Reliable protective coatings for severe environments
Thermal protection systems based on oxide coatings are widely used in gas turbine engines for aircraft propulsion and thermal power generation. The benefits of these coatings reside in their ability to inhibit degradation of the underlying structural superalloy component by establishing a thermal gradient. To meet the emerging demand for carbon dioxide (CO2) emission reduction, the application of temperature resistant coatings to internally cooled components has improved the thermal efficiencies of gas turbines. A university team from California joined with European counterparts in HIPERCOAT project to investigate the mechanisms governing the stability and reliability of thermal barrier coatings (TBCs). These systems are inherently metastable and their durability is limited because of their susceptibility to erosion caused by sequential small particle impacts. The plastic deformation resistance was explored by means of a novel impression probe designed for TBCs with columnar microstructures created by electron beam physical vapour deposition techniques. For testing the spherical indentation, the sample coating was deposited on a rigid alumina substrate and inserted into a high temperature servo-hydraulic loading system. The utility of the experimental information was dependent on a numerical procedure for deconvoluting aspects of stress response to load displacements measurements. The modelling method elucidates the extent of plastic deformation and densification, as well as column distortions caused by impression. By embodying salient constituent properties such as contact and intercolumnar friction, it furthermore provides the essential tools for estimating deformation heterogeneities observed experimentally. While initial efforts focused on thermal barrier coatings, the results should be applicable to a wider range of systems for which structural integrity must be sustained under severe environmental conditions.
