Causes of electric voltage in bearing steel rolling contact at uninterrupted electric current flow: (a) WLI, SEM, and XPS surface characterization, (b) development of electric contacts from nano- to micrometer scales

The motivation is to find out the reason behind the voltage increase (see (b)) of a rolling contact (see (a)), which represents a ball-raceway contact in a real bearing. The contacting rings in (a) are etched via an acid to remove any additional oxidation and contamination. It is shown that the tribo-oxidation (see (d)), due to the mechanical friction between the asperities in the contact point SAL (see (c)), is responsible for the contact voltage increase. • Quantifies contact voltage (∼0.5 V) in rolling steel contacts under current. • Links friction-induced surface oxidation to increased contact resistance. • Uses WLI, SEM/EDS and XPS to map surface topography and oxide chemistry. • Analytical nm–µm a-spot model explains and reproduces measured U–I curves. • Shows ballistic → diffusive electron transport at nanoscale raises resistance. The sometimes-inevitable electric bearing current flow through the bearings of a drive system may damage the bearing raceway surface. Understanding the electrical behavior of rolling contacts is essential to calculate these damaging bearing currents. In this paper, the electrical contact between two steel rolling rings is investigated at dry condition (without lubrication). The findings are applicable to the mixed lubrication as well. The roughness and topography of the raceway surface is characterised via White Light Interferometry (WLI) and Secondary Electron Microscopy (SEM), and the surface composition via X–ray Photoelectron Spectroscopy (XPS) and Energy Dispersive Spectroscopy (EDS). The electrical resistance of a local metallic contact with a radius of nm-range to µm-range is analytically calculated. The analytical findings are used to derive the U-I-characteristic of the contact. The calculated U-I-characteristic of the contact is verified with the measurements. Finally, the time signals of the measured contact voltage and current are analysed based on the analytical findings.