Why is viscosity less important during boundary lubrication?

In the context of lubrication, boundary lubrication is a condition where sliding surfaces are not completely separated by a fluid film, meaning friction is determined by the properties of the surfaces themselves and the chemical nature of the lubricant rather than its viscosity.12

While fluid-film lubrication relies heavily on the viscous properties of a fluid to generate pressure and separate surfaces, boundary lubrication occurs when such a film is insufficient, often during the starting and stopping of machines. In these instances, the chemical composition of the lubricant becomes the dominant factor in reducing friction and wear, as it must protect the surfaces through mechanisms like forming a protective film to neutralize corrosive materials or preventing solid-to-solid contact. Consequently, because the surfaces are in closer proximity and the fluid is not acting as a load-bearing hydrodynamic medium, the viscosity of the lubricant is significantly less important than its chemical interactions with the metal surfaces.132