Development of a Hydroxyapatite Coating on AZ31 Magnesium Alloy for Use in Biodegradable Implants

Abstract

Known to have poor corrosion resistance for general engineering applications, magnesium alloys have recently attracted great interest among researchers for use as biodegradable osteo-implants. The extremely low corrosion resistance of these alloys, which has been a major drawback for its use as light-weight surgical implants, is an advantage point for the application of biodegradable implants. Ideally, biodegradable implants should have a controllable dissolution rate, or in other words, a delayed corrosion reaction on its surface to maintain mechanical integrity until the bone restores itself. Thereafter, the implant should be degraded over time in a controlled manner and reabsorbed safely into the body as the healing process is completed – so that no secondary surgery is needed to remove the implants. The main challenge in this research work is to investigate an approach of improving corrosion resistance of AZ31 Magnesium alloy, by introducing a corrosion-rate controlling layer between the implant material and the body fluid. This research work explores the use of plasma spray as the deposition method for applying the hydroxyapatite (HAp) coatings on AZ31 substrates. The spray parameters used in all trials have successfully led to production of a porous coating structure specifically designed for corrosion-rate controlling layer for biodegradable implants. Corrosion-tested samples were characterised to evaluate their degradation behaviour to establish an understanding of the corrosion mechanism of the designed coating system in this work. An improved corrosion resistance has been achieved by applying a plasma-sprayed HAp coating on bare AZ31 material, in which the corrosion rate is slower than that of uncoated material. Additionally, the carbonate treatment of the HAp-coated AZ31 has successfully enhanced the corrosion characteristic of the designed coating system. Despite the fact that AZ31 is corroding due to the presence of corrosive medium at the substrate-coating interface area, the coating is still adherent to allow a controlled degradation mechanism - thus preventing a catastrophic failure of the designed coating system.