What is Tribology?

Though seldom known to the masses, tribology is one of the most important fields of study in the modern world, affecting every facet of our daily lives in ways commonly overlooked.

For those in the field of engineering and mechanical design, a strong understanding of tribological principles is vital, so this article will explore the field’s evolution, fundamental concepts, and more, to provide a clear understanding of this seminal subject.
 

What is tribology?

Let’s begin by answering the pivotal question of this article. 

Tribology is the multidisciplinary science and technology of interacting surfaces in relative motion. It encompasses the study of friction, wear, lubrication, and their practical applications across engineering fields.

Far from a niche topic, tribology impacts mechanical performance in sectors such as automotive, aerospace, energy generation, manufacturing, biomedical engineering, and renewable energy. 

Tribology's impact has become especially significant in modern engineering thanks to increased machinery complexity, energy conservation needs, and global sustainability.
 

The history of tribology

Humanity’s interaction with tribology actually predates written history.

In the Palaeolithic and Neolithic eras, ancient humans would use frictional heating for fire-making and shaping materials, such as producing bracelets, or performing primitive dental procedures via drilling and filling.

Civilisations such as Ancient Egypt and Sumer would use lubricants in large construction projects. Paintings dated to 1880 BCE depicted workers pouring water on sand ahead of sledges carrying statues, showing concepts of surface friction modification and lubrication.

True study began during the Renaissance. Leonardo da Vinci recorded laws of friction, the proportionality of friction force to load, and its independence from surface area, ahead of formal articulations nearly two centuries later.

In the 18th and 19th centuries, Charles-Augustin de Coulomb, Osborne Reynolds, and Beauchamp Tower further developed laws of friction and hydrodynamic lubrication, with the latter developing the Reynolds equation, central to the mathematical modelling of fluid films in bearings and gears.

The term “tribology” itself, from the Greek word “tribos” meaning “I rub,” was first coined in the 1966 Jost Report which highlighted the economic costs of friction, wear, and corrosion. 

Tribological inefficiencies were shown to account for more than 1% of GDP in some industrialised countries, with costs arising from energy losses, increased maintenance, and premature component failures. 

In the years since, tribology has ballooned into a fully interdisciplinary science, with subfields including biotribology, nanotribology, space tribology, tribotronics, and green tribology. 

With reports now estimating tribological contacts account for around 23% of total global energy consumption, and efficient practices potentially providing annual energy savings of up to 40%, this field is arguably one of the most important to the future of engineering.
 

What are the fundamental concepts of tribology?

Tribology primarily concerns itself with three interconnected factors:

• Friction - the resistive force that opposes relative motion between contacting surfaces
• Wear - the progressive loss or alteration of material from a surface due to mechanical action
• Lubrication - the application of substances or strategies to lower friction, moderate wear, and reduced heat or corrosion at interacting surfaces

We will delve further into these three core pillars of tribology shortly, but first we must understand the concepts of tribofilm and tribosystems.

The concept of tribofilm (thin protective layers spontaneously formed (chemically, physically, or tribochemically) at the interface) is another key factor for understanding how surfaces evolve and protect themselves under stress.

Modern tribology gains insight across scales and disciplines. Friction, for instance, arises from atomic-scale adhesive forces and manifests as macroscopic resistance, while lubrication embodies hydrodynamics, viscoelasticity, and surface chemistry. 

Modelling, experimentation, and diagnostics require both system-level abstraction and detailed surface science.

A tribosystem is defined as any physical system constituted by at least two interacting surfaces (triboelements) in relative motion, plus all environmental and operational factors influencing the interaction. 

Describing a tribosystem involves explicit enumeration of the following:

• Inputs - such as load, speed, temperature, environment
• Structure - nature and properties of the contacting elements
• Outputs - motion, energy, forces
• Losses - primarily friction and wear

Adopting this view is important for designing, diagnosing, and optimising tribological performance. 

For example, in a bearing, the tribosystem includes the shaft (material, surface finish), the bearing (material, geometry), lubricant (type, viscosity), and operating environment (load, temperature, contaminants).
 

The laws of friction

The word friction comes from the Latin “frictionem”, which means rubbing. In tribology, this is determined by a set of empirical laws, first established by Guillaume Amontons in the 17th century, then later refined by Coulomb:

• The First Law of Amontons - the friction force is proportional to the normal load applied
• The Second Law of Amontons - the friction force is independent of the apparent contact area between surfaces
• The Third Law of Coulomb - kinetic friction is independent of sliding velocity

These laws generally hold under dry, unlubricated contact and for metals or hard ceramics, but breakdown occurs with polymers, at very high/low speeds, or in nanoscopic systems where, for instance, adhesion dominates.

At the microscopic level, Bowden and Tabor established that the real contact area (determined by surface asperities and modulus) controls friction, not the visible (apparent) area. 

The relationship between normal load and real contact area leads directly to friction laws.

The three types of friction are as follows:

• Static friction applies when surfaces are at rest relative to each other; it is typically higher than kinetic friction, leading to “stiction”
• Kinetic (dynamic) friction is the force during motion, further broken down into sliding and rolling friction
• Rolling friction is generally much lower than sliding friction, crucial for the efficiency of rolling element bearings

Their corresponding friction coefficients are denoted by µs, µk, and µr, and usually follow this order:

µs > µk > µr

Recent progress in nanotribology has revealed phenomena such as superlubricity (which we will discuss further shortly) is often achieved using special surface structures or lubricants, such as graphene or diamond-like carbon films.
 

The mechanisms of wear

Wear is the process by which material is removed, transferred, or transformed at interacting surfaces due to mechanical or chemical effects. 

Unlike failure by fracture, wear usually occurs progressively, though severe forms can be catastrophic. 

Wear is typically characterised by volume loss per unit distance slid (known as the specific wear rate) or through material mass loss. 

Empirical laws, such as Archard’s law, relate wear volume to normal force, sliding distance, and material hardness.

Principal wear mechanisms include:

• Adhesive wear - local bonding leads to material transfer and the eventual detachment of fragments. Adhesive wear is especially prevalent on metal-on-metal contacts where boundary lubrication fails, and can rapidly degrade surfaces by forming micro-welds that subsequently shear off
• Abrasive wear - hard particles or rough protuberances cut or dig into softer surfaces, a common occurrence where external contaminants are present or where wear debris is not swiftly removed
• Fatigue wear - surface or sub-surface cracking from cyclic loading leads to flaking or pitting, and is commonly observed in bearings and gear teeth
• Corrosive/oxidative wear - chemical or electrochemical reactions at the interface weaken material, often producing brittle oxide scales or debris that aggravate other wear forms
• Fretting and erosive wear - small-amplitude oscillatory motion (fretting) and material removal by fluid or particle impingement (erosion) further aggravate the other categories

The rate and pattern of each wear mechanism depend on load, velocity, environment, materials involved, and lubricant effectiveness.
 

The categories of lubrication

Lubrication, the science of intervening between surfaces to control friction and wear, is the last of the central components of tribology.

Contrary to belief, lubrication extends beyond oils or fats to any fluid material characterised by viscosity.

It can be classified into categories, each determined by the thickness of the lubricant film relative to surface roughness (the λ parameter):

• Boundary lubrication - in which the load is carried by the surface asperities rather than by the lubricant. Friction and wear depend on chemical additives, surface films, and molecular interactions
• Mixed lubrication - in which partial lubricant film exists, but some asperity contact remains. Both hydrodynamic and boundary effects are present
• Hydrodynamic lubrication - in which surfaces are entirely separated by a pressure-bearing, viscous fluid film. Friction is minimal, determined by the fluid’s rheology and operating conditions
• Elastohydrodynamic lubrication (EHL) - similar to hydrodynamic lubrication, but occurs under higher pressure/rolling contacts, so surfaces elastically deform, increasing the lubricant film thickness

The applications of lubricants extend beyond friction reducers. They cool surfaces, protect against corrosion, forbid contamination, and enable energy-efficient operation.
 

Surface engineering

Success in tribological systems hinges greatly on the right combination of surface engineering and materials. 

Wear, friction, and lubrication demands rarely align perfectly across operational phases, so surface properties must be tuned for each regime.

Surface coatings, such as diamond-like carbon (DLC), titanium nitride (TiN), or chromium nitride (CrN) are common solutions for reducing wear and friction. 

Their deposition uses methods like physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma-assisted processes, nitriding, carburising, and boriding. 

Many coatings now include multi-layer or graded architectures for enhanced durability, self-healing capabilities, or specialised friction profiles (superlubricity).

The table below outlines the key tribological surface coatings, and their properties:

Coating type	Application fields	Key properties
DLC (Diamond-like carbon)	Automotive, biomedical, electronics	Superlow friction, chemical inertness, biocompatible
TiN, CrN, TiAlN	Cutting tools, aerospace, gears	Hard, wear-resistant, oxidation stability
Polycrystalline diamond (PCD)	Machining, electronics	Extreme hardness, thermal conductivity
Self-lubricating nanocomposites	Automotive, power tools	Embedded lubrication phases

Selection of base materials (steels, ceramics, polymers, composites) requires a careful match between hardness, toughness, chemical reactivity, and counterface performance. 

For instance, metals with a high surface energy often show strong adhesion, while hexagonal-closed-packed materials may have lower adhesive wear.

Modern tools such as scanning electron microscopy (SEM), atomic force microscopy (AFM), nanoindentation, and in-situ spectroscopy allow for real-time observation of wear particle formation, tribofilm growth, and chemical changes on operating surfaces.
 

Tribological testing and measurement techniques

Reliable tribological design must be grounded in robust, reproducible testing.

Tribometers replicate simplified tribosystem contacts in controlled settings, measuring friction coefficients, wear rates, and lubrication effectiveness under various loads, speeds, temperatures, and environmental conditions. 

Common configurations include:

• Pin-on-disk (PoD) - a stationary pin (spherical, cylindrical, or conical) is loaded against a rotating disk. This configuration - standardised in ASTM G99 and G133 - compares materials, coatings, and lubricants in both dry and lubricated contacts, allowing for measurement of friction versus time, wear track profilometry, and failure onset
• Ball-on-flat/block-on-ring - this method is useful for rolling or sliding simulations
• Four-ball wear tester - three stationary balls form a cradle for a rotating ball. Zone of wear scar is compared, often for lubricant performance assessment
• Reciprocating tribometers - mimic linear sliding contacts, such as in engine pistons

Modern tribometers measure not only friction and wear, but also temperature, humidity, electrical contact resistance, and sometimes lubricant chemistry in real time.

Testing setups may replicate high vacuum (space applications), physiological fluids (biomedical), or aggressive chemical regimes (corrosive environments) to make sure that tribological systems are robust to real-world challenges.
 

Advancements in Tribology

Tribology is a living science. In the past decade alone, fresh insights into surface behaviour, materials, and simulation techniques have opened new frontiers:
 

Superlubricity

Superlubricity describes the condition in which the coefficient of friction approaches zero. 

This phenomenon occurs when incommensurate contact between atomically smooth surfaces (most famously graphite or molybdenum disulphide (MoS₂)) where lattice misalignment prevents the usual interlocking of asperities. 

Lab tests found that sliding contacts of few-layer graphene on graphite have achieved friction coefficients below 0.001.

Diamond-like carbon (DLC) coatings are a practical bridge to real-world superlubricity. By depositing an amorphous carbon film rich in sp² bonding, DLC creates a low-shear-strength interface that resists adhesion. 

Under boundary-lubrication conditions, DLC surfaces can sustain extremely low friction and dramatically reduced wear rates. 

Studies have extended these benefits by doping DLC with elements such as fluorine or tungsten, altering the tribofilm chemistry for specific operating temperatures and contact pressures.

Beyond hard coatings, researchers are exploring liquid-mediated superlubricity. Aqueous solutions containing long-chain polymers or ionic liquids can form self-assembled monolayers on metallic and ceramic surfaces, achieving ultra-low friction in the boundary regime. 

As additive chemistry becomes more sophisticated, superlubricity may migrate from the lab into applications ranging from micro-electromechanical systems (MEMS) to automotive drivetrains.

 

Biotribology 

The study of friction and wear in biological systems, known as biotribology, has advanced in parallel with medical implant technology. 

Early hip and knee prostheses relied on metal-on-polyethylene bearings, which suffered from long-term wear debris and osteolysis. Today, highly cross-linked ultrahigh-molecular-weight polyethylene (UHMWPE) and ceramic-on-ceramic articulations have extended implant lifetimes beyond 20 years. 

Surface texturing at the microscale (arrays of dimples or grooves on femoral heads) achieves hydrodynamic lubrication by trapping synovial fluid, reducing point contact stresses.

Sub-fields such as skin tribology and oral tribology have also flourished. 

By characterising the frictional behaviour of skin against fabrics, researchers are optimising sportswear, wound dressings and haptic feedback in prosthetic limbs. 

In oral tribology, the interaction between saliva, enamel, and food particles dictates toothpaste formulation and dietary strategies to minimise dental wear. Tools like atomic force microscopy and rheometers now allow measurement of friction and adhesion at the nano-to-micro scale.
 

High-temperature tribology 

As industries push machinery into ever-higher temperature regimes, a new class of lubricants and surface treatments has emerged. 

High-temperature tribology typically covers 300–1,000°C - a range where conventional oils and greases fail. Solid lubricants such as tungsten disulphide (WS₂), calcium fluoride (CaF₂) and graphite remain stable at extreme heat, forming protective tribofilms that sustain sliding contact.

Advanced ceramic coatings can provide both thermal insulation and a hard interface that resists wear. Recent innovations include functionally graded coatings that transition from a metallic bond coat to a ceramic topcoat, reducing thermal mismatch and spallation. 

Researchers are also embedding ceramic-solid-lubricant composites within these layers, allowing for self-lubricating behaviour at temperatures up to 900 °C. These solutions are critical for next-generation turbines and rocket engines, where minimal frictional losses could easily lead directly into fuel efficiency and service life.
 

Computational tribology 

The complexity of real tribosystems has driven the rise of computational tribology. 

By coupling finite-element analysis (FEA), computational fluid dynamics (CFD) and molecular dynamics (MD), engineers can now predict friction, wear, and lubrication performance long before the first prototype is built.

FEA models simulate contact stresses and lubricant film thickness in journal bearings and gears, enabling optimisation of surface patterns and oil-flow channels. 

MD simulations, on the other hand, show how atomic roughness and lubricant chemistries interact under shear, guiding the design of novel boundary additives. 

Bridging these scales, meso-scale lattice Boltzmann methods capture lubricant distribution in textured surfaces.

Recently, machine-learning algorithms have been applied to vast tribological datasets to identify patterns and predict component life. Digital twins, fed by real-time sensor data, adjust operating parameters to maintain lubrication regimes and alert maintenance teams before wear thresholds are exceeded. 

Together, these computational advances are propelling tribology into the era of predictive maintenance and intelligent surfaces.
 

Make your business more productive with our tribology services

Tribology remains a field of foundational relevance for engineering, industry, and sustainability. Its principles and technologies quietly but critically influence machine design, energy efficiency, health, and environmental impact.

Regardless of industry, if you use machinery, vehicles, or any type of system that incorporates interacting components, surfaces, and lubrication, you can benefit from our expert tribology services.

As part of the Brookes Bell group of companies, Neale Consulting Engineers draws upon the knowledge and experience of a multidisciplinary team that includes metallurgists, tribologists, materials scientists and more.

With decades of experience in tribology, our team is on hand to assist you with the ultimate aim of using applied mechanical engineering to improve your business and its operations.

To see how our tribology services can assist you, get in touch with our team today.

Contact Neale Consulting Engineers today

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