Bimodal Distribution of Microstructure and Mechanical Properties of Plasma Sprayed Nanostructured Al2O3―13wt% TiO2 Coatings

Nanostructured Al2O3―13wt% TiO2 coatings were prepared by plasma spraying agglomerated nanocrystalline powders formed via spray drying method. The phase composition, microstructure, microhardness and fracture toughness of the coatings were investigated. It is found that the nanostructured coatings exhibited a unique bimodal microstructure consisting of two distinct regions: one of the regions is Fully Melted (FM) and solidified as lamellar structure, and the other is Partially Melted (PM) particulate microstructure with embedded nano or sub―micron particles retained from the starting powders. Furthermore, the percentage of PM region, which is proportional to unmelted α―Al2O3 nanoparticles, in the coatings can be controlled by the critical plasma spray parameter. Such a characteristic of blended microstructure of the coatings is clearly confirmed from a bimodal distribution of their mechanical properties. The Weibull statistical analysis shows that the microhardness and fracture toughness of the coatings present a bimodal distribution, the mean value and the dispersity of microhardness of PM region are lower than those of FM region, but the mean value and the dispersity of fracture toughness of PM region are both higher than those of FM region.