What are the three basic types of lubrication?

There are three basic varieties of lubrication:

• Fluid-film lubrication: This occurs when a fluid film completely separates sliding surfaces, preventing direct contact. The pressure within the film, which supports the load, can be generated externally (hydrostatic) or by the motion and shape of the surfaces themselves (hydrodynamic).1
• Boundary lubrication: This condition exists between unlubricated sliding and fluid-film lubrication. Here, friction is determined by the properties of the surfaces and the chemical nature of the lubricant rather than the lubricant's viscosity. It is common during the starting and stopping of machinery.23
• Solid lubrication: This involves using a film of solid material—such as inorganic compounds (e.g., graphite or molybdenum disulfide), organic compounds, or soft metals—to reduce friction between surfaces.4