Influence of original powders on the microstructure and properties of thermal barrier coatings deposited by supersonic atmospheric plasma spraying, Part I: Microstructure

In this work, two types of yttria-stabilised zirconia (YSZ) powders, a microsized powder and a reconstituted nanostructured powder, were used as the original feedstock for depositing thermal barrier coatings (TBCs) using a high-efficiency supersonic atmospheric plasma spraying (SAPS) system. The effect of the original powder on the coating microstructure was studied by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results indicated that the microsized powder was fully melted in the plasma jet and that the as-sprayed conventional coating (named MC) was composed of regular-shaped tetragonal ZrO2 with grain size of 200–500 nm. However, the cross-section morphology of the water-quenched powders revealed that the reconstituted nanostructured powder was partially melted during plasma spraying and that the as-sprayed nanostructured coating (named NC) exhibited a multi-modal microstructure that mainly consisted of unmelted nanoparticles (30–50 nm) and nanograins (60–110 nm), with the latter being the main microstructure of the coating. One visible polycrystalline region consisting of 10 nm grains was also found in NC. In addition, due to the full melting of the microsized powder in the plasma jet, MC exhibited a lower porosity and higher microhardness and Weibull modulus compared with those of NC. In the following paper (Part II), the thermo-mechanical properties, such as thermal shock resistance, oxidation resistance and thermal insulation performance, of the above two coating types will be further studied.