Laser Shock Adhesion Test (LASAT) of EB-PVD TBCs: Towards an industrial application

The assessment of the interface strength of EB-PVD thermal barrier coating (TBC) is a key issue to control the production and better understand the ceramic spallation that will occur during life duration of coated turbine blades. The Laser Shock Adhesion Test (LASAT) involving bi-dimensional shock wave propagation, namely the LASAT-2D, consists in measuring the interfacial crack diameter when implementing a set of laser shocks with increased laser power densities. From the resulting LASAT-2D plots, adhesion strength could be obtained from tensile stress calculated at the interface with a finite element model of shockwave propagation combined to interface cracking criterion. This work is focusing on the qualitative approach to compare adhesion on TBC systems directly through LASAT-2D plots. Two different testing modes were investigated: the conventional MS mode when the laser irradiation is carried out on the metallic side of a TBC coupon and the newly developed CS mode for which the laser shock is implemented onto the ceramic side. The interfacial crack is revealed by the presence of a spot that could be measured on a top-view optical image of the ceramic. In this work, the actual relationship between spot diameter and interfacial inner crack size is clearly established for both MS and CS modes. It was achieved experimentally by examinations of SEM cross-sections or non-destructive piezospectroscopic measurements involving lifetime decay map of the photoluminescence signal. From LASAT-2D curves, the role of the laser conditions like laser shock diameter and substrate thickness is discussed. This discussion is illustrated through the results of the qualitative IASAT-2D method applied on two different TBC samples (as-coated and thermally-cycled). The final objective of this work is to enable a sound and relevant application of the LASAT on TBC systems for both modes. With this goal, the LASAT-2D CS was successfully applied directly on a TBC coated turbine blade. (C) 2013 Elsevier B.V. All rights reserved.