Currently, a large portion of consumed energy is used to overcome friction. Design of low friction components is a primary goal in building a sustainable society. Superlubricity, the state of ultralow friction (<0.01), has already been achieved in various systems ranging from atomic to microscales. In these systems, the attempt is made to substitute the high-shear strength interface by the low-shear strength to accommodate for the sliding.
In classical lubrication theory, the friction reduction is performed through the separation of the sliding surfaces by the lubricant film. The high shear strength solid contact interface is substituted by the low shear-strength liquid and the friction is reduced. However, the shear strength of conventional lubricants is still high to achieve superlubricity (friction less than 0.01 is hardly achievable).
Recently, researchers from Tsinghua University presented a work on reduction of friction by attachment of hydrogen ions to the rubbing surfaces. An ultra-low friction coefficient of was achieved between and in presence of ethylene glycol. Generation of ultra-low shear strength hydration layer was claimed to be responsible for superlubricity regime.
During the running-in, the surfaces absorb hydrogen ions by means of tribochemical reaction induced by normal and shear pressures. The resultant hydrated layer is low-shear strength, but also delicate. It cannot carry much of the load and therefore, the researchers used a low-viscous lubricant to separate the surfaces by a thin film. This film protects the hydrogen layer from being worn, whereas hydrogen layer accommodates the difference in the wall velocity by its low shear strength and the resultant friction becomes low.
The researchers showed, that the proposed mechanism is applicable for any surfaces capable of absorbing hydrogen and aqueous lubricants with low viscosity .They used aqueous solutions of glycerol, dimethyl sulfoxide, 1.4 butanediol and 1.5 pentanediol and super low friction was achieved in all cases. Various pairs of materials were rubbed to show independence of the superlubricity on the surface properties.
The same group of researchers reported previously the generation of hydrogen layer, but the importance of the classical lubrication was not emphasized and the mechanism was not clarified until now. With the current work, a clear picture was drawn and the discussed mechanism can be used in the design of low friction systems.
The details of the research can be found in the original article by Jinjin Li, Chenhui Zhang, Mingming Deng & Jianbin Luo, “Reduction of friction stress of ethylene glycol by attached hydrogen ions”.