Microstructural characterization of plasma sprayed TiO2 functional coating with gradient of crystal grain size

The paper aims at development of a coating with a gradient in grain size. A first layer was deposited directly onto stainless steel substrate by plasma spraying of a coarse TiO2 powder. This layer had thickness in the range from 30 to 50 μm. A second layer was deposited on the first one using plasma spraying of different aqueous suspensions of two TiO2 pigments. The layer had thickness smaller than 10 μm. Morphologies of TiO2 powder and pigments were characterized using a scanning electron microscope (SEM) and their sizes were tested with a laser sizer. Phase analysis of powder and pigments, made by X-ray diffraction (XRD), enabled only rutile phase to be discovered. A mixture of rutile and anatase was found in the suspension sprayed coatings. The presence of anatase in sprayed coatings was an interesting and original result, which was also confirmed by Raman spectroscopy. The formation of this phase was tentatively explained by a high cooling rate of small splats. The suspension plasma spraying was optimized in order to obtain homogeneous and dense deposits. The morphology of sprayed coatings was characterized using SEM. Crystal grain sizes were estimated using XRD. The sizes were in the range of a few tenths to one hundred nanometers in the initial powders. They were clearly smaller in the coatings sprayed using coarse powder but remained quite similar in suspension sprayed layers. Finally, X-ray photoelectron spectroscopy (XPS) was used to analyze quantitatively the chemical composition of powders and coatings.