Thermal activation of Ti(1-x)Au(x) thin films with enhanced hardness and biocompatibility

Abstract

The lifetime of orthopaedic implants can be extended by coating the softer Ti6Al4V alloy with harder biocompatible thin films. In this work, thin films of Ti(1-x)Au(x) are grown on Ti6Al4V and glass substrates by magnetron sputtering in the entire x = 0–1 range, before their key biomechanical properties are performance tuned by thermal activation. For the first time, we explore the effect of in-situ substrate heating versus ex-situ post-deposition heat-treatment, on development of mechanical and biocompatibility performance in Ti–Au films. A ∼250% increase in hardness is achieved for Ti–Au films compared to bulk Ti6Al4V and a ∼40% improvement from 8.8 GPa as-grown to 11.9 and 12.3 GPa with in-situ and ex-situ heat-treatment respectively, is corelated to changes in structural, morphological and chemical properties, providing insights into the origins of super-hardness in the Ti rich regions of these materials. X-ray diffraction reveals that as-grown films are in nanocrystalline states of Ti–Au intermetallic phases and thermal activation leads to emergence of mechanically hard Ti–Au intermetallics, with films prepared by in-situ substrate heating having enhanced crystalline quality. Surface morphology images show clear changes in grain size, shape and surface roughness following thermal activation, while elemental analysis reveals that in-situ substrate heating is better for development of oxide free Ti3Au β-phases. All tested Ti–Au films are non-cytotoxic against L929 mouse fibroblast cells, while extremely low leached ion concentrations confirm their biocompatibility. With peak hardness performance tuned to >12 GPa and excellent biocompatibility, Ti–Au films have potential as a future coating technology for load bearing medical implants.