Hydroxyapatite-coated bioalloy surfaces: current status and future challenges

The present review contains salient features of the most widely used methods for depositing hydroxyapatite (HA) coatings on bioalloy substrates. These methods range from the ubiquitous plasma spraying in air to the recently introduced ion beam enhanced deposition technique. Special attention is paid to the residual stress in the coating that arises from various sources and whose magnitude impacts the adhesion of the coating to the substrate and, hence, the efficacy of the coated part (see, for example, the in vivo performance of HA-coated orthopaedic and dental implants). Thus, the plethora of methods for estimating residual stresses in HA-coated systems are critically examined. The review concludes with a detailed presentation of ten areas for future research work.