Decrease Friction with Hydrogen Ions

HydratedLayerLubrication

HydratedLayerLubrication

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  \mu=0.004 was achieved between  SiO_2 and  Si_3N_4 in presence of ethylene glycol. Generation of ultra-low shear strength hydration layer  \mu_r=0.0002 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”.

Credit for image: Jinjin Li, Chenhui Zhang, Mingming Deng & Jianbin Luo, “Reduction of friction stress of ethylene glycol by attached hydrogen ions”.

Aydar Akchurin
About Aydar Akchurin 35 Articles
Editor-in-Chief, PhD (Tribology), Researcher at ASML, Eindhoven, the Netherlands. Expertise in modeling of lubrication, friction and wear.

3 Comments

  1. “Superlubricity, the state of ultralow friction (<0.01), has already been achieved in various systems . . . . In these systems, the attempt is made to substitute the high-shear strength interface by the low-shear strength to accommodate for the sliding."

    As said elsewhere, this is the terminally-flawed EHL (AW) concept rearing its ugly head again.

    Sacrificial barrier coatings OVER surface asperities will never work. Proof of this inconvenient truth can be found in the last 60 years of lubrication science/history. "Superlubricity" in a liquid lubricant regime can only be had in the absence of surface asperities – period. The "key" to this is achieving perfection of the interacting friction surfaces, not covering the imperfections under a layer of low-shear stuff that simply follows the existing (rough and uneven) surface contours.

    That said, hydrogen ions on certain surfaces can be a very good thing. Hydrogen ions on the surface of graphene can be a really good thing . . . especially if that graphene is coating the metal friction surface! The "trick" is getting hydrogen in its monoatomic (nascent) H⁺ form, not its molecular (H₂) form. Nascent hydrogen is a byproduct of SYNGAS generation during pyrolysis of carbonaceous materials. This is part of Phantaslube® ISS-ISN technology. The nascent hydrogen is created in-situ (during the sacrificial pyrolysis event) to coat the as-synthesized graphene layer (hydrogenated graphene).

    There is undoubtedly hydrogen covering the exposed graphene surfaces with Phantaslube® technology. Thusly, "superlubricity" can be achieved on the usable macroscale (outside of the lab), by having the permanently smooth interacting surfaces covered in hydrogenated graphene as well.

  2. I meant here that any low friction can only be achieved by the decrease of the shear resistance in the contact. Addition of graphene or hydrogen, decrease of surface roughness are needed to eliminate this shear strength. I agree that smoother surfaces would be a better option to decrease friction and in this study, they also worked on relatively smooth surfaces.

    • That is my point Aydar. The researchers always use super-smooth substrate materials because the “superlubricity” exhibited in these (laboratory) systems requires such surfaces, else they cannot achieve the desired effect. This brings into question the validity of these studies outside of the lab, as machine internals (without Phantaslube technology) are not atomically smooth.

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