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Kevlar fabric composites with nano-Si3N4 and submicron size WS2 fillers
Tribology International 80 (2014) 172–178
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Tribological behavior of hybrid PTFE/Kevlar fabric composites with nano-Si3N4 and submicron size WS2 fillers HuLin Li, ZhongWei Yin n, Dan Jiang, YaJun Huo, YuQing Cui Shanghai Jiao Tong University, School of Mechanical Engineering, Shanghai 200240, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 March 2014 Received in revised form 23 June 2014 Accepted 9 July 2014 Available online 23 July 2014
Hybrid PTFE/Kevlar fabric composite specimens were prepared with nano-Si3N4 and/or submicron size WS2 as fillers. The tribological behaviors of these composites were studied. The morphologies of the worn surface, transfer film and debris were analyzed by means of scanning electron microscopy. In addition, an energy-dispersive X-ray spectrometer was used for analysis of the elemental distribution and content in the transfer film. The results indicate that single nano-Si3N4 fillers can effectively reduce the wear rate of composites, but they do not reduce the friction coefficient. Hybrid Si3N4 and WS2 fillers can significantly reduce the wear rate and friction coefficient of composites. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Hybrid PTFE/Kevlar fabric composite Nanoparticle Transfer film Friction and wear
1. Introduction Fabric composites play an important role in materials and mechanical engineering, not only for their ease in manufacturing and low unit cost but also for their potentially excellent tribological performance in engineered forms; therefore, they are very attractive materials for use as advanced bearing liner materials. Hybrid PTFE/Kevlar fabric composites, which are woven out of PTFE and Kevlar fibers, are one of the most popularly used bearing liner materials. The good lubrication of the PTFE fiber side was used as the friction surface, and the Kevlar fiber side, with its good mechanical properties, was used as the binding surface [1–3]. However, it is well known that PTFE fibers exhibit a high wear rate in the pristine form; as a result, hybrid PTFE/Kevlar fabric composites would also not satisfy most of the tribological requirements. Therefore, a modification is necessary to improve the tribological properties of hybrid PTFE/Kevlar fabric composites. Many researchers have developed polymer-based nanocomposites for tribological applications [4–8]. Li et al. filled PTFE with nanoparticles of ZnO and found that using nanometer ZnO powders as a filler in PTFE changed the microstructure of PTFE and prevented the destruction of PTFE banded structures during the friction process [9]. Sawyer et al. studied the wear and friction properties of nanoalumina-filled PTFE composites and found that at filler concentrations of 20 wt%, the wear resistance improved by 600 [10]. Sebastian et al. studied the tribological behavior of polyphenylensulfide composites filled with short carbon fibers and/or carbon nanotubes. They found
n
Corresponding author. E-mail address: [email protected] (Z. Yin).
http://dx.doi.org/10.1016/j.triboint.2014.07.006 0301-679X/& 2014 Elsevier Ltd. All rights reserved.
that due to the fast formation of a groove-filling transfer film on the counterpart, these composites exhibited an excellent tribological property [8]. Wang et al. filled polyetheretherketone (PEEK) with various weight fractions of nano-Si3N4 and found that nano-Si3N4 particles could significantly improve the wear resistance and reduce the friction coefficient. This improvement was closely related to the improved characteristics of the transfer film [11]. Chang et al. observed the tribological properties of polyamide composites filled with TiO2 nanoparticles, short carbon fibers, and graphite flakes. It was found that nano-TiO2 could effectively reduce the frictional coefficient and wear rate. One mechanism of the remarkable improvement of the tribological properties of the composites was the positive rolling effect of nanoparticles between the material pairs [12,13]. Recently, some research on nanoparticle-filled fabric composites revealed their significant potential in producing materials with good comprehensive friction properties [14–19]. Professor Zhang and his coworkers at the Lanzhou Institute of Chemical Physics have performed a significant amount of work in the area of fabric composites filled with nanoparticles. Su et al. investigated the tribological properties of nano-TiO2 and nano-ZnO-filled hybrid glass/PTFE fabric composites, and the results indicated that the incorporation of TiO2 and ZnO nanoparticles could significantly improve the wear resistance of the composites [14–17]. Zhang et al. studied the influence of nano-TiO2 and nano-SiO2 on the tribological behavior of hybrid PTFE/Kevlar fabric/phenolic composites. The incorporation of TiO2 or SiO2 nanoparticles was found to reduce the wear rate of the fabric composite at elevated temperatures, but it did not have an important influence on the wear resistance of the composites at room temperature. Nanoparticles can be effective in reducing the wear rate of fabric composites, and in some cases, the coefficient of friction can also be reduced. However, in practice, fabric composites filled with
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Fig. 1. Image of the PTFE/Kevlar fabric.
Table 1 Properties of nano-Si3N4 and submicron size WS2. Materials
Si3N4
WS2
Grain size (nm) Purity (%) Specific surface area (m²/g) Tap density (g/cm3) Bulk density (g/cm3)
80–100 99.9 71.0 1.14 0.49
600 99.9 3.2 2.04 0.44
only nano-fillers still do not meet the practical requirements, due to their relatively high friction coefficient. Thus, a combination of various functional fillers is required. Unfortunately, few studies have been performed to investigate such combinations. This paper therefore focuses on the study of the tribological behavior of hybrid PTFE/Kevlar fabric composites with a combination of nanoSi3N4 and submicron size WS2 fillers.
2. Experimental section 2.1. Materials and specimen preparation The PTFE/Kevlar fabric used in this study was woven from PTFE fibers and Kevlar fibers, and a picture of the fabric is shown in Fig. 1. As shown in Fig. 1, the two sides of the fabric have different proportions of PTFE and Kevlar. The front face was rich in PTFE fiber and was always used as a friction surface. The back face was rich in Kevlar fiber and was used as a binding surface. In this way, the low friction of PTFE and high strength of Kevlar were combined to a great extent [1]. The fillers used in this study are commercially available, and the material properties are listed in Table 1. Due to the very small diameter of nano-Si3N4, nano-Si3N4 has a much higher surface area-to-volume ratio than do larger-sized particles, and their surface energy is high. Thus, the adhesive force between the nanoparticles is strong, and the particles are easily agglomerated [14]. To prevent agglomeration of the particles, ultrasonic vibration was used to disperse the nano-Si3N4 and submicron size WS2 particles in a modified polyimide resin. The hybrid PTFE/Kevlar fabrics were ultrasonically cleaned in an acetone bath for 1 h, boiled 30 min in distilled water, dried in an oven at 80 1C for 1 h, and then the dried hybrid PTFE/Kevlar fabrics were weighed by a precision balance (accuracy: 0.1 mg). The fillers were mixed uniformly with modified polyimide adhesive resin at the proper mass fractions with the assistance of ultrasonic stirring. Afterwards, the hybrid PTFE/Kevlar fabrics were immersed in the mixed adhesive containing the fillers. Subsequently, the immersed PTFE/Kevlar fabrics were dried in a nitrogen-purged environment at a rate of 50 1C/h to 130 1C, where they were held for 1 h.
Fig. 2. The schematics of the test assembly.
The specimens were then cooled at a rate of 50 1C/h back to room temperature. After this, the specimens were weighed and the relative mass fraction of fabrics was calculated. The immersion was repeated several times until the relative mass fraction of fabrics was 707 5%. And then, a laboratory press was used to consolidate the fabrics under pressure of 5 MPa at 210 1C for 30 min. Finally, the filled PTFE/Kevlar fabric composites (which were composed of the PTFE/Kevlar fabric, the adhesive resin, and the fillers) and unfilled PTFE/Kevlar fabric composites were affixed onto the 440c stainless steel using the modified polyimide adhesive resin and then cured at 210 1C for 15 min under pressure of 0.2–0.3 MPa. 2.2. Tribological test Tribological tests were performed using a pin-on-disc tribometer (RTEC MFT-5000, made in the USA) under dry friction conditions. All of the experiments were performed under laboratory conditions (temperature, T ¼25 1C, relative humidity 50%). Fig. 2 shows the schematics of the test assembly. In the pin-ondisc tester, a stationary steel pin slid against a rotating steel disc that was affixed with the PTFE/Kevlar fabric composite specimens. The flat-ended bearing steel pin (diameter of 3.3 mm) was secured to the load arm with a chuck. The distance between the center of the pin and the center of the disc was 16 mm. The pin remained over the disc with two degrees of freedom: the vertical one for normal load application by direct contact with the disc and the horizontal one for friction measurement. The sliding was performed under dry friction conditions at a sliding speed of 1 m/s and at a normal load of 513 N (60 MPa), and the sliding distance used was 1.8 km. For self-lubricating fabric composites these contact pressures are typical for industrial applications. The normal load and friction force were measured by normal force sensor (range: 50–5000 N, resolution: 0.25 N) and friction force sensor (range: 1–100 N, resolution: 0.005 N), and the frequency of sampling is 1000 Hz. The friction coefficient m of the
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specimen were calculated by the relationship m ¼F/P, where F is the friction force (N), P is the normal load (N). At the end of each test, the wear volume loss of the PTFE/Kevlar fabric composites was obtained by measuring the cross-sectional area of the wear scar and the wear depth using the KEYENCE VK-X200 laser microscopic 3D & profile measurement apparatus (accuracy: 0.012 μm, made in Japan). To ensure statistically relevant results, each sample was taken three cross-sectional measurements equispaced distributed along the wear scar to determine the general average of volume loss occurring. Meanwhile, in order to reduce the influence of initial surface characteristics on the measurement accuracy the average height of the initial surface were used as the measuring datum (base line).The wear data were finally presented as the wear coefficients in well-known units (mm3/Nm). The uncertainty of volume loss (uv) is determined by the following equation [20]: 2πR uv ¼ pffiffiffiffi σ A N
ð1Þ
where R is the nominal radius of the wear scar, N is the number of scans and σ A is the standard deviation of the measured areas. The schematic diagram of the measurement technique is shown in Fig. 3. Prior to the tests, the pins were polished using 1000 grit paper to an average roughness of Ra ¼0.1–0.2 μm, and then the pins were cleaned with acetone and dried with a laboratory wipe. Each experiment used a new pin and a new PTFE/Kevlar fabric sample. To ensure statistically relevant results, each experiment was repeated several times. Thus, every data point in the diagrams represents an average of three repeated tests. After the experiments, all the samples (the surface of the pins, the wear scar of the PTFE/Kevlar fabrics and its debris) were examined by Scanning electron microscopy (SEM), and the ones shown were representative of the general samples. In addition, Energy dispersive x-ray spectroscopy (EDS) was used for the determination of the elemental distribution and content in the transfer films.
3. Results and discussion 3.1. Friction and wear properties Fig. 4a shows the variation of the friction coefficient and the wear rate of hybrid PTFE/Kevlar fabric composites containing different contents of Si3N4 nanoparticles under the load of 513 N (60 MPa) at the speed of 1 m/s. The use of the fillers remarkably reduced the wear rate of the composites. Compared to unfilled PTFE/Kevlar fabric composites, the composites with 12.5 wt% nano-Si3N4 exhibit the best wear resistance. In contrast, with the increase in the content of filler, the friction coefficient increased. Contrary to the above case, submicron size WS2 filler was favorable for the reduction of the friction coefficient and the wear rate
Fig. 3. Schematic diagram of the measurement technique.
of composites, as shown in Fig. 4b. When the content of filler increased to 7.5 wt%, the composites exhibited lower wear rate, and the minimum friction coefficient. Therefore, this content was selected to investigate the combined effects of two types of fillers on the tribological properties of hybrid PTFE/Kevlar fabric composites. Fig. 4c shows the variation of the friction coefficient and the wear rate of the PTFE/Kevlar fabric composite filled with hybrid nano-Si3N4 and submicron size WS2 fillers (the content of WS2 is a constant value of 7.5 wt%, and the content of Si3N4 is varied from 2.5 wt% to 12.5 wt%). The friction condition was the same as that mentioned above. The wear rate was found to sharply decrease with the increase in the content of Si3N4 nanoparticles, and the composites with 7.5 wt% WS2 þ 12.5 wt% nano-Si3N4 exhibits the lowest wear rate. Fig. 4d and e shows the variations of the friction coefficient and the wear rate of all types of composites. It can be observed from the figures that hybrid fillers exhibit the best wear resistance and friction properties. 3.2. SEM observations of the worn surfaces Fig. 5a–d shows the SEM images of the worn surface of unfilled and WS2 þSi3N4-filled PTFE/Kevlar fabric composites. Fig. 5a and b shows the SEM image of the worn surface of unfilled PTFE/Kevlar fabric composites. A large number of PTFE and Kevlar fibers significantly peel, and there are many long and deep cracks on the worn surface, which is indicative of severe wear. Fig. 5c and d shows the SEM images of the worn surfaces of 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled composites. The worn surface of the WS2 þ Si3N4-filled composites is quite smooth, and the fibers are strongly combined with the matrix. This observation indicates that the addition of WS2 and Si3N4 filler in the hybrid PTFE/Kevlar fabric composite results in strong adhesion of fibers to the matrix, which results in the strengthening of the mechanical properties of the fabric composite. Therefore, the wear resistance is significantly improved. 3.3. SEM and EDS analysis of the transfer films It is well known that achieving the appropriate characteristics of the transfer film is a very important factor affecting the tribological properties of polymer composites. To better understand the role of fillers in tribological processes, SEM was used to explore the morphologies of the transfer films formed on the steel pin surface for unfilled and filled hybrid PTFE/Kevlar fabric composites. In addition, an energy-dispersive X-ray spectrometer (EDS) was used for analysis of the elemental distribution and content in the transfer films. The results of the EDS elemental analysis are shown in Fig. 6. The F element and O element were found in the transfer films for unfilled fabric composites. The F element, O element, N element and Si element were found in transfer films for 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites. These findings indicate that the transfer of PTFE, resin and filler occurred during sliding [21,22]. It is also observed from Fig. 6 that the F element and O element content in the transfer film for unfilled composites are higher and that the Fe element content is lower than that of the filled composites. This observation might suggest that the transfer film for unfilled composites formed on the steel pin is thicker than that of the 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites. It can be seen from Fig. 6a that the transfer film formed on the unfilled composites is characterized as thick and lumpy. This type of transfer film has poor adhesive properties with the substrate and was easily peeled off from the counterface. Therefore, it can be inferred that adding nanoparticles to composites can significantly improve the performance of the transfer film formed on the
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Fig. 4. Variation of the friction coefficient/wear rate and their standard deviation of the hybrid PTFE/Kevlar fabric composites with different filler contents.
counterface, which results in a much lower wear rate for the filled composite in comparison with the unfilled one. The SEM image of the transfer film formed on the pin's surface is shown in Fig. 7. For unfilled composites, the transfer film formed on the pin's surface is thick and lumpy. According to former research, this type of transfer film was easily scaled off of the counterface and formed debris during wear process, which resulted in the high wear rate of unfilled hybrid PTFE/Kevlar fabric composites. Whereas a uniform and thin transfer film is formed on the pin sliding against the 7.5 wt% WS2 þ 12.5 Si3N4-filled hybrid PTFE/Kevlar fabric composites. The thin and uniform transfer film will firmly adhere onto the counterface and is not easily scaled off during sliding. The tenacious transfer film can prevent the direct
contact between the steel pin and fabric composites, thereby significantly reducing the wear rate. Moreover, by careful observation of Fig. 7d, we find that there are many nanoparticles on the transfer film surface; those nanoparticles between the material pairs may have acted as roller bearings and further reduced the wear rate [12,14,23].
3.4. SEM examination of the wear debris Debris is a product of the frictional experiment, and analyzing debris is helpful to understand the tribological mechanisms. Fig. 8 shows the SEM images of the wear debris of unfilled hybrid
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Fig. 5. Scanning electron micrographs of the worn surfaces of hybrid PTFE/Kevlar fabric composites: (a) unfilled, (b) magnification of (a), (c) filled with 7.5 wt% WS2 þ 12.5 wt% Si3N4, and (d) magnification of (c).
Fig. 6. EDS analysis of the transfer film formed on a steel pin: (a) unfilled and (b) filled with 7.5 wt% WS2 þ 12.5 wt% Si3N4.
PTFE/Kevlar fabric composites and 7.5 wt% WS2 þ12.5 wt% Si3N4filled composites. It can be observed from Fig. 8a and b that unfilled composites generate debris that mainly consists of wide ribbon debris and lumpy slabs, which is indicative of delamination. Contrary to the unfilled composites, the wear debris of 7.5 wt% WS2 þ12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites is much smaller and mainly consists of tiny flakes and filamentous wear debris. This observation indicates that adding nanoparticles to PTFE/Kevlar fabric composites can restrain the formation of larger debris. Moreover, wide ribbon and lumpy slab debris cannot easily fill in the grooves on the fabric surface, and hence, they pile up during repeated sliding and subsequently are discarded as debris. Wear debris composed of tiny flakes is easily trapped in the gap of a worn surface and can repair the damaged surface, and this trapped debris can be considered as a secondary source of lubricant that permeates to the surface to reduce the friction and retard the galling during sliding. Therefore, adding hybrid fillers to PTFE/Kevlar fabric composites can sharply improve the friction and wear properties of the composites.
4. Conclusion This work presents investigations on the tribological behavior of hybrid PTFE/Kevlar fabric composites with nano-Si3N4 and/or submicron size WS2 filler samples against bearing steel at dry friction conditions. The following conclusions can be drawn from the present study: (a) It was found that under the experimental conditions, Si3N4 nanoparticles could effectively reduce the wear rate of hybrid PTFE/Kevlar fabric composites. (b) Submicron size WS2 filler was favorable for the reduction of the friction coefficient and the wear rate of composites compared to the unfilled samples; the 7.5 wt% WS2 filled composites exhibited lowest friction coefficient, and the value is 0.045. (c) The wear rate and friction coefficient of 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites were much less than those of the unfilled composites. The wear rate was reduced greatly, from 8.75 10 14 m3/(Nm) to
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Fig. 7. The scanning electron micrographs of the transfer film formed on the counterpart pin: (a) unfilled, (b) magnification of (a), (c) filled with 7.5 wt% WS2 and 12.5 wt% Si3N4, and (d) magnification of (c).
Fig. 8. SEM images of the wear debris of hybrid PTFE/Kevlar fabric composites: (a) lumpy slabs debris (unfilled), (b) wide ribbon debris (unfilled), (c) tiny flake debris (filled) and (d) filamentous debris (filled).
0.41 10 14 m3/(Nm), and the uncertainty of wear rate of unfilled composites and the 7.5 wt% WS2 þ12.5 wt% Si3N4-filled composites are 0.069 10 14 m3/(Nm) and 0.073 10 14 m3/(Nm), respectively. The friction coefficient also decreased from unfilled (0.059) to filled with hybrid fillers (0.040). Thus, hybrid fillers could achieve the desired comprehensive tribological properties in dry sliding.
(d) The transfer films of 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites were thin, uniform and full of nanoparticles. The tenacious transfer film can prevent the direct contact between the steel pin and the fabric composites, and those nanoparticles between the material pairs are thought to have acted as roller bearings, thereby further reducing the wear rate.
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(e) The wear debris of 7.5 wt% WS2 and 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites was much smaller than that of the unfilled composites. This observation indicated that adding nanoparticles to PTFE/Kevlar fabric composites could restrain the formation of larger debris. Moreover, tiny wear debris was easily trapped in the gap of a worn surface and can repair the damaged surface. In addition, the trapped debris could be considered as a secondary source of lubricant.
Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant no. 51175341). References [1] Zhang ZZ, Zhang HJ, Guo F, Wang K, Jiang W. Enhanced wear resistance of hybrid PTFE/Kevlar fabric–phenolic composite by cryogenic treatment. J Mater Sci 2009;44:6199–205. [2] Biswas SK, Vijayan K. Friction and wear of PTFE—a review. Wear 1992;158: 193–211. [3] Penn L, Larsen F. Physicochemical properties of Kevlar 49 fiber. J Appl Polym Sci 1979;23:59–73. [4] 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. [5] Burris DL, Sawyer WG. Improved wear resistance in alumina-PTFE nanocomposites with irregular shaped nanoparticles. Wear 2006;260:915–8. [6] Wetzel B, Haupert F, Friendrich K, Zhang MQ, Rong MZ. Impact and wear resistance of polymer nanocomposites at low filler content. Polym Eng Sci 2002;42:1919–27. [7] Schwartz CJ, Bahadur S. Studies on the tribological behavior and transfer filmcounterface bond strength for polyphenylene sulfide filled with nanoscale alumina particles. Wear 2000;237:261–73.
[8] Sebastian R, Noll A, Zhang G, Burkhart T, Wetzel B. Friction and wear of PPS/ CNT nanocomposites with formation of electrically isolating transfer films. Tribol Int 2013;64:187–95. [9] Li F, Hu KA, Li JL, Zhao BY. The friction and wear characteristics of nanometer ZnO filled polytetrafluoroethylene. Wear 2002;249:877–82. [10] Sawyer WG, Freudenberg KD, Praveen B, Schadler LS. A study on the friction and wear behavior of PTFE filled with alumina nanoparticles. Wear 2003;254:573–80. [11] Wang QH, Xu JF, Shen WC, Liu WM. An investigation of the friction and wear properties of nanometer Si3N4 filled PEEK. Wear 1996;196:82–6. [12] Chang L, Zhang Z, Zhang H, Schlarb AK. On the sliding wear of nanoparticle filled polyamide 66 composites. Compos Sci Technol 2006;66:3188–98. [13] Chang L, Friedrich K. Enhancement effect of nanoparticles on the sliding wear of short fiber-reinforced polymer composites: a critical discussion of wear mechanisms. Tribol Int 2010;43:2355–64. [14] Su FH, Zhang ZZ, Liu WM. Tribological behavior of hybrid glass/PTFE fabric composites with phenolic resin binder and nano-TiO2 filler. Wear 2008;264: 562–70. [15] Zhang HJ, Zhang ZZ, Guo F. A study on the sliding wear of hybrid PTFE/Kevlar fabric/phenolic composites filled with nanoparticles of TiO2 and SiO2. Tribol Trans 2010;53:678–83. [16] Zhang HJ, Zhang ZZ, Guo F, Jiang W, Wang K. Friction and wear of hybrid PTFE/ Kevlar fabric composite filled with ZnO nanoparticles sliding against steel, copper, and aluminum. Tribol Trans 2009;52:833–40. [17] Su FH, Zhang ZZ, Liu WM. Friction and wear behavior of hybrid glass/PTFE fabric composite reinforced with surface modified nanometer ZnO. Wear 2008;265:311–8. [18] Zhang HJ, Zhang ZZ, Guo F, Liu WM. Friction and wear behavior of the hybrid PTFE/cotton fabric composites filled with TiO2 nanoparticles and modified TiO2 nanoparticles. Polym Eng Sci 2009;49:115–22. [19] Zhang HJ, Zhang ZZ, Guo F, Wang K, Jiang W. Enhanced wear properties of hybrid PTFE/cotton fabric composites filled with functionalized multi-walled carbon nanotubes. Mater Chem Phys 2009;116:183–90. [20] Colbert RS, Krick BA, Dunn AC, Vail JR, Argibay N, Sawyer WG. Uncertainty in pin-on-disk wear volume measurements using surface scanning techniques. Tribol Lett 2011;42:129–31. [21] Lai SQ, Li TS, Liu XJ, Lv RG. A study on the friction and wear behavior of PTFE filled with acid treated nano-attapulgite. Macromol Mater Eng 2004;289: 916–22. [22] Yamane M, Stolarski TA, Tobe S. Influence of counter material on friction and wear performance of PTFE–metal binary coatings. Tribol Int 2008;41:269–81. [23] Jawahar P, Gnanamoorthy R, Balasubramanian M. Tribological behaviour of clay–thermoset polyester nanocomposites. Wear 2006;261:835–40.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Tribological behavior of hybrid PTFE/Kevlar fabric composites with nano-Si3N4 and submicron size WS2 fillers HuLin Li, ZhongWei Yin n, Dan Jiang, YaJun Huo, YuQing Cui Shanghai Jiao Tong University, School of Mechanical Engineering, Shanghai 200240, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 March 2014 Received in revised form 23 June 2014 Accepted 9 July 2014 Available online 23 July 2014
Hybrid PTFE/Kevlar fabric composite specimens were prepared with nano-Si3N4 and/or submicron size WS2 as fillers. The tribological behaviors of these composites were studied. The morphologies of the worn surface, transfer film and debris were analyzed by means of scanning electron microscopy. In addition, an energy-dispersive X-ray spectrometer was used for analysis of the elemental distribution and content in the transfer film. The results indicate that single nano-Si3N4 fillers can effectively reduce the wear rate of composites, but they do not reduce the friction coefficient. Hybrid Si3N4 and WS2 fillers can significantly reduce the wear rate and friction coefficient of composites. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Hybrid PTFE/Kevlar fabric composite Nanoparticle Transfer film Friction and wear
1. Introduction Fabric composites play an important role in materials and mechanical engineering, not only for their ease in manufacturing and low unit cost but also for their potentially excellent tribological performance in engineered forms; therefore, they are very attractive materials for use as advanced bearing liner materials. Hybrid PTFE/Kevlar fabric composites, which are woven out of PTFE and Kevlar fibers, are one of the most popularly used bearing liner materials. The good lubrication of the PTFE fiber side was used as the friction surface, and the Kevlar fiber side, with its good mechanical properties, was used as the binding surface [1–3]. However, it is well known that PTFE fibers exhibit a high wear rate in the pristine form; as a result, hybrid PTFE/Kevlar fabric composites would also not satisfy most of the tribological requirements. Therefore, a modification is necessary to improve the tribological properties of hybrid PTFE/Kevlar fabric composites. Many researchers have developed polymer-based nanocomposites for tribological applications [4–8]. Li et al. filled PTFE with nanoparticles of ZnO and found that using nanometer ZnO powders as a filler in PTFE changed the microstructure of PTFE and prevented the destruction of PTFE banded structures during the friction process [9]. Sawyer et al. studied the wear and friction properties of nanoalumina-filled PTFE composites and found that at filler concentrations of 20 wt%, the wear resistance improved by 600 [10]. Sebastian et al. studied the tribological behavior of polyphenylensulfide composites filled with short carbon fibers and/or carbon nanotubes. They found
n
Corresponding author. E-mail address: [email protected] (Z. Yin).
http://dx.doi.org/10.1016/j.triboint.2014.07.006 0301-679X/& 2014 Elsevier Ltd. All rights reserved.
that due to the fast formation of a groove-filling transfer film on the counterpart, these composites exhibited an excellent tribological property [8]. Wang et al. filled polyetheretherketone (PEEK) with various weight fractions of nano-Si3N4 and found that nano-Si3N4 particles could significantly improve the wear resistance and reduce the friction coefficient. This improvement was closely related to the improved characteristics of the transfer film [11]. Chang et al. observed the tribological properties of polyamide composites filled with TiO2 nanoparticles, short carbon fibers, and graphite flakes. It was found that nano-TiO2 could effectively reduce the frictional coefficient and wear rate. One mechanism of the remarkable improvement of the tribological properties of the composites was the positive rolling effect of nanoparticles between the material pairs [12,13]. Recently, some research on nanoparticle-filled fabric composites revealed their significant potential in producing materials with good comprehensive friction properties [14–19]. Professor Zhang and his coworkers at the Lanzhou Institute of Chemical Physics have performed a significant amount of work in the area of fabric composites filled with nanoparticles. Su et al. investigated the tribological properties of nano-TiO2 and nano-ZnO-filled hybrid glass/PTFE fabric composites, and the results indicated that the incorporation of TiO2 and ZnO nanoparticles could significantly improve the wear resistance of the composites [14–17]. Zhang et al. studied the influence of nano-TiO2 and nano-SiO2 on the tribological behavior of hybrid PTFE/Kevlar fabric/phenolic composites. The incorporation of TiO2 or SiO2 nanoparticles was found to reduce the wear rate of the fabric composite at elevated temperatures, but it did not have an important influence on the wear resistance of the composites at room temperature. Nanoparticles can be effective in reducing the wear rate of fabric composites, and in some cases, the coefficient of friction can also be reduced. However, in practice, fabric composites filled with
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173
Fig. 1. Image of the PTFE/Kevlar fabric.
Table 1 Properties of nano-Si3N4 and submicron size WS2. Materials
Si3N4
WS2
Grain size (nm) Purity (%) Specific surface area (m²/g) Tap density (g/cm3) Bulk density (g/cm3)
80–100 99.9 71.0 1.14 0.49
600 99.9 3.2 2.04 0.44
only nano-fillers still do not meet the practical requirements, due to their relatively high friction coefficient. Thus, a combination of various functional fillers is required. Unfortunately, few studies have been performed to investigate such combinations. This paper therefore focuses on the study of the tribological behavior of hybrid PTFE/Kevlar fabric composites with a combination of nanoSi3N4 and submicron size WS2 fillers.
2. Experimental section 2.1. Materials and specimen preparation The PTFE/Kevlar fabric used in this study was woven from PTFE fibers and Kevlar fibers, and a picture of the fabric is shown in Fig. 1. As shown in Fig. 1, the two sides of the fabric have different proportions of PTFE and Kevlar. The front face was rich in PTFE fiber and was always used as a friction surface. The back face was rich in Kevlar fiber and was used as a binding surface. In this way, the low friction of PTFE and high strength of Kevlar were combined to a great extent [1]. The fillers used in this study are commercially available, and the material properties are listed in Table 1. Due to the very small diameter of nano-Si3N4, nano-Si3N4 has a much higher surface area-to-volume ratio than do larger-sized particles, and their surface energy is high. Thus, the adhesive force between the nanoparticles is strong, and the particles are easily agglomerated [14]. To prevent agglomeration of the particles, ultrasonic vibration was used to disperse the nano-Si3N4 and submicron size WS2 particles in a modified polyimide resin. The hybrid PTFE/Kevlar fabrics were ultrasonically cleaned in an acetone bath for 1 h, boiled 30 min in distilled water, dried in an oven at 80 1C for 1 h, and then the dried hybrid PTFE/Kevlar fabrics were weighed by a precision balance (accuracy: 0.1 mg). The fillers were mixed uniformly with modified polyimide adhesive resin at the proper mass fractions with the assistance of ultrasonic stirring. Afterwards, the hybrid PTFE/Kevlar fabrics were immersed in the mixed adhesive containing the fillers. Subsequently, the immersed PTFE/Kevlar fabrics were dried in a nitrogen-purged environment at a rate of 50 1C/h to 130 1C, where they were held for 1 h.
Fig. 2. The schematics of the test assembly.
The specimens were then cooled at a rate of 50 1C/h back to room temperature. After this, the specimens were weighed and the relative mass fraction of fabrics was calculated. The immersion was repeated several times until the relative mass fraction of fabrics was 707 5%. And then, a laboratory press was used to consolidate the fabrics under pressure of 5 MPa at 210 1C for 30 min. Finally, the filled PTFE/Kevlar fabric composites (which were composed of the PTFE/Kevlar fabric, the adhesive resin, and the fillers) and unfilled PTFE/Kevlar fabric composites were affixed onto the 440c stainless steel using the modified polyimide adhesive resin and then cured at 210 1C for 15 min under pressure of 0.2–0.3 MPa. 2.2. Tribological test Tribological tests were performed using a pin-on-disc tribometer (RTEC MFT-5000, made in the USA) under dry friction conditions. All of the experiments were performed under laboratory conditions (temperature, T ¼25 1C, relative humidity 50%). Fig. 2 shows the schematics of the test assembly. In the pin-ondisc tester, a stationary steel pin slid against a rotating steel disc that was affixed with the PTFE/Kevlar fabric composite specimens. The flat-ended bearing steel pin (diameter of 3.3 mm) was secured to the load arm with a chuck. The distance between the center of the pin and the center of the disc was 16 mm. The pin remained over the disc with two degrees of freedom: the vertical one for normal load application by direct contact with the disc and the horizontal one for friction measurement. The sliding was performed under dry friction conditions at a sliding speed of 1 m/s and at a normal load of 513 N (60 MPa), and the sliding distance used was 1.8 km. For self-lubricating fabric composites these contact pressures are typical for industrial applications. The normal load and friction force were measured by normal force sensor (range: 50–5000 N, resolution: 0.25 N) and friction force sensor (range: 1–100 N, resolution: 0.005 N), and the frequency of sampling is 1000 Hz. The friction coefficient m of the
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specimen were calculated by the relationship m ¼F/P, where F is the friction force (N), P is the normal load (N). At the end of each test, the wear volume loss of the PTFE/Kevlar fabric composites was obtained by measuring the cross-sectional area of the wear scar and the wear depth using the KEYENCE VK-X200 laser microscopic 3D & profile measurement apparatus (accuracy: 0.012 μm, made in Japan). To ensure statistically relevant results, each sample was taken three cross-sectional measurements equispaced distributed along the wear scar to determine the general average of volume loss occurring. Meanwhile, in order to reduce the influence of initial surface characteristics on the measurement accuracy the average height of the initial surface were used as the measuring datum (base line).The wear data were finally presented as the wear coefficients in well-known units (mm3/Nm). The uncertainty of volume loss (uv) is determined by the following equation [20]: 2πR uv ¼ pffiffiffiffi σ A N
ð1Þ
where R is the nominal radius of the wear scar, N is the number of scans and σ A is the standard deviation of the measured areas. The schematic diagram of the measurement technique is shown in Fig. 3. Prior to the tests, the pins were polished using 1000 grit paper to an average roughness of Ra ¼0.1–0.2 μm, and then the pins were cleaned with acetone and dried with a laboratory wipe. Each experiment used a new pin and a new PTFE/Kevlar fabric sample. To ensure statistically relevant results, each experiment was repeated several times. Thus, every data point in the diagrams represents an average of three repeated tests. After the experiments, all the samples (the surface of the pins, the wear scar of the PTFE/Kevlar fabrics and its debris) were examined by Scanning electron microscopy (SEM), and the ones shown were representative of the general samples. In addition, Energy dispersive x-ray spectroscopy (EDS) was used for the determination of the elemental distribution and content in the transfer films.
3. Results and discussion 3.1. Friction and wear properties Fig. 4a shows the variation of the friction coefficient and the wear rate of hybrid PTFE/Kevlar fabric composites containing different contents of Si3N4 nanoparticles under the load of 513 N (60 MPa) at the speed of 1 m/s. The use of the fillers remarkably reduced the wear rate of the composites. Compared to unfilled PTFE/Kevlar fabric composites, the composites with 12.5 wt% nano-Si3N4 exhibit the best wear resistance. In contrast, with the increase in the content of filler, the friction coefficient increased. Contrary to the above case, submicron size WS2 filler was favorable for the reduction of the friction coefficient and the wear rate
Fig. 3. Schematic diagram of the measurement technique.
of composites, as shown in Fig. 4b. When the content of filler increased to 7.5 wt%, the composites exhibited lower wear rate, and the minimum friction coefficient. Therefore, this content was selected to investigate the combined effects of two types of fillers on the tribological properties of hybrid PTFE/Kevlar fabric composites. Fig. 4c shows the variation of the friction coefficient and the wear rate of the PTFE/Kevlar fabric composite filled with hybrid nano-Si3N4 and submicron size WS2 fillers (the content of WS2 is a constant value of 7.5 wt%, and the content of Si3N4 is varied from 2.5 wt% to 12.5 wt%). The friction condition was the same as that mentioned above. The wear rate was found to sharply decrease with the increase in the content of Si3N4 nanoparticles, and the composites with 7.5 wt% WS2 þ 12.5 wt% nano-Si3N4 exhibits the lowest wear rate. Fig. 4d and e shows the variations of the friction coefficient and the wear rate of all types of composites. It can be observed from the figures that hybrid fillers exhibit the best wear resistance and friction properties. 3.2. SEM observations of the worn surfaces Fig. 5a–d shows the SEM images of the worn surface of unfilled and WS2 þSi3N4-filled PTFE/Kevlar fabric composites. Fig. 5a and b shows the SEM image of the worn surface of unfilled PTFE/Kevlar fabric composites. A large number of PTFE and Kevlar fibers significantly peel, and there are many long and deep cracks on the worn surface, which is indicative of severe wear. Fig. 5c and d shows the SEM images of the worn surfaces of 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled composites. The worn surface of the WS2 þ Si3N4-filled composites is quite smooth, and the fibers are strongly combined with the matrix. This observation indicates that the addition of WS2 and Si3N4 filler in the hybrid PTFE/Kevlar fabric composite results in strong adhesion of fibers to the matrix, which results in the strengthening of the mechanical properties of the fabric composite. Therefore, the wear resistance is significantly improved. 3.3. SEM and EDS analysis of the transfer films It is well known that achieving the appropriate characteristics of the transfer film is a very important factor affecting the tribological properties of polymer composites. To better understand the role of fillers in tribological processes, SEM was used to explore the morphologies of the transfer films formed on the steel pin surface for unfilled and filled hybrid PTFE/Kevlar fabric composites. In addition, an energy-dispersive X-ray spectrometer (EDS) was used for analysis of the elemental distribution and content in the transfer films. The results of the EDS elemental analysis are shown in Fig. 6. The F element and O element were found in the transfer films for unfilled fabric composites. The F element, O element, N element and Si element were found in transfer films for 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites. These findings indicate that the transfer of PTFE, resin and filler occurred during sliding [21,22]. It is also observed from Fig. 6 that the F element and O element content in the transfer film for unfilled composites are higher and that the Fe element content is lower than that of the filled composites. This observation might suggest that the transfer film for unfilled composites formed on the steel pin is thicker than that of the 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites. It can be seen from Fig. 6a that the transfer film formed on the unfilled composites is characterized as thick and lumpy. This type of transfer film has poor adhesive properties with the substrate and was easily peeled off from the counterface. Therefore, it can be inferred that adding nanoparticles to composites can significantly improve the performance of the transfer film formed on the
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Fig. 4. Variation of the friction coefficient/wear rate and their standard deviation of the hybrid PTFE/Kevlar fabric composites with different filler contents.
counterface, which results in a much lower wear rate for the filled composite in comparison with the unfilled one. The SEM image of the transfer film formed on the pin's surface is shown in Fig. 7. For unfilled composites, the transfer film formed on the pin's surface is thick and lumpy. According to former research, this type of transfer film was easily scaled off of the counterface and formed debris during wear process, which resulted in the high wear rate of unfilled hybrid PTFE/Kevlar fabric composites. Whereas a uniform and thin transfer film is formed on the pin sliding against the 7.5 wt% WS2 þ 12.5 Si3N4-filled hybrid PTFE/Kevlar fabric composites. The thin and uniform transfer film will firmly adhere onto the counterface and is not easily scaled off during sliding. The tenacious transfer film can prevent the direct
contact between the steel pin and fabric composites, thereby significantly reducing the wear rate. Moreover, by careful observation of Fig. 7d, we find that there are many nanoparticles on the transfer film surface; those nanoparticles between the material pairs may have acted as roller bearings and further reduced the wear rate [12,14,23].
3.4. SEM examination of the wear debris Debris is a product of the frictional experiment, and analyzing debris is helpful to understand the tribological mechanisms. Fig. 8 shows the SEM images of the wear debris of unfilled hybrid
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Fig. 5. Scanning electron micrographs of the worn surfaces of hybrid PTFE/Kevlar fabric composites: (a) unfilled, (b) magnification of (a), (c) filled with 7.5 wt% WS2 þ 12.5 wt% Si3N4, and (d) magnification of (c).
Fig. 6. EDS analysis of the transfer film formed on a steel pin: (a) unfilled and (b) filled with 7.5 wt% WS2 þ 12.5 wt% Si3N4.
PTFE/Kevlar fabric composites and 7.5 wt% WS2 þ12.5 wt% Si3N4filled composites. It can be observed from Fig. 8a and b that unfilled composites generate debris that mainly consists of wide ribbon debris and lumpy slabs, which is indicative of delamination. Contrary to the unfilled composites, the wear debris of 7.5 wt% WS2 þ12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites is much smaller and mainly consists of tiny flakes and filamentous wear debris. This observation indicates that adding nanoparticles to PTFE/Kevlar fabric composites can restrain the formation of larger debris. Moreover, wide ribbon and lumpy slab debris cannot easily fill in the grooves on the fabric surface, and hence, they pile up during repeated sliding and subsequently are discarded as debris. Wear debris composed of tiny flakes is easily trapped in the gap of a worn surface and can repair the damaged surface, and this trapped debris can be considered as a secondary source of lubricant that permeates to the surface to reduce the friction and retard the galling during sliding. Therefore, adding hybrid fillers to PTFE/Kevlar fabric composites can sharply improve the friction and wear properties of the composites.
4. Conclusion This work presents investigations on the tribological behavior of hybrid PTFE/Kevlar fabric composites with nano-Si3N4 and/or submicron size WS2 filler samples against bearing steel at dry friction conditions. The following conclusions can be drawn from the present study: (a) It was found that under the experimental conditions, Si3N4 nanoparticles could effectively reduce the wear rate of hybrid PTFE/Kevlar fabric composites. (b) Submicron size WS2 filler was favorable for the reduction of the friction coefficient and the wear rate of composites compared to the unfilled samples; the 7.5 wt% WS2 filled composites exhibited lowest friction coefficient, and the value is 0.045. (c) The wear rate and friction coefficient of 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites were much less than those of the unfilled composites. The wear rate was reduced greatly, from 8.75 10 14 m3/(Nm) to
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Fig. 7. The scanning electron micrographs of the transfer film formed on the counterpart pin: (a) unfilled, (b) magnification of (a), (c) filled with 7.5 wt% WS2 and 12.5 wt% Si3N4, and (d) magnification of (c).
Fig. 8. SEM images of the wear debris of hybrid PTFE/Kevlar fabric composites: (a) lumpy slabs debris (unfilled), (b) wide ribbon debris (unfilled), (c) tiny flake debris (filled) and (d) filamentous debris (filled).
0.41 10 14 m3/(Nm), and the uncertainty of wear rate of unfilled composites and the 7.5 wt% WS2 þ12.5 wt% Si3N4-filled composites are 0.069 10 14 m3/(Nm) and 0.073 10 14 m3/(Nm), respectively. The friction coefficient also decreased from unfilled (0.059) to filled with hybrid fillers (0.040). Thus, hybrid fillers could achieve the desired comprehensive tribological properties in dry sliding.
(d) The transfer films of 7.5 wt% WS2 þ 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites were thin, uniform and full of nanoparticles. The tenacious transfer film can prevent the direct contact between the steel pin and the fabric composites, and those nanoparticles between the material pairs are thought to have acted as roller bearings, thereby further reducing the wear rate.
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(e) The wear debris of 7.5 wt% WS2 and 12.5 wt% Si3N4-filled hybrid PTFE/Kevlar fabric composites was much smaller than that of the unfilled composites. This observation indicated that adding nanoparticles to PTFE/Kevlar fabric composites could restrain the formation of larger debris. Moreover, tiny wear debris was easily trapped in the gap of a worn surface and can repair the damaged surface. In addition, the trapped debris could be considered as a secondary source of lubricant.
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