Research on Improving the Tribological Properties of Bearings under Cryogenic Conditions

Bearings, as one of the most central components of rotating equipment, are widely used both in servo steering gear, aerospace and other fields. However, for extremely cryogenic conditions, the life of bearings is low, so it is necessary to carry out the tribological design of bearings to improve its cryogenic performance. In this paper, four kinds of optimized and improved plain bearings are designed. The tribological tests of these four bearings under extreme cryogenic working conditions were carried out using the developed bearing simulation test rig. The results show that under the same load and rotational speed conditions, the preparation of micro-weave structure on the surface of the shaft tile or the preparation of composite coating on the surface of the journal can improve the friction and wear performance of the bearings to a certain extent. When both methods are used at the same time, the friction reduction and anti-friction wear effect of the sliding bearing is better. This study not only provides theoretical guidance and technical support for improving the cryogenic tribological performance of bearings, but also has important practical significance for the future development of rotating equipment in the low-temperature field.


Introduction
With the progress of science and technology and the development of aerospace technology, cryogenic tribology is more and more widely used.Especially for rotating equipment, such as servo servos, rotating arms and other equipment.Among them, bearings, as one of the most critical components of rotating equipment, have gradually received more and more attention from researchers in the field of cryogenic research [1][2][3].
According to research, more than 40% of rotating machinery failures are caused by bearing failures [4][5][6].Especially in the extremely cryogenic working environment, bearings will experience serious wear and lead to failure after a short period of operation.Bearing failure not only causes serious harm to the safe operation of the whole equipment, but also consumes a large amount of time and economic costs, so it is necessary to improve the bearing life by improving the tribological performance of bearings under cryogenic operating conditions.
To improve the friction and wear performance of bearings under cryogenic conditions, many scholars have made many efforts.For example, NASA Lewis Research Center, the German Federal Institute for Materials Research and Testing has long been engaged in the tribological research of equipment materials in the field of space, such as the space drive mechanism, the lunar rover rotating parts bearings and so on [7][8][9].In terms of coating preparation, Lan et al. [10] from Texas A&M University also experimentally prepared an ATSP coating configured from two polymers.The results showed that the coating exhibited a good friction reduction and anti-wear effect due to the generation of a transfer layer with good lubricity on the surface of the composite coating when rubbed under cryogenic drying conditions.In addition, it is known from the bionic perspective that the friction resistance and lubrication effect of the material surface are closely related to the surface texture structure [11][12][13].Scientific studies have shown that the use of special processing technology to prepare a layer of micrometer-level texture structure on the material surface can effectively improve the friction and wear performance of the substrate surface.
Therefore, to improve the friction and wear performance of bearings under cryogenic conditions, four kinds of optimized and improved bearings are designed to further clarify the influence law of load and rotational speed on the friction and wear characteristics of these four kinds of bearings.Finally, the bearing type with the best tribological performance is optimized and its corresponding friction reduction and anti-wear mechanisms are revealed.

Materials
Four types of plain bearings are designed and machined for the shaft tile and journal components, and the bearing types are shown in table 1.As shown in table 1, the type 1 bearings have shingles and journals made of Invar 36 alloy without micro-texture and G95Cr18 steel without composite coating, respectively.For type 2 bearings, Invar 36 alloy with micro-texture is used for the bearing pads, and the type of micro-texture selected is groove-channel micro-texture, and the journals are made of G95Cr18 steel without composite coating.Type 3 bearings have shingles of Invar 36 alloy without micro-texture and journals of G95Cr18 steel with composite coating of 30% Ni60A/70% WC-12Co carbide composite coating.Type 4 bearings have Invar 36 alloy with microstructure and G95Cr18 steel journals with composite coating.Before the test, all bearing specimens were fully ground and polished on the surface of the axial tile and the surface of the journal.Then the surface roughness of the inner surface of the axial tile and the surface of the journal were measured using a three-dimensional profilometer (VK-X250K, KEYENCE).The measurement results are shown in table 2. The results show that the surface hardness and surface roughness of the bearing shingles and journals comply with the bearing design standards.

Test Process
To further study the friction and wear performance of the four types of tribologically designed bearings under cryogenic operating conditions, the developed bearing simulation test rig is used to carry out friction and wear tests on the four types of bearings under different load and speed conditions, and the specific test parameters are shown in table 3. The test equipment was carried out using a developed bearing simulation test platform [14].Before the test, the cryogenic circulation system switch was first turned on to pre-cool the test chamber for 30 min to provide a more stable cryogenic environment.Then the motor switch is turned on to start the bearing performance test.
Afterward, slices were taken from the worn parts of the bearing tiles and journals.The samples were then ultrasonically cleaned in 99.7% ethanol solution for 30 min, and after the surface of the specimens was completely dried, the surface of the wear marks was observed and analyzed by scanning electron microscope (SEM, Quanta 200F).In addition, FA1604 N electronic analytical balance (accuracy of 0.1 mg) was used to measure the weights of journals and shingles before and after wear to find out their wear amounts.

Tribological Experimental Results
Figure 1 (a) and figure 1 (b) show the time-varying friction coefficient curves of the friction surfaces of the four types of plain bearings under the load conditions of 100 N and 700 N, respectively.As can be seen from the figures, the friction coefficient curves of the above four types of bearings remain relatively stable under different load conditions.
Under the load condition of 100 N, the friction coefficients of type 2 and type 4 bearings are lower, around 0.052 and 0.049 respectively, and the amplitude of fluctuation of the friction curves is smaller.Although the fluctuation of the friction coefficient curve of type 3 bearings is smaller, the amplitude of the friction coefficient is larger, which is due to the friction of the interface of the friction pair mixed with wear debris during the operation of the bearings, resulting in the friction of the interface increases.The friction curves of type 1 bearings are more stable in the pre-wear stage, but the amplitude of the friction curves increases when the bearings are operated up to about 65 h.The larger fluctuation of the friction curves in the late stage of the bearing operation indicates that the wear of the bearings increases in this stage, which leads to the decrease of the stability of the friction curves.
When the load is increased to 700 N, the stability of the friction curve of type 1 bearings is seriously reduced, and the amplitude of the curve is larger, which is due to the fact that with the increase of load, the self-lubricating film becomes thinner under the effect of larger shear stress and Hertzian contact stress, resulting in a further increase in interfacial friction.The coefficient of friction of Type 2 bearings gradually increased after running to about 95 h.This was due to the increase of abrasive debris generated at the interface with the increase of load, and some of the abrasive debris overflowed from the micro-texture grooves leading to the increase of friction at the interface.However, under this condition, the friction coefficients of Type 3 and Type 4 bearings were lower, at about 0.51 and 0.41, respectively, due to the fact that the migration efficiency of the transfer film on the surface of the composite coating increased with the increase of load, resulting in the friction at the interface decreasing, and the friction coefficients of the Type 4 bearings were even lower, due to the fact that the micro-knitted structure on the inner surface of the axle shanks of the Type 4 bearings efficiently collected and transferred the air bubbles and abraded debris mixed in the interface. .In summary, it is found that type 2, type 3 and type 4 bearings have better friction reduction characteristics under LNG cryogenic medium conditions, among which, type 4 bearings have better friction reduction effect.

Wear Mechanism
Figure 2 shows the SEM morphology of the surface of the axial tile of the four types of bearings after running for 100 h under the conditions of 1500 r/min and 100 N, respectively.From figure 2 (a), it can be found that there are more abrasive chips and dimensional furrows on the wear surface of the axial shingles of type 1 bearings.And there is a large area of peeling layer, indicating that abrasive wear and fatigue wear occurred at the interface of the friction pair under this condition.From figure 2 (b), it can be found that the size and number of furrows on the wear surface of the type 2 bearing are significantly reduced, but the number of abrasive chips at the interface increases.This is because the journal material of Type 2 bearing is G95Cr18 steel, and the material surface is more prone to fatigue and spalling under cryogenic conditions and generates a higher number of abrasive chips.When the abrasive chips in the micro-texture grooves on the surface of the journal are overfilled and overflowed, the friction at the interface is further increased and more abrasive chips and spalling are produced.From figure 2 (c), it can be found that a small amount of abrasive chips, furrows and spalling layer exists on the wear surface of the type 3 bearing shaft tile.And a small amount of cracks appeared in some areas, this is because the journal surface of type 3 bearings is prepared with 30% Ni60A/70% WC-12Co composite coating, and a transfer layer is produced on the surface of the composite coating when the journal rubs against the surface of the axle tile under this condition, which effectively reduces the generation of interfacial abrasive chips, thus reducing the wear of the friction sub-surface.From figure 2 (d), it can be found that the wear degree of the surface of the axial tile of the type 4 bearing is the lowest, and there are only fewer peeling layers and slight furrows on the surface of the wear marks.This is due to the fact that on the basis of type 3 bearings, the groove-channel type microtexture structure on the inner surface of the axial shingles better collects and transfers the abrasive debris and air bubbles mixed at the interface, and also releases the heat generated at the interface, which effectively reduces the interfacial wear.

Conclusion
This paper carries out the friction and wear test of the four types of bearings under service conditions.Further clarified the load, speed on the four types of bearings friction and wear characteristics of the influence of the law.The bearing types with the best tribological performance are finally derived.And revealed its corresponding friction reduction and anti-wear mechanism, the specific conclusions are as follows: (1) Under the same load and rotational speed conditions, the preparation of groove-channel type micro-texture structure on the surface of the shaft tile or 30% Ni60A/70% WC-12Co composite coating on the surface of the journal can improve the friction and wear performance of the bearings to a certain extent.When the two methods are used simultaneously, the friction reduction and antifriction effect of the sliding bearings are better, indicating that the two methods play a better synergistic effect when applied simultaneously, which inhibits the occurrence of abrasive wear and fatigue wear on the surface of the friction pair, and effectively improves the stability and service life of the bearings.
(2) On the one hand, the micro-texture can effectively collect and transfer the abrasive debris mixed at the interface during the friction process.This method not only releases the heat accumulated in the interface region, but also improves the stability of the bearing and better inhibits the occurrence of abrasive wear on the surface of the friction vice.On the other hand, when the composite coating and journal surface sliding friction, the composite coating surface generated a transfer layer, resulting in a

Figure 1 .
The time-varying curves of friction coefficient of four kinds of plain bearings under 1500 r/min and different loads.

Figure 2 .
Figure 2. SEM morphologies of the four kinds of bearing journal surfaces at 1500 r/min and 100 N.The wear rates of the shingles and their paired journals of the four types of bearings under different load and speed conditions are shown in figure3.From figure3(a) and figure3(c), it can be found that the wear of type 1 bearing shingles and journals is at a high level under all four sets of load conditions and increases with the load.The wear amounts of type 4 bearing journal and bearing tile are the lowest under the four sets of load conditions, which is due to the better synergistic effect of the composite coating on the journal surface and the micro-weave structure on the inner surface of the bearing tile, both of which show better friction reduction and anti-friction effects, effectively reducing the bearing wear.From figure 3 (b) and figure3(d), it can be found that the wear of the type 4 bearing's axial tile and journal is still the lowest, and decreases with the increase of speed, which indicates that the type 4 bearing has a better anti-wear effect.

Figure 3 .
Figure 3.The wear loss of the four kinds of bearings under different load and speed conditions.

Table 1 .
Type of the plain bearing.

Table 2 .
Surface roughness of bearing bush and journal.

Table 3 .
Friction and wear test parameters.