mixedlubrication

Mixed lubrication is a lubrication regime in tribology in which a lubricating film between two sliding surfaces is not thick enough to fully separate them, yet is not negligible. In this regime, part of the load is supported by the liquid film while another part is carried by direct contact of surface asperities. This leads to a combination of hydrodynamic effects and asperity interactions, and friction and wear are influenced by both mechanisms.

Causes and occurrence: mixed lubrication commonly occurs at moderate speeds and loads, where the lubricant film thickness is comparable to the scale of surface roughness. It is typical in engines, bearings, gears, piston rings, seals, and other sliding or rolling contacts. The precise boundary between mixed and hydrodynamic or boundary conditions depends on factors such as viscosity, temperature, surface roughness, load, and speed. The film thickness relative to roughness is often discussed using a lambda parameter, reflecting how much load is supported by the fluid film versus asperity contact.

Friction and wear characteristics: because asperities still touch, friction in mixed lubrication tends to be higher than in full fluid separation, and wear can occur if contacts are sustained. Lubricant additives (friction modifiers and anti-wear agents) and surface coatings can reduce asperity wear, while maintaining some fluid film protection. Surface engineering, proper lubrication selection, and operating conditions that promote thicker films can shift operation toward hydrodynamic lubrication.

Applications and management: understanding mixed lubrication is important for bearings, gears, cam-driven mechanisms, and seals. Managing it involves balancing viscosity, temperature, speed, load, and surface finish, using additives and coatings, and applying appropriate lubrication regimes to minimize wear while controlling friction.