EIS, Mott Schottky and EFM analysis of the electrochemical stability and dielectric properties of Ca-P-Ag and Ca-P-Si-Ag coatings obtained by plasma electrolytic oxidation in Ti6Al4V

Abstract

Ti6Al4V ELI (ASTM F136) alloy is one of the most used alloys in devices for osteosynthesis and joint replacement. However, the properties of this alloy can be improved regarding biocompatibility and the osteointegration with the bone tissue through coatings. The aim of this study was the evaluation of the electrochemical behavior of a coating obtained by plasma electrolytic oxidation on Ti6Al4V ELI, using electrolyte solutions enriched with PO 4 2-, Ca +2, Si +4 ions and various concentrations of Ag +1. The coatings were characterized by electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) in a three-electrode cell with a simulated physiological solution. The electrochemical behavior was compared with electrostatic force microscopy (EFM). The EIS results showed improved stability properties in corrosion for Ca-P-Ag coatings, compared with Ca-P-Si-Ag. Nyquist and Bode Spectra showed relaxations related to charge transference to the double electrochemical layer and the reflex of the microstructural changes and conductivity of the coating given by its composition. According to with M-S results, all the samples showed an n-type conductivity increased with silver inclusion. The potential, the phase, and amplitude of the coatings with silver inclusion compared to uncoated Ti6Al4V were observed by EFM. RESUMEN:El Ti6Al4V ELI (ASTM F136) es una de las aleaciones más empleadas en dispositivos de osteosíntesis y reemplazo articular. Sin embargo, las propiedades de esta aleación pueden ser mejoradas respecto a su biocompatibilidad y osteointegración con el tejido óseo a través de recubrimientos. El objetivo de este trabajo fue evaluar el comportamiento electroquímico de un recubrimiento obtenido por oxidación por plasma electrolítico sobre Ti6Al4V ELI empleando soluciones electrolíticas enriquecidas con iones de PO 4 2-, Ca +2 , Si +4 y varias concentraciones de Ag +1. Los recubrimientos se caracterizaron mediante espectroscopia de impedancia electroquímica (EIS) y Mott-Schottky (M-S) en una celda de tres electrodos con una solución fisiológica simulada. El comportamiento electroquímico se contrastó con microscopía de fuerza electrostática (EFM), donde se analizó el potencial eléctrico del recubrimiento. Los resultados de EIS mostraron mejores propiedades de estabilidad frente a la corrosión en los sustratos recubiertos con Ca-P-Ag, comparados con los recubiertos con Ca-P-Si-Ag. Los espectros de Nyquist y Bode mostraron relajaciones relacionadas con la transferencia de carga hacia la doble capa electroquímica como reflejo de los cambios microestructurales y de conductividad de los recubrimientos, dada por la presencia particular en cada caso, de los elementos contenidos en estos. De acuerdo con los resultados de M-S, todas las muestras ensayadas presentaron un comportamiento con dopado tipo n, cuya conductividad incrementó con la inclusión de plata. Mediante EFM se observaron mayores contrastes en el potencial, fase y la amplitud de los recubrimientos con mayor cantidad de plata respecto al Ti6Al...

Evolution of anodised titanium for implant applications

“…To date, considering the growth of the nano-thick oxide film (Figure 8(c)) in the Ti/TNTs interface (Figure 8(b): ox 1 to ox 2 , as observed elsewhere [290,320,334]) together with the existence of tribofilm-laminated porous walls on top of the TNTs (Figure 8(d)), the fitting model as shown in Figures 8(e) and 8(f) has been shown to be the most appropriate electronic fitting (lowest accuracy) ever reported for the anodised titanium [144,286,330,332].…”

“…The anodised titanium is a structurally an advanced semiconductor [87,167,332,335,336]. Precise electronic fitting of its electrochemical characteristics, which could reduce the high-sensitivity of χ 2 , could result in improved analysis of the porous anodic properties of the crystallised TiO 2 semiconductor, validating the measured surface stability in the electrochemical circuit, which was neglected in other studies [138,141,144,189,209,214,331,332,333]. From most of these corrosion studies, the increase of current density in the passive region of TiO x was owing to the formation of metastable oxides such as Ti 3 O in the anodised layers, which were transformed to a more stable titanium oxide, i.e., TiO 2 when the voltage in the test circuit was increased.…”

“…However, throughout previous studies [ 138 , 141 , 144 , 189 , 209 , 214 , 331 , 332 ], it can be seen that the building blocks (interconnected coalesced subsurface layers adjacent to the electrolyte interfaces) that create windows (pore walls to electrolyte interfaces) in electrode/electrolyte interfaces circuitry are rarely considered to be double-layer structure of porous anodised titanium, as shown in Figure 8 (e) [ 286 ] ( similar illustration was found elsewhere [ 39 , 333 ]). As a result, studies have produced higher statistical functions (χ 2 ) in their electronic fittings [ 306 ], yet the issue is that some of the tests did not report their χ 2 fitting accuracy [ 138 , 189 , 209 ], and thus it is hard to be certain that the integrity of their circuit fitting prior to the EIS calibration of the testing being optimised.…”

“…The anodised titanium is a structurally an advanced semiconductor [ 87 , 167 , 332 , 335 , 336 ]. Precise electronic fitting of its electrochemical characteristics, which could reduce the high-sensitivity of χ 2 , could result in improved analysis of the porous anodic properties of the crystallised TiO 2 semiconductor, validating the measured surface stability in the electrochemical circuit, which was neglected in other studies [ 138 , 141 , 144 , 189 , 209 , 214 , 331 , 332 , 333 ].…”