Self-powered, online, highly sensitive lubricating oil acidity monitoring driven by a triboelectric sensor

Abstract

Lubricating oil as the “lifeblood” of industry can improve the performance and longevity of machines by orders of magnitude. However, the acidity of oils occurring during operation always results in fatal failures of machine elements. It is a great challenge to monitor oil acidity online in a highly sensitive machine system. In this study, a self-powered, real-time, online triboelectric sensor was developed for detecting oil acidity based on oil-solid triboelectricity (OA-TENG). By engineering a tribo-surface molecular structure with fluorocarbon chains and amino groups, the contact angle of the optimized triboelectric surface changed to ∼155°, which was much higher than that of commercial polytetrafluoroethylene (PTFE, ∼40°). Furthermore, the real-time adsorption of aged oils on the optimized surface was achieved using quartz crystal microgravimetry with dissipation (QCM-D) for oil-solid contact. It was found that the amino group on the surface has the capability of capturing oxygen-containing contaminants generated during aging through hydrogen and covalent bonds, which is able to adsorb with another 0.3 μg cm⁻¹ compared with that without amino groups. Thus, typical acidity contaminants in aged oils can be successfully monitored. The triboelectric output of the optimized OA-TENG was similar to that of the PTFE-based OA-TENG, but the monitoring sensitivity of the optimized OA-TENG (∼0.62 V per TAN) was much higher than that of the PTFE-based OA-TENG (∼0.2 V per TAN). The triboelectric sensor's sensitivity reported in this study was also much better than that of the previous method for monitoring oil acidity (∼0.25 V per TAN). This work not only provides new pathways for oil condition monitoring with high sensitivity but also greatly expands the applicability of TENGs.