Tribology: Of the origins of tribology

Frictional forces and their impact have long fascinated humanity. A host of references show that from the earliest times our ancestors were looking for ways to harness and use the force of friction. The kindling of fire, crank and stone axe as the first simple tools: everything is based on friction. Even the first machines, bow drill and potter's wheel, which originated 5,000 to 6,000 years ago, were based on the principle of friction. The use of rollers and slides to reduce friction when transporting heavy loads is a well-known example of tribology. The earliest documents on the use of wheels to reduce friction date back to the year 650 BC. Today's roller bearings have their origins in the so-called "rolling elements" that were used back in ancient Egypt to build the Pyramids and allowed blocks of stone weighing tonnes to be transported over long distances. However the first specific scientific examination of tribology only came much later with Leonardo da Vinci (1452-1519), who was the first to carry out studies on friction on the horizontal and inclined plane and on wear on slide bearings. Since 1966 in professional circles, the technical term "tribology" is more common. It comes from the Greek terms tribein = rub and logia = knowledge of, and is first mentioned in connection with the Jost Report, a study commissioned by the British Government on wear and tear. Since then, tribology and tribological systems are the terms used when speaking in the context of friction, wear and lubrication.

Friction and lubrication have gone together since time immemorial: This is also evidenced by numerous early discoveries. Back in the past, the Chinese were searching for effective lubricants to reduce friction and initially used water as a lubricant and then later a mixture of vegetable oils and lead. Likewise, the ancient Egyptians knew about the friction-reducing effect of lubricants and lubrication reduced the crew required to pull by 50% by lubricating the underside of the Pharaoh’s throne. They also lubricated their chariots with animal fats or a combination of olive oil and lime flour. However, the real breakthrough for lubricants did not come until the beginning of the industrial revolution, as the increasing industrial development boosted the demand for volume and quality of lubricants. Vegetable and animal oils were gradually replaced by mineral oils, which were obtained by distillation and refining of crude oil, shale and coal.

Leonardo da Vinci studied the static friction coefficient of static friction on an inclined plane and determined the value as f = 1/4. Also, the first and second friction law, the so-called laws of dry friction are attributed to him. These state that the frictional force is proportional to the normal force and independent of the apparent contact area, and depends not only on the adhesion but also on the abrasion. The abrasion has a particularly big impact when the rougher friction partner consists of a harder material, or if there is abrasion in the form of hard oxidised metal particles in the joint. In 1490, in the roller bearing, he replaced the flexible connection between two parts almost exclusively with balls, thereby producing much lower friction. In so doing, he discovered that the friction decreases if the balls do not touch and then developed separators that enabled the balls to move freely.

The French physicist and governor of Lille Guillaume Amontons (1663-1705) carried out investigations in the field of mixed friction. He noted that the friction force depends on the normal force and recognised the surface roughness as the cause of friction occurring. For him, frictional forces were based on mechanical-geometric interlocking of uneven areas. The positive lock of the micro-elevations inhibits the relative movement, with the result that a friction force contrary to the direction of movement occurs. Amontons defined the friction coefficient as f = 1/3 and in 1699 presented the laws of tribology discovered by Leonardo da Vinci to the Académie Royale in Paris.

The natural philosopher John Theophilus Desaguliers (1683-1744) developed an explanatory model for friction, in which he attributed friction forces to the influence of cohesion or adhesion. He also noted that greater friction occurs with better polished surfaces, and that two well-polished and firmly pressed together lead bodies can only be separated again with very great force. He attributed friction occurring back to the influence of cohesion and adhesion, but could not reconcile this idea with the quantitative tribological friction laws.

The English naturalist and administrator Sir Isaac Newton (1643-1727) defined the material parameters of dynamic viscosity: The theory of adhesion, or assumption of a molecular-mechanical cause of friction came about, which was already assumed by Desaguliers as a partial cause of friction. These theories of adhesion were later significantly expanded in the 20s and 30s of this century by Bowden and Tabor.

The Swiss mathematician and physicist Leonhard Euler (1707-1783) investigated the friction on inclined planes. He noted that the static friction is about twice as large as the sliding friction and introduced the friction coefficient "μ", which is now known in tribology as "f". The friction coefficient of metals is measured on polished surfaces to exclude a mechanical interlock as far as possible. The adhesion and cohesion forces between the two materials are crucial.

The French physicist Charles-Augustin de Coulomb (1736-1806) took Amontons thoughts on surface roughness and mixed friction further and looked at the correlation between force to be expended horizontally and weight. According to Coulomb, the friction coefficient of a surface does not depend on the load. This means that the frictional force is proportional to the weight and area-independent, since it is only a function of the average inclination angle of the roughness. The smoother the surface, the lower in his opinion the friction should be: A thesis, which is only partly correct according to the current state of research.

Tribology, as we know it today, began after the First World War, when high loads, speed and temperatures indicated an increasing stress on the friction pairs and thereby necessitated a physical adaptation of lubricants. Pour point and viscosity index improvers, oxidation and corrosion inhibitors came about, simultaneously the development of synthetic oils started. The use of metal ceramic became increasingly important: surfaces made of metal ceramics and ceramic cutting materials ensure greater efficiency and durability of industrial bearings and gears. Using modern and innovative oil additives like DuraGear^® or PowerShot^® (link to landing page), the surfaces of bearings, gears and internal combustion engines can even be upgraded subsequently in the friction process to metal-ceramic surfaces.