What type of lubrication occurs when surfaces make contact due to inadequate film thickness?

Boundary lubrication occurs when the lubricating film is too thin to completely separate the sliding surfaces, leading to direct contact between them. In this scenario, the surfaces can experience increased friction and wear due to the inadequate film thickness that fails to maintain a full fluid layer.

During boundary lubrication, additives in the lubricant, such as anti-wear agents and friction modifiers, are crucial as they help create a protective barrier on the surface. This additional layer aids in reducing wear and minimizing friction even when the lubricant film is insufficient for hydrodynamic conditions.

In contrast, hydrodynamic lubrication and elastohydrodynamic lubrication require a sufficient lubricant film thickness to maintain separation between surfaces, while fluid film lubrication also refers to fully separating surfaces with a complete film. Since these types provide separation and prevent contact, they do not apply in this case of inadequate film thickness.