Fretting wear mechanism of antifriction coating with selenium on aluminum alloy

. The article considers the issues associated with structure and mechanism of destruction of the antifriction coating with selenium on an aluminum alloy during fretting wear in contact with a steel ball. The process of fretting wear of the coupling under the influence of vibrations in heating conditions of up to 200 0 C is simulated. The influence of load on the wear resistance index is analyzed. It has been established that an increase in load leads to a decrease in fretting wear of the coating. Testing of an aluminum alloy sample under the same conditions showed that an increase in load leads to an increase in wear resistance. Based on the study, it is proposed to protect the workpiece from vibration to prevent the destruction of the anti-friction coating during


Introduction
Fixed supports of mechanisms and press connections fail because of contact surfaces wear due to vibration, temperature changes during thermal shocks, periodic force effects causing a relative shift of the contacting surfaces within the limits of preliminary displacement.The time intervals of vibration exposure are very different; these can be years, as for example during press landings on the axles of railway transport, or tens of minutes, as for example in the case of destruction of solid lubricant antifriction coatings during logistics operations for delivering a spacecraft to its destination, while in operation mode, the workpiece can heat up to 200 0 C. Due to the relevance of the protection problem against fretting wear, many studies, mainly experimental, are being carried out to study the contact interaction of surfaces under conditions of a stationary contact subject to vibration.Experimental studies of fretting wear are mainly carried out on test setup in which the contact of the samples represents a kinematic coupling of higher grade.The work [1] presents the results of an experimental study of the fatigue properties of a fluluminium alloy of the Al-Si-Cu system during fretting.It has been shown that different states of surface roughness lead to completely different damage mechanisms.The influence of microstructure on surface wear is noted.In [2], fretting wear of a rivet joint of an aluminum alloy in contact with carbon fiber was studied.The effect of heat accumulation caused by friction on the joints destruction is considered.It has been shown that fretting wear is influenced by the sensitivity of the mechanical properties of materials to the external environment.The place where the connection fails at low loads is the aluminum plate; at high loads, the carbon fiber is destroyed.Work [3] is devoted to studying the influence of fretting wear on the fatigue strength of multi-core wires in underwater power cables for offshore wind energy.An analysis of fretting wear of individual wires in a cable was performed using the finite element method.The closeness of the calculations results of frettig wear with the data of wires fatigue tests during bending has been established.It has been shown that the service life depends on the aerohydrodynamic load, wear, sliding mode and wire diameter.Similar results of the behavior of the fretting wear process in multi-core wires of an overhead power line were observed in [4], as well as in cable suspension of bridges [5].Work [6] is devoted to studying the effect of temperature on fretting wear of the steel-Inconel 718 alloy contact.Experiments were carried out in the temperature range 25 ℃ < T < 400 ℃ under conditions of displacement amplitude from 15 to 100 μm.It was found that wear was predominantly combined with fatigue wear and spalling.The highest friction coefficient occurred at a temperature of 100 0C.As the temperature increased, adhesive and oxidative wear dominated.Works [7] studied the evolution of surface damage and cracks depending on the number of cycles of fretting wear tests in relation to press joints on axes.It was established that the redistribution of stresses resulting from fretting wear led to the occurrence of cracks.As a result of fretting wear, the surface layer is removed, in which, during pressing, residual compressive stresses are formed, which contributed to the occurrence of cracks on the contact surface.A method for predicting the initiation of cracks in the surfaces contact is proposed.The use of metal selenides as solid lubricants has shown the technology to be highly effective for protecting friction surfaces from wear [8].In [9], it was experimentally established that, under normal sliding conditions, the tribological behavior of friction surfaces treated in selenium vapor can be described within the framework of the molecular mechanical theory of friction based on the fatigue wear mechanism.The resistance of high-temperature antifriction coatings based on selenium applied to aluminum alloys has not been sufficiently studied.
The purpose of the work is to study the wear mechanism of an antifriction coating with selenium on an aluminum alloy under fretting conditions in fixed joints with kinematic coupling of higher grade when heated to 200 0 C.

Materials and equipment
The experiments were carried out on a deformed corrosion-resistant aluminum alloy of the AlMn system Al -Mn. with the coating of the selenium antifriction coating; samples are made in the form of plates in size 70x25x8.Counterbody was a ball of ШХ15 steel with a diameter of 11.4 mm.

Equipment and technology
Aluminum alloy samples were processed in the selenium vapors at the installation [10].The structure of the coating was studied on an electronic microscope SNE-4500M Plus with the prefix The QUANTAX EDS System (Germany).
The mechanical properties of the coating were determined by the method of kinetic microindention on the MNT_Z_AE_000 device CSM Instruments according to the ISO/DIS 14577_1: 2002 standard with Vickers indenter.
Testing for fretting wear are performed according to the sphere-plane contact model at heating conditions up to 200 0 C on the device with electromagnetic loading of the console beam with a fixed ball sample.

Results
In the vacuum installation chamber, the sample was suspended vertically over the source of selenium vapors, which is technologically convenient, but there is an inhomogeneity of the covered coating (Fig. 1).In the lower part of the sample facing the source of selenium vapors and located close to it, an inhomogeneous larger bubble structure is formed, probably void.The elemental composition of the coating is shown in Fig. 2.

Fig. 2. The elemental composition of the coating
The quantitative characteristics of the elemental composition are given in table 1.The quantitative ratio between aluminum and selenium suggests that the good lubricating action of the coating is based on the formation of the aluminum selenide structure.The influence of manganese on the tribological properties of this coating composition is improbable.Figure 2 shows a diagram of kinetic micro-indention of the coating.The initial plateau of the pressing line reflects the deformation of the bubble structure into which the indenter "fails".The steep section of the ascending pressing line and the unloading line characterize the coating as a material with high plasticity.The plasticity of the coating contributes to the increased resistance of the material to repeated deformation, which means an increase in low-cycle fatigue.Coating hardness is 95 kgf/mm 2 .
In accordance with the accepted methodology, the friction coupling was heated to a temperature of 200 0 С.Each fretting wear test lasted for 30 minutes.Wear was determined by the gravimetric method based on the difference in weight before and after the experiment, with subsequent recalculation to the value of linear wear.After completion of the tests, an imprint with a trace of the frictional interaction of the samples was photographed by means of an electron microscope.This made it possible to determine the vibration amplitude and to calculate the friction path taking into account the vibration frequency (50 Hz).The measure of an aluminum sample durability is the estimated dimensionless wear resistance index D (ГОСТ 23.

Discussion
The amount of fretting wear of flat aluminum samples in contact with a ball indenter is influenced by two main factors: normal load and contact rigidity.The size of the contact area and the actual pressure depend on the normal load.The influence of contact rigidity affects the vibration amplitude, especially in installations with an elastic element in the form of a cantilever beam, which is loaded by an electromagnet.The greater the load, the smaller the vibration amplitude.Accordingly, the friction path is smaller for the same test duration.The wear mechanisms of aluminum surfaces in the original as-received condition and those coated with selenium are significantly different.Uncoated samples wear mainly due to adhesive interaction, and the role of contact rigidity is negligible.The wear mechanism of the antifriction coating with selenium is different.Fretting mainly involves chipping and entrainment of coating particles.In this case, the magnitude of the friction path plays a major role.An increase in the normal load on the friction coupling reduces the vibration amplitude and the friction path, so the durability of the coating increases.The formation of wear particles occurs as a result of flaking and chipping of material in the form of flakes during fatigue wear.Taking into account the low resistance of the antifriction coating with selenium on aluminum alloys, it is necessary to ensure that during logistics operations the contact of the parts on which the antifriction coating is applied is fixed.

Conclusion
1. Model experiments were carried out to determine the wear resistance of a steel sample during fretting in the contact of parts forming a kinematic coupling of higher grade under heating conditions of up to 200 °C.It has been established that, in relation to the contact of steel surfaces, temperature is the main factor influencing the wear resistance of a friction coupling under fretting wear conditions.The destruction of the friction surface occurs as a result of the separation of material in the form of flakes during fatigue wear.2. A methodology has been proposed for protecting friction couplings from fretting wear under operating conditions of the interface, under which the use of lubricants is impossible.

Fig. 1 .
Fig. 1.Photo of the sample surface with the coating of selenium.

Fig, 3 .
Fig, 3. The kinetic diagram of pressing the Vickers pyramid into the coating of selenium.

Figure 4 Fig. 4 .
Figure4shows the dependence of the load on the wear resistance index D under conditions of fretting wear of the AlMn aluminum alloy in contact with a steel ball.

Table 1 .
The quantitative characteristics of the elemental composition