**3. Analysis of results of the experimental researches**

The complex physical, mechanical and tribo-chemical processes proceeding in the contact zone of interacting surfaces at direct impact of the environmental conditions raise the problems whose solution demands many-sided approach to these processes. There are many works devoted to these problems [36–38] but they are not solved properly yet. Namely, prediction of the friction coefficient in the contact zone, its control and character of influence of many parameters on its variation are still problematic.

At heavy working conditions, when destruction of the third body is irreversible and scuffing is spread over the factual contact area of the whole surface relative displacement of the surfaces causes sharp increase of the shear stresses, corresponding deformations, values and instability of the friction forces and rupture of the seized places. Strength of the seized places may exceed the strength of the interacting bodies because of which the material pulled out from one surface can form a wear product or can be transferred on the other surface and attached to it that is followed by development of the scuffing process.

Multiple repetition of the shear deformation generated on the surfaces (that sharply decreases towards the depth) causes appearance of cracks on the surfaces, their development and fatigue damage, superficial plastic deformations and lamination. The area of each seized place in the contact zone depends on its power and thermal load; initial micro-geometry of the surfaces; value, velocity and resistance of the deformation etc. Therefore, various working conditions are characterized by corresponding variation of the tribological parameters, namely friction forces, amplitude and frequency of their variable component, wear intensity and roughness of the surfaces. Development of these processes leads to the catastrophic wear due to scuffing.

At low velocities of interacting surfaces, the thermal load of the factual contact zone, velocity of the surface and environment tribo-chemical reaction and resistance of deformation decrease and time of the thermal action and thickness of the superficial heated up layer increase. In such conditions, at destruction of the third body, due to rupture of the seized places, the jerks of low frequency and high amplitudes and sharp instability of the friction coefficient take place and relatively large-size asperities (pits, scratches, asperities, cracks and layers) appear on the surfaces. This is correspondingly reflected on the damage type and roughness of the surfaces.

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

At high velocities of interacting surfaces, despite several works in this area [39–43], some problems have not yet been resolved. The thermal load of the factual contact zone, velocity of the surface, tribo-chemical reaction of the environment and resistance of deformation increase, whereas time of the thermal action and thickness of the superficial heated up layer decrease. In such conditions, at destruction of the third body, due to rupture of the seized places, the jerks of high frequency and comparatively low amplitudes and instability of the friction coefficient take place and relatively small-size asperities (pits, scratches, asperities, cracks and layers) appear on the surfaces. This is correspondingly reflected on the damage type and roughness of the surfaces. Under the conditions of our experiments at a high rolling speed (more than 40 m/s), traces of fatigue damage and scuffing are not visually observed, however, a high wear rate remains.

Thus, destruction of the third body causes sharp worsening of tribological properties of the interacting surfaces and necessary condition of its avoidance is separation of these surfaces from each other by continuous third body with due properties.

It was ascertained by the experimental researches that destruction of the third body begins in individual points of the factual contact zone that is revealed by appearance of signs of the scuffing in these points. Restoration of the individual damaged points was often observed at unchanged operational conditions but at worsening, the operational conditions the superficial damage quantity increased and multiple damages appeared. At further worsening the operational conditions, a narrow strip of damage is generated spreading afterwards over the whole surface that causes worsening of the tribological parameters and catastrophic wear. The above-mentioned damage stages of the third body are shown in **Figure 12**.

Usually the friction process proceeds at presence of the continuous or discontinuous (restorable or progressively destructible) third body stipulating the character of variation of the friction coefficient. Experimentally it was revealed that to negative friction corresponds the continuous or discontinuous but restorable third body; to neutral friction – multiple seizures of the interacting surfaces and to positive friction – increasing scuffing process that is spread on the whole surface. In **Figure 13** is shown variation of the tractive (friction) force with creep [37].

As it were shown by our experimental researches, at presence of the continuous third body increase of the relative sliding velocity leads to increase of the friction power and contact temperature; decrease of the lubricant viscosity, film thickness and friction force (**Figure 13**, "negative friction"), stable (or smoothly variable) friction torque and low destruction rate of the surfaces. Worsening of the working conditions caused by the partial, non-progressive damage of the third body in the separate unit places corresponds to the separate small impulses of the friction moment. Destruction of the third body in the multiple places leads to the multiple damage of the third body, multiple adhesive junctions of micro-asperities, disruption of these junctions, and a bit little increased impulses of the friction torque and to "neutral friction."

**Figure 12.**

*The damage stages of the interacting surfaces. (a) Unit seizures; (b) multiple seizures; (c) seizures on the narrow strip; (d) seizures on the whole area.*

film) is technically difficult, its presence in the contact zone is indicated by the magnitude and stability of the friction coefficient. Further worsening of the work-

A particular instability of the friction coefficient was observed at low velocities and high loads: intensive impulses of low frequency were marked and the scuffing marks of significant sizes – scratches and pits were noticed on the rollers surfaces. With increase of the velocity, the time of dwelling of the surfaces in the real contact zone and duration of the thermal impact, values of the amplitude of the friction force variable component decrease; the frequency increases and the individual impulses turn into noise. With further increase of the velocity the friction process is progressed, the temperature on the actual contact area of the interacting surfaces reaches the metal melting point, tonality of the noise rises and turns into whistle and when the frequency exceeds 20 KHz it becomes

The complex physical, mechanical and tribo-chemical processes proceeding in the contact zone of interacting surfaces at direct impact of the environmental conditions raise the problems whose solution demands many-sided approach to these processes. There are many works devoted to these problems [36–38] but they are not solved properly yet. Namely, prediction of the friction coefficient in the contact zone, its control and character of influence of many parameters on its

At heavy working conditions, when destruction of the third body is irreversible and scuffing is spread over the factual contact area of the whole surface relative

corresponding deformations, values and instability of the friction forces and rupture of the seized places. Strength of the seized places may exceed the strength of the interacting bodies because of which the material pulled out from one surface can form a wear product or can be transferred on the other surface and attached to

Multiple repetition of the shear deformation generated on the surfaces (that sharply decreases towards the depth) causes appearance of cracks on the surfaces, their development and fatigue damage, superficial plastic deformations and lamination. The area of each seized place in the contact zone depends on its power and thermal load; initial micro-geometry of the surfaces; value, velocity and resistance of the deformation etc. Therefore, various working conditions are characterized by corresponding variation of the tribological parameters, namely friction forces, amplitude and frequency of their variable component, wear intensity and roughness of the surfaces. Development of these processes leads to the catastrophic wear

At low velocities of interacting surfaces, the thermal load of the factual contact zone, velocity of the surface and environment tribo-chemical reaction and resistance of deformation decrease and time of the thermal action and thickness of the superficial heated up layer increase. In such conditions, at destruction of the third body, due to rupture of the seized places, the jerks of low frequency and high amplitudes and sharp instability of the friction coefficient take place and relatively large-size asperities (pits, scratches, asperities, cracks and layers) appear on the surfaces. This is correspondingly reflected on the damage type and roughness of the

displacement of the surfaces causes sharp increase of the shear stresses,

it that is followed by development of the scuffing process.

ing conditions leads to destruction of the third body in individual places of

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

**3. Analysis of results of the experimental researches**

interacting surfaces.

imperceptible for man.

variation are still problematic.

due to scuffing.

surfaces.

**140**

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

oscillogram of the friction torque and may be predicted on the base of results of the experimental researches. The typical damage types of this zone are fatigue, plastic deformation, adhesive wear and limited rate of scuffing and correspond to "severe"

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

At further increase of the relative sliding velocity, destruction of the third body becomes irreversible and extending and multiple seizures becomes uninterrupted (causing scuffing) and they propagate on the whole width of the interacting surfaces. The typical damage types of this zone are scuffing, plastic deformation and fatigue (zone 3, **Figure 14a**, and "catastrophic" wear rate **Figure 14b**). In this case,

Destruction of the third body makes especially heavy the working conditions of the interacting surfaces and is characterized by increased instability, high wear rate

At low velocities, time of dwelling of individual places of the surfaces in the contact zone and power and thermal actions, variable components of the friction torque and scales of the superficial damage increase and inversely, decrease with

For each operational mode and frictional pairs, this stipulates corresponding

The methods of calculation of the contact zone power and thermal loads, friction coefficient, wear rate etc., are characterized by low informativeness and precision. This complicates prediction and realization of proper tribological properties of surfaces at various working conditions that prevents machines from reliable and

For heavy loaded interacting surfaces are typical destruction of the third body, direct contact of the surfaces and cohesion. The shearing forces, rate of the adhesive and fatigue wear rise sharply in the contact zone at such conditions and friction

The mentioned types of wear in the contact zone are the results of quite different processes proceeding simultaneously. Besides, identification of the wear type according to the wear signs is often ambiguous that hinders selection of methods for

Dependence of tribological properties of the interacting surfaces on the properties of the third body and degree of its destruction were ascertained on the base of

At existence of a continuous third body between interacting surfaces, tribological properties of the contact zone are stipulated by the properties of the third body and at existence of a discontinuous third body, tribological properties of the contact zone are mainly stipulated by the properties of the third body and degree of its

The signs of onset and development of the third body destruction and a criterion of its destruction are given there. The reasons of the negative, neutral and positive friction, mild, severe and catastrophic wear and types of surface damage at various

**4. Estimation of stability of the third body on the base of EHD theory of**

The most complete mathematical model of lubrication is the elastohydrodynamic (EHD) theory of lubrication [44]. The effectiveness of the EHD theory of lubrication is described by ratio λ or film parameter [45], which is the ratio of

the scuffing can be avalanche in nature that quickly disables the machine.

('catastrophic wear"), vibrations and noise, change of structure and micro-

increasing speed, although the high wear rate is maintained.

forces become instable causing the vibrations and noise.

wear rate (**Figure 14b**).

effective operation.

its decrease.

destruction.

**lubrication**

**143**

geometry of the surfaces at operation etc.

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

micro-geometry and tribological properties.

results of the experimental researches.

relative sliding velocities are revealed.

**Figure 13.** *Friction/creep relationship.*

At progressive damage of the third body, the friction torque increases and corresponds to positive friction. Our experimental researches have shown that in other equal conditions the variation of the friction coefficient mainly depends on degree of destruction of the third body. Therefore, preservation of the third body between interacting surfaces and avoidance the scuffing, has a crucial importance for decrease of the friction coefficient, wear rate, etc. This issue became burning especially for wheels and rails in the last 50 years and many works appeared that are devoted to enhancing stability of the wheel flanges against the operational impacts.

In **Figure 14** are shown dependences of the friction factor and various damage types on relative sliding velocity (a) and of the wear rate (types) on slip (b) [32].

Three zones can be distinguished in **Figure 14a**. The low relative sliding velocity, full separation of the interacting surfaces and continuous third body provide high wear resistance of the interacting surfaces and relatively stable friction coefficient (zone 1, **Figure 14a**) that corresponds to "mild" [32] wear rate (**Figure 14b**). In such conditions, the main damage types are the fatigue and plastic deformations.

Small increase of the sliding velocity leads to appearance of small damage sources in multiple places and emergence of small surges of the friction torque (zone 2, **Figure 14a**). The rise of the third body destruction, as well as the magnitude of the friction coefficient and its instability, are clearly reflected in the

#### **Figure 14.**

*Dependences of the friction factor and various damage types on relative sliding velocity (a) and of the wear rate (types) on slip (b).*

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

oscillogram of the friction torque and may be predicted on the base of results of the experimental researches. The typical damage types of this zone are fatigue, plastic deformation, adhesive wear and limited rate of scuffing and correspond to "severe" wear rate (**Figure 14b**).

At further increase of the relative sliding velocity, destruction of the third body becomes irreversible and extending and multiple seizures becomes uninterrupted (causing scuffing) and they propagate on the whole width of the interacting surfaces. The typical damage types of this zone are scuffing, plastic deformation and fatigue (zone 3, **Figure 14a**, and "catastrophic" wear rate **Figure 14b**). In this case, the scuffing can be avalanche in nature that quickly disables the machine.

Destruction of the third body makes especially heavy the working conditions of the interacting surfaces and is characterized by increased instability, high wear rate ('catastrophic wear"), vibrations and noise, change of structure and microgeometry of the surfaces at operation etc.

At low velocities, time of dwelling of individual places of the surfaces in the contact zone and power and thermal actions, variable components of the friction torque and scales of the superficial damage increase and inversely, decrease with increasing speed, although the high wear rate is maintained.

For each operational mode and frictional pairs, this stipulates corresponding micro-geometry and tribological properties.

The methods of calculation of the contact zone power and thermal loads, friction coefficient, wear rate etc., are characterized by low informativeness and precision. This complicates prediction and realization of proper tribological properties of surfaces at various working conditions that prevents machines from reliable and effective operation.

For heavy loaded interacting surfaces are typical destruction of the third body, direct contact of the surfaces and cohesion. The shearing forces, rate of the adhesive and fatigue wear rise sharply in the contact zone at such conditions and friction forces become instable causing the vibrations and noise.

The mentioned types of wear in the contact zone are the results of quite different processes proceeding simultaneously. Besides, identification of the wear type according to the wear signs is often ambiguous that hinders selection of methods for its decrease.

Dependence of tribological properties of the interacting surfaces on the properties of the third body and degree of its destruction were ascertained on the base of results of the experimental researches.

At existence of a continuous third body between interacting surfaces, tribological properties of the contact zone are stipulated by the properties of the third body and at existence of a discontinuous third body, tribological properties of the contact zone are mainly stipulated by the properties of the third body and degree of its destruction.

The signs of onset and development of the third body destruction and a criterion of its destruction are given there. The reasons of the negative, neutral and positive friction, mild, severe and catastrophic wear and types of surface damage at various relative sliding velocities are revealed.
