An In Situ Method for Simultaneous Friction Measurements and Imaging of Interfacial Tribochemical Film Growth in Lubricated Contacts

Tribological investigations of macroscopic lubricated sliding contacts are critical for a wide range of industrial applications including automotive engines, gears, bearings, and any other contacting surfaces in relative motion. However, the inability of existing techniques to access buried sliding interfaces with high spatial resolution inhibits the development of fundamental insights into the tribological processes at play. Here we demonstrate a novel and general in situ method, based on atomic force microscopy (AFM), in which micrometer-scale spherical probes are attached to a standard microfabricated AFM cantilever which is then slid over a substrate while immersed in a liquid lubricant. In this case, steel colloidal probes and steel substrates were used, and the contact was immersed in a commercial polyalphaolefin oil with zinc dialkyl dithiophosphate (ZDDP) additive at both room temperature and 100 °C, but the method can be used for a broad range of material combinations, lubricants, and temperatures. We demonstrate that the in situ measurements of friction force and the morphological evolution of the tribochemical films on the substrate can be simultaneously achieved with nanometer-level spatial resolution. In addition, we demonstrate that the sliding zone is readily accessible for further characterization with higher spatial resolution using standard AFM probes with nanometer-scale tip radii. Ex situ characterization of the micrometer-scale probe and the sample is also feasible, which is demonstrated by acquiring high-resolution AFM topographic imaging of the final state of the probe.