There are three primary lubrication regimes, which are boundary, mixed, and hydrodynamic lubrication. Lubrication regimes describe the type of lubrication film that is created under specific operating conditions and is dependent on the degree of contact between surfaces.
During boundary lubrication, opposing surfaces meet with little or no oil film separation. In this regime, damage is prevented by protective additives that promote sliding rather than welding of surface asperities. This regime typically occurs at low speeds.
Imagine you were driving a speed boat. If you gave the boat just a little bit of throttle the nose of the boat would go up and the back of the boat would be angled in the water. When half of the boat is still in the water, it creates drag. The same thing would happen on water skis when you’re first trying to get up and going in the water. This is a great example of boundary lubrication.
During hydrodynamic lubrication, moving parts are completely separated by a viscous fluid film. This regime typically occurs at high speeds. Back to our previous example, imagine now the boat has been at maximum throttle for long enough that you reach full speed. At full speed, the nose of the boat comes down and you move so fast that you just skim over the top layer of water. While we definitely do not recommend trying this at home, this example demonstrates hydrodynamic lubrication.
Mixed lubrication occurs during the transition from low to high speed operation when boundary and hydrodynamic conditions coincide, the asperities of bounding surfaces will extend through the film and occasionally come in contact.
Finally, imagine you gave your boat maximum throttle. At first, the boats nose will come down a bit as the boat is accelerating, but you haven’t reached the maximum speed the boat can go. At this point, your water skis have balanced and you are cruising along under similar conditions to mixed lubrication.