Microstructure investigation of plasma sprayed alumina 13 weight percent titania coatings from nanocrystalline feed powders

Date of Completion

January 2002


Engineering, Materials Science




The development of constituent phases and microstructure in air plasma sprayed alumina (Al2O3) thirteen weight percent titania (TiO2) coatings from reconstituted nanocrystalline feed powder (nanopowder) was investigated as a function of processing conditions and compared to a conventional coating created from micron-sized feed powder. The microstructure of the nanopowder coating was found to consist of a mixture of two distinct regions; one was completely melted and quenched as splats, and the other partially melted with a particulate microstructure retained from the starting powder. The melted regions predominantly consisted of γ-Al2O 3 with dissolved Ti4+ that varied in morphology and size from a few nanometers to several microns. Based upon these experimental observations, a theory is developed that explains the mechanism behind the morphological differences observed across the melted splat structures. In addition, differences between the melted structures in the nanopowder coatings and the conventional coating are explained using this theory. The partially melted regions were found to be primarily submicrometer-sized α-Al2O 3 particles in an amorphous TiO2 matrix containing small amounts of γ-Al2O3 with dissolved Ti4+ . For the nanopowder coatings, the ratio of the melted splat microstructure to the particulate microstructure and thus the ratio of the γ-Al 2O3 to α-Al2O3 can be controlled by a plasma spray parameter, defined as the critical plasma spray parameter (CPSP). The conventional coating microstructure was insensitive to CPSP, consisting primarily of fully melted splats. The ability to vary the microstructure of the nanopowder coating through CPSP has resulted in bimodal distributions of microstructure and grain size that have allowed these coatings to significantly surpass the conventional coating in several measures of mechanical properties. ^