Damage mechanisms of EB-PVD thermal barrier coatings under combined CMAS and erosion loads

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Damage of thermal barrier coatings (TBCs) in aero-engines and stationary gas turbines by deposits of calcia-magnesia-alumina-silica (CMAS) has gained increasing importance. At lower operating temper-atures erosion is another important damage phenomenon. The ultimate goal of this project is un-changed: to reveal the combined damage by CMAS and particle erosion, to get systematic correlations between the underlying parameters, and to discover the damage mechanisms of the TBCs under this complex loading situation. Therefore, the impact of CMAS infiltration on the erosion behavior was studied systematically for EB-PVD TBCs. In the first funding period it was found that the microstructure of 7YSZ has a large impact on CMAS infiltration kinetics, erosion resistance, and on the combined damage. Interestingly, the erosion resistance of infiltrated TBCs under perpendicular particle impact was often improved at the expense of mechanical integrity of the TBC. This is also valid for novel CMAS-resistant coatings such as Gd-zirconate and Al2O3. The chemistry of both deposit and TBC, and infiltration depth that would be different under a temperature gradient rule the formation of reaction phases and damage progression in the TBCs. First analytical models for the progress of infiltration, calculation of phases (Calphad), and measurements of mechanical properties correlate qualitatively well with experiments and open opportunities for improving the protective nature of the coatings and prediction of damage. Based on the results from the first funding period, both infiltration and erosion resistance shall be improved simultaneously during the second and final project phase. This will be achieved by using EB-PVD 65%-YZ (ZrO2-65 wt% Y2O3) TBCs and dense or graded SHVOF-Al2O3 coatings on top of 7YSZ. 3D simulation models based on real microstructures shall integrate the influence of morphology on infiltration, unfold more scientific based possibilities for further improvements and reveal the damage mechanisms in more depth. Infiltration by a reduced number of deposits will be performed under cyclic conditions in a temperature gradient to provide more realistic stress states in the coating. For the erosion experiments variation of both impact angle and particle size are within the main focus to simulate the damage progression under more realistic conditions that are similar to a turbine. The preliminary erosion model that quantifies various factors affecting the erosion behavior will be enlarged by incorporating additional aspects; for most of them sufficient data are not yet available. This erosion model will then reveal the complex interaction between all factors.