Fundamental and practical evaluation of plasma electrolytic oxidation coatings of titanium

Abstract

Plasma electrolytic oxidation (PEO) is a unique coating technology, which enables fabrication of high performance ceramic surfaces on the light metals, with thickness, topography, composition and structure controllable by the process parameters. Here the growth of anodic coatings on titanium, important due to their biocompatibility and potential use in dental and orthopaedic implants, is investigated at a fundamental level. The work has achieved major impact in understanding coating growth from original tracer studies. For the first time discrete thickening at sites of dielectric breakdown, local growth at the metal/oxide interface and destruction of earlier formed oxide were shown, which offered a new perspective on coating processes, including those aspects affecting coating efficiency – a critical feature in a relatively high cost technology. Furthermore, based on the fundamental findings, routes to tailored coating compositions for enhanced biocompatibility have been developed, and carried through to in vitro evaluation. Fabrication of amorphous oxide, which is beneficial to biocompatibility, was achieved through anodising at controlled current densities and with use of complexing agents. These promote uniform coating composition, with major increase in both the electrolyte element content in the coating and the coating pore size. Corrosion tests of the coatings revealed low titanium release into physiological solution, which is important to avoid adverse tissue reaction. In vitro tests confirmed good attachment of human osteoblast cells to the fabricated titanium oxide coatings, expressed as a well spread cell morphology with cytoplasmic projection development. The work is proceeding with a focus on optimising coatings for in vivo implant trials.