Boundary Lubrication:
In boundary lubrication, the lubricant molecules form a monolayer on the surfaces of the contacting asperities (high points on the surface). These lubricant molecules physically separate the asperities and prevent direct metal-to-metal contact. The boundary layer is typically formed by polar molecules that have strong chemical affinity to metal surfaces.
Characteristics of Boundary Lubrication:
1. Direct Contact: In boundary lubrication, the asperities on the surfaces of the contacting materials come into direct contact, resulting in higher frictional forces.
2. Lubricant Film Thickness: The lubricant film thickness in boundary lubrication is extremely thin, often comparable to the molecular size of the lubricant.
3. High Pressure and Temperature: Boundary lubrication occurs under high-pressure and high-temperature conditions where the lubricant film is likely to rupture.
4. Additives: Boundary lubricants often contain additives known as extreme pressure (EP) or anti-wear additives. These additives react chemically with the metal surfaces to create protective layers that reduce wear and tear.
Examples of Boundary Lubrication:
Boundary lubrication is commonly encountered in situations where the load is heavy and the lubricant film is unable to fully separate the contacting surfaces, such as in gears, cams, and rolling element bearings under extreme conditions.
Film Lubrication:
In film lubrication, the lubricant creates a complete and separating layer between the opposing surfaces, effectively preventing direct contact between the asperities. The load is supported by the hydrodynamic or hydrostatic pressure generated by the lubricant film. The lubricant film thickness in film lubrication is generally several times the size of the surface roughness.
Characteristics of Film Lubrication:
1. No Direct Contact: Film lubrication is characterized by the absence of direct contact between the opposing surfaces. Instead, the lubricant film separates the surfaces, significantly reducing friction.
2. Lubricant Film Thickness: The lubricant film thickness in film lubrication is significantly thicker compared to boundary lubrication and is typically in the range of micrometers to hundreds of micrometers.
3. Pressure and Temperature: Film lubrication works efficiently at lower pressures and temperatures where the lubricant film is able to maintain its integrity and provide sufficient separation.
4. Viscosity: The viscosity of the lubricant plays a crucial role in film lubrication. Higher viscosity lubricants tend to form thicker films, providing better separation and reduced friction.
Examples of Film Lubrication:
Film lubrication is commonly observed in journal bearings, high-speed gears, hydrodynamic thrust bearings, and many engine components where a continuous and uninterrupted lubricant film is maintained.
In summary, boundary lubrication involves direct contact between asperities, utilizes thin lubricant films, and is suitable for high-pressure and high-temperature conditions. On the other hand, film lubrication prevents direct contact, forms thicker lubricant films, and is effective at lower pressures and temperatures. The selection of appropriate lubrication regime depends on the specific application, load, speed, and desired performance.