Dense to Columnar Microstructure Evolution of Ba(Mg1/3Ta2/3)O3 TBCs Through Tailoring of Suspension Plasma Spray Conditions

Inductively-coupled plasma spraying is used to deposit Ba(Mg1/3Ta2/3)O3 (BMT) thermal barrier coatings. Three distinct solutions/suspensions were chosen as precursors: 1. Ba and Mg nitrates + Ta oxide in water and ethanol; 2. Ba and Mg nitrates + Ta ethoxide in ethanol; 3. Ba and Mg acetates + Ta ethoxide in ethanol and acetic acid. The substrate surface was either sandblasted or polished. By combining the appropriate precursor formulation, droplet size, feeding rate, spraying distance, spraying direction and substrate preparation, it is possible to tune the BMT coating microstructure from dense splat-like layers to columns. Of those parameters, the spraying distance and the precursor formulation are considered the most crucial. By changing the spraying direction, shadow effects formed a pseudo-columnar structure, which only occurred on a rough substrate surface. The combination of a homogeneous precursor solution, short spray distance and sandblasted substrate lead to a columnar structure. In particular, the vertical grains grew across 2 or 3 splat layers, which makes this BMT coating significantly different from the cluster structure commonly observed in suspension plasma spraying. The thermal gradient between the coating top surface and the substrate bottom surface is likely responsible for the formation of vertical grains.