The primary purpose of lubrication is to create an acceptable lubricant film to sufficiently keep the two moving surfaces apart while allowing them to move with reduced friction. This is the ideal condition, but a lubricant can undergo a couple of different regimes before it achieves this full film format.
The figure below shows the overall relationship between film thickness and the related regime and the associated regime relationships with the coefficient of friction.
Boundary Lubrication
At startup or rest, lubricants are usually residing in the sump. For this example, let us think about a car at rest. Since the vehicle has not moved, all the oil should have been drained and settled in its sump at the bottom of the engine. When the car starts, all the parts on the inside will begin moving.
Only after it starts does the oil begin its swift journey from the bottom of the sump to all the moving parts. That means that there is a delay between the oil getting to perform its function or reaching the moving parts.
In boundary lubrication, the oil has not fully formed its film, and there isn’t adequate separation of the asperities.
During boundary lubrication, the oil has not fully formed its film, and there isn’t adequate separation of the asperities. In this state, wear can still occur, and it is in this state that most wear occurs. A similar situation occurs during equipment shutdown, where the components also experience this boundary state of lubrication.
The figure at the side shows the various film conditions. In boundary lubrication (c), the asperities touch, whereas they are fully separated in (a).
Surface-active additives are critical for boundary lubrication and become activated under certain conditions. One of the most popular additives is EP (Extreme pressure) additives, which become activated when temperatures are increased (usually as a result of increased friction).
A surface film is typically formed during boundary lubrication. This can be the result of physical adsorption (physisorption), Chemical adsorption, or Chemical reactions involving or not involving stearate.
Physical adsorption occurs under mild sliding conditions with light loads and low temperatures. Chemical adsorption (chemisorption), stronger than physisorption, occurs when fatty acids react with metals to form soaps, which may or may not be attached to the surface.
On the other hand, chemical reactions that do not involve a substrate allow for slightly stronger bonds than chemisorption. However, with phosphorus-containing compounds, the phosphorus exists in a soluble carrier molecule that degrades at elevated temperatures, plates out on the metal surfaces, and forms a phosphorus soap (typically found in the Antiwear additive packages).
The last and strongest bonds to protect the surface are the chemical reactions involving a substrate where sulfide layers are formed on the surface. These provide low friction and good adhesive wear resistance5.
Mixed Lubrication
This state of lubrication exists as the lubricant transitions between Boundary and Full-Film lubrication. Its average film thickness is less than 1 but greater than 0.01μm. Some exposed asperities and roller element bearings can still experience this state during their start-stop cycles or if they are experiencing excessive or shock loads. These thin films are exposed to high shear conditions, leading to increased temperatures and reducing the lubricants’ viscosity6.
During this state, antiwear and EP additives protect the surfaces (similar to boundary lubrication). Most lubricants transition through this phase, and the additive packages must be able to help protect the surfaces.
Hydrodynamic Lubrication
During this regime, the two surfaces are usually fully separated. They are thick hydrodynamic fluid films that tend to be more than 0.001 inches (25μm) in depth, experiencing pressures between 50-300psi7. Ideally, friction only results from the shearing forces of a viscous lubricant8.
In this state, the surfaces are conformal, meaning that the angles between the intersecting surfaces remain unchanged. It is important to remember this, as it differentiates the hydrodynamic regime from the elastohydrodynamic regime. As shown in the figure below, the coefficient of friction changes for the various regimes, with the hydrodynamic regime having the lowest value.
Elastohydrodynamic Lubrication (EHL)
One of the main defining factors with EHL is that the oil’s viscosity must increase as the pressure on the oil increases, such that a supporting film must be established at the very high-pressure contact areas. Due to the pressure of the lubricant, elastic deformation of the two surfaces in contact will occur. These films are thin, typically around 10-50 μinches (0.25 – 1.25μm).
The surfaces in EHL are non-conformal (unlike Hydrodynamic lubrication), and the asperities of the contacting surfaces do not touch. However, the high pressures can deform either of the contacting surfaces to ensure that a full fluid film is maintained. This can increase the coefficient of friction.
Find out more in the full article featured in Precision Lubrication Magazine.
