Friction

Friction is a fundamental mechanical force that opposes the relative motion of solid surfaces, fluid layers or deforming material elements. Present in everyday phenomena and critical to engineering applications, friction has shaped scientific understanding for more than two millennia. The study of friction, lubrication and wear—collectively known as tribology—reveals the complex interactions that occur whenever surfaces come into contact and energy is dissipated.

Nature and Origins of Friction

Friction arises from a range of microscopic and macroscopic mechanisms. When two surfaces meet, their irregularities or asperities interlock, deform or shear as motion begins. This contact creates resistance to movement. Additional contributors include adhesion between surface molecules, plastic deformation of microstructures and triboelectric effects. As bodies slide past each other, part of the mechanical energy is transformed into heat or used to alter the structural properties of the materials involved. The total dissipated energy per unit distance corresponds to the frictional force that must be overcome for motion to continue.
Because friction depends on complex, often non-linear interactions, deriving its behaviour from first principles is challenging. For this reason, empirical models and laboratory observations remain central to tribological research.

Types of Friction

Frictional behaviour varies depending on the nature of the surfaces, the medium separating them and the mode of motion. Key types include:
Dry frictionDry friction resists relative lateral motion between two solid surfaces.

• Static friction acts before motion begins and is typically greater than kinetic friction.
• Kinetic friction (or sliding friction) acts during movement.Apart from interactions at the atomic scale, dry friction is largely governed by the interlocking and deformation of asperities.

Fluid frictionFluid friction occurs between layers of a viscous fluid moving relative to one another. It plays a principal role in hydrodynamics and is described by relations derived from fluid mechanics, including Reynolds’ formulation of viscous flow.
Lubricated frictionLubricated friction is a specialised form of fluid friction in which a fluid film separates two solid bodies. Lubricants reduce direct surface contact, lowering wear and improving efficiency in machinery.
Skin frictionSkin friction forms part of aerodynamic or hydrodynamic drag, arising from the interaction between a moving fluid and a solid surface.
Internal frictionInternal friction resists deformation within a solid material. It influences mechanical damping, elasticity and the stability of structures under dynamic loads.
These categories illustrate friction’s pervasive role across mechanical systems, from microscopic interactions to large-scale engineering challenges.

Historical Development of Friction Theory

Interest in friction dates back to antiquity. Ancient Greek and Roman authors noted differences between static and kinetic resistance, and they explored techniques to reduce wear in construction and machinery.
Early foundationsBy the late fifteenth century, Leonardo da Vinci formulated what would later be recognised as the classical laws of dry friction, although his findings remained unpublished for centuries. Guillaume Amontons independently rediscovered these principles in 1699, identifying the proportionality between normal load and frictional force and emphasising the role of surface roughness.
Eighteenth-century advancesJesuit scientist John Theophilus Desaguliers introduced adhesion as a key factor, arguing that microscopic sticking contributes to friction. Charles-Augustin de Coulomb expanded tribological theory in 1785 by studying how material properties, surface coatings, normal force and contact duration influence friction. He also distinguished between static and kinetic friction in a formal manner.
Nineteenth-century experimental workResearchers sought to resolve inconsistencies in earlier models. John Leslie provided a time-dependent explanation based on the flattening of asperities. Developments in thermodynamics, particularly by Benjamin Thompson and James Prescott Joule, demonstrated that friction converts mechanical work into heat, establishing friction as a central example of irreversible processes.
Osborne Reynolds’ work on viscous flow in 1866 completed the classical understanding of fluid friction, forming the basis of modern lubrication theory.
Twentieth-century discoveriesFleeming Jenkin, James Alfred Ewing and George H. Bryan further investigated transitions between static and kinetic friction. Max Planck later emphasised frictional heating as a key thermodynamic phenomenon. By mid-century, Frank Philip Bowden and David Tabor revolutionised friction research by demonstrating that the true contact area between rough surfaces is far smaller than the apparent area. With the invention of the atomic force microscope in the 1980s, researchers could observe friction at atomic scales, revealing that frictional behaviour depends on interfacial shear stress and atomic-scale contact areas.

Energy Dissipation, Wear and Efficiency

Friction plays a vital role in mechanical performance. While it enables essential functions such as braking and gripping, it also causes wear, which can degrade components and shorten operational life. Globally, friction-related energy losses account for a significant proportion of industrial energy consumption, often estimated at around one-fifth of total usage. This highlights the need for advanced materials, surface treatments and lubrication strategies to enhance energy efficiency.
The dissipative nature of friction also contributes to heat generation and structural changes that influence material behaviour over time. These processes are highly dependent on the microscopic features of surfaces and the environmental conditions present during contact.

Laws of Dry Friction

Classical empirical laws describe the behaviour of sliding friction under many conditions:

• Amontons’ First Law states that the frictional force is directly proportional to the normal load.
• Amontons’ Second Law asserts that the frictional force is independent of the apparent contact area between surfaces.
• Coulomb’s Law extends these principles by distinguishing between static and kinetic friction, each characterised by different coefficients.

Although these laws apply primarily to dry, rigid surfaces and moderate conditions, they form the foundation of engineering analysis and remain widely used in design and modelling.

Modern Tribology and Applications

Tribology today integrates physics, materials science, chemistry and engineering to analyse friction, lubrication and wear. Applications span mechanical engineering, nanotechnology, biomedical devices and geophysics. Advances in surface engineering, such as coatings, polymers and nanostructured materials, aim to reduce frictional losses and extend the lifespan of mechanical systems.