**2. Fundamentals of tribology**

## **2.1 Surfaces in contact**

The friction and wear are mainly dependent on the characteristics of two sliding surfaces. The difficulty to predict and to clarify with more accuracy such phenomena reveals the complex nature of the surfaces, which can be evaluated through material properties such as microstructure, presence of organic molecules and oxides, water vapor, geometrical irregularities and other impurities which can be adsorbed from the atmosphere. Hence, while the two bodies are coming in to closer contact, the significant features of their sliding surfaces define the nature of the interaction, which includes mechanical character, with the development of a stress-strain on the sliding area, with the strong establishment of physical or chemical bonds [6]. To calculate the contact stresses, the smooth surface concept can be introduced, i.e., the surfaces are free from geometrical irregularities. Generally, the formation of smooth surfaces is difficult at a molecular level. The relation for contact stresses and deformations can be obtained through theoretical analysis which is developed by Hertz for linear elastic bodies. This can be employed while the two bodies are in frictional or elastic contact, with the assumption that the contact body radius is higher while compared with contact zone size.

## *2.1.1 Elastic contact*

The viewpoint of geometrical, the contact between two solid bodies can be classified in to conformal or nonconformal as shown in **Figure 1**. From the **Figure 1 (a)**, it can be observed that the conformal contact happens while mating surfaces fit closely together. This kind of contact can be seen while bearing sliding on shafts and between wire and tool in drawing processes. The **Figure 1 (b)** shows the contact between two bodies which is nonconformal and this can be theoretically occurred. For example, with the presence of point contact in rolling bearing (between seat and ball), whereas a line contact happens in gears (between tooth and tooth). In another case, the contact area has a limited extension and it can be easily determined.

#### *2.1.2 Viscoelastic contact*

In the case of polymers, the deformation behavior can be occurred that is affected by plastic, elastic and viscoelastic processes. For example, the polypropylene (PP) sphere hard pressed on the transparent plane with the function of contact deformation displacement (δ) and time. In polymer plate the viscosity influence is

#### **Figure 1.**

*The conformal contact between a plane and the base of a cylinder (a) and nonconformal contact between a plane and sphere (b).*

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**Figure 2.**

*Friction, Lubrication, and Wear*

*2.1.3 Elastic and plastic contacts*

in elastic deformation only.

**2.2 Friction**

*DOI: http://dx.doi.org/10.5772/intechopen.93796*

and time while polypropylene sliding on a plane.

stage; however, it must be an elastic-plastic condition [8].

these kinds of working atmosphere, the friction is reduced [9].

*Polypropylene sphere sliding on a plane with respect to load displacement and time.*

mainly noticeable while holding the amorphous phase and the heat may be higher than glass transition temperature. Further, the loss of energy is related to the viscoelastic loading and unloading processes. This energy dissipation may create the temperature on the material due to lesser thermal conductivity of material. However, the creation of plastic deformation is stable. Besides, the plastic processes and viscoelastic are based on the temperature and while increasing the temperature their intensity also increased [7]. **Figure 2** shows the various loads displacement

The material behaves with ductile way; the provided contact load can be induced the plastic deformation. At the same time, the equivalent stress at the critical point influence the material uniaxial yield stress. In this case, the material is not as elastic

**Figure 3** illuminates the elastic-plastic contact in detail with sphere and plane contact. In this case, the sphere has a higher hardness while compared with plane. If the applied load is increases, the plastic zone size can be increased. The applied load is taken out while the contact pressure with the below specific limit, then with the same magnitude of additional load, which are applied possibly, results the increase

The frictional forces can be recognized as good or bad, without this friction, there is no possibility to use vehicle tires on a road, walking on the road or pickup objects. In some cases, such as machine application like clutches, vehicle brakes and transmission of power (belt drives), friction is increased. But, in many cases like rotating and sliding components such as seals and bearings, friction is unwanted. The higher friction makes more material loss (i.e., wear rate) and energy loss. In

The term friction is called as the force resisting the relative motion of two mating surfaces in contact with a fluid. The two sliding surfaces move relative to each

#### *Friction, Lubrication, and Wear DOI: http://dx.doi.org/10.5772/intechopen.93796*

mainly noticeable while holding the amorphous phase and the heat may be higher than glass transition temperature. Further, the loss of energy is related to the viscoelastic loading and unloading processes. This energy dissipation may create the temperature on the material due to lesser thermal conductivity of material. However, the creation of plastic deformation is stable. Besides, the plastic processes and viscoelastic are based on the temperature and while increasing the temperature their intensity also increased [7]. **Figure 2** shows the various loads displacement and time while polypropylene sliding on a plane.
