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Tribological behavior of polyethersulfone-reinforced polytetrafluoroethylene composite under dry sliding condition
Tribology International 86 (2015) 17–27
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Tribological behavior of polyethersulfone-reinforced polytetrafluoroethylene composite under dry sliding condition Zhen Zuo a, Laizhou Song b,c, Yulin Yang a,b,n a
College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China Aviation Key Laboratory of Science and Technology on Generic Technology of Self-Lubricating Spherical Plain Bearing, Yanshan University, Qinhuangdao 066004, China c College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 October 2014 Received in revised form 28 November 2014 Accepted 19 January 2015 Available online 28 January 2015
Polyethersulfone (PES) as an addition was blended into polytetrafluoroethylene (PTFE) to improve the tribological property of this type of polymer. The performances of anti-friction and wear resistance of the PES/PTFE composites are far better than that of the virgin PTFE. At the friction interface, the tribological behavior of the PES/PTFE composite is governed by the interaction between the PTFE transfer film and the PES layer. Of all the PES/PTFE composites, the composite with the PES addition of 40 wt% exhibits the best tribological property, attributing to the coexistences of the uniform PTFE transfer film and the continuous PES layer. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Polymeric composites Wear and friction Transfer film High temperature
1. Introduction Polymer composites have received enormous attention in the field of tribology, due to their excellent advantages of self-lubrication, chemical stability, light weight and low cost [1,2]. Polytetrafluoroethylene (PTFE), being one of them, has been extensively investigated as a type of self-lubricating polymer [3,4]. However, PTFE is not suitable for the practical applications alone because of its high wear rate, and thus the wear resistance of PTFE should be enhanced by the incorporations of other materials [5,6]. The technique of polymer blending is an effective method to obtain the PTFE-based composites with excellent tribological performances [7–10]. Polyethersulfone (PES) as a conventional polymer shows a remarkable resistance to heat, and its glass transition temperature is much higher than that of PTFE. Furthermore, the mechanical property, creep resistance and light weight property of PES are also better than that of PTFE. In view of these outstanding properties, PES can be competent for the application as a reinforcing polymer employed to improve the antifriction and wear resistance of PTFE. In general, PTFE latex was blended with the PES to prepare the PTFE/PES composites (weight fraction of PTFEr40 wt%), then the thermal and mechanical properties of these composites were studied [11,12]. PTFE as an excellent solid lubricant was also n Corresponding author at: College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China. Tel.: þ863358057062; fax: þ863358057062. E-mail address: [email protected] (Y. Yang).
http://dx.doi.org/10.1016/j.triboint.2015.01.019 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
incorporated into some composites such as glass fiber/PES and carbon fabric-PES composites [13,14], to improve the tribological behaviors of them. In these studies, PES was the polymer matrix and PTFE was employed as a type of filler. To our knowledge, insufficient work has been focused on the investigation in the tribological properties of the PTFE-type composites, considering PES employed as a reinforcing addition. The goal of this study is to reveal the influence of PES addition on the tribological properties of PTFE composites under various sliding conditions, i.e. different normal loads, sliding velocities and ambient temperatures. Also, Fourier transform infrared spectroscopy with an attenuated total reflectance module (ATR-FTIR), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), differential scanning calorimetry (DSC), and thermogravimetry (TG) were employed to characterize the PES/PTFE composites.
2. Materials and methods 2.1. Materials PTFE powder (CGM031; Zhonghao Chenguang Research Institute of Chemical Industry, Sichuan, China) and PES powder (Jida High Performance Materials Co. Ltd., Jilin, China) were used as received to prepare the lower specimen for the tribological tests. The absolute ethanol of analytical grade was supplied by Jingchun Reagent Co. Ltd. (Shanghai, China). Q235 carbon steel was employed as the
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FN and L are the normal load (N) and sliding distance (m), respectively. In order to guarantee the accuracy of Δm in every quality measurement, the composites were dried at 100 1C for 90 min to remove the moisture derived from the ambient air. Prior to the tribological tests, the Q235 steel specimen employed as the upper specimen was polished with the silicon carbide paper (grit #1000, Kailai Abrasive Technology Co. Ltd., Qingdao, China) for 2 min and then washed in absolute ethanol with sonication for 5 min. Finally, the steel specimen was rinsed with deionized water and dried. Each test was performed three times under identical conditions to ensure the reproducibility of the experimental data. 2.4. Morphology characterization
Fig. 1. Diagram of MMU-5G friction pairs: 1 spindle, 2 upper torque pin, 3 lower torque pin, 4 lower specimen base, 5 temperature thermocouple, 6 fixing bolt, 7 upper specimen, 8 lower specimen, 9 heating furnace.
upper specimen, and its weight composition (wt%) received was: C 0.17, Mn 0.53, Si 0.28, P 0.02, S 0.02, and Fe 98.98. 2.2. Preparation of PES/PTFE specimens PES and PTFE powders were added into the absolute ethanol of 50 ml and magnetic stirred (HJ-5 multi-functional magnetic stirrer, Huanxi Medical Instrument Co. Ltd., Guangzhou, China) for 30 min at room temperature, to ensure the powders dispersed homogeneously. After that, the mixed powders were filtered and dried at 80 1C to evaporate the residual absolute ethanol. The molding processes of virgin PTFE and PES/PTFE composites were as follows: the mixed powders were pressed at room temperature and 40 MPa for 10 min. The temperature was then increased to 375 1C at a heating rate of 1 1C/min. The samples were sintered at this temperature for 90 min and then cooled naturally to room temperature. Both dosages of PES and PTFE in PES/PTFE composites were described by the weight fraction (wt%). Herein, the PES/PTFE composites with PES additions of 10%, 20%, 30% and 40% were represented as A, B, C and D, respectively. In addition, the molding process of virgin PES was as follows: firstly, the PES powders were pressed at room temperature and 40 MPa for 10 min. Then the pressure was decreased to 0.15 MPa, and the temperature was increased to 260 1C with a heating rate of 3 1C/min. The PES sample was sintered at this temperature for 90 min and then cooled naturally to room temperature. 2.3. Tribological tests A ring-on-disk friction and wear tester (MMU-5G, Sida Tester Co. Ltd., Jinan, China) was used to evaluate the tribological properties of PES/PTFE composites under the dry sliding conditions. The friction pairs involving both the upper and lower specimens were shown in Fig. 1, and the area of contact surface was 144.44 mm2. Parameters of five normal loads (400, 800, 1000, 1200 and 1400 N, i.e. 2.77, 5.54, 6.93, 8.31 and 9.70 MPa), four sliding speeds (0.482, 0.750, 1.003, 1.204 m/s), and five ambient temperatures (25, 65, 100, 150, 200 1C) were chosen for the series of friction tests. The friction time was controlled as 60 min, and the values in the last 10 min were selected to calculate the average friction coefficients. The masses of PES/PTFE composites before and after the tribological tests were weighed using an electronic balance (0.0001 g accuracy) to calculate the specific wear rate k (m3/Nm) [15]: k ¼ ΔV =F N L ¼ Δm=ρF N L
ð1Þ
where ΔV and Δm are the volume loss (m ) and the mass loss (g) of the composite; ρ is the density (g/m3) of the composite. 3
The morphologies of the worn surfaces of PES/PTFE composites and Q235 steel specimens were characterized by a scanning electron microscope (SEM, S-3400N, Hitachi, Japan) equipped with an energy dispersive spectrometer (EDS). Before the observations, the surfaces of polymer samples were sputtered with gold powders. 2.5. ATR-FTIR and thermal analyses A FTIR spectrometer (E55 þ þ FRA106, Bruker Inc., Karlsruhe, Germany) with an attenuated total reflectance (ATR) module was used to identify changes of the functional groups on the worn surface of PES/PTFE composites. Differential scanning calorimetry (DSC) and thermogravimetric (TGA) analyses were performed using a simultaneous thermal analyzer (STA449C/6/G, Netzsch, Bavaria, Germany). The samples were placed in a ceramic crucible and heated to 800 1C with a heating rate of 10 1C/min; the argon filled with a flow rate of 20 ml/min was used as the protective gas. 3. Results and discussion 3.1. Tribological tests The friction coefficient μ and specific wear rate k of the virgin PTFE, PES/PTFE composites (Figs. 2 and 3), and the virgin PES (Fig. 3) were measured. As indicated by Fig. 2, the friction coefficient of specimens tends to be stable within 60 min, suggesting that 60 min is sufficient for the composites to reach the steady wear stage. The values of μ for the four PES/PTFE composites are lower than 0.12 (Fig. 3), which are also much lower than that of PES (0.306) and PTFE (0.148). k of these composites decreases with the increasing dosage of PES. Compared with the specific wear rates of PTFE (k¼ 1.03 10–12 m3/Nm), k of the sample D (5.5 10–16 m3/Nm) decreases by a factor of 1873. Moreover, k of the PES/PTFE composite is also much lower than that of virgin PES (5.59 10–12 m3/Nm). In comparison with the wear rate of polyetheretherketone (PEEK)/PTFE composite [9], potassium titanate whiskers reinforced PEEK/PTFE composite [16] and carbon fabric (CF)/PES composite [17], the PES/ PTFE composite shows the fascinating anti-wear property in view of its low value of k. Therefore, both the friction and wear performances of PTFE are greatly improved due to the addition of PES. 3.1.1. Effect of normal load The influences of normal loads (400, 800, 1000, 1200 and 1400 N, i.e. 2.77, 5.54, 6.93, 8.31 and 9.70 MPa) on the tribological properties of PES/PTFE composites at the sliding velocity of 0.482 m/s were tested. When the value of normal load is lower than 1200, μ of the composite decreases with the increase in load (Fig. 4). μ of the sample A still decreases with the increase of load when the load is higher than 1200 N; but μ of samples B–D rise dramatically, indicating the deterioration of their friction performances. The wear behaviors of composites also become worse when the load reached 1200 N
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Fig. 2. Friction coefficient of virgin PTFE and PES/PTFE composites (load: 400 N, sliding velocity: 0.48 m/s).
Fig. 3. Friction coefficient and wear rate of virgin PES, virgin PTFE and PES/PTFE composites (load: 400 N, sliding velocity: 0.48 m/s).
Fig. 4. Friction coefficient of PES/PTFE composites under various normal loads.
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Fig. 5. Wear rate of PES/PTFE composites under various normal loads.
Fig. 6. Friction coefficient and wear rate of PES/PTFE composites at different sliding speeds.
Table 1 Details of different PV values. PV (MPa m/s)
Code
Nominal pressure (MPa)
Sliding velocity (m/s)
2.67
P1-1 P1-2
2.77 5.54
0.96 0.48
3.34
P2-1 P2-2
2.77 6.93
1.20 0.48
(Fig. 5). As the load was 400, 800 and 1000 N, k reduces with the increasing dosage of PES. Among the four samples, the sample D exhibits the lowest k (ko7.0 10–16 m3/Nm). Whereas, k of the sample D rises remarkably as the load was higher than 1200 N, and an increasing factor higher than two orders of magnitude can be observed at this load. It can be concluded that a critical load ranging from 1000 to 1200 N will be ascertained, below which μ changes regularly as a function of load, but the change of k is unremarkable. When the load is higher than these critical loads, the changing tendency of μ becomes irregularly; the composites are not competent for the practical applications due to the high values of k. Moreover, the addition of PES also shows a significant influence on the variations of μ and k. As the load was lower than 1000 N, the changes of μ for the four composites are unremarkable, and the sample D displays the best wear resistance. When the load is higher than 1200 N, however, μ and k rise with the increasing dosage of PES.
3.1.2. Effect of sliding velocity The sliding velocity also affects the friction and wear behaviors of PES/PTFE composites (Fig. 6). With the increase of sliding velocity, the value of μ reduces firstly and then rises to a high value. k increases with the increasing of sliding velocity, and a noticeable increase can be observed as the velocity ranged from 0.75 to 1.003 m/s. Under an identical sliding velocity (0.75 and 1.003 m/s), the variation of μ can be ignored; but k varies greatly. With the increase of PES dosage, k decreases at the velocities in the range of 0.482–0.750 m/s; on the contrary, it increases at the velocities ranging from 1.003 to 1.204 m/s. Therefore, a critical sliding velocity in the range of 0.75–1.003 m/s can also be ascertained.
3.1.3. Effect of load and velocity under an identical pressure velocity (PV) value The influences of normal load and sliding velocity on the tribological properties of PES/PTFE composites at an identical PV value were compared. Two PV values were chosen for the tests, and two load/velocity systems were considered for each PV (Table 1). μ of these two systems follows the order: μP1-2 o μP1-1 and μP2-2 o μP2-1 (Fig. 7). The same trend can be observed for the k; values of k for the sample D at P1-2 and P2-2 are three orders of magnitude lower than that at P1-1 and P2-1. Therefore, the friction and wear performances of PES/PTFE composites at high loads are far better than that at high speeds. It can be deduced that PES/PTFE composites are more suitable for the applications in high load-low velocity systems rather than the low load-high velocity systems.
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Fig. 7. Effect of load and velocity on the tribological properties of PES/PTFE composites under an identical PV value.
Table 2 Details of tribological tests under high ambient temperatures. Lower specimen
Ambient temperature (1C)
Normal load (N)
Sliding velocity (m/ s)
40% PES
257 5 657 5 1007 3 1507 3 2007 3
400 400 400 400 400
0.48 0.48 0.48 0.48 0.48
3.1.4. Effect of high ambient temperature The friction and wear performances of PES/PTFE composites at different temperatures were also evaluated. The composite with 40 wt% PES was chosen for the tribological tests at different ambient temperatures (Table 2). As shown in Fig. 8, the variation of μ is negligible when the ambient temperature is lower than 200 1C. Values of k at the temperatures of 100 and 150 1C are slightly higher than that at 25 and 65 1C; but the values of k (ko1.3 10–15 m3/Nm) are still low. However, μ and k rise remarkably when the ambient temperature reaches 200 1C, indicating the severe wear properties of the composites. It can be accepted that a critical ambient temperature ranging 150–200 1C is ascertained. Based on the analyses mentioned above, the critical parameters of load, velocity and ambient temperature are confirmed. The friction and wear performances of PES/PTFE composites will become worse when these parameters are beyond the critical values of them.
absorption peaks at 636, 1147 and 1201 cm 1 assigned to the –CF2– groups, and two peaks at 1576 and 1483 cm 1 owing to the aromatic benzene ring can be observed, suggesting the existences of PES and PTFE. After the friction tests (Fig. 9b–e), the detected peaks of the abraded surfaces for the four composites are in good accordance with that of pure PES; however, no obvious peak attributed to the PTFE can be found. This indicates that PES is the major polymer remained on the worn surface, thereby forming a residual PES layer.
3.2. ATR-FTIR analysis
3.3. Morphology characterization
ATR-FTIR can be employed to identify the functional groups on the surface layers of samples [18–20], which was adopted to investigate the chemical groups existing on the worn surfaces of PES/PTFE composites. In the ATR-FTIR spectrum of PTFE (Fig. 9a), three absorption peaks observed at 1201, 1147 and 636 cm 1 can be assigned to the asymmetric stretch, symmetric stretch and wagging of –CF2– groups, respectively [21]. The sharp peak at 1144 cm 1 in the spectrum of PES is attributed to the symmetric stretching of – SO2– group [22]. Both the two adjacent bands at 1576 cm 1 and 1483 cm 1 are owned to the typical aromatic benzene ring [23]. ATR-FTIR spectra of the contact surfaces for the composite specimens are illustrated in Fig. 9b–e. Before the friction tests, three
3.3.1. Morphology of worn surface under an identical sliding condition The micrographs of the worn surfaces of PES/PTFE composites are shown in Fig. 10. Many obvious furrows and indentations (arrow) can be observed on the worn surface of sample A (Fig. 10a), but no obvious furrows could be found for the worn surfaces of samples B–D (Fig. 10b–d). Thus, the surface of sample A will be abraded more easily than that of B–D, consistent with the results of friction tests (Fig. 5). It can be observed that many fragments are scattered on the discontinuous worn surfaces of B and C, but the distance between fragments of sample C is shorter than that of B (double-headed arrow), indicating that the wear
Fig. 8. The friction coefficient and wear rate as a function of the ambient temperature for sample D (load: 400 N, velocity: 0.48 m/s).
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Fig. 9. ATR-FTIR spectra of pure polymers (a) and PES/PTFE composites: (b) 10% PES, (c) 20% PES, (d) 30% PES, (e) 40% PES.
resistance of C is better than that of B. Compared with the discontinue PES layers existing on the worn surfaces of other three samples, the worn surface of sample D is more continuous and no large cracks can be found. For this reason, the PES layer on the worn surface of sample D is hard to be abraded, resulting in its lowest wear rate.
When sliding against metals, it is well known that the PTFE transfer film can be formed on the metallic counterpart, and it will play a major role in reducing the friction coefficient and wear rate of the PTFE-based composite [15,24]. The morphologies and chemical components of the abraded surfaces of Q235 steels were investigated by SEM (Fig. 11) and EDS (Fig. 12). Samples of this carbon steel
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Fig. 10. SEM micrographs of the worn surface of PES/PTFE composites after the friction tests (load: 1000 N, velocity: 0.48 m/s): (a) 10% PES, (b) 20% PES, (c) 30% PES, (d) 40% PES.
Fig. 11. SEM micrographs of the worn surface for Q235 steel after the friction tests (load: 1000 N, velocity: 0.48 m/s): (a) 10% PES, (b) 20% PES, (c) 30% PES, (d) 40% PES.
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Fig. 12. EDS analysis of the worn surface for Q235 steel before (a) and after the friction tests (load: 1000 N, velocity: 0.48 m/s): (b) 10% PES, (c) 20% PES, (d) 30% PES, (e) 40% PES.
sliding against the PES/PTFE composites with PES additions of 10%, 20%, 30%, and 40% are denoted as E, F, G and H, respectively. The results of EDS demonstrate that a portion of PTFE is transferred to the steel, forming a PTFE transfer film. But PES cannot be found in this transfer film except for sample E, indicating that PES cannot be easily incorporated into this transfer film. The PTFE transfer films formed on the surfaces of samples F–G are thin and uniform (Fig. 11b–d), which could effectively reduce the friction coefficients and wear rates of these composites [25–27]. On the contrary, a thick and nonuniform transfer film is observed on the surface of sample E, and the lump wear fragments adhering on the surface of the steel can also be found (arrow, Fig. 11a). PES may be involved in these large scale wear debris adhered on the surface of sample E. For this reason, the transfer film formed on the surface of sample E consists both PTFE and PES. In contrast with the PTFE transfer films of samples B–D, this thick and lumpy transfer film containing PES and PTFE decreases the wear resistance of sample A. Based on the results of SEM and EDS, we confirm that a portion of PTFE is transferred to the surface of the carbon steel, forming the PTFE transfer film. However, in most situations, PES cannot be transferred and it will retain on the worn surfaces of the PES/PTFE composites, leading to the existence of a PES layer. To some extent, the tribological behaviors of PES/PTFE composites sliding against the carbon steel are governed by the interaction between the PTFE transfer film and the residual PES layer. Also, this interaction will be the cause for the excellent performances of anti-friction and wear resistance for PES/PTFE composites.
3.3.2. Morphology of worn surface under different sliding conditions In addition to the morphologies of worn surface under an identical sliding condition, the worn surfaces related to the composite with 40%
PES under various sliding conditions were also characterized. Four sliding conditions were chosen for the characterizations: (a) 400 N, 0.482 m/s (below all critical parameters); (b) 400 N, 0.482 m/s, 200 1C (above critical temperature); (c) 1400 N, 0.482 m/s (above critical load); (d) 400 N, 1.204 m/s (above critical velocity). The morphologies of the worn surface for PES/PTFE composite (40% PES) after four different friction tests are illustrated in Fig. 13. Below the critical parameters, the worn surface is dense, smoothing, and no obvious furrow can be found (Fig. 13a). Conversely, many cracks can be observed on the worn surface when the sliding parameter is higher than the critical values of temperature (Fig. 13b), load (Fig. 13c), and velocity (Fig. 13d), suggesting that the composites may be easy to be abraded from the surface, consequently resulting in their high wear rate under these conditions. The morphologies and chemical components of the abraded surfaces of Q235 steels were investigated by SEM (Fig. 14) and EDS (Fig. 15). As shown in Fig. 14, the steel surfaces are all uniform, continuous, and no wear fragments can be observed. On the other hand, the EDS results reveal that F element can be found on the steel surface under 400 N and 0.482 m/s (Fig. 15a), indicating the formation of PTFE transfer film. Nevertheless, F element cannot be found for the other three conditions, demonstrating the inexistence of PTFE transfer film. Therefore, it may be deduced that PTFE transfer film cannot be formed on the steel surface, when the sliding parameters are higher than the critical values. 3.4. Thermal analysis In the friction process, it has been reported that 90–95% of the mechanical energy is translated into heat [28,29]. The heat accumulated in the friction interface leads to the increase in interfacial temperature, thereby plays a dominant influence on
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Fig. 13. SEM micrographs of the worn surface of PES/PTFE composite (40% PES) after different friction tests: (a) 400 N, 0.48 m/s; (b) 400 N, 0.48 m/s, 200 1C; (c) 1400 N, 0.48 m/s; (d) 400 N, 1.20 m/s.
Fig. 14. SEM micrographs of the worn surface for Q235 steel (sliding against composite with 40% PES) after different friction tests: (a) 400 N, 0.48 m/s; (b) 400 N, 0.48 m/s, 200 1C; (c) 1400 N, 0.48 m/s; (d) 400 N, 1.20 m/s.
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Fig. 15. EDS analysis of the worn surface for Q235 steel (sliding against composite with 40% PES) after different friction tests: (a) 400 N, 0.48 m/s; (b) 400 N, 0.48 m/s, 200 1C; (c) 1400 N, 0.48 m/s; (d) 400 N, 1.20 m/s.
Fig. 16. DSC (a) and TGA (b) curves of PES, PTFE and PES/PTFE composites.
the tribological behavior of polymers. In general, the glass transition temperature (Tg) extensively affects the applications of polymers. The deformation of polymers increases significantly when the interfacial temperature reaches Tg. Tg of PES and PTFE is 125 and 233 1C, respectively, which was analyzed by the DSC measurements. The sharp peak appearing at 330 1C (Fig. 16a) can be attributed to the melt of PTFE. The reinforcing role of PES is crucial to the tribological property of PES/PTFE composites. The PES layer formed on the surface of the composite can be easily deformed when the load, velocity and ambient temperature are higher than the critical values, which may be due to that the interfacial temperature is higher than the Tg of PES. Under these sliding conditions, the friction and wear performances of the composites tend to become worse dramatically. In addition, the elevated ambient temperatures can also cause the remarkable changes of μ and k, and both of them increase abruptly when the ambient temperature reaches 200 1C. Under the ambient temperature of 200 1C, it may be deduced that the interfacial temperature is higher than the Tg of PES (233 1C), due to the fact that the heat is generated in the frictional interface. To some extent, this may be the cause of the deterioration of the tribological properties for PES/PTFE composites.
The TGA data of PES, PTFE and PES/PTFE composites are illustrated in Fig. 16b. The initial decomposition temperature of PTFE is around 510 1C, and a sharp decrease in its weight can be observed at 600 1C. The complete decomposition of PTFE takes place at 630 1C. The initial and the rapid decomposition of PES occur at 450 1C and 550 1C, respectively. After the test, PES with a residual mass of 41 wt% can be obtained. The four PES/PTFE composites start to decompose at 500 1C; the significant weight losses of them occur at the temperatures ranging from 550 to 590 1C. The residual masses of samples A, B, C and D are 5.2%, 11.0%, 16.9%, and 20.9%, respectively. Although the initial decomposition temperature of PES is lower than that of PTFE, the initial decomposition temperatures of PES/PTFE composites show an unremarkable difference in compassion with that of PTFE. It can be confirmed that the existent PTFE with a high decomposition temperature is benefit for the stabilities of the PES/PTFE composites.
4. Conclusion ATR-FTIR, SEM, EDS, DSC, and TG combined with the tribological tests were employed to study the friction and wear behaviors
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of PES/PTFE composites under the dry sliding conditions. The following conclusions can be drawn: (1) The tribological property of PTFE is improved effectively by the blending of PES. PES/PTFE composites will be more suitable for the applications in high load-low velocity systems rather than low load-high velocity systems. (2) During the friction process, PTFE transfer film is formed on the Q235 steel surface; whereas PES is remained on the worn surface of PES/PTFE composites, thereby forming a PES layer. The tribological behavior between the steel and composites is governed by the interaction between the PTFE transfer film and residual PES layer, resulting in the excellent friction and wear behaviors of the PES/PTFE composites. (3) The heat accumulated in the friction interface may be the primary cause for the confirmations of critical values for load, velocity and ambient temperature. When the sliding parameters are higher than the critical values, the PES can be easily deformed because the interfacial temperature reaches its Tg. Consequently, the friction coefficient and wear rate of the PES/ PTFE composites rise dramatically. (4) Below the critical sliding conditions, the composite with 40% PES illustrates the best tribological property (μ o0.11, ko 7.0 10–16 m3/Nm). This can be attributed to the coexistence of the uniform PTFE transfer film and the most continuous PES layer.
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[9] Lu ZP, Friedrich K. On sliding friction and wear of PEEK and its composites. Wear 1995;181:624–31. [10] Theiler G, Hübner W, Gradt T, Klein P, Friedrich K. Friction and wear of PTFE composites at cryogenic temperatures. Tribol Int 2002;35:449–58. [11] Antonioli D, Laus M, Sparnacci K, Deregibuset S, Kapeliouchko V, Poggio T, et al. Preparation and thermal characterization of PTFE/PES nanocomposites. Macromol Symp 2012;311:70–6. [12] Righetti MC, Boggioni A, Laus M, Antonioli D, Sparnacci K, Boarino L. Thermal and mechanical properties of PES/PTFE composites and nanocomposites. J Appl Polym Sci 2013;130:3624–33. [13] Bijwe J, Rajesh JJ, Jeyakumar A, Ghosh A, Tewari US. Influence of solid lubricants and fibre reinforcement on wear behaviour of polyethersulphone. Tribol Int 2000;33:697–706. [14] Sharma M, Bijwe J. Surface designing of carbon fabric polymer composites with nano and micron sized PTFE particles. J Mater Sci 2012;47:4928–35. [15] Krick BA, Ewin JJ, Blackman GS, Junk CP, Sawyer WG. Environmental dependence of ultra-low wear behavior of polytetrafluoroethylene (PTFE) and alumina composites suggests tribochemical mechanisms. Tribol Int 2012;51:42–6. [16] Xie GY, Zhuang GS, Sui GX, Yang R. Tribological behavior of PEEK/PTFE composites reinforced with potassium titanate whiskers. Wear 2010;268: 424–30. [17] Sharma M, Bijwe J. Influence of molecular weight on performance properties of polyethersulphone and its composites with carbon fabric. Wear 2012;274:388–94. [18] Pellegrin B, Prulho R, Rivaton A, Thérias S, Gardette JL, Gaudichet-Maurin E, et al. Multi-scale analysis of hypochlorite induced PES/PVP ultrafiltration membranes degradation. J Mater Sci 2013;447:287–96. [19] Hughes J, Ayoko G, Collett S, Golding G. Rapid quantification of methamphetamine: using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and chemometrics. PloS One 2013;8:e69609. [20] Dogan A, Lasch P, Neuschl C, Millrose MK, Alberts R, Schughart K, et al. ATRFTIR spectroscopy reveals genomic loci regulating the tissue response in high fat diet fed BXD recombinant inbred mouse strains. Genomics 2013;14:386. [21] Satyaprasad A, Jain V, Nema SK. Deposition of superhydrophobic nanostructured Teflon-like coating using expanding plasma arc. Appl Surf Sci 2007;253:5462–6. [22] Peyravi M, Rahimpour A, Jahanshahi M, Javadi A, Shockravi A. Tailoring the surface properties of PES ultrafiltration membranes to reduce the fouling resistance using synthesized hydrophilic copolymer. Microporous Mesoporous Mater 2012;160:114–25. [23] Saedi S, Madaeni SS, Hassanzadeh K, Shamsabadi AA, Laki S. The effect of polyurethane on the structure and performance of PES membrane for separation of carbon dioxide from methane. J Ind Eng Chem 2014;20:1916–29. [24] Ye J, Khare HS, Burris DL. Transfer film evolution and its role in promoting ultra-low wear of a PTFE nanocomposite. Wear 2013;297:1095–102. [25] Conte M, Igartua A. Study of PTFE composites tribological behavior. Wear 2012;296:568–74. [26] Podgornik B, Borovšak U, Megušar F, Košir K. Performance of low-friction coatings in helium environments. Surf Coat Technol 2012;206:4651–8. [27] Xie GY, Zhuang GS, Sui GX, Yang R. Tribological behavior of PEEK/PTFE composites reinforced with potassium titanate whiskers. Wear 2010;268:424–30. [28] Majcherczak D, Dufrenoy P, Berthier Y. Tribological, thermal and mechanical coupling aspects of the dry sliding contact. Tribol Int 2007;40:834–43. [29] Myshkin NK, Petrokovets MI, Kovalev AV. Tribology of polymers: adhesion, friction, wear, and mass-transfer. Tribol Int 2006;38:910–21.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Tribological behavior of polyethersulfone-reinforced polytetrafluoroethylene composite under dry sliding condition Zhen Zuo a, Laizhou Song b,c, Yulin Yang a,b,n a
College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China Aviation Key Laboratory of Science and Technology on Generic Technology of Self-Lubricating Spherical Plain Bearing, Yanshan University, Qinhuangdao 066004, China c College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 October 2014 Received in revised form 28 November 2014 Accepted 19 January 2015 Available online 28 January 2015
Polyethersulfone (PES) as an addition was blended into polytetrafluoroethylene (PTFE) to improve the tribological property of this type of polymer. The performances of anti-friction and wear resistance of the PES/PTFE composites are far better than that of the virgin PTFE. At the friction interface, the tribological behavior of the PES/PTFE composite is governed by the interaction between the PTFE transfer film and the PES layer. Of all the PES/PTFE composites, the composite with the PES addition of 40 wt% exhibits the best tribological property, attributing to the coexistences of the uniform PTFE transfer film and the continuous PES layer. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Polymeric composites Wear and friction Transfer film High temperature
1. Introduction Polymer composites have received enormous attention in the field of tribology, due to their excellent advantages of self-lubrication, chemical stability, light weight and low cost [1,2]. Polytetrafluoroethylene (PTFE), being one of them, has been extensively investigated as a type of self-lubricating polymer [3,4]. However, PTFE is not suitable for the practical applications alone because of its high wear rate, and thus the wear resistance of PTFE should be enhanced by the incorporations of other materials [5,6]. The technique of polymer blending is an effective method to obtain the PTFE-based composites with excellent tribological performances [7–10]. Polyethersulfone (PES) as a conventional polymer shows a remarkable resistance to heat, and its glass transition temperature is much higher than that of PTFE. Furthermore, the mechanical property, creep resistance and light weight property of PES are also better than that of PTFE. In view of these outstanding properties, PES can be competent for the application as a reinforcing polymer employed to improve the antifriction and wear resistance of PTFE. In general, PTFE latex was blended with the PES to prepare the PTFE/PES composites (weight fraction of PTFEr40 wt%), then the thermal and mechanical properties of these composites were studied [11,12]. PTFE as an excellent solid lubricant was also n Corresponding author at: College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China. Tel.: þ863358057062; fax: þ863358057062. E-mail address: [email protected] (Y. Yang).
http://dx.doi.org/10.1016/j.triboint.2015.01.019 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
incorporated into some composites such as glass fiber/PES and carbon fabric-PES composites [13,14], to improve the tribological behaviors of them. In these studies, PES was the polymer matrix and PTFE was employed as a type of filler. To our knowledge, insufficient work has been focused on the investigation in the tribological properties of the PTFE-type composites, considering PES employed as a reinforcing addition. The goal of this study is to reveal the influence of PES addition on the tribological properties of PTFE composites under various sliding conditions, i.e. different normal loads, sliding velocities and ambient temperatures. Also, Fourier transform infrared spectroscopy with an attenuated total reflectance module (ATR-FTIR), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), differential scanning calorimetry (DSC), and thermogravimetry (TG) were employed to characterize the PES/PTFE composites.
2. Materials and methods 2.1. Materials PTFE powder (CGM031; Zhonghao Chenguang Research Institute of Chemical Industry, Sichuan, China) and PES powder (Jida High Performance Materials Co. Ltd., Jilin, China) were used as received to prepare the lower specimen for the tribological tests. The absolute ethanol of analytical grade was supplied by Jingchun Reagent Co. Ltd. (Shanghai, China). Q235 carbon steel was employed as the
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FN and L are the normal load (N) and sliding distance (m), respectively. In order to guarantee the accuracy of Δm in every quality measurement, the composites were dried at 100 1C for 90 min to remove the moisture derived from the ambient air. Prior to the tribological tests, the Q235 steel specimen employed as the upper specimen was polished with the silicon carbide paper (grit #1000, Kailai Abrasive Technology Co. Ltd., Qingdao, China) for 2 min and then washed in absolute ethanol with sonication for 5 min. Finally, the steel specimen was rinsed with deionized water and dried. Each test was performed three times under identical conditions to ensure the reproducibility of the experimental data. 2.4. Morphology characterization
Fig. 1. Diagram of MMU-5G friction pairs: 1 spindle, 2 upper torque pin, 3 lower torque pin, 4 lower specimen base, 5 temperature thermocouple, 6 fixing bolt, 7 upper specimen, 8 lower specimen, 9 heating furnace.
upper specimen, and its weight composition (wt%) received was: C 0.17, Mn 0.53, Si 0.28, P 0.02, S 0.02, and Fe 98.98. 2.2. Preparation of PES/PTFE specimens PES and PTFE powders were added into the absolute ethanol of 50 ml and magnetic stirred (HJ-5 multi-functional magnetic stirrer, Huanxi Medical Instrument Co. Ltd., Guangzhou, China) for 30 min at room temperature, to ensure the powders dispersed homogeneously. After that, the mixed powders were filtered and dried at 80 1C to evaporate the residual absolute ethanol. The molding processes of virgin PTFE and PES/PTFE composites were as follows: the mixed powders were pressed at room temperature and 40 MPa for 10 min. The temperature was then increased to 375 1C at a heating rate of 1 1C/min. The samples were sintered at this temperature for 90 min and then cooled naturally to room temperature. Both dosages of PES and PTFE in PES/PTFE composites were described by the weight fraction (wt%). Herein, the PES/PTFE composites with PES additions of 10%, 20%, 30% and 40% were represented as A, B, C and D, respectively. In addition, the molding process of virgin PES was as follows: firstly, the PES powders were pressed at room temperature and 40 MPa for 10 min. Then the pressure was decreased to 0.15 MPa, and the temperature was increased to 260 1C with a heating rate of 3 1C/min. The PES sample was sintered at this temperature for 90 min and then cooled naturally to room temperature. 2.3. Tribological tests A ring-on-disk friction and wear tester (MMU-5G, Sida Tester Co. Ltd., Jinan, China) was used to evaluate the tribological properties of PES/PTFE composites under the dry sliding conditions. The friction pairs involving both the upper and lower specimens were shown in Fig. 1, and the area of contact surface was 144.44 mm2. Parameters of five normal loads (400, 800, 1000, 1200 and 1400 N, i.e. 2.77, 5.54, 6.93, 8.31 and 9.70 MPa), four sliding speeds (0.482, 0.750, 1.003, 1.204 m/s), and five ambient temperatures (25, 65, 100, 150, 200 1C) were chosen for the series of friction tests. The friction time was controlled as 60 min, and the values in the last 10 min were selected to calculate the average friction coefficients. The masses of PES/PTFE composites before and after the tribological tests were weighed using an electronic balance (0.0001 g accuracy) to calculate the specific wear rate k (m3/Nm) [15]: k ¼ ΔV =F N L ¼ Δm=ρF N L
ð1Þ
where ΔV and Δm are the volume loss (m ) and the mass loss (g) of the composite; ρ is the density (g/m3) of the composite. 3
The morphologies of the worn surfaces of PES/PTFE composites and Q235 steel specimens were characterized by a scanning electron microscope (SEM, S-3400N, Hitachi, Japan) equipped with an energy dispersive spectrometer (EDS). Before the observations, the surfaces of polymer samples were sputtered with gold powders. 2.5. ATR-FTIR and thermal analyses A FTIR spectrometer (E55 þ þ FRA106, Bruker Inc., Karlsruhe, Germany) with an attenuated total reflectance (ATR) module was used to identify changes of the functional groups on the worn surface of PES/PTFE composites. Differential scanning calorimetry (DSC) and thermogravimetric (TGA) analyses were performed using a simultaneous thermal analyzer (STA449C/6/G, Netzsch, Bavaria, Germany). The samples were placed in a ceramic crucible and heated to 800 1C with a heating rate of 10 1C/min; the argon filled with a flow rate of 20 ml/min was used as the protective gas. 3. Results and discussion 3.1. Tribological tests The friction coefficient μ and specific wear rate k of the virgin PTFE, PES/PTFE composites (Figs. 2 and 3), and the virgin PES (Fig. 3) were measured. As indicated by Fig. 2, the friction coefficient of specimens tends to be stable within 60 min, suggesting that 60 min is sufficient for the composites to reach the steady wear stage. The values of μ for the four PES/PTFE composites are lower than 0.12 (Fig. 3), which are also much lower than that of PES (0.306) and PTFE (0.148). k of these composites decreases with the increasing dosage of PES. Compared with the specific wear rates of PTFE (k¼ 1.03 10–12 m3/Nm), k of the sample D (5.5 10–16 m3/Nm) decreases by a factor of 1873. Moreover, k of the PES/PTFE composite is also much lower than that of virgin PES (5.59 10–12 m3/Nm). In comparison with the wear rate of polyetheretherketone (PEEK)/PTFE composite [9], potassium titanate whiskers reinforced PEEK/PTFE composite [16] and carbon fabric (CF)/PES composite [17], the PES/ PTFE composite shows the fascinating anti-wear property in view of its low value of k. Therefore, both the friction and wear performances of PTFE are greatly improved due to the addition of PES. 3.1.1. Effect of normal load The influences of normal loads (400, 800, 1000, 1200 and 1400 N, i.e. 2.77, 5.54, 6.93, 8.31 and 9.70 MPa) on the tribological properties of PES/PTFE composites at the sliding velocity of 0.482 m/s were tested. When the value of normal load is lower than 1200, μ of the composite decreases with the increase in load (Fig. 4). μ of the sample A still decreases with the increase of load when the load is higher than 1200 N; but μ of samples B–D rise dramatically, indicating the deterioration of their friction performances. The wear behaviors of composites also become worse when the load reached 1200 N
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Fig. 2. Friction coefficient of virgin PTFE and PES/PTFE composites (load: 400 N, sliding velocity: 0.48 m/s).
Fig. 3. Friction coefficient and wear rate of virgin PES, virgin PTFE and PES/PTFE composites (load: 400 N, sliding velocity: 0.48 m/s).
Fig. 4. Friction coefficient of PES/PTFE composites under various normal loads.
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Fig. 5. Wear rate of PES/PTFE composites under various normal loads.
Fig. 6. Friction coefficient and wear rate of PES/PTFE composites at different sliding speeds.
Table 1 Details of different PV values. PV (MPa m/s)
Code
Nominal pressure (MPa)
Sliding velocity (m/s)
2.67
P1-1 P1-2
2.77 5.54
0.96 0.48
3.34
P2-1 P2-2
2.77 6.93
1.20 0.48
(Fig. 5). As the load was 400, 800 and 1000 N, k reduces with the increasing dosage of PES. Among the four samples, the sample D exhibits the lowest k (ko7.0 10–16 m3/Nm). Whereas, k of the sample D rises remarkably as the load was higher than 1200 N, and an increasing factor higher than two orders of magnitude can be observed at this load. It can be concluded that a critical load ranging from 1000 to 1200 N will be ascertained, below which μ changes regularly as a function of load, but the change of k is unremarkable. When the load is higher than these critical loads, the changing tendency of μ becomes irregularly; the composites are not competent for the practical applications due to the high values of k. Moreover, the addition of PES also shows a significant influence on the variations of μ and k. As the load was lower than 1000 N, the changes of μ for the four composites are unremarkable, and the sample D displays the best wear resistance. When the load is higher than 1200 N, however, μ and k rise with the increasing dosage of PES.
3.1.2. Effect of sliding velocity The sliding velocity also affects the friction and wear behaviors of PES/PTFE composites (Fig. 6). With the increase of sliding velocity, the value of μ reduces firstly and then rises to a high value. k increases with the increasing of sliding velocity, and a noticeable increase can be observed as the velocity ranged from 0.75 to 1.003 m/s. Under an identical sliding velocity (0.75 and 1.003 m/s), the variation of μ can be ignored; but k varies greatly. With the increase of PES dosage, k decreases at the velocities in the range of 0.482–0.750 m/s; on the contrary, it increases at the velocities ranging from 1.003 to 1.204 m/s. Therefore, a critical sliding velocity in the range of 0.75–1.003 m/s can also be ascertained.
3.1.3. Effect of load and velocity under an identical pressure velocity (PV) value The influences of normal load and sliding velocity on the tribological properties of PES/PTFE composites at an identical PV value were compared. Two PV values were chosen for the tests, and two load/velocity systems were considered for each PV (Table 1). μ of these two systems follows the order: μP1-2 o μP1-1 and μP2-2 o μP2-1 (Fig. 7). The same trend can be observed for the k; values of k for the sample D at P1-2 and P2-2 are three orders of magnitude lower than that at P1-1 and P2-1. Therefore, the friction and wear performances of PES/PTFE composites at high loads are far better than that at high speeds. It can be deduced that PES/PTFE composites are more suitable for the applications in high load-low velocity systems rather than the low load-high velocity systems.
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Fig. 7. Effect of load and velocity on the tribological properties of PES/PTFE composites under an identical PV value.
Table 2 Details of tribological tests under high ambient temperatures. Lower specimen
Ambient temperature (1C)
Normal load (N)
Sliding velocity (m/ s)
40% PES
257 5 657 5 1007 3 1507 3 2007 3
400 400 400 400 400
0.48 0.48 0.48 0.48 0.48
3.1.4. Effect of high ambient temperature The friction and wear performances of PES/PTFE composites at different temperatures were also evaluated. The composite with 40 wt% PES was chosen for the tribological tests at different ambient temperatures (Table 2). As shown in Fig. 8, the variation of μ is negligible when the ambient temperature is lower than 200 1C. Values of k at the temperatures of 100 and 150 1C are slightly higher than that at 25 and 65 1C; but the values of k (ko1.3 10–15 m3/Nm) are still low. However, μ and k rise remarkably when the ambient temperature reaches 200 1C, indicating the severe wear properties of the composites. It can be accepted that a critical ambient temperature ranging 150–200 1C is ascertained. Based on the analyses mentioned above, the critical parameters of load, velocity and ambient temperature are confirmed. The friction and wear performances of PES/PTFE composites will become worse when these parameters are beyond the critical values of them.
absorption peaks at 636, 1147 and 1201 cm 1 assigned to the –CF2– groups, and two peaks at 1576 and 1483 cm 1 owing to the aromatic benzene ring can be observed, suggesting the existences of PES and PTFE. After the friction tests (Fig. 9b–e), the detected peaks of the abraded surfaces for the four composites are in good accordance with that of pure PES; however, no obvious peak attributed to the PTFE can be found. This indicates that PES is the major polymer remained on the worn surface, thereby forming a residual PES layer.
3.2. ATR-FTIR analysis
3.3. Morphology characterization
ATR-FTIR can be employed to identify the functional groups on the surface layers of samples [18–20], which was adopted to investigate the chemical groups existing on the worn surfaces of PES/PTFE composites. In the ATR-FTIR spectrum of PTFE (Fig. 9a), three absorption peaks observed at 1201, 1147 and 636 cm 1 can be assigned to the asymmetric stretch, symmetric stretch and wagging of –CF2– groups, respectively [21]. The sharp peak at 1144 cm 1 in the spectrum of PES is attributed to the symmetric stretching of – SO2– group [22]. Both the two adjacent bands at 1576 cm 1 and 1483 cm 1 are owned to the typical aromatic benzene ring [23]. ATR-FTIR spectra of the contact surfaces for the composite specimens are illustrated in Fig. 9b–e. Before the friction tests, three
3.3.1. Morphology of worn surface under an identical sliding condition The micrographs of the worn surfaces of PES/PTFE composites are shown in Fig. 10. Many obvious furrows and indentations (arrow) can be observed on the worn surface of sample A (Fig. 10a), but no obvious furrows could be found for the worn surfaces of samples B–D (Fig. 10b–d). Thus, the surface of sample A will be abraded more easily than that of B–D, consistent with the results of friction tests (Fig. 5). It can be observed that many fragments are scattered on the discontinuous worn surfaces of B and C, but the distance between fragments of sample C is shorter than that of B (double-headed arrow), indicating that the wear
Fig. 8. The friction coefficient and wear rate as a function of the ambient temperature for sample D (load: 400 N, velocity: 0.48 m/s).
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Fig. 9. ATR-FTIR spectra of pure polymers (a) and PES/PTFE composites: (b) 10% PES, (c) 20% PES, (d) 30% PES, (e) 40% PES.
resistance of C is better than that of B. Compared with the discontinue PES layers existing on the worn surfaces of other three samples, the worn surface of sample D is more continuous and no large cracks can be found. For this reason, the PES layer on the worn surface of sample D is hard to be abraded, resulting in its lowest wear rate.
When sliding against metals, it is well known that the PTFE transfer film can be formed on the metallic counterpart, and it will play a major role in reducing the friction coefficient and wear rate of the PTFE-based composite [15,24]. The morphologies and chemical components of the abraded surfaces of Q235 steels were investigated by SEM (Fig. 11) and EDS (Fig. 12). Samples of this carbon steel
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Fig. 10. SEM micrographs of the worn surface of PES/PTFE composites after the friction tests (load: 1000 N, velocity: 0.48 m/s): (a) 10% PES, (b) 20% PES, (c) 30% PES, (d) 40% PES.
Fig. 11. SEM micrographs of the worn surface for Q235 steel after the friction tests (load: 1000 N, velocity: 0.48 m/s): (a) 10% PES, (b) 20% PES, (c) 30% PES, (d) 40% PES.
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Fig. 12. EDS analysis of the worn surface for Q235 steel before (a) and after the friction tests (load: 1000 N, velocity: 0.48 m/s): (b) 10% PES, (c) 20% PES, (d) 30% PES, (e) 40% PES.
sliding against the PES/PTFE composites with PES additions of 10%, 20%, 30%, and 40% are denoted as E, F, G and H, respectively. The results of EDS demonstrate that a portion of PTFE is transferred to the steel, forming a PTFE transfer film. But PES cannot be found in this transfer film except for sample E, indicating that PES cannot be easily incorporated into this transfer film. The PTFE transfer films formed on the surfaces of samples F–G are thin and uniform (Fig. 11b–d), which could effectively reduce the friction coefficients and wear rates of these composites [25–27]. On the contrary, a thick and nonuniform transfer film is observed on the surface of sample E, and the lump wear fragments adhering on the surface of the steel can also be found (arrow, Fig. 11a). PES may be involved in these large scale wear debris adhered on the surface of sample E. For this reason, the transfer film formed on the surface of sample E consists both PTFE and PES. In contrast with the PTFE transfer films of samples B–D, this thick and lumpy transfer film containing PES and PTFE decreases the wear resistance of sample A. Based on the results of SEM and EDS, we confirm that a portion of PTFE is transferred to the surface of the carbon steel, forming the PTFE transfer film. However, in most situations, PES cannot be transferred and it will retain on the worn surfaces of the PES/PTFE composites, leading to the existence of a PES layer. To some extent, the tribological behaviors of PES/PTFE composites sliding against the carbon steel are governed by the interaction between the PTFE transfer film and the residual PES layer. Also, this interaction will be the cause for the excellent performances of anti-friction and wear resistance for PES/PTFE composites.
3.3.2. Morphology of worn surface under different sliding conditions In addition to the morphologies of worn surface under an identical sliding condition, the worn surfaces related to the composite with 40%
PES under various sliding conditions were also characterized. Four sliding conditions were chosen for the characterizations: (a) 400 N, 0.482 m/s (below all critical parameters); (b) 400 N, 0.482 m/s, 200 1C (above critical temperature); (c) 1400 N, 0.482 m/s (above critical load); (d) 400 N, 1.204 m/s (above critical velocity). The morphologies of the worn surface for PES/PTFE composite (40% PES) after four different friction tests are illustrated in Fig. 13. Below the critical parameters, the worn surface is dense, smoothing, and no obvious furrow can be found (Fig. 13a). Conversely, many cracks can be observed on the worn surface when the sliding parameter is higher than the critical values of temperature (Fig. 13b), load (Fig. 13c), and velocity (Fig. 13d), suggesting that the composites may be easy to be abraded from the surface, consequently resulting in their high wear rate under these conditions. The morphologies and chemical components of the abraded surfaces of Q235 steels were investigated by SEM (Fig. 14) and EDS (Fig. 15). As shown in Fig. 14, the steel surfaces are all uniform, continuous, and no wear fragments can be observed. On the other hand, the EDS results reveal that F element can be found on the steel surface under 400 N and 0.482 m/s (Fig. 15a), indicating the formation of PTFE transfer film. Nevertheless, F element cannot be found for the other three conditions, demonstrating the inexistence of PTFE transfer film. Therefore, it may be deduced that PTFE transfer film cannot be formed on the steel surface, when the sliding parameters are higher than the critical values. 3.4. Thermal analysis In the friction process, it has been reported that 90–95% of the mechanical energy is translated into heat [28,29]. The heat accumulated in the friction interface leads to the increase in interfacial temperature, thereby plays a dominant influence on
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Fig. 13. SEM micrographs of the worn surface of PES/PTFE composite (40% PES) after different friction tests: (a) 400 N, 0.48 m/s; (b) 400 N, 0.48 m/s, 200 1C; (c) 1400 N, 0.48 m/s; (d) 400 N, 1.20 m/s.
Fig. 14. SEM micrographs of the worn surface for Q235 steel (sliding against composite with 40% PES) after different friction tests: (a) 400 N, 0.48 m/s; (b) 400 N, 0.48 m/s, 200 1C; (c) 1400 N, 0.48 m/s; (d) 400 N, 1.20 m/s.
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Fig. 15. EDS analysis of the worn surface for Q235 steel (sliding against composite with 40% PES) after different friction tests: (a) 400 N, 0.48 m/s; (b) 400 N, 0.48 m/s, 200 1C; (c) 1400 N, 0.48 m/s; (d) 400 N, 1.20 m/s.
Fig. 16. DSC (a) and TGA (b) curves of PES, PTFE and PES/PTFE composites.
the tribological behavior of polymers. In general, the glass transition temperature (Tg) extensively affects the applications of polymers. The deformation of polymers increases significantly when the interfacial temperature reaches Tg. Tg of PES and PTFE is 125 and 233 1C, respectively, which was analyzed by the DSC measurements. The sharp peak appearing at 330 1C (Fig. 16a) can be attributed to the melt of PTFE. The reinforcing role of PES is crucial to the tribological property of PES/PTFE composites. The PES layer formed on the surface of the composite can be easily deformed when the load, velocity and ambient temperature are higher than the critical values, which may be due to that the interfacial temperature is higher than the Tg of PES. Under these sliding conditions, the friction and wear performances of the composites tend to become worse dramatically. In addition, the elevated ambient temperatures can also cause the remarkable changes of μ and k, and both of them increase abruptly when the ambient temperature reaches 200 1C. Under the ambient temperature of 200 1C, it may be deduced that the interfacial temperature is higher than the Tg of PES (233 1C), due to the fact that the heat is generated in the frictional interface. To some extent, this may be the cause of the deterioration of the tribological properties for PES/PTFE composites.
The TGA data of PES, PTFE and PES/PTFE composites are illustrated in Fig. 16b. The initial decomposition temperature of PTFE is around 510 1C, and a sharp decrease in its weight can be observed at 600 1C. The complete decomposition of PTFE takes place at 630 1C. The initial and the rapid decomposition of PES occur at 450 1C and 550 1C, respectively. After the test, PES with a residual mass of 41 wt% can be obtained. The four PES/PTFE composites start to decompose at 500 1C; the significant weight losses of them occur at the temperatures ranging from 550 to 590 1C. The residual masses of samples A, B, C and D are 5.2%, 11.0%, 16.9%, and 20.9%, respectively. Although the initial decomposition temperature of PES is lower than that of PTFE, the initial decomposition temperatures of PES/PTFE composites show an unremarkable difference in compassion with that of PTFE. It can be confirmed that the existent PTFE with a high decomposition temperature is benefit for the stabilities of the PES/PTFE composites.
4. Conclusion ATR-FTIR, SEM, EDS, DSC, and TG combined with the tribological tests were employed to study the friction and wear behaviors
Z. Zuo et al. / Tribology International 86 (2015) 17–27
of PES/PTFE composites under the dry sliding conditions. The following conclusions can be drawn: (1) The tribological property of PTFE is improved effectively by the blending of PES. PES/PTFE composites will be more suitable for the applications in high load-low velocity systems rather than low load-high velocity systems. (2) During the friction process, PTFE transfer film is formed on the Q235 steel surface; whereas PES is remained on the worn surface of PES/PTFE composites, thereby forming a PES layer. The tribological behavior between the steel and composites is governed by the interaction between the PTFE transfer film and residual PES layer, resulting in the excellent friction and wear behaviors of the PES/PTFE composites. (3) The heat accumulated in the friction interface may be the primary cause for the confirmations of critical values for load, velocity and ambient temperature. When the sliding parameters are higher than the critical values, the PES can be easily deformed because the interfacial temperature reaches its Tg. Consequently, the friction coefficient and wear rate of the PES/ PTFE composites rise dramatically. (4) Below the critical sliding conditions, the composite with 40% PES illustrates the best tribological property (μ o0.11, ko 7.0 10–16 m3/Nm). This can be attributed to the coexistence of the uniform PTFE transfer film and the most continuous PES layer.
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