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Tribology is an area of science which plays a key role in many mechanical systems to describe how the various components of a system interact together. It is used factor to determine how well a system is working and can be used to deduce whether components of a system need replacing. In this article, we look at the fundamental principles of tribology.
What is Tribology?
Tribology is a multidisciplinary area which lies at the interface of science and engineering to better understand how surfaces interact with each other and with their surrounding environment. Technically speaking, it is the interaction of surfaces in relative motion.
There are many parts to tribology. The three areas of friction, lubrication and wear make up the fundamental areas of tribology (as these are the most important). However, the study of tribology also encompasses areas of contact mechanics, surface damage processes and surface optimization processes. These three fundamental areas are the most important because these are the properties and interactions that help to tailor the tribological system, as well as aiding in optimizing the efficiency, performance and reliability of a system.
The effects of tribology are realised when two or more surfaces meet each other, be it liquid-liquid, liquid-solid, or otherwise. There are many ways in which the surfaces can interact with each other, and the tribological interactions can take the form of frictional forces, surfaces adhering to each other, surfaces being kept apart from each other and stress or strain at a surface contact point.
However, tribology is not always straightforward, because a surface is not a simple entity. Adding to this, tribology is often most important in applications involving lubricants/oils and solid components, such as in industrial machinery and automotive parts. The presence of multiple phases under a mechanically shearing environment can cause the surfaces to attract contaminants, which adds more complex dynamics alongside the variety of materials/chemicals (and the different thicknesses of these materials) used in these systems.
Aside from the many components of a tribological system, surfaces are not flat, despite being depicted as such in many diagrammatical representations. In fact, surfaces are very rough. Whilst this is sometimes visible with the naked eye, this surface roughness goes much deeper and can be present at the nano and Angstrom levels.
Surface roughness plays an important role in tribology. From a physical point of view, the surface roughness can play a role in determining the contact area of a surface, the contact stresses on a surface, the lubrication pathways and the contact angle between a solid and liquid surface. From a chemical perspective, the surface roughness can have an impact on chemical compatibility, shear strength and lubrication properties of a surface.
Friction is the force, i.e. the resistance to relative motion, between two objects in contact with each other. The frictional forces occur at the interface between the two surfaces. There are two types of interactions that occur when two surfaces meet, these are molecular adhesion and mechanical abrasion. Molecular adhesion interactions are governed by electrostatic interactions, Van der Waals forces and metallic bonds (when there are metallic components involved), whereas mechanical abrasion is governed by elastic and plastic deformations of the surface(s).
There are many factors that contribute to the friction including the contact area, the types of materials used, the geometry of the surfaces, the applied stresses, the surface roughness, the sliding speed of the surfaces, and the surrounding environment. However, whilst there are many factors that contribute to the friction, only the normal load and frictional force are required to evaluate the frictional coefficient between surfaces.
Lubrication is a property exhibited by a material (be it a fluid or a layered solid material) to reduce friction, minimize the amount of wear in a system, transport contaminants away from the surface of components, and provide cooling within a system.
The lubrication properties of a material are often governed by the thickness of the film present at the surface of another material, and the surface roughness of the material that is being lubricated. There are many different factors that contribute to how effective a lubricant is, and these include the viscosity and viscosity-index of a lubricant, the pressure-viscosity index of a lubricant, the reactivity with the surface, the boundary film-forming properties between the surfaces and the shear strength of the lubricant.
Wear is the degradation of a material as result of its environment. This most commonly occurs in solid materials through prolonged usage and poor lubrication, which causes the displacement of material from the main component, thus resulting in a less-effective material. Wear can also happen in lubricants through a loss of activity arising from over-use, corrosion and impurities.
For solid components, the most common types of wear are abrasive wear, adhesive wear, scratching and scuffing, the formation of cavities, impacts with other components, fretting arising from oxidation, cracks appearing from stresses and microstructural inhomogeneity, and fatigue through prolonged contact. It should be noted that, just like friction, wear is a systematic property and not a material property, and it is not uncommon for multiple wear modes to occur simultaneously.