Friction, in physics, is the contact force that acts whenever two bodies collide and there is a tendency to move. It is generated by the roughness of the bodies. The frictional force is always parallel to the surfaces in interaction and generally contrary to the relative movement between them. Friction can be desirable or undesirable. For example, in the contact between the tire and the pavement and between the pads and the brake disc, high friction is desirable. However, in applications such as operating motors or equipment with bearings and gears, high friction is undesirable. This is because friction causes wear and generates heat that can lead to premature failure of machine parts in operation. The frictional heat not only causes wear, but is also considered a waste, as it does not result in useful work. Proper lubrication of the equipment is important so that its integrity is preserved throughout its useful life.

Types of Friction

Friction can be divided into two categories: internal and external.

The internal friction results from the friction between the lubricant molecules, described as viscosity.

External friction is the mechanical force that resists movement or prevents movement between sliding or rotating surfaces.

Types of external friction:

1. Sliding friction - it is friction in a pure sliding motion, without bearing and without rotation. This type of friction generates greater wear.
2. Rolling Friction - is the friction generated by the rotating contact. In roller bearings, rolling friction occurs mainly between rolling elements and raceways, while sliding friction occurs between rolling elements and the cage. The main cause of friction in roller bearings is slipping in the contact zones between the rolling elements and the raceways.
3. Solid Friction - this occurs when two solids have direct contact with each other without a separating layer. If conventional materials are involved, the friction coefficients and wear rates are high. Lubrication technology tries to eliminate this condition.
4. Friction Limit - solid friction, in which the surfaces of the friction partners are covered by a molecular lubricant film that has no load capacity. The lubricant influences the friction and wear characteristics. One of the most important objectives in the development of lubricants is the creation of such layers of limit friction in a variety of dynamic, geometric and thermal conditions. These layers are of great importance in practical applications, when thick, long-lasting lubricant films to separate two surfaces are technically impossible. Lubricating boundary films are created from surface active substances and their chemical reaction products. Adsorption, chemisorption and tribochemical reactions also play significant roles.

   Although boundary friction is often attributed to solid friction, the difference is of great importance for the development of lubricants and the understanding of lubrication and wear processes, especially when boundary friction layers are formed by lubricants.

5. Fluid Friction - the friction value is determined by the shear of the fluid. The load capacity of the fluid film prevents direct contact between the two solid surfaces. This film is hydrostatically formed. From the point of view of lubricants, it is known as hydrodynamic lubrication. Liquid friction is caused by resistance to friction, because of the rheological properties of the fluid.
6. Mixed friction - it happens when the limit friction is combined with the fluid friction. From the point of view of lubricant technology, this form of friction requires sufficient load-bearing boundary layers to form. Machine elements that are normally hydrodynamically lubricated experience mixed friction when starting and stopping.

Wear

Wear is the loss of material from a solid surface. Wear can appear in several ways, depending on the material of the interacting contact surfaces, the environment and the operating conditions.

Adhesive

This is the most complex of the wear mechanisms. Molecular and atomic interactions between the two frictional partners can pull particles of material from contact faces. This is preceded by the formation of a point joint between the two materials, a process that is often called cold welding. Adhesive wear occurs under lubricated conditions when the hydrodynamic or elastohydrodynamic (EHD) film is so thin that the surface roughness penetrates the film.

Abrasive

It is the most common type of wear. Abrasive contact occurs mainly when one body is in contact with another, harder body. Contamination of lubricants by abrasive particles, such as dust, can also cause abrasive wear. Abrasion causes the surface peaks to break or remove more abrasive particles from the surface. This form of wear can be combined with other wear mechanisms.

Surface Fatigue

Surface fatigue is the result of periodic impacts on contact zones. The frequent increase in punctual impact leads to surface fatigue, which is the result of material fatigue. A thick, separating lubricating film minimizes or eliminates this problem. Machine elements that are subject to periodically severe loads in the contact zone are particularly prone to this type of damage.

Erosive

Erosive wear, which is a loss of material from a solid surface, occurs during impact on a surface by a fluid containing solid particles.

Fretting

Fretting wear occurs when there is a very small relative oscillatory movement between two surfaces in contact. Often, the intention is for the surfaces to be fixed, but due to, for example, vibrations, some movement still occurs. The term fretting is often used to denote damage mechanisms, such as friction fatigue, friction wear and friction corrosion.

Cavitation

Materials can be damaged by the implosion of gas and vapor bubbles entrained in lubricating oils or hydraulic fluids. In systems that carry lubricants, the elimination of entrained air, low boiling substances or the use of active surface components reduces gas bubbles and thus reduces cavitation.

The condition of the surfaces affected by cavitation can be further damaged by corrosion. This process can be controlled by the use of specific inhibitors.

Corrosion

This type of wear results from the attack of the lubricant or corrosive contaminants, such as salts, water and acids, on the metal surfaces. These materials cause a chemical or electrochemical reaction with the metal.

Corrosion can occur uniformly on a contact surface or selectively on surface inclusions, grain contours and between different materials, etc.