Silicon nitride based coatings for biomedical implants

Abstract: Silicon nitride-based coatings have been proposed as a means to reduce the release of metal ions and debris from metallic implants. This reduction in combination with the biocompatible debris from silicon nitride could result in a less adverse immune response. However, as the literature shows, introduction of coatings for biomedical implants is challenging and so far only a few alternatives have made it to market.This thesis focuses on the composition of silicon nitride-based coatings deposited using physical vapor deposition methods and its effect on material properties. Firstly, the effect of the nitrogen content was evaluated through the deposition of silicon nitride coatings using different nitrogen gas flow and substrate temperatures in a HiPIMS system. The highest nitrogen content was obtained by selecting a nitrogen-to-argon gas flow of 0.3 and substrate temperature of 350°C. This in turn resulted in the highest hardness, 25.9±2.1 GPa, which could prevent surface damage and this could in turn prevent wear.Further investigation aimed at improving adhesion, deposition rate and dissolution behaviour, and therefore investigated the effect of specific alloying elements (Fe, C, Cr and Nb) on hardness, elastic modulus, surface properties, dissolution and biocompatibility. In order to screen a range of compositions in an efficient manner the coatings used for this were either SiFeCN or SiCrNbN deposited with compositional gradients. The silicon nitride coatings with the incorporation of Fe and C (SiFeCN coatings) were found to reduce ion release from the underlying substrate and the coating itself when compared to pure silicon nitride coatings. While the SiFeCN coatings reduced the ion release from the underlying metal, the coating surface showed clear signs of dissolution after 14 days in simulated body fluid. The coatings alloyed with Cr and Nb showed a further reduction in ion release from the coating itself. The ion release correlated negatively with the Cr content in the coating, which could be due to the passivating properties of the surface oxide of these coatings.The work presented here shows the importance of the nitrogen content and its influence on the mechanical properties of silicon nitride coatings. In addition, the studies reveal a possibility to tune coating properties such as dissolution rate, while maintaining the biocompatibility, through alloying with elements Fe, C, Cr and Nb. In summary, this thesis has contributed to a further understanding of the influence of composition on the behaviour of silicon nitride coatings intended for biomedical applications.