An Optical Emission Spectroscopy Study of Plasma-Precursor Interactions in Solution Precursor Plasma Spray

Solution precursor plasma spray (SPPS) provides a finer microstructure and a better control of the coating chemistry when compared to the conventional plasma spray of microsized powders. Nevertheless, liquid injection brings a new complexity to the process since the injected precursors are undergoing several physical and chemical transformations before impinging the substrate: the solvent evaporates, increasing the solution concentration, and then, the precursor will precipitate, melt and partially evaporate. The resulting coating microstructure highly depends on the history of the droplets inside the plasma. As such, achieving a better control of SPPS requires a thorough understanding of these mechanisms. Optical emission spectroscopy (OES) being a non-invasive tool, it is used to observe several atomic and molecular emission lines, enabling the study of the precursor transformations inside the plasma. In this study, SPPS is employed to deposit coatings of barium titanate (BaTiO3), a material with piezoelectrical properties. The precursors consist in a solution of titanium butoxide, barium acetate, ethanol and acetic acid. Different precursor compositions are studied to discern their effect on the plasma and on the coating morphology. For example, one can follow the evaporation and the decomposition of titanium butoxide across the plasma jet.