*2.3.1.2 Hydrodynamic lubrication*

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 manufacturing industry [19, 20].

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

#### **Figure 6.**

*Coefficient of friction and lubricant film parameter as a function of Stribeck curve showing various lubrication regimes observed in fluid lubrication without an external pumping agency.*

occurred while starting and ending up with lesser sliding speeds. The behavior of the interaction is directed through the lubricant bulk properties i.e., viscosity, and the frictional force can be developed based on the shearing of the viscous lubricant. Besides, the corrosive wear can be occurred on the bearing surfaces due to the presence of lubrication. The adhesive wear also was possible while initial and ending up the processes. The corrosive wear can be reduced through lubricant precipitation and formation of film on the bearing surface.

**193**

*Friction, Lubrication, and Wear*

*2.3.2 Boundary lubrication*

*2.3.3 Mixed lubrication*

**2.4 Wear**

*2.4.1 Introduction*

quasi-hydrodynamic lubrication [21].

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

minor impact on the wear and friction behavior [22, 23].

While increasing the load, the fluid viscosity or speed is decreased in the Stribeck graph as shown in **Figure 6**. In this situation, the friction coefficient can be increased as high as 0.1 or higher than this. This situation may also arise in a starved contact. When the solid surfaces are nearby that surface contact between the multimolecular or monomolecular films of gases or liquids and the dense solid asperities may rule the contact. For more understanding, the cross section of the films and the asperity area contacts is shown in **Figure 6** [21]. Further, with the absence of gases and boundary lubricants (without oxide films), the produced frictional forces may be higher than one. In case with failure in boundary lubrication, causes corrosive and adhesive wear. Generally, the boundary lubricants can easily form the sheared film on the bearing surfaces. Therefore, this formed shear film can be reduced the corrosive and adhesive wear. The major physical properties of the films are hardness, melting point and shear strength. The other properties are cohesion, tenacity or adhesion and formation rates. Also, the viscosity of the lubricant can show a

The transition between the boundary lubrication and the hydrodynamic regimes

Generally, the term wear is defined as material removal or surface damage on the one or two surfaces while rolling, sliding or impact motion relative to one another. Particularly, the wear happens through surface interactions at asperities. While two objects/components in relative motion, the material can be displaced from the interacting surfaces. Consequently, the properties of the material may be changed at least or interface region. But, there is a possibility for less or no material losses. Then, the displaced material can be removed from the interacting surfaces and may cause the material transfer to the counterpart surface or may break as small wear debris. When material transfer from bulk to counterpart, the net mass or volume loss of the interacting surface is zero while the bulk material surface is worn. The wear loss leads the real material loss, and this may occur sometimes independently. Generally, the wear is a system output and it is not a material property. In addition to that the working atmosphere affect the interface wear. In some cases, mistakenly assumed that the higher frictional force displays the increase in wear rate. For example, the polymers and solid lubricant interfaces showed with higher wear and lesser friction, whereas ceramic material showed the lower wear but moderate

is a gray zone known as mixed lubrication. In this regime, the both mechanisms such as boundary lubrication and hydrodynamic lubrication may be in operational condition. There might be possible for the higher solid contacts, however, a minimum portion of the bearing surface leftover through partial hydrodynamic film. The hard solid interaction between the new metal surfaces can cause to a wear debris formation, adhesion of particle with counterpart, metal transfer from bulk to counterpart and eventual seizure. But, in the case of liquid condition, the chemically formed films protect the surfaces from adhesion during the sliding experiment. This mixed regime is called as thin film lubrication, partial fluid and
