Correlation between Water Film Thickness and Tribological Behavior of Polymer Brush in Aqueous Lubrication

Research on macroscopic tribological performance of polymer brush was conducted on friction tester and nanoscale film thickness measuring device with ball-disc contact pairs, where steel balls and glass discs were grafted by hydrophilic polymer brush-polymethacrylic acid-3-sulfonic acid propyl potassium salt (PSPMA) respectively using surface-initiated atom radical transfer polymerization technology (SI-ARTP) and deionized water as a lubricant, the aqueous lubrication mechanism of polymer brush was explored under the intervention of hydrodynamic effect. Optical interferometry technique was used to observe actual distribution of water film in the contact area at low entrainment speed (4 mm/s), it was found that the shape of water film adjacent to both sides of raceway changed from a circular shape in the initial state to a distribution along entrainment speed direction with time, which confirmed that polymer brush formed a layer of stable aqueous film by continuously harvesting the surrounding water molecules. The transition of lubrication regime was achieved by controlling entrainment speed continuously increasing from 1 mm/s to 512 mm/s, it was considered to be in thin film lubrication regime at lower speed, where film thickness was independent of velocity and was stable at about 35 nm, and hydration effect of polymer brush was responsible for the establishment of effective film in contact; when velocity was greater than 32 mm/s, it transformed into elastic hydrodynamic lubrication, and the measured value of film thickness was higher than that of film thickness predicted by isoelastic film thickness formula (2~12 nm) added by film thickness due to hydration effect (~35 nm), which meant that polymer brush presented lubrication enhancement effect under hydrodynamic lubrication, which was resulted from synergistic effect of hydration effect and hydrodynamic effect.