Charge pattern detection through electrostatic array sensing

As reported in previous work, electrostatic charge induced by contact potential difference (CPD) arising from oxidational wear can be detected by electrostatic button sensors. However, to detect signs of localised wear and to investigate the mechanisms involved, the charge distribution on worn regions needs to be measured. Therefore, this paper investigates the evolution of surface charge during wear processes, its interplay with friction, and the correlation between charge distribution and surface chemistry. An electrostatic bar sensor and an array sensor were initially calibrated using CPD patterns generated by dissimilar metal inserts in steel plates. After appropriate signal processing, the sensor outputs exhibited excellent agreement with the electric field strength modelled using COMSOL Multiphysics. Subsequently, the bar sensor was integrated into a reciprocating tribometer for in-situ detection of oxidational wear of steel-on-steel rubbing contacts, followed by ex-situ array sensing of worn regions. Positive picocoulomb-level surface charge during the evolution of oxidational wear were successfully detected by the bar sensor and were found to correlate to changes in friction coefficient. The array sensor effectively mapped the distribution of surface charge. EDS mapping suggested patchy formation of Fe3O4 layers over the worn areas, and these patches correlated to the surface charge map. Increased electrostatic charge levels were associated with higher concentrations of oxidational wear. Therefore, this paper evaluates the potential of electrostatic array sensors to spatially resolve surface charge patterns induced by surface chemistry transformations, which enables the monitoring of localised and smaller-scaled machinery component deterioration and provides additional information for machinery diagnosis.