Microstructural characteristics and formation mechanism of Al2O3–13 wt.% TiO2 coatings plasma-sprayed with nanostructured agglomerated powders

Nanostructured Al2O3–13 wt.% TiO2 coatings were prepared by plasma spraying with agglomerated powders. The microstructural characteristics of the feedstock and the coating were investigated by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). Moreover, the microstructural formation mechanism of the coating was analyzed. The results show that the ceramic coating consists of both fully melted regions and partially melted regions, and the fully melted region has a lamellar-like structure as the conventional coating. In terms of microstructures, the partially melted regions can be divided into liquid-phase sintered regions (a three-dimensional net or skeleton-like structure: Al2O3-rich submicron particles embedded in the TiO2-rich matrix) and solid-phase sintered regions (remained nano-particles). The fully melted region, liquid-phase sintered region and solid-phase sintered region of the coating derive from the region of the feedstock, where the corresponding temperature during plasma spraying is beyond 2045 °C (melting point of Al2O3), between 1840 °C (melting point of TiO2) and 2045 °C and below 1840 °C, respectively. The formation of the solid-phase and liquid-phase sintered regions in the partially melted region is attributed to the melting point difference of A12O3 and TiO2.