method, a residual stress measurement experiment is conducted, and the results show that this method can be applied to measure the residual stress in TBCs.

Interfacial Residual Stress Analysis of Thermal Spray Coatings by Miniature Ring-Core Cutting Combined with DIC Method

The residual stress in thermal barrier coatings (TBCs) fabricated from coating deposition plays a vital role in the coating design and processing parameters optimization. The main objective of the present work is to determine the interfacial residual stress in TBCs by means of miniature ring-core cutting and the digital image correlation (DIC) method. Both the ring-core cutting and the dot pattern used for DIC deformation measurement are implemented by the focused ion beam (FIB) milling on the cross-section of a coating. A finite element model (FEM) is developed to simulate the ring-core cutting process. From the FEM, the calibration coefficients are determined for general applications. The surface of the ring-core containing dot patterns is recorded before and after the FIB milling process. DIC technique is then performed to calculate the surface displacement caused by the release of residual stresses due to the cutting. Results demonstrate that the interfacial residual stress in TBCs is nearly in a uniaxial stress state and has a tendency to be compressive toward the interface. Finally, essential aspects of the technique are discussed.

Effect of Young's modulus evolution on residual stress measurement of thermal barrier coatings by X-ray diffraction

Subjected to thermal cycling, the apparent Young's modulus of air plasma-sprayed (APS) 8 wt.% Y 2O 3-stabilized ZrO 2 (8YSZ) thermal barrier coatings (TBCs) was measured by nanoindentation. Owing to the effects of sintering and porous microstructure, the apparent Young's modulus follows a Weibull distribution and changes from 50 to 93 GPa with an increase of thermal cycling. The evolution of residual stresses in the top coating of an 8YSZ TBC system was determined by X-ray diffraction (XRD). The residual stresses derived from the XRD data are well consistent with that obtained by the Vickers indention. It is shown that the evolution of Young's modulus plays an important role in improving the measurement precision of residual stresses in TBCs by XRD.

Residual Stress and Damage Evolution in TBCs by Optical Method

In recently years, ruby fluorescence spectroscopy has been demonstrated as a powerful technique for monitoring residual stress evolution in the thermally grown oxide scale in thermal barrier coatings(TBC) systems. The measured residual stresses, in turn can be used to monitor evolution of damage in the coatings. Effective use of this technology for real time damage monitoring require the identification of strength in measured stresses that can be used as indicators of damage evolution.the present work focuses on studying the evolution of residual stresses in TBC systems during oxidation. The coating are atmospheric plasma sprayed (APS), the residual stress were measured at different oxidation time and to identify critical features so as to be used as indicators of failure in TBCs.

724 プラズマ溶射 TBC の残留応力

To improve the thermal efficiency of a gas turbine, thermal barrier coatings (TBC) have been developed to protect superalloy components. The damage in TBC is often caused by mechanical and thermal residual stress in TBC. Therefore, it is very important to evaluate residual stress distribution in TBC coating system. In this study, the residual stress of TBC has been measured by Raman spectroscopy. The testing material is a plasma-sprayed TBC containing 8% Y_2O_3 on a stainless steel. To understand the residual stress near interface, the TBC dissolved from the substrate has also been used to measure residual stress. The sharp raman peak exists at a frequency of 640 (cm)^<-1> for the tetragonal zirconia. It has been found that compressive stress in TBC system can be released in the TBC without substrate. However, this occurs with a thickness up to 100 μm.