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

Abstract

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. The LASAT has shown to introduce very low scatter in delamination/buckling results when laser flux is low enough to avoid any cracking within the ceramic layer [2]. Thus, we choose to determine interfacial toughness from the specimen after LASAT leading to buckling without ceramic cracking. To determine the interfacial toughness between the ceramic layer and the substrate, a large variety of experimental techniques has been employed, including light profilometer, photoluminescence piezo-spectroscopy, infra-red imaging and image analysis. All these techniques enable to determine the precise 3D morphology of the ceramic blister obtained after LASAT and the size of interfacial crack. Further, mechanical elastic analysis of the blister shape is used to calculate the toughness of an interfacial crack at arrest as well as the residual stresses. At last but not least, 3D shape of the crack tip is measured by FIB and slice tomography to validate the size of the process zone and to determine the range of error including the global chain of measurements to the final evaluation of interfacial toughness.