Plasma nitriding and duplex coating of biocompatible surfaces for low friction and wear resistant applications

Abstract

The 316L austenitic stainless steels, Ti-6A1-4V and Ultrahigh molecular weight polyethylene (UHMWP) are very attractive and well known materials for diverse engineering applications due to their many superior properties. More specifically, these materials have acquired much attention in biomedical applications due to their excellent biocompatibility. However, the practical use of these materials are limited in many cases by the need to attain specific tribological (that means, low' friction and wear rate) and mechanical (that means, high hardness, high loadbearing capacity) properties. The long-term outcome of the total joint replacement is currently limited by the breakdown of the implant by wear processes. Therefore, much research has been carried out in this area to reduce the incidence of wear. The goal of this project was to enhance the mechanical and tribological properties of the biocompatible surface by the plasma nitriding and duplex coating process. The works consisted of three phases. In the first phase, in order to acquire a preliminary approximation of the different process parameters, finite volume and finite element methods have been employed to determine (i) the gas mixture pattern inside the sputtering chamber and (ii) the thermal stress inside the coating, with and without considering graded interlaycr respectively. In the second phase of the work, a new plasma nitriding process has been used using an existing carbon based coating system. Plasma nitriding was performed on biomaterials substrates where a Saddle field neutral fast atom beam source and an auxiliary heater was used for substrate heating. It has been observed that low temperature plasma nitriding of the stainless steel samples produced a precipitation and black layer free, hard, thick and load supporting nitridcd layer within a short processing time which was suitable for subsequent coating deposition as well. Plasma nitriding has also been carried out on Ti-6A1-4V and non-conductivc UHMWP biomaterials. In both cases plasma nitriding treatment increased the untreated substrate hardness and wear properties substantially. In the third phase o f the w'ork, the additional improvement of mechanical and tribological properties of the solid lubricant based coatings (MoS2 and DLC), a continuous and discontinuous duplex coating system has been investigated. The main focus has been on the improvement of the load bearing capacity of the coating-substratc system. For the first time a continuous duplex process is being introduced consisting of the plasma nitriding followed by in situ deposition of the DLC biomedical coating. The process has been successfully carried out in a single process chamber (PECVD based Saddle field neutral fast atom beam source) without any interlaycr or post nitriding treatment. Rockwell C indention results confirmed the improvement of the adhesion in the duplex treated coating samples compared to the non-duplex treated sample. Duplex treatment significantly increased the composite hardness and reduced the plastic deformation of the substrate. The Pin on disk tests showed that the duplex treatment increase the overall wear properties of the coating compared to the non-duplex coating. In addition, a discontinuous duplex treatment consisting of plasma nitriding and TiN+MoS, coatings with and without a graded interlaycr on the stainless steel substrate were investigated. Low-temperature plasma nitriding was performed with the newly developed process (Saddle field fast atom beam source) and coatings were deposited by closed-field unbalanced magnetron sputtering. Results showed that the graded interlaycr and plasma nitnding had a great influence on the load bearing and overall tribological properties of the coating-substratc system