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Tribology

Definition:

Tribology is defined as the science and engineering of surface phenomena such as friction, wear, lubrication, adhesion, surface fatigue, erosion etc. Tribological design and materials selection play vital roles in the performance, operation and durability of all mechanical machines. Even our body faces tribological issues and problems especially in joints.

History:

The word "Tribology" for the study of friction, wear and lubrication was coined in the mid-1960s as a result of the growing recognition of the importance of these phenomena in engineering machines. However, tracing back the history, we may find that the understanding of the role of friction in man-made machine certainly goes back to the ancient Egyptians (ca. 2500 BC). Overcoming friction in moving large pieces of stones and statues must have been a great challenge for them. Some carvings and images of that time tell that the use of water might have given some reduction in the friction and thus less work for the slaves who had to carry out this monumental task of building the Pyramids. Also, there is a clear indication of the use of lubricated wooden planks to reduce contact area in moding very large pieces of stones or statues. A situation much closer to that of today's time was faced in the case of the lubrication of the chariot wheel hubs, for war or other usages. Around 1500 BC, the art of chariot building was quite advanced in Egypt and the use of leather and animal fats to lubricate the wheel hub was generally pacticed.

The Development of Friction theories:

The very first theory of friction came out of Da Vinci's experimental work in the 16th century. By conducting very simple sliding of rectangular blocks of materials he proposed; 1) The area of contact has no effect on friction;(2) If the load of an object is doubled, its friction will also be doubled. The first observation basically says that friction is a function of the weight of the body and not how large the contact area is. In simple words, pulling a brick on a surface will require same force regardless whether we have placed the brick on the surface with its broader side as the base or the narrower side. The second observation of Da Vinci gives the well known friction law of direct proportionality between the normal laod and the friction force. The constant of this proportionality is known as the coefficient of friction or COF.

The Da Vinci's observations were finally decoded when Amonton in 1699 provided what are now knows as Laws of Friction and often referred as Amonton's Law:

(1) Coefficient of friction = [(Normal force including the weight of the object)/(Friction force experienced in sliding this object on a surface)]

(2) Friction force is independent of the area of contact

Eighty two years later in 1781, Coulomb proposed the third law of friction which says that friction is independent of the relative sliding velocity between two objects. This law now bears Coulomb's name (it may be mentioned here that this law is not valid now as the coefficient of friction does depend on the relative motion).

The above laws and observations basically dealt with dry or very partially (boundary) lubricated conditions when the relative speeds were still very low. However, when there was full liquid lubrication between contacting bodies and under some constraints of the movement of the lubricant, Tower (1883) found that it was possible to produce a continuous fluid film between two solid surfaces. This came to be known as the hydrodynamic lubrication (HDL). Tower's experimental observations were later theoretically proved by N. Petrove (1883) and more famously by Reynolds (1886) and the equation for the fluid film formation equation is known as the Reynolds equation. Fluid film formation is a result of the viscosity of the lubricant and the relative speed. Hence, it is important that these two parameters should be high. The normal pressure acting between the solids tries to bring the two solids in contact and hence should have low value in any lubricated contact such as those of bearings. Stribeck (1902) later produced a plot of the coefficient of friction as a function of the ratio [(viscosity x speed)/Apparent normal pressure] and found that this curve can clearly show at least three regimes of lubrication; boundary lubrication, mixed lubrication and hydrodynamic lubrication. Hydrodynamic lubrication, which also includes elastohydrodynamic lubrication (EHL) condition for compliant contacts, provides extremely low COF in liquid lubricated condition. One may add here that the science and technology of liquid lubrication is very complex which requires much intervention of chemistry to tweak each lubricant for each application.

The understanding of friction between solids was made clearer after the development of the junction growth theory by Bowden and Tabor (1950). This is also known as the adhesion theory of friction. It states that friction is a result of the true contact area between the solids. The true area of contact is far less in comparison to the apparent contact area. True area of contact is a sum of the all micro-contacts at the asperities of the two solids and will be dependent on the yield strength of the materials. A soft material such as rubber would give near complete contact meaning true area of contact will be equal to the apparent area of contact.

We have also learnt that friction is also dependent upon the surface properties of the solids involved such as surface energy, interatomic attractive forces between the two solids, roughness, molecular architecture for lubricious nature of the solids, surface roughness etc. The exact nature of these effects will depend upon the normal load involved and other operational variables such as frictional heating.

Theories of Wear

While the science of friction has attracted much attention from physicists and engineers alike, wear is only considered an engineer's dilemma. It is an undesirable consequence of friction and many other phenomena that occur on the surface. High friction leads to wear because of the high shear stress on the surfaces. The softer of the two interacting surfaces will start wearing out. Frictional heating is also a strong contributor to the weakening (softening, melting, oxidation etc) of the surfaces. Adhesion due to high surface energies will also have material removal effects. Thus, based on the causes of wear, there are many wear mechanisms proposed such as abrasive wear, adhesive wear, surface fatigue wear, chemical wear, oxidative wear, corrosive wear, erosive wear etc.

Lubrication

If friction and wear may be considered as the problems then lubrication is one of the solutions. About 2600 years BC, ancient Eygiptians used water to lubricate the soil over which they dragged the colosus and heavy stones to build pyramids. Later people used animal fats to lubricate chariot wheels. These inventions, though very simple in nature, have given birth to may later inventions and the industrial revolution. It is one thing to design a car, it is another to run it without a lubricant. In today's context lubricants are all natural or synthetic long chain organic materials. They provide lubrication by virtue of their viscosity and ability to withstand high bearing pressure and frictional heat. Lubricants simply separate two solid surfaces from intimate contact when in relative motion against each other. This ability to separate surfaces and thus bear the load applied increases with the relative speed if there is a converging gap between the solid surfaces and the motion forces the lubricant to flow into this flow constraint. As a result, the rise in the hydrodynamic pressure will be sufficient to take the bearing pressure or the load applied between the two surfaces. Surfaces can also be protected by coatings and some surfaces modifications and treatment. This area of knowledge is known as surface engineering in tribology.