The impact of microstructure refinement and passive layers formation on the antibacterial activity of lead-free brass

The antibacterial properties of lead-free brasses, which are interesting from their possible application in hospital infrastructure, plumbing systems, and the food industry, have not yet been investigated. Therefore, this paper investigates the antibacterial properties of micrograined and nanostructured lead-free brass CuZn21Si3P, consisting of α and κ’ phases, against Staphylococcus aureus in adhesion tests. The nanostructured brass CuZn21Si3P has been produced by high pressure torsion for the first time, and its microstructure consists of nanotwins and nanograins. A wide range of analytical techniques such as X-ray photoelectron spectroscopy, doppler broadening positron annihilation spectroscopy, variable energy positron lifetime spectroscopy, and focused ion beam combined with scanning transmission electron microscopy enabled revealing the microstructure, defects density, surface roughness, passive films thickness and their chemical composition. It was shown that microstructure refinement has a negligible impact on the passive layer chemical composition and thickness, but mostly affects the quantity and distribution of oxides. The significantly enhanced antibacterial properties of nanostructured CuZn21Si3P compared to the annealed one might be related to the smaller size of zinc and copper oxide islands and their more defective structure, leading to easier zinc and copper ion release. S. aureus adhesion seems independent of phase composition in the case of annealed CuZn21Si3P. However, after HPT, one can notice that bacteria tend to adhere less to α phase. The interesting finding is also that the nanoscale surface roughness, as measured locally from STEM images in the case of nanostructured CuZn21Si3P, might also make the bacteria adhesion more difficult.