Effect of electrolyte composition on the surface characteristics of plasma electrolytic oxidation coatings over Ti[sbnd]40Nb alloy

Abstract

The present work aims to develop corrosion-resistant and cytocompatible coatings on Ti40Nb alloy by the plasma electrolytic oxidation (PEO) process. Usually, PEO coatings on niobium substrate with superior surface characteristics are obtained using a silicate-based electrolyte system, and a phosphate-based electrolyte system is used for titanium substrate. Till date, electrolyte system composition is not optimised for the PEO coatings on Ti40Nb alloys. To study the influence of electrolyte composition on PEO coating properties over Ti40Nb alloy, three different concentrations of phosphate and silicate (75:25, 50:50, and 25:75) and a fixed amount of KOH were used, and their surface characteristics and corrosion properties were evaluated in this study. We found that coatings formed with an equal proportion of phosphate and silicate in an electrolyte system are dense, compact, thick and corrosion-resistant. A coating aimed for high bioactivity is also developed by adding hydroxyapatite (HA) nanoparticles to an equal proportion (50:50) electrolyte system. After the successful development of the PEO coatings over Ti40Nb alloy, its phase composition, surface morphological features, wettability, scratch resistance, and corrosion properties are examined. The cell viability and live/dead cell morphology of L-929 fibroblast cells were evaluated for untreated Ti40Nb alloy, PEO-treated Ti40Nb alloy, and Ti40Nb alloy PEO-treated with hydroxyapatite nanoparticles. All surface-modified samples exhibited better corrosion resistance than the untreated alloy. Ti40Nb alloy PEO treated with phosphate: silicate electrolyte system (50:50) exhibited better scratch resistance, higher wettability, bioactivity, favourable chemical composition, and surface morphological features than the other ratios of phosphate and silicate (25:75 and 75:25). Meanwhile, Ti40Nb alloy PEO treated with hydroxyapatite nanoparticles addition showed better cytocompatibility than without HA addition.