*2.2.3 Polymer surfaces*

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, and these two effects are nearly proportional.

**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.

**Figure 4.** *Coefficient of friction and applied load for alumina sliding on the alumina.*

**Figure 5.** *Work of adhesion and friction coefficient for some polymers sliding against PA 6.*

#### **2.3 Lubrication**

Two solid components/parts are sliding between surfaces is normally considered through a maximum coefficient of friction and higher wear rate because of the surface properties such as reactivity, lesser hardness, mutual solubility and higher surface energy. The clean surfaces without any rust is freely adsorb traces of other substances from the atmosphere. In addition, the newly manufactured surfaces normally produce the lesser wear and coefficient of friction while compared with clean surfaces. However, there may be chances of external material on the interface of bulk material which can increase the coefficient of friction during continuous sliding process. Hence, the lubricants can be applied to reduce the wear rate and coefficient of friction [8]. The term called lubrication can be applied to two various conditions: namely solid lubrication and fluid film lubrication (liquid or gaseous).

**191**

*Friction, Lubrication, and Wear*

extreme working atmosphere.

film parameter [17].

*2.3.1.1 Hydrostatic lubrication*

*2.3.1.2 Hydrodynamic lubrication*

manufacturing industry [19, 20].

*2.3.1 Fluid film lubrication regimes*

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

In any kind of material, the solid lubricant such as solid film and powder was used to protect the sliding surface from the unexpected damages during the sliding process and reduce the wear rate and coefficient of friction. The solid lubricants were used in sliding applications. For example, the bearing was operated with low speeds and higher loads, and the hydrodynamically lubricated bearings demanding the start and stop processes. The solid lubricant holds the higher variety of material which can produce the lower wear rate and coefficient of friction [15]. In addition to that the hard materials also were used as lubricant to reduce the friction and wear in

A thick film of lubrication was maintained in the region between two solid surfaces with no relative motion or lesser motion through an external pumping agency is called as hydrostatic lubrication [16]. The lubrication regimes which are noticed in fluid lubrication with self-acting can be recognized in the Stribeck curve as shown in **Figure 6**. This graph provides the hypothetical fluid-lubricated bearing structure with coefficient of friction as a function of the rotational speed (N) and absolute viscosity (η) divided by the applied load (P). This curve has a lowest with providing the recommendation that higher than one lubrication system is presented. In some cases, the lubrication regimes can be recognized through lubricant

In hydrostatic bearings, the supporting load on the thicker film provided from an external source, a pump, which can induce the fluid pressure toward the film. Based on this reason, those kinds of bearings are called externally pressurized. Generally, the hydrostatic bearings are considered for usage in compressible and incompressible fluids. Subsequently, the hydrostatic bearings are no need of any relative motion on the surface of bearings to create the load supporting pressures as it essential in the hydrostatic bearings [18]. Further, this type of hydrostatic bearings is used in application with no relative motion or lesser motion between the sliding surfaces. Besides, the hydrostatic bearings offer great stiffness though, this type of lubrication needed the high-pressure equipment and pumps for the cleaning

The hydrodynamic lubrication is called as thick film or fluid film lubrication. The convergent type of bearings starts to move in the direction of longitudinal from the initial position, a less thickness of layer is pulled due to viscous entrainment. Further, it is compressed between the two bearing surfaces and generating the necessary pressure to support the load with no other external devices as indicates in **Figure 6**. This hydrodynamic lubrication mechanism is necessary for the effective working of the hydrodynamic journal and the thrust bearings are extensively used in the modern

In addition, the hydrodynamic lubrication films thickness is ranging from 5 to 500 μm and referred as the ideal lubricated contact condition. Also, the friction coefficient of the hydrodynamic contacts is as lesser as 0.001 which represents the **Figure 6**. Sometimes the frictional force can be increased slightly while increase in sliding speed due to the viscous drag. Generally, the physical interaction can be

of fluid, which is occupying more space with higher cost.

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

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

*Coefficient of friction and applied load for alumina sliding on the alumina.*

Two solid components/parts are sliding between surfaces is normally considered

through a maximum coefficient of friction and higher wear rate because of the surface properties such as reactivity, lesser hardness, mutual solubility and higher surface energy. The clean surfaces without any rust is freely adsorb traces of other substances from the atmosphere. In addition, the newly manufactured surfaces normally produce the lesser wear and coefficient of friction while compared with clean surfaces. However, there may be chances of external material on the interface of bulk material which can increase the coefficient of friction during continuous sliding process. Hence, the lubricants can be applied to reduce the wear rate and coefficient of friction [8]. The term called lubrication can be applied to two various conditions: namely solid lubrication and fluid film lubrication (liquid or gaseous).

*Work of adhesion and friction coefficient for some polymers sliding against PA 6.*

**190**

**2.3 Lubrication**

**Figure 5.**

**Figure 4.**

In any kind of material, the solid lubricant such as solid film and powder was used to protect the sliding surface from the unexpected damages during the sliding process and reduce the wear rate and coefficient of friction. The solid lubricants were used in sliding applications. For example, the bearing was operated with low speeds and higher loads, and the hydrodynamically lubricated bearings demanding the start and stop processes. The solid lubricant holds the higher variety of material which can produce the lower wear rate and coefficient of friction [15]. In addition to that the hard materials also were used as lubricant to reduce the friction and wear in extreme working atmosphere.
