**1. Introduction**

Between the interacting surfaces can be continuous or discontinuous third body. Until 70s of the last century the oil layer of hydrodynamic generation existent in the contact zone, was considered as a parameter determining a working capacity of the heavy loaded frictional contact. Many experimental and theoretical works are devoted to study of thickness of this layer [1–5]. An approximate (digital) solution of the elasto-hydrodynamic problem considering thermal processes is given in the work [6], where the temperature, pressure and thickness of the oil layer between the cylinders interacting with the rolling-sliding friction, are determined. However, in spite of many attempts, ascertainment of the reliable relations between the thickness of the oil layer and tribological properties of the contact zone turned out to be problematic [7]. The supplements to the lubricants developed in succeeding years and technical means of study the processes proceeding in the contact zone have radically widened direction of the researches.

The fundamentals of materials science and contact mechanics are developed in works [8–10] and in recent years a new direction of tribology – nano-tribology appeared [11, 12]. New materials were created (graphene etc.) [13]. For tribological modeling are used the methods of mechanics and multiphysics [14–18], methods of finite and boundary elements [19–21], discrete dynamics of dispositions [22], and atomistic methods [23]. However, in spite of this, some engineer aspects of the problems of tribology are not yet properly studied and their solution needs additional researches.

At common operational conditions, various types of boundary films - products of interaction with the environment that prevent the direct contact of rubbing surfaces, cover these surfaces with thin layers. Depending on the friction conditions, properties of the surfaces and environment, these layers may have various tribological properties that will have the great influence on the boundary friction [24–26]. This is confirmed by the results of the experimental researches in the inert gas environment and vacuum, that excludes the possibility of oxidation during friction. Under such conditions, the seizure and intensive wear rate are observed. To prevent these undesirable phenomena, it is necessary to provide the presence of the third body in the contact zone with due properties, control of the friction factor and protection of the third body from destruction.

material, variation of the surface structure and physical and mechanical character-

PτAasp) [24], where τ is effective strength on shear of the actual contact area of interacting surfaces; Aasp – seizure area of the actual contact that depends on the thermal load of the contact zone, thickness of the heated up layer, properties of the

Hence, the friction forces depend on the contact area in both cases, when the

The surfaces are the weakest places of the rigid body from which their destruction begins [29]. Displacement of the coupled places of surfaces relative to each other causes sharp increase of the shear stresses and corresponding deformations, value and instability of the friction forces and rupture of the coupled places. It is possible in this case transfer of the pulled out material from on surface on the other, sharp change of roughness of these surfaces and development of the process of catastrophic wear – scuffing. The shear deformation generated on the surface sharply decreases towards the depth and multiple repetition of such processes results in superficial plastic deformations, lamination and fatigue damage

The damage scales and dominant types in such cases depend on the working conditions. Thus, for providing the interacting surfaces with due tribological properties, their separation from each other by the continuous third body with

It should be noted that various types of surface take place simultaneously and proceed with various intensity and a dominant type of damage ascertained visually. The experimental researches have shown that damage intensity and type, of

*The scheme of the surface plastic deformation (a); appearance of cracks and lamination (b); appearance of*

places of actual contact) depend on the total area of the actual contacts Ff = ψ

*A New Concept of the Mechanism of Variation of Tribological Properties of the Machine…*

surfaces are separated from each other by the third body fully or partially.

The friction forces between interacting surfaces (at lack of the third body in the

istics and damage of various types proceeding simultaneously.

surfaces and environment of individual micro-asperities etc.

(

**Figure 1.**

*Types of interaction of the surfaces.*

*DOI: http://dx.doi.org/10.5772/intechopen.93825*

(**Figure 2**) [30, 31].

**Figure 2.**

**133**

*fatigue pits (c).*

corresponding properties is necessary.

When the interacting surfaces are separated by the continuous third body, the friction forces mainly depend on the rheological properties of the third body [25, 26] or on the third body viscosity and area of the contact zone: F = f(η, *<sup>Δ</sup><sup>v</sup> <sup>Δ</sup>x*, S), where S is area of the contact zone; *η* — viscosity; *<sup>Δ</sup><sup>v</sup> <sup>Δ</sup>x*- velocity gradient.

Usually, the surfaces are covered with various types of natural and artificial coatings, which represent the components of the third body in the contact zone of the interacting surfaces, are subjected to heavy power and thermal loads. This causes deformations of these coatings, their destruction, activation of the physical and chemical processes proceeding between them and the surfaces and generation of new coatings. Thus, during the interaction of surfaces, the processes of the third body destruction and restoration takes place in the contact zone continuously. When the intensity of destruction of the third body is greater than the intensity of its restoration, the amount of the micro-asperities coming into direct interaction leads to seizure and the wear rate increase because of various kinds of surface damage.

A part of micro-asperities of the heavy loaded interacting surfaces are in direct contact with each other causing their seizure and the remaining part interact with each other through the third body that is schematically shown in **Figure 1**.

For heavy loaded interacting surfaces is typical seizure. This can happen when continuity of the third body is disrupted in individual places; the parts of the direct contact are cleansed from various coatings and boundary layers and are approached to each other at the distance of several atom diameters. As molecular dynamics [27] and atom microscope [28] show, in such conditions they will attract each other generating electron-pair bindings.

Adhesive approach to the friction means invasion of micro-asperities into each other in the contact zone, their close contact without the third body and adhesive scuffing of micro-asperities. The thermal effects accompanying the process have direct influence on the deformation area and value, volume of the deformed

*A New Concept of the Mechanism of Variation of Tribological Properties of the Machine… DOI: http://dx.doi.org/10.5772/intechopen.93825*

**Figure 1.**

in spite of many attempts, ascertainment of the reliable relations between the thickness of the oil layer and tribological properties of the contact zone turned out to be problematic [7]. The supplements to the lubricants developed in succeeding years and technical means of study the processes proceeding in the contact zone

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

The fundamentals of materials science and contact mechanics are developed in works [8–10] and in recent years a new direction of tribology – nano-tribology appeared [11, 12]. New materials were created (graphene etc.) [13]. For tribological modeling are used the methods of mechanics and multiphysics [14–18], methods of finite and boundary elements [19–21], discrete dynamics of dispositions [22], and atomistic methods [23]. However, in spite of this, some engineer aspects of the problems of tribology are not yet properly studied and their solution needs

At common operational conditions, various types of boundary films - products of interaction with the environment that prevent the direct contact of rubbing surfaces, cover these surfaces with thin layers. Depending on the friction conditions, properties of the surfaces and environment, these layers may have various tribological properties that will have the great influence on the boundary friction [24–26]. This is confirmed by the results of the experimental researches in the inert gas environment and vacuum, that excludes the possibility of oxidation during friction. Under such conditions, the seizure and intensive wear rate are observed. To prevent these undesirable phenomena, it is necessary to provide the presence of the third body in the contact zone with due properties, control of the friction factor

When the interacting surfaces are separated by the continuous third body, the

Usually, the surfaces are covered with various types of natural and artificial coatings, which represent the components of the third body in the contact zone of the interacting surfaces, are subjected to heavy power and thermal loads. This causes deformations of these coatings, their destruction, activation of the physical and chemical processes proceeding between them and the surfaces and generation of new coatings. Thus, during the interaction of surfaces, the processes of the third body destruction and restoration takes place in the contact zone continuously. When the intensity of destruction of the third body is greater than the intensity of its restoration, the amount of the micro-asperities coming into direct interaction leads to seizure and the wear rate increase because of various kinds of surface

A part of micro-asperities of the heavy loaded interacting surfaces are in direct contact with each other causing their seizure and the remaining part interact with each other through the third body that is schematically shown in **Figure 1**.

For heavy loaded interacting surfaces is typical seizure. This can happen when continuity of the third body is disrupted in individual places; the parts of the direct contact are cleansed from various coatings and boundary layers and are approached to each other at the distance of several atom diameters. As molecular dynamics [27] and atom microscope [28] show, in such conditions they will attract each other

Adhesive approach to the friction means invasion of micro-asperities into each other in the contact zone, their close contact without the third body and adhesive scuffing of micro-asperities. The thermal effects accompanying the process have direct influence on the deformation area and value, volume of the deformed

*<sup>Δ</sup>x*, S),

*<sup>Δ</sup>x*- velocity gradient.

friction forces mainly depend on the rheological properties of the third body [25, 26] or on the third body viscosity and area of the contact zone: F = f(η, *<sup>Δ</sup><sup>v</sup>*

have radically widened direction of the researches.

and protection of the third body from destruction.

where S is area of the contact zone; *η* — viscosity; *<sup>Δ</sup><sup>v</sup>*

additional researches.

damage.

**132**

generating electron-pair bindings.

*Types of interaction of the surfaces.*

material, variation of the surface structure and physical and mechanical characteristics and damage of various types proceeding simultaneously.

The friction forces between interacting surfaces (at lack of the third body in the places of actual contact) depend on the total area of the actual contacts Ff = ψ ( PτAasp) [24], where τ is effective strength on shear of the actual contact area of interacting surfaces; Aasp – seizure area of the actual contact that depends on the thermal load of the contact zone, thickness of the heated up layer, properties of the surfaces and environment of individual micro-asperities etc.

Hence, the friction forces depend on the contact area in both cases, when the surfaces are separated from each other by the third body fully or partially.

The surfaces are the weakest places of the rigid body from which their destruction begins [29]. Displacement of the coupled places of surfaces relative to each other causes sharp increase of the shear stresses and corresponding deformations, value and instability of the friction forces and rupture of the coupled places. It is possible in this case transfer of the pulled out material from on surface on the other, sharp change of roughness of these surfaces and development of the process of catastrophic wear – scuffing. The shear deformation generated on the surface sharply decreases towards the depth and multiple repetition of such processes results in superficial plastic deformations, lamination and fatigue damage (**Figure 2**) [30, 31].

The damage scales and dominant types in such cases depend on the working conditions. Thus, for providing the interacting surfaces with due tribological properties, their separation from each other by the continuous third body with corresponding properties is necessary.

It should be noted that various types of surface take place simultaneously and proceed with various intensity and a dominant type of damage ascertained visually. The experimental researches have shown that damage intensity and type, of

**Figure 2.**

*The scheme of the surface plastic deformation (a); appearance of cracks and lamination (b); appearance of fatigue pits (c).*

interacting surfaces are especially sensitive to the relative sliding velocity and shear stresses. Thereat, at low total and relative sliding velocities of the surfaces, when power of the thermal action, velocity and resistance of the shear deformation in the contact zone are comparatively small, stability of the third body and its resistance to scuffing are high and a main type of damage is fatigue wear [4]. With increase of the total and relative sliding velocities of surfaces, thermal load of the actual contact zone and destruction intensity of the third body increases. However, time of action of this load, thickness of the heated up layer and sizes of micro-asperities generated because of the scuffing and subsequent rupture of the seized places, decrease. Such phenomena take place on tread surfaces of the train wheel, near the pitch point of the gear drives, in the rolling bearings etc. (**Figure 3**). At increase of the relative sliding velocity, share of the adhesive wear and scuffing increases and it often becomes a dominant type of damage. For example, a steering surface of the train wheel, places of tooth profile of the gear drive distant from the pitch point, cam mechanisms etc.

Various interacting surfaces of machines should have different tribological properties: tooth gear drives, cam mechanisms, guides of various types etc., should have stable and as small as possible friction coefficient (≤ 0.1) and friction clutch and brakes – comparatively high and stable friction coefficient (0.25-0.4). Especially should be noted operational peculiarities of the wheel and rail interacting surfaces. The existent profiles of wheels and rails can be divided into the tread surfaces (which take part in the "free" rolling, traction and braking) and steering surfaces (the wheel flange and rail gauge, which take part in the steering mainly in curves and prevent the wheel-set from derailment). The flange root can roll on the rail corner, and it can take part in traction, braking and steering

*A New Concept of the Mechanism of Variation of Tribological Properties of the Machine…*

But traction (braking) and steering require mutually excluding properties and the "ideal" value of the friction coefficient (μ < 0,1) in the contact zone of the

As it is seen from **Figure 5**, the power and thermal loads of tread surfaces are relatively low. At working of wheels in the modes of traction and braking, the lateral displacement, rotation about vertical axis and skidding, sliding velocity and distance increase. The flange root and rail corner in the contact zone are characterized by the increased creeping, that at destruction of the third body results in the

For interacting surfaces of some mechanisms, such as tooth gear drives, cam mechanisms, wheel and rail etc., the main types of wear are adhesive wear (and its heavy form – scuffing, whose nature is not studied sufficiently and under heavy working conditions it is followed by sharp increase of the friction coefficient instability and wear rate or catastrophic wear) and fatigue wear, that proceed

*The ideal values of the friction coefficients and stress distribution in the contact zone of the wheel and rail*

flange root and the rail corner is not acceptable for both cases.

increased shearing stresses and temperatures.

*DOI: http://dx.doi.org/10.5772/intechopen.93825*

simultaneously and are quite different processes.

*Components of the wheel and rail interacting surfaces.*

(**Figure 4**).

**Figure 4.**

**Figure 5.**

**135**

*according to [32] and the thermal loads.*

For avoiding the above-mentioned non-desirable phenomena, providing the contact zone with the third body having due properties, its protection against destruction and control of the friction coefficient are necessary. However, despite the great number of scientific works this direction could not attract due attention of the scientists until today.

Variation of tribological properties of the surfaces is a result of various mechanical, physical and chemical processes proceeding simultaneously in the contact zone whose essence and mechanism of action are not properly studied [20–22]. This complicates control of the mentioned processes that needs consideration of many factors acting simultaneously. Such factors are:


**Figure 3.**

*The damage types: (a) train wheel with fatigue damage of the tread surface and adhesive wear (scuffing) of the flange; (b) gear wheel with the traces of scuffing on the tooth face; (c) inner ring of the rolling bearing with the traces of fatigue damage.*

*A New Concept of the Mechanism of Variation of Tribological Properties of the Machine… DOI: http://dx.doi.org/10.5772/intechopen.93825*

Various interacting surfaces of machines should have different tribological properties: tooth gear drives, cam mechanisms, guides of various types etc., should have stable and as small as possible friction coefficient (≤ 0.1) and friction clutch and brakes – comparatively high and stable friction coefficient (0.25-0.4).

Especially should be noted operational peculiarities of the wheel and rail interacting surfaces. The existent profiles of wheels and rails can be divided into the tread surfaces (which take part in the "free" rolling, traction and braking) and steering surfaces (the wheel flange and rail gauge, which take part in the steering mainly in curves and prevent the wheel-set from derailment). The flange root can roll on the rail corner, and it can take part in traction, braking and steering (**Figure 4**).

But traction (braking) and steering require mutually excluding properties and the "ideal" value of the friction coefficient (μ < 0,1) in the contact zone of the flange root and the rail corner is not acceptable for both cases.

As it is seen from **Figure 5**, the power and thermal loads of tread surfaces are relatively low. At working of wheels in the modes of traction and braking, the lateral displacement, rotation about vertical axis and skidding, sliding velocity and distance increase. The flange root and rail corner in the contact zone are characterized by the increased creeping, that at destruction of the third body results in the increased shearing stresses and temperatures.

For interacting surfaces of some mechanisms, such as tooth gear drives, cam mechanisms, wheel and rail etc., the main types of wear are adhesive wear (and its heavy form – scuffing, whose nature is not studied sufficiently and under heavy working conditions it is followed by sharp increase of the friction coefficient instability and wear rate or catastrophic wear) and fatigue wear, that proceed simultaneously and are quite different processes.

**Figure 4.** *Components of the wheel and rail interacting surfaces.*

#### **Figure 5.**

*The ideal values of the friction coefficients and stress distribution in the contact zone of the wheel and rail according to [32] and the thermal loads.*

interacting surfaces are especially sensitive to the relative sliding velocity and shear stresses. Thereat, at low total and relative sliding velocities of the surfaces, when power of the thermal action, velocity and resistance of the shear deformation in the contact zone are comparatively small, stability of the third body and its resistance to scuffing are high and a main type of damage is fatigue wear [4]. With increase of the total and relative sliding velocities of surfaces, thermal load of the actual contact zone and destruction intensity of the third body increases. However, time of action of this load, thickness of the heated up layer and sizes of micro-asperities generated because of the scuffing and subsequent rupture of the seized places, decrease. Such phenomena take place on tread surfaces of the train wheel, near the pitch point of the gear drives, in the rolling bearings etc. (**Figure 3**). At increase of the relative sliding velocity, share of the adhesive wear and scuffing increases and it often becomes a dominant type of damage. For example, a steering surface of the train wheel, places of tooth profile of the gear drive distant from the pitch point, cam

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

For avoiding the above-mentioned non-desirable phenomena, providing the contact zone with the third body having due properties, its protection against destruction and control of the friction coefficient are necessary. However, despite the great number of scientific works this direction could not attract due attention of

Variation of tribological properties of the surfaces is a result of various mechanical, physical and chemical processes proceeding simultaneously in the contact zone whose essence and mechanism of action are not properly studied [20–22]. This complicates control of the mentioned processes that needs consideration of many

• Initial tribological properties of the third body and surfaces; influence of interaction of the friction modifier and other materials existent in the contact zone and the surfaces on the properties and stability of the third body and

• Structural, physical and mechanical peculiarities and tendency to scuffing of the clean (juvenile) surfaces in the places of the third body destruction;

• Influence of the contact zone working conditions on the wear type and rate,

*The damage types: (a) train wheel with fatigue damage of the tread surface and adhesive wear (scuffing) of the flange; (b) gear wheel with the traces of scuffing on the tooth face; (c) inner ring of the rolling bearing with the*

mechanisms etc.

the scientists until today.

surfaces.

**Figure 3.**

**134**

*traces of fatigue damage.*

factors acting simultaneously. Such factors are:

destruction peculiarities of the seized places;

variation of the micro- and macro-geometry etc.

For revealing the factors influencing tribological properties of the interacting surfaces, the experimental researches were carried out on the high-speed and serial twin-disk machines.
