An experimental and molecular dynamics study of the superlubricity enabled by hydration lubrication

Recently, hydration lubrication has emerged as a promising approach to macroscale liquid superlubricity. However, the reported methods till now exhibit a long running-in process that features high and fluctuating friction evolution. Herein, we have employed glycerol aqueous solutions as the model to reveal the mechanism governing the achievement of superlow friction by hydration lubrication, aiming to erase the long and disturbing running-in. Through experimental investigations and molecular dynamics simulations, it is found that the initial high friction of dilute glycerol aqueous solution with the water/glycerol weight ratio of 2.0 is due to the ineffective film formation between the tribopairs, and the superlow low friction after the long running-in is attributed to the synergetic effect of increased viscosity due to water evaporation, increased film thickness due to higher speeds, reduced contact pressure due to the wear of friction materials, and the adsorbed molecular layers on steel surfaces. To improve the initial lubricating condition that is responsible for the high coefficient of friction at the start of the test, the formula has been modified with the water/glycerol weight ratio reduced to 0.2, and its friction performances have been further investigated. Surprisingly, remarkable and robust superlow friction appears almost as soon as the test starts. Molecular dynamics sliding simulation reveals the formation of shearing layers or hydration layers near the surfaces and the load-supporting layers between them. The results in this work are helpful to expand the understanding of the superlubricity mechanism enabled by hydration lubrication, and it also provides a novel strategy to liquid superlubricity using aqueous media that features vanished running-in.