Erosion Behaviour of Thermal Barrier Coatings

Abstract: Thermal barrier coatings (TBCs) are advanced material systems used in the hot sections of gas turbines. The TBCs are designed to provide insulation against hot gases by a ceramic top coat and to provide oxidation and corrosion resistance by a metallic bond coat. As the operating environment is harsh and complex, the TBC often requires stricter material properties. Failure of TBCs can limit the longevity of the turbine severely. In this study, failure caused by erosion has been the main focus. The erosion behaviour of TBCs processed by atmospheric plasma spay (APS), electron beam physical vapour deposition (EB-PVD), and plasma spray physical vapour deposition (PS-PVD) has been studied by an experimental investigation and a literature study. The erosion performance of different TBCs was studied by conducting erosion tests under 90° and 15° alumina particle impact (50 μm) and measuring the weight loss and thickness loss of the ceramic top coat. Variables affecting the erosion behaviour were studied by means of scanning electron microscopy (SEM), investigating the microstructure, the erosion damage, porosity content, and column density. Hardness tests were also conducted to investigate a potential correlation between hardness and erosion performance. It was evident that the 8YSZ top coat processed by EB-PVD had higher erosion resistance than APS, which in turn had higher erosion resistance than PS-PVD. Their microstructures are significantly different, resulting in different erosion failure mechanisms. APS TBCs have a splat-on-splat lamellar microstructure, and the failure mechanismsare ploughing of furrows, splat boundary failure, and tunneling via pores. In contrast, EB-PVD TBCs have columnar microstructure and fail by near-surface cracking. The investigated PS-PVD TBC had a feathery columnar microstructure, containing many large grain boundaries and flaws, making grain boundary failure the governing mechanism. The APS and EB-PVD TBCs impacted at a 90° angle had significantly higher erosion rates than those eroded at 15°, which also was reported in literature. However, the opposite was observed for the PS-PVD TBCs. The level of porosity and hardness of the TBC top coat was found to affect the erosion rate, even though no evident correlations could be observed in this study. No factor alone was found to dictate the erosion behaviour of the investigated TBCs. Based on the literature study and findings in the experimental study, a TBC with good erosion performance has, in general, low porosity, few defects, high hardness and high fracture toughness. Specifically for APS TBCs, good splat bonding is favourable and for EB-PVD and PS-PVD it recommended to have high column density, columns orthogonal to the substrate, and low gap width between the columns.