Towards multi-functional stainless steel surface: plasma surface alloying with N, Ag and Cu

Abstract

Hospital-acquired infections, a large proportion of which are derived from contact transmission, represent a massive global challenge. It has been proved that surface modification of biomaterials with Ag or Cu has evolved as a potentially effective method for preventing bacterial proliferation on the devices surfaces. However, thin antimicrobial coatings on materials such as austenitic stainless steels can be easily worn and removed in relative motion with other surfaces. The purpose of this study is to develop multi-functional stainless steel surfaces which combine greatly improved wear resistance, at least maintain corrosion resistance and provide long-lasting, high efficacy, antimicrobial effects. In this thesis a series of surface engineering technologies, including active screen co-alloying, active screen plasma duplex alloying and double glow plus active screen duplex plasma alloying, were developed for surface alloying stainless steel with Ag or Cu and N; the phase constitution, microstructure, composition, and surface roughness of the alloyed surfaces were fully characterized, and the surface hardness, wear resistance, bonding strength, antimicrobial efficiency and corrosion behaviour of the treated surfaces were evaluated. In addition, further inspection of the wear mechanisms and corrosion mechanisms were conducted on post-exposure surfaces. It was found that the adhesive wear mechanism of austenite can be reduced by this alloying combination and the wear resistance was improved by up to 1000 times, and the Ag/Cu alloyed surface was bactericidal and growth-inhibitive for many pathogens including E. coli NCTC 10418 and S. epidermidis NCTC 11047 effectively up to 99%/6h. The mechanism of bactericidal efficiency of Ag/Cu is found dependent on the structure of the bacterial membrane and a higher efficiency of antibacterial agents is found associated with the higher elemental concentration of copper and silver. With regard to corrosion, it is affected largely by the configuration of surface structure and several corrosion mechanisms were evolved. One principal conclusion was that it is feasible to generate long lasting antimicrobial stainless steel surface to fulfil growing demands from industry for practically robust multifunctional medical device surfaces.