Novel antimicrobial stainless steel : metallurgical route and applications

Abstract

Stainless steel (SS) is one of the most widely used affordable materials in hospitals, food industries, and public areas but suffers from a lack of antimicrobial properties. The addition of some alloying elements such as Ag, Cu, and Zn can offer a long-term antimicrobial property for regular SS, killing the pathogen bacteria and viruses on the surface. The present thesis aims to study the effect of microstructures and chemical composition on the antimicrobial properties of SS. Two major SS kinds are covered, i.e., Ag-containing SS and Cu-containing SS. Firstly, the effect of the size and distribution of Ag phases on the antibacterial properties of Ag-containing SS were studied. Ag-containing SS with tailored average distances between the Ag phases inside the SS matrix is prepared via the designed powder metallurgy (PM) method. It allows controlling the contact between bacteria and Ag phases. The study suggests that in addition to the released Ag ion concentration, the contact killing mechanism can significantly affect the antibacterial properties of Ag-containing SS. Next, the antibacterial properties of Ag-containing SS under different bacterial densities were investigated. Because of the extremely low solid solubility of Ag within SS, the current casting Ag-containing SS fails to obtain an excellent antibacterial rate at elevated bacterial densities. As indicated by the importance of contact killing, this work proposes that a higher density of Ag-rich particles in SS can provide better antibacterial properties. Consequently, the work employs the ball milling technique to introduce a high density of nano-sized Ag particles in the SS matrix. It is found that this Ag-containing SS has exhibited an improved antibacterial property against high bacterial densities compared with the casting Ag-containing SS though the Ag content is the same. The present thesis demonstrates the superiority of intensive nano-sized Ag particles for the antibacterial performances of Ag-containing SS. Hence, it is vital to develop a cost-effective way for fabrication. For solutions, the thesis proposes a novel method to prepare Ag-containing SS using the gas atomization process. The resulted Ag-containing SS powder and sintered bulk alloy contain a high density of nano-sized Ag particles in the steel matrix, confirmed by the small angle neutron scattering (SANS). It is believed that these fine Ag particles are precipitated from the steel liquid in the fast cooling process of gas atomization, as Ag barely has a solubility in solid SS. Previous studies have mainly focused on the antibacterial properties of SS. Therefore, this thesis section intends to investigate if traditional SS can be modified to inactivate both pathogen bacteria and viruses. The stabilities of typical viruses and bacteria on the surface of Cu-containing SS, pure Cu, Ag-containing SS, and pure Ag are evaluated. It is discovered that pure Ag and Ag-containing SS surfaces do not display apparent inhibitory effects on the testing viruses. In comparison, pure Cu and Cu-containing SS with a high Cu content exhibit significant antiviral properties. Significantly, this developed antimicrobial SS is the first anti-COVID-19 SS, which may help reduce the risk of accidental infection in public areas.