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The lubricity and reinforcement of carbon fibers in polyimide at high temperatures
Tribology International 101 (2016) 291–300
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Tribology International journal homepage: www.elsevier.com/locate/triboint
The lubricity and reinforcement of carbon fibers in polyimide at high temperatures Fengxia Dong a,b, Guoliang Hou a, Fengxiang Cao a,b, Fengyuan Yan a,n, Liang Liu a,b, Jianzhang Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 February 2016 Received in revised form 26 April 2016 Accepted 28 April 2016 Available online 29 April 2016
The friction and wear behaviors of neat PI and carbon fibers reinforced polyimide (CF/PI) composites were investigated at various temperatures. The results showed that the introduction of carbon fibers could greatly improve the wear resistance in the whole temperature range, while the friction coefficients strongly depended on sliding temperatures. The lubricity of carbon fibers was only found occurring at high temperatures of 180–260 °C, which can be attributed to the graphitization of carbon fibers that promotes the generation of friction and transfer films with excellent lubricity on the worn surfaces. This study is expected to provide guidance for the application of carbon fibers both as lubricating and reinforcing additives in polymer matrix sliding at high temperatures. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Polyimide Carbon fibers High temperature Lubricity
1. Introduction As compared to metallic and ceramic materials, the favorable use of polymer materials in dry sliding is indicated by their excellent performances, including self-lubricating and anti-wear abilities, low density, high strength and wide performance tailorability [1–5]. However, due to the low thermal stability and loss of mechanical properties at high temperatures, many technical polymers, such as polyethylene terephthalate (PET), polyamides (PA) and polyethylene (PE) cannot be widely used under extreme sliding conditions [6,7]. As one of the most common engineering materials with both linear or cyclic imides group and aromatic groups, thermoplastic polyimide (TPI), possessing excellent mechanical, chemical properties and thermal stability, has received a great deal of attention as a potential candidate for use in tribological systems under harsh conditions, such as high temperatures, high normal loads, and/or high sliding velocities [8–11]. For all that, there are also some problems that usually encountered in TPI applications: (I) the friction coefficient (μ) is relatively high, and (II) the wear rate (Ws) is also high because of the brittleness, making TPI is not suitable to be used as self-lubricating materials directly, especially at high temperatures. In order to improve the mechanical and tribological characteristics, polyimide-based composites have been developed in recent years by adding some n
Corresponding author. E-mail address: [email protected] (F. Yan).
http://dx.doi.org/10.1016/j.triboint.2016.04.035 0301-679X/& 2016 Elsevier Ltd. All rights reserved.
appropriate additives, such as solid lubricants, nanoparticles or fibers into polyimide matrix. Carbon fibers (CFs), combining high specific strength and modulus, damping capacity, excellent thermal stability and conductivity, and potentially lubricating ability, has been widely used as reinforcements and/or lubrications in tribological applications of polymer composites [12–15]. Generally, they can improve the wear resistance of composites because of the increase of loading capability, while it is unclear under what conditions CFs can induce low friction coefficient [16–18]. According to Wang and Zhang, the incorporation of carbon fibers can significantly improve the friction-reducing and anti-wear abilities of polyimide under dry sliding conditions [19], while Li and Cheng found the incorporation of carbon fibers into polyimide can generally reduce the wear rate of composites, but may either increase or reduce the friction coefficient with different carbon fibers contents [20]. Unfortunately, these researches mainly focus on the friction and wear behaviors of CF/PI composites at room temperature, which still cannot satisfy the requirements for their use at high temperatures. Until now, only several researches have been conducted to investigate the influence of sliding temperature on the tribological properties of CF/PI composites [10,21]. Apparently, there still is large room to systematically investigate and fully understand the wear behavior, combined with the wear mechanism of CF/PI composites sliding at high temperatures, especially the lubrication mechanism of CFs. Bearing those perspectives in mind, in this research polyimide composites reinforced with carbon fibers were fabricated by hot press molding technique and the friction and wear behaviors of neat PI and CF/PI composites were investigated sliding against steel ball at various
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temperatures. In addition, the mechanisms of high improvements in friction-reducing and anti-wear abilities at high temperatures were deeply discussed in relation to their mechanical properties, wornsurface features and formation of friction and transfer films. The present research, hopefully, is to provide references for application of CF/PI composites as self-lubricating materials at high temperatures, as well as the application of CFs as lubricating additives in polymer composites at high temperatures.
2. Experimental 2.1. Materials and specimens Thermoplastic polyimide powder (YS-20) with an average particle size of 30 μm and a density of 1.38 g/cm3 was purchased from Shanghai Synthetic Resin Institute, (China). PAN-based carbon fibers with length of 28–56 μm and diameter of 7 μm and a density of 1.77 g/cm3 were provided by Nanjing Fiber-glass Research and Design Institute (China). The samples of neat PI and CF/PI composites with a fixed volume content of carbon fibers of 10% were fabricated by means of mechanical mixing followed by hot pressing technique. Specifically, the neat PI powder was mixed with carbon fibers by simply mixing at room temperature, then the neat PI powder or mixed powders were filled into a mold, compressed and heated up to 375 °C and held at 30 MPa for 60 min to allow full compression and sintering. After cooled in the stove in air and released from the mold, target specimens with different dimensions were obtained.
Fig. 1. Contact schematic diagram for unidirectional rotational sliding friction.
2.2. Test apparatus and experimental procedures Friction and wear tests were carried out in a ball-on-disk contact configuration with a high temperature friction and wear testing machine (CSEM-THT07-135). A contact schematic diagram of the frictional couple is shown in Fig. 1. The polymer samples (Ra is about 0.03 μm) were used as the lower specimens, and commercially available GCr15 (AISI 52100) steel balls with a diameter of 3 mm (hardness is about 9 GPa, Ra is about 0.02 μm) were used as the upper specimens. The sliding was performed with a sliding speed of 0.3 m/s (rotational speed: 573 r/min), load of 5 N, and duration of 30 min. Prior to each test, the stainless steel balls were ultrasonically cleaned with acetone for 30 min to remove the oil thoroughly on the surfaces. The friction coefficient curves were recorded automatically with a computer connected to the friction and wear tester. The wear volume loss was measured with a NanoMap three dimensional (3D) contact surface mapping profile. The specific wear rate (mm3/Nm) was calculated as below: K¼
V F US
where V is the wear volume loss (mm3), F is the normal load (N), and S is the total sliding distance (m). Tests were carried out in ambient air with relative humidity of 1072%. Three repeated friction and wear tests were carried out for each specimen and the average of the three repeated test values was reported in this paper. 2.3. Characterization Thermal gravimetric analysis (TGA) was performed under N2 atmosphere with a NETZSCH STA449C thermal analyzer from 30 to 900 °C with a heating rate of 10 °C/min. Dynamic mechanical analysis (DMA) was carried out in nitrogen on a NETZSCH DMA-242C analyzer. The specimens were rectangular bars (60 mm 10 mm 4 mm) and performed under a three-point bending mode from 30 to 350 °C at a heating rate of 3 °C /min and a frequency of 1 Hz.
Fig. 2. TGA thermograms of neat PI (PI-1) and CF/PI (PI-2) composites.
Table 1 Data on TGA and flexural strength of neat PI (PI-1) and CF/PI (PI-2) composites. Material
TGA
PI-1 PI-2
Flexural strength (MPa)
Tda (°C)
T5b (°C)
T10c (°C)
Rwd (%)
545 550
555 560
570 573
55 62
1427 3 1717 2
a
Td: the onset decomposition temperature. T5: the temperature at 5 wt% of weight loss. c T10: the temperature at 10 wt% of weight loss. d Rw: residual weight retention at 900 °C. b
The flexural strength of samples was determined using a DY35 universal testing machine with a span of 64 mm and crosshead speed of 2 mm/min. The specimens were 80 mm 10 mm 4 mm and the test surface was 80 mm 10 mm. The specific flexural strength ( σf ) of specimens was calculated as below:
σf ¼
3FL 2bh
2
Where F is the maximum load (N), L is the span length (mm), b is the width of the specimen (mm), and h is the thickness of the specimen (mm). At least three measurements were conducted for each sample in the bending test. The worn surfaces of PI and CF/PI composites were examined using a JEM-5600LV (JEOL, Japan) scanning electron microscope
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(SEM). In order to increase the resolution for SEM analysis, all the tested polymeric samples were ion plated with a gold coating to render them electrical conductance. The transfer films formed on steel ball surfaces were qualitatively studied by an OLYMPUS optical microscope, and their detailed bonding structures were characterized by Jobin-Y von HR-800 Raman spectrometer equipped with an argon-ion laser source at an excitation wavelength of 532 nm.
3. Results and discussion
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seen clearly that the storage modulus of neat PI and CF/PI composites both exhibited relative high values below glass transition temperature, while that sharply decreased to very low values in the rubbery state. For clarity, Fig. 3c and d shows the storage modulus of PI and CF/PI composites versus temperature in both glassy and rubbery state. At the temperatures of 150–260 °C, the storage modulus of CF/PI composites was improved by 40–52% compared to neat PI. More importantly, the storage modulus of composites is 3400 MPa at 260 °C, while that of neat PI is only 2200 MPa. Even at the temperatures of 280–350 °C, the storage modulus of composites also increased obviously though the increment became less and less with
3.1. Thermal analysis and reinforcing effect of carbon fibers As thermal stability is a key parameter for polymer composites used in high-tech field, TGA measurements were conducted for neat PI (PI-1) and CF/PI composites (PI-2) and the results were presented in Fig. 2, according data listed in Table 1. It is found that the addition of carbon fibers slightly improved the thermal stability of polyimide matrix, as the onset decomposition temperature increased 5 °C for CF/ PI composites, and so did the temperature at 5 wt% of weight loss (Table 1). The residual weight retention at 900 °C was 62% for CF/PI composites, also higher than that of 55% for neat PI. In order to further explore the thermal mechanical properties of neat PI and CF/PI composites, dynamic mechanical analysis experiments were undertaken in nitrogen from 30 °C to 350 °C. As shown in Fig. 3a, the peak temperatures of tanδ for neat PI and CF/PI composites are 270 °C and 273 °C, respectively, corresponding to the glass transition temperature (Tg). That is to say, the presence of carbon fibers hardly affected the glass transition temperature. From Fig. 3b, it can be
Fig. 4. Scanning electron microscope image of the fracture surface of CF/PI composites.
Fig. 3. Tanδ (a) and dynamic storage modulus (b, c, d) of neat PI (PI-1) and CF/PI (PI-2) composites as a function of temperature.
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temperature increasing (Fig. 3d). Also as expected, the flexural strength of composites was clearly improved from 142 to 171 MPa with the addition of 10 vol% carbon fibers (Table 1). Therefore, it can be concluded that the incorporated carbon fibers helps to slightly enhance the thermal stability and greatly improve the mechanical properties of neat PI. This can be attributed to the reinforcement effect of carbon fibers, which are uniformly dispersed into polyimide resin matrix and make the composites exhibit a “reinforced concrete structure” (see Fig. 4). Since the PAN-based carbon fibers have good thermal conductivity and temperature resistance, the uniform and interlaced dispersion make it possible to impart the excellent thermal conductivity of carbon fibers to composites, and it is known that the enhanced thermal conductivity of composites can facilitate heat transport and thus increase its thermal stability. Meanwhile, the PANbased carbon fibers have much higher tensile modulus, strength and compressive strength than polymers. So their uniform dispersion and anisotropic orientation can efficiently transfer stress from polymer matrix to the filler, thereby enhancing the mechanical properties of CF/PI composites. 3.2. Friction and wear behavior The online-measured friction coefficients as a function of sliding time at different temperatures were plotted in Fig. 5. It can be seen that the friction coefficients of neat PI were relatively stable without
significant changes between the running-in and steady-state conditions over the entire temperature range. In contrast, the friction coefficients of CF/PI composites displayed two sliding regimes as function of temperatures. At low temperatures (r140 °C), the friction coefficients were relatively high, and the values in the steady-state period were slightly higher than those in the running-in period. Whereas at high temperatures, the coefficients were very low with the values in the steady-state period far lower than those in the running-in period, just as shown in Fig. 5b. Moreover, it is worth noting that the transition time into low values became shorter and shorter with the temperature increasing and the sliding behavior at 180 °C is instable with some characteristic spikes on the onlinemeasured friction curves after which these spikes became lacking and nearly disappeared with the temperature increasing up to 260 °C. The average friction coefficients of neat PI and CF/PI composites at different temperatures were given in Fig. 6a. At low temperatures (r140 °C), the friction coefficients of neat PI changed slightly ranging from 0.23 to 0.25, while those for CF/PI composites kept relatively high values of around 0.4, indicating that the carbon fibers did not exhibit lubricating ability in the low temperature regions. The main possible reasons can be described as follows: (I) the shear modulus for PAN-based carbon fibers was higher than that of PI matrix, which could increase the shear-force between frictional pairs during sliding process and finally caused higher friction coefficients; (II) some broken and pulled-out carbon fibers easily pressed into PI matrix with
Fig. 5. Online-measured friction coefficient as a function of sliding time at different temperatures: (a) PI, (b) CF/PI composites.
Fig. 6. Friction coefficient and specific wear rate of all samples sliding at different temperatures.
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low hardness, thereby hindering the relative movement of the frictional pairs to some extent and increasing the friction. With further increasing temperature to 180–260 °C, the friction coefficient of neat PI increased gradually to the maximum value of 0.35 (at 260 °C). This trend agreed well with those published by Samyn et al. [22] and Tanaka et al. [23], but in contrast to the results reported by Samyn et al. for semi-thermosetting polyimide [24] and Cong et al. [25], who found lower friction coefficient at high temperatures above 180 °C with the cylinder-on-plate contact configuration (the cylinder made of polyimide used as upper specimens). This can be explained by the differences in thermal mechanical properties of polyimide and contact configuration. Above all, the present polyimide behaves as thermoplastic, which is prone to overload, plastic deformation and softening during high temperature friction tests due to the loss of mechanical strength under the synergistic effect of high bulk temperature and frictional heating. On the other hand, the present ball-on-disk contact configuration with polyimide samples as lower specimens made the polyimide suffer from more severe plastic deformation owing to high contact stress and shear stress. As a result, the steel ball surface asperities penetrated the polyimide surface more deeply, making the relative motion of frictional pairs more difficult, meanwhile, the softening of thermoplastic polyimide increased the bonding strength of contact asperities and enhanced the interface adhesion between PI
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and metal, thereby resulting in higher friction coefficient at 220 and 260 °C. For CF/PI composites, strong transitions into low friction coefficients around 0.1 were noticed for as the elevated temperature rises to 180–260 °C, showing excellent self-lubricating ability thereat. As shown in Fig. 6b, the wear rates of neat PI and CF/PI composites were also strongly dependent on sliding temperatures. Over the entire temperature range, the wear rates of neat PI were far higher than those for composites and increased with increasing temperature. When sliding at room temperature, the specific wear rate was 6.26 10 5 mm3/Nm, while that increased more than 10 times to 8.55 10 4 mm3/Nm as sliding at 260 °C. This can be ascribed to the serious plastic deformation and softening of polyimide caused by the heat from frictional contact at high temperatures. In contrast, due to the enhancement of thermal stability and mechanical properties caused by carbon fibers, the resistance to plastic deformation and softening for CF/PI composites increased, thus resulting in lower wear rates. Moreover, the wear rates of CF/PI composites also exhibited two regimes with transition occurring at 180 °C, in agreement with the friction coefficients. Specifically, at low temperatures, the wear rates of composites increased from 8.97 10 6 mm3/Nm (room temperature) to 6.38 10 5 mm3/Nm (140 °C), indicating the wear resistance of composite were enhanced by 2–7 times compared to neat PI. While at 180 °C, the wear rate of composites sharply decreased to
Fig. 7. SEM observation of worn surface of PI at different temperatures: (a) RT, (b) 100 °C, (c) 140 °C, (d) 180 °C, (e) 220 °C, (f) 260 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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6 10 6 mm3/Nm, which was only 2.35% of that for pure PI. The main reason should be the significant decrease of shear stress in the sliding interface resulted from self-lubricating property. As the elevated temperature rises to 220 °C, the wear rate of composites (7.72 10 6 mm3/Nm) was reduced by 498.5% in comparison with neat PI (6 10 4 mm3/Nm). In other words, the wear resistance was enhanced more than 70 times. Even at the maximum applied temperature (260 °C), which is close to the Tg of PI and CF/PI composites, the wear rate was still reduced by 498%, indicating the wear resistance was also enhanced more than 50 times. Overall, the incorporation of carbon fibers can greatly improve the tribological properties of PI, especially at high temperatures. This could be of significance in terms of the application of CF/PI composites as a selflubricating and wear-resistant polymer material at elevated temperatures. 3.3. Worn surface and wear mechanism Microscopic observations of the worn surfaces of polymers and GCr15 steel counterparts can be beneficial to explore the relevant friction and wear mechanisms for PI and CF/PI composites. Fig. 7 showed the worn surfaces of PI sliding at different temperatures. It is found that the worn surface at room temperature was very smooth and characterized with many furrows (Fig. 7a), indicating that abrasive wear was the main wear mechanism for neat PI. At 100 °C, apart from many furrows, some spalling points also appeared on the worn surface (Fig. 7b), suggesting that abrasive wear and fatigue wear were the main wear mechanism. With further increase of temperature, serious cracks perpendicular to the sliding directions (indicated by the red arrow) occurred on the worn surfaces (Fig. 7c–f), revealing the fatigue wear aggravated. Moreover, the optical micrographs of worn surfaces of steel counterparts at 100 °C and 220 °C were shown in Fig. 8. It is evident that more separate polyimide debris particles appeared on the worn surface at 220 °C, suggesting that adhesion wear also aggravated at higher temperature, which agreed well with the research results of wear rate mentioned in Fig. 6b. Meanwhile, fatigue wear and adhesion wear could cause the increase of friction coefficients at high
temperatures owing to the increased roughness of polyimide worn surface and enhanced adhesive strength between PI and metal. However, those separate polyimide debris particles on the counterpart surface could be easily removed by cotton ball (Fig. 8b and d), meaning that the bonding strength between polyimide debris particles and steel counterpart was very low. The worn surfaces of CF/PI composites were shown in Fig. 9. It is obviously found that the worn surface of CF/PI composites at room temperature was characterized with many furrows, combined with some broken and pulled-out carbon fibers, leaving several holes (Fig. 9a). This reveals that the CF/PI composites mainly suffered from abrasive wear and fatigue wear during the sliding process. As temperature increasing, the above phenomenon became more and more serious, indicating the degree of wear, especially fatigue wear, aggravated (Fig. 9b–d), which can well explain the increases of friction coefficients and wear rates from room temperature to 140 °C mentioned in Fig. 6. Interestingly, the furrows on the worn surface seemed to disappear as increasing temperature to 180 °C, instead, the worn surface looked very smooth with a friction film layer (Fig. 9e). More importantly, the friction film layer became more intact and continuous at 220 °C and 260 °C (Fig. 9f and g), which could more effectively protect the covered material from wear, consequently leading to much lower specific wear rate. Fig. 10 presented the optical micrographs of worn surfaces of steel counterparts sliding against CF/PI composites at various temperatures. It is found that the formation of transfer film was also dominantly controlled by temperature. As shown in Fig. 10a–c, there were only a few separate flakes on the worn surfaces of steel counterparts at low temperatures, which can be also removed by cleaning with cotton ball, similar to the cases of neat PI. With increasing temperature to 140 °C, little transfer film was visually observed on the steel ball surface, although it was very rough and discontinuous. Whereas at temperatures of 180–260 °C, the films were distinctly different, showing lumpy and smooth characteristics and parallel to the sliding directions. More importantly, they could not be removed by cleaning with cotton ball. Therefore, we can draw a conclusion that the films can generate more easily at higher temperatures and exhibit stronger
Fig. 8. Optical micrograph of counterface sliding against neat PI at (a, b)100 °C, and (c, d) 220 °C ((a, c) as received; (b, d) cleaned by cotton ball).
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Fig. 9. SEM observation of worn surface of CF/PI composites at various temperatures: (a) RT, (b) 60 °C, (c) 100 °C, (d) 140 °C, (e) 180 °C, (f) 220 °C, (g) 260 °C.
adhesion strength to the counterfaces. In addition, the relative poor adhesion strength of transfer films might be the main reason for the appearance of some characteristic spikes on the online-measured friction curves at 180 °C (see Fig. 5b). Based on the above research and analysis, it could be obviously found that the decreases of friction coefficients and wear rates for CF/ PI composites occurring at 180–260 °C were closely related to the formation of friction films and transfer films. To further explore the wear mechanism and better explain the excellent lubricating ability of the films formed at high temperatures, the detailed bonding structures of transfer film and carbon fibers were characterized by Raman
spectroscopy. As shown in Fig. 11, two sharp peaks were clearly observed in both Raman spectra independent of wear test, one at approximately 1375 cm 1 corresponding to the defect-induced structure (the D band) and the other at about 1595 cm 1 corresponding to ordered graphitic structure (the G band) [26,27]. However, the peak intensity ratio of the D to G band (ID/IG) of transfer film was lower than that of carbon fibers, suggesting an increase of graphitic-like crystallize structure in the transfer film [28]. This indicates that under the action of high pressure caused by point contact and friction-induced high shear stress and temperature, further graphitization of carbon fibers has occurred on the worn surface during
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Fig. 10. Optical micrograph of steel counterface sliding against composite at various temperatures: (a) RT, (b) 60 °C, (c) 100 °C, (d) 140 °C, (e) 180 °C, (f) 220 °C, (g) 260 °C.
the wear process. It is well known that graphite has low shear resistance through its molecular structure with hexagonal planes, in which the electrons between carbon atoms are held together with weak van der Waals bonds. Meanwhile, due to the water supply into sliding interface caused by imidisation reaction of PI, the dangling bonds in graphite could be deactivated through chemisorption of water, leading to lower shear resistance [24,29]. Therefore, the further graphitized carbon fibers could more easily cleave into sheets by shear force and formed as friction and transfer films with good lubricity on the counterfaces. Moreover, owing to the high adhesive force between graphite and solid surfaces [30,31], as well as the high cohesive force
of wear debris onto the worn surface caused by plastic deformation and softening of polymer matrix [32], the friction and transfer films exhibited stronger adhesion to the sliding surfaces. Under this condition, the contact between composites specimens and steel balls would be transformed from “polymer-steel” contact into “film–film” contact, which reduced the “direct contact” between the frictional counterparts and made friction occur in the internal of lubricating films, consequently reducing the contact pressure and resulting in lower friction coefficient [33–35]. Certainly, the faster generated lubricating films could cause the shorter transition time into very low and stable values at higher temperatures.
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Fig. 11. Raman spectra of (a) carbon fiber and (b) the transfer film formed at 220 °C.
On the other hand, the reduction in “direct contact” between the frictional pairs and the decrease of shear stress in the sliding interface can protect the CF/PI composites sample surface from further abrasive and adhesive damage, accordingly resulting in lower wear rate at high temperatures. What is more, the reinforcement of carbon fibers can be also one of the necessary factors for increasing the wear resistance at high temperatures. As shown in Fig. 3c, the CF/PI composites maintained much higher storage modulus than neat PI at high temperatures of 150–260 °C, effectively improving the resistance to plowing and plastic deformation for the substrate, which was also beneficial to the retaining of lubricating films. Consequently, the huge improvement in friction-reducing and wear resistance at high temperatures can be achieved.
4. Conclusion In this study, PAN-based carbon fibers were utilized to prepare CF/PI composites, and the sliding wear behaviors were investigated at various temperatures with neat PI as a comparison. The main conclusions can be drawn as follows: (1) The sliding behaviors of CF/PI composites displayed two friction regimes versus sliding temperature. At low temperatures (RT 140 °C), the addition of carbon fibers remarkably reduced the wear rates of composites, whereas increased the friction coefficients. Nevertheless, at high temperatures (180– 260 °C), the CF/PI composites exhibited super excellent tribological properties with extremely low and stable friction coefficients and huge improvement in wear resistance. (2) The remarkably decrease of friction coefficients for CF/PI composites at 180–260 °C is closely related to the excellent lubricity of friction and transfer films, the formation of which is promoted by the graphitization of carbon fibers during high temperature sliding process. (3) The huge improvement in wear resistance for CF/PI composites at high temperatures can be attributed to some necessary factors, such as the lubricity and protection of friction and transfer films, as well as the reinforcement of carbon fibers. This composite material may be a promising candidate with excellent self-lubricating and anti-wear properties for use at elevated temperatures.
Acknowledgment The authors greatly acknowledge the reviewers for their detailed, rigorous and helpful comments.
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Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
The lubricity and reinforcement of carbon fibers in polyimide at high temperatures Fengxia Dong a,b, Guoliang Hou a, Fengxiang Cao a,b, Fengyuan Yan a,n, Liang Liu a,b, Jianzhang Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 February 2016 Received in revised form 26 April 2016 Accepted 28 April 2016 Available online 29 April 2016
The friction and wear behaviors of neat PI and carbon fibers reinforced polyimide (CF/PI) composites were investigated at various temperatures. The results showed that the introduction of carbon fibers could greatly improve the wear resistance in the whole temperature range, while the friction coefficients strongly depended on sliding temperatures. The lubricity of carbon fibers was only found occurring at high temperatures of 180–260 °C, which can be attributed to the graphitization of carbon fibers that promotes the generation of friction and transfer films with excellent lubricity on the worn surfaces. This study is expected to provide guidance for the application of carbon fibers both as lubricating and reinforcing additives in polymer matrix sliding at high temperatures. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Polyimide Carbon fibers High temperature Lubricity
1. Introduction As compared to metallic and ceramic materials, the favorable use of polymer materials in dry sliding is indicated by their excellent performances, including self-lubricating and anti-wear abilities, low density, high strength and wide performance tailorability [1–5]. However, due to the low thermal stability and loss of mechanical properties at high temperatures, many technical polymers, such as polyethylene terephthalate (PET), polyamides (PA) and polyethylene (PE) cannot be widely used under extreme sliding conditions [6,7]. As one of the most common engineering materials with both linear or cyclic imides group and aromatic groups, thermoplastic polyimide (TPI), possessing excellent mechanical, chemical properties and thermal stability, has received a great deal of attention as a potential candidate for use in tribological systems under harsh conditions, such as high temperatures, high normal loads, and/or high sliding velocities [8–11]. For all that, there are also some problems that usually encountered in TPI applications: (I) the friction coefficient (μ) is relatively high, and (II) the wear rate (Ws) is also high because of the brittleness, making TPI is not suitable to be used as self-lubricating materials directly, especially at high temperatures. In order to improve the mechanical and tribological characteristics, polyimide-based composites have been developed in recent years by adding some n
Corresponding author. E-mail address: [email protected] (F. Yan).
http://dx.doi.org/10.1016/j.triboint.2016.04.035 0301-679X/& 2016 Elsevier Ltd. All rights reserved.
appropriate additives, such as solid lubricants, nanoparticles or fibers into polyimide matrix. Carbon fibers (CFs), combining high specific strength and modulus, damping capacity, excellent thermal stability and conductivity, and potentially lubricating ability, has been widely used as reinforcements and/or lubrications in tribological applications of polymer composites [12–15]. Generally, they can improve the wear resistance of composites because of the increase of loading capability, while it is unclear under what conditions CFs can induce low friction coefficient [16–18]. According to Wang and Zhang, the incorporation of carbon fibers can significantly improve the friction-reducing and anti-wear abilities of polyimide under dry sliding conditions [19], while Li and Cheng found the incorporation of carbon fibers into polyimide can generally reduce the wear rate of composites, but may either increase or reduce the friction coefficient with different carbon fibers contents [20]. Unfortunately, these researches mainly focus on the friction and wear behaviors of CF/PI composites at room temperature, which still cannot satisfy the requirements for their use at high temperatures. Until now, only several researches have been conducted to investigate the influence of sliding temperature on the tribological properties of CF/PI composites [10,21]. Apparently, there still is large room to systematically investigate and fully understand the wear behavior, combined with the wear mechanism of CF/PI composites sliding at high temperatures, especially the lubrication mechanism of CFs. Bearing those perspectives in mind, in this research polyimide composites reinforced with carbon fibers were fabricated by hot press molding technique and the friction and wear behaviors of neat PI and CF/PI composites were investigated sliding against steel ball at various
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temperatures. In addition, the mechanisms of high improvements in friction-reducing and anti-wear abilities at high temperatures were deeply discussed in relation to their mechanical properties, wornsurface features and formation of friction and transfer films. The present research, hopefully, is to provide references for application of CF/PI composites as self-lubricating materials at high temperatures, as well as the application of CFs as lubricating additives in polymer composites at high temperatures.
2. Experimental 2.1. Materials and specimens Thermoplastic polyimide powder (YS-20) with an average particle size of 30 μm and a density of 1.38 g/cm3 was purchased from Shanghai Synthetic Resin Institute, (China). PAN-based carbon fibers with length of 28–56 μm and diameter of 7 μm and a density of 1.77 g/cm3 were provided by Nanjing Fiber-glass Research and Design Institute (China). The samples of neat PI and CF/PI composites with a fixed volume content of carbon fibers of 10% were fabricated by means of mechanical mixing followed by hot pressing technique. Specifically, the neat PI powder was mixed with carbon fibers by simply mixing at room temperature, then the neat PI powder or mixed powders were filled into a mold, compressed and heated up to 375 °C and held at 30 MPa for 60 min to allow full compression and sintering. After cooled in the stove in air and released from the mold, target specimens with different dimensions were obtained.
Fig. 1. Contact schematic diagram for unidirectional rotational sliding friction.
2.2. Test apparatus and experimental procedures Friction and wear tests were carried out in a ball-on-disk contact configuration with a high temperature friction and wear testing machine (CSEM-THT07-135). A contact schematic diagram of the frictional couple is shown in Fig. 1. The polymer samples (Ra is about 0.03 μm) were used as the lower specimens, and commercially available GCr15 (AISI 52100) steel balls with a diameter of 3 mm (hardness is about 9 GPa, Ra is about 0.02 μm) were used as the upper specimens. The sliding was performed with a sliding speed of 0.3 m/s (rotational speed: 573 r/min), load of 5 N, and duration of 30 min. Prior to each test, the stainless steel balls were ultrasonically cleaned with acetone for 30 min to remove the oil thoroughly on the surfaces. The friction coefficient curves were recorded automatically with a computer connected to the friction and wear tester. The wear volume loss was measured with a NanoMap three dimensional (3D) contact surface mapping profile. The specific wear rate (mm3/Nm) was calculated as below: K¼
V F US
where V is the wear volume loss (mm3), F is the normal load (N), and S is the total sliding distance (m). Tests were carried out in ambient air with relative humidity of 1072%. Three repeated friction and wear tests were carried out for each specimen and the average of the three repeated test values was reported in this paper. 2.3. Characterization Thermal gravimetric analysis (TGA) was performed under N2 atmosphere with a NETZSCH STA449C thermal analyzer from 30 to 900 °C with a heating rate of 10 °C/min. Dynamic mechanical analysis (DMA) was carried out in nitrogen on a NETZSCH DMA-242C analyzer. The specimens were rectangular bars (60 mm 10 mm 4 mm) and performed under a three-point bending mode from 30 to 350 °C at a heating rate of 3 °C /min and a frequency of 1 Hz.
Fig. 2. TGA thermograms of neat PI (PI-1) and CF/PI (PI-2) composites.
Table 1 Data on TGA and flexural strength of neat PI (PI-1) and CF/PI (PI-2) composites. Material
TGA
PI-1 PI-2
Flexural strength (MPa)
Tda (°C)
T5b (°C)
T10c (°C)
Rwd (%)
545 550
555 560
570 573
55 62
1427 3 1717 2
a
Td: the onset decomposition temperature. T5: the temperature at 5 wt% of weight loss. c T10: the temperature at 10 wt% of weight loss. d Rw: residual weight retention at 900 °C. b
The flexural strength of samples was determined using a DY35 universal testing machine with a span of 64 mm and crosshead speed of 2 mm/min. The specimens were 80 mm 10 mm 4 mm and the test surface was 80 mm 10 mm. The specific flexural strength ( σf ) of specimens was calculated as below:
σf ¼
3FL 2bh
2
Where F is the maximum load (N), L is the span length (mm), b is the width of the specimen (mm), and h is the thickness of the specimen (mm). At least three measurements were conducted for each sample in the bending test. The worn surfaces of PI and CF/PI composites were examined using a JEM-5600LV (JEOL, Japan) scanning electron microscope
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(SEM). In order to increase the resolution for SEM analysis, all the tested polymeric samples were ion plated with a gold coating to render them electrical conductance. The transfer films formed on steel ball surfaces were qualitatively studied by an OLYMPUS optical microscope, and their detailed bonding structures were characterized by Jobin-Y von HR-800 Raman spectrometer equipped with an argon-ion laser source at an excitation wavelength of 532 nm.
3. Results and discussion
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seen clearly that the storage modulus of neat PI and CF/PI composites both exhibited relative high values below glass transition temperature, while that sharply decreased to very low values in the rubbery state. For clarity, Fig. 3c and d shows the storage modulus of PI and CF/PI composites versus temperature in both glassy and rubbery state. At the temperatures of 150–260 °C, the storage modulus of CF/PI composites was improved by 40–52% compared to neat PI. More importantly, the storage modulus of composites is 3400 MPa at 260 °C, while that of neat PI is only 2200 MPa. Even at the temperatures of 280–350 °C, the storage modulus of composites also increased obviously though the increment became less and less with
3.1. Thermal analysis and reinforcing effect of carbon fibers As thermal stability is a key parameter for polymer composites used in high-tech field, TGA measurements were conducted for neat PI (PI-1) and CF/PI composites (PI-2) and the results were presented in Fig. 2, according data listed in Table 1. It is found that the addition of carbon fibers slightly improved the thermal stability of polyimide matrix, as the onset decomposition temperature increased 5 °C for CF/ PI composites, and so did the temperature at 5 wt% of weight loss (Table 1). The residual weight retention at 900 °C was 62% for CF/PI composites, also higher than that of 55% for neat PI. In order to further explore the thermal mechanical properties of neat PI and CF/PI composites, dynamic mechanical analysis experiments were undertaken in nitrogen from 30 °C to 350 °C. As shown in Fig. 3a, the peak temperatures of tanδ for neat PI and CF/PI composites are 270 °C and 273 °C, respectively, corresponding to the glass transition temperature (Tg). That is to say, the presence of carbon fibers hardly affected the glass transition temperature. From Fig. 3b, it can be
Fig. 4. Scanning electron microscope image of the fracture surface of CF/PI composites.
Fig. 3. Tanδ (a) and dynamic storage modulus (b, c, d) of neat PI (PI-1) and CF/PI (PI-2) composites as a function of temperature.
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temperature increasing (Fig. 3d). Also as expected, the flexural strength of composites was clearly improved from 142 to 171 MPa with the addition of 10 vol% carbon fibers (Table 1). Therefore, it can be concluded that the incorporated carbon fibers helps to slightly enhance the thermal stability and greatly improve the mechanical properties of neat PI. This can be attributed to the reinforcement effect of carbon fibers, which are uniformly dispersed into polyimide resin matrix and make the composites exhibit a “reinforced concrete structure” (see Fig. 4). Since the PAN-based carbon fibers have good thermal conductivity and temperature resistance, the uniform and interlaced dispersion make it possible to impart the excellent thermal conductivity of carbon fibers to composites, and it is known that the enhanced thermal conductivity of composites can facilitate heat transport and thus increase its thermal stability. Meanwhile, the PANbased carbon fibers have much higher tensile modulus, strength and compressive strength than polymers. So their uniform dispersion and anisotropic orientation can efficiently transfer stress from polymer matrix to the filler, thereby enhancing the mechanical properties of CF/PI composites. 3.2. Friction and wear behavior The online-measured friction coefficients as a function of sliding time at different temperatures were plotted in Fig. 5. It can be seen that the friction coefficients of neat PI were relatively stable without
significant changes between the running-in and steady-state conditions over the entire temperature range. In contrast, the friction coefficients of CF/PI composites displayed two sliding regimes as function of temperatures. At low temperatures (r140 °C), the friction coefficients were relatively high, and the values in the steady-state period were slightly higher than those in the running-in period. Whereas at high temperatures, the coefficients were very low with the values in the steady-state period far lower than those in the running-in period, just as shown in Fig. 5b. Moreover, it is worth noting that the transition time into low values became shorter and shorter with the temperature increasing and the sliding behavior at 180 °C is instable with some characteristic spikes on the onlinemeasured friction curves after which these spikes became lacking and nearly disappeared with the temperature increasing up to 260 °C. The average friction coefficients of neat PI and CF/PI composites at different temperatures were given in Fig. 6a. At low temperatures (r140 °C), the friction coefficients of neat PI changed slightly ranging from 0.23 to 0.25, while those for CF/PI composites kept relatively high values of around 0.4, indicating that the carbon fibers did not exhibit lubricating ability in the low temperature regions. The main possible reasons can be described as follows: (I) the shear modulus for PAN-based carbon fibers was higher than that of PI matrix, which could increase the shear-force between frictional pairs during sliding process and finally caused higher friction coefficients; (II) some broken and pulled-out carbon fibers easily pressed into PI matrix with
Fig. 5. Online-measured friction coefficient as a function of sliding time at different temperatures: (a) PI, (b) CF/PI composites.
Fig. 6. Friction coefficient and specific wear rate of all samples sliding at different temperatures.
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low hardness, thereby hindering the relative movement of the frictional pairs to some extent and increasing the friction. With further increasing temperature to 180–260 °C, the friction coefficient of neat PI increased gradually to the maximum value of 0.35 (at 260 °C). This trend agreed well with those published by Samyn et al. [22] and Tanaka et al. [23], but in contrast to the results reported by Samyn et al. for semi-thermosetting polyimide [24] and Cong et al. [25], who found lower friction coefficient at high temperatures above 180 °C with the cylinder-on-plate contact configuration (the cylinder made of polyimide used as upper specimens). This can be explained by the differences in thermal mechanical properties of polyimide and contact configuration. Above all, the present polyimide behaves as thermoplastic, which is prone to overload, plastic deformation and softening during high temperature friction tests due to the loss of mechanical strength under the synergistic effect of high bulk temperature and frictional heating. On the other hand, the present ball-on-disk contact configuration with polyimide samples as lower specimens made the polyimide suffer from more severe plastic deformation owing to high contact stress and shear stress. As a result, the steel ball surface asperities penetrated the polyimide surface more deeply, making the relative motion of frictional pairs more difficult, meanwhile, the softening of thermoplastic polyimide increased the bonding strength of contact asperities and enhanced the interface adhesion between PI
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and metal, thereby resulting in higher friction coefficient at 220 and 260 °C. For CF/PI composites, strong transitions into low friction coefficients around 0.1 were noticed for as the elevated temperature rises to 180–260 °C, showing excellent self-lubricating ability thereat. As shown in Fig. 6b, the wear rates of neat PI and CF/PI composites were also strongly dependent on sliding temperatures. Over the entire temperature range, the wear rates of neat PI were far higher than those for composites and increased with increasing temperature. When sliding at room temperature, the specific wear rate was 6.26 10 5 mm3/Nm, while that increased more than 10 times to 8.55 10 4 mm3/Nm as sliding at 260 °C. This can be ascribed to the serious plastic deformation and softening of polyimide caused by the heat from frictional contact at high temperatures. In contrast, due to the enhancement of thermal stability and mechanical properties caused by carbon fibers, the resistance to plastic deformation and softening for CF/PI composites increased, thus resulting in lower wear rates. Moreover, the wear rates of CF/PI composites also exhibited two regimes with transition occurring at 180 °C, in agreement with the friction coefficients. Specifically, at low temperatures, the wear rates of composites increased from 8.97 10 6 mm3/Nm (room temperature) to 6.38 10 5 mm3/Nm (140 °C), indicating the wear resistance of composite were enhanced by 2–7 times compared to neat PI. While at 180 °C, the wear rate of composites sharply decreased to
Fig. 7. SEM observation of worn surface of PI at different temperatures: (a) RT, (b) 100 °C, (c) 140 °C, (d) 180 °C, (e) 220 °C, (f) 260 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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6 10 6 mm3/Nm, which was only 2.35% of that for pure PI. The main reason should be the significant decrease of shear stress in the sliding interface resulted from self-lubricating property. As the elevated temperature rises to 220 °C, the wear rate of composites (7.72 10 6 mm3/Nm) was reduced by 498.5% in comparison with neat PI (6 10 4 mm3/Nm). In other words, the wear resistance was enhanced more than 70 times. Even at the maximum applied temperature (260 °C), which is close to the Tg of PI and CF/PI composites, the wear rate was still reduced by 498%, indicating the wear resistance was also enhanced more than 50 times. Overall, the incorporation of carbon fibers can greatly improve the tribological properties of PI, especially at high temperatures. This could be of significance in terms of the application of CF/PI composites as a selflubricating and wear-resistant polymer material at elevated temperatures. 3.3. Worn surface and wear mechanism Microscopic observations of the worn surfaces of polymers and GCr15 steel counterparts can be beneficial to explore the relevant friction and wear mechanisms for PI and CF/PI composites. Fig. 7 showed the worn surfaces of PI sliding at different temperatures. It is found that the worn surface at room temperature was very smooth and characterized with many furrows (Fig. 7a), indicating that abrasive wear was the main wear mechanism for neat PI. At 100 °C, apart from many furrows, some spalling points also appeared on the worn surface (Fig. 7b), suggesting that abrasive wear and fatigue wear were the main wear mechanism. With further increase of temperature, serious cracks perpendicular to the sliding directions (indicated by the red arrow) occurred on the worn surfaces (Fig. 7c–f), revealing the fatigue wear aggravated. Moreover, the optical micrographs of worn surfaces of steel counterparts at 100 °C and 220 °C were shown in Fig. 8. It is evident that more separate polyimide debris particles appeared on the worn surface at 220 °C, suggesting that adhesion wear also aggravated at higher temperature, which agreed well with the research results of wear rate mentioned in Fig. 6b. Meanwhile, fatigue wear and adhesion wear could cause the increase of friction coefficients at high
temperatures owing to the increased roughness of polyimide worn surface and enhanced adhesive strength between PI and metal. However, those separate polyimide debris particles on the counterpart surface could be easily removed by cotton ball (Fig. 8b and d), meaning that the bonding strength between polyimide debris particles and steel counterpart was very low. The worn surfaces of CF/PI composites were shown in Fig. 9. It is obviously found that the worn surface of CF/PI composites at room temperature was characterized with many furrows, combined with some broken and pulled-out carbon fibers, leaving several holes (Fig. 9a). This reveals that the CF/PI composites mainly suffered from abrasive wear and fatigue wear during the sliding process. As temperature increasing, the above phenomenon became more and more serious, indicating the degree of wear, especially fatigue wear, aggravated (Fig. 9b–d), which can well explain the increases of friction coefficients and wear rates from room temperature to 140 °C mentioned in Fig. 6. Interestingly, the furrows on the worn surface seemed to disappear as increasing temperature to 180 °C, instead, the worn surface looked very smooth with a friction film layer (Fig. 9e). More importantly, the friction film layer became more intact and continuous at 220 °C and 260 °C (Fig. 9f and g), which could more effectively protect the covered material from wear, consequently leading to much lower specific wear rate. Fig. 10 presented the optical micrographs of worn surfaces of steel counterparts sliding against CF/PI composites at various temperatures. It is found that the formation of transfer film was also dominantly controlled by temperature. As shown in Fig. 10a–c, there were only a few separate flakes on the worn surfaces of steel counterparts at low temperatures, which can be also removed by cleaning with cotton ball, similar to the cases of neat PI. With increasing temperature to 140 °C, little transfer film was visually observed on the steel ball surface, although it was very rough and discontinuous. Whereas at temperatures of 180–260 °C, the films were distinctly different, showing lumpy and smooth characteristics and parallel to the sliding directions. More importantly, they could not be removed by cleaning with cotton ball. Therefore, we can draw a conclusion that the films can generate more easily at higher temperatures and exhibit stronger
Fig. 8. Optical micrograph of counterface sliding against neat PI at (a, b)100 °C, and (c, d) 220 °C ((a, c) as received; (b, d) cleaned by cotton ball).
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Fig. 9. SEM observation of worn surface of CF/PI composites at various temperatures: (a) RT, (b) 60 °C, (c) 100 °C, (d) 140 °C, (e) 180 °C, (f) 220 °C, (g) 260 °C.
adhesion strength to the counterfaces. In addition, the relative poor adhesion strength of transfer films might be the main reason for the appearance of some characteristic spikes on the online-measured friction curves at 180 °C (see Fig. 5b). Based on the above research and analysis, it could be obviously found that the decreases of friction coefficients and wear rates for CF/ PI composites occurring at 180–260 °C were closely related to the formation of friction films and transfer films. To further explore the wear mechanism and better explain the excellent lubricating ability of the films formed at high temperatures, the detailed bonding structures of transfer film and carbon fibers were characterized by Raman
spectroscopy. As shown in Fig. 11, two sharp peaks were clearly observed in both Raman spectra independent of wear test, one at approximately 1375 cm 1 corresponding to the defect-induced structure (the D band) and the other at about 1595 cm 1 corresponding to ordered graphitic structure (the G band) [26,27]. However, the peak intensity ratio of the D to G band (ID/IG) of transfer film was lower than that of carbon fibers, suggesting an increase of graphitic-like crystallize structure in the transfer film [28]. This indicates that under the action of high pressure caused by point contact and friction-induced high shear stress and temperature, further graphitization of carbon fibers has occurred on the worn surface during
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Fig. 10. Optical micrograph of steel counterface sliding against composite at various temperatures: (a) RT, (b) 60 °C, (c) 100 °C, (d) 140 °C, (e) 180 °C, (f) 220 °C, (g) 260 °C.
the wear process. It is well known that graphite has low shear resistance through its molecular structure with hexagonal planes, in which the electrons between carbon atoms are held together with weak van der Waals bonds. Meanwhile, due to the water supply into sliding interface caused by imidisation reaction of PI, the dangling bonds in graphite could be deactivated through chemisorption of water, leading to lower shear resistance [24,29]. Therefore, the further graphitized carbon fibers could more easily cleave into sheets by shear force and formed as friction and transfer films with good lubricity on the counterfaces. Moreover, owing to the high adhesive force between graphite and solid surfaces [30,31], as well as the high cohesive force
of wear debris onto the worn surface caused by plastic deformation and softening of polymer matrix [32], the friction and transfer films exhibited stronger adhesion to the sliding surfaces. Under this condition, the contact between composites specimens and steel balls would be transformed from “polymer-steel” contact into “film–film” contact, which reduced the “direct contact” between the frictional counterparts and made friction occur in the internal of lubricating films, consequently reducing the contact pressure and resulting in lower friction coefficient [33–35]. Certainly, the faster generated lubricating films could cause the shorter transition time into very low and stable values at higher temperatures.
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Fig. 11. Raman spectra of (a) carbon fiber and (b) the transfer film formed at 220 °C.
On the other hand, the reduction in “direct contact” between the frictional pairs and the decrease of shear stress in the sliding interface can protect the CF/PI composites sample surface from further abrasive and adhesive damage, accordingly resulting in lower wear rate at high temperatures. What is more, the reinforcement of carbon fibers can be also one of the necessary factors for increasing the wear resistance at high temperatures. As shown in Fig. 3c, the CF/PI composites maintained much higher storage modulus than neat PI at high temperatures of 150–260 °C, effectively improving the resistance to plowing and plastic deformation for the substrate, which was also beneficial to the retaining of lubricating films. Consequently, the huge improvement in friction-reducing and wear resistance at high temperatures can be achieved.
4. Conclusion In this study, PAN-based carbon fibers were utilized to prepare CF/PI composites, and the sliding wear behaviors were investigated at various temperatures with neat PI as a comparison. The main conclusions can be drawn as follows: (1) The sliding behaviors of CF/PI composites displayed two friction regimes versus sliding temperature. At low temperatures (RT 140 °C), the addition of carbon fibers remarkably reduced the wear rates of composites, whereas increased the friction coefficients. Nevertheless, at high temperatures (180– 260 °C), the CF/PI composites exhibited super excellent tribological properties with extremely low and stable friction coefficients and huge improvement in wear resistance. (2) The remarkably decrease of friction coefficients for CF/PI composites at 180–260 °C is closely related to the excellent lubricity of friction and transfer films, the formation of which is promoted by the graphitization of carbon fibers during high temperature sliding process. (3) The huge improvement in wear resistance for CF/PI composites at high temperatures can be attributed to some necessary factors, such as the lubricity and protection of friction and transfer films, as well as the reinforcement of carbon fibers. This composite material may be a promising candidate with excellent self-lubricating and anti-wear properties for use at elevated temperatures.
Acknowledgment The authors greatly acknowledge the reviewers for their detailed, rigorous and helpful comments.
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