Development of Porous Coatings Enriched with Magnesium and Zinc Obtained by DC Plasma Electrolytic Oxidation

Coatings with developed surface stereometry, being based on a porous system, may be obtained by plasma electrolytic oxidation, PEO (micro arc oxidation, MAO). In this paper, we present novel porous coatings, which may be used, e.g., in micromachine's biocompatible sensors' housing, obtained in electrolytes containing magnesium nitrate hexahydrate Mg(NO₃)₂·6H₂O and/or zinc nitrate hexahydrate Zn(NO₃)₂·6H₂O in concentrated phosphoric acid H₃PO₄ (85% w/w). Complementary techniques are used for coatings' surface characterization, such as scanning electron microscopy (SEM), for surface imaging as well as for chemical semi-quantitative analysis via energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES), and X-ray powder diffraction (XRD). The results have shown that increasing contents of salts (here, 250 g/L Mg(NO₃)₂·6H₂O and 250 g/L Zn(NO₃)₂·6H₂O) in electrolyte result in increasing of Mg/P and Zn/P ratios, as well as coating thickness. It was also found that by increasing the PEO voltage, the Zn/P and Mg/P ratios increase as well. In addition, the analysis of XPS spectra revealed the existence in 10 nm top of coating magnesium (Mg²⁺), zinc (Zn²⁺), titanium (Ti⁴⁺), and phosphorus compounds (PO₄^3-, or HPO₄^2-, or H₂PO₄^-, or P₂O₇^4-).