Self-organization Effects on Tribosystems when Lubricated with a Metal-plating Additive „Valena”

The external appearance of self-organization is very low friction and wear, and in some cases there is lack of wear, so a friction effect without wear is observed. In the present study are examined tribosystems of different materials - steel, spheroidal graphite cast iron micro alloyed with 0.051% of tin, bronze and other materials under conditions of border lubrication with grease “Litol 24” with a metal-plated additive “Valena”. A unique multifunctional tribotester has been developed that allows varying load and slide speed across wide ranges and research different types of contact - point, ring, plane. Studies have been carried out on different friction modes and results have been obtained for friction coefficient and the parameters of wear of the specimens and the counter body depend of the friction times. Conditions have been established to produce a No-wear effect due to the formation of a metal-plated copper film on the surfaces of the body and the counter body.


Introduction
Generally acknowledged self-organization is connected with the spontaneous transition of the open non-equilibrium systems from less complicated and more chaotic forms to more complicated and more аranged forms [1,2,3]. In the non-equilibrium thermodynamics, the Belgian physic-chemist, Nobel Prize winner Ilya Prigozhin has proved, that in the open natural systems independently of their nature: physical, chemical, biological and social there exist possibility for their sustaining in stable nonequilibrium and the latter takes the towards elaboration in their internal structure and their structural enrichment. This evolutionary state of the systems might be supported with the help of stationary energetically and mass streams. Similar conditions are accomplished for processes which are carried out spontaneously and in the human society to the spontaneous actions are added the impacts with set purpose from the side of the humans. The strongly non-equilibrium open system forms structures on the basis of the mechanism of self-organization. In the scientific literature these structures are known as dissipative structures. The term for the latter comes from its basic purpose and function to protect 9th International Conference on Tribology (Balkantrib '17) IOP Publishing IOP Conf. Series: Materials Science and Engineering 295 (2018) 012026 doi: 10.1088/1757-899X/295/1/012026 the system from outer powerful energetic flows by scattering them. Dissipative structures exist and they are developing themselves only in non-equilibrium state of the open systems. The tribological systems are assigned to the open thermodynamically systems. They are characterized with exchange of substances, energy and information with the environment. In the tribology are well known many effects, which are assigned to the self-organization in the tribosystems: effect of anomalously friction, registered by Silin after the contact surfaces have been irradiated with alfa particles; effect of selective transfer of material from one to the other surfaces under friction; effect of processing of the roughness and the connected triboprocesses of friction; effect of Rebinder, which is expressed with the influence of special surfactants designed to decrease of the surface tension; auto vibrations in contact systems, etc. Special attention of many scientists and basically of the authors of this publication is the phenomenon no-wear effect under friction. This effect has been discovered in 1956 in the Soviet union by a group of researchers under the scientific guidance of D. Garkunov [4÷8]. The effect is observed in cases of friction and it is a result of mechanical and physical-chemical processes, leading to auto-compensation of the wear and decrease of friction. Specific in the essence of this effect is the building and functioning of a protective film where is realized diffusion -vacational of deformations which are proceeding without accumulation of defects as opposed to the fatigue processes. As a result of the processes of self-organization during the friction are formed self-recoverable metal-polymeric structures -serving and servitic films with thickness of 100 nm, where the deformation-plastic phenomena are materialized without accumulation of defects in the crystal structure on the surface. The processes of formation of such surfaces is carried out under stringent combination of certain conditions -composition and properties of the contacting surfaces; composition and properties of the lubricating material; characteristics of the deformation field; value of the relative pressure; friction rate; contact temperatures. Generally, the basic factor which has an influence for the simultaneous action of tribo-recovering and tribo-coordination processes leading of self-organization is the chaotic auto-vibration of copper containing products in the lubricating material [9÷12]. The self-organization is the basic mechanism for non-wear friction and it is a result of the evolution changes of the contact, which are conditioned by the achievement of stationary (stable) condition of the tribosystem with minimal friction and maximal wear resistance [13÷17]. Especially expressive for the self-organization are the strong non-equilibrium states of the system where the energetic and material flows from the outside are non-linear functions of the temperature gradients and the concentration of the substances. The purpose of the present paper is to be studied the influence of the metal-plating additive Valena on the intensity of the wear of the tribosystems from different materials and different friction regimes.

Materials
The influence of metal-plating composite additive Valena on wear of different materials during friction in grease Litol 24 is studied. Possibilities for obtaining the selective transfer of materials effect are to be found.

Experimental results and analysis
The materials are tested by means of the device shown in Figure 1. A multifunctional set of accessories is developed, which allows simultaneous testing of wear and seizure through 180 о rotation of the body, with possibility to reconstruct different kinematic situations. The set is located on a common fundament ( Figure 2). The elements of the set are: 1 -fundament; 2 -vertical bearing frame of the chamber 4 for testing of wear; 3 -protective screen; 5 -mandrill; 6 -bearing partition; 7 -chamber for testing the seizure of specimens and oils with and without additive. Three spheres 8 with diameter 11.4 mm are located in chamber 7, which are loaded with load Р through the sphere with equal diameter. The spheres 8 are fixed and covered with lubricant. The test rig allows three cases of kinematic interactions: firstly, when the upper arbor rotates, and the spheres in the chamber are fixed; secondly, when the upper arbor is immovable, and chamber 8 with the spheres rotates; and the third case, simultaneous rotation of the arbor and the chamber with different revolutions and opposite directions. The chamber is driven by motor 14 through the belt drive 12, 11, 10. The motor is located in case 13.  The interaction between specimen and counter body occurs in an annular contact with nominal area Аа = 44.75 × 10 -6 m 2 . The space above the annular contact is filled with lubricant, providing permanent penetration in the contact zone, and avoiding thus the regime of boundary and/or dry friction (Figure 2а).
The tests are carried out at two regimes of friction: the first one is realized at low load and high number of revolutions, i.e., big peripheral sliding speed, and the second one -at high load and small peripheral sliding speed. Wear is tested at every friction regime in two cases: lubrication with grease Litol 24 without Valena and lubrication with grease Litol 24 with Valena ( Table 6). The time of friction for all tests is 40 min.  Experimental results for the three contact systems at friction without and with Valena additive in both regimes of friction are presented below.   leads to higher wear compared with that in grease without Valena additive. After that period, wear decreases, the specimen's mass increases, and that is due to the film deposited on the cast iron specimen's surface ( Figure 3).   (Figure 6). Visual observation shows clearly metal-plating film with reddish colour on the surface of the spheroidal graphite cast iron specimen and on the surface of the steel counter body (Figure 7а, b). The microstructure and the composition of the metal-plating film on specimens' surfaces are studied by scanning electronic microscope EVO MAX 10 ZEISS. Figure 8 shows the microstructure and the composition of the deposited metal-plating film on the surface of spheroidal graphite cast iron GJS 600-3 specimen. The composition of the deposited film shows presence of the following elements: iron, copper, aluminum, nickel, chromium and chlorine.   Metal-plating copper films on the specimen of spheroidal graphite cast iron and on the counter body of steel form almost simultaneously. Figure 11 shows the microstructure and the composition of the metal-plating film on the surface of the counter body (С22).

a) b)
c) d) Figure 11. Microstructure (а), composition (b), microstructure with elements Cu (c), and Fe (d) in the metalplating film on the surface of the counter body (С22), after friction at P = 10 N; n = 2300 min -1 . Roughness profiles of the surface before and after friction are obtained in radial direction of annular copper film (Figures 12, 13). The roughness of the surface before friction is Ra = 0.466 μm, and after friction the roughness of the deposited copper film is Ra = 0.196 μm.  The width of the copper film is determined by the profilogram. It is 6 mm, i.e., twice larger than the width of the ring. The process of selective transfer occurs not only in the contact between the specimens, but also out of the contact zone. This fact is also found in other authors' studies [20,30,35,38]. It is seen in the profilogram's indication from 5 to 11.5 mm.
Copper film thickness is no uniformly distributed both in radial direction and along the length of the trail. The average thickness film value is 3 μm.
Contact system "Steel -Steel" (Ст4кп -С22) Figures 14 and 15 give the wear curves for specimen of steel Ст4кп in both regimes of friction. The influence of Valena on the character of variation is almost analogous with that in the case of spheroidal graphite cast iron micro alloyed by tin. In both regimes of friction is observed deposition of metal plating copper film upon the specimen and the counter body, which shows the presence of selective transfer process. For the lighter regime of friction -Р = 10 N; n = 2300 min -1 , effect of no-wear effect is observed, as it is in the case of spheroidal graphite cast iron specimens. The copper film thickness on the counter body is 2.5 μm.    Contact system "Bronze -Steel" (БрАЖ9-4 -С22) Significant wear decrease is observed during friction of bronze-steel couple in the first regime Р = 10 N; n = 2300 min -1 , there is, however, no effect of specimen's mass increase as it was in the first two cases. Small wear of the bronze specimen is indicator that a selective transfer process. The thin copper film protects the surface from oxidation and destruction ( Figure 18).   In the case of the second regime of friction Р = 20 N; n = 870 min-1, quite high value of bronze wear is observed in the process of running-in when Valena additive available ( Figure 19). In that case, there are no favourable conditions for selective transfer occurrence. The wear increase with load increase is explained with the character of the electrochemical contact processes. Microscopic study of the surface shows that cavities with liberated hydrogenare formed in some sections under the cathode copper, which causes detachment of copper flakes. As other authors' studies also show, saturation of the surface with hydrogen takes place under these conditions, which causes brittleness in some sections of the surface. The process of extraction of alloying elements from bronze and steel is more intensive at higher load. The extracted copper transferred to the steel surface does not succeed to crystallize, and forms crumbly copper film, which is not able to protect surfaces from wear [1,3,14]. One more feature affects the processes in the above considered case. The plastic deformation of the surface film increases under higher pressure, the real contact area increases, and respectively, the contact interstices decrease. Lubricant penetration in contact is hindered and direct mechanical interaction between asperities occurs. So, lubricant and additive lose their functional ability. Boundary friction and intensive wear take place. The parameter wear intensity is announced below as criterion of comparative assessment for the tested materials. It is determined by the formula: where h = m/ρ.Aa is thickness of the worn film during friction way L = 1 m. Experimental results about wear intensity of the three materials are given in Table 7.  Figure 22 shows diagram of wear intensity for the materials at friction Regime I.

Conclusions
The basic results of the present paper are: 1. Experimental data are gathered for the influence of metal-plating additive Valena on the contact characteristic of 3 types of: "GJS 600-3 -С22" (Spheroidal graphite Cast iron with 0.051%Sn -Steel), "Ст4кп -С22" (Steel -Steel) и "БрАЖ9-4 -С22" (Bronze -Steel) and their functioning at 2 friction regimes. 2. It has been found out that the additive Valena in grease Litol 24 in all three tribosystems the intensity of wear decreases and processes of selective transfer of the occur in both friction regimes. 3. In tribosystems "GJS 600-3 -С22" and "Ст4кп -С22" in friction regime at a contact pressure of P a =0.22 MPa and revolutions of n = 2300 min -1 (Regime I) is observed a no-wear effect under friction. This effect is due to a proceeding mechanism of self-organization, where a dissipative structure is build up in the form of protective servovitic film with thickness of 2.5÷3 µm. The servovitic protective film is formed on the contact surfaces of the body and antibody of the tribosystem. The microstructure investigations of the microstructures and the chemical composition of the protective film with scanning electronic microscope shows the presence of the chemical element copper (Cu). The contact temperature decreased 2 to 4 times compared to the one in friction where there is no additive "Valena".