Structural Lubricity: A Recent Overview

Tribology is largely concerned with understanding of the (friction) forces acting at the interface between objects moving against each other (Menezes et al., 2013). Over the recent years there has been significant progress in understanding and controlling friction, and in the beginning of 1990s, a new state of frictionless state was found – superlubricity.

Recently, an international group of researchers published an overview article on the topic of superlubricity. In the article “Structural lubricity in soft and hard matter systems” by Vanossi, Bechinger, and Urbakh (2020), the authors gave an overview of the studies and recent research devoted to superlubricity.

One of the ways to achieve superlubricity is to use liquid lubrication. Lubricant additives, polymer brushes, ionic liquids, or hydration layers may be used to reach this frictionless state. Vanossi, Bechinger, and Urbakh (2020) further note that apart from the lubricated cases superlubricity can also be achieved in case of well-defined mating interfaces. This state occurs due to structural incommensurability of the atomic lattice. The atomic mismatch reduces interlocking of the surfaces and hence reduce the force needed to unlock them and, in effect, the surface slide on each other with a lower frictional force. This concept is illustrated in figure 1 below.

Design of superlubic interfaces is essential to reduce the energy dissipated in sliding contacts, minimizing wear and tear. Despite the significant practical potential of superlubricity, this process is hindered by many effects, especially when scaling up towards macroscopic contacts. Elasticity of the contacting bodies is one of such obstacles.  It has been theoretically found that even for incommensurate surfaces there is a threshold size of a contact, above which the elastic deformations may become sufficiently large to locally deform surfaces to a commensurate configuration. The loss of incommensurability brings pinning and increase in friction. However, Vanossi, Bechinger, and Urbakh (2020) continue to note that the negative impact of elasticity can be reduced by generating multiple weakly connected contacts at the interface.

The superlubricity is also affected by the edges of the surfaces and not just the sliding interface. According to Vanossi, Bechinger, and Urbakh (2020), the edges of surfaces have lower pinning than the other part of the surface area; as a result, the bonds of the edges have higher flexibility, which means more friction during movement. Hence, the edges may be acting as the pinning sites and bringing the superlubricity down. In addition, edges of contacts are more exposed to the contamination which also destroys the superlubricity state.

Scaling up structural superlubricity towards the macroscopic world is also challenged by the polycrystalline nature of materials at macroscopic scales, external conditions, such as load, sliding speed, environmental conditions, etc.

Authors of the article emphasize the importance of experimental investigation of frictional contacts to help to explore energy dissipation mechanisms and to help with development of the theoretical models. Atomic force microscopy, surface-force apparatus, and quartz-crystal microbalance techniques are the mostly used techniques in nanotribology. They allow researchers to get crucial, but frequently averaged, physical quantities. However, there are ways to perform experiments with cold ions or colloidal particles in electro-optical fields. These experiments shed light at the single-particle level shear processes with unprecedented real-time resolution. The results from such experiments can be directly compared to the existing theoretical models, e.g., the most widely accepted single particle Frenkel–Kontorova model.

At the conclusion, authors underline the interplay of experiments, theory and simulations in recent advances of nanoscale tribology, the progress in understanding of the fundamental atomic-scale mechanisms of superlubricity and its scaling up to macroscopic contacts.

Further information: Structural lubricity in soft and hard matter systems, Andrea Vanossi, Clemens Bechinger & Michael Urbakh, https://doi.org/10.1038/s41467-020-18429-1.

References

1. Ludema, K. C., & Ajayi, L. (2018). Friction, wear, lubrication: a textbook in tribology. CRC press.
2. Menezes, P. L., Nosonovsky, M., Ingole, S. P., Kailas, S. V., & Lovell, M. R. (Eds.). (2013). Tribology for scientists and engineers (pp. 3-4). New York: Springer.
3. Vanossi, A., Bechinger, C., & Urbakh, M. (2020). Structural lubricity in soft and hard matter systems. Nature Communications, 11(1), 1-11.