Step-by-step investigation of degradation mechanisms induced by CMAS attack on YSZ materials for TBC applications

Over the past decades, Thermal Barrier Coatings (TBCs) have become essential parts in gas turbine engines. In working conditions, TBCs are subject to many kinds of degradation (erosion, foreign object damage (F.O.D), oxidation, etc.) which deteriorate integrity and mechanical properties of the whole system. Moreover, with the aim to increase the turbine inlet temperature, a new type of damage has been highlighted: corrosion by molten Calcium-Magnesium-Alumino Silicates, better known as CMAS. In this paper, interactions between yttria-stabilized zirconia (YSZ) materials synthesized via sol-gel process and synthetic CMAS powder were investigated via a step-by-step methodology. The approach was conducted starting from the more severe conditions of interactions and then gradually gets closer to the interactions taking place in service. It was proved that CMAS can induce faster densification of the ceramic leading to a loss of strain tolerance of the protective coating. Besides, a dissolution/re-precipitation mechanism can also take place between YSZ and CMAS leading to the transformation of the initial tetragonal yttria-stabilized zirconia into globular particles of monoclinic zirconia. CMAS were also found to infiltrate the entire thickness of both EBPVD and sol-gel YSZ coatings at 1250 °C for 1 h. Nevertheless, the original non-oriented microstructure provided by sol-gel route leads to a different way of interaction due to the high reactivity of sol-gel precursors and materials. The behaviors of EBPVD and sol-gel coatings under CMAS exposure are discussed in this paper.