Tribological properties of oil-impregnated polyimide in double-contact friction under micro-oil lubrication conditions

Oil-impregnated porous polyimide (iPPI) materials are usually used as retainer for bearings. In these bearings, balls and rings, balls and retainers are two different kinds of contact. In this paper, the friction and wear properties of iPPI were investigated using steel (disc)—steel (ball)—iPPI (pin) double-contact friction test rig for simulating the actual contact in bearings. The results show that compared with that of iPPI—steel single contact, the friction coefficient of iPPI—steel in double contacts is lower and decreases with the amount of additional oil. The surface of iPPI in single contact suffers more wear compared with that in double contacts. Different from single contact, the worn surfaces of iPPI in double contacts are blackened. The Raman spectra of worn surfaces of balls and discs indicate that α-Fe2O3 and Fe3O4 were formed during rubbing of the double contacts. Many nanoscale iron oxide particles are found on the worn surfaces of iPPI in double contacts; on the contrary, few particles could be found on the surface in single contact. In double-contact friction, the nanoscale wear debris penetrates inside the iPPI material through the process of extruding and recycling of oil, which is the mechanism of the blackening of the iPPI worn surfaces. The studies show that the double-contact friction method is a new and effective method to study the friction in bearings, especially for those with polymer retainer.


Introduction
Polyimide (PI) has attracted extensive attention in the field of aerospace, microelectronics, and tribology because of its excellent mechanical properties and stability [1][2][3]. PI can be made into porous PI (PPI) by cold pressing and hot sintering process methods. The micro pores in PPI material could be used to store lubricating oil [4][5][6]. Oil-impregnated PPI (iPPI) has excellent tribological properties and can keep a high oil content, after centrifugation [7][8][9], which has been widely used in the retainer of micro-oil lubricated bearings used in space [10][11][12][13].
Recently, researches are focused on the improvement of iPPI material. Shao et al. [14] designed a cobweb-like structural system for improving the oil-content and oil-retention of iPPI material. They found that the oil impregnated in iPPI can be released under stimuli and sucked back when removing stimuli. Zhang et al. [15] proposed a chemical method to improve silicon oil retention of iPPI material, and the results indicated that the modification could not only change the wettability of PI from hydrophilicity to hydrophobicity but also enhance the silicon oil retention from 52% to 87%. Lv et al. [16] prepared two kinds of lubricating composites to fill in PPI and studied the tribological properties before and after proton irradiation using a ball-on-disc tribometer. Ruan et al. [17] obtained a smart lubrication material by filling a supramolecular oleogel into macroporous PI, and the material exhibits both high oil storage and recyclable lubricant releasing/reabsorbing. Wang et al. [18] researched the effect of porosity on friction properties of PPI materials and found that higher porosity can store more oil but lead to higher contact pressures.
Bearing retainer is the most important application of iPPI material. Most of the aforementioned studies about friction and wear tests of iPPI material were carried on standard ball-on-disc or pin-on-disc friction test machine; however, the friction in bearings is more complicated [19][20][21][22]. To investigate the friction of bearing retainer, Russell et al. [23] proposed a novel test rig for replicating the position and dynamics of a ball and retainer of a bearing in operation while measuring the friction between the two bodies. This rig could provide insight into the mixture of oil and air inside the retainer pockets. However, in ball bearings, especially in these with polymer retainer, ball-ring and ball-retainer are two different kinds of contact pairs with disparate properties. The two kinds of contact affect each other during friction, and this effect is difficult to be carried out using standard friction test machines.
For iPPI retainer, the blackening of worn surface is commonly found in actual application, but this blackening worn surface is difficult to be reproduced using the standard single-contact friction test machine. Therefore, the wear of iPPI retainer in actual bearings needs to be further studied. The lack of wear blackening mechanism of iPPI retainer makes the improving direction of iPPI material unclear.
Herein, in order to simulate the friction in bearings, a double-contact friction test rig is made. The tribological properties of iPPI material in double contacts are studied for the first time in this paper. The double contacts consist of steel ball, steel disc (simulating outer and inner ring), and iPPI pin (simulating retainer). The steel ball is set between iPPI material and steel disc. The ball rotates and slides against both iPPI pin and steel disc during friction test. The interactions of the two contacts in friction coefficient and worn surface are investigated. The mechanism of blackening of the iPPI worn surfaces, commonly found in bearings, is revealed. This paper gives a better understanding of friction and wear properties of iPPI material used for bearing retainer and proposes a new double-contact friction test method for bearings.

Double-contact friction test rig
The double-contact friction test rig used in this paper was researched and developed by our research group, as shown in Fig. 1. The counterweight is used to make the load zero before the load weight applying. The force transducer is fixed between a rigid beam and a flexible beam, used to measure the frictional force of upper contact. The rigid beam and flexible beam or similar structure used to measure frictional force were commonly seen in some commercial friction test machines. A three-dimensional (3D) transducer www.Springer.com/journal/40544 | Friction was fixed on the lifting table, and the disc clamper was fixed on the 3D transducer. The 3D transducer was used to measure the load and frictional force of lower contact. The parameters of the force transducer and 3D transducer are shown in Table 1. Considering the systematic error of the double-contact machine and according to calibration results, the measuring error of friction force of upper contact is within ±0.015 N, and that of lower contact is within ±0.05 N. The steel ball was fixed on the end of a shaft, which connects the motor through an elastic coupling. When friction was tested with the double contacts, the ball rotated, and the pin and disc remained stationary. The double-contact details are shown in Fig. 1(b). During friction tests, the maximum temperatures of the two contacts were measured using a high-precision infrared camera (PI640, Optris). The temperature nephogram of the double contacts is shown in Fig. 1(c). Area 1 is the steel ball and iPPI pin contact, and area 2 is the steel ball and steel disc contact.

Materials
The PPI pins, balls, and discs used in this paper are shown in Figs. 2(a), 2(b), and 2(c), respectively. The PPI material was machined into pins with a diameter of 6 mm and a height of 7 mm.
The cross section of iPPI material was observed using a scanning electron microscope (SEM; SU5000, Hitachi), and the interwoven pores can be found, as shown in Fig. 2(d). Note that the cross section was obtained by punching a PPI rod into two parts. The surface of PPI pin after polishing was also observed, and a few pores can be found. The pore size and porosity of PPI material were examined by an automatic mercury porosimeter (AutoPore IV 9500, Micromeritics Instrument Corporation). The distributions of pore size and intrusion and extrusion processes are shown in Figs. 2(e) and 2(f), respectively. The median pore size of PPI is about 1.65 μm, and the porosity is 17.48%. The pins were polished by the sandpaper (7000#) to remove turning marks. After polishing and cleaning, the PPI pin was immersed in PAO4 oil (Durasyn 164, INEOS) and put in a vacuum oven at 80 °C for 20 h. Then the oil content, η, can be calculated using Eq. (1), which is about 9.9%. The iPPI pin was centrifuged using a centrifugal machine (with a speed of 3,000 r/min  and a centrifugal radius of 100 mm) for 1 h, and the weight of pin was measured every 10 min. The oil retention of iPPI pin, γ, can be obtained by Eq. (2), as shown in Fig. 2(g). After 1 h of centrifugation, the γ was still 92.5%.
where m 0 refers to the mass of the sample before oil-impregnated, m 1 refers to the mass of the sample after oil-impregnated, and m t is the mass of the oilimpregnated sample after centrifugation for t min.
The material of drilled balls and discs is GCr15 (American Iron and Steel Institute (AISI) 52100) steel. The diameter of balls is 19.05 mm. The diameter and height of discs are 10 and 3 mm, respectively. The parameters of the specimens are shown in Table 2.

Friction coefficients
In space bearings, besides the micro-oil impregnated in the iPPI retainer, little oil will also be supplied to maintain stable lubrication. In this paper, different amounts of additional oil were added on steel ball surfaces for double-contact friction. The amounts of additional oil are 0, 5, 10, and 20 μL, which are represented by DC-0 μL, DC-5 μL, DC-10 μL, and DC-20 μL, respectively. iPPI pin and steel ball single contact with no addition oil (single contact, SC-0 μL) were also tested for comparison. According to the work condition of bearings, the load of lower contact (steel ball and steel disc) was set as 30 N. The load of upper contact (steel ball and iPPI pin) was set as 19.8 N (2 kg). The contact radii, average contact pressures, and indentations of the two contacts were calculated using Hertzian contact, as shown in Table 3. Note that the contact pressure of retainer and ball in a bearing may be smaller than the load used. In order to accelerate the wear of iPPI material, a larger load was applied in this study. The friction coefficient and temperature curves of both single contact and double contacts are shown in Fig. 3. The average values of friction coefficient and temperature are shown in Fig. 4. From the results, it can be found that the friction coefficient of iPPI-steel single contact is the highest, while the temperature is lower than that of iPPI-steel in double contacts under the same lubricating condition (with no additional oil). Temperature rising could make the oil be squeezed out more easily, which may be one of the reasons for the higher friction coefficient of single contact. The friction between steel and polymer is affected by the roughness of steel. Interestingly, small roughness does not always result in low friction [24,25]. In doublecontact friction, the roughness of ball surface would be higher than that in single contact, and relatively large roughness may reduce the friction of iPPI-steel contacts, which is another reason for the lower friction coefficient of double contacts. Both the friction coefficient and temperature of iPPI-steel are higher than those of steel-steel in double contacts. The friction coefficient of iPPI-steel in double contacts decreases with the amounts of additional oil, while the friction coefficient of steel-steel first decreases, and then changes little after 10 μL (Fig. 4(a)). The temperature of double contacts first drops sharply, and then stays stable after 5 μL (Fig. 4(b)). Comparing the curves of friction coefficient, the friction coefficients of upper contact and lower contact change in the consistent steps, which indicates that the upper contact and lower contact affect each other. The good or bad lubrication of one contact will reduce or increase friction of the

Stribeck curves
Stribeck curves were tested after running for 20 h, as shown in Fig. 5. The speeds were set from 0.2 to 2.8 m/s with a 0.2 m/s interval when the speed is higher than 0.2 m/s. Because the Stribeck curves drop sharply at low speeds, the friction coefficients at 0.01, 0.02, 0.05, and 0.1 m/s were also measured to provide details.
Therefore, every Stribeck curves consists of 18 points. At every speed, the test lasts for 15 s, and the data from 4 to 13 s were used to calculate the average friction coefficient at that speed. With no additional oil for both single contact and double contacts, the Stribeck curves have a little tortuous. Nevertheless, it can be found that the oil impregnated in iPPI pin could lubricant the steel-steel surface effectively. With the addition of oil, the Stribeck curves behave smooth-The friction coefficient first decreases rapidly, and then changes little or rises slightly with the sliding speed. The friction coefficients of iPPI-steel are always higher than those of steel-steel.

Worn surfaces
The worn surfaces of iPPI pins, steel balls, and steel discs were observed by a stereo microscope and a laser confocal microscope. Comparing with the worn surfaces of iPPI in iPPI-steel single contact, the iPPI surfaces in double contacts are blackened obviously, as well as the worn surfaces of steel balls and steel discs. Basically, the colors of the worn areas of the three contact surfaces are similar, indicating that the components that blacken the worn surfaces may be the same.
The profiles of the worn surfaces are measured by the laser confocal microscope (VK-X200, Keyence), as shown in Fig. 6. The profiles of worn surfaces on steel discs are circular arcs, while the profiles of iPPI pins appear flat due to plastic deformation. In doublecontact friction, the wear heights of iPPI and steel discs decrease with the increase of additional oil. Considering that the test lasted for 20 h, the wear of disc, as well as iPPI, is quite small, even with no additional oil in double-contact friction, indicating that the iPPI could effectively lubricate the steel-steel surface. Another interesting result is that iPPI pins in single-contact friction have the highest wear height, which means that in double-contact friction, the wear of iPPI pin is reduced by steel-steel contact. From Fig. 7, it can be found that the worn surface in double contacts looks darker. Wear debris coming from the both contacts is stacked on ball surfaces. The stacked debris forms a protect layer on worn surface, which reduces the wear height of iPPI.
In order to find out the composition of black matter, the Raman spectra of ball surfaces and disc surfaces were tested using a confocal Raman microscope (inVia, Renishaw), as shown in Fig. 8. The spectra of worn surfaces on discs and balls in double-contact friction are similar. The peaks are in good agreement with the characteristic peaks of α-Fe 2 O 3 and Fe 3 O 4 . The peaks at 228, 294, 414, and 1,322 cm −1 are commonly assigned to α-Fe 2 O 3 , and the peak at 665 cm −1 is commonly assigned to Fe 3 O 4 . These peaks are not seen in the spectra of the cleaned balls and discs. Comparing with the spectra of PI, very little PI could be found on one disc and one ball (DC-0 μL of disc and DC-10 μL of ball), but it does not indicate that the PI was not transferred in other surfaces of balls and discs, because the transferred PI is very little and not evenly distributed on the ball and disc surface. Therefore, it can be concluded that the black matter is the mixture of mostly iron oxide and little PI.
The worn surfaces of iPPI pin were observed by the SEM to find more details, as shown in Fig. 9. Before observation, the oil impregnated in the iPPI material www.Springer.com/journal/40544 | Friction  should be removed before observation, otherwise the oil will interfere with the observation. In this paper, the iPPI pins were immersed in petrol ether for 48 h to remove the oil. It can be found from Fig. 9 that the worn surface of iPPI in single contact is quite clean, while a lot of white particles could be found on the worn surfaces in double contacts. After adding more oil, the number of white particles decreased. The worn surface of iPPI under 5 μL additional oil is magnified, as shown in Fig. 9(f). A lot of white particles could be found. The EDS test results of the area located in the red square are shown in Table 4. Fe element accounts for 11.26 wt%, which verified that the black matter on worn surfaces is iron oxide. The size of the whitest particles is below 100 nm. The small particles could penetrate inside the iPPI material, that is why not only the wear area of iPPI pin but also the area around the wear area is blackened (Fig. 7).  Figure 10 shows the white particles around the micro pores. It could be found that a large number of small white particles are distributed around the pore. The particle is much smaller than the pore, so the particles (consisting of α-Fe 2 O 3 and Fe 3 O 4 ) easily enter the pores and penetrate inside the PPI material. Note that the particles are reduced significantly after extra oil was added ( Fig. 10(a) and (b)), and the worn surface of PPI in single contact is quite clean with few particles found (Fig. 10(c)). Moreover, the pores are difficult to  www.Springer.com/journal/40544 | Friction be blocked by the particles due to smaller size and stronger penetrability, which means that the blackening of PPI surface has little effect on the oil release during the lifetime of bearing, because severe wear of balls and grooves is prohibited in the service life of bearing.

Discussion
In bearings with polymer retainer, ball-ring and ballretainer are two different kinds of contact pairs with disparate properties. The interactions of the two kinds of contacts give different results from single-contact friction. For bearings with iPPI retainer, the worn surface of the retainer is always blackened; but using standard friction test machine, it is difficult to find the blackened wear surface, which makes the wear mechanism of the iPPI retainer unclear. In this paper, in order to investigate the friction and wear mechanisms of iPPI material used for bearing retainer, a doublecontact friction test rig was used to test tribological properties of iPPI material.
According to the test results, the worn surface of iPPI material in double-contact friction is blackened notably, which is significantly different from the worn surface in single-contact friction. This double-contact friction method is closer to the friction in actual bearings. The friction process is shown in Fig. 11.
According to the Raman spectra and EDS test results, very little PI debris was found on both ball and disc surfaces (Figs. 8(a) DC-0 μL and 8(b) DC-10 μL), and the Fe element was also found on iPPI surface (Table 4). Hence, both the iPPI-steel contact and steel-steel contact produce wear debris under micro oil lubrication condition. The iPPI debris and steel debris are mixed during friction looping. As shown in Figs. 9 and 10, the wear debris is quite small (below 100 nm), while the pore size of PPI material is around 1.65 μm. The micro-oil impregnated in the PPI material is extruded during friction and transferred to the surface of ball and disc, and then on the other side, the oil will be recycled into the PPI material [14,17]. Therefore, the debris can penetrate inside thre iPPI material by the extrusion and recycling of the micro-oil impregnated in the iPPI. That is why the worn surface of the iPPI material is blackened, as shown in Fig. 11(b). Moreover, the mixed wear debris is also crushed on the worn surface of ball, making the surface look black (Fig. 7).
Adding additional oil on disc surface could reduce the blackening of worn surface, because the wear debris from the two contacts was reduced. From the friction coefficient and Stribeck curves (Figs. 3 and 5, respectively), after adding 10 μL of oil, the double contacts are well lubricated, and more oil does not help to reduce friction, but the iPPI surface is still blackened (Fig. 7). It seems that the blackening of worn surface of iPPI material in double-contact friction (or in bearings) is inevitable. Hence, it is necessary to know that the blackening of iPPI surface is good or bad for the friction of the double contacts.
First of all, the mixed wear debris from a protect layer on ball surface and disc surface could reduce the wear of contact surfaces under micro-lubrication condition. The excess debris penetrates inside the iPPI material, preventing too much debris from staying on contact surfaces, which is called good embeddability. In this respect, the blackening of worn surface is beneficial to the friction of the double contacts. The blackening of iPPI surface is also an indicator of the wear of steel-steel contact.
On the other hand, that the small debris penetrates inside the iPPI may block the tiny pores and hinder the extrusion of oil, which is bad for the friction. Considering that the pore size of iPPI material is relatively larger than the size of debris, and the porosity is about 10%, the oil is not easy to be blocked. However, the blocking effect of debris on oil extrusion cannot be ignored if the iPPI material is used for a quite long time.
| https://mc03.manuscriptcentral.com/friction In summary, the iPPI material is easy to be blackened in double-contact friction due to good embeddability, which is helpful for keeping the friction pairs stable. The blackening degree is related to the lubrication condition of steel-steel contact. The blackening degree of iPPI surface can be an indicator of the wear of steel-steel contact-If the steel-steel contact (or ball and grooves in bearings) is not well lubricated, the worn surface of iPPI material will be blackened seriously. Considering that most of the oil comes from iPPI material in the micro-oil lubricated bearings, the porosity of PPI material should be high to store oil. However, a high porosity results in a significant reduction in material strength. The balance of porosity and strength is important for the design of iPPI materials. Moreover, a high porosity usually leads to low oil retention, which should also be considered in the design of iPPI material for long-term lubrication.

Conclusions
In this paper, the friction and wear properties of iPPI material were studied using a double-contact test rig. The friction coefficients and Stribeck curves were tested, and the worn surfaces were observed and analyzed. Based on the results, the blackening mechanisms of iPPI material were revealed. The critical findings are concluded as follows: 1) The oil impregnated in iPPI material can lubricate the steel-steel contact effectively in double-contact friction. The friction coefficient of steel-steel contact is below 0.08 even with no additional oil. With the increase of additional oil, the friction coefficient first decreases, and then changes little after adding 10 μL of oil. This result shows that in double-contact friction containing iPPI material, a little oil can lubricate the surfaces well.
2) The blackening of iPPI worn surfaces results from the good embeddability of iPPI material. The mixed wear debris, consisting of mostly iron oxide (α-Fe 2 O 3 and Fe 3 O 4 ) and little PI, is transferred between the two contacts and penetrated inside the iPPI material through the process of extruding and recycling of oil, which makes the worn surface be blackened. The mixed wear debris could form a protect layer on ball and disc surface, and excess debris could penetrate inside the iPPI material, preventing too much debris from staying on contact surfaces. The blackening of iPPI material is a self-protection behavior of the double contacts, despite the blocking pores effect after long time sliding.
3) The double-contact friction method is a new and effective method to study the friction bearings. In bearings, balls, and rings, balls and retainer are two different kinds of contact. The interaction of the two kinds of contact makes the friction in bearings special and different from those in single contact. The worn surface of iPPI material in double-contact friction is blackened notably, which is significantly different from the worn surface in single-contact friction. The results proved the validity of double-contact test method.