In lubrication, what does the term 'boundary lubrication' refer to?

Boundary lubrication refers to a specific situation in lubrication where there is minimal fluid present, resulting in intermittent direct metal-to-metal contact. This typically occurs at low speeds or under high loads, where the lubricant film might not be sufficient to fully separate the surfaces. In boundary lubrication, the surfaces are primarily protected by the chemical properties of the lubricant, which forms a thin protective film that can reduce wear despite the presence of direct contact between the metal surfaces.

In this state, the lubricant may rely more on additives, such as anti-wear agents, to create a protective layer at the contact points. This contrasts with other lubrication regimes, such as full fluid film lubrication where a complete fluid layer exists and entirely separates the two surfaces from contact, minimizing wear and friction significantly. High-speed lubrication or lubrication that relies solely on solid lubricants operates under different principles that do not fit the boundary lubrication definition.