Structure-property correlations in multilayer hydrogenated amorphous carbon films enabling long-lasting stable superlubricity under ultra-high contact stress

Hydrogenated amorphous carbon (a-C:H) films demonstrate significant potential as solid lubricant in engineering applications. However, the instability of lubrication performance under high-load conditions remains a significant challenge, primarily attributed to combined effects, such as high intrinsic stress and poor interfacial adhesion. In this paper, a group of multilayer a-C:H films were fabricated, exhibiting excellent tribological performances under high-load conditions with a maximum Hertzian contact stress of 3.19 GPa in dry nitrogen environment. One of the multilayer films demonstrated exceptional durability over 100,000 cycles with a stable super-low coefficient of friction (COF) of 0.005 and an ultra-low wear rate of 5.32 × 10⁻⁹ mm³/N·m. It is revealed that the film intrinsic stress exerts a decisive influence on the friction stability and wear resistance. The low internal stress, high wear resistance index (H/E) and elevated plasticity index (H³/E²) of this film fundamentally explain the superior wear resistance and enhanced load-bearing capacity. Microstructural characterization of the sliding interface reveals that the hydrogen passivation mechanism, rather than structural ordering transformation, is likely the dominant factor to sustain superlubricity. The synergistic effect of high hydrogen content and superior mechanical properties guarantees durable lubrication capabilities. These findings establish fundamental design guidelines for implementing carbon-based lubricative coatings in extreme load-bearing applications.