Fig. 1 (a) Research on the regulation of textured surface and friction mechanisms
(b) Interfacial optimization mechanism for ultra-low friction under solid-liquid composite conditions
Fig. 2 Comparative study of selective hydrogenation under dry friction and oil
Recently, Prof. Xiaowei Li’s team from School of Materials Science and Physics has made a series of advances in the regulation of surface texturing and friction mechanisms of a-C coatings. Under the double pressure of energy and environment, especially the proposal of carbon peak and carbon neutrality and the development trend of modern manufacturing precision and miniaturization, the traditional lubricating grease or solid lubrication coating has been difficult to meet the practical application needs under multi-factor coupling working conditions, and there is an urgent need to develop high-hardness, high load-bearing, low friction, multi-environmental adaptability of high-performance lubrication materials and technologies. Amorphous carbon (a-C) coating has high hardness, low friction, corrosion resistance and other functional characteristics, and can achieve a large area of controllable preparation in the green, dry low-temperature physical/chemical vapor deposition technology. As a key moving parts of the ideal surface modification of the new material, amorphous carbon, in line with the energy saving, consumption reduction, emission reduction of the country’s major strategic needs, has become one of the research hotspots of tribology field research.
Firstly, focusing on the key scientific and technical issues of the unknown intrinsic mechanism caused by parameter optimization and real-time characterization of the surface texture of a-C coatings, a systematic study on the screening of different texture parameters and lubrication mechanism under dry friction/oil lubrication conditions was carried out. The study revealed the promotion of hydrodynamic lubrication by the surface texture and interfacial cross-linking induced by the C-C bonding under dry friction; and revealed that the increase in the depth of the texture led to the transition of the interfacial lubrication status and the change of its width determines the effective degree of the hydrodynamic lubrication state (Fig. 1a). It is further found that the lubrication performance of the optimal parameter system depends on the competition between the degree of interfacial passivation and the hydrodynamic lubrication effect under coupled conditions with different oil contents and contact pressures; meanwhile, the collapse of the texture layer under extreme pressures causes abnormal lubricant flow, which adversely affects the friction reduction (Fig. 1b). The above conclusions provide theoretical guidance for the design and application of the surface structure of a-C solid lubrication coatings in microelectromechanical and disk protection. The related research results were summarized in papers entitled “Friction dependence on the textured structure of an amorphous carbon surface: A reactive molecular dynamics study” (DOI：10.1016/j.apsusc.2022.155584) and “Atomistic insights into interfacial optimization mechanism for achieving ultralow-friction amorphous carbon films under solid-liquid composite conditions” (DOI：10.1021/acsami.3c12838) and were published in the international journals Applied Surface Science (JCR I, CAS II Top Journal, Impact Factor 6.7) and ACS Applied Materials & Interfaces (JCR I, CAS II Top Journal, Impact Factor 9.5), respectively.
Second, the selective introduction of hydrogen atoms to the textured surface reduces the degree of cross-linking at the interface, thereby significantly improving the frictional stability of the interface (Fig. 2). Under dry friction conditions, the selective introduction of hydrogen atoms blunts the friction interface and reduces the degree of cross-linking between the surfaces of the self-assembled a-C coatings, and a large number of hydrogen atoms aggregates at the friction interface occur with the increase of the surface hydrogen content, generating high tensile stresses; in particular, the -CH clusters detached from the textured a-C surfaces are re-bonded with the upper friction sub-a-C coatings, which enhances the repulsive effect between hydrogen atoms and thus leading to a sharp decrease in the coefficient of friction. Under oil lubrication conditions, for the intrinsically textured a-C system, the shielding effect of the lubricant on the surface of the self-assembled a-C coating makes the friction behavior dominated by hydrodynamic lubrication, but the friction coefficient decreases by up to 63% with the selective introduction of hydrogen atoms into the raised sites of the textured surface. That is because of the fact that the added hydrogen atoms occupy the active sites in the raised surfaces of the textured a-C coatings, which promotes the passivation of the friction interfaces, and the strong repulsion with the hydrogen atoms of the lubricant results in the formation of a dense lubricant film on the surface of the friction sub-surface, and the friction interface is transformed from oil/oil to oil/hydrogenated bumps. The related research results were summarized in papers entitled “Unraveling the friction response from selective hydrogenation of textured amorphous carbon surface” (DOI: 10.1016/j.apsusc.2022.156246), “Friction reactions induced by selective hydrogenation of textured surface under lubricant condition” (DOI: 10.1007/s40544-023-0772-4) and were published in the international journals of Applied Surface Science (JCR I, CAS II Top Journal, Impact Factor 6.7), Friction (JCR I. Friction (JCR I, CAS I Top Journal, impact factor 6.8).
The research papers published from the above studies all list China University of Mining and Technology as the first affiliation, with Du Naizhou, a postgraduate from School of Materials Science and Physics, as the first author and Professor Li Xiaowei as the corresponding author.
