Macroscale superlubricity induced by film-forming polymer brush-grafted colloidal additives

Hypothesis: Modifying surfaces with concentrated polymer brushes (CPBs) is an effective way to reduce friction of tribo-pairs lubricated with liquids. We investigate the hypothesis that colloids grafted with CPBs (hybrid colloids) can deposit onto tribo-substrates by varying the solvent quality with respect to the polymer, in order to obtain ultra-low coefficients of friction (CoFs), so-called superlubricity.Experiments: Hybrid colloids are synthesized and characterized, and a dynamic light scattering compares their swellings in aqueous solutions of glycerol or polyethylene glycol. A mini-traction machine with vis-coelastic tribo-pairs is used for lubrication experiments. Adsorption of colloids and film structures are tested using a quartz crystal microbalance and an atomic force microscope.Findings: The solvent controls whether hybrid colloids spontaneously adsorb to the substrate under qui-escent conditions or require contact forces to enable (tribo-)deposition. In both cases, the friction in the boundary-mixed lubrication regimes is lower upon increasing the degree of swelling of CPBs and upon increasing coverage of deposited colloids. The greatest lubrication enhancement and surface coverage occur for the spontaneously adsorbed colloids, with ultra-low CoFs of order 10-3 over a large range of speeds. The results demonstrate the potential for hybrid colloids to be used as solvent dispersible fric-tion modifier additives.CO 2022 Elsevier Inc. All rights reserved.

Automated summary

It is hypothesized that modifying surfaces with concentrated polymer brushes (CPBs) can effectively reduce the friction of tribo-pairs lubricated with liquids. The study investigates the deposition of hybrid colloids grafted with CPBs on tribo-substrates by varying the solvent quality, aiming to achieve ultra-low coefficients of friction (CoFs) or superlubricity. The experiments reveal that the degree of swelling of CPBs and the coverage of deposited colloids play a crucial role in the lubrication enhancement, with the spontaneously adsorbed colloids exhibiting the greatest improvement.