Abb. 1: Internally cooled gas turbine blade
Thermal barrier coatings (TBC) with a ceramic topcoat are widely used for protecting highly loaded gas turbine components against overheating. For example, on internally cooled turbine blades the ceramic topcoat maintains a high temperature difference between the outer surface and the underlying metallic substrate. Mainly used as a bandage, TBC systems have a high potential with respect to higher turbine inlet temperatures. This potential can be fully exhausted as long as these coatings can reliably withstand their extreme thermal and mechanical fatigue loading for an economically acceptable time. Demonstration of TBC reliability requires realistic testing to ensure that lifetime determining damage mechanisms under testing conditions are the same as in real engines.
Service loading of TBC systems
Most severe loading conditions for TBC systems occur in rotating gas turbine blades of the first turbine stage (Fig. 1). Simultaneous mechanical and thermal loads occur in every flight cycle with maximum temperatures at the ceramic surfaces exceeding 1100°C. Additionally, the high thermal gradient over the wall of the turbine blade induces high multiaxial stresses, which are determined by the resultant strain and the mismatch of the physical properties of the substrate and layer materials.
Test facilities for Thermal Gradient Mechanical Fatigue (TGMF)
For simulation of the service conditions of rotating gas turbine blades in aircraft engines two unique testing facilities have been designed and built. The thermal and mechanical fatigue load occurring during an entire flight cycle, including start, cruising, landing and shut off, can be applied on test specimens (Fig.2). Besides simultaneous cyclic thermal and mechanical loading, a high thermal gradient over the wall of tubular specimens can be applied by external radiation heating and internal air cooling. Because of the controlled thermal gradient over the specimen wall the test cycles are called ‘thermal gradient mechanical fatigue’ or TGMF-cycles. High heating and cooling rates are produced by a radiation furnace with concentrating mirrors combined with a shutter technique.
Failure and damage features
The combined thermal and mechanical loading results in local microstructural and chemical changes and damages, which are analysed by means of optical and scanning electron microscopy including microanalytic methods. Damages are observed especially near the interface between ceramic and metal but also at the cooled surface by oxidation assisted fatigue cracking. In service one important failure mode of TBC systems is spallation of the coating near the interface between ceramic topcoat and underlying oxidation protection layer, the so called bond coat. In laboratory the complex TGMF cycle results in specific damages, e.g. enhanced TBC spallation due to fatigue cracking parallel to the metal/ceramic interface Because of their specific feature we called this fatigue cracks ‘smiley cracks’ (Fig. 3). Analyses of damage features combined with mechanical analyses by means of finite element methods are used to elucidate the damage mechanisms in service.
Lifetime assessment and damage parameter
Failure of the TBC by spallation of the coating requires in realistic testing a high number of about 5000 to 10000 cycles, comparable to the number of flights in service thatch the TBC system has to survive. In order to achieve reasonable testing times for lifetime assessment, the damage status as function of the loading history needs to be characterized in terms of life consumption long before failure. For the case of final failure by spallation of the ceramic top- coat, the use the apparent interfacial fracture toughness as damage parameter. Several methods for measuring the apparent fracture toughness of brittle coatings are under investigation with respect to their application to TBC systems.