Interfacial toughness evolution under thermal cycling by laser shock and mechanical testing of an EB-PVD coating system

One of the major challenges for coatings on superalloys is to keep adherence during aging, where damage is mostly driven by thermal cycling. On the other hand, the methodology of the evaluation of the interfacial toughness should be consistent with in service loading. Recently, the use of LAser Shock Adhesion Test (LASAT) has shown its capability for both ranking different coating solutions and evaluating the evolution of a given coating as a function of aging [1-2]. The intent of this paper is to demonstrate the ability of LASAT to reproduce damage mechanisms observed under quasi-static in plane mechanical testing and to propose a general methodology to assess interfacial toughness evolution based on LASAT measurements. The material chosen in this study is a partially Y2O3 stabilized EB-PVD zirconia layer coating deposited by Electron Beam – Physical Vapor Deposition (EB-PVD) onto a first generation Ni base superalloy. Aging has been performed using thermal cycling under laboratory air. Degradation of the coating system due to ageing is quantitatively assessed by LASAT and accompanied by different microstructural analysis methods. For LASAT, if laser flux is below a threshold, no delamination occurs. When increasing laser flux above this threshold, a systematic sequence is observed: i) delamination without buckling of the ceramic layer, ii) delamination and buckling, iii) partial cracking of the ceramic layer, and iv) spallation [1-2]. These different states are also achieved in compressive quasi-static testing and assessed by means of local strain measurement using digital image correlation technique [3]. Aging is evaluated through the evolution of both the delamination and the buckling behavior induced by the LASAT method or critical strain at ceramic spallation under compressive static load. Please click Additional Files below to see the full abstract

Crack morphology in a columnar thermal barrier coating system

For high temperature application, EB-PVD ceramic layers are commonly used as thermal barrier coating. During thermal transients, the thermal expansion mismatch between coating and substrate drives failure of the TBC mainly by interfacial cracking. Laser Shock Adhesion Test (LASAT) provides stresses at the ceramic/metal interface enabling controlled interfacial cracking [1-2]. For achieving a clear understanding of the influence of local morphology on interfacial toughness, this study aims at characterizing the 3D morphology of a crack at the interface between metal and an EB-PVD TBC having a columnar structure. Please click Additional Files below to see the full abstract

Thermo-mechanical Analysis of Blister Damage in Eb-pvd Tbc System: Experiments and Modeling

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3D multiscale segmentation and morphological analysis of x-ray microtomography from cold-sprayed coatings

International audienceX-ray microtomography from cold-sprayed coatings brings a new insight on this deposition process. A noise-tolerant segmentation algorithm is introduced, based on the combination of two segmentations: a deterministic multiscale segmentation and a stochastic segmentation. The stochastic approach uses random Poisson lines as markers. Results on a X-ray microtomographic image of aluminium particles are presented and validated

A composite approach to Al2O3 based plasma-sprayed coatings

25-29 septembreInternational audienceThermally-sprayed ceramic coatings such as plasma-sprayed alumina show a composite microstructure actually due to the presence of defects such as pores, inter-lamellar and intralamellar cracks. These 2nd phase-typed features influence the mechanical behavior and electrical insulation of the coating dramatically. In this study, a composite approach to the microstructure of plasma-sprayed alumina was developed for the optimising of component properties such as electrical gaps used in the oil industry. This approach consisted of a Finite Element Analysis (FEA) of thermo-mechanical and electrical properties from simulated microstructures. Series of composite microstructures were tested, i.e that of air plasma-sprayed (APS) alumina basically plus those obtained by addition of glass or resin using co-spraying and impregnation respectively. Various degrees of porosity and cracks could be obtained from different spraying conditions and by subsequent laser surface remelting. Every composite microstructure was studied using quantitative image analysis of series of SEM cross-sections. Electrochemical Impedance Spectroscopy (EIS) in NaCl solution was also performed to characterize the level of connected pores and the resulting electrical insulating properties. From experiments, a Finite Element Model (FEM) based on the actual microstructure was developed. The latter was simulated with involving of all significant features, such as phase distribution, porosity and defects. This simulation was developed to optimize the composite microstructure to meet industrial applications

SiC-based/EBC Coating systems investigated by LASAT (LAser Shock Adhesion Test)

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Influence du cisaillement macroscopique sur la propagation d'une fissure située à l'interface d'un revêtement céramique et d'un substrat métallique

International audienceLes dépôts des revêtements projetés sont le plus souvent réalisés à chaud (jet plasma, faisceau d'électron...) ou avec des échauffements locaux importants au cours du dépôt (cold spray). Lors de l'utilisation des pièces, la température étant généralement différente de la température de dépôt, l'écart de coefficients de dilatation thermique entre les couches du système conduit à un état de ..

Etude par choc laser de l'adhérence de barrières thermiques aéronautiques

National audienceL'estimation de la durée de vie des barrières thermiques aéronautiques (BT) déposées sur les aubes de turbines haute pression nécessite de comprendre les mécanismes induisant l'endommagement entre la sous-couche et la zircone. Le LASAT, LAser Shock Adhesion Test ou essai d'adhérence par choc laser, est une technique exploitant tout son potentiel dans le cas des BTs, en particulier par sa rapidité, simplicité et au comportement optique de la couche de zircone. Ce dernier avantage permet de dimensionner l'endommagement généré sans réaliser de coupes métallographiques. Un nouveau protocole de l'essai LASAT a été mis en place utilisant les ondes de choc bidimensionnelles et exploitant la dépendance entre l'intensité du choc appliqué, le niveau d'adhérence du dépôt, et la dimension de la fissure générée

Laser Shock Adhesion Test (LASAT) of electron beam physical vapor deposited thermal barrier coatings (EB-PVD TBCs)

International audienceDamage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock