Tribology Explained

Tribology, the science of friction, wear, and lubrication, is a vital but often overlooked field that impacts our daily lives in profound ways. From the smooth operation of car engines to the durability of prosthetic joints, tribology plays a crucial role in reducing energy consumption, extending the lifespan of machinery, and enhancing human comfort and safety. In this article, we will delve into the fascinating world of tribology, exploring its significance, key principles, and real-world applications that make it an indispensable science in our modern world.

Friction is the resistance to relative motion between two bodies in contact. It is not a material property but a system property. Scientists believe it occurs due to the electromagnetic attraction between charged particles in two surfaces that are touching. There are several types of friction, including:

• Static Friction, which occurs when two objects are not moving relative to each other (i.e., like a chair on the ground);
• Rolling Friction, which occurs when two objects move relative to each other and one “rolls” on the other (i.e., a car’s wheels on the ground)
• Kinetic Friction, which occurs when two objects mov relative to each other and rub together (i.e., a person sliding down a slide);
• Sliding Friction, which occurs when two objects rub against each other (i.e., placing a book flat on a table and moving it around); and
• Fluid Friction, which occurs when a solid object moves through a liquid or gas (i.e., a kite moving through the air).

Friction is not considered a fundamental force. It is a non-conservative force meaning that work done against friction is path dependent.

Wear is the gradual removal, damaging or displacement of material at solid surfaces. When it comes to wear, the common types are:

1. Abrasive Wear, which occurs when a hard, rough surface slides across a softer surface
2. Adhesive Wear, which occurs due to unwanted displacement and attachment of wear debris from one surface to another;
3. Fretting Wear, which occurs due to repeated cyclical rubbing between two surfaces
4. Erosive Wear, which occurs when solid or liquid particles impinge against the surface of an object
5. Surface Fatigue, which occurs when the surface of a material is weakened by cyclic loading; and Corrosion/Oxidation Wear, which occurs due to chemical reactions between worn materials and a corroding medium.

Given the various types, tribology also illustrates how Wear can undergo many changes in time or undergo changes in operational conditions.

 

Tribology and Lubrication

Lubrication is the control of friction and wear by introducing a friction-reducing film between moving surfaces in contact. This film, aka lubricant, can be a solid, fluid or plastic substance with oil and grease being the most common.

Lubricants have several functions, including reducing friction, preventing wear, protecting equipment from corrosion, controlling temperature and contamination, transmitting power and providing a fluid seal.

When it comes to lubrication, there are three different types referred to as regimes: boundary, mixed and full film. Boundary lubrication exists where there are frequent starts and stops, and also where shock-loading conditions are present.

For example, some oils contain additives – such as extreme pressure (EP) or anti-wear (AW) – to help protect surfaces in case full films cannot be recognized due to load, speed or other factors. These EP and/or AW additives adhere to the metal surfaces to form a “sacrificial layer that protects the metal from wear” (Cash, “What is Lubrication?”).

Full-film lubrication exists in two forms: hydrodynamic and elastohydrodynamic. Hydrodynamic lubrication (HL) occurs when two surfaces in sliding motion are fully separated by a film of fluid. Elastohydrodynamic lubrication (EHL) is very similar to HL but occurs when the surfaces are in a rolling motion (relative to each other). EHL gets its name from the film’s property of elastically deforming the rolling surface to lubricate it, and the film layer in EHL conditions is much thinner than that of HL. This results in greater pressure on the film.

Mixed lubrication, which is a combination between boundary and hydrodynamic lubrication, is when “the bulk of the surfaces are separated by a lubricating layer [and] the asperities still make contact with each other” (Cash, “What is Lubrication?”). As with boundary lubrication, mixed lubrication might also contain additives to create a protective layer on the metal.

There are several fundamental concepts in Tribology, such as Tribosystem, Tribofilm and Stribeck Curve. A Tribosystem is defined as a Tribological System composed of at least two contacting bodies and any environmental factor affecting their interaction (“Tribosystem,” Wikipedia). It is essential for tribologists to understand Tribological Systems as it allows them to create and execute Tribological Tests. Another important element of the Tribosystem revolves around the use of Tribological Coatings, as it relates to ferrous and non-ferrous materials.

For example, a diamond-like carbon (DLC) or hydrocarbon coating might be applied to a component or components for reducing friction and protecting against wear. Physical vapor deposition (PVD) coating is another example and involves producing a thin coating by subjecting material from a condensed phase to a vapor phase, resulting in a thin film condensed phase. This is different than chemical vapor deposition (CVD) which produces high-performance solid materials under vacuum. Other Tribological Coatings include:

• Cathodic arc deposition (Arc-PVD)
• Titanium carbo-nitride (TiCN)
• Titanium nitride (TiN)
• Titanium aluminum nitride (TiAIN)
• Chromium nitride (CrN)
• Zirconium Nitride (ZrN)
• Titanium Diboride (TiB2)
• Polycrystalline Diamond (PCD)

Tribofilm, or Tribofilms, are films produced on surfaces and play an integral part in reducing or minimizing Friction and Wear in lubricated systems. Tribofilms are also referred to as boundary lubricant films, boundary lubricating films, tribo-boundary films or boundary films (“Tribofilm,” Wikipedia).

The Stribeck Curve is a graph showing how friction in fluid-lubricated contacts is a non-linear function of lubricant viscosity, entrainment velocity and contact load (“Stribeck curve,” Wikipedia). It is named after Richard Stribeck, a German mechanical engineer, who first described the concept in 1902. This graph showcases how the generation of lubricant films are critical in the reduction of friction and wear of machine parts.

 

History of Tribology

The word “Tribology” comes from the Greek word tribos meaning rubbing, translating the word literally into the “science of rubbing”. While the study of the concept dates to Leonardo da Vinci and his studies on the laws of friction, the word “Tribology” was not widely used until Peter H. Jost, a British mechanical engineer, coined the term in the March 9, 1966 Jost Report.

Jost is considered the founder of the discipline of Tribology and from his report, a greater spotlight was placed on the subject. It called for the establishment of Institutes of Tribology, along with the publication of a handbook on tribo-design and engineering.

In an interview conducted by Jim Fitch, founder of Noria Corporation, Jost was asked to describe Tribology’s conception and he pinpointed that moment to September 1964 at the Joint Iron and Steel Institute/IMechE Lubrication and Wear Group Conference on Lubrication in Iron and Steel Works in Cardiff.

It was at this conference where failures were discussed, particularly in broken steel mill machinery and equipment. After this, Jost was asked to form a committee to “investigate the question of lubrication education, research and the needs of industry” (Fitch, “Interview with Luminary Professor H. Peter Jost”).

Shortly after publication of the Jost Report, the Committee on Tribology was formally established on September 26, 1966 and was charged with several duties, including:

• Advising the minster of technology on measures to effect technological progress and economic savings in the sphere of tribology
• Advising government departments and other bodies on matters associated with tribology
• Examining and recommending to industry the latest techniques on tribology
• Reporting to the minster of technology annually on its own activities and on trends and developments in tribology considered to be of technological or economic significance to the nation

Tribology has since become an interdisciplinary area linked with biology, chemistry, engineering, materials science, mathematics and physics.

 

Applications

Historically, tribology is applied to the most common rolling or sliding components, which are bearings, gears, cams, brakes and seals. These common elements are used in a variety of machines that have relative motion and require some sliding motion and/or rotational motion. This early focus on enhancing operation and extending the life of industrial machinery has evolved into other applications where it has made a major impact on a variety of applications.

 

Application in Industry

When it comes to research, tribology ranges from macro to nano scales. While it was traditionally concentrated on transport and manufacturing sectors, it has grown more diversified over the decades and can be divided into the following fields:

Classical Tribology

As the name suggests, Classical Tribology focuses on friction and wear in machine elements – rolling-element bearings, gears, plain bearings, brakes, clutches, wheels, etc. – as well as manufacturing processes.

Biotribology

With Biotribology, research focuses on lubrication in biological systems such as human hip and knee joints. In fact, one of the most striking examples of Biotribology is with total hip replacements, which “replace the body’s natural ball-and-socket joint with a very smooth metallic (stainless steel or cobalt-chromium alloy) ball at the head of the femur, articulating in a cup in the pelvis made from ultra-high molecular weight polyethylene” (Hutching, “Fifty Years of Tribology”).

Green Tribology

Green Tribology, which was also introduced by Peter H. Jost, looks at minimizing the environmental impact, including ways to reduce tribological losses by using technologies with minimal impact on the environment.

Geotribology

With Geotribology, the focus is on studying friction, wear and lubrication of geological systems such as faults and glaciers (Wikipedia). As a new facet of Tribology, Geotribology is gaining momentum in the scientific world particularly in its abilities to analyze fault slips.

Nanotribology

With the development and commercialization of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), the field of Nanotribology has emerged as a strong focus. This particular application studies tribological phenomena at a nanoscopic scale, which refers to structures with a length scale applicable to nanotechnology. Nanotribology has gotten a boost in its research since the invention of Atomic Force Microscopy (AFM), which is a high-resolution form of scanning probe microscopy (SPM).

Tribotronics

Another industry application is Tribotronics, which is a facet of research combining machine elements and electronic components to create active tribological systems and increase a machine’s efficiency and lifetime.

Computational Tribology

With Computational Tribology, the aim is modeling the behavior of tribological systems by combining several disciplines such as contact mechanics (i.e., the study of the deformation of solids that touch each other at one or more points), fracture mechanics (i.e., the study of the proliferation of cracks in materials) and computational fluid dynamics (i.e., the study of using numerical analysis and data structures to solve and analyze problems involving fluid flows).

Space Tribology

Space Tribology looks at tribological systems with the ability to operate under the harsh environmental conditions of outer space, particularly due to extreme temperature fluctuations.

Open System Tribology

Open System Tribology studies tribological systems exposed to and affected by the natural environment.  

Importance of Tribology

In the beginning, Tribology and Tribology research was focused on the design and effective lubrication of machine components such as bearings. Over time, there has been a shift in tribology’s focus to include several aspects of modern technology.

Traditional applications, which fall under Classical Tribology, highlight the importance of tribology as it pertains to the sliding surfaces in most mechanical components, which are critical to energy efficiency and maximum life expectancy of those components. Within the transportation industry, traditional tribology research focused on reliability but in more modern times, the focus has shifted towards energy consumption and increased efficiency, resulting in more complex lubricants. For example, Tribology can reduce carbon dioxide emissions by increasing energy efficiency.

In addition to the transportation industry, tribology has played a vital role in the manufacturing sector, particularly in metal-forming operations. Understanding tribology in manufacturing is important as it can increase productivity while reducing costs. Two other sectors where tribology has significant importance is power generation and residential.

The importance of tribology has only increased over time. In fact, Congressional Representatives Tim Ryan (D-Ohio), Dan Lipinski (D-Illinois) and Mike Doyle (D-Pennsylvania) introduced H.Res.306 on May 2, 2017, to magnify the importance of tribology. Still awaiting further action, H.Res.306 “recognizes the impact of tribology … on the United States economy and competitiveness in providing solutions to critical technical problems in various industries.”

This legislation also “encourages federal agencies to develop and install programs related to tribology” … “encourages the formation of public-private partnerships to advance fundamental research and speed up the development of Tribology-related products” … and “encourages the National Academy of Engineering to conduct a survey on the status of tribology research in academia and government laboratories and to recommend a course of action to accelerate innovations in tribology” (Ryan “H.Res.306”).

 

Lubrication and Tribology

Lubrication and tribology go hand in hand. The use of lubricants dates back to images from ancient civilizations in China and Egypt showing the application of lubricants to reduce friction from dragging heavy stones used in building.

Lubricants are used to separate two sliding surfaces, minimizing direct surface contact and reducing tool wear and power requirements. Lubricants also conduct heat and contaminants away from the interface. The majority of lubricants are liquids, composed of oil and additives; but, some lubricants are gases and solids.

Before selecting the proper lubricant, the tribological system needs to be identified. These identifying markers, or parameters, include the type of motion, speed, temperatures, load and operating environment.

Type of motion is the first parameter of the tribological system. The motion can be sliding, which requires hydrodynamic lubrication (HL) theory for analysis, or rolling, which would require elastohydrodynamic lubrication (EHL) theory.

The different between those two is simple; HL theory focuses on reducing friction and/or wear of rubbing solids by adding the proper lubricant that goes between the rubbing solids, creating a thin liquid film (Tribonet.org).

As discussed, EHL is a type of HL where significant elastic deformation of the surface takes place, drastically altering the shape and thickness of the separating lubricant film (Tribonet.org).

In some cases, the motion is a combination of sliding and rolling, which occurs in certain rolling-element bearings, such as tapered roller bearing. In this instance, the lubricant’s chemistry would need to be fine-tuned for optimal performance.

The second parameter of the tribological system is speed, which can be divided into three categories: fast, moderate and slow. In determining the ranges for speed categories, it is essential to know the Stribeck curve and how to calculate it. As stated, the Stribeck Curve is a graph that shows how friction in fluid-lubricated contacts is a non-linear function of lubricant viscosity, entrainment velocity and contact load. By knowing the speed of the contact, the proper lubricant can be selected to reduce friction.

Temperature is the third tribological parameter and a vital one as all lubricants have specific temperature ranges for optimal performance and effectiveness. Due to its chemistries, some lubricants operate within a broad temperature range, while others perform optimally at lower temperatures. By identifying the tribological system’s temperature, a tribo-engineer is able to accurately select a lubricant(s) that will enable a machine to achieve optimum operating life and performance.

Next on the list is the load, which is an important component affecting the lubricant requirement. If there is a light load, a lubricant designed to minimize fluid friction – while still providing metal-to-metal friction protection – would be needed since the application is sensitive to frictional torque.

Adversely, if there is a heavily loaded application, one would need to select a lubricant containing specific additives to protect against extreme wear, galling and pitting.

The final parameter is the operating environment. If the tribosystem is operating in an environment subject to moisture or water, the required lubricant needs to be resistant to water washout or contamination, as well as provide good anti-corrosion properties. When the application exists in an environment containing chemical liquids and vapors, the required lubricant must be resistant to these chemicals and vapors.

If the application’s environment is in a vacuum or partial vacuum, the application’s atmospheric pressure must be within the operational limits of the lubricant and above its vapor pressure at the operating temperature (Lauer, “Tribology: the Key to Proper Lubricant Selection”).

After identifying the system and its parameters, the tribo-engineer (or lubrication engineer) employs different lubricant chemistries to determine the optimal lubricant for the application. In addition to choosing lubricants based on chemistry, the tribo-engineer also needs to analyze the application based on the tribological system, which includes an analysis on speed factors, elastohydrodynamic (EHD) lubrication, bearing-life calculations, extreme-pressure lubrication, emergency lubrication and other special application requirements.

 

Tribological Test Methods

In the past (and even today), testing tribological properties of materials involved building entire systems to run specialized tests such as field tests, bench tests, component tests and model tests.

Model tests, for example, allow for film thickness, friction and wear measurements to be conducted. Other tests include electrical and interferometry methods, the pin-on-disc test, the twin disc test, a reciprocating test and a rotary tribotest.

Tribology testing should always be designed and carried out to meet a defined need, and it is essential as it results in vital information regarding any failure mechanisms of mechanical components.

 

What is a Tribologist?

A Tribologist is a student of, or expert in, the field of Tribology. Leonardo da Vinci is one of earliest and most famous Tribologists for his studies on the laws of friction. Peter H. Jost, British mechanical engineer and author of the ground-breaking Jost Report, coined the term “Tribology” in 1966 and is considered the father of the discipline.

Those involved in the development, maintenance, and continual improvement of lubrication programs are Tribologists by extension.

 

For further reading on tribology, please see our article, "A Comprehensive Exploration of Tribology: Unveiling the Historical Evolution."