Ultra-high pressure superlubricity enabled by a hydrogenated carbon film/Ag-doped transition metal disulfide heterogeneous interface

Superlubricity is a state of motion characterized by near-zero friction and negligible wear on tribo-affected materials. It represents a groundbreaking technological approach to mitigating friction-induced material degradation and mechanical equipment failure. From a surface engineering perspective, achieving superlubricity relies heavily on the design and development of both bulk and surface tribo-materials. Solid superlubricity can be achieved under specific conditions, such as ultra-high vacuum or dry inert gaseous environments, and offers distinct advantages, including the ability to sustain high normal loads and extreme temperatures. Diamond-like carbon and layered materials such as molybdenum disulfide can achieve superlubricity through their inherent surface characteristics. However, in the practical operating conditions of mechanical components, the complex and often harsh contact environments present significant challenges for a simple, homogeneous lubricant to sustain exceptional lubricity. Heterogeneous systems composed of at least two types of lubricants offer a promising solution.

This research investigates a heterogeneous tribo-interface composed of hard hydrogenated carbon films and nanocrystalline-doped transition metal disulfides. The hard hydrogenated carbon films were synthesized by an ion beam deposition system using hydrocarbon gaseous sources as processed precursor. Specific molecular structure such as aromatic-ring species like methylbenzene (C₇H₈) was chosen for tuning the superior property and surface passivation capacity in the film. The correlation between the sp²/sp³ ratio and hydrogen content in the carbon matrix can be controlled by the pulse-biased ion energy. The silver-doped MoS₂ or WS₂ films were prepared by the ion beam assisted magnetron sputtering method. Multilayered structures were established by alternatively depositing each individual layers using different modes. Afterwards, the study focuses on characterizing the mechanical properties, nanostructures, and tribological behaviors of the system.

The macroscale superlubricity performance and its influencing parameters, particularly the contact pressures ranging from 0 to 3.2 GPa, are analyzed for the tribo-systems GLCH/WS₂-Ag and SUJ2/WS₂-Ag. A superlow friction coefficient (COF < 0.01) was achieved for a wide range of contact pressure, generally with the decreasing evolution trend as the gradual increasement in the applied normal load regardless of the counterface materials. The heterogeneous sliding interfaces are even capable of bearing a maximum Hertz contact pressure of 7.78 GPa, corresponding to an average value of 3.2 GPa. The duration test further verifies the robustness of the system with a prolonged sliding life-span in the term of 330 000 reciprocating cycles (1 353 m) along with a very low material wear rate. The in-depth analysis of the morphologies and nanostructures of tribofilms at the contact interface reveals the stress-induced evolution of graphene-like carbon transfer layers and WS₂-derived shear bands occurred along the sliding interface.

The above results emphasize that the in-situ formed composite structure provides a synergetic lubrication effect for the maintenance of a superlubricity state in harsh contract conditions. These findings clarify the mechanisms underlying the in-situ formation and ordering of multiple lubricating phases, enabling superlubricity under ultra-high contact pressures.