**2. The experimental researches into variation of tribological properties of the interacting surfaces**

#### **2.1 Research into tribological properties on the high-speed twin disk machine**

A great number of scientific works are devoted to ascertainment of laws of variation of tribological properties of the interacting surfaces and with perfection of machines, actuality of such works increases. Despite the considerable quantity of works in this direction, the expected results are not obtained yet. The unexpected and catastrophic failure, unlike fatigue, corrosion and other slowly progressing wear types, are subjected some heavy loaded interacting surfaces of gear teeth, cams and followers, sleeve bearings etc. The wheel and rail contact zone is characterized by heavy operational conditions [33] (direct impact of the environmental conditions, high relative sliding and contact stresses) that enhances adhesive and fatigue processes. The wheel and rail contact zone is characterized by the heavy operational conditions (direct impact of the environmental conditions, high relative sliding and contact stresses) that enhances adhesive and fatigue processes raise the problems to be solved for many-sided study of these processes.

Experimental research was performed at rolling of discs with up to 20% of sliding. The rollers had diameters of 40 mm and widths of 10 and 12 mm. The tests were performed at single application of the friction modifier on the interacting surface of the rollers. After certain number of revolutions, a thin layer of the friction modifier (FM) was destroyed that was revealed by sharp increase of the friction moment and initial signs of scuffing on the surfaces. Without repeated feeding of the friction modifier the damage process were progressed. The rollers with various degrees of damage are shown in **Figure 7**: (a) with initial signs of damage; (b) damage in the form of a narrow strip; (c) damage of the whole

*The twin disk machine model MT1 and measuring means: 1 - twin disk machine, 2 - tribo-elements, 3 - the*

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

The graphs of dependences of the friction coefficient and number of revolutions

The graphs of dependences of the friction coefficient and number of revolutions

of rollers until appearance of the first signs of scuffing on the contact stress for initial linear contact of disks are shown in **Figure 8**. It is seen from these graphs that for the initial linear contact, when the contact stress is in the range of 0.65-0.77 GPa increase of the contact stress leads to decrease of the friction coefficient. It can also be seen that increase of the contact stress leads to decrease of number of revolutions

of rollers until appearance of the first signs of scuffing on the contact stress for initial point contact of disks and anti-frictional friction modifiers are shown in

*The stages of damage of the interacting surfaces: (a) damage in the separate points; (b) damage in the form of*

until the destruction of the third body and onset of scuffing.

*the narrow strip; (c) damage on the whole area of the contacting surfaces.*

*wear products, 4 - tester, 5 - personal computer, 6 – vibrometer.*

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

contacting area.

**Figure 6.**

**Figure 9**.

**Figure 7.**

**137**

For some heavy loaded interacting surfaces of machines are typical unpredictable change of tribological properties and sharp increase of the friction coefficient and wear intensity, so called catastrophic wear. As main cause of the latter is considered the heaviest form of the adhesive wear – scuffing [4] that is not properly studied yet [34] and whose signs are appearance of pits and scratches on the surfaces and transfer of the material from one surface on the other. The various aspects of the complex physical, tribo-chemical and mechanical processes proceeding in the contact zone are not properly studied yet that is accordingly reflected on the operation quality and resource. As an example can be cited interaction of the wheel and rail that occurs on: the tread surfaces during rolling, traction and braking; steering surfaces mainly in curves; flange root and rail corner at rolling, traction, braking and steering. The friction coefficient for wheel-rail interaction can vary in the range 0.05 - 0.8. The values of the friction coefficient for the tread and steering surfaces must be correspondingly in the ranges of 0.25-0.4 and <0.1 [32]. The optimal value of the friction factor for tread surfaces is 0.35 [32] and for steering surfaces - as low as possible. The scuffing on the wheel and rail steering surfaces causes rise of the friction coefficient, energy consumed on rolling, vibrations, nose, wear intensity and probability of derailment.

For more detailed study of the properties and state of the third body in the contact zone we performed the experimental researches on the twin disk machine MT 1 (**Figure 6**) with the use of existing lubricants and ecologically friendly friction modifiers, developed by us.

The tests were performed at single application of the friction modifier on the rolling surface of the roller. After certain number of revolutions, a thin layer of the friction modifier was destroyed that was revealed by sharp increase of the friction moment and initial signs of scuffing on the surfaces. Without repeated feeding the friction modifier, the damage process was progressed. The rollers with various degree of damage are shown in **Figure 7**: (a) with initial signs of damage; (b) damage in the form of a narrow strip; (c) damage on the whole contacting area.

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

#### **Figure 6.**

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

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

**2. The experimental researches into variation of tribological properties**

**2.1 Research into tribological properties on the high-speed twin disk machine**

A great number of scientific works are devoted to ascertainment of laws of variation of tribological properties of the interacting surfaces and with perfection of machines, actuality of such works increases. Despite the considerable quantity of works in this direction, the expected results are not obtained yet. The unexpected and catastrophic failure, unlike fatigue, corrosion and other slowly progressing wear types, are subjected some heavy loaded interacting surfaces of gear teeth, cams and followers, sleeve bearings etc. The wheel and rail contact zone is characterized by heavy operational conditions [33] (direct impact of the environmental conditions, high relative sliding and contact stresses) that enhances adhesive and fatigue processes. The wheel and rail contact zone is characterized by the heavy operational conditions (direct impact of the environmental conditions, high relative sliding and contact stresses) that enhances adhesive and fatigue processes raise the

problems to be solved for many-sided study of these processes.

wear intensity and probability of derailment.

friction modifiers, developed by us.

contacting area.

**136**

For some heavy loaded interacting surfaces of machines are typical unpredictable change of tribological properties and sharp increase of the friction coefficient and wear intensity, so called catastrophic wear. As main cause of the latter is considered the heaviest form of the adhesive wear – scuffing [4] that is not properly studied yet [34] and whose signs are appearance of pits and scratches on the surfaces and transfer of the material from one surface on the other. The various aspects of the complex physical, tribo-chemical and mechanical processes proceeding in the contact zone are not properly studied yet that is accordingly reflected on the operation quality and resource. As an example can be cited interaction of the wheel and rail that occurs on: the tread surfaces during rolling, traction and braking; steering surfaces mainly in curves; flange root and rail corner at rolling, traction, braking and steering. The friction coefficient for wheel-rail interaction can vary in the range 0.05 - 0.8. The values of the friction coefficient for the tread and steering surfaces must be correspondingly in the ranges of 0.25-0.4 and <0.1 [32]. The optimal value of the friction factor for tread surfaces is 0.35 [32] and for steering surfaces - as low as possible. The scuffing on the wheel and rail steering surfaces causes rise of the friction coefficient, energy consumed on rolling, vibrations, nose,

For more detailed study of the properties and state of the third body in the contact zone we performed the experimental researches on the twin disk machine MT 1 (**Figure 6**) with the use of existing lubricants and ecologically friendly

The tests were performed at single application of the friction modifier on the rolling surface of the roller. After certain number of revolutions, a thin layer of the friction modifier was destroyed that was revealed by sharp increase of the friction moment and initial signs of scuffing on the surfaces. Without repeated feeding the friction modifier, the damage process was progressed. The rollers with various degree of damage are shown in **Figure 7**: (a) with initial signs of damage; (b) damage in the form of a narrow strip; (c) damage on the whole

twin-disk machines.

**of the interacting surfaces**

*The twin disk machine model MT1 and measuring means: 1 - twin disk machine, 2 - tribo-elements, 3 - the wear products, 4 - tester, 5 - personal computer, 6 – vibrometer.*

Experimental research was performed at rolling of discs with up to 20% of sliding. The rollers had diameters of 40 mm and widths of 10 and 12 mm. The tests were performed at single application of the friction modifier on the interacting surface of the rollers. After certain number of revolutions, a thin layer of the friction modifier (FM) was destroyed that was revealed by sharp increase of the friction moment and initial signs of scuffing on the surfaces. Without repeated feeding of the friction modifier the damage process were progressed. The rollers with various degrees of damage are shown in **Figure 7**: (a) with initial signs of damage; (b) damage in the form of a narrow strip; (c) damage of the whole contacting area.

The graphs of dependences of the friction coefficient and number of revolutions of rollers until appearance of the first signs of scuffing on the contact stress for initial linear contact of disks are shown in **Figure 8**. It is seen from these graphs that for the initial linear contact, when the contact stress is in the range of 0.65-0.77 GPa increase of the contact stress leads to decrease of the friction coefficient. It can also be seen that increase of the contact stress leads to decrease of number of revolutions until the destruction of the third body and onset of scuffing.

The graphs of dependences of the friction coefficient and number of revolutions of rollers until appearance of the first signs of scuffing on the contact stress for initial point contact of disks and anti-frictional friction modifiers are shown in **Figure 9**.

**Figure 7.**

*The stages of damage of the interacting surfaces: (a) damage in the separate points; (b) damage in the form of the narrow strip; (c) damage on the whole area of the contacting surfaces.*

executed with the use of the high-speed roller machine with independent drive of

*High speed twin disk machine (a), experimental pieces (b) and a working surface of the roller with traces of*

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

• Diameters of rollers 183 mm and 143,3 mm; width of rollers 12 mm and 17 mm;

;

During experiments at the given loading and rolling velocity, the friction torque, sliding velocity and lubricant film thickness were measured. For measurement of speeds of rotation was utilized magnetic pickups, for measurement of the friction torque was utilized the strain gage transducer and contactless skate. The lubricant film thickness was measured by capacitance method [35]. The beginning of scuffing was revealed by the surges of the friction moment. Development of the friction process was accompanied by sharp rise of temperature and characteristic noise.

The studies have shown that with increase of the rolling speed, the thickness of the lubricating film initially increases (in our case up to 14 m/s) and then decreases

With increase of the sliding velocity, sharp decrease of the lubricated film thickness is observed. Though measurement of the particularly thin film (boundary

*Dependence of relative lubricant film thickness (h/R), linear scuffing load (Pllsc) and coefficient of friction (f) until the appearance of the first signs of scuffing from rolling speed (Vr) and sliding velocity (Vs) at various viscosities (ν) of lubricants: (1) h/R = <sup>φ</sup>(Vr), Pll = 106 H/m; <sup>ν</sup> = 157 cSt, (2) h/R = <sup>φ</sup>(Vsl), Pll = 2 <sup>10</sup><sup>6</sup> H/m, ν = 157 cSt, Vr = 50 m/s, (3) Pllsc = φ(Vsl), ν = 49 cSt, Vr = 50 m/s, (4) Pllsc = φ(Vr), ν = 157 cSt, Vsl = 22 m/s, (5) f = <sup>φ</sup>(Vsl), Pll = 1.5 106 H/m, <sup>ν</sup> = 49 cSt; Vr = 50 m/s, (6) f = <sup>φ</sup>(Vr), Pll = 106 H/m, <sup>ν</sup> = 157 cSt.*

rollers. Conditions of the experiments and measured sizes were:

*scuffing (c) at total speed of rolling 7 m/s, sliding speeds of 3 m/s, linear load 100 N/m.*

• Rolling speed – up to 70 m/s;

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

• Sliding velocity- up to 35 m/s;

slightly.

**Figure 11.**

**139**

**Figure 10.**

• Contact pressure – 5 105 – 2 106 N/м;

Results of experimental research are shown in **Figure 11**.

• Dynamic viscosity 49-140 mNs/m<sup>2</sup>

#### **Figure 8.**

*Dependences of friction coefficients (a) and numbers of revolutions (b) until appearance of the first signs of scuffing on the contact stress for initial linear contact of disks and different anti-frictional friction modifiers.*

**Figure 9.**

*Dependences of friction coefficients (a) and numbers of revolutions (b) until appearance of the first signs of destruction of the third body (first signs of scuffing) on the contact stress for initial point contact of disks and three different frictional FM-s.*

When the contact stress is in the range of 2.42-3.96 GPa the friction coefficient increases with increase of the contact stress. It can also be seen that increase of the contact stress leads to decrease of the number of revolutions until the destruction of the third body and onset of scuffing more intensive than in the previous case.

#### **2.2 Research into tribological properties on the high-speed twin disk machine**

At high working velocities, the maximal power and thermal stresses approach to the surfaces and intensity of the third body destruction/restoration and sensitivity of the contact zone tribological properties to working conditions, increase. To promote the mentioned problem, the experimental researches were carried out on the high-speed twin disk machine with independent drive of rollers (**Figure 10**).

During experiments were studied the character of the wear process of working surfaces, influence of various parameters on the lubricant film thickness and friction coefficient at the use of popular mineral lubricants. The researches were

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

**Figure 10.**

*High speed twin disk machine (a), experimental pieces (b) and a working surface of the roller with traces of scuffing (c) at total speed of rolling 7 m/s, sliding speeds of 3 m/s, linear load 100 N/m.*

executed with the use of the high-speed roller machine with independent drive of rollers. Conditions of the experiments and measured sizes were:


During experiments at the given loading and rolling velocity, the friction torque, sliding velocity and lubricant film thickness were measured. For measurement of speeds of rotation was utilized magnetic pickups, for measurement of the friction torque was utilized the strain gage transducer and contactless skate. The lubricant film thickness was measured by capacitance method [35]. The beginning of scuffing was revealed by the surges of the friction moment. Development of the friction process was accompanied by sharp rise of temperature and characteristic noise. Results of experimental research are shown in **Figure 11**.

The studies have shown that with increase of the rolling speed, the thickness of the lubricating film initially increases (in our case up to 14 m/s) and then decreases slightly.

With increase of the sliding velocity, sharp decrease of the lubricated film thickness is observed. Though measurement of the particularly thin film (boundary

#### **Figure 11.**

When the contact stress is in the range of 2.42-3.96 GPa the friction coefficient increases with increase of the contact stress. It can also be seen that increase of the contact stress leads to decrease of the number of revolutions until the destruction of the third body and onset of scuffing more intensive than in the previous case.

*Dependences of friction coefficients (a) and numbers of revolutions (b) until appearance of the first signs of destruction of the third body (first signs of scuffing) on the contact stress for initial point contact of disks and*

*Dependences of friction coefficients (a) and numbers of revolutions (b) until appearance of the first signs of scuffing on the contact stress for initial linear contact of disks and different anti-frictional friction modifiers.*

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

**Figure 8.**

**Figure 9.**

**138**

*three different frictional FM-s.*

**2.2 Research into tribological properties on the high-speed twin disk machine**

surfaces, influence of various parameters on the lubricant film thickness and friction coefficient at the use of popular mineral lubricants. The researches were

At high working velocities, the maximal power and thermal stresses approach to the surfaces and intensity of the third body destruction/restoration and sensitivity of the contact zone tribological properties to working conditions, increase. To promote the mentioned problem, the experimental researches were carried out on the high-speed twin disk machine with independent drive of rollers (**Figure 10**). During experiments were studied the character of the wear process of working

*Dependence of relative lubricant film thickness (h/R), linear scuffing load (Pllsc) and coefficient of friction (f) until the appearance of the first signs of scuffing from rolling speed (Vr) and sliding velocity (Vs) at various viscosities (ν) of lubricants: (1) h/R = <sup>φ</sup>(Vr), Pll = 106 H/m; <sup>ν</sup> = 157 cSt, (2) h/R = <sup>φ</sup>(Vsl), Pll = 2 <sup>10</sup><sup>6</sup> H/m, ν = 157 cSt, Vr = 50 m/s, (3) Pllsc = φ(Vsl), ν = 49 cSt, Vr = 50 m/s, (4) Pllsc = φ(Vr), ν = 157 cSt, Vsl = 22 m/s, (5) f = <sup>φ</sup>(Vsl), Pll = 1.5 106 H/m, <sup>ν</sup> = 49 cSt; Vr = 50 m/s, (6) f = <sup>φ</sup>(Vr), Pll = 106 H/m, <sup>ν</sup> = 157 cSt.*

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 working conditions leads to destruction of the third body in individual places of interacting surfaces.

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

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

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

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

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

**Figure 13** is shown variation of the tractive (friction) force with creep [37].

*The damage stages of the interacting surfaces. (a) Unit seizures; (b) multiple seizures; (c) seizures on the*

visually observed, however, a high wear rate remains.

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

properties.

to "neutral friction."

*narrow strip; (d) seizures on the whole area.*

**Figure 12.**

**141**

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 imperceptible for man.
