Ordered graphite breaks superlubricity rule as friction vanishes at forbidden angle

Researchers at Tsinghua University's Center for Nano and Micro Mechanics have discovered a new configuration of graphite that challenges the established understanding of superlubricity. Traditionally, superlubricity occurs when two crystalline surfaces are misaligned, causing atomic interactions to cancel out and reduce friction. However, the team observed near-zero friction at specific twist angles of 21.8° and 38.2° in a perfectly ordered graphite interface, a phenomenon previously thought impossible.

This finding has significant implications for reducing energy waste in mechanical systems. Friction accounts for about one-third of fuel energy loss in moving devices. Superlubricity in two-dimensional materials like graphite and hexagonal boron nitride offers a path to near-zero friction in precision devices such as microelectromechanical systems (MEMS) and hard disk drives. The challenge has been maintaining stability, as conventional superlubric interfaces tend to relax into ordered states over time, negating the frictionless benefit. The discovery of a stable, ordered configuration that remains superlubric could transform engineering approaches.

The Tsinghua team systematically varied the twist angle in graphite layers and measured friction at each configuration. They found that the (2,1) commensurate moiré pattern at 21.8° and 38.2° exhibited load-independent friction, a hallmark of superlubricity. This behavior contrasts with the conventional friction observed at 0° alignment and other commensurate patterns, highlighting the unique properties of the (2,1) configuration.

Theoretical calculations suggest that the small unit cell size of the (2,1) pattern prevents structural relaxation into larger domains, which typically generate friction. The potential energy surface for sliding is nearly flat, with minimal corrugation, allowing for almost complete force cancellation within the unit cell. This results in friction behavior akin to the superlubric state, despite the ordered nature of the interface.

The implications extend beyond graphite to other hexagonal two-dimensional materials like hexagonal boron nitride and molybdenum disulfide (MoS₂). These materials share the hexagonal lattice geometry that allows for commensurate moiré patterns. Whether similar friction anomalies occur in their (2,1) configurations remains to be seen.

The Tsinghua study opens new avenues for designing superlubric surfaces that are stable and ordered, potentially overcoming the instability issues that have hindered the practical application of superlubricity. Future research will need to address open questions about the generalizability of the (2,1) effect, the influence of contact size, temperature, and defects, and the broader applicability to other materials.