The role of PVD sputtered PTFE and Al2O3 thin films in the development of damage tolerant coating systems

Abstract

The surfaces of artificial joints are susceptible to premature wear which reduces their service life leading to the increased risk of revision arthroplasty. Tribological coatings having low coefficients of friction (CoF) and which are also biocompatible are therefore required to minimise such risks. Metal matrix composite (MMC) coatings exhibiting a modified toughness structure, thus reducing the potential to crack were successfully fabricated by cold spray (CS) technology. The coatings comprised of an ultra-low wear polytetrafluoroethylene (PTFE) + alumina (Al2O3) composite, firstly delineated by Burris and Sawyer in 2006, were prepared on cold sprayed substrate and the functionality of such coatings in terms of enhanced frictional properties was investigated. In this study, PTFE + Al2O3 thin film coatings were prepared by physical vapour deposition (PVD) and incorporated with cold sprayed MMC coatings to produce a damage tolerant coating system designed to extend the service life of artificial movable joint surfaces. Surface characterisation analysis indicated the formation of a PTFE-Al2O3 nanocomposite forming between the PTFE and Al2O3. This composite thin film exhibited excellent friction reducing ability up to an applied load of 7 N. Regardless of whether the Ti-6Al is manufactured by a layer-by-layer approach or co-sputtering method, the CoF is reduced to below 0.1 at the start of the test. Furthermore, the results reveal that the PVD sputtered PTFE + Al2O3 thin film functionalises the cold sprayed surface by offering a self-lubricating effect and that PVD combined with CS is reliable for devising material systems operating in high-wear applications.