Hans-Peter Bossmann

Manufacturing Optimization for Bondcoat/Thermal Barrier Coating Systems

A reliable lifetime prediction rule for bondcoat/thermal barrier coating (BC/TBC) coated parts in gas turbine operation is necessary to determine remnant service life. The specimens investigated were coated with MCrAlY plus yttria partially stabilized zirconia applied by vacuum plasma spraying and atmospheric plasma spraying processes, respectively. The performances of these laboratory specimens were statistically assessed, combining long term oxidation testing with thermal cycling, thus superimposing thermomechanical loading on the laboratory specimens to more accurately represent engine conditions. A design of experiment (DOE) approach was used for manufacturing optimization of the BC/TBC system . The life of the coating system is influenced by several manufacturing parameters such as BC thickness, BC roughness, TBC thickness, TBC porosity, and TBC stiffness. Specimens with a suitable variation in these parameters were produced to ensure a balanced test matrix of fractional factorial DOE. Based on results derived from laboratory testing the specifically tailored parts, first and second order effects of manufacturing parameters on lifetime were quantified. The findings revealed that the second order effects (the interaction of manufacturing parameters) were more important on the lifetime of the BC/TBC system than the corresponding first order effect (single parameter). For instance, the variation in BC thickness or BC roughness led to a scatter of lifetimes of 10% and 60%, respectively whereas their interaction resulted in a scatter of lifetime of 150% for the same range of coating parameters. Further examples of such pairings are also demonstrated. Finally, a lifetime prediction for three quality classes (high, medium, and low qualities) has been demonstrated. The difference in achievable lifetime highlights the importance of manufacturing parameters in determining the life of the BC/TBC system.

Probabilistic Lifetime Prediction of Thermal Barrier Coating Systems Depending on Manufacturing Scatter

The assessment of Bondcoat/Thermal barrier coating systems is an inherent part of the lifing process of gas turbine component. On the one hand, coatings are considered in the constitutive modelling — e.g. in the thermal model and for the prediction of eigenfrequencies of gas turbine blades. On the other hand, the influence of the coating system on the lifetime of the part (target cyclic life and target operation hours) needs to be assessed. This paper addresses the prediction of coating lifetime. Lifing models of Bondcoat/Thermal barrier coating systems (BC/TBC) are commonly built using isothermal furnace cyclic tests (FCT). The lifetime of the BC/TBC under such test conditions has been shown to depend on multiple coating parameters like TBC thickness, TBC porosity, BC thickness, BC roughness, and also on testing temperature. For example, the TBC life (defined as time to partial TBC spallation) is reduced with increasing temperature, with increasing TBC thickness and decreasing porosity and BC roughness. When operating in a gas turbine (GT), the TBC surface temperature and the BC temperature depend on engine operating conditions, heat transfer of combustion gas and cooling air, coating microstructure and thickness. For instance, a TBC with high porosity typically demonstrates a lower thermal conductivity than that with low porosity. For otherwise same boundary conditions, the BC temperature will decrease with increasing TBC porosity and increasing TBC thickness. The benefit of having a high coating porosity observed in FCT is further amplified by its impact on reducing the BC temperature in GT operation. To the contrary, the positive impact of a reduced TBC thickness observed in FCT is reduced by its negative impact on an increased BC temperature during GT operation. Taking these effects into account a probabilistic lifing model is proposed based on Monte Carlo simulations. Using this model the impact of the manufacturing scatter on the BC/TBC life can be assessed, and enables improved manufacturing by focusing on those parameters that are most critical for coating lifetime.Copyright

EVOLUTION OF THERMAL BARRIER COATING STRAIN TOLERANCE DURING ENGINE OPERATION AND ITS APPLICATION TO LIFING MODELS

Models for thermal barrier coating lifetime prediction are often based on bondcoat oxidation models leading to an end of life criterion either based on bondcoat full consumption or a critical thermally grown oxide thickness. Such models can be satisfactory on turbine parts where the most common coating delamination modes are black or grey failure which are linked to the bondcoat behaviour. Such models are not reliable for combustor parts with thick thermal barrier coating systems where the most common life limiting factor is the formation of cracks appearing in the ceramic layer few tens of microns above the bondcoat interface. This behaviour is linked to the TBC layer mechanical properties and should be described by a model taking into account the evolution of the TBC mechanical properties during engine operation, the mechanical loads in the ceramic layer and a crack propagation model in the TBC. A study of the strain tolerance of TBC from combustor parts after engine operation was performed by taking samples from combustor liners at various locations having different TBC surface temperature. The strain tolerance of TBC samples was measured by four-point bending and correlated with the TBC microstructure and various engine operation parameters. It was shown that the TBC microstructure has an influence on TBC strain tolerance, and that the evolution of the TBC strain tolerance during engine operation is linked to the TBC temperature as well as the operating hours. The data have been used to develop a predictive model of the evolution of the TBC strain tolerance during engine operation. This model allows optimization of parts reconditioning interval, and provides tools for determining the residual life of coated components.

Burner Rig Testing of Thermal Barrier Coatings for Lifetime Prediction

Thermal barrier coating lifetime prediction has been commonly performed using furnace cyclic test results. This testing method causes coating failures driven by the bondcoat oxidation. This allows definition of lifetime prediction models representative of the field experience for thin thermal barrier coating systems where the difference between the bondcoat temperature and the coating surface are limited to 100–200 °C. Thick thermal barrier coating systems can experience coating surface temperatures 500 °C higher than the bondcoat temperature. In such cases sintering and phase transformations in the ceramic layers can also affect the coating lifetime. For this reason cyclic test methods like thermal gradient burner rig and laser heat-flux tests have been developed. They allow to test a coating system with surface temperatures >1400 °C while keeping bondcoat temperature <900 °C. The main issue of such tests is the often limited samples statistic, the reproducibility of the test conditions, and the coating failure mode that is not representative of the field experience. In Alstom, a burner rig test has been developed to solve these issues. It allows to test in parallel 10 samples, with a closed loop control system allowing live adjustment of the heat and cooling air input to keep an individually controlled constant thermal gradient with a homogeneous temperature distribution on the sample surface. Modeling of the test has been performed to understand the coating failure mechanism and to adapt the testing conditions such to get a failure mechanism closer to the relevant degradation mechanisms experienced in the field. Testing of coatings coming from the same production batch in various test campaign shows a low scatter in test results confirming that the burner rig test design allowed solving the test reproducibility and samples statistics issues. Examples will be shown how this burner rig test can be used for the development of lifetime prediction rules for thermal barrier coating systems.© 2014 ASME