Phase and microstructural stability of solution precursor plasma sprayed thermal barrier coatings

The phase and microstructural stability of thermal barrier coatings (TBCs) deposited using the solution precursor plasma spray (SPPS) process is studied as a function of thermal cycles at 1121 °C. In the SPPS process, an aqueous chemical precursor feedstock, that results in ZrO2–7 wt.% Y2O3 (7YSZ) ceramic coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The resulting coating has the desired non-transformable tetragonal phase structure and this phase is stable throughout the thermal cycling test. SPPS TBCs consist of ultra-fine splats and unmelted particles that include some non-pyrolyzed precursor. The non-pyrolyzed precursor in the coating was observed to decompose and crystallize during initial thermal cycling, which results in an increase of coating hardness. No sign of sintering of the ultra-fine splats in the coating was observed after 1090 cycles. The spacing of through-coating-thickness cracks (160–190 μm) does not change with thermal cycling, while the opening of cracks increases from around 0.4 μm at as-sprayed state to 1.7 μm at 40 cycles, and to 3.1 μm at 800 cycles.