Fretting-wear behavior of aluminum alloys with antifriction temperature-resistant coating based on selenium

. The article considers the issues associated with the resistance to vibrations and elevated temperatures of the antifriction high-temperature coating obtained by processing an aluminum alloy in selenium vapor. The statement about the low resistance of solid lubricant coatings to fretting wear is substantiated. The quality control of the antifriction coating is carried out using an electron microscope, and the mechanical properties of the surface layer of the samples are determined by the method of kinetic microindentation. The experiments were carried out on a testing bench with a cantilevered plate driven by an electromagnet, at the end of which a steel ball contacting with an aluminum alloy plate is installed. The fretting wear of a coating on a plate in contact with a steel ball is experimentally studied depending on the normal load. Particular attention is paid to the stability of maintaining a given temperature regime. Based on the study, it was found that the anti-friction coating has a low resistance to vibrations.


Introduction
In a number of problem solving in spacecraft, friction units with delayed actuation are used, in which cold diffusion welding is possible in open space conditions.To prevent the consequences of this phenomenon, various coatings are used, including antifriction ones.It has been experimentally established that such coatings are not resistant to fretting wear, which occurs when spacecraft (SC) are launched into the target orbit, due to vibrations and dynamic effects on the deployment units.Also, vibration loads are possible when performing logistics operations to deliver the product to the launch site.The European Space Agency has carried out a study on the effect of fretting wear on the adhesive force produced by fretting.It has been established that solid lubricating coatings are poorly preserved under fretting wear conditions, and a damaged coating is ineffective in preventing surfaces from seizing in a vacuum, especially during long-term flights to other planets in the solar system.In particular, the destruction of the anti-friction coating due to fretting wear during transportation to the cosmodrome and launching into the transfer orbit, the SC GALILEO antenna could not open after 1.5 years of stay in space [1][2].Therefore, much attention is paid to the problem of adhesion of materials.Due to the variety of phenomena occurring in the area of contact of bodies, most studies of fretting wear of friction couplings are carried out experimentally.The contact of a friction couple is simulated mainly in the form of coupling with a higher kinematic pair, in almost all industries [3][4][5][6][7][8][9].In [10], the resistance of coatings to fretting wear was studied on an installation that simulates a vibration load similar to that arising when a SC is put into orbit.The experiments were carried out on samples of aluminum alloy plates.It has been experimentally established that for the purpose of eliminating welding, traditionally used solid lubricating coatings based on molybdenum disulfide from fretting wear are erased to the full thickness.The European Space Agency (ESA) confirms this conclusion.A series of studies on the development of a technology for processing tribotechnical materials in selenium vapor has been completed.The high efficiency of this hardening treatment for improving antifriction materials operating at high temperatures has been established.However, the question of the resistance of the antifriction coating under the influence of vibrations has been little studied.The aim of the study is to determine the resistance of high-temperature anti-friction coatings of aluminum alloys obtained by processing in selenium vapor.

Materials and equipment
The tests were carried out on the aluminum alloy ALMn in the delivered state and after treatment in selenium vapor according to the technology [11][12].Al-Mn alloy: deformable, corrosion-resistant, weldable aluminum alloy.The chemical composition of the ALMn alloy (GOST 4784-97) is shown in Table 1.Aluminum alloy AlMn has high corrosion resistance, ductility and good weldability.The material is hardened by heat treatment and there are no magnetic properties.In the annealed condition AlMnM the AlMn alloy has a corrosion resistance similar to that of pure aluminum.The AlMn grade alloy is used for the production of lightly loaded welded assemblies in various fields of technology.

Equipment
The antifriction coating was applied by treating aluminum samples in selenium vapor.The quality of the applied coating was controlled using an SNE-4500M Plus microscope with The QUANTAX EDS System attachment.The mechanical properties of the sample surfaces were determined by kinetic microindentation on an MNT_Z_AE_000 device from CSM Instruments according to ISO/DIS 14577_1:2002 standard with a Vickers indenter.
The testing bench was upgraded to carry out fretting wear tests at elevated temperatures.A stable heating of the sample up to 200 °C was provided, a temperature stabilization and control device was developed and a new sample loading system was manufactured.A layout of the testing bench is shown in Figure 1.

Results
Modification of the sample surface was carried out by heating HFC in selenium vapor.A film of presumably Al2Se3 selenide was obtained (Figure 3).The surface morphology was studied from electron photographs.The coating is homogeneous, without breaks.The AlMn aluminum alloy contains manganese, so a film with a more complex chemical composition may be formed.To compare the resistance to fretting wear, hard aluminum alloy Д16 was chosen.The choice of aluminum alloys is due to the large number of applications in the process of spacecraft design.Antifriction coatings, which can be destroyed during transportation and during the launch of the devices, are used to improve their tribological properties.The treatment of materials in selenium vapor has shown high efficiency in improving the antifriction properties [13].
Table 2 shows the Brinell hardness and Vickers microhardness numbers, which are determined by indentation of a steel ball with a diameter of 10 mm at a load of 1000 kgf The hardness of the surface layer of the AlMn alloy and the antifriction film was determined on a ПМТ-3 microhardness tester by indentation of the Vickers pyramid: in the initial state, HV0.1 21; after treatment in selenium vapor, HV0.1 27.
In accordance with the accepted methodology of testing for fretting wear after the completion of sample preparation (pre-machining, washing of samples) before the start of the experiments, the samples were weighed.The effect of temperature on fretting wear was assessed by comparison with experimental data obtained at room temperature (Figure 4).The test sample was installed in the holders, when testing at elevated temperatures heating was turned on, and when the temperature reached 200 °C the sample was loaded with a given force and the vibrator was turned on at a frequency of 50 Hz with an amplitude of 3 mm.After 30 min, the bench was switched off and the sample was weighed.The effect of temperature and load on sample wear is shown in Figure 5. Experiments have shown that the antifriction coating obtained by the treatment of an aluminum alloy in selenium vapor has low resistance to fretting wear.At low loads (69 N) on a steel ball, fretting wear is 7 times greater than without coating.Under load (115 N), the coating is pressed through and the fretting wear reaches the values of untreated aluminum samples.

Conclusions
1.An analysis of literature data on emergencies associated with fretting wear of friction units in spacecraft was carried out.2. The effect of fretting on the durability of high-temperature antifriction coatings of aluminum alloys obtained by treatment in selenium vapors has been studied.3. Improvement of the hardening technology during processing in selenium vapors in relation to the production of antifriction coatings of aluminum alloys has been completed.4. Works to modernize the testing bench in terms of improving the sample heating system and the loading device were carried out. 5.It has been experimentally established that the anti-friction coating has a low resistance to vibrations and its use in space technology requires protection from vibrations at all stages of launching a spacecraft into a target orbit.

E3SFig. 1 .
Fig. 1.Testing bench for fretting wear testsTest samples are made in the form of plates 70x25x8 mm in size (Figure2).

Table 2 .
Hardness of aluminum alloys samples.