Macroscale superlubricity on carbon coated metallic surfaces

Globally, ~100 million terajoule of energy is required to reduce friction, accounting to a fifth of energy generated annually. About 1 – 4% of the Gross Domestic Product (GDP) of industrialized economies is spent on friction and wear phenomena. The global aerospace lubricant market was valued at USD 2.22 billion in 2021, a large part of which is used to overcome friction. Superlubricity is a state of low coefficient of friction near zero, achievable at the micron and nano scales by the application of graphene and its variance, due to their excellent lubrication properties. However, depositing these carbon-based coatings on substrates at the macroscale is challenging and expensive. The mechanisms of macroscale superlubricity is also not fully understood. In this work, carbon is deposited on metallic substrates, using a novel low-cost high temperature treatment at 900^oC. Wear and friction behaviour using ball-on-disk configuration on the bulk coated metallic substrates were investigated. Microstructural features and Raman scattering of the as received and worn surfaces were characterized. The results showed sustained macroscale superlubricity on the bulk metallic surfaces. This was attributed to the coated carbon film that provided the needed incommensurability for superlubricity. The wear rates of the treated samples were drastically reduced as well. The underlying tribo-oxidation and stress induced tribo-transformation controlled interactions and mechanisms are elucidated, before exploring the implications of the current results for the design of robust and next generation multifunctional carbon-based coatings, with superlubricious property at the macroscale, for the aerospace, space exploration, automobile, machinery and robotic industries.