Which form of lubrication is characterized by a thick film preventing direct contact between surfaces?

• A. Boundaries
• B. Elastohydrodynamic
• C. Hydrodynamic
• D. Solid-film

Answer: (C)

In hydrodynamic lubrication, a full fluid film forms between the mating surfaces, so they don’t touch. As the parts slide past each other, the lubricant is drawn into the converging gap and pressure builds in the liquid, lifting the surfaces apart. The film thickness, which is large compared with surface roughness, separates the surfaces and carries the load. This thickness increases with higher lubricant viscosity and faster relative motion, and it decreases if the load rises or speed drops, risking contact.

Other forms don’t describe this same thick fluid separation. Boundary lubrication involves only a very thin adsorbed film and can still allow surface contact; solid-film lubrication relies on a solid coating rather than a liquid film; elastohydrodynamic lubrication is a high-pressure, elastic-deformation regime of hydrodynamic lubrication, but the essential idea here is the thick liquid film that fully separates the surfaces.