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phenolic composites
ARTICLE IN PRESS Tribology International 42 (2009) 243– 249
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of plasma treatment of Kevlar fabric on the tribological behavior of Kevlar fabric/phenolic composites Fang Guo a,b, Zhao-Zhu Zhang a,, Wei-Min Liu a, Feng-Hua Su a,b, Hui-Juan Zhang a,b a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Graduate School of Chinese Academy of Science, Beijing 100039, China
a r t i c l e in fo
abstract
Article history: Received 26 January 2007 Received in revised form 5 May 2008 Accepted 12 June 2008 Available online 21 July 2008
Pure and plasma-treated Kevlar fabrics were used to prepare Kevlar fabric/phenolic composites by consecutive dipping of the fabric in phenolic adhesive resin. The friction and wear performance of the resulting composites has been evaluated in a pin-on-disk wear tester at various dry-sliding conditions. The surface changes occurring on Kevlar fibers treated with air–plasma were analyzed by using X-ray photoelectron spectroscope (XPS), Fourier transform infrared spectroscope (FT-IR) and scanning electron microscope (SEM). Moreover, the impact of air–plasma treatment time and power on the friction and wear behavior of Kevlar fabric/phenolic composites composed of the air–plasma-treated Kevlar fabrics was systematically studied. It was found that plasma treatment can significantly improve the tribological performance of the prepared Kevlar fabric/phenolic composites; the best performance was after a plasma treatment at 50 W for 15 min. The plasma treatment generates oxygenic and nitrogenous groups on the surface of the fabric, coupled with an increase of the surface roughness, strengthening the bond between the Kevlar fabric and phenolic adhesive resin and hence improving the tribological properties of the Kevlar fabric/phenolic composites. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Composites Kevlar fabric Phenolic resin Air–plasma treatment Friction and wear
1. Introduction Under extreme friction conditions, conventional polymer composites as bearing liner materials can hardly be effective for antiwear and friction reduction, for example, under heavy load. Fabric composites composed of fabric as the matrix and adhesive resin as the binder have been considered as an advanced bearing liner material for tribological application in many high-tech industries such as aerospace, aviation, automobile, etc. owing to their low density, high strength, high modulus, excellent chemical stability and antiwear ability [1,2]. In the case of polymer matrix composites, the friction and wear properties of fiber or fabric reinforced composites have been systematically discussed in many papers [3–9] in the last few decades. The tribological performance of fabric composites as bearing liner materials with adhesive resin as binder, however, has not been systematically evaluated [10,11], especially Kevlar fabric/phenolic composites composed of air–plasma-treated Kevlar fabric and phenolic adhesive resin as binder. Kevlar fiber, characterized by high tenacity, high modulus and strength, and low electrical conductivity compared with metallic and carbon fibers, is widely used in the manufacture of advanced
Corresponding author. Tel.: +86 931 4968098.
E-mail address: [email protected] (Z.-Z. Zhang). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.06.004
composites [12,13]. It is well known that the mechanical and tribological properties of the fiber-reinforced composites depend on the effectiveness of the interaction between the fiber and matrix [14,15]. In terms of the fabric/adhesive resin composites, Su et al. [16,17] found that the attributes of the fabric/adhesive resin interface are critical for the quality of the fabric; however, the adhesion between Kevlar fibers and most resins is poor because of the high crystallinity and smooth surface of the Kevlar fiber resulting in chemical inertness. In order to improve the binding of the Kevlar fabric/adhesive resin system, a variety of fiber surface modifications have been developed, such as plasma treatment and chemical modification, etc. [18]. The application of cold plasma in the treatment of polymeric materials has become increasingly important recently [19–21]. It can improve the surface quality of polymers without affecting their bulk properties. Plasma treatment modifies the uppermost atomic layers of the material surface without affecting its main characteristics. Most researchers employ the plasma treatment technique to increase the roughness of fiber surface [22] and consequently the fiber/matrix adhesion, but sacrifice the composite toughness [23]. Based upon that earlier work we anticipate that the interfacial adhesion between Kevlar fabric and phenolic resin can be significantly improved by treating the Kevlar fabric with air–plasma, which may greatly enhance the tribological performance of the prepared Kevlar fabric/phenolic composites with plasma-treated Kevlar fabric. The present work is to extend
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F. Guo et al. / Tribology International 42 (2009) 243–249
the application of Kevlar fabric/phenolic composites in dry-sliding bearings.
2. Experimental 2.1. Materials The Kevlar fabrics (weave: plain, area density: 260 g/m2) used in this study were woven from Kevlar fibers (Kevlar 49, fiber fineness: 800 Denier), which were purchased from Du Pont, Inc., USA. The adhesive resin (phenolic resin) was provided by Shanghai Xing-guang Chemical Plant, China. 2.2. Specimen preparation The Kevlar fabrics were cleaned by Soxhlet, continuous extraction in petroleum ether, and then acetone, and dried for 24 h at 80 1C. The cleaned Kevlar fabrics were then bombarded with air–plasma at given power for different time periods at low pressure (0.2–0.4 mbar) on a PREP5 plasma apparatus produced by Gala Co., Germany. The untreated (abridged as KF-I) and air–plasma-treated (abridged as KF-II) Kevlar fabrics were then used to prepare the Kevlar fabric/phenolic composites by dipping of the fabrics in the phenolic adhesive resin. The relative mass fraction of the Kevlar fabric in the Kevlar fabric/phenolic composites is 6575%. Finally, the Kevlar fabric/phenolic composites with the air–plasma-treated Kevlar fabric (abridged as KFC-II) were bonded on the AISI-1045 steel disk (surface roughness Ra ¼ 0.45 mm) with phenolic resin and then cured at 180 1C for 2 h under a certain pressure. For comparison, the Kevlar fabric/phenolic composites with the untreated Kevlar fabric (abridged as KFC-I) were prepared in the same way.
performed at ambient temperature, a load between 141 and 251 N, speed of 0.26 m/s and over a period of 2 h under dry conditions. At the end of each test, the wear depth of the specimen was obtained using a micrometer with 0.001 mm resolution. Eight points were measured around the wear track and the average value was used as wear depth. The wear track was annular. The diameter of the pin, the distance between the center of the pin and axis, and wear depth were known; thus the wear volume loss (V) of the specimen could be calculated. The wear performance was expressed in terms of wear rate (w, m3 (N m)1) as follows: w ¼ V/p L, where V is the wear volume loss in m3; P is the load in Newton; and L the sliding distance in m. The friction coefficient was the ratio of the measured friction force and load. It was measured from the frictional torque gained by a load cell sensor, which was acquired directly by the computer running the friction measure software. Each experiment was carried out three times and the average value was used; the relative errors were 710%. 2.4. Analysis of Kevlar fabric and composites Kevlar fabric surfaces had morphological changes after they were treated with air–plasma. The interfaces between the Kevlar fabric and adhesive resin of the resulting composites and worn surfaces of the composites were analyzed using a JSM-5600LV scanning electron microscope (SEM). The chemical changes occurring on the Kevlar fibers after air–plasma treatment were analyzed on an ESCALAB 210 X-ray photoelectron spectroscope (XPS) and Fourier transform-infrared spectroscope (FT-IR).
3. Results and discussion 3.1. XPS analysis of untreated and plasma-treated Kevlar fabric
2.3. Friction and wear test The friction and wear behavior of Kevlar fabric composites was investigated using a Xuanwu-III pin-on-disk friction and wear tester (Fig. 1). The pin-on-disk tester consisted of loading a stationary pin sliding against a rotating disk which was affixed with the Kevlar fabric/phenolic composites. The flat-ended AISI-1045 pin (diameter 2 mm) was fixed to the load arm with a chuck. The distance between the center of the pin and axis was 12.5 mm. The pin stayed over the disk with two degrees of freedom: a vertical one, which allowed normal load application by direct contact with the disk, and a horizontal one, for friction measurement. Prior to the tests, the pin was polished with 350, 700 and 900 grade water-proof abrasive papers to a surface roughness Ra ¼ 0.15 mm, and then cleaned with acetone. The sliding was
load
Kevlar fibers are generally considered to be composed of poly (para-phenylene terephthalamide) (PPTA), the chemical formula of which can be written as
The concentration of reactive chemical groups is limited due to the chemical structure of the macromolecules. The cold-gas air–plasma treatment applied here can effectively modify the surface structure of the Kevlar fibers with no sacrifice of their bulk properties. It is comprehensible that the radicals, ions and photons in the air–plasma collide with the polymer, breaking covalent bonds and creating free radicals on the polymer surface. The polymer’s free radicals react with other species in the plasma
Table 1 Surface composition of KF-I and KF-II determined by XPS
pin fabric composite
rotating disk
electric furnace
rotating direction Fig. 1. Schematic diagram of pin-on-disk wear tester.
Sample
Element
Pk Area
Xa (%)
Atomic ratio
KF-I
C1s O1s N1s
36,883 17,125 6673
73.6 17.4 9.0
O/C, 0.236; N/C, 0.122
KF-II
C1s O1s N1s
50,655 42,320 7076
65.8 28.0 6.2
O/C, 0.425; N/C, 0.094
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environment to introduce functional chemical groups. As a result, we can expect that –COOH, –OH, and –NH2 groups may be generated on the surface of the Kevlar fabric as it is bombarded with air–plasma. XPS allows us to analyze the chemical changes occurring on Kevlar fibers during plasma treatment. Table 1 gives a general survey of the surface atomic distribution of the interested
245
elements for the two samples. Parameter Pk Area, is the measured photoelectron intensity for one XPS line. Parameter Xa is the relative atomic concentration. After air–plasma treatment, the O/C ratio has increased to 0.425, indicating an effective modification of the fiber surface. The significant increase of the surface oxygen content observed in the fibers may lead to a better fiber/resin adhesion owing to its high chemical activity.
Fig. 2. C1s, O1s and N1s spectra of KF-I and KF-II.
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More information about the two samples could be obtained by deconvolution of C1s, O1s and N1s peaks. Fig. 2 shows the C1s, O1s and N1s spectra of the untreated and plasma-treated Kevlar fiber, respectively. The most significant differences between the spectra
of plasma-treated and untreated fibers are the increase of –COOH and –PhNH2 and the appearance of –C–OH component. These changes suggest that the plasma treatment involves both the hydrolysis of the –C–N bond in the amide group, creating –NH2 and –COOH functional groups, and the hydroxylation of the benzene ring, creating –C–OH groups. These changes can correspond to the increase in O/C reported in Table 1. 3.2. IR analysis of Kevlar fabric
Fig. 3. FT-IR spectra of KF-I and KF-II.
In order to investigate the possible change of chemical composition of the Kevlar fabric bombarded with air–plasma at 50 W for 15 min, FT-IR spectroscopy measurements in the mid infrared (4000–400 cm1) were performed. FT-IR spectra were recorded on powder samples, which were obtained from the cut Kevlar fibers, dispersed in dry KBr using Bruker IFS/66v. As shown in Fig. 3, the stretching vibration of N–H at about 3500 cm1 and the stretching vibrations of methylene and methyl groups at 2850 and 2925 cm1 in KF-II decreased in comparison with KF-I. It proved that the sizing agent on the original Kevlar fabric had been cleaned as it was bombarded with air–plasma. At the same time, the stretching vibration of O–H at 3000–3500 cm1 and the CQO stretching vibration peak at 1650 cm1 had a higher intensity in KF-II than that in KF-I, which indicated that the Kevlar fabric had been oxidized and etched by the plasma treatment. As a result, we can infer that the Kevlar fabric has been cleaned and oxidized by air–plasma treatment, which caused many active chemical groups produced on the surface of Kevlar fabric. The significant increase of the active groups on the fiber surface might lead to better fiber/resin adhesion. 3.3. Tribological properties of the Kevlar fabric composites
Fig. 4. Variations of friction coefficient and wear rate with power of plasma treatment (treatment time: 3 min, load: 141 N).
Fig. 5. Variations of friction coefficient and wear rate with treatment time (treatment power: 50 W, load: 141 N).
The impact of air–plasma bombarded power on the friction coefficient and wear rate of the Kevlar fabric/phenolic composites at 141.12 N is shown in Fig. 4 (treatment time: 3 min). It is noticed that the friction coefficient of the composites increases slightly with increasing air–plasma bombarded power up to 70 W and then decreases. A decrease in wear rate of the composites is observed for tests conducted from 30 to 50 W, followed by an increase with increasing of power. Conclusively, we select 50 W as the air–plasma bombarded power. The effect of plasma treatment time at 50 W on the friction coefficient and wear rate of the Kevlar fabric/phenolic composites at 141 N is shown in Fig. 5. We can see that the friction coefficient and wear rate of the composites initially decrease and then increase with the increase of the air–plasma treatment time. As a conclusion, the Kevlar fabric/phenolic composites composed of the air–plasma-treated Kevlar fabric at 50 W for 15 min can obtain the optimal friction coefficient and wear rate. The air–plasma-treated Kevlar fabrics in Kevlar fabric/phenolic composites in the following discussion are treated at 50 W for 15 min. Fig. 6 illustrates that air–plasma treatment of Kevlar fabric can greatly improve the friction-reducing and wear-resistant ability of Kevlar fabric/phenolic composites under different loads, especially under high load. For the Kevlar fabric/phenolic composites with untreated Kevlar fabric, the friction coefficient and wear rate are relatively large and the friction system was destroyed at a load higher than 220 N. Meanwhile, it is found that the friction coefficient decreases with increasing load up to 188 N and then increases with a further increase of the load. However, the friction coefficient of the Kevlar fabric/phenolic composites with the air–plasma-treated Kevlar fabric decreased with increases of load for the entire range of loads (141–251 N). The wear rates of
ARTICLE IN PRESS F. Guo et al. / Tribology International 42 (2009) 243–249
the Kevlar fabric/phenolic composites with the untreated and air–plasma-treated Kevlar fabric all increase with the increase of the load.
Fig. 6. Variation of friction coefficient and wear rate with load for KFC-I and KFC-II (treatment time: 15 min, power: 50 W).
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3.4. SEM analysis Besides chemical interactions, the Kevlar fiber/resin interfacial behavior should also result from morphological changes in the fiber surfaces brought about by the plasma treatment. Fig. 7 shows the morphology of untreated and plasma-treated Kevlar fibers. A clear increase in surface roughness can be observed after the plasma treatment at 50 W for 15 min (Fig. 7(b)), with some small globular-like microstructures substituting the smooth surfaces presenting on the fresh fibers (Fig. 7(a)). This should enhance the mechanical interlocking of the resin on the fibers’ surface. When treatment time is 25 min, the increase of the surface reactive groups on the fiber surface causes significant damage to the bulk properties of the fibers (Fig. 7(c)), thus diminishing the friction and wear properties of the Kevlar/ phenolic composites correspondingly. The interface between the Kevlar fabric and adhesive resin of Kevlar fabric composites with untreated and air–plasma-treated Kevlar fabric is shown in Fig. 8. As shown in Fig. 8(a), the untreated Kevlar fibers cannot tightly combine with the adhesive resin; there are obvious interfacial gaps, which could account for the poor wear resistance of the Kevlar fabric composites with the untreated Kevlar fabrics. In contrast, the plasma-treated Kevlar fibers bind so tightly with the adhesive resin that we can hardly see any interfacial gaps (Fig. 8(b)). In a word, the compositions and surface properties of the Kevlar fabric after being bombarded with air–plasma have been changed. Some active groups were introduced on the surface of the Kevlar fabric, coupled with an increase of the surface roughness, which enhances the adhesion between the Kevlar fabric and phenolic adhesive. As a result, the tribological performance of the Kevlar fabric/phenolic composites has been significantly improved after the Kevlar fabric was bombarded with air–plasma under a certain condition. Fig. 9 shows the SEM morphologies of the worn surfaces of Kevlar fabric/phenolic composites with untreated and plasmatreated Kevlar fabrics. It is seen that the KFC-I after sliding is characterized by pulling-out and exposure of the Kevlar fibers (Fig. 9(a)), which indicates the KFC-I experiences severe damage. With the KFC-II (Fig. 9(b)), there are relatively fewer Kevlar fibers
Fig. 7. SEM images of surface of fresh Kevlar fibers (a), and plasma-treated Kevlar fibers (b) (treatment time: 15 min, power: 50 W) and (c) (treatment time: 25 min, power: 50 W).
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Fig. 8. SEM images of interface between Kevlar fabrics and adhesive resin of Kevlar fabric/phenolic composites with untreated (a) and plasma-treated Kevlar fabrics (b) (treatment time: 15 min, power: 50 W).
Fig. 9. SEM images of worn surfaces of KFC-I (a) and KFC-II (b) (load: 204 N, testing time: 2 h).
pulled out and cut from the composites in the worn surface. This indirectly indicates that the plasma treatment can strengthen the interface adhesion between the Kevlar fabric and adhesive and hence to improve the friction reduction and antiwear ability of the composites.
4. Conclusion The air–plasma treatment of Kevlar fabric increases the antiwear, friction-reducing abilities and load-carrying capacity of Kevlar fabric/phenolic composites. The composites composed of Kevlar fabric treated with air–plasma at 50 W for 15 min can obtain the lowest friction coefficient and wear rate. The compositions and surface properties of the Kevlar fabric after being bombarded with air–plasma have been changed. Some active groups were introduced on the surface of Kevlar fabric and the roughness of the fiber surface increased, which enhances the adhesion between the Kevlar fabric and phenolic adhesive. As a result, the tribological performance of the Kevlar fabric/phenolic composites with air–plasma-treated Kevlar fabrics has been improved significantly.
Acknowledgments We are grateful to Prof. Jiazheng Zhao for his help in SEM analysis. We acknowledge financial support from the ‘‘973’’ project of China (Grant no. 2007 CB607601).
References [1] Tercan M, Asi O, Aktas- A. An experimental investigation of the bearing strength of weft-knitted 1 1 rib glass fiber composites. Compos Struct 2007;78:392–6.
[2] Lua J. Thermal–mechanical cell model for unbalanced plain weave woven fabric composites. Compos Part A—Appl S 2007;38:1019–37. [3] Bijwe J, Rajesh JJ, Jeyakumar A. Influence of solid lubricants and fiber reinforcement on wear behaviour of polyethersulphone. Tribol Int 2000;33: 697–706. [4] Bijwe J, Rattan R, Fahim M. Abrasive wear performance of carbon fabric reinforced polyetherimide composites: influence of content and orientation of fabric. Tribol Int 2007;40:844–54. [5] Friedrich K. Friction and wear of polymer composites. Amsterdam: Elsevier Science Publishers; 1986. [6] Yamamoto Y, Hashimoto M. Friction and wear of water lubricated PEEK and PPS sliding contacts part 2. Composites with carbon or glass fiber. Wear 2004;257:181–9. [7] Friedrich K, Zhang Z, Schlarb AK. Effects of various fillers on the sliding wear of polymer composites. Compos Sci Technol 2005;65:2329–43. [8] Jacobs O, Friedrich K, Marom G, Schulte K. Fretting wear performance of glass-, carbon-, and aramid-fibre/epoxy and peek composites. Wear 1990;135: 207–16. [9] Mimaroglu A, Unal H, Arda T. Friction and wear performance of pure and glass fibre reinforced polyetherimide on polymer and steel counterface materials. Wear 2007;262:1407–13. [10] Liu XJ, Li TS, Yang SR. Study of the tribological properties of Kevlar fabric. Tribology (Chinese) 1999;19:23–7. [11] Li TQ, Wang CY. Effect factors and improving methods of the properties of aramid fiber and its composite. Polym Mater Sci Eng (Chinese) 2003;19: 5–9. [12] Yue CY, Padmanabhan K. International studies on surface modified Kevlar fiber/epoxy matrix composites. Compos Part B—Eng 1999;30: 205–17. [13] Lin TK, Wu SJ, Lai JG, Shyu SS. The effect of chemical treatment on reinforcement/matrix interaction in Kevlar-fiber/bismaleimide composites. Compos Sci Technol 2000;60:1873–8. [14] C - o¨keliler D, Erkut S, Zemek J. Modification of glass fibers to improve reinforcement: a plasma polymerization technique. Dent Mater 2007;23: 335–42. [15] Yang Y, Lu CX, Su XL. Effect of nano-SiO2 modified emulsion sizing on the interfacial adhesion of carbon fibers reinforced composites. Mater Lett 2007;61:3601–4. [16] Su FH, Zhang ZZ, Guo F. Tribological properties of the composites made of pure and plasma treated-Nomex fabrics. Wear 2006;261:293–300. [17] Su FH, Zhang ZZ, Guo F. Study on the friction and wear properties of the composites made of surface modified-Nomex fabrics. Mater Sci Eng A 2006; 416:126–33. [18] Park JM, Kim DS, Kim SR. Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar
ARTICLE IN PRESS F. Guo et al. / Tribology International 42 (2009) 243–249
fibers/epoxy composites using micromechanical techniques and surface wettability. J Colloid Interface Sci 2003;264:431–45. [19] McCord M. Atmospheric pressure plasma modification of textile surfaces: nylon 66, polypropylene, and ultra high modulus polyethylene. In: Proceedings of the second AUTEX conference; 2002. p. 313–26. [20] Tsal PP. Surface modification of fabrics using a one-atmosphere glow discharge plasma to improve fabric wettability. Text Res J 1997;67:359–69.
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[21] Huang FL, Wei QF, Wang XQ. Dynamic contact angles and morphology of PP fibers treated with plasma. Polym Test 2006;25:22–7. [22] Li RZ, Ye L, Mai YW. Application of plasma technologies in reinforced–reinforced polymer composites: a review of recent developments. Compos Part A—Appl S 1997;28:73–86. [23] Cech V. New progress in composite interfaces: a use of plasma technologies. In: Proceedings of the FRC 2000, Newcastle 2000:246–52.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of plasma treatment of Kevlar fabric on the tribological behavior of Kevlar fabric/phenolic composites Fang Guo a,b, Zhao-Zhu Zhang a,, Wei-Min Liu a, Feng-Hua Su a,b, Hui-Juan Zhang a,b a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Graduate School of Chinese Academy of Science, Beijing 100039, China
a r t i c l e in fo
abstract
Article history: Received 26 January 2007 Received in revised form 5 May 2008 Accepted 12 June 2008 Available online 21 July 2008
Pure and plasma-treated Kevlar fabrics were used to prepare Kevlar fabric/phenolic composites by consecutive dipping of the fabric in phenolic adhesive resin. The friction and wear performance of the resulting composites has been evaluated in a pin-on-disk wear tester at various dry-sliding conditions. The surface changes occurring on Kevlar fibers treated with air–plasma were analyzed by using X-ray photoelectron spectroscope (XPS), Fourier transform infrared spectroscope (FT-IR) and scanning electron microscope (SEM). Moreover, the impact of air–plasma treatment time and power on the friction and wear behavior of Kevlar fabric/phenolic composites composed of the air–plasma-treated Kevlar fabrics was systematically studied. It was found that plasma treatment can significantly improve the tribological performance of the prepared Kevlar fabric/phenolic composites; the best performance was after a plasma treatment at 50 W for 15 min. The plasma treatment generates oxygenic and nitrogenous groups on the surface of the fabric, coupled with an increase of the surface roughness, strengthening the bond between the Kevlar fabric and phenolic adhesive resin and hence improving the tribological properties of the Kevlar fabric/phenolic composites. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Composites Kevlar fabric Phenolic resin Air–plasma treatment Friction and wear
1. Introduction Under extreme friction conditions, conventional polymer composites as bearing liner materials can hardly be effective for antiwear and friction reduction, for example, under heavy load. Fabric composites composed of fabric as the matrix and adhesive resin as the binder have been considered as an advanced bearing liner material for tribological application in many high-tech industries such as aerospace, aviation, automobile, etc. owing to their low density, high strength, high modulus, excellent chemical stability and antiwear ability [1,2]. In the case of polymer matrix composites, the friction and wear properties of fiber or fabric reinforced composites have been systematically discussed in many papers [3–9] in the last few decades. The tribological performance of fabric composites as bearing liner materials with adhesive resin as binder, however, has not been systematically evaluated [10,11], especially Kevlar fabric/phenolic composites composed of air–plasma-treated Kevlar fabric and phenolic adhesive resin as binder. Kevlar fiber, characterized by high tenacity, high modulus and strength, and low electrical conductivity compared with metallic and carbon fibers, is widely used in the manufacture of advanced
Corresponding author. Tel.: +86 931 4968098.
E-mail address: [email protected] (Z.-Z. Zhang). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.06.004
composites [12,13]. It is well known that the mechanical and tribological properties of the fiber-reinforced composites depend on the effectiveness of the interaction between the fiber and matrix [14,15]. In terms of the fabric/adhesive resin composites, Su et al. [16,17] found that the attributes of the fabric/adhesive resin interface are critical for the quality of the fabric; however, the adhesion between Kevlar fibers and most resins is poor because of the high crystallinity and smooth surface of the Kevlar fiber resulting in chemical inertness. In order to improve the binding of the Kevlar fabric/adhesive resin system, a variety of fiber surface modifications have been developed, such as plasma treatment and chemical modification, etc. [18]. The application of cold plasma in the treatment of polymeric materials has become increasingly important recently [19–21]. It can improve the surface quality of polymers without affecting their bulk properties. Plasma treatment modifies the uppermost atomic layers of the material surface without affecting its main characteristics. Most researchers employ the plasma treatment technique to increase the roughness of fiber surface [22] and consequently the fiber/matrix adhesion, but sacrifice the composite toughness [23]. Based upon that earlier work we anticipate that the interfacial adhesion between Kevlar fabric and phenolic resin can be significantly improved by treating the Kevlar fabric with air–plasma, which may greatly enhance the tribological performance of the prepared Kevlar fabric/phenolic composites with plasma-treated Kevlar fabric. The present work is to extend
ARTICLE IN PRESS 244
F. Guo et al. / Tribology International 42 (2009) 243–249
the application of Kevlar fabric/phenolic composites in dry-sliding bearings.
2. Experimental 2.1. Materials The Kevlar fabrics (weave: plain, area density: 260 g/m2) used in this study were woven from Kevlar fibers (Kevlar 49, fiber fineness: 800 Denier), which were purchased from Du Pont, Inc., USA. The adhesive resin (phenolic resin) was provided by Shanghai Xing-guang Chemical Plant, China. 2.2. Specimen preparation The Kevlar fabrics were cleaned by Soxhlet, continuous extraction in petroleum ether, and then acetone, and dried for 24 h at 80 1C. The cleaned Kevlar fabrics were then bombarded with air–plasma at given power for different time periods at low pressure (0.2–0.4 mbar) on a PREP5 plasma apparatus produced by Gala Co., Germany. The untreated (abridged as KF-I) and air–plasma-treated (abridged as KF-II) Kevlar fabrics were then used to prepare the Kevlar fabric/phenolic composites by dipping of the fabrics in the phenolic adhesive resin. The relative mass fraction of the Kevlar fabric in the Kevlar fabric/phenolic composites is 6575%. Finally, the Kevlar fabric/phenolic composites with the air–plasma-treated Kevlar fabric (abridged as KFC-II) were bonded on the AISI-1045 steel disk (surface roughness Ra ¼ 0.45 mm) with phenolic resin and then cured at 180 1C for 2 h under a certain pressure. For comparison, the Kevlar fabric/phenolic composites with the untreated Kevlar fabric (abridged as KFC-I) were prepared in the same way.
performed at ambient temperature, a load between 141 and 251 N, speed of 0.26 m/s and over a period of 2 h under dry conditions. At the end of each test, the wear depth of the specimen was obtained using a micrometer with 0.001 mm resolution. Eight points were measured around the wear track and the average value was used as wear depth. The wear track was annular. The diameter of the pin, the distance between the center of the pin and axis, and wear depth were known; thus the wear volume loss (V) of the specimen could be calculated. The wear performance was expressed in terms of wear rate (w, m3 (N m)1) as follows: w ¼ V/p L, where V is the wear volume loss in m3; P is the load in Newton; and L the sliding distance in m. The friction coefficient was the ratio of the measured friction force and load. It was measured from the frictional torque gained by a load cell sensor, which was acquired directly by the computer running the friction measure software. Each experiment was carried out three times and the average value was used; the relative errors were 710%. 2.4. Analysis of Kevlar fabric and composites Kevlar fabric surfaces had morphological changes after they were treated with air–plasma. The interfaces between the Kevlar fabric and adhesive resin of the resulting composites and worn surfaces of the composites were analyzed using a JSM-5600LV scanning electron microscope (SEM). The chemical changes occurring on the Kevlar fibers after air–plasma treatment were analyzed on an ESCALAB 210 X-ray photoelectron spectroscope (XPS) and Fourier transform-infrared spectroscope (FT-IR).
3. Results and discussion 3.1. XPS analysis of untreated and plasma-treated Kevlar fabric
2.3. Friction and wear test The friction and wear behavior of Kevlar fabric composites was investigated using a Xuanwu-III pin-on-disk friction and wear tester (Fig. 1). The pin-on-disk tester consisted of loading a stationary pin sliding against a rotating disk which was affixed with the Kevlar fabric/phenolic composites. The flat-ended AISI-1045 pin (diameter 2 mm) was fixed to the load arm with a chuck. The distance between the center of the pin and axis was 12.5 mm. The pin stayed over the disk with two degrees of freedom: a vertical one, which allowed normal load application by direct contact with the disk, and a horizontal one, for friction measurement. Prior to the tests, the pin was polished with 350, 700 and 900 grade water-proof abrasive papers to a surface roughness Ra ¼ 0.15 mm, and then cleaned with acetone. The sliding was
load
Kevlar fibers are generally considered to be composed of poly (para-phenylene terephthalamide) (PPTA), the chemical formula of which can be written as
The concentration of reactive chemical groups is limited due to the chemical structure of the macromolecules. The cold-gas air–plasma treatment applied here can effectively modify the surface structure of the Kevlar fibers with no sacrifice of their bulk properties. It is comprehensible that the radicals, ions and photons in the air–plasma collide with the polymer, breaking covalent bonds and creating free radicals on the polymer surface. The polymer’s free radicals react with other species in the plasma
Table 1 Surface composition of KF-I and KF-II determined by XPS
pin fabric composite
rotating disk
electric furnace
rotating direction Fig. 1. Schematic diagram of pin-on-disk wear tester.
Sample
Element
Pk Area
Xa (%)
Atomic ratio
KF-I
C1s O1s N1s
36,883 17,125 6673
73.6 17.4 9.0
O/C, 0.236; N/C, 0.122
KF-II
C1s O1s N1s
50,655 42,320 7076
65.8 28.0 6.2
O/C, 0.425; N/C, 0.094
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environment to introduce functional chemical groups. As a result, we can expect that –COOH, –OH, and –NH2 groups may be generated on the surface of the Kevlar fabric as it is bombarded with air–plasma. XPS allows us to analyze the chemical changes occurring on Kevlar fibers during plasma treatment. Table 1 gives a general survey of the surface atomic distribution of the interested
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elements for the two samples. Parameter Pk Area, is the measured photoelectron intensity for one XPS line. Parameter Xa is the relative atomic concentration. After air–plasma treatment, the O/C ratio has increased to 0.425, indicating an effective modification of the fiber surface. The significant increase of the surface oxygen content observed in the fibers may lead to a better fiber/resin adhesion owing to its high chemical activity.
Fig. 2. C1s, O1s and N1s spectra of KF-I and KF-II.
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More information about the two samples could be obtained by deconvolution of C1s, O1s and N1s peaks. Fig. 2 shows the C1s, O1s and N1s spectra of the untreated and plasma-treated Kevlar fiber, respectively. The most significant differences between the spectra
of plasma-treated and untreated fibers are the increase of –COOH and –PhNH2 and the appearance of –C–OH component. These changes suggest that the plasma treatment involves both the hydrolysis of the –C–N bond in the amide group, creating –NH2 and –COOH functional groups, and the hydroxylation of the benzene ring, creating –C–OH groups. These changes can correspond to the increase in O/C reported in Table 1. 3.2. IR analysis of Kevlar fabric
Fig. 3. FT-IR spectra of KF-I and KF-II.
In order to investigate the possible change of chemical composition of the Kevlar fabric bombarded with air–plasma at 50 W for 15 min, FT-IR spectroscopy measurements in the mid infrared (4000–400 cm1) were performed. FT-IR spectra were recorded on powder samples, which were obtained from the cut Kevlar fibers, dispersed in dry KBr using Bruker IFS/66v. As shown in Fig. 3, the stretching vibration of N–H at about 3500 cm1 and the stretching vibrations of methylene and methyl groups at 2850 and 2925 cm1 in KF-II decreased in comparison with KF-I. It proved that the sizing agent on the original Kevlar fabric had been cleaned as it was bombarded with air–plasma. At the same time, the stretching vibration of O–H at 3000–3500 cm1 and the CQO stretching vibration peak at 1650 cm1 had a higher intensity in KF-II than that in KF-I, which indicated that the Kevlar fabric had been oxidized and etched by the plasma treatment. As a result, we can infer that the Kevlar fabric has been cleaned and oxidized by air–plasma treatment, which caused many active chemical groups produced on the surface of Kevlar fabric. The significant increase of the active groups on the fiber surface might lead to better fiber/resin adhesion. 3.3. Tribological properties of the Kevlar fabric composites
Fig. 4. Variations of friction coefficient and wear rate with power of plasma treatment (treatment time: 3 min, load: 141 N).
Fig. 5. Variations of friction coefficient and wear rate with treatment time (treatment power: 50 W, load: 141 N).
The impact of air–plasma bombarded power on the friction coefficient and wear rate of the Kevlar fabric/phenolic composites at 141.12 N is shown in Fig. 4 (treatment time: 3 min). It is noticed that the friction coefficient of the composites increases slightly with increasing air–plasma bombarded power up to 70 W and then decreases. A decrease in wear rate of the composites is observed for tests conducted from 30 to 50 W, followed by an increase with increasing of power. Conclusively, we select 50 W as the air–plasma bombarded power. The effect of plasma treatment time at 50 W on the friction coefficient and wear rate of the Kevlar fabric/phenolic composites at 141 N is shown in Fig. 5. We can see that the friction coefficient and wear rate of the composites initially decrease and then increase with the increase of the air–plasma treatment time. As a conclusion, the Kevlar fabric/phenolic composites composed of the air–plasma-treated Kevlar fabric at 50 W for 15 min can obtain the optimal friction coefficient and wear rate. The air–plasma-treated Kevlar fabrics in Kevlar fabric/phenolic composites in the following discussion are treated at 50 W for 15 min. Fig. 6 illustrates that air–plasma treatment of Kevlar fabric can greatly improve the friction-reducing and wear-resistant ability of Kevlar fabric/phenolic composites under different loads, especially under high load. For the Kevlar fabric/phenolic composites with untreated Kevlar fabric, the friction coefficient and wear rate are relatively large and the friction system was destroyed at a load higher than 220 N. Meanwhile, it is found that the friction coefficient decreases with increasing load up to 188 N and then increases with a further increase of the load. However, the friction coefficient of the Kevlar fabric/phenolic composites with the air–plasma-treated Kevlar fabric decreased with increases of load for the entire range of loads (141–251 N). The wear rates of
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the Kevlar fabric/phenolic composites with the untreated and air–plasma-treated Kevlar fabric all increase with the increase of the load.
Fig. 6. Variation of friction coefficient and wear rate with load for KFC-I and KFC-II (treatment time: 15 min, power: 50 W).
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3.4. SEM analysis Besides chemical interactions, the Kevlar fiber/resin interfacial behavior should also result from morphological changes in the fiber surfaces brought about by the plasma treatment. Fig. 7 shows the morphology of untreated and plasma-treated Kevlar fibers. A clear increase in surface roughness can be observed after the plasma treatment at 50 W for 15 min (Fig. 7(b)), with some small globular-like microstructures substituting the smooth surfaces presenting on the fresh fibers (Fig. 7(a)). This should enhance the mechanical interlocking of the resin on the fibers’ surface. When treatment time is 25 min, the increase of the surface reactive groups on the fiber surface causes significant damage to the bulk properties of the fibers (Fig. 7(c)), thus diminishing the friction and wear properties of the Kevlar/ phenolic composites correspondingly. The interface between the Kevlar fabric and adhesive resin of Kevlar fabric composites with untreated and air–plasma-treated Kevlar fabric is shown in Fig. 8. As shown in Fig. 8(a), the untreated Kevlar fibers cannot tightly combine with the adhesive resin; there are obvious interfacial gaps, which could account for the poor wear resistance of the Kevlar fabric composites with the untreated Kevlar fabrics. In contrast, the plasma-treated Kevlar fibers bind so tightly with the adhesive resin that we can hardly see any interfacial gaps (Fig. 8(b)). In a word, the compositions and surface properties of the Kevlar fabric after being bombarded with air–plasma have been changed. Some active groups were introduced on the surface of the Kevlar fabric, coupled with an increase of the surface roughness, which enhances the adhesion between the Kevlar fabric and phenolic adhesive. As a result, the tribological performance of the Kevlar fabric/phenolic composites has been significantly improved after the Kevlar fabric was bombarded with air–plasma under a certain condition. Fig. 9 shows the SEM morphologies of the worn surfaces of Kevlar fabric/phenolic composites with untreated and plasmatreated Kevlar fabrics. It is seen that the KFC-I after sliding is characterized by pulling-out and exposure of the Kevlar fibers (Fig. 9(a)), which indicates the KFC-I experiences severe damage. With the KFC-II (Fig. 9(b)), there are relatively fewer Kevlar fibers
Fig. 7. SEM images of surface of fresh Kevlar fibers (a), and plasma-treated Kevlar fibers (b) (treatment time: 15 min, power: 50 W) and (c) (treatment time: 25 min, power: 50 W).
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Fig. 8. SEM images of interface between Kevlar fabrics and adhesive resin of Kevlar fabric/phenolic composites with untreated (a) and plasma-treated Kevlar fabrics (b) (treatment time: 15 min, power: 50 W).
Fig. 9. SEM images of worn surfaces of KFC-I (a) and KFC-II (b) (load: 204 N, testing time: 2 h).
pulled out and cut from the composites in the worn surface. This indirectly indicates that the plasma treatment can strengthen the interface adhesion between the Kevlar fabric and adhesive and hence to improve the friction reduction and antiwear ability of the composites.
4. Conclusion The air–plasma treatment of Kevlar fabric increases the antiwear, friction-reducing abilities and load-carrying capacity of Kevlar fabric/phenolic composites. The composites composed of Kevlar fabric treated with air–plasma at 50 W for 15 min can obtain the lowest friction coefficient and wear rate. The compositions and surface properties of the Kevlar fabric after being bombarded with air–plasma have been changed. Some active groups were introduced on the surface of Kevlar fabric and the roughness of the fiber surface increased, which enhances the adhesion between the Kevlar fabric and phenolic adhesive. As a result, the tribological performance of the Kevlar fabric/phenolic composites with air–plasma-treated Kevlar fabrics has been improved significantly.
Acknowledgments We are grateful to Prof. Jiazheng Zhao for his help in SEM analysis. We acknowledge financial support from the ‘‘973’’ project of China (Grant no. 2007 CB607601).
References [1] Tercan M, Asi O, Aktas- A. An experimental investigation of the bearing strength of weft-knitted 1 1 rib glass fiber composites. Compos Struct 2007;78:392–6.
[2] Lua J. Thermal–mechanical cell model for unbalanced plain weave woven fabric composites. Compos Part A—Appl S 2007;38:1019–37. [3] Bijwe J, Rajesh JJ, Jeyakumar A. Influence of solid lubricants and fiber reinforcement on wear behaviour of polyethersulphone. Tribol Int 2000;33: 697–706. [4] Bijwe J, Rattan R, Fahim M. Abrasive wear performance of carbon fabric reinforced polyetherimide composites: influence of content and orientation of fabric. Tribol Int 2007;40:844–54. [5] Friedrich K. Friction and wear of polymer composites. Amsterdam: Elsevier Science Publishers; 1986. [6] Yamamoto Y, Hashimoto M. Friction and wear of water lubricated PEEK and PPS sliding contacts part 2. Composites with carbon or glass fiber. Wear 2004;257:181–9. [7] Friedrich K, Zhang Z, Schlarb AK. Effects of various fillers on the sliding wear of polymer composites. Compos Sci Technol 2005;65:2329–43. [8] Jacobs O, Friedrich K, Marom G, Schulte K. Fretting wear performance of glass-, carbon-, and aramid-fibre/epoxy and peek composites. Wear 1990;135: 207–16. [9] Mimaroglu A, Unal H, Arda T. Friction and wear performance of pure and glass fibre reinforced polyetherimide on polymer and steel counterface materials. Wear 2007;262:1407–13. [10] Liu XJ, Li TS, Yang SR. Study of the tribological properties of Kevlar fabric. Tribology (Chinese) 1999;19:23–7. [11] Li TQ, Wang CY. Effect factors and improving methods of the properties of aramid fiber and its composite. Polym Mater Sci Eng (Chinese) 2003;19: 5–9. [12] Yue CY, Padmanabhan K. International studies on surface modified Kevlar fiber/epoxy matrix composites. Compos Part B—Eng 1999;30: 205–17. [13] Lin TK, Wu SJ, Lai JG, Shyu SS. The effect of chemical treatment on reinforcement/matrix interaction in Kevlar-fiber/bismaleimide composites. Compos Sci Technol 2000;60:1873–8. [14] C - o¨keliler D, Erkut S, Zemek J. Modification of glass fibers to improve reinforcement: a plasma polymerization technique. Dent Mater 2007;23: 335–42. [15] Yang Y, Lu CX, Su XL. Effect of nano-SiO2 modified emulsion sizing on the interfacial adhesion of carbon fibers reinforced composites. Mater Lett 2007;61:3601–4. [16] Su FH, Zhang ZZ, Guo F. Tribological properties of the composites made of pure and plasma treated-Nomex fabrics. Wear 2006;261:293–300. [17] Su FH, Zhang ZZ, Guo F. Study on the friction and wear properties of the composites made of surface modified-Nomex fabrics. Mater Sci Eng A 2006; 416:126–33. [18] Park JM, Kim DS, Kim SR. Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar
ARTICLE IN PRESS F. Guo et al. / Tribology International 42 (2009) 243–249
fibers/epoxy composites using micromechanical techniques and surface wettability. J Colloid Interface Sci 2003;264:431–45. [19] McCord M. Atmospheric pressure plasma modification of textile surfaces: nylon 66, polypropylene, and ultra high modulus polyethylene. In: Proceedings of the second AUTEX conference; 2002. p. 313–26. [20] Tsal PP. Surface modification of fabrics using a one-atmosphere glow discharge plasma to improve fabric wettability. Text Res J 1997;67:359–69.
249
[21] Huang FL, Wei QF, Wang XQ. Dynamic contact angles and morphology of PP fibers treated with plasma. Polym Test 2006;25:22–7. [22] Li RZ, Ye L, Mai YW. Application of plasma technologies in reinforced–reinforced polymer composites: a review of recent developments. Compos Part A—Appl S 1997;28:73–86. [23] Cech V. New progress in composite interfaces: a use of plasma technologies. In: Proceedings of the FRC 2000, Newcastle 2000:246–52.