Tribology

Friction, wear and lubrication

Tribology studies interacting surfaces in relative motion. Friction is resistance to motion, wear is progressive surface material loss or damage, and lubrication changes the contact conditions. Low friction does not automatically mean low wear, and neither can be selected from material names alone.

Simple friction model

In dry Coulomb friction, the static tangential force adjusts up to a limiting magnitude μ_s N. During sliding, a simple model uses F = μ_k N opposite relative sliding, where N is normal force. Friction coefficients depend on the surface pair, finish, contamination, speed, temperature and lubrication; they are not universal material constants.

For steady sliding with constant friction force and relative speed v, mechanical power dissipated is P = Fv. This energy becomes heat distributed between the bodies and surroundings. Contact temperature requires a separate thermal calculation.

Lubrication regimes

• Boundary lubrication: asperity interactions and surface films dominate much of the behavior.
• Mixed lubrication: both asperity contacts and a fluid film support the load.
• Full-film lubrication: the surfaces are substantially separated by lubricant. Hydrodynamic pressure is generated by motion and geometry, whereas hydrostatic lubrication uses an external pressure supply.
• Elastohydrodynamic lubrication: elastic contact deformation and pressure-dependent lubricant properties are important, commonly in concentrated rolling contacts.

A Stribeck curve illustrates how friction can change across regimes as a suitable viscosity–speed–load parameter changes. The axes and parameter definition depend on the contact model; it is not a universal friction-versus-speed curve independent of load and viscosity.

Wear mechanisms

Adhesive wear involves junction formation and material transfer; abrasive wear involves hard asperities or particles cutting/ploughing a surface. Surface fatigue can create pitting or spalling under repeated contact, and tribochemical processes change surface films. Several mechanisms can operate together.

A simple wear estimate

The Archard model for a specified sliding-wear regime is V_wear = K W s/H, where W is normal load in newtons, s is sliding distance in metres, H is the appropriate softer-surface hardness in pascals and K is a dimensionless wear coefficient calibrated for the contact and conditions. It is an empirical estimate, not a universal service-life law. Some data use a dimensional specific wear rate instead; do not substitute it as dimensionless K.

Worked example

A sliding contact carries W = 100 N over s = 1000 m. Supplied values for the specified regime are K = 1e-5 and H = 1e9 Pa.

V_wear = (1e-5)(100)(1000)/(1e9) = 1e-9 m³ = 1 mm³.

If wear is idealized as uniform over an apparent area of 100 mm², the mean depth is 1/100 = 0.01 mm = 10 micrometres. Actual local depth can differ substantially if contact and wear are nonuniform.

Separately, with an assumed sliding friction coefficient 0.1 and speed 0.5 m/s, friction force is 10 N and dissipated mechanical power is 5 W. This power does not follow from K; friction and wear coefficients describe different measurements.

Design checks

Choose lubricant properties and supply conditions for the actual operating range. Check starting/stopping, contamination, surface roughness, sealing, heat rejection and contact stress. A viscosity increase may improve film formation but can raise viscous drag. A model fitted in one wear mechanism should not be extrapolated through a transition to another.

Quick check

Doubling sliding distance doubles predicted Archard wear volume only if load, hardness and wear coefficient remain applicable. A static friction force is not automatically μ_s N; that value is the limiting force at impending slip.