Characterization of porous phosphate coatings enriched with calcium, magnesium, zinc and copper created on CP titanium grade 2 by plasma electrolytic oxidation

Abstract

In the paper, the effect of voltage increase (from 500 VDC up to 650 VDC) on the structure and chemical composition of the porous coating on titanium made by Plasma Electrolytic Oxidation is presented. Phosphates-based coatings enriched with calcium, magnesium, zinc, and copper in electrolyte based on 1 L of 85% concentrated H3PO4, with additions of Ca(NO3)2·4H2O, and Mg(NO3)2·6H2O, and Zn(NO3)2·6H2O, and Cu(NO3)2·3H2O, are described. The morphology and chemical and phase composition are evaluated using SEM, EDS, XRD, XPS, GDOES, and CLSM. Based on these analyses, it was found that PEO coatings are porous and enriched with calcium, magnesium, zinc and copper. They consist mainly of the amorphous phase, which is more visible for higher voltages; this is correlated with an increase in the total PEO coating thickness (the higher the voltage, the thicker the PEO coating). However, for 650 VDC, an amorphous phase and titanium substrate were also recorded, with a signal from Ti2P2O7 crystalline that was not observed for lower voltages. It was also found that all obtained coatings may be divided into three sub-layers, i.e., porous, semiporous, and transitional.