Sino-Swiss Bilateral Project: Bridging the gap between macro and nanotribology

The group at Lanzhou Institute of Chemical Physics (LICP) is proficient in the development of solid-like lubricants but lacks the understanding of microscopic mechanisms. The group at the University of Basel (UBAS) has expertise in high-resolution force microscopy and performs nano- and micro-tribological experiments. The goal of the project was to bridge the gap between macrotribology and micro-and nanotribology. The idea was to perform experiments on solid-like lubricants from LICP, which were investigated on the macro-, micro-and nanoscale. The emphasis was put on the onion-like fullerenes, which are supposed to be the essential ingredient for superlubricity as observed by the LICP group.

After the first visits of the Swiss partners in China and of the Chinese partners in Switzerland, the communication was improved and the preparation, exchange and characterization of samples were successfully done. An in-depth discussion led to a better understanding of the fundamental mechanism of superlubricity, which meant that coatings could be prepared with minimum friction and energy losses.

The University of Basel was responsible for the nanoscale characterization of the tribological coatings. The sample preparation was done in China, and the transfer of the samples was successfully made with the help of the Chinese partners.

The fullerene-like hydrogenated carbon films (FL-C:H) films were prepared in Lanzhou. Tribological properties at the nanoscale were investigated by friction force microscopy in dry nitrogen and under ambient conditions were done in Switzerland. This film is considered to be composed of a binary structure: FL-C and a-C areas. The former structure induces a lower friction coefficient, lower adhesive force and higher surface roughness, while the latter behaves just the opposite. It was obvious that the low friction is determined by the ratio of both FL-C and a-C structures. To understand the mechanism, it should shed light on the excellent hardness and elastic modulus, which behaves elastically with less energy dissipation during frictional sliding. Since higher hydrogen fraction is verified to impede the formation of FL-C structures, the further plan is to reduce the hydrogen concentration during the film preparation, such as utilizing the unsaturated hydrocarbon as the reaction gas. In contrast to DLC films, the FL-C:H films are rather independent of hydrogen to perform excellent lubricity and durability, which promotes the application range of this special carbon film in the ambient environment. The results of this joint study are published in [Liu2020a] and summarized in the PhD thesis of Zhao Liu [Liu2020b]. Zhao Liu (a Chinese Student, matriculated at the University of Basel and supported by this project) successfully finished his PhD thesis at the University of Basel in September 2020.

[Liu2020a] Zhao Liu, Yongfu Wang, Thilo Glatzel, Antoine Hinaut, Junyan Zhang, and Ernst Meyer, Low Friction at the Nanoscale of Hydrogenated Fullerene-Like Carbon Films, Coatings, 10, (7), (2020)

[Liu2020b] Z. Liu, “ Correlation of 2D-layer Superstructures with Nanofriction”, PhD thesis, University of Basel, 18. September 2020

The bilateral cooperation was ideal because the expertise of the partners complemented each other. The partners in China have the know-how to prepare the tribological coatings, and the partners in Switzerland were able to characterize the samples by nanoscale techniques. In such a way, we gained a deeper understanding of the mechanisms.