*2.1.3 Elastic and plastic contacts*

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

**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 in elastic deformation only.

### **2.2 Friction**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

radius is higher while compared with contact zone size.

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

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.

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

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

**2. Fundamentals of tribology**

**2.1 Surfaces in contact**

*2.1.1 Elastic contact*

*2.1.2 Viscoelastic contact*

**186**

**Figure 1.**

*plane and sphere (b).*

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

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

**Figure 2.** *Polypropylene sphere sliding on a plane with respect to load displacement and time.*

**Figure 3.**

*Elasto-plastic (a) complete plastic, (b) brittle-type contact, and (c) between plane and a sphere.*

other, the friction between the mating surfaces converts the kinetic energy in to heat or thermal energy. Generally, the term used to describe the friction is the coefficient of friction. It is denoted with dimensionless scalar value (μ) and explaining the ratio of the friction between two mating surfaces (F) and the applied force on them (Fn). It is described in Eq. (1):

$$\mathbf{F} = \mu \left( \mathbf{F}\_n \right) \tag{1}$$

**189**

*Friction, Lubrication, and Wear*

*2.2.2 Ceramic surfaces*

*2.2.3 Polymer surfaces*

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

which is lesser than the pure steel-steel pairs.

and applied load for alumina sliding on the alumina.

and these two effects are nearly proportional.

plastically with relatively lower temperature. Hence, while sliding the asperity junctions deform with lesser intensity and the adhesion forces is not developed at the junctions [11]. While sliding between two metal alloys, the coefficient of friction is to be lesser than in the corresponding pure metals. In the case of bronze material (Cu-8% Sn), while sliding in dry condition, the coefficient of friction is around 0.6 while the typical value of 1 was recorded for the Cu/Cu pair. In the case of steel, the coefficient of friction value is around 0.6–0.8 while using two steel alloys, i.e.,

In ceramics material the contact at the asperities is normally mixed. This may be fully in elastic while the surface roughness shows with less [6]. Else, if the surface roughness is high, it may be shows with plastic stage. Generally, the coefficient of friction must be independent for the normal applied load in elastic contacts. For example, the alumina balls sliding on the alumina surface and the friction showed around 0.4. It can be noted that the friction coefficient in ceramics material with dry atmosphere is lesser around 0.3–0.7 while applying the minimum load with less than 200°C temperature. The obtained frictional values are relatively similar to metal alloys and this may seem to be quite surprise. In case, the ceramics material is characterized with higher hardness and elastic modulus, and lesser values of the surface energy [12]. Besides, the surface energy of ceramics material is reduced through the surface reactions with water vapor and the presence of other substances on the working atmosphere. Hence, the lesser frictional force can be expected from the sliding wear experiment. Though, while continuous sliding with real contact area is notably increases with increase in friction. Further, the applied load is considered as major input parameter in the ceramic material. If the applied load is increased, the brittle contact may establish, and this will increase the coefficient of friction as much as high around 0.8. Specifically, the brittle contact can be occurred while the tangential stresses owing to higher friction due to the occurrence of critical microcracks on the surfaces. This type of microcracks can be seen on the ceramics surfaces normally and producing the defects such as porosity, flaws and inclusions. The cracks can be initiated from the development of asperity because of continuous applied load during the tribological study. **Figure 4** represents the coefficient of friction

The polymer like polytetrafluoroethilene (PTFE) is produces very low friction around below 0.1 while sliding on the same material or other metals. Thus, this material behaves as solid lubricant while sliding with counterpart [13]. Generally, most of the polymer material friction coefficient ranges from 0.2 to 1 while sliding in dry condition. In the case of the work of adhesion in polymer was lesser than in ceramics and metals. However, their stiffness and hardness of the material is lesser,

**Figure 5** indicates the correlation between adhesion and coefficient of friction for various polymers sliding against PA. These experiments were conducted on flat to flat surface contacts with lower sliding speed of 0.24 μm/s. So that the thermal effects on the polymers can be avoided [14]. It can be observed that the coefficient of friction was increased with the work of adhesion. In this kind of working condition, the adhesion was considered as a most important factor in friction determination. In the case of point or line interactions, the produced deformations may be

higher and therefore the effect of viscoelastic can play a major role.

In a sliding condition, it is normally required to monitor or calculate the frictional behavior during the experiment. The changes in friction with wear data normally offer the beneficial data regarding modeling and mechanisms. The friction is generally classified as two categories: (a) static friction and (b) dynamic friction. In the case of two objects is not sliding each other is called as static friction. In another hand, both the mating objects sliding relatively to each other is called as dynamic friction [10]. **Table 1** shows the required parameters which need to be considered while performing wear experiments.

#### *2.2.1 Metal surfaces*

Generally, the asperities contact in metal surfaces is normally plastic. The metals such as titanium, cobalt, magnesium with hcp (hexagonal closed packed) crystal lattice provides the coefficient of friction nearly 0.5 while sliding against themselves. In addition to that the hcp metals possess a reduced ability to deform


#### **Table 1.**

*Necessary input parameters to be considered for wear experiment.*

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

plastically with relatively lower temperature. Hence, while sliding the asperity junctions deform with lesser intensity and the adhesion forces is not developed at the junctions [11]. While sliding between two metal alloys, the coefficient of friction is to be lesser than in the corresponding pure metals. In the case of bronze material (Cu-8% Sn), while sliding in dry condition, the coefficient of friction is around 0.6 while the typical value of 1 was recorded for the Cu/Cu pair. In the case of steel, the coefficient of friction value is around 0.6–0.8 while using two steel alloys, i.e., which is lesser than the pure steel-steel pairs.
