Direct Simulation of CMAS Infiltrating TBC Microstructure

CMAS, a category of atmospheric debris, causes erosion of thermal barrier coatings in aircraft engines. The CMAS melts, and infiltrates the microstructure of these coatings. Understanding this infiltration process is key to develop methods to prevent infiltration and mitigate damage to the coating. Conventional wisdom classifies the infiltration process as a capillary-driven flow. To evaluate this conventional wisdom, the process was directly resolved numerically using a finite-volume, multiphase, volume-of-fluid simulation. Results from this simulation method were compared to experiments and analytical models. Results show that the Ohnesorge Number drives the equilibrium point of the capillary-driven flow, where larger Ohnesorge Number lead to shorter infiltration depth. The results disagreed with experiments and the analytical pipe models, but agreed well with a differential equation model based on capillary flow in a rectangular microchannel, and a new proposed model that is tuned specifically for feathery coating geometries. This finding supports the idea that capillary flow is not the only dominant feature in the infiltration process, and other phenomenon, such as chemical processes, may have first-order effects on the flow physics.