Tribology: Friction, Wear, & Lubrication

Tribology is the study of interacting surfaces in motion relative to one another within a system of moving parts. It has three main aspects: friction, wear and lubrication. Renaissance thinker Leonardo da Vinci is widely regarded as the father of tribology as he developed some of the first rudimentary ideas around the subject. Physicist Guillaume Amontons and engineer Charles-Augustin Coulomb, both Frenchmen, expanded upon the tribological body of knowledge and developed further theories, some of which are still relevant to date.

The field of tribology is highly complex as it involves a host of disciplines and academic fields, including mathematics, physics, chemistry, biology, material science and engineering. As such, tribology has numerous subfields, such as biotribology, nanotribology, tribotronics, and more. 

In this article, you will learn about:

• What tribology is
• The significance of tribology
• The materials used in tribology
• The challenges involved in tribology 

What is tribology?

Tribology has several subfields, but it would be prudent to study it from its main underlying processes: friction, wear and lubrication. The essence of tribology is essentially minimising (or in some cases, maximising) friction and wear through the right type of lubrication in a given system that involves moving parts. Since there are many different types of such systems, each involving different materials and requiring different types of lubrication, it is apparent how complicated the study of tribology can become.

What is friction?

Friction is a force that acts in the opposite direction of the motion of two interacting surfaces. This interface may be between two separate bodies or internally within one body. Friction, F, can be defined as the product of an applied load, L, and a constant called the coefficient of friction, μ, which is unique to the interaction between surfaces in contact and not intrinsic to the materials that make up the surfaces.

`F=\mu \cdot L`

There are two main types of friction: static friction and kinetic friction. These indicate the friction between surfaces at rest or in relative motion, respectively. Friction can also be further categorised into sliding friction, rolling friction and fluid friction. Energy used to overcome friction accounts for a significant reduction in efficiency in devices, machines and systems.

What is wear?

Wear is the gradual deterioration of one or all the solid surfaces in contact as they undergo relative motion against each other. Simply put, friction causes wear, and the higher the friction, the higher the wear. There are several mechanisms by which wear occurs. A surface may experience wear due to one or more of these mechanisms simultaneously. 

The mechanisms of wear can be classified into four main categories [1]:

1. Adhesive wear: is wear caused by the adhesion between two material surfaces as they impinge against each other. This bonding may cause a loss of material from one surface and a gain in the other or a loss from both surfaces.
2. Abrasive wear: is wear caused by the surface of a harder material as it presses against a softer material under load, causing a gradual and continuous degradation of the softer material. 
3. Fatigue wear: is wear caused by localised stress between two surfaces that is repeated a large number of times leading to crack initiation and propagation. After a critical number of cycles, debris is formed as material chips off the surfaces from the cracks.
4. Corrosive wear: is the deterioration of a material’s surface due to the corrosive environment that it is exposed to. Corrosive wear may occur in dry or wet conditions as long as the environment has corrosive conditions.

What is lubrication?

Lubricants are substances used to control friction and wear between two moving surfaces in contact with each other. They can be solid, liquid, air or a hybrid of these states. Since there is no truly flat surface, two surfaces rubbing against each other will interlock on a microscopic level. The introduction of a lubricant aims to reduce or eliminate the contact between these two rough surfaces via a lubricating film.

Lubrication used to be an imprecise science that relied heavily on experience and empiricism. For most applications, general-purpose lubricants were sufficient. However, there have been great strides in lubrication driven by the extremely demanding conditions in contemporary applications. 

There are five main factors to consider when selecting lubricants [2]:

1. Type of motion involved (sliding or rolling or a combination of both)
2. Speed of motion
3. Working temperature range
4. Load
5. Operating environment (presence of moisture, chemicals or corrosive liquids)

The above conditions are getting more stringent with the demands of newer applications. As a result, specific fields such as superlubricity, solid coatings, bio-lubrication, and molecular lubrication are under research.

• Explore and compare the different solid-lubricant materials

The significance of tribology

Research into the impact of friction and wear and, by extension, tribology on worldwide energy consumption was conducted by Kenneth Holmberg and Ali Erdemir in 2017. Their study focused on four major energy-consuming sectors: manufacturing, transportation, power generation and residential. Here are some of the conclusions they came up with [3]:

• Almost a quarter of the total energy consumed in the world comes from tribological contacts, mainly in efforts to minimise friction and in manufacturing replacements of worn-out parts and equipment.
• If the advances in tribological research are properly utilised, the potential reduction of total energy lost due to friction and wear could reach 40% in the next 15 years.
• Among the four energy-consuming sectors, transportation stands to gain the most from the proper application of breakthroughs in tribological research (25% in the short term and 55% in the long term) followed by power generation (20% in the short term and 40% in the long term). The list is rounded up with the manufacturing sector and the residential sector with 25% and 20% energy savings in the long term, respectively.
• Tribology research and its proper application can reduce carbon dioxide emissions worldwide by 1.46 billion tons and 3.14 billion tons in the short and long terms, respectively. These reductions in emissions could also lead to cost savings to the tune of 450 billion Euros and 970 billion Euros over the short and long terms, respectively.

Materials used in tribology

Lubricating materials may be gaseous, liquid, solid or a hybrid of these states. Their primary function is to control friction and wear, but they also help control temperature rise, minimise corrosion, transmit power, wash out debris, dampen shocks, and more. 

The use of liquid lubricants is relatively well known. They are made mainly from oils (derived from petroleum, plant, animal and synthetic sources). Some lubricants are semi-liquid/plastic (also known as grease) and are also quite commonly used. Solid lubricants are less common but extremely useful in specific demanding applications. They are rarely used as is and are often added to other lubricants to impart particularly useful properties to them. Examples of solid lubricants are graphite, mica, and lead carbonate [4]. Metal sulfides are also used as solid lubricants, including bismuth sulfide, tungsten sulfide, copper sulfide, tin sulfide, zinc sulfide and iron sulfide. The table below shows a list of metal sulfides' key properties.

• Explore and compare the different metal-sulfide solid lubricants

 

Table 1. Properties of common solid metal sulfide lubricants

	Tribotecc® BIS 84 Bismuth Sulfide	Tribotecc® WS 2 Tungsten Disulfide	Tribotecc® CB 500 Copper-sulfide	Tribotecc® Ferrostar
Thermal diffusivity (mm²/s)	0.252	0.58	1.343	0.25
Oxidation threshold (°C)	400	400	400	475
Volatile share (%)	0.1	0.1	0.5	0.2
Melting point (°C)	685	1250	1100	1188
Specific heat capacity (J/(kg·K))	280	324	324	714
Thermal conductivity (W/(m·K))	0.379	1.131	1.131	0.59
Electrical resistivity (Ω·m)	16	0.5	0.000007
Electrical conductivity (S/m)	0.061	2	140000
Max service temperature (°C)	400	400	400	475
Particle size D90 (μm)	5	7	40
Particle size D50 (μm)	2	3	30
Density (g/cm³)	6.8	7.5	4.9	5

 

Solid lubricants have properties and advantages that make them suitable for specific applications, some of which include [5]:

• They have high stability under high pressure, high vacuum, and in elevated or cryogenic temperatures
• They have high dissipation of heat when used in conjunction with lubricants that have high thermal conductivities
• They have high resistance to abrasion and deterioration in extreme environments, such as high-radiation, high-dust, and high-reactivity environments
• They are more suitable where high speeds, high and intermittent loadings are involved
• They allow for less complex design since leakages and delivery systems are not of concern
• They allow for easier system maintenance and servicing liquid, solid or a hybrid of these states. 

Challenges in tribology

One of the major challenges faced in tribology is its low visibility, largely due to its multidisciplinary nature requiring expertise in varying fields to fully grasp the big picture. Due to this, it is also very difficult to develop problem-solving models that can accurately predict and simulate tribological processes. 

Another challenge is the sheer complexity of the field in itself. A lot of progress that has been made has come from empirical methods of trial and error and improvements born of multiple experimental iterations rather than a complete and thorough understanding of the underlying tribological mechanisms. The way forward is nothing but increased research and an agglomeration of all the component fields into one field so that tribology can stand on its own [6].

[1] Kato, K. "Classification of wear mechanisms/models." Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 216.6 (2002): 349-355.

[2] Lauer, Dennis, and KLUBER LUBRICATION. "Tribology: The key to proper lubricant selection." Machinery Lubrication, Klüber Lubrication 3 (2008).

[3] Holmberg, Kenneth, and Ali Erdemir. "Influence of tribology on global energy consumption, costs and emissions." Friction 5.3 (2017): 263-284.

[4] Vishwakarma, Suraj. “Lubrication System.” DIT University (2015).

[5] Sunil, Takalapally, et al. "A critical review on solid lubricants." Int. J. Mech. Eng. Technol 7 (2016): 193-199.

[6] Popov, Valentin L. "Is tribology approaching its golden age? Grand challenges in engineering education and tribological research." Frontiers in Mechanical Engineering 4 (2018): 16.