Phototribology of copper surfaces

• Green-light illumination enhances friction on copper across multiple length scales. • Friction increase arises from photoinduced electronic excitations at the surface. • UV exposure promotes copper oxidation with negligible impact on friction. • Phototribology enables active, reversible, multiscale control of friction. A comprehensive understanding of the factors influencing friction is essential for advancing surface engineering technologies aimed at optimizing tribological performance. However, the effect of light exposure on the friction coefficient behavior of metallic surfaces remains insufficiently explored in tribological research. This work investigates the phototribological behavior of copper surfaces under different illumination conditions (dark, UV, and green light). Analyses of the experimental results indicate a significant increase in the substrate friction force when exposed to green illumination, while UV illumination condition produced negligible changes, both compared to dark either micro- or nanoscale. The increase in friction force under green illumination is attributed to electronic excitations modifying surface electronic structure, representing a novel observation in the relatively unexplored field of phototribology applied to this promising material. Substrate characterization was performed using spectroscopic and diffraction techniques to assess the physicochemical properties. The friction response was evaluated using nano-scratch testing and lateral force microscopy, allowing assessment through different methods and at multiple scales. XPS analyses performed on copper substrates subjected to different illumination conditions show that only exposure to UV illumination induces significant chemical changes on the surface, leading to increased oxidation of the substrate. Accordingly, higher concentrations of metallic copper are observed under green illumination, resulting in a temporary increase in the friction force between the tip and the substrate. These findings demonstrate the potential for active friction control via phototribology, with implications for surface engineering and the optimization of tribological systems across various scales.