The effect of lateral size of few-layer graphene oxide sheets as aqueous lubricant additives

In this work, few-layer graphene oxide (GO) sheets are investigated as aqueous lubricant additives between ceramic surfaces. Three batches of GO sheets with various lateral sizes were selected, and the lateral sizes were mainly within the ranges of 0.5±0.2, 5.9±2.1, and 59.1±17.1 μm. The weight concentration of the GO sheets in the aqueous lubricant ranged from 0.0005 to 0.8 wt%. The lubrication regime for the friction tests was kept at boundary lubrication. The GO sheets can enhance lubricity by entering the contact area and preventing the sliding surfaces from contacting each other directly, and lubricity is determined by the coverage of the contact area. For each batch of GO sheets, as the concentration increases, the coverage rate of the contact area increases; thus, the coefficient of friction (COF) and wear volume decrease. However, when the GO sheet concentration is very high, the COF approaches a stable value since the contact area is already fully covered by GO sheets, but the wear volume increases slightly due to the high acidity. Moreover, GO sheets with larger lateral sizes can lead to a smoother contact interface. Therefore, at the same concentration, GO sheets with larger lateral sizes can lead to lower COFs and wear volumes. These findings provide a general strategy for improving the performance of lubricants with two-dimensional (2D) material additives in a broad range of mechanical applications.