Neue Verfahren zur Analyse des Verformungs- und Schädigungsverhaltens von MCrAlY-Schichten im Wärmedämmschichtsystem

Thermal barrier coatings applied to thermally highly loaded areas of land-based gas turbines represent a promising tool to increase efficiency and to reduce emissions in the conventional electric power production. A basic requirement to achieve reliable use of thermal barrier coatings is an improved understanding of deformation and damage within the coating system and of the respective influencing factors. The present work is concerned with the application of new experimental procedures to a thermal barrier coating system, composed of an APS- or EB-PVD-TBC (partially stabilised Zirconia) and a VPS bond coat (48,3% Ni, 21,1% Co, 17,1% Cr, 12,6% Al, 0,61% Y, 0,4% Hf) on the single crystal alloy CMSX-4. The creep properties within the coating system of the MCrAlY bond coat have been investigated using a double shear creep testing assembly, especially developed for thermal barrier coatings. Parameters to describe primary and secondary creep for the temperature range 750°C - 1050°C were acquired on specimens before and after annealing at 1050°C. The influence of temperature and creep deformation on the microstructure was additionally evaluated. Based on these results, a set of data describing the deformation behaviour of the MCrAlY coating was established to be used in finite element models. Accompanying three and four point bending tests, a combination of two non-destructive diagnostic procedures, acoustic emission analysis and optical in-situ observations by CCD camera, was used to study mechanically induced damage evolution of the coating system. Time and location of crack initiation, as well as crack propagation path up to macroscopic failure could be assigned for tensile and compression loading at both, room temperature and 950°C. In addition some of the specimens have been annealed over 300 hours at 1050°C prior to testing. Critical strain values were determined in correlation to the damage evolution. A TMF testing device was build up, designed to simulate the service loading of turbine blades in the lab as a superposition of cyclic thermal and cyclic mechanical loads. The progressive method of Pulse Thermography has been used to detect hidden delamination cracks within the TBC. To design the experimental simulation, a dwell time at high temperature was integrated into the thermomechanical cycle, which led to an activation of oxidation processes at the interface. This was shown to be a necessary requirement in order to simulate the failure type of the TBC system, known from gas turbines