Superlubricity: The energy saving superhero for new products

The article is sponsored by Bruker.

Friction is the great obstacle to overcome when testing new products with moving parts. While friction can never disappear completely, it may be possible to achieve ultralow friction: a state called superlubricity, where the friction between two materials sliding against each other virtually vanishes.

Since the word was coined in the early 1900s after being theorised by Motohisa Hirano, it attracted growing interest due to its potential energy saving and environmental benefits that can be applied to mechanical, automotive, electronic and biomedical industries.

Thanks to advancing tribology research we now have experimental evidence of superlubricity. While earlier research focused on nano- and micron-scales, there is now growing interest in larger scale tests for high-pressure industrial applications [1].

There two main types of superlubricity. When solids provide lubrication with no liquids we have solid superlubricity, in which contact pressures of 1 GPa have been achieved for the microscale and 2.5 GPa at the nanoscale. For liquid superlubricity, 2D materials combine with liquid lubricants. At large scales required for most commercial and medical applications, liquid superlubricity offers more potential, but so far, contact pressures have been limited to below 0.3GPa [2].

How to achieve superlubricity

Whether liquid lubricants are involved or not, to reach this state, two materials must slip against each other enough so that friction is drastically reduced. Ultrathin sheets of certain materials can reach superlubricity, such as graphene, when dry or coated with oil, due to its tightly packed molecule-thick sheets.

A form of phosphorus oxide can also form molecular-thin sheets. A recent Nature publication by Ren et al. (2021) shows that when coated in water, the sheets help to achieve liquid superlubricity. Research in the field has faced obstacles in reaching liquid superlubricity at an ultrahigh contact pressure, but the co-authors of the published study demonstrated that superlubricity can be realised under a contact pressure of 1193 MPa with lubricated partially oxidized black phosphorus nanosheets [3].

Like graphene, the phosphorus oxide material is also layered, but particularly attracts large amounts of water, where each sheet holds onto water molecules across its area. When the solids slide against each other, so do the water layers with very low friction. These slippery liquid layers resist pressure when squeezed, achieving superlubricity under ultrahigh contact pressures, offering potential for diverse industrial uses.

Bruker’s Universal Mechanical Tester

To understand how materials function together under pressure scientists use tribometers to measure friction and tribotesters to test and simulate the interactions. In the study by Ren et al. the friction experiments were conducted using Bruker’s Universal Mechanical Tester (UMT), a micro-tribotester that reveals the tribology and properties of materials under real-world conditions. Bruker provides a complete tribology lab in one, with test configurations from the nano- to macro-scale. This tool can assist engineers to work to achieve superlubricity in new products, saving energy and boosting efficiency.

Superlubricity has important implications for sustainability and healthcare. Energy can be saved in electric cars, from the wheel bearings to the motor, and in the biomedical industry, devices like artificial joints can be improved. With our global sustainability goals and growing healthcare needs, there has never been a more important time for engineers to leverage superlubricity, because the more we reduce friction, the longer lasting devices will be and more energy can be saved.

References

1. Hod, O., Meyer, E., Zheng, Q. et al. Structural superlubricity and ultralow friction across the length scales. Nature 563, 485–492 (2018). DOI: 10.1038/s41586-018-0704-z
2. Xiangyu G, Jinjin Li, Jianbin L. Macroscale Superlubricity Achieved With Various Liquid Molecules: A Review. Frontiers in Mechanical Engineering. 5, p2 (2019). DOI: 10.3389/fmech.2019.00002
3. Ren, X., Yang, X., Xie, G. et al. Superlubricity under ultrahigh contact pressure enabled by partially oxidized black phosphorus nanosheets. npj 2D Mater Appl 5, 44 (2021). DOI: 10.1038/s41699-021-00225-0