Simulation of stresses in TBC-EBC coating systems for ceramic components in gas turbines

Thermal barrier coatings (TBCs) and environmental barrier coatings (EBCs) improve the efficiency and durability of turbine blades. Plasma-sprayed yttria-stabilized zirconia (YSZ) coupled with dense, chemical vapor deposited functionally graded mullite is a promising TBC-EBC system. A computational model is constructed to consider effects of functional grading of the EBC, coating geometry, as well as density and depth of vertical cracks in the TBC on the stress distribution in the coating system. Functional grading of the EBC coating is found to reduce interfacial σxx mismatch at the interfaces of the coating system. The ratio of EBC to TBC thicknesses has a much smaller effect compared with the effect of grading. Vertical cracks increase TBC compliance but introduce substantial σxx fluctuations in the EBC, especially when cracks are through-thickness. Increased spacing between cracks is accompanied by greater σxx fluctuation. While TBC stresses are minimized with through-thickness and more distanced cracks, this geometry is not ideal for the EBC. The absence of cracks produces the best results for the EBC layer. If cracks are present, sub through-thickness, closely spaced cracks achieve the lowest magnitude and most uniform stresses in the EBC.