Dry and wet friction: why the wet side already has names you know
Boundary, mixed and full-film are lubrication regimes and also friction types. Same three words answering two different questions.
Two kinds of friction
This is a simplified view of some heavy physics. There are two kinds of friction: dry and wet. Dry friction happens without a lubricant film and comes as sliding or rolling friction. Wet friction happens when a lubricant is present, and its types share their names with the lubrication regimes: boundary, mixed and full fluid.
Boundary friction
There is almost no real film between the surfaces, so friction still comes mostly from high points (asperities) touching. It is the same mechanism as dry friction, only with a thin additive-built film on the metal instead of nothing. It scales with load and does not care much about speed.
Mixed friction
Part of the load is carried by asperity contact and part by a thin hydrodynamic film that is starting to build. You are blending boundary-style and fluid-style friction in whatever ratio the film thickness allows, and that ratio shifts fast as speed changes. That is why mixed sits on the steepest, most sensitive part of the Stribeck curve.
Full-fluid friction
Contact stops mattering. The surfaces are completely separated, so what you measure is the oil resisting shear: pure viscous drag. In a full film, friction is proportional to viscosity and speed and depends very little on load, the opposite of dry friction.
A short history
- 1883: an English engineer, Beauchamp Tower, testing a railway axle bearing, found oil forcing its way out of a lubrication hole hard enough to push out the cork plugging it. A full oil film carries real, measurable pressure.
- 1883: a Russian engineer, Nikolai Petroff, described the friction side of the same story.
- 1886: Osborne Reynolds supplied the mathematics, which is still the foundation of hydrodynamic lubrication theory.
One thing, described two ways
The lubrication regime tells you how much film is between the surfaces. The friction type tells you how that film affects the forces you measure. They are the same three words answering different questions.