Achieving superlubricity in DLC films by controlling bulk, surface, and tribochemistry

Superlubricity refers to a sliding regime in which contacting surfaces move over one another without generating much adhesion or friction [1]. From a practical application point of view, this will be the most ideal tribological situation for many moving mechanical systems mainly because friction consumes large amounts of energy and causes greenhouse gas emissions [2]. Superlubric sliding can also improve performance and durability of these systems. In this paper, we attempt to provide an overview of how controlled or targeted bulk, surface, or tribochemistry can lead to superlubricity in diamond-like carbon (DLC) films. Specifically, we show that how providing hydrogen into bulk and near surface regions as well as to sliding contact interfaces of DLC films can lead to super-low friction and wear. Incorporation of hydrogen into bulk DLC or near surface regions can be done during deposition or through hydrogen plasma treatment after the deposition. Hydrogen can also be fed into the sliding contact interfaces of DLCs during tribological testing to reduce friction. Due to favorable tribochemical interactions, these interfaces become very rich in hydrogen and thus provide super-low friction after a brief run-in period. Regardless of the method used, when sliding surfaces of DLC films are enriched in hydrogen, they then provide some of the lowest friction coefficients (i.e., down to 0.001). Time-of-flight secondary ion mass spectrometer (TOF-SIMS) is used to gather evidence on the extent and nature of tribochemical interactions with hydrogen. Based on the tribological and surface analytical findings, we provide a mechanistic model for the critical role of hydrogen on superlubricity of DLC films.