boundarylubrication

Boundary lubrication is a tribological regime in which the lubricating film is so thin that surface asperities from two contacting bodies interact directly, and friction and wear are governed primarily by interfacial processes rather than bulk fluid shear. This regime occurs when film thickness is on the order of a few molecular layers or less, typically during starting, at high load, low sliding speed, elevated temperature, or in the presence of surface contamination.

Boundary films form by adsorption or chemical bonding of molecules to the contacting surfaces, creating boundary layers that reduce direct metal-to-metal contact. These films can be monolayers or multilayers and may comprise polar organic compounds, polymeric coatings, or inorganic solids. Common boundary materials include fatty acids and phosphates, organophosphorus compounds, and solid lubricants such as molybdenum disulfide and graphite. Additive packages in lubricants are designed to promote rapid adsorption, film stability, and sacrificial wear protection under boundary conditions.

Performance in boundary lubrication depends on factors such as surface chemistry, roughness, temperature, pressure, and shear rate, as well as lubricant composition and contaminant levels. The regime is typically mapped on a Stribeck-like curve as a transition between full-film (hydrodynamic or elastohydrodynamic) lubrication and mixed modes.

Applications include engine valve trains, gears and bearings under load, and other high-contact components where full fluid separation is not feasible. Measurement and assessment rely on tribometry and surface analysis to evaluate friction, wear, and boundary-film integrity. Ongoing challenges involve film stability at extreme conditions, compatibility with materials, and environmental considerations in additive design.