Progresses on cryo-tribology: lubrication mechanisms, detection methods and applications

Tribology at cryogenic temperatures has attracted much attention since the 1950s with the acceleration of its applications in high-tech equipment such as cryogenic wind tunnels, liquid fuel rockets, space infrared telescopes, superconducting devices, and planetary exploration, which require solid lubrication for moving parts at low temperatures down to 4 K in cryogenic liquid, gaseous, or vacuum environments. Herein, the research progress regarding cryo-tribology is reviewed. The tribological properties and mechanisms of solid lubricants listed as carbon materials, molybdenum disulfide, polymers, and polymer-based composites with decreasing temperature are summarized. The friction coefficient increases with decreasing temperature induced by thermally activated processes. The mechanism of transfer film formation should be considered as a significant way to enhance the tribological properties of solid lubricants. In addition, applications of solid lubrication on moving parts under cryogenic conditions, such as spherical plain bearings and roller bearings, are introduced. The technology for tribological testing of materials and bearings at cryogenic temperatures is summarized, where the environmental control, motion and loading realization, as well as friction and wear measurement together in a low-temperature environment, result in the difficulties and challenges of the low-temperature tribotester. In particular, novel technologies and tribotesters have been developed for tribotests and tribological studies of solid lubricants, spherical plain bearings, and roller bearings, overcoming limitations regarding cooling in vacuum and resolution of friction measurement, among others, and concentrating on in-situ observation of friction interface. These not only promote a deep understanding of friction and wear mechanism at low temperatures, but also provide insights into the performance of moving parts or components in cryogenic applications.


Introduction
With the development and innovation of high-tech fields, including the military [3], aviation [4,5], aerospace [6][7][8][9], and superconducting and energy industries [2,10,11], demand is increasing for the operation of more advanced equipment at low (<273.15 K) and cryogenic (<120 K) temperatures, e.g. in cryogenic wind tunnels, space infrared telescopes, and cryogenic liquid rockets, as shown in figure 1. In the field of aerospace, researchers from the National Aeronautics and Space Administration (NASA) have concluded that many mechanical failures that occurred on spacecraft are caused by tribological problems [12]. In particular, lubrication under low-temperature conditions for key moving parts, such as plain bearings, roller bearings, sliders, and ball screws, is vital for the safe and reliable operation of advanced equipment. However, common liquid lubricants and greases are ineffective at temperatures lower than approximately 150 K owing to solidification or high viscosity, and the only choice for the lubrication of these moving parts is solid lubrication [13]. This mainly includes polymers or polymer-based composites and coatings, for example polytetrafluoroethylene (PTFE), inorganic materials and coatings such as molybdenum disulfide (MoS 2 ), graphite, diamond-like carbon (DLC) film, and soft metal. Therefore, study of the low-temperature tribology of solid materials and lubricants, as well as the invention of novel solid lubricants for low-temperature applications, are particularly important for the fields mentioned above.
Under the motivation of application demands and researchers' curiosity about the in-depth mechanism of temperaturedependent friction and wear, theoretical research on tribology at low temperatures and solid lubricants for cryogenic applications have been initiated since the 1950s, as depicted in figure 1. After proposing the adhesion theory of friction, Bowden, Tabor, and his colleagues at the University of Cambridge [14,15] examined the applicability of their theory on plastics and metals at low temperatures, and the results showed that the mechanical properties, particularly shear strength, indentation hardness, and yield pressure, determine the friction behavior of the material at low temperatures. Furthermore, they studied the distinction of material transfer and friction between moderate and low temperatures [16], and the relatively large fragment transfer of PTFE accounts for high friction at low temperatures. In the 1960s, Bartenev and El'kin [17] from the Lenin Moscow State Pedagogical Institute established the relationship between mechanical loss and friction force from 73 to 423 K for rubberlike polymeric material, which exhibited nonmonotonic variation with temperature. Since the 1960s, owing to the demands of bearings and seals in rocket engines operating directly in cryogenic propellants, such as liquid hydrogen and oxygen, NASA [9,[18][19][20][21] have invented and investigated many polymer-based composite materials and soft metal coatings for cryogenic tribological usage; among these, the laminated-glass cloth with the PTFE binder exhibited the best wear resistance [9], and the transfer mechanism of self-lubricating material for the roller bearing was examined [18]. From the 1980s to the end of the 20th century, Michael, Rabinowicz, and Iwasa [22,23] of the Massachusetts Institute of Technology (MIT) in the U.S.A combined the theory of adhesion friction and material transfer by studying the tribological performance of polymers and polymeric composites with fillers of solid lubricants and fabrics or particles at 4.2, 77, and 293 K. Their results indicated that the difference in transfer film formation between polymers and polymeric composites leads to dissimilarity in the friction mechanism. It is worth mentioning that these studies and other investigations on coating materials such as DLC [24] and MoS 2 [11] at cryogenic temperatures by researchers at Iwate University in Japan [10,11,25] and B. Verkin Institute for Low Temperature Physics and Engineering in Ukraine [24,26,27] were promoted by demands on cryogenic superconducting devices in this period.
From the 2000s to the present, the multiplex applications of low-temperature lubrication in rocket engines, cryogenic infrared optical mechanisms, liquid fuel transportation, cryogenic wide tunnels, and superconducting devices, among others, have generated increased interest in tribology studies at low temperatures. Gradt and Theiler et al of the Federal Institute for Materials Research and Testing (BAM) in Germany conducted a series of tribological studies on solid lubricants, including polymer and polymeric composite [28][29][30][31][32][33][34][35][36], DLC [37], and MoS 2 [38], and summarized the relationship between the mechanical properties and tribological performance of polymeric materials [30], as well as noted the effect of tribochemistry on the formation of transfer films at low temperatures [36]. Sawyer and Burris et al of the University of Florida [39][40][41][42][43][44][45][46] and Burton et al of the University of California in the U.S.A [47][48][49] studied the thermal activation phenomenon of PTFE bulk material [39,40,42,[45][46][47], MoS 2 [43,44,46,49], and DLC [48] coatings under decreasing temperature, and discussed the relationship between the tribological properties and relaxation transition of the polymer, wear and thermal process of the coating. Furthermore, our research clarified the influence of low temperature on the transfer film formation of polymeric materials under macro [5,50] and micro [51,52] dimensions. Researchers of aerospace agencies in Japan [53][54][55][56], Ecole Centrale de Lyon and the Université de Lyon in France [57,58], the University of Alaska Fairbanks in the U.S.A [59,60], Harbin Institute of Technology [3,6,[61][62][63][64][65][66], Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences [67][68][69][70] have contributed to the development of tribology theory at low temperatures and the expansion of solid lubricants for low-temperature applications. Overall, theoretical tribology studies at low temperatures have gradually concentrated on atomic and molecular scales with coupling of mechanical, physical, and chemical processes, compared with macro and asperity scales previously, which mainly focus on mechanical and physical processes.
Although technically important and extensively studied, the influence of low temperature on the friction and wear of solid lubricants is complex, and the existing results from the literature are sometimes different and even contradictory because History and progress on tribological research considering solid lubrication at low temperatures, motivated by its applications in military and aviation (the image marked with 'a' was adapted from www.nasa.gov/centers/dryden/multimedia/imagegallery/F-104/EC88-0029-04.html, NASA), cryogenic rockets (the image marked with 'b' was adapted from www.nasa.gov/feature/apollo-4-was-first-everlaunch-from-nasas-kennedy-space-center, NASA), space stations and satellites (the image marked with 'c' was adapted from www.nasa. gov/audience/forstudents/k-4/more_to_explore/International-Space-Station.html, NASA), cryogenic wind tunnels (the image marked with 'd' was reproduced from [1], US Government/Springer Nature, with permission from SNCSC), superconducting and fusion (image marked with 'e' was reproduced from [2], © 2006 IAEA, Vienna), space infrared telescopes (image marked with 'f' was adapted from www.nasa. gov/feature/jpl/webb-telescope-s-coldest-instrument-reaches-operating-temperature, NASA GSFC/CIL/Adriana Manrique Gutierrez), and planetary exploration (image marked with 'g' was adapted from https://mars.nasa.gov/msl/home/, NASA of the different test equipment, methods, environment, and materials. The effect of low temperature on the friction and wear of solid lubricants is not yet fully understood. Therefore, it is urgent to summarize and clarify the existing research results to provide a reference for subsequent research.
The accomplishments achieved in tribological research at low temperatures accompanied the development of tribotest technologies under extreme environments of low temperatures, which brought technical difficulties to tribotests, including low-temperature environment control, friction and wear measurement under low temperature and wide temperature range, as well as motion and loading simulation under extreme temperatures, among others. Most of the lowtemperature tribotests mentioned above were conducted using homemade or customized and modified tribotesters because of the complexity in simulating the extreme conditions of tribological tests. Summarizing and sorting out the existing tribological test technologies and inventing new methods and equipment for tribological studies at low temperatures are vital to promote the further development of cryogenic solid lubricants, as well as to reveal the nature of temperaturedependent friction. Meanwhile, tribological research at the application level, regarding plain bearings [2,5,[71][72][73] or roller bearings [6,9,[74][75][76], for example, is equally important compared with theoretical research on solid lubricants at low temperatures, to clarify the relationship between lubricants, structural thermal deformation, and internal stress under real working conditions for complex tribological components applied in cryogenic devices. The progress of these studies is also inseparable from test technology and equipment to evaluate and verify the performance of tribological components such as bearings under service conditions, which is vital for realizing the application of solid lubricants at low temperatures.
This study reviews the literature related to tribological research at low temperatures since the 1950s. First, the existing circumstances of tribotest technologies at low temperatures are summarized. Second, the progress in tribological research on solid lubricants is detailed considering the types of materials and mechanism of friction. The third part introduces several application cases and testing technologies for solid-lubricated bearings at low temperatures. Novel tribotest technologies and equipment for low-temperature tribotests are described. The problems to be solved in the present research are highlighted, and the prospects for future research directions are described.

Low-temperature environment for tribological tests
Tribological tests of solid lubricants at low temperatures are usually carried out in cryogenic liquid [8,10,11,19,24,28,77], low-temperature gas [5,39,46,58,[78][79][80][81], vacuum [34, 35, 44, 47-50, 55, 57] environments according to the application environment and temperature, and the tribological performance can be affected by the test medium physically and chemically. For example, boiling of cryogenic liquids at the friction interface because of friction heat produced during tests leads to a significant decrease in the cooling ability of cryogenic liquids due to the formation of a gaseous film with lower thermal conductivity, which is particularly serious in a liquid helium environment; thus, the effect of low temperatures was more clearly detected at low sliding speeds [28,30]. In addition, the hydrodynamic effect of cryogenic liquids such as liquid nitrogen was discussed in a previous study, in which the effect was small because the viscosity of the cryogens is too low to establish a hydrodynamic film of sufficient thickness to separate the two surfaces [82], but the influence of the cryogenic fluid on heat dissipation and removal of wear debris should be considered [30]. However, the chemical interaction between the test material and active cryogenic liquids such as hydrogen and oxygen affects the friction properties either through chemical processes of reduction or oxidation [36]. In gaseous media, the chemical effect of the test media on lubrication is significant and dependent on the tribochemical activity of the lubricant. In addition, phase transitions (e.g. freezing, desublimation) of different gases (e.g. water, oxygen, and nitrogen) on the contact region at low temperatures should be noticed, which could cause changes in media content or transformation of the friction interface owing to the existence of freezing gas [40,42,58,79,83]. Therefore, repeated replacement of the required test gas and content monitoring is necessary to ensure the consistency of the gaseous environment at low temperatures. Furthermore, various physical properties of the gas medium, such as thermal conductivity, affect the tribological properties of materials at low temperatures [31]. Vacuum environments, particularly ultrahigh vacuum (UHV) environments, can eliminate the interference of environmental media with the tribotest results to a great extent; however, high-speed tribotests may be inaccurate because of the serious temperature rise caused by high friction heat and the lack of a heat transfer medium. To obtain the actual temperature of the friction interface in a vacuum environment, the heat conduction model of the contact area can be established as a static heat transfer problem under low-speed conditions, and the rise of temperature at the contact interface mainly depends on the thermal conductivity of the two rubbing objects in vacuum [49]. Most notably, the outgassing of materials in vacuum also affects their tribological performance, and the influence of different vacuum degrees as well as adsorbed gases on the contact regions should not be ignored at decreased temperatures [84].
For tribotests in cryogenic liquids, the test temperature was considered to be its boiling point, which can be controlled to change continuously for tests in gaseous or vacuum environments. There are two methods for tribological tests at various low temperatures in summary, namely 'varied temperature tests' [39,46,47] and 'independent temperature tests' [35,36,83]. The former was conducted at different temperatures with the same sample and wear tracks, while the latter was conducted with independent samples or tracks. The run-in process is commonly performed to reach a stable state of friction before 'varied temperature tests' [85] because of the running-in phenomenon (coefficients of friction (COFs) evolved with test time or distance) caused by the unstable friction interface at the beginning of friction for some materials. Subsequently, the temperature is controlled to the required value, and tribological tests can be conducted under constant or varied temperatures, even circulating variable temperatures; thus, considerable time and refrigerant can be saved. Generally, the results of 'varied temperature tests' and 'independent temperature tests' are approximately the same for the same material, but there are exceptions [32,33]. Table 1 compares the configurations and specifications of typical low-temperature tribotesters from published literature, including contact forms of pin-on-disk or ball-on-disk with rotation or reciprocating motion, while figure 2 details the design of some tribotesters. In conclusion, low-temperature environment control, motion and loading, and measurement of friction and wear at low temperatures are key technologies for the design and construction of low-temperature tribotesters.

Tribological testing technology at low temperatures
For the realization of low temperatures (i.e. as low as 4.2 K (liquid helium)), as shown in figure 3(a), cryogenic liquids (liquid nitrogen of 77 K, liquid hydrogen of 20 K, liquid helium of 4.2 K, liquid oxygen of 90 K, among others) can be directly used as the refrigerant by immersing the friction pair, which is easy to implement with low cost and high cooling power. In addition, low-temperature gas can be used as the cooling medium, and continuous temperature control can be realized by mixing warm and cold gas or intermittent injection of cryogenic liquids [39,46,79]. Nitrogen or helium media are generally used, as illustrated in table 1. Iwasa et al [25] performed a fretting friction and wear test in a helium environment, considering the wide temperature range of 4.2-293 K, using the evaporation process of liquid helium, as depicted in figure 2(a). It is much more difficult to realize a low temperature in a vacuum environment because of the lack of a heat-transfer medium, and only heat conduction and radiation can be achieved, as shown in figure 3(a). For reciprocating tests, cooling of the moving sample can be achieved through both conduction and radiation. The efficacy of direct cooling by cryogenic liquid is the highest, whereas the supply of cryogenic liquid can be realized through a flexible pipeline (bellows) for reciprocating tests to solve the dynamic seal problem of cryogenic liquid in vacuum. For example, the low-temperature and vacuum reciprocating tribometer developed by researchers at Ecole Centrale de Lyon in France [57] used liquid nitrogen to cool the moving sample in vacuum directly, and the minimum temperature reached 81 K, as shown in figure 2(b). Similarly, flexible thermal straps for heat conduction from the cold source of the cryogenic liquid container or cryogenerator have also been used in reciprocating tribotesters, but with low cooling power, taking an example of the reciprocating tribometer developed by the University of Florida [44] with the minimum temperature of 173 K, as shown in figure 2(c). For tribological tests in the form of rotation, a thermal shield filled with cryogenic liquid for heat radiation is mostly used for the cooling of moving samples [50] because it is difficult to transport cryogenic liquid to the rotating shaft or connect the rotating shaft and cold source in good contact for heat conduction, and radiation heating is usually used to control the test temperature. However, the cooling power of radiation is very low, and it is difficult to perform tribological tests with large loads and high speeds. The ultra-low-temperature vacuum tribometer developed by the University of California [47] adopted multistage radiation refrigeration with small heat leakage to realize the temperature range of 4-450 K in vacuum, allowing tribotests to be conducted with low load (mN level) and low friction heat, as shown in figure 2(d). In recent years, a special feed through was developed to allow cold gas, power, and temperature data to pass through a rotating interface by Dynamic Sealing Technologies, Inc. (DSTI), which was applied to a high-vacuum low-temperature tribometer constructed by The Aerospace Corporation [86] to realize rotating tribological tests at temperatures as low as 100 K in high vacuum.
Compared with conventional tribometers, the problems of sealing and motion feeding to the low-temperature environment need to be considered, particularly in vacuum, when some motion mechanisms (such as driving motors) are placed outside of the low-temperature environment to solve problems such as thermal deformation, thermal gradient, thermal insulation, lubrication, and heat dissipation when mechanisms are placed in a low-temperature or vacuum environment. There are two loading methods, including weight and a combination of the elastic structure and liner stage, as shown in figure 3(b). Loading through the weight is simple without the requirement of load control; however, it is difficult to apply a heavy load, and adjustment of the load is inconvenient. The load is closed-loop controlled over a wide range by controlling the deformation of the elastic structure by the liner stage, according to the measurement results of the load cell when using the elastic structure and liner stage configuration.
Measurement of the COF with high precision and resolution is the most difficult and vital part of low-temperature tribometers. In addition to decoupling problems of friction and load force similar to conventional tribometers, the influence of low and varied temperatures on two-dimensional force measurement should be considered, as the elastic structure stiffness of the force sensor changes with temperature, which causes large sensor drift and seriously affects the measurement accuracy [92][93][94]. Calibration of the force sensor at different temperatures can be conducted to eliminate the influence of various temperatures; however, the process is complex and cumbersome. Moreover, the repeatability and accuracy of the results are questionable because of the uncertain consistency and uniformity of the temperature field of the elastic structure. Therefore, it is necessary to place the force sensor in a stable ambient environment by feeding the force out of the low-temperature test environment and using the force-decoupling mechanism for measurement, as shown in figure 3(c). However, the sealing structure (particularly for a vacuum with a high airtightness requirement), through which the force to be measured is fed out, and the support bearing in the decoupling mechanism will inevitably bring force measurement errors, and hence calibration is needed. In addition, a novel method of COF measurement was raised by scholars in the University of California [47], who calculated the dynamic COF through measuring the acceleration of the slider using a high-speed camera when the slider slid on the slope in vacuum and cryogenic temperatures; and the static friction can be measured by changing the slope angle, as shown in figure 2(d).
The measurement of wear is also important, and is normally accompanied by measuring the mass or volume loss after the tribotests, as well as the linear wear during the friction process; however, the influence of thermal deformation should be noted for low-temperature tests. For materials with a low wear rate, a pin with a spherical counterface can be used for wear tests by measuring the diameter of wear scars after the experiment [48]. Furthermore, the in-situ observation and characterization of the morphology and composition on the friction surface at the same time as the tribological test is highly valuable for studying the evolution process of the friction interface at low temperatures, as well as to avoid the physical or chemical change of the friction surface during departure from the low-temperature environment. For example, in-situ x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were applied to a tribotester developed by Ecole Centrale de Lyon, France [57], as shown in figure 2(b).

Novel configuration of tribotester for low-temperature tribological tests of material
Regarding the above technical problems, we proposed a measurement method for the COF with high resolution under extreme conditions, such as a low and wide temperature range in high vacuum [91], which solves the problems of cooling efficiency of rotating samples in a vacuum environment, narrow load range, and low resolution of friction measurement in the existing low-temperature tribometer, as shown in figure 3(d).
Accordingly, low-pressure helium sealing and convection cooling technology, temperature measurement, and control of the rotating shaft in vacuum were employed to realize the tribotest environment at temperatures from 60 to 450 K in a high vacuum of 10 −4 Pa. A cryogenic tribotester was designed for industrial demands to conduct rotation-mode tribological experiments. In contrast to most conventional reciprocatingmode tribotesters, cooling a rotating disk is extremely challenging. To obtain a test temperature as low as possible for the rotating tribotests, a highly efficient cooling method of heat convection through low-pressure helium gas of ∼100 Pa between the cold bearing block and rotating shaft with a disk sample was used, as shown in figure 3(d). More specifically, a GM cryocooler (Giffod/McMahon) was applied as the cold source, thermal straps were used to transfer heat from the bearing block of the rotating shaft, low-pressure helium gas was filled in the sealed space between the bearing block and rotating shaft to cool the shaft by heat convection, and the hollow shaft was designed to realize the measurement and control of temperature with a combination of conductive slip rings. We note that two extreme conditions can be achieved in this system: (1) vacuum of 10 −5 -10 −6 Pa and temperature of 170 K through heat radiation, and (2) temperature of 60 K and vacuum of 10 −4 Pa. For the latter case, the chamber will be first pumped to 10 −5 -10 −6 Pa, then the GM cryocooler will start to work, the cold head of which could be cooled to 20 K. Meanwhile, high-purity helium gas was introduced into the chamber between the cold bearing block and rotating shaft, and the disk sample was cooled to 60 K, but the vacuum will increase to 10 −4 Pa due to the leak of helium gas. It is considered that the pressure of 10 −4 Pa is a relatively poor vacuum normally, while the adsorbed active gas molecules, such as H 2 O and O 2 , in the contact region could significantly affect the friction and wear of the material, and the sticking of those gas molecules on surfaces will increase at low temperatures [95][96][97]. In contrast to the literature, the main content of trace gas molecules in our system was helium, which is inert and shows less effect on friction and wear. Most gas molecules might be absorbed in the coldest part of the system, which is the cold head of the GM cryocooler, as mentioned above. To evaluate the influence of trace gas molecules on the experiment, the gas partial pressures of the vacuum were measured using a residual gas analyzer, and the influence of adsorbed gas molecules on friction and wear was expected to be investigated via in-situ observations such as infrared spectroscopy. In the future, the UHV environment and other precise in-situ characterization devices, such as SEM/EDS and XPS, will be considered for the improvement of this system. Furthermore, to maintain the vacuum of the system and reduce the influence of gas molecule adsorption as much as possible, baking and cleaning of the vacuum chamber should be performed frequently.
Another novel technology is high-resolution measurement of the COF by adopting radial and axial magnetic bearings to decouple the load and friction force completely by floating the shaft without friction. Furthermore, a long elastic cantilever structure for heat isolation and loading, as well as a level structure (measurement arm and floating shaft) for force transfer, were designed, as shown in figure 3(d). The friction pin or ball was mounted at the end of the cantilever, which was subjected to a vertical load force and a friction force normal to the screen. The load and friction force were transferred and decoupled by the floating shaft structure as a load force, which can be measured by the load sensor, and the torque of friction, which can be measured by a high-resolution force sensor contacting the measurement arm or torque sensor connected to the floating shaft. It is worth mentioning that the force to be measured is transmitted to the end of the level by this design, which is far from the low-temperature test area, and a stable temperature for force measurement can be achieved. Using force or torque sensors with different ranges and elastic cantilevers with different stiffnesses, the load range of 1-500 N is realized, and the theoretical resolution of the friction force can reach 100 µN. In this system, the position of the pin or ball on the rotating disk (the friction radius of the pin/ball-on-disk test) can be adjusted by the X stage inside the vacuum chamber, and the test load can be controlled through the movement of the Z stage in the vertical direction. The system is also equipped with an optical, Raman, and Fourier transform infrared (FTIR) microscope to realize in-situ observation of the friction counterface online under extreme test conditions. Further, a sliding electrical contact measurement device was designed to measure the contact resistance using the four-wire method under extreme conditions, aiming to solve the problem of sliding electrical contact in space industries.

Tribological properties and friction and wear mechanism of solid lubricants at low temperatures
Solid lubricants include inorganic materials and coatings, as well as polymers and polymer-based composite materials/ coatings. Table 2 lists the tribological properties of solid lubricants with decreasing temperature from the previous literature, which are discussed in detail in sections 3.1 and 3.2. Meanwhile, the mechanisms of temperature-dependent friction are summarized in section 3.3, namely physical and mechanical effects, transfer film and tribochemistry, thermally activated process, relaxation and transition of the polymer.

Inorganic materials and coatings
3.1.1. Carbon materials. Graphite is a typical solid lubricant with low interlayer shear strength owing to the layer-lattice structure of carbon, and is believed to facilitate the interlamellar shearing of graphite crystals in the presence of water vapor and oxygen, while exhibiting high friction in vacuum or dry environments [101], which should be noted when conducting low-temperature experiments. The temperature dependence of the COF for graphite was investigated by Iwasa et al [25], and the results showed a peak COF value within the temperature range of 100-200 K when sliding against graphite, diamond, and alumina. The hardness of the material was considered to contribute to this variation, but the deep mechanism is still unclear. Zhao et al [41] evaluated the kinetic friction between highly oriented pyrolytic graphite and a silicon nitride probe tip through atomic force microscopy in vacuum, and interpreted that the activated mechanism of energy dissipation during sliding accounted for the sharp increase in friction from 280 to 140 K.
Diamond and DLC films are promising solid lubricants for low friction, particularly DLC films [102] with superlubricity properties by hydrogen passivation and graphitization mechanisms. However, the friction and wear of DLC coatings are very sensitive to test media in which a reductive or inert environment is beneficial for decreasing the friction of DLC coatings and achieving super-low friction [103], while oxidation of the coating surface is disadvantageous for excellent tribological performance [104]. The influence of temperature on the friction of diamond and DLC films is related to grain size and hydrogen content [48]. The COF of microcrystalline diamond films was high and independent of temperature [25], while ultra-nanocrystalline films exhibited a significant increase in friction from 220 to 120 K owing to the inhibition of the thermally activated hydrogen transport process. Regarding DLC films, a high hydrogen content (25%) provided sufficient hydrogen passivation for the fresh surface exposed to wear; thus, low COFs of approximately 0.06 and 0.03 were achieved at 170 K and at room temperature, respectively, as shown in figure 4(a). However, the running-in process and wear performance of DLC films are still affected by the temperature [78]. Most hard DLC films (hydrogen-containing a-C: H, metal-doped Me-C: H) failed rapidly owing to the mismatch of the thermal expansion coefficient [83] and descending low-temperature toughness, while DLC (Si-C: H) with a silicon gradient-doped structure showed better lubrication performance at low temperatures [37]. In addition, the friction and wear of DLC films at low temperatures are also related to the surface roughness and counterface, and lower friction and wear were obtained when self-paired [24,60].

MoS 2 .
MoS 2 shows excellent lubrication performance owing to its lamellar hexagonal crystal structure with weak van der Waals forces between layers formed during the running process under high-stress conditions [46]. The tribological properties of molybdenum sulfide films are related to the test medium, which usually exhibits high friction and a short wear life under humid environmental conditions [38]. An obvious temperature dependence of friction was observed in the oxidizing environment of the MoS 1.6 coating; specifically, the friction increased and service life decreased with decreasing temperature, which is related to the inhibited flow of the third body circulating between the first bodies and reintroducing into the contact region, as well as increased detachment of particles owing to superficial tribological transformations, oxidation, cracking, and adhesion, while the temperature did not affect the friction value under nitrogen gas [58]. Several studies of pure MoS 2 coatings have also shown that the COF rarely changes or slightly increases with a decrease in temperature in inert or vacuum environments [11,46,69], which suggests that weak thermally activated interlayer sliding of MoS 2 occurs with an activation energy of approximately 1 kJ mol −1 below 273 K [46]. To strengthen the normally soft MoS 2 coating and improve the friction performance of MoS 2 in humid environments, metals and their compounds are typically doped [38,49,105]. However, strong thermally activated friction has been observed after doping (such as MoS 2 /Ti [43,49], MoS 2 /Cr [38], MoS 2 /Ni [43], MoS 2 /Sb 2 O 3 [43], MoS 2 /C/Sb 2 O 3 [43], MoS 2 /Au/Sb 2 O 3 [43,85]), as can be seen in figure 4(b); the value of the activation energy is related to the changes in the shear interface [85] and wear rate [43] caused by the blocking of dopants on the sliding interface. In addition, it is necessary to solve the problem of poor matching of thermal expansion between the coating and the substrate, which results in large internal stress and coating failure when the MoS 2 coating is used in ultra-low-temperature environments such as LHe and LH 2 [38].

Other inorganic materials.
Soft metals such as gold, silver, and lead have also been used as low-temperature solid lubricants [9]. For face-centered cubic metals such as gold, silver, copper, and nickel, there is a critical temperature above which the COF is independent of temperature; below this temperature, the COF decreases with decreasing temperature, and tearing occurs in the wear track [98]. A study [57] was also conducted for ice films, in which friction decreased when the temperature increased from 120 to 163 K in a high-vacuum environment, which may be related to a change in the ice surface chemistry; however, the mechanism remains a mystery. WS 2 coatings show decreasing friction at low temperatures up to 143 K with the COF of 0.005 in UHV (3 × 10 −7 Pa) from Iwaki's result [55]. However, the mechanism of this superlubricity at low temperatures is unclear, and thus is worthy of attention for tribology and its applications at low temperatures. Finally, little temperature dependence was found for most hard materials, such as steel and ceramics, and the effect of wear on friction is greater than that of temperature [11,47,61,106,107].

Polymers or polymer-based composite materials and coatings
3.2.1. PTFE. PTFE has been widely used as a solid lubricant because of the weak van der Waals and negative charge repulsion of fluorine atoms between spiral PTFE molecule chains [52]. The transfer film formed on the counterface is considered to result in excellent friction and wear performance of PTFE or PTFE-based composites by sliding oriented PTFE molecules at a moderate temperature [16,[108][109][110][111], while the transfer of relatively large fragments of PTFE was found at low temperatures and the friction increased [16]. The tribochemistry of the chelation reaction between radicals of the broken PTFE molecule chain and the metal counterface accounted for the formation of a transfer film in humid and oxidized environments [52,112,113], and the influence of low temperature on this process requires further investigation. Additional studies on PTFE revealed nonmonotonic variations in friction and wear with temperature, which are closely related to the relaxation and transition of PTFE molecules at T α around 390 K, T β around 293 K, and T γ around 170 K [39,40,46,70]. It is assumed that the internal deformation of PTFE at temperatures below T γ takes place within amorphous regions with relatively athermal performance, whereas deformation around T β occurs primarily in crystalline regions with strong thermal activation [46]. Thus, the COF of PTFE increases with decreasing temperature until approximately 200 K, and the COF of PTFE shows a downward tendency with a further decrease in temperature owing to the enhanced mechanical properties [39,40,46,70], as shown in figure 4(c). The maximum value of the wear rate at 273 K is related to the quality of the transfer film as well as the enhanced mechanical properties of PTFE [30,42,70]. In addition, the friction and wear of polymer blends with main PTFE composition, such as PTFE/polyetheretherketone (PEEK), PTFE/epoxy and other PTFEbased composites, is similar with pure PTFE [39,42,46].

Other polymers.
Polychlorotrifluoroethylene (PCTFE) [23,77], ultra-high molecular weight polyethylene (UHMWPE) [22,23,99], polypropylene [23], polymethylmethacrylate [23], polyimide (PI) [10,36,70,81], and PEEK [36,70] have also been considered for lowtemperature lubrication. The increasing modulus and hardness at low temperatures, which are related to the relaxation and transition of polymers, slightly improve the friction and wear performance of most polymers owing to the lack of transfer film formation at both moderate and low temperatures [10,23,70,78], such as PCTFE [23], PI [78], PEEK [36,70], and epoxy [10], as shown in figure 4(d). Meanwhile, friction and wear follow the adhesive wear mechanism with a third-power relation, which is generally much better at cryogenic temperatures owing to viscoelastic-plastic deformation at moderate temperatures [23]. Deteriorated brittleness of polymers at low temperatures, which is related to the molecular structure of the polymer [83,88], may cause severe abrasive wear and high friction, taking the example of UHMWPE [99]. In addition, the tribochemical activity and transfer film formation of the polymer in an active media environment were influenced by the low temperature; further, the tribological properties were affected. For example, in PI, the friction and wear properties deteriorate in liquid hydrogen owing to the inhibition of the tribochemical reaction; thus, there is a lack of uniform transfer film compared with that in hydrogen in an ambient environment [36], as shown in figure 4(e). However, some studies have shown that tribochemical reactions also occur at low temperatures owing to the high contact stress [50] and friction heat [77].

Polymer-based composites and coatings.
Solid lubricants such as PTFE, MoS 2 , and graphite, and fillers such as carbon fibers, SiO 2 , and Al 2 O 3 , were considered to improve the mechanical and tribological performance of polymers at low temperatures. The friction in most composites is mainly affected by the polymer matrix [29]. This is similar to pure polymers in that the improvement of the mechanical properties of the polymer matrix composite at low temperatures [4,67] leads to a reduction in friction and wear [22, 28-31, 62, 63, 79, 100] when there is no obvious transfer film formation, both at moderate and low temperatures. Studies have shown that PEEK-based composites exhibit better tribological properties than PTFE-based composites [30,64]. The type of filler affects the tribological performance of composites at low temperatures [114]. Some polymers filled with fibers and particles usually have high friction or wear at low temperatures owing to the inhibition of transfer film formation [23] and increasing interfacial adhesion of fresh metal surfaces exposed by abrasive wear of filled particles or hard fragments of worn fibers [11]. The formation of a transfer film is crucial to the change in mechanism from the adhesion theory of friction for a polymer filled with a solid lubricant, and a weak correlation between friction and the wear coefficient was found [11]. However, the effect of low temperature on the formation of the transfer film remains controversial. The friction and wear properties deteriorate at low temperatures for graphite-filled polymers, which accounts for the failure to form a uniform transfer film by inhibition of the tribochemical reaction at low temperatures [36]. In contrast, a thin and uniform transfer film was formed for the PEEK-based composites filled with MoS 2 , PTFE, and carbon fiber after tribotests at low temperatures in a vacuum environment; thus, excellent friction properties were obtained [32][33][34][35]50], as shown in figure 4(f). Similarly, aromatic thermosetting polyester (ATSP) coatings showed an obvious run-in process with a sharp decrease in the COF at the low temperature of 113 K under the high load of 10 N, and a distinctive transfer layer formed on the steel ball; however, the mechanism is still unclear [80]. According to a previous study on carbon-fiber-filled PEEK composites, dry nitrogen gas and low-temperature environments both accounted for low friction and wear compared to humid air environments [79]. In the case of polymer-based composite coatings filled with MoS 2 , the COF at low temperatures is greater than that at room temperature, whereas the wear life is related to the binder material [27,69].

Physical and mechanical effects.
The adhesion theory of friction was originally applied by Bowden and Tabor to metals [115] and plastics [14], which focuses on the shearing of junctions formed by strong adhesion at the actual regions of contact. The adhesion component of the COF µ a can be expressed as: where τ s denotes the shear strength of the junction, N denotes the normal load, and A r denotes the real contact area. According to the models of contact mechanics [30,116], the real contact area A r is proportional to the elastic modulus E for elastic contact, as shown in equation (2): For plastic materials, the real contact area A r can be expressed as: where H is the hardness and p y is the yield pressure. With a decrease in temperature, the elastic modulus (E), hardness (H), and yield pressure (p y ) increase for most solid lubricants; thus, a small real contact area and low value of the adhesion component of the COF are obtained, as shown in figure 5(a), which is accepted by most scholars. When considering the deformation part of the COF (µ d ), which can be expressed as equation (4) [117], the same conclusion can be drawn: Here, ξ depends on the characteristics of the surface, tan δ is the mechanical loss factor, and p is the average normal stress. However, the brittleness of the material increases at low temperatures, and deteriorated brittleness causes severe wear and high friction [83,88], as well as coating fracture failure.

Transfer film and tribochemistry.
Transfer films play a very important role in the low friction and wear performance of solid lubricants, particularly for polymers and polymerbased composites [16,108,[118][119][120][121]. Many studies have shown that the formation of a transfer film produces low shear between the layered structure of solid lubricants such as PTFE [16,120], MoS 2 [51], and graphite [102,104]; thus, excellent self-lubricating properties were obtained. However, the effect of low temperature on transfer film formation is controversial, as shown in figure 5(b). The formation of a transfer film is strongly related to tribochemical reactions such as chain section, chelating reaction between carboxylic group and metal substrate, as well as structure orientation, among others [112,113,[122][123][124][125][126]. Transfer films strongly adhere to the counterface and have low shearing force. However, these films are usually inhibited at low temperatures and affected by the change in test media at decreased temperature, which may be reactants of tribochemical reactions. Taking examples of pure PI as well as graphite-filled PEEK and PI [36], high friction occurred when sliding in liquid hydrogen because the inhibition of the tribochemical reaction further deteriorated the transfer film. However, tribochemical reactions can also be activated at low temperatures owing to friction heating [77] and high local stress [35]. Thus, the transfer film exhibited good tribological performance at low temperatures. Studies on ATSP-based coatings [80] and PEEK/PTFE hybrid composites filled with MoS 2 , carbon fiber, graphite, among others [50], acquired superior friction properties under high normal stress and low-temperature conditions due to the formation of a thin and uniform transfer film. These results suggest that efficient lubrication at low temperatures can be achieved by controlling the transfer film formation behavior; however, sufficient understanding of the mechanism is still lacking.

Thermally activated process.
Friction is a complex physical, mechanical, and chemical process in which molecular and atomic motions [41,48,127] and the breakage and formation of chemical bonds [36,104,128] are affected by thermal activation. Recently, studies on PTFE [40,42,46], MoS 2 [43,46,49,127], and graphite [41] have revealed the phenomenon of thermally activated friction, and the relationship between the COF and temperature can be described by the Arrhenius function, as shown in equation (5): Here, T 0 is the reference temperature (generally room temperature), R is the universal gas constant, T is the temperature, µ (T) is the COF at T, and Ea is the activation energy, which is approximately 3.7-10 kJ mol −1 for PTFE [39,40,42,46,129], 10.8 kJ mol −1 for highly oriented pyrolytic graphite [41], 1 kJ mol −1 for pure MoS 2 [46], and 2.6-10 kJ mol −1 for doped MoS 2 [43,85]. The increased effective barriers for sliding and lower probability of crossing the energy barrier in the sliding interface during friction lead to increasing friction between solid lubricants when cooling further [40,50]. Meanwhile, the transition from thermal to athermal friction occurred at various temperatures owing to changes in the sliding interface, interfacial wear, or other mechanisms [46,127]. In addition, other thermally activated processes, such as hydrogen transport and passivation processes for DLC films [48] and tribochemical reactions during the formation of the transfer film [36], are responsible for increasing friction at low temperatures, which is a major obstacle to improving the lowtemperature tribological performance of solid lubricants, as shown in figure 5(c).

Relaxation and transition of polymer.
The tribological performance of polymeric materials at low temperatures is much more complex because of their diverse molecular structures and relaxations. With a decrease in temperature, the molecular motion of the polymer gradually froze below its glass transition temperature from molecular segments to side chains [101], which seriously affects the mechanical and tribological properties of the polymeric material. Meanwhile, the molecular mobility was obviously suppressed at low temperatures. Moreover, the molecular chains experienced extensive entanglement and became more rigid, which resulted in a less oriented interface and brittle-like shear interface, further increasing or nonmonotonic friction [45,52]. Macroscopically, the storage modulus and loss factor of the polymeric material varied significantly with decreasing temperature, which can be measured via dynamic mechanical analysis [70], as shown in figure 5(d). Generally, the peak of the loss factor with temperature is correlated to the relaxation and transition of polymeric materials, and the turning point of the COF usually appears at the relaxation and transition temperatures, such as PTFE [46] and ATSP-based coatings [80]. Furthermore, according to previous studies, the relaxation and transition of polymeric materials can lead to changes in the deformation and shear region inside the polymer, transition of thermally activated friction to athermally [46], transformation of the sliding interface of coatings [57], and friction mechanism [17], among others.  [46,48,52], respectively), and (d) relaxation and transition of polymer (data of curve reused from [46,70]).

Self-lubricating spherical plain bearing
A self-lubricating spherical plain bearing is a type of plain bearing with a spherical sliding surface for self-aligning and heavy-load working conditions. The bearing consists of an inner ring, outer ring, and self-lubricating liner made of fabric generally woven by PTFE fibers and reinforced fibers, such as Nomex [131] and Kevlar [132,133] fibers, most commonly, or composite material molded by injection [134,135], as shown in figure 6(a). The formation of a PTFE transfer film on the inner ring surface and lubricating film on the liner surface is vital to the low friction and wear of PTFE fabric liner lubricated spherical plain bearings at ambient temperature [136,137]. A typical application of a self-lubricating spherical plain bearing at low temperatures is the cryogenic wind tunnel, which is constructed to test the aerodynamic characteristics of larger aircraft at high Reynolds numbers by reducing the temperature of the nitrogen flow to 89 K. There are only two sets of large-scale cryogenic wind tunnels in service around the world, namely, the National Transonic Facility in the United States [145] and the European Transonic Wind Tunnel in Germany [130], with the third under construction in China [5,146]. A large number of self-lubricating spherical plain bearings have been used in cryogenic wind tunnels [5], including the flexible plate mechanism of the nozzle section, the model support mechanism of the test section, and the adjustable central flap mechanism of the second throat section. Taking the flexible plate mechanism as an example, the bending shape of the flexible plate was controlled by the motion of the actuators to adjust the test Mach number, and the spherical plain bearings connected the flexible plate and actuators; thus, an axial oscillation motion was produced for the bearing. Considering the large thermal deformation of the flexible plate, which is in direct contact with the cryogenic flow, compared with the small thermal deformation of the frame structure, a lateral oscillation occurred for the hinges during the operation of the wind tunnel. Therefore, the spherical plain bearing is very suitable for this working condition, as shown in figure 6(a). To ensure motion accuracy and prevent jamming of the mechanism, it is required that the spherical plain bearing has low and stable friction torque and reliable lubrication at cryogenic and wide temperature ranges.
However, with a decrease in temperature, the COF of the PTFE lubricated spherical plain bearing increased until the temperature reached approximately 173 K, after which the COF increased slowly and even decreased with decreasing temperature [5], which is similar to the temperature dependence of friction under material level tests for PTFE reported by Babuska [46]. The mechanism was illustrated as the friction was affected by thermally activated friction and improved mechanical properties of PTFE at low temperatures. Therefore, there is an urgent need to develop advanced solid lubricants and to investigate the mechanisms of friction at low temperatures.
To evaluate the tribological performance of self-lubricating spherical plain bearings at low temperatures, bearing tribotesters have been developed. Table 3 summarizes the reported low-temperature tribotester for spherical plain bearings, which includes the configuration of a single bearing and four bearing tests, as shown in figure 6(b). The temperature range of the existing single-bearing test configuration is narrow; otherwise, the accuracy of the friction torque will be significantly affected by the high friction of the support roller bearings at low temperatures; whereas, the friction of a single bearing is impossible to distinguish in the four-bearing test because the measured friction torque is the total torque of the four testing bearings. Herein, we propose a novel configuration of a lowtemperature bearing tribotester [141][142][143] to solve the problems of a narrow temperature range for a single bearing test through a long cantilever thermal insulated shaft to improve the accuracy of friction measurement by maintaining the temperature of the support bearing in a normal range, as illustrated in figure 6(c). Meanwhile, a mutually perpendicular three-axis with two-axis motion and one-axis loading structure was designed to simulate the actual service conditions of the bearing; high-power liquid nitrogen cooling and temperature control in both vacuum (up to 5 × 10 −5 Pa) and nitrogen gas (19-101 kPa) environments were realized to obtain a test temperature range from 100 to 450 K, and noncontact wear as well as in-situ clearance measurements were achieved through a laser interference displacement sensor over a wide temperature range to investigate the influence of low temperature on wear and bearing structure deformation.
In addition, a visualization of in-situ observations on the bearing sliding surface was proposed to deeply study the formation and evolution of transfer films at low temperatures [144,147], which is believed to be the key to the lubrication of self-lubricating spherical plain bearings. The in-situ observation was carried out innovatively using a semi-outer ring spherical plain bearing and a specialized long working distance FTIR microscope through the vacuum chamber, as shown in figure 6(d). Thus, the outer surface of the inner ring can be observed optically, and the transferred material can be distinguished online by the infrared spectrum during the friction and wear tests at low temperatures in a vacuum or nitrogen environment. Accordingly, inaccuracies such as oxidation caused by leaving the test environment of traditional offline characterization are avoided.

Solid-lubricated roller bearing
Solid-lubricated rolling bearings were designed for extreme work conditions, such as vacuum and low-temperature environments, owing to the failure of oil or grease lubrication, which is realized through solid lubricating coatings such as MoS 2 [148,149], graphite [150], and soft metals [7,74], on the grooves of the bearing and self-lubricating cage commonly made of PTFE-based polymeric composites to overcome the shortcomings of poor strength and thermal conductivity for pure PTFE, as shown in figure 7(a). A transfer film was formed on the surface of the rollers when sliding against the polymeric cage or coated grooves and then transferred to the inner and outer races, thus lubricating the bearings [9,18,151,152].
The turbopump of cryogenic liquid fuel rockets is a traditional application of solid-lubricated roller bearings, requiring highly reliable performance in liquid hydrogen (20 K) and liquid oxygen (90 K) with heavy load and high speed [20,75,153,154], as shown in figure 7(b). Research has shown that laminated-glass cloth with the PTFE binder exhibits the best wear resistance at low temperatures, and was chosen as the bearing cage material of Space Shuttle main engines [18,20]. In recent years, with the development of reusable rockets, higher requirements have been proposed for the turbopump bearing of cryogenic liquid fuel rockets [75].
The space infrared telescope is an important application of solid-lubricated rolling bearings at cryogenic temperatures in a vacuum environment, which was launched to observe and  study the origin and cosmogony of all kinds of celestial bodies such as planets, stars, and galaxies in the universe in the infrared band [155]. To effectively observe infrared targets in deep space and reduce noise during observation, it is necessary to cool the optical system of the telescope as much as possible [156][157][158][159][160][161]. The James Webb Space Telescope (JWST) of the U.S.A, launched in 2021, works at cryogenic temperatures below 50 K [155,162], representing the highest level of space infrared telescope technology, as shown in figure 7(b). Cryogenic actuators were used to control the position and angle of 18 separate segments, which were combined into the main mirror of the telescope. The reliability of these actuators directly determines the success of the mission, which depends on the performance of a specially designed solid-lubricated rolling bearing under cryogenic operating conditions [163][164][165]. In addition, wheel mechanisms are designed to meet various observation requirements and satisfy the demands of long service life, low driving power, high positioning accuracy, and withstanding serious vibration during launch, which are closely related to the performance of the support roller bearing [166]. For example, the filter wheel mechanism, MoS 2 /TiC, and gold coating-lubricated bearings were used in the JWST and Herschel telescope of the European Space Agency (ESA), respectively, in which the bearings should achieve lubrication below 30 K [7,[167][168][169][170]. In April 2022, the JWST successfully completed the preliminary adjustment, and the first photo was released in July. Solid-lubricated roller bearings have also attracted considerable attention in the field of planetary exploration owing to their satisfactory performance over a wide temperature range, as shown in figure 7(b). For example, the bearings in the mobility actuators of the Perseverance Mars rover were initially drylubricated as cold as 138 K without heating. However, the insufficient life of dry lubricated gears has led to the replacement of solid lubrication by grease lubrication and heating control above 203 K [177][178][179][180][181]. In future, deep-space exploration such as the satellites of Titan and Ganymede requires lubrication at cryogenic temperatures down to 73 K, and the best choice should be solid lubrication. Further studies and development are thus valuable.
To carry out the test and research of solid-lubricated bearings at cryogenic temperatures, NASA [18], the ESA [161], and other institutions [53,175,176,182] have built rollerbearing testers for simulating cryogenic environments, including cryogenic liquid and vacuum, as shown in table 4 and figures 7(c) and (d). The realization of the cryogenic process in a vacuum environment mainly depends on the outer ring cooling and inner ring insulation, such as the bearing testers introduced by ESA, depicted in figures 7(c) and (e). A few testers can realize the combined axial and radial loading, typically shown in figures 7(d) and (e), which was developed by the Korea Institute of Science and Technology. However, it is difficult to realize the high-precision measurement of ultralow COF for roller bearings at cryogenic temperatures because of the influence of friction on load bearings, as illustrated in figure 7(e). In this study, we designed a roller bearing tester to realize high-resolution measurement of the ultra-low COF (∼0.001) under an extreme environment of 100-450 K in high vacuum (10 −5 Pa) through radial and axial electromagnetic loading combined with the force measurement method using  [172], Copyright (2015), with permission from Elsevier, Reproduced from [7]. Image stated to be in the public domain and (b) its applications on Space Shuttle, James Webb Space Telescope, Mars Rover. Reproduced from [https://images. nasa.gov/details-51j-s-003]. Image stated to be in the public domain, Reproduced from [154]. Image stated to be in the public domain, Reproduced from [https://www.flickr.com/photos/nasawebbtelescope/51412207042]. Image stated to be in the public domain, Reproduced from [https://www.flickr.com/photos/nasawebbtelescope/13291410214/]. Image stated to be in the public domain, Reproduced from [173]. © 2015. The Astronomical Society of the Pacific. All rights reserved, Reproduced from [https://mars.nasa.gov/mars2020/spacecraft/rover/]. Image stated to be in the public domain, Reproduced from [174]. Image stated to be in the public domain. Roller bearing friction torque measurement devices introduced by (c) ESA (image reprinted from with permission from the author), Reproduced with permission from [161], and (d) Korea Institute of Science and Technology, Reprinted from [175,176], Copyright (2019), with permission from Elsevier, for the cryogenic application. (e) Existing configurations of roller bearing tester for low-temperature conditions and (f) a novel tester to realize the tribotest of solid-lubricated roller bearing with high accuracy measurement of COF at cryogenic temperatures in vacuum. Level for torque measurement a lever structure [184], as shown in figure 7(f). The shaft and bearing fixture were simultaneously cooled by liquid nitrogen, and the temperature was measured and controlled simultaneously to reduce the temperature difference between the inner and outer races of the bearing. The tester can provide an effective means for performance testing and high-precision measurement of the COF of a solid-lubricated roller bearing under extreme conditions in a cryogenic vacuum environment.

Other applications
In addition to bearings, solid lubrication at low temperatures has also been applied in moving parts such as guide rails [185], gears [161,186], and valves [187]. Sliding between MoS 2 coated support structure and superconducting coils caused by differences in thermal deformation in Tokamak [2] and stellarator [72,73] devices have caused demand for low friction at 4.2 K in vacuum to reduce the internal stress of the structure and avoid the annealing of the superconducting coil by friction heat. In contrast, requirements have also been proposed for wearresistant and high-friction materials used in ultrasonic motors [188], inchworm mechanism [189,190], and other piezoelectric driving mechanisms [191,192] for cryogenic usage. Thus, the low-temperature wear resistance and performance of ultrasonic motors can be improved using PTFE-based composites filled with glass fiber, MoS 2 , copper particles, nano-silica, and other material [65,66,188].

Summary
This review outlines the research progress on tribology at low temperatures, including low-temperature tribological test technology, properties and friction mechanisms of solid lubrication at decreasing temperatures, and the application status of solid lubrication in cryogenic environments.
First, the tribological test methods and technologies for materials at low temperatures were summarized and discussed. A vacuum is an ideal environment for low-temperature tribotests, but it is more difficult to realize environmental control, motion, and loading, as well as friction and wear measurement, compared with gaseous and cryogenic liquid environments. Novel technologies of low-pressure helium convection cooling technology in vacuum, high-resolution friction measurement technology by force decoupling of magnetic bearings, and force transfer of level structures were developed and adopted to construct a tribotester system to achieve tribotests in a high vacuum of 10 −4 Pa and the temperature range of 60-450 K, with the load range of 1-500 N and a theoretical resolution of 100 µN for friction force. In addition, in-situ observation and sliding electrical contact online measurements under extreme environments were achieved, aiming to promote a deeper understanding of friction, wear, and sliding electrical contact at low temperatures.
Second, studies on friction and wear properties and the mechanism of solid lubrication were described, including inorganic materials such as carbon materials, MoS 2 , and polymeric materials such as PTFE and PTFE-based composites.
Increased friction is commonly observed at low temperatures, particularly for polymers and polymer matrix composites, which are governed by the mechanisms of adhesive wear, thermal activation, and transfer film formation and are essentially affected by the relaxation and transition of polymer molecules. Based on previous studies, it is possible to improve the tribological performance of solid lubricants by promoting the formation of transfer films against thermal activation at low temperatures; however, this requires more in-depth studies.
Finally, solid lubrication addresses the lubrication problem in advanced cryogenic equipment such as cryogenic wind tunnels, cryogenic liquid fuel rockets, and space infrared telescopes, the most common of which is a solid-lubricated bearing; therefore, testing technology and testing of bearings at low temperatures are important. A spherical plain bearing tribotester and an ultra-low friction tester of a roller bearing for a low-temperature test were designed to realize in-situ observation of the bearing friction surface and measurement of the bearing clearance and high-precision measurement of the ultra-low COF of the rolling bearing, respectively, based on existing test technology. The construction of advanced low-temperature bearing testers is expected to provide effective tools for theoretical research, design, and performance optimization of solid lubricated bearings for future cryogenic applications.