Investigation of porous coatings obtained on Ti-Nb-Zr-Sn alloy biomaterial by plasma electrolytic oxidation: characterisation and modelling

In the paper, the fabrication method and characteristics of porous coatings on Ti-Nb-Zr-Sn alloy biomaterial obtained by plasma electrolytic oxidation (PEO) are presented. The PEO process was performed at two voltages of 180 +/- 10 and 450 +/- 10 V, respectively, during 3 min of treatment in the electrolyte based on orthophosphoric acid with copper II nitrate of initial temperature of 20 +/- 2 A degrees C. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS) and 2D roughness measurements were performed on the samples. The study results indicate an enrichment of the porous layer (18 and 21 mu m thick, for 180 and 450 V, respectively) in two elements, P and Cu, coming from the electrolyte used. The analysis performed based on the SEM, EDS, GDOES and XPS results obtained shows that after the PEO treatment, three sub-layers of the coating can be distinguished and separated and two models are proposed to fit these findings. It was found that both the contents of copper and phosphorus in the surface layer as well as the thickness of porous coating can be controlled to some extent by the PEO parameters. The greatest achievement of the presented work is the lack of toxic tin inside the top surface layer of the porous coatings as well as the enrichment of the coatings with copper ions up to 2.3 at%. In authors' opinion, the finding of the transition layer enriched within hydrogen and nitrogen ions can be interpreted by the presence of molecules of phosphoric acid and copper nitrate occurring in that sub-layer. This is a great advancement in the field of identification of the layers obtained by PEO.