Friction

Friction is the resistance that opposes relative motion between surfaces, fluids or bodies in contact. It is not a fundamental force by itself; it emerges from surface roughness, adhesion, deformation, lubricant shear, chemical interactions and energy dissipation. Friction can be useful for grip and braking, but excessive friction wastes energy, generates heat and accelerates wear.

• Key takeaway: Friction acts opposite to relative motion or the tendency of motion.
• Key takeaway: Dry friction is usually controlled by surface roughness, real contact area, adhesion and deformation.
• Key takeaway: Lubricated friction is often controlled by lubricant-film shear and the lubrication regime.
• Key takeaway: Reducing harmful friction is one of the main goals of tribology because friction losses reduce efficiency and increase wear.

Table of Contents

• What is friction?
• Friction definition
• Why friction happens
• Dry friction vs lubricated friction
• Laws of friction
• Static, kinetic and rolling friction
• Coefficient of friction
• How to control friction
• FAQs

What is friction?

Friction is a force acting opposite to the direction of relative motion. Friction arises at the interface between bodies, but it may also develop inside fluids or deforming bodies. Examples of the latter include air friction and hydrodynamic friction, where resistance appears between moving fluid layers.

Friction is not a fundamental force in the same sense as gravity or electromagnetism. It is a macroscopic result of many microscopic interactions: elastic and plastic deformation, surface roughness, adhesion, capillary forces, van der Waals forces, chemical bonding, lubricant shear and third-body particles. Because these mechanisms dissipate energy, friction is a non-conservative force.

In engineering, friction is both a problem and a feature. Brakes, tires, clutches and walking require friction. Bearings, gears, engines and seals usually need friction to be controlled or reduced. This is why tribology studies friction together with wear and lubrication.

Friction definition

Friction is the tangential resistance force that opposes relative sliding, rolling or flow between contacting bodies or material layers. In a simple dry sliding case, friction is often written as:

F = μN

Here, F is friction force, μ is the coefficient of friction and N is the normal load. This simple equation is useful, but it hides the complex physics behind friction. The value of μ is not a universal material constant; it depends on surface roughness, contact pressure, speed, temperature, humidity, lubricant chemistry, contamination and wear state.

Friction losses reduce the energy efficiency of many mechanical devices. A reported estimate for passenger cars found that only a fraction of potential fuel energy is used to move the car, while direct frictional losses account for a large share of the energy budget. One of the major challenges of modern tribology is therefore to develop technologies that reduce harmful friction and increase energy efficiency.

Why friction happens

For most engineering surfaces, dry friction is strongly affected by surface roughness. Even mirror-like surfaces are not perfectly flat at micro- and nano-scales. They contain asperities: small peaks and valleys that carry the real contact load.

AFM height profile and cross-section of clean glass plate

When a rough surface slides against another rough surface, asperities can interlock, deform, fracture or adhere. The classical egg-carton analogy illustrates mechanical interlocking: surfaces with repeated hills and valleys resist sliding because they must climb, deform or break through each other.

Egg carton type surfaces. Source: https://en.wikipedia.org/wiki/Superlubricity

However, making a surface atomically smooth does not automatically eliminate friction. When surfaces become extremely smooth and clean, adhesion can increase. Van der Waals interactions, capillary bridges and chemical bonding may raise friction even when mechanical roughness is low. This is one reason friction control requires both surface engineering and lubrication, not just polishing.

Dry friction vs lubricated friction

Friction can be dry or lubricated. Dry friction occurs when two solid surfaces carry load with little or no lubricant film separating them. It is usually dominated by asperity contact, adhesion, deformation and wear-particle interactions.

Lubricated friction occurs when a lubricant separates the surfaces partly or fully. Depending on film thickness and surface roughness, the contact may operate in boundary, mixed, elastohydrodynamic or hydrodynamic lubrication. TriboNet’s pages on hydrodynamic lubrication, elastohydrodynamic lubrication, mixed lubrication and the Reynolds equation provide the deeper lubrication background.

Lubricant Film (From: Lubrication for Industry, by Ken Bannister, Industrial Press)

In full-film lubrication, the friction force is determined mainly by shear within the lubricant rather than by direct shearing of solid asperities. This generally reduces friction and wear. However, many real contacts operate in mixed or boundary regimes, especially during startup, shutdown, low speed, high load or lubricant starvation. In those cases, additives, surface finish and material pairings become critical.

Laws of friction

Leonardo da Vinci performed an early systematic analysis of friction. Later, Amontons and Coulomb formulated laws that are still used for introductory dry-friction models. These laws are simplifications, but they remain useful for many engineering estimates.

1. Amontons’ First Law: the force of friction is approximately proportional to the applied normal load.
2. Amontons’ Second Law: the force of friction is approximately independent of the apparent area of contact.
3. Coulomb’s Law of Friction: kinetic friction is approximately independent of sliding velocity.

These laws work best for simple dry contacts over limited ranges. They can fail when adhesion, plasticity, lubrication, temperature, contamination, surface films, vibration or severe wear dominate the contact behavior.

Static, kinetic and rolling friction

• Static friction acts before sliding begins. It resists the start of motion and can adjust up to a maximum value.
• Kinetic friction acts during sliding. It is often lower than the maximum static friction, although this depends on materials and conditions.
• Rolling friction or rolling resistance occurs when a body rolls over a surface. It is usually caused by deformation, hysteresis, micro-slip and surface losses.

In real machines, these friction types can overlap. A rolling bearing, for example, may include rolling resistance, sliding at contacts, lubricant shear, cage friction and seal friction.

Coefficient of friction

The coefficient of friction, μ, is a convenient way to compare contact pairs, but it should be used carefully. A tabulated value for steel-on-steel, rubber-on-asphalt or polymer-on-metal is only a starting point. The measured coefficient can change with speed, pressure, roughness, oxidation, humidity, lubrication, temperature and running-in.

For practical design, engineers usually need test data under representative conditions. Tribological tests such as pin-on-disc, ball-on-disc, block-on-ring and reciprocating wear tests can measure friction and wear under controlled load, speed, temperature and lubricant conditions.

How to control friction

1. Choose compatible materials: avoid adhesive pairs when low friction is required.
2. Control roughness: optimize surface texture rather than assuming smoother is always better.
3. Use lubrication: select viscosity, additives and delivery method for the operating regime.
4. Apply coatings: DLC, MoS₂, nitrides and other coatings can reduce friction in specific environments.
5. Manage load and speed: avoid operating points that drive the contact into damaging mixed or boundary regimes.
6. Prevent contamination: abrasive particles can increase friction and wear dramatically.
7. Design for heat removal: frictional heat can change lubricant viscosity, material properties and oxidation behavior.

In advanced cases, researchers also study superlubricity, where friction coefficients can fall far below typical lubricated engineering values. Superlubricity is promising, but it is highly sensitive to surface structure, chemistry, environment and scale.

FAQs

Is friction always bad?

No. Friction is essential for walking, braking, gripping, clutches and tires. The engineering goal is not always to minimize friction, but to control it for the application.

Why does friction produce heat?

Friction dissipates mechanical energy through deformation, adhesion, fracture, lubricant shear and microscopic slip. Much of that dissipated energy becomes heat.

Can friction be zero?

In practical engineering contacts, friction is rarely zero. Very low friction is possible under special conditions such as full-film lubrication or superlubricity, but maintaining those conditions can be difficult.

What is the difference between friction and wear?

Friction is resistance to motion. Wear is material loss or surface damage. They are related but not identical: a contact can have high friction with little wear, or low friction with significant wear depending on mechanisms.

Last updated: May 14, 2026.