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

In this paper, 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 are mainly within the range of 0.5±0.2 μm, 5.9±2.1 μm, and 59.1±17.1 μm, respectively. The weight concentration of GO sheets in the aqueous lubricant ranges from 0.0005 mass % to 0.8 mass %. The lubrication regime for the friction tests was kept at boundary lubrication. It is found that, the GO sheets can enhance the lubricity by entering the contact area and preventing the sliding surfaces from contacting each other directly, and the 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, and thus the coefficient of friction (COF) and the wear volume decrease. However, when the GO sheets concentration is at a very high level, 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, it is found that, the GO sheets with larger lateral size can lead to a smoother contact interface. Therefore, with the same concentration, the GO sheets with larger lateral size can lead to lower COF and wear volume. The findings provide a general strategy for improving the performance of lubricants with 2D material additives in a broad range of mechanical applications.