Using Molecular and Atomic Emission Spectroscopy to Better Understand Suspension Plasma Spray

Plasma spray processes have increased in complexity with the introduction of suspensions (SPS) and solutions precursors (SPPS), both aiming at depositing coatings with columnar or nanostructured features. Prior to solid particle heating and melting, one has to consider liquid precursor atomization, vaporization, particle aggregation or precursor precipitation sub-processes. Often, the liquid carrier or solvent vaporization consumes a large amount of energy, cooling down the plasma jet and making the thermal treatment less efficient. In this study, we are using optical emission spectroscopy (OES) to monitor some of these sub-processes and to increase our understanding of plasma-precursor interactions in SPS and SPPS. The precursors, a suspension of TiO2 nanoparticles in water without or with barium acetate in solution, is fed axially within an Ar-O2 plasma generated by an inductively-coupled RF plasma torch. Precursor decomposition is observed through molecular and atomic emission using a spectrometer. Raw OES data is processed to build atomic and molecular species temperature and relative density maps, while the deposited TiO2 and BaTiO3 coatings are characterized by X-ray diffraction and scanning electron microscopy for phase and morphology. Interestingly, our results emphasize the importance of relative density maps to correlate the coating microstructure with the SPS process parameters.