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

Abstract

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 (PO43−, or HPO42−, or H2PO4−, or P2O74−).

Metal Ions Supported Porous Coatings by Using AC Plasma Electrolytic Oxidation Processing

“…The scanning electron microscopy images, with magnification of 4000× show that there are significant differences between the coatings fabricated under different cathodic conditions (−35 V and −135 V) and the same anodic voltage of +400 V, as presented in Figure 2 . Those samples obtained under +400/−35 V have a porosity similar to those obtained under DC conditions [ 17 , 18 , 19 ]. The coatings fabricated at −35 V cathodic voltage revealed less cracked surfaces than those obtained at −135 V. The usage of a cathodic voltage of −35 V provides coatings with a “more uniform” porous structure, but still with the observed regions that differ regarding pores size and shape ( Figure 3 and Figure 4 ).…”

“…It should be noted that fabrication of coatings of the same electrochemical properties is possible in electrolytes of lesser amount of salt in the solution and lower PEO voltage. Several years of research have shown that it is possible to obtain in DC-PEO process the calcium–phosphate coatings (hydroxyapatite-like structures) [ 18 ], which can be enriched with magnesium ions [ 19 ], resulting in faster wound healing, as well as antibacterial zinc [ 20 ] and copper [ 21 ] ions. In addition, it was found that the cerium-doped hydroxyapatite coatings have antimicrobial properties [ 22 ].…”

Phosphate Porous Coatings Enriched with Selected Elements via PEO Treatment on Titanium and Its Alloys: A Review

“…On the other hand, authors use with success phosphoric-based electrolytes to fabricate coatings containing copper under Direct Current Plasma Electrolytic Oxidation (DC-PEO) condition [171][172][173][174][175][176][177][178][179][180][181][182], which was implemented through usage of electrolytes contain dissolved copper(II) nitrate in concentrated (85 wt.%) orthophosphoric acid. It should be pointed out that this type of electrolyte allows to prepare porous Cu-enriched coatings not only on titanium, but also on its alloys, such as Ti-Nb-Zr-Sn [183] or nitinol [184], as well as on Ti6Al4V [185,186], TNZ [186].…”

Novel Porous Phosphorus–Calcium–Magnesium Coatings on Titanium with Copper or Zinc Obtained by DC Plasma Electrolytic Oxidation: Fabrication and Characterization

“…It should also be pointed out that in hydroxyapatite-like structures it is possible to substitute the Ca 2+ ions for Ca 2+ , Mg 2+ , Cu 2+ , and Zn 2+ , as well as OH – for Cu + , which will be used in the fabrication of novel PEO coatings. The porous calcium–phosphate coatings obtained on titanium [ 117 , 118 , 119 ] and enriched with biocompatible magnesium, which causes faster wound healing [ 120 , 121 , 122 , 123 , 124 , 125 ], as well as antibacterial zinc [ 126 , 127 , 128 , 129 , 130 , 131 , 132 ] and copper [ 133 , 134 , 135 , 136 ], may be used as biomaterial, which will be fully accepted by the tissue environment.…”