Microstructure of suspension plasma sprayed multilayer coatings of hydroxyapatite and titanium oxide

The paper aims at development of the multilayer coatings of Ca5(PO4)3OH (hydroxyapatite, HA) and TiO2 onto titanium substrate using suspension plasma spraying. Two types of multilayer coatings were tested: (i) duplex coatings with suspension plasma sprayed HA layer onto TiO2 one of total thickness in the range 10 to 20 μm; (ii) gradient coatings with suspension sprayed TiO2 at bottom part of coating and gradually with changing chemical composition to achieve HA on its top of total thickness ranging from 30 to 40 μm. The aqueous suspensions were formulated with the use of a fine commercial TiO2 pigment and by milling of a coarse commercial HA powder. Morphologies of TiO2 and HA powders and pigments were characterized using a scanning electron microscope (SEM) and their granulometry was found with a laser sizer. Phase analysis of sprayed coatings was made by X-ray diffraction (XRD) and confirmed by micro-Raman spectroscopy. The morphology of sprayed coatings was characterized using SEM. Electron microprobe analysis (EMPA) was used to observe the elements distribution in tested coatings. Finally, X-ray photoelectron spectroscopy (XPS) was used to analyze quantitatively the chemical composition of powders and coatings and to find the Ca/P ratio.