**4. "Zero-wear" effect: selective transfer**

It seems expedient, at least briefly, to consider how the achievements that were obtained in the study of self-organizing tribo-systems, and in particular, the "zerowear" (effect of wearlessness/effect of selective transfer—ST), play a role in the circle of tasks which green tribology is designed to solve.

The effect of ST in friction was registered as opening in 1966, with a priority in 1956. The authors of this discovery – D.N. Garkunov and I.V. Kragelsky – stated that the essence of the observed phenomenon as follows: "…that in the friction of couple copper alloys-steel under boundary lubrication, eliminating the oxidation of copper, there is a phenomenon of ST of a solid solution of copper from copper-alloy to steel and its transfer backwards from steel to copper alloy, with a reduction of the friction coefficient as liquid lubrication and leads to a significant reduction in wear of the friction pair…" [4].

In the closing years of the XX century the "zero-wear" effect is defined as one of the examples of self-organization in frictional interaction in tribological systems [41, 42], and since then, a synergistic approach at his description has become essential.

Classical tribo-system for realizing of ST is a system of "copper alloy (bronze or brass) – aqueous or alcoholic solution of glycerol – steel". The evolution of the tribological properties of this system visually demonstrated the self-organization in friction in ST mode, which is expressed in the ultra-low frequency vibrations of the friction coefficient and of the size of the rubbing bodies (**Figure 6** [42]).

Self-organization in the ST mode during friction is the consequence of the complex tribo-chemical reactions and physico-chemical processes occurred in the area of frictional contact, which lead to the manifestation of unique tribological characteristics: super-antifrictional (friction coefficient ~ 10−3) and without wear (intensity wear ~10−15). This condition of tribo-system was provided by a protective nanocrystalline servovite film made of soft metal with unusual combination of mechanical properties [43]. According to the results of nanoindentation, such a film has "super-hardness" at compression and "super-fluidity" at shear [44].

Within the framework of the I.V. Kragelsky's molecular-mechanical theory, the providing extremely low friction coefficients and practical absence of wear during friction of solids is possible either at spontaneous generation of wear autocompensation systems or in the case of friction of perfectly smooth two-dimensional crystals, in which show up only molecular component of the friction force that occurs, such as, during friction of graphene [45].

In the engineering practice, the auto-compensation systems of wear during friction in the ST regime, usually are formed by selecting (a) the materials of tribocoupling, (b) a composition of lubricants, and (c) a construction of the friction units. As a result of successful material science and engineering solutions, tribosystems are capable of self-organization, in which the process of frictional interaction

#### **Figure 6.**

*The evolution of the tribological properties (1 – friction moment, 2 – the linear wear) in tribo-system brass-glycerol-steel. A (a, b) – running-in; c, d – transition mode; and e – the ST mode [42].*

moved to the nanocrystalline quasi-liquid [3], and thus provides the friction coefficient, which is characterized for hydrodynamic friction, forming nanoclusters with almost perfect crystals, that leads to increases in load capacity and wear resistance of the friction surfaces.

In practice, "zero-wear" functioning of friction is achieved most often by application of metal-plating lubricants in the real friction units: oils, plastic lubricant, self-lubricating materials and coatings [3].

The mechanism of "zero-wear" effect during friction does not follow from the existing theoretical conclusions about the nature of the frictional interaction. Therefore, none of the attempts to propose developed in detail and experimentally substantiated scientific approach to explaining the "zero-wear" effect is currently generally recognized, although works aimed at clarifying the causes of friction without wear have been underway for more than half a century, during which reliable experimental facts have been accumulated and consistent approaches have been proposed that allowed to qualitatively explain the evolution of the tribotechnical characteristics of friction pairs during realization of the ST.

Currently, it has been reliably established that the composition, thickness and properties of servovite film during frictional interaction continuously change so that the extrapolated to the infinity friction surface is a pure copper (**Figure 7**), whose stability during friction is provided by the absorption of surfactants from the lubricating medium [42, 46].

Detailed studies on tribochemical reactions, as well as the evolution of the chemical composition of the servovite film on friction surfaces in the "zero-wear" regime, made it possible to characterize in detail the products arising during friction and to establish their role in the mechanisms of formation of boundary layers during self-organization of not only the classical tribosystem "copper-glycerinsteel", but also a number of more effective tribosystems using other lubricants, such as aqueous solutions of polyhydric alcohols, solutions of sucrose, glucose, galactose and other carbohydrates [3].

String of sequential and parallel chemical reactions: tribo-oxidation, tribocoordination, tribo-restoration, tribo-reducing decay of coordination compounds, tribo-polymerization, tribo-clusterization and others, etc., accompanying, and/or generating vibrational tribo-chemical reaction, vibrational electrical and

**57**

*Cu*<sup>0</sup>

*Green Tribology*

**Figure 7.**

*DOI: http://dx.doi.org/10.5772/intechopen.94510*

electrochemical effects, vibration of the size of rubbing bodies and tribological

*AFM (3D-visualization) of the surface of the servovite transfer film (brass on steel in glycerol) [42].*

In the most general case, the evolution of the open tribo-system "copper alloyglycerin-steel", classical for the realization of the "zero-wear" effect, from the thermodynamically equilibrium state of rest under constant external initial conditions (P, V, T) to the friction regime without wear always starts with high (more than 0.1) values of the friction coefficient and large running-in wear, which leads to an increase in the energy intensity of the frictional contact zone and triggers complex physicochemical transformations in the lubricating medium and on the contacting surfaces of copper alloy and steel. At the same time, in the initial period of time, the friction of the copper alloy against steel in glycerin does not differ in nature from the boundary friction, which manifests itself both in the tribo-technical, electrical and electrochemical contact characteristics. The products of wear accumulating at this time in glycerin have a very wide particle size distribution from 10−7 to 10−3 m and are almost exclusively particles softer from contact bodies of copper alloy. Wear, repeatedly increasing surfaces of copper alloy in the tribo-system, leads to the predominant role of topochemical and tribo-chemical effects, both in the lubricant composition and on the friction surfaces, which reflect in the tribo- and topochemical oxidation of glycerin with the accumulation of a wide range of oxygen-containing surfactants (aldehydes, ketones, carboxylic acids, ethers and esters, as well as oligomeric and polymeric products of their further transformations). Parallel to this, the formation of complex compounds (tribo-coordination) occurs both on the surface of wear particles and on the friction surfaces, and the soluble coordination

characteristics of friction pairs - that's the one, is far from complete.

metal compounds accumulate in the solution [3, 47–49].

*- 2e = Cu*+2 (in the case of bronze) or zinc *Zn*<sup>0</sup>

Leading of tribo-chemical mechanisms on electrochemical reasons on the friction surface and on the surface of wear of particles is oxidation of copper

in result of its selective dissolution. Other metals that are part of the friction alloys such as Fe, Sn, Pb, etc., are also subjected to tribo-oxidation with the formation of metal-containing products, so that in the lubricating medium and on the frictional surface simultaneously there is a wide gamma of products, in which the explicitly pronounced tendency in the initial period is the accumulation of oxidized forms of different metals and oxidation products of the lubricant. An important event at this stage of evolution is the accumulation in the lubricating medium and reducing the size of the metallic wear particles with a simultaneous change in their composition due to above reasons. This leads ultimately to the accumulation of metal-containing products upto critical concentrations and thus, to the change

*-2e = Zn*+2 (in the case of brass)

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

moved to the nanocrystalline quasi-liquid [3], and thus provides the friction coefficient, which is characterized for hydrodynamic friction, forming nanoclusters with almost perfect crystals, that leads to increases in load capacity and wear resistance

*The evolution of the tribological properties (1 – friction moment, 2 – the linear wear) in tribo-system brass-glycerol-steel. A (a, b) – running-in; c, d – transition mode; and e – the ST mode [42].*

In practice, "zero-wear" functioning of friction is achieved most often by application of metal-plating lubricants in the real friction units: oils, plastic lubricant,

The mechanism of "zero-wear" effect during friction does not follow from the existing theoretical conclusions about the nature of the frictional interaction. Therefore, none of the attempts to propose developed in detail and experimentally substantiated scientific approach to explaining the "zero-wear" effect is currently generally recognized, although works aimed at clarifying the causes of friction without wear have been underway for more than half a century, during which reliable experimental facts have been accumulated and consistent approaches have been proposed that allowed to qualitatively explain the evolution of the tribo-

Currently, it has been reliably established that the composition, thickness and properties of servovite film during frictional interaction continuously change so that the extrapolated to the infinity friction surface is a pure copper (**Figure 7**), whose stability during friction is provided by the absorption of surfactants from the

Detailed studies on tribochemical reactions, as well as the evolution of the chemical composition of the servovite film on friction surfaces in the "zero-wear" regime, made it possible to characterize in detail the products arising during friction and to establish their role in the mechanisms of formation of boundary layers during self-organization of not only the classical tribosystem "copper-glycerinsteel", but also a number of more effective tribosystems using other lubricants, such as aqueous solutions of polyhydric alcohols, solutions of sucrose, glucose, galactose

String of sequential and parallel chemical reactions: tribo-oxidation, tribocoordination, tribo-restoration, tribo-reducing decay of coordination compounds, tribo-polymerization, tribo-clusterization and others, etc., accompanying, and/or

generating vibrational tribo-chemical reaction, vibrational electrical and

technical characteristics of friction pairs during realization of the ST.

**56**

of the friction surfaces.

**Figure 6.**

lubricating medium [42, 46].

and other carbohydrates [3].

self-lubricating materials and coatings [3].

electrochemical effects, vibration of the size of rubbing bodies and tribological characteristics of friction pairs - that's the one, is far from complete.

In the most general case, the evolution of the open tribo-system "copper alloyglycerin-steel", classical for the realization of the "zero-wear" effect, from the thermodynamically equilibrium state of rest under constant external initial conditions (P, V, T) to the friction regime without wear always starts with high (more than 0.1) values of the friction coefficient and large running-in wear, which leads to an increase in the energy intensity of the frictional contact zone and triggers complex physicochemical transformations in the lubricating medium and on the contacting surfaces of copper alloy and steel. At the same time, in the initial period of time, the friction of the copper alloy against steel in glycerin does not differ in nature from the boundary friction, which manifests itself both in the tribo-technical, electrical and electrochemical contact characteristics. The products of wear accumulating at this time in glycerin have a very wide particle size distribution from 10−7 to 10−3 m and are almost exclusively particles softer from contact bodies of copper alloy. Wear, repeatedly increasing surfaces of copper alloy in the tribo-system, leads to the predominant role of topochemical and tribo-chemical effects, both in the lubricant composition and on the friction surfaces, which reflect in the tribo- and topochemical oxidation of glycerin with the accumulation of a wide range of oxygen-containing surfactants (aldehydes, ketones, carboxylic acids, ethers and esters, as well as oligomeric and polymeric products of their further transformations). Parallel to this, the formation of complex compounds (tribo-coordination) occurs both on the surface of wear particles and on the friction surfaces, and the soluble coordination metal compounds accumulate in the solution [3, 47–49].

Leading of tribo-chemical mechanisms on electrochemical reasons on the friction surface and on the surface of wear of particles is oxidation of copper *Cu*<sup>0</sup> *- 2e = Cu*+2 (in the case of bronze) or zinc *Zn*<sup>0</sup> *-2e = Zn*+2 (in the case of brass) in result of its selective dissolution. Other metals that are part of the friction alloys such as Fe, Sn, Pb, etc., are also subjected to tribo-oxidation with the formation of metal-containing products, so that in the lubricating medium and on the frictional surface simultaneously there is a wide gamma of products, in which the explicitly pronounced tendency in the initial period is the accumulation of oxidized forms of different metals and oxidation products of the lubricant. An important event at this stage of evolution is the accumulation in the lubricating medium and reducing the size of the metallic wear particles with a simultaneous change in their composition due to above reasons. This leads ultimately to the accumulation of metal-containing products upto critical concentrations and thus, to the change

of lubricant composition in becoming a metal-plating lubricant. There is a radical change of physico-chemical, electrochemical and tribological situation on the friction surfaces and in the zone of frictional contact. Tribo-system in the course of long enough evolution (in the laboratory it is about 103 m of sliding distance) reaches the bifurcation point with transition either to "zero-wear" friction regime or to the regime of the catastrophic wear.

In the transition and functioning of the tribo-system under ST, both contacting surfaces in friction as copper alloy and steel have the same composition and structure. This is another paradox of ST and an unusual combination of materials of the rubbing surfaces. It has been observed that during friction of the same materials (usually in the friction units dissimilar metals and alloys are combined) record-breaking parameters of frictional interaction can be achieved, wherein the self-organization of frictional systems was achieved by the special structure of surface layers.

In the transition from boundary friction to "zero-wear" friction, due to the non-equilibrium character of the processes occurring in the tribo-system (since the system is far enough from the position of thermodynamic equilibrium), and their description by systems of nonlinear differential equations, oscillatory mechanisms begin to appear, which associated with both tribo-chemical transformations in the contact zone, for example, with fluctuations in the concentration of coppercontaining products in the lubricant, and with the electrical, electrochemical and tribological characteristics of the contact (**Figure 8**) [50]. Observing this type of oscillations, which always accompany friction in the "zero-wear" regime, prove the manifestation of the self-organization in friction, as well as the transition and functioning of the tribological system in one of the stationary states.

The transient regime from boundary to "zero-wear" friction lasts significantly less than the boundary friction regime, but at this time there are main events that lead to the unique tribological characteristics of tribo-system. It is in this transition that the ordering process occurs, which associated with the formation of a servovite film on the friction surface. The servovite film is formed under nonequilibrium, non-isothermal and topographically unequal conditions, which leads to inevitable differences in its composition and properties in different places of frictional contact. Nevertheless, formation of the film is always due to mutually complementary

#### **Figure 8.**

*Fluctuations in the transient regime of "boundary friction – ST" in the friction pair "AISI 1045 Steel–AISI 1045 Steel" in the lubricant of copper nanocluster in glycerol. 1 – load; 2 – friction coefficient; 3 – electrical resistance of contact [50].*

**59**

**Figure 9.**

*Green Tribology*

*DOI: http://dx.doi.org/10.5772/intechopen.94510*

less activated areas on the frictional contact surfaces.

in the metal crystal lattice [43].

properties of the system.

processes of tribo- and electrochemical reduction of coordination compounds of soft metals on the friction surface, clustering their reduced forms, and optimization depending on the friction regimes (P, V, T), sizes (in the nanoscale) and the shape (triaxial ellipsoid) of the clusters in two ways "top-down" and "bottom-up" followed by the direct deposition of metal nanoclusters on the contact surfaces due to tribo-electrochemical effects. The formation of servovite film begins on the individual most active sections of the steel surface, which leads to reducing the friction coefficient and a decrease in the energy density the friction unit. Finally, it is accompanied by a decrease in wear and a transfer of the film formation process to

Any system thermodynamically approaches to one of many possible stationary states, the choice of which is caused solely by the initial conditions. It should be noted that the trajectory of the tribo-system during evolution into the "zero-wear" regime is always strictly individual and can never be reproduced in detail. If the tribo-system self-organizes, which in the thermodynamic description is characterized by an increase in entropy and ordering, then its tribological and physicochemical characteristics in a stationary state become almost unchanged (**Figure 9**) [50]. This is due to the fact that a servovite film, formed from individual atoms and their small clusters, has a nanocrystalline structure and, on the one hand, is superstrong in compression, since its nanoparticles are fragments of almost ideal crystals, and on the other hand, the film is quasi-liquid and superplastic under tension and shear due to much weaker interactions between nanoparticles than between atoms

In this regime, the system can function until continuously accumulating external disturbances or changing external conditions transfer it to a new stationary state, which may be characterized by other and not necessarily higher tribo-technical characteristics, which makes the practical implementation of "zero-wear" in real

At the same time, even with a partial realization of "zero-wear" friction, the effects can be impressive, since when functioning under self-organization conditions and with a slight change in external conditions, the transition to a nearby stationary state is accompanied, as a rule, by a slight change in the tribo-technical

Thus, the application of "zero-wear" effect in engineering practice opens a real opportunity for the design of friction units with significantly increased durability and ultra-high efficiency in terms of friction losses in moving machine interfaces. The "zero-wear" effect in the friction fully fits into the presentation and concepts of

*Stationary regime in the realization of ST in friction pair "AISI 1045 Steel–AISI 1045 Steel" with lubricant of copper nanocluster in glycerol. 1 – load, 2 – friction coefficient, 3 – electrical resistance of contact [50].*

machines and mechanisms very complex and not always justified event.

#### *Green Tribology DOI: http://dx.doi.org/10.5772/intechopen.94510*

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

of long enough evolution (in the laboratory it is about 103

or to the regime of the catastrophic wear.

surface layers.

of lubricant composition in becoming a metal-plating lubricant. There is a radical change of physico-chemical, electrochemical and tribological situation on the friction surfaces and in the zone of frictional contact. Tribo-system in the course

reaches the bifurcation point with transition either to "zero-wear" friction regime

In the transition and functioning of the tribo-system under ST, both contacting surfaces in friction as copper alloy and steel have the same composition and structure. This is another paradox of ST and an unusual combination of materials of the rubbing surfaces. It has been observed that during friction of the same materials (usually in the friction units dissimilar metals and alloys are combined) record-breaking parameters of frictional interaction can be achieved, wherein the self-organization of frictional systems was achieved by the special structure of

In the transition from boundary friction to "zero-wear" friction, due to the non-equilibrium character of the processes occurring in the tribo-system (since the system is far enough from the position of thermodynamic equilibrium), and their description by systems of nonlinear differential equations, oscillatory mechanisms begin to appear, which associated with both tribo-chemical transformations in the contact zone, for example, with fluctuations in the concentration of coppercontaining products in the lubricant, and with the electrical, electrochemical and tribological characteristics of the contact (**Figure 8**) [50]. Observing this type of oscillations, which always accompany friction in the "zero-wear" regime, prove the manifestation of the self-organization in friction, as well as the transition and

The transient regime from boundary to "zero-wear" friction lasts significantly less than the boundary friction regime, but at this time there are main events that lead to the unique tribological characteristics of tribo-system. It is in this transition that the ordering process occurs, which associated with the formation of a servovite film on the friction surface. The servovite film is formed under nonequilibrium, non-isothermal and topographically unequal conditions, which leads to inevitable differences in its composition and properties in different places of frictional contact. Nevertheless, formation of the film is always due to mutually complementary

functioning of the tribological system in one of the stationary states.

m of sliding distance)

**58**

**Figure 8.**

*of contact [50].*

*Fluctuations in the transient regime of "boundary friction – ST" in the friction pair "AISI 1045 Steel–AISI 1045 Steel" in the lubricant of copper nanocluster in glycerol. 1 – load; 2 – friction coefficient; 3 – electrical resistance*  processes of tribo- and electrochemical reduction of coordination compounds of soft metals on the friction surface, clustering their reduced forms, and optimization depending on the friction regimes (P, V, T), sizes (in the nanoscale) and the shape (triaxial ellipsoid) of the clusters in two ways "top-down" and "bottom-up" followed by the direct deposition of metal nanoclusters on the contact surfaces due to tribo-electrochemical effects. The formation of servovite film begins on the individual most active sections of the steel surface, which leads to reducing the friction coefficient and a decrease in the energy density the friction unit. Finally, it is accompanied by a decrease in wear and a transfer of the film formation process to less activated areas on the frictional contact surfaces.

Any system thermodynamically approaches to one of many possible stationary states, the choice of which is caused solely by the initial conditions. It should be noted that the trajectory of the tribo-system during evolution into the "zero-wear" regime is always strictly individual and can never be reproduced in detail. If the tribo-system self-organizes, which in the thermodynamic description is characterized by an increase in entropy and ordering, then its tribological and physicochemical characteristics in a stationary state become almost unchanged (**Figure 9**) [50].

This is due to the fact that a servovite film, formed from individual atoms and their small clusters, has a nanocrystalline structure and, on the one hand, is superstrong in compression, since its nanoparticles are fragments of almost ideal crystals, and on the other hand, the film is quasi-liquid and superplastic under tension and shear due to much weaker interactions between nanoparticles than between atoms in the metal crystal lattice [43].

In this regime, the system can function until continuously accumulating external disturbances or changing external conditions transfer it to a new stationary state, which may be characterized by other and not necessarily higher tribo-technical characteristics, which makes the practical implementation of "zero-wear" in real machines and mechanisms very complex and not always justified event.

At the same time, even with a partial realization of "zero-wear" friction, the effects can be impressive, since when functioning under self-organization conditions and with a slight change in external conditions, the transition to a nearby stationary state is accompanied, as a rule, by a slight change in the tribo-technical properties of the system.

Thus, the application of "zero-wear" effect in engineering practice opens a real opportunity for the design of friction units with significantly increased durability and ultra-high efficiency in terms of friction losses in moving machine interfaces. The "zero-wear" effect in the friction fully fits into the presentation and concepts of

#### **Figure 9.**

*Stationary regime in the realization of ST in friction pair "AISI 1045 Steel–AISI 1045 Steel" with lubricant of copper nanocluster in glycerol. 1 – load, 2 – friction coefficient, 3 – electrical resistance of contact [50].*

green tribology and should be considered as the real embodiment in the theory and practice of modern engineering.
