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Tribological and mechanical properties of Nomex fabric composites filled with polyfluo 150 wax and nano-SiO2
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 102–110 www.elsevier.com/locate/compscitech
Tribological and mechanical properties of Nomex fabric composites filled with polyfluo 150 wax and nano-SiO2 Feng-Hua Su a
a,b
, Zhao-Zhu Zhang
a,*
, Wei-Min Liu
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School, Chinese Academy of Sciences, PR China Received 27 October 2005; received in revised form 16 March 2006; accepted 29 March 2006 Available online 22 May 2006
Abstract Nomex fabric composites filled with the particulates of polyfluo150 wax (PFW) and nanoparticles of SiO2, respectively, were prepared by dip-coating of Nomex fabric in a phenolic resin containing particulates to be incorporated and the successive curing. The friction and wear behaviors of the pure and filled-Nomex fabric composites sliding against AISI-1045 steel in a pin-on-disk configuration were evaluated on a Xuanwu-III high temperature friction and wear tester. The structure of the composites, and the morphologies of the worn surfaces and of the counterpart steel pins were analyzed by means of scanning electron microscopy. The adhesion and tensile strength of the unfilled, PFW or nano-SiO2 filled Nomex fabric composites were evaluated with a DY35 universal material tester. The results showed that the addition of PFW and nano-SiO2 significantly improved the wear resistance and decreased the friction coefficient, moreover the PFW as a filler is better than nano-SiO2. The improved tribological performance of filled-Nomex fabric composites when compared with the unfilled one, can be attributed to the self-properties of filler, such as the self-lubricative of PFW, the bonding strength between the Nomex fabric and the adhesive resin adopted with the different particles and the special characteristic of transfer film. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nomex fabric composite; A. Polyfluo 150 wax and nano-SiO2; B. Adhesion and tensile strengths; B. Friction and wear behaviors
1. Introduction In order to improve the wear-resistance and frictionreducing abilities of the polymers, fiber reinforcement, such as glass and carbon fiber, were frequently applied, which can generally enhance the mechanical properties of the composites and result in an superior tribological performance [1–4]. Kevlar fiber combines a high specific modulus and strength, and moreover exhibits low electrical conductivity as compared with metallic and carbon fibers. In the recent decade, many studies have been carried out on the mechanical and tribological properties of Kevlar fiber filled with polymer [5–7]. Among these Kevlar fibers, Nomex
*
Corresponding author. Tel.: +86 931 4968098. E-mail address: [email protected] (Z.-Z. Zhang).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.03.029
fiber is increasing attracting considerable scientific and technological interest by virtue of their thermal resistance. The fabric composites, composed of fabric as the matrix, and adhesive resin as the binder, which can be cohered with metal surface in the presence of a adhesive resin, have been considered to be a good candidate material for tribological application, such as aircraft brake disk and linings [8–10], due to the composites with good self-lubricity, anti-wear ability, and load-carrying capacity, as well as the low density. Unfortunately, few studied have been reported on the friction and wears behaviors of the fabric composites [11–14], especially the tribological study of Nomex fabric composites. The tribological properties of polymers filled with solid lubricants and nano-particles to improve the wear resistance and friction reducing of polymer composites have been extensively investigated [15–20]. However the study on the friction and wear properties of Nomex fabric
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composites reinforced with solid lubricants and nano-particles has not been reported in any journal. PFW (polyfluo 150 wax), the special mixture of polytetrafluoroethylene and polyethylene, have the good potential as lubricant material because of its low surface energy [12]. Owing to its specific properties such as high surface activity and energy and small size effect, the inorganic nano-particles filled with fabric composites is of significance to improve their tribological and mechanical properties [13,12]. With this perspective in mind, PFW and nano-SiO2 were selected to fill Nomex fabric composites so as to obtain the superior tribological performance of Nomex fabric composites. This article deals with the preparation of Nomex fabric composites filled with PFW, nano-SiO2, respectively. The friction and wear properties of the composites under different condition were also evaluated. 2. Experiment 2.1. Materials The Nomex fabric, which is plain weave with a weight/ area of 1.37 g/cm2, was knitted with the Nomex fibers from the Du Pont Plant. The adhesive resin, 204 phenolic resin adhesive, was provided by the Shanghai Xing-guang Chemical Plant, China. PFW (polyfluo-150 wax, diameter <15 lm, melting point 112 C) was provided by Micro Powder Inc., USA. SiO2 (diameter about 20 nm, determining by a TEM) was prepared in our lab. The surface of AISI-1045 steel disc to be coated by the Nomex fabric composites was mechanically polished by the 280 and 350 grade waterproof abrasive papers to a surface roughness of Ra = 0.45–0.80 lm. 2.2. Preparation of Nomex fabric composites PFW and nano-SiO2 were evenly dispersed in the phenolic resin adhesive at proper mass fractions with the assistant of magnetical stirring and ultrasonic stirring. Then the Nomex fabric after pre-treatment (dipped in acetone for 24 h, followed by boiling in distilled water for 10 min and drying in an oven for 4 h at 80 C) was immersed in the mixed adhesive to allow the coating by the adhesive mixture containing the filler. The immersion of the Nomex fabric in the mixed adhesive and the successive drying of the coated Nomex fabric around 60 C were repeated to allow the generation of the Nomex fabric composites saturated with the mixed adhesive resin and about 400 lm thick. Finally, the filled Nomex fabric (abridged as NFC) with proper mass fraction particles were affixed on the AISI1045 steel surface using the pure phenolic resin adhesive and curing at 180 C for 2 h under 0.15 MPa. The unfilled Nomex fabric composite was prepared in the same manner except that no particles fillers were introduced into the phenolic resin as the binder. The cross-section picture of the pure and filled-Nomex fabric composites were analyzed with JSM-5600LV scanning electron micrograph.
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2.3. Adhesion and tensile strength test The tensile strength of the single strip of 120 mm · 15 mm Nomex fabric composite was determined on a DY35 universal materials test machine at a constant speed of 10 mm/min. Thus a single strip Nomex fabric composite was clamped and fixed vertically and then pulled until it was broken. The force at the breakage point was cited as the pulled out force FP that was determined from the recorded load–displacement curve. The accurate thickness D and width B of the Nomex fabric composites were measured with a vernier caliper, and the tensile strength s of the Nomex fabric composites was calculated as s = FP/B Æ D. Before carrying out the adhesion test, the Nomex fabric composites was cut into strips of 20 mm · 12.5 mm, with a thickness of about 400 lm. Then the strip was affixed between two AISI-1045 steel plates using phenolic resin adhesive and cured at 180 C for 2 h under 0.15 MPa. The adhesion strength between the Nomex fabric composite, adhesive resin and the AISI-1045 steel was also determined on the universal materials test machine. The resulting two stainless steel plates containing the strip Nomex fabric composites were clamped and fixed vertically and pulled at a constant speed of 50 mm/min, until the two stainless steel plates containing the strip Nomex fabric composites were pulled apart. The force at this point was cited as the pulled out force P that was determined from the recorded load–displacement curve. The exact length L and width B of the Nomex fabric composites were measured on a vernier caliper. The adhesion strength sA between the Nomex fabric composites and the adhesive resin on the stainless steel plates was calculated as sA = P/L Æ B. Five replica tests were carried out for each specimen and the averaged results of the five replica tests were reported. (The relative errors to measure the adhesion and tensile strength are ±5%.) 2.4. Friction and wear test The friction and wear behaviors of the Nomex fabric composites adhered on the stainless steel disc sliding against AISI-1045 steel pin of a diameter 3 mm were evaluated on an Xuanwu-III high temperature friction and wear tester. Fig. 1 shows the schematic diagram of the test rig. Prior to the tests, the pin was successively mechanically polished with 350, 700, and 900 grade waterproof abrasive paper, to a surface roughness Ra = 0.15 lm, and then cleaned with acetone. The sliding was performed under ambient condition at a sliding velocity of 0.256 m/s, and a normal load within 235.2–600.0 N, a temperature of 20–240 C and over a period of 2 h except for the otherwise indication. At the end of each test, the disc was cleaned and dried, then its wear volume loss (V) was obtained by measuring the wear scar and wear depth on a micrometer (±0.001 mm). The wear rate was obtained from dividing the wear volume loss by the normal load and sliding distance, and it was still calculated using the same formula
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Fig. 1. Schematic diagram of the pin-on-disc (pin:u 3.0 mm and disc: u 44.0 mm) friction and wear tester: P, applied load; 1, counterpart pin; 2, NFC coated AISI-1045 steel disc; 3, electric furnace; and 4, thermocouple.
(w = V/p Æ L) even the tested composites sample was unable to endure sliding for 2 h. The friction coefficients can obtain directly from the computer with the friction-measured software that has translated the frictional torque to friction coefficient accurately. The contact temperature of the worn surface was monitored by a thermocouple position on the edge of the counterpart pin. The environmental temperature of frictional condition was controlled with the electric furnace and was monitored with a thermocouple in the furnace. Each experiment was carried out three times and average value was considered. (The relative errors to measure the friction coefficient and wear volume loss are ±5%.) The morphology of the worn composite surfaces was analyzed on a JSM-5600LV scanning electron microscope (SEM).
in composites greatly decrease the friction coefficient and wear rate of the Nomex fabric composite. However nanoSiO2 filler with a mass fraction of 4% only slightly changes the friction coefficient as compared with the unfilled Nomex fabric composite, but exhibit the optimum improvement of the anti-wear abilities. And it is clearly observed that PFW as a filler is better than nano-SiO2 in obtaining the best tribological property of Nomex fabric composite. The comparison of the maximal loading abilities (a series of friction and wear tests at a fixed velocity of 0.256 m/s with various loads conducted under dry sliding for a test duration up to 120 min, the largest normal load at which the composites can endure is cited as the maximal loading ability) of the unfilled and particle filler-filled Nomex fabric composites is shown in Fig. 2. It is clearly seen that the composites filled with 4% of nano-SiO2 and 20% of PFW show higher maximal loading abilities than the unfilled one. The latter can endure a load as high as 525.0 N, which is much higher than that of the unfilled one (302.5 N). The effects of the PFW content on friction coefficient and wear rate of Nomex fabric composites at 470.4 N and room temperature is shown in Table 2. The unfilled Nomex fabric composite cannot endure sliding for over 5 min under the load of 470.4 N but the Nomex fabric composites filled with different contents of PFW can endure sliding for over 2 h under the same condition, which indicates that the PFW as a filler can greatly enhance the
3. Results
Table 1 Comparison of the friction coefficient, wear rate and the contact temperature of unfilled and filled Nomex fabric composites at 274.4 N under room temperature
3.1. Friction and wear behaviors
Composite
Table 1 shows the coefficient of friction, wear rate and the contact temperature of the unfilled and filled-Nomex fabric composites at 274.4 N under room temperature. As seen in Table 1, PFW filler with a mass fraction of 20%
Friction coefficient (l)
Wear rate/10 14 m3 (N m) 1
Contact temperature (C)
Unfilled NFC 20%PFW/NFC 4%nano-SiO2/NFC
0.136 0.057 0.128
3.59 0.75 1.96
62 40 52
0
10
NFC+4% nano-SiO2
NFC+20% PFW
Unfilled NFC
0
100
200
300
400
500
The Maximal loading ability /N Fig. 2. Comparison of maximal loading ability of unfilled and filled Nomex fabric composites at room temperature.
F.-H. Su et al. / Composites Science and Technology 67 (2007) 102–110 Table 2 Effect of PFW contents on friction coefficient and wear rate of Nomex fabric composite filled with PFW at 470.4 N under room temperature Composites
Friction coefficient (l)
Wear rate/10
Unfilled NFC 10%PFW/NFC 20%PFW/NFC 30%PFW/NFC 40%PFW/NFC
– 0.051 0.044 0.042 0.040
– 1.72 1.11 2.24 4.35
14
m3 (N m)
1
105
Table 3 Effect of nano-SiO2 contents on the friction coefficient and wear rate of Nomex fabric composites filled with nano-SiO2 at 274.4.4 N and room temperature 14
m3 (N m)
1
Composites
Friction coefficient (l)
Wear rate/10
‘‘–’’ represents t the composites cannot slide for over 5 min under this condition.
Unfilled NFC 2%Nano-SiO2/NFC 4%Nano-SiO2/NFC 6%Nano-SiO2/NFC 8% nano-SiO2/NFC
0.136 0.130 0.128 0.127 0.133
3.59 2.75 1.96 3.01 3.93
friction and wear behaviors of Nomex fabric composites. From Table 2, we could see that the friction coefficient of the composites decrease with the increasing PFW content, but the corresponding wear rate decreases initially with increasing content of PFW up to 20%, and then, gradually increases as PFW content further increased above 20%. Table 3 shows the effect of nano-SiO2 content on the friction coefficient and wear rate of nano-SiO2 filled-Nomex fabric composites at 274.4 N under room temperature. It is seen that the friction coefficient of the nano-SiO2 filledNomex fabric composite change very slightly as compared with the pure Nomex fabric composite. However the
inclusion of nano-SiO2 leads to a decrease in the wear rate of the composites and the smallest wear rate is obtained with a filler mass fraction of 4% for the nano-SiO2-filled Nomex fabric composites. The composite filled with 8% nano-SiO2 even have a wear rate larger than the unfilled one. In order to illustrate the friction and wear mechanism of the unfilled and filled-Nomex fabric composites, the typical variations in friction coefficient of the composites with sliding time at typical loads of 274.4 and 313.6 N are shown in Fig. 3. We can find that the frictional process contains the severe friction process and the smooth friction process, and the friction coefficient at the severe friction coefficient
Fig. 3. Typical friction curves with sliding time of the unfilled and filled-Nomex fabric composites at typical loads of 274.4 and 313.6 N (room temperature).
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is much higher than that at the smooth friction process. At the load of 274.4 N for the unfilled and 4%nano-SiO2 filled Nomex fabric composites, the severe-smooth friction process transition completes rapidly within 10 min (see Fig. 3(a)). However at the load of 313.6 N the friction process of the unfilled Nomex fabric composites is at the persistent severe friction, but the severe-smooth friction process transition also completes rapidly within 20 min of 4%nano-SiO2-filled Nomex fabric composites (see Fig. 3(b)), which correspond to the improved anti-wear abilities of the 4%nano-SiO2 filled Nomex fabric composites. In addition, the 20%PFW filled-Nomex fabric composites exhibited constant smooth friction process at 274.4 and 313.6 N, which indicates that PFW as filler can obviously reduce the friction force and at last obtain the lowest friction coefficient and wear rate. Variation of friction coefficient and wear rate of the unfilled and filled-Nomex fabric composites with load under room temperature are shown in Fig. 4. It is seen that the unfilled Nomex fabric composites show a slightly increased friction coefficient and wear rate with increasing load up to 274.4 N and then show a rapidly increased friction coefficient and wear rate from 274.4 to 313.6 N, which is related with the friction and wear mechanism of the composites. But the friction coefficient of the 4%nano-SiO2 and
Unfilled NFC 4%nano-SiO2 /NFC 20% PFW/NFC
0.20 0.16 0.12 0.08 0.04 235.2
313.6
392.0
Wear rate /10 -14m3(N.m)-1
*
0.24
Friction coefficient µ
20%PFW filled Nomex fabric composites decrease with the increase of load. However the wear rate of 4%nano-SiO2 filled-Nomex fabric composites increase gradually with increasing load and the wear rate of 20%PFW filled Nomex fabric composites do not show obviously change with increasing load. The Nomex fabric composites filled with 20% PFW exhibited the lowest friction coefficient and wear rate, followed by 4%nano-SiO2 under the identical wear condition. Unfilled and 20%PFW filled-Nomex fabric composites are selected as example to investigate the effect of the environmental temperature on the friction and wear properties of composites. Fig. 5 shows the effect of the environmental temperature on the friction coefficient and wears rate of the unfilled and 20%PFW filled-Nomex fabric composites at 274.4 N. It can be obviously seen that the friction coefficient and wear rate of the 20%PFW-filled Nomex fabric composites is much lower than that of the unfilled Nomex fabric composites under different temperature, which indicate that PFW as filler can improve the wear-resistance, friction-reducing and thermal stability of the composites. The friction coefficient of the unfilled and 20%PFW-filled Nomex fabric composites decreased initially with the increasing temperature up to 180 C, and then rapidly increased from 180 to 240 C. However, the wear rate of
18 16
* Unfilled NFC 4%nano-SiO2 /NFC 20%PFW/NFC
4 3 2 1 0 235.2
470.4
313.6
Apllied load /N
392.0
470.4
548.8
Apllied load /N
Fig. 4. Variation in friction coefficient and wear rate of unfilled and filled Nomex fabric composites with load at room temperature. (The data marked with * represents the composites cannot slide for 120 min under this condition.)
unfilled NFC 20% PFW/NFC
Friction coefficient
0.16 0.14
*
0.12 0.10 0.08 0.06
*
0.04 0
60
120
180
240
Environmental temperature /°C
Wear rate /10-14m3(N.m)-1
0.18
30 25 20 15
*
Unfilled NFC 20% PFW/NFC
*
4 3 2 1 0 0
60
120
180
240
Environmental temperature /°C
Fig. 5. Effect of the environmental temperature on the friction coefficient and wear rate of unfilled and 20%PFW-filled NFC at 274.4 N. (The data marked with * represent that the composites cannot slide for 120 min under this condition.)
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the unfilled and 20%PFW-filled composites increased gradually with the increase of the environmental temperature up to 180 C, and then rapidly increased from 180 to 240 C. The rapidly increase of the friction coefficient and wear rate from 180 to 240 C are owing to the catastrophic destruction of the composites through wearing process at evaluated high temperature [12,13]. 4. Discussion 4.1. Tensile and adhesion strength Tensile strength indirectly represents the bonding strength between the adhesive, fabric-fiber and filler particles, which directly affect the structural integrity of the fabric composites and finally affect the friction and wear properties of the composites. And the adhesion strength between the Nomex fabric composite, adhesive resin and the AISI-1045 steel directly influence on the friction and wear properties of fabric composites because many fabric composites with the low adhesion strength might be peeled off from the steel disc through the wearing process. Table 4 shows the tensile and adhesion strength of the unfilled and filled Nomex fabric composites. Table 4 Adhesion and tensile strength of the unfilled and filled Nomex fabric composites Composite
Mechanical properties (MPa) Tensile strength
Adhesion strength
Unfilled NFC 20%PFW/NFC 40%PFW/NFC 4%nano-SiO2/NFC 8%nano-SiO2/NFC
41.1 69.6 57.0 65.1 54.7
10.3 15.9 13.2 13.7 11.1
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It is seen that different contents of PFW and nano-SiO2, as the fillers, can enhance the tensile and adhesion strength of the composites. And the Nomex fabric composites filled with 20% PFW can obtain the largest adhesion and tensile strength, which correspond to the best anti-wear and friction-reducing abilities. Our previous studies on the structure of PFW with IR have found that some active groups such as mC@O and mO–H generated during the curing of the PFW in fabric composites at 180 C [12]. Because these active groups generated, the proper content of PFW as filler reinforcing Nomex fabric composites help to increase the bonding strength among Nomex fabric and the adhesive resin. However, when the composites were filled with much excessive content of the PFW, the particles tended to conglomerate. The agglomeration of excessive content of PFW would affect the structure of the Nomex fabric composites, as a result the tensile and adhesion strength of the Nomex fabric composites filled with 40% PFW is obvious lower than the 20%PFW-filled Nomex fabric composites (see Fig. 6). Moreover, the Nomex fabric composites filled with 4%nano-SiO2 also shows the proper enhancement of tensile and adhesion strength. This corresponds to the better wearresistance of the filled Nomex fabric composites than that of the unfilled one. It is supposed that the enhanced adhesion and tensile strength is related to the large surface energy and activity of nano-particles, which contribute to increase the interfacial boding among the fillers, the Nomex fabric matrix and the adhesive resin, and accordingly improve the anti-wear abilities of the Nomex fabric composites. However, when the nano-SiO2 particulates were incorporated in the fabric composite at a much excessive content, the nano-particulates would also tend to conglomerate, and subsequently the corresponding structural integrity of the composites would be affected (see Fig. 6). Because of the destroyed structure of Nomex fabric
Fig. 6. SEM pictures of the cross section of: (a) unfilled NFC, (b) 20%PFW/NFC, (c) 40%PFW/NFC, (d) 4%nano-SiO2/NFC and (e) 8%Nano-SiO2/ NFC.
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composites filled with excessive nano-SiO2, the composites would have poor friction-reducing and anti-wear properties (see Table 3). 4.2. SEM analysis of the structure In order to verify the difference of the tribological properties, the adhesion strength and the tensile strength of the unfilled and different particles filled-Nomex fabric composites. Fig. 6 shows the SEM micrograph of the cross-section of these composites, which directly reflect structural integrity of the composites, namely, the interfacial bonding strength between the Nomex fabric, the adhesive resin doped with filler particles. It is obviously observed that some holes and cracks exist in the pure Nomex fabric composites, and the Nomex fiber cannot compactly bond with the adhesive (see Fig. 6(a)), which correspond to the poor tribological and mechanical properties of the unfilled composites. However, the Nomex fabrics bond well with the adhesive resin doped with 20% PFW (see Fig. 6(b)) and 4%nano-SiO2 (see Fig. 6(c)) respectively. Surprisingly, the interface between Nomex fabric and adhesive resin of the 20%PFW filled-Nomex fabric composites become blurry (see Fig. 6(b)), which indicate that the proper content PFW does contribute to the increase of bonding strength between the Nomex fabric and the adhesive resin. This could be rational to understand why 20%PFW-filled Nomex fabric composites obtain the best tribological properties, largest adhesion and tensile strength
as compared with the unfilled and nano-SiO2 filled-Nomex fabric composites. But when the content of PFW and nanoSiO2 fillers are 40% and 8%, respectively, some holes and cracks are obviously observed, and the Nomex fiber cannot compactly compact with the adhesive doped with excessive filler (see Figs. 6(d) and (e)), particularly doped with the 8%nano-SiO2. As a result, the structure of the composites filled with excessive PFW and nano-SiO2 would deteriorate, and then result in the lower adhesion and tensile strength and hence the poor tribological performance. 4.3. SEM analysis of the worn surfaces Fig. 7 shows the SEM pictures of the worn surfaces of the unfilled, 4%nano-SiO2 and 20%PFW filled Nomex fabric composites at 313.6 N and room temperature. It is seen that most Nomex fibers are pulled out and cut from the composites matrix on the worn surface of unfilled Nomex fabric composites after sliding for 40 min (see Fig. 7(a)), which indicate that the unfilled Nomex fabric composites experienced severe peeling off as it slide against the steel. However, the worn surface of 4%nano-SiO2 filled Nomex fabric composites are characterized with a little pulledout and cut fibers (see Fig. 7(b)), which agree well with the friction and wear behaviors of the composites filled with 4%nano-SiO2. Contrary to the above, the worn surface of the 20%PFW-filled Nomex fabric composites is very smooth and the pulling-out and exposure of the Nomex fiber are nearly invisible (see Fig. 7(c)), which indicate that
Fig. 7. SEM pictures of the worn surface of: (a) Unfilled NFC at 40 min; (b) 4%nano-SiO2/NFC at 120 min; and (c) 20%PFW/NFC at 120 min (313.6 N and room temperature).
Fig. 8. SEM morphologies of the worn steel surfaces sliding against: (a) Unfilled NFC; (b) 4%nano-SiO2/NFC; and (c) 20%PFW/NFC (274.4 N, room temperature, 2 h).
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the PFW particles effectively incorporated in the Nomex fabric composites and improved the friction and wear properties of the Nomex fabric composites. The SEM morphologies of the worn surfaces of the counterpart pin sliding against the unfilled, 4%nanoSiO2 and 20%PFW filled Nomex fabric composites at 274.4 N and room temperature are shown in Fig. 8, respectively. We can find that the transfer film on the pin against the unfilled Nomex fabric composites is characterized with thick, rough and discontinuous film, and the transfer film was easy scaled off during wear process (see Fig. 8(a)), which corresponds to the poor mechanical strength and the poor wear-resistance of the unfilled Nomex fabric composites. However the transfer film on the counterpart pin surface sliding 4%nano-SiO2 filled Nomex fabric composites is thin and uniform but show some signs of scuffing (see Fig. 8(b)), which agree well with the increased anti-wear abilities and maximal loading ability of 4%nano-SiO2 filled Nomex fabric composites as compared with the unfilled one. Contrary to the above, the transfer film on the counterpart pin sliding against 20%PFW filled-Nomex fabric composites is much smooth and uniform and showed no signs of scuffing (see Fig. 8(c)), which conform to the best mechanical and tribological properties and the largest adhesion and tensile strength of this composites. 5. Summary The incorporation of PFW with different contents in the Nomex fabric composites can significantly enhance the anti-wear and friction-reducing abilities and increase the adhesion and tensile strength of the composites. Nano-SiO2 at optimum content can enhance the anti-wear abilities, the maximal loading abilities and increase the adhesion and tensile strength of the Nomex fabric composites. The Nomex fabric composites filled with 20%PFW exhibits the lowest friction coefficient and wear rate, largest adhesion and tensile strength and largest maximal loading abilities. PFW, as a filler with melting point 112 C, can homogeneously disperse in Nomex fabric composites because some active groups generate during the curing of the PFW-filled Nomex fabric composites at 180 C. These active groups contribute to the increase of the bonding strength between the Nomex fabric and the adhesive, and hence to increase the adhesion strength and tensile strength and finally improve the tribological properties. At the same time, the self-lubricity of PFW can significantly reduce the friction and shear force, and also contribute to the increase of friction-reducing and anti-wear abilities of PFW-filled Nomex fabric composites. Moreover, Nomex fabric composites filled with a proper content of nano-SiO2 exhibit larger adhesion and tensile strength and better anti-wear abilities than that of the unfilled Nomex fabric composites owing to specific properties of nano-SiO2 such as high surface activity and
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energy and small size effect. The difference in the characters of the transfer film on the surfaces of counterpart pin sliding against unfilled and different particles filled-Nomex fabric composites also account for the differences in the friction and wear behaviors of the unfilled, 4% of nanoSiO2 and 20% of PFW filled-Nomex fabric composites. In a word, the character of the transfer film and the difference in composites structure, coupled with the difference in the properties of the particles themselves, account for the difference in the wear resistance and friction-reduction of Nomex fabric composites filled with PFW and nano-SiO2. Acknowledgements We thank Prof. Jiazheng Zhao for carrying out SEM analysis. We acknowledged financial support from the National Nature Science Foundation of China (50421502). References [1] Friedrich K, Zhang Z, Schlarb AK. Effects of various fillers on the sliding wear of polymer composites. Compos Sci Technol 2005;65(15):2329–43. [2] Jia JH, Chen JM, Zhou HD, Hu LT, Chen L. Comparative investigation on the wear and transfer behaviors of carbon fiber reinforced polymer composites under dry sliding and water lubrication. Compos Sci Technol 2005;65(7):1139–47. [3] Zhang H, Zhang Z. Comparison of short carbon fibre surface treatments on epoxy composites: II. Enhancement of the wear resistance. Compos Sci Technol 2004;64(13):2031–8. [4] Luo RY. Fabrication of carbon/carbon composites by electrified perform beating CVI method. Carbon 2002;40(11):2693–701. [5] Huang BZ, Hu XZh, Liu J. Modelling of inter-laminar toughening from chopped Kevlar fibers. Compos Sci Technol 2004;64(13): 2165–75. [6] Park BY, Kim SC. A study of the interlaminar fracture toughness of a carbon-fiber/epoxy composite containing surface-modified short kevlar fibers. Compos Sci Technol 1998;58(3):389–400. [7] Pihtili H, Tosum NH. Effect of load and speed on the wear behaviour of woven glass cloth and aramid fiber-reinforced composites. Wear 2002;252(11):979–84. [8] Lancaster JK, Bay RG. Selecting and use of wear tests for coatings. ASTM, STP 1982;769:92–9. [9] Blanchet TA, Kennedy FE. Sliding wear mechanism of polytetrafluoroethylene (PTFE) and PTFE composites. Wear 1992;153(1):229–43. [10] Craig WD. PTFE bearings. Lubricat Eng 1964;20(12):456–62. [11] Su FH, Zhang ZZ, Wang K, Jiang W, Liu WM. Tribological and mechanical properties of the composites made of carbon fabrics modified with various methods. Compos Part A 2005;36(12): 1601–17. [12] Zhang ZZ, Su FH, Wang K, Jiang W, Men XH, Liu WM. Study on the friction and wear properties of carbon fabric composites reinforced with micro- and nano-particles. Mater Sci Eng A 2005;404(1):251–8. [13] Su FH, Zhang ZZ, Liu WM. Study on the friction and wear properties of glass fabric composites filled with nano- and microparticles under different conditions. Mater Sci Eng A 2005;392(1): 359–65. [14] Ren ZH, Wang QH, Wu ZD. Research on tribological properties of wear-resistant polytetrafluoroethylene fabric. Tribology (In Chinese) 2002;22:193–8.
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[18] Wang QH, Xue QJ, Liu WM, Chen JM. The friction and wear characteristics of nanometer SiC and polytetrafluoroethylene filled polyetheretherketone. Wear 2000;243(1):140–6. [19] Shi G, Zhang MQ, Rong MZ, Wetzel B, Friedrich K. Friction and wear of low nanometer Si3N4 filled epoxy composites. Wear 2003;254(7):784–96. [20] Li F, Hu KA, Li JL, Zhao BY. The friction and wear characteristics of nanometer ZnO filled polytetrafluoroethylene. Wear 2002;249(10): 877–82.
Tribological and mechanical properties of Nomex fabric composites filled with polyfluo 150 wax and nano-SiO2 Feng-Hua Su a
a,b
, Zhao-Zhu Zhang
a,*
, Wei-Min Liu
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School, Chinese Academy of Sciences, PR China Received 27 October 2005; received in revised form 16 March 2006; accepted 29 March 2006 Available online 22 May 2006
Abstract Nomex fabric composites filled with the particulates of polyfluo150 wax (PFW) and nanoparticles of SiO2, respectively, were prepared by dip-coating of Nomex fabric in a phenolic resin containing particulates to be incorporated and the successive curing. The friction and wear behaviors of the pure and filled-Nomex fabric composites sliding against AISI-1045 steel in a pin-on-disk configuration were evaluated on a Xuanwu-III high temperature friction and wear tester. The structure of the composites, and the morphologies of the worn surfaces and of the counterpart steel pins were analyzed by means of scanning electron microscopy. The adhesion and tensile strength of the unfilled, PFW or nano-SiO2 filled Nomex fabric composites were evaluated with a DY35 universal material tester. The results showed that the addition of PFW and nano-SiO2 significantly improved the wear resistance and decreased the friction coefficient, moreover the PFW as a filler is better than nano-SiO2. The improved tribological performance of filled-Nomex fabric composites when compared with the unfilled one, can be attributed to the self-properties of filler, such as the self-lubricative of PFW, the bonding strength between the Nomex fabric and the adhesive resin adopted with the different particles and the special characteristic of transfer film. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nomex fabric composite; A. Polyfluo 150 wax and nano-SiO2; B. Adhesion and tensile strengths; B. Friction and wear behaviors
1. Introduction In order to improve the wear-resistance and frictionreducing abilities of the polymers, fiber reinforcement, such as glass and carbon fiber, were frequently applied, which can generally enhance the mechanical properties of the composites and result in an superior tribological performance [1–4]. Kevlar fiber combines a high specific modulus and strength, and moreover exhibits low electrical conductivity as compared with metallic and carbon fibers. In the recent decade, many studies have been carried out on the mechanical and tribological properties of Kevlar fiber filled with polymer [5–7]. Among these Kevlar fibers, Nomex
*
Corresponding author. Tel.: +86 931 4968098. E-mail address: [email protected] (Z.-Z. Zhang).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.03.029
fiber is increasing attracting considerable scientific and technological interest by virtue of their thermal resistance. The fabric composites, composed of fabric as the matrix, and adhesive resin as the binder, which can be cohered with metal surface in the presence of a adhesive resin, have been considered to be a good candidate material for tribological application, such as aircraft brake disk and linings [8–10], due to the composites with good self-lubricity, anti-wear ability, and load-carrying capacity, as well as the low density. Unfortunately, few studied have been reported on the friction and wears behaviors of the fabric composites [11–14], especially the tribological study of Nomex fabric composites. The tribological properties of polymers filled with solid lubricants and nano-particles to improve the wear resistance and friction reducing of polymer composites have been extensively investigated [15–20]. However the study on the friction and wear properties of Nomex fabric
F.-H. Su et al. / Composites Science and Technology 67 (2007) 102–110
composites reinforced with solid lubricants and nano-particles has not been reported in any journal. PFW (polyfluo 150 wax), the special mixture of polytetrafluoroethylene and polyethylene, have the good potential as lubricant material because of its low surface energy [12]. Owing to its specific properties such as high surface activity and energy and small size effect, the inorganic nano-particles filled with fabric composites is of significance to improve their tribological and mechanical properties [13,12]. With this perspective in mind, PFW and nano-SiO2 were selected to fill Nomex fabric composites so as to obtain the superior tribological performance of Nomex fabric composites. This article deals with the preparation of Nomex fabric composites filled with PFW, nano-SiO2, respectively. The friction and wear properties of the composites under different condition were also evaluated. 2. Experiment 2.1. Materials The Nomex fabric, which is plain weave with a weight/ area of 1.37 g/cm2, was knitted with the Nomex fibers from the Du Pont Plant. The adhesive resin, 204 phenolic resin adhesive, was provided by the Shanghai Xing-guang Chemical Plant, China. PFW (polyfluo-150 wax, diameter <15 lm, melting point 112 C) was provided by Micro Powder Inc., USA. SiO2 (diameter about 20 nm, determining by a TEM) was prepared in our lab. The surface of AISI-1045 steel disc to be coated by the Nomex fabric composites was mechanically polished by the 280 and 350 grade waterproof abrasive papers to a surface roughness of Ra = 0.45–0.80 lm. 2.2. Preparation of Nomex fabric composites PFW and nano-SiO2 were evenly dispersed in the phenolic resin adhesive at proper mass fractions with the assistant of magnetical stirring and ultrasonic stirring. Then the Nomex fabric after pre-treatment (dipped in acetone for 24 h, followed by boiling in distilled water for 10 min and drying in an oven for 4 h at 80 C) was immersed in the mixed adhesive to allow the coating by the adhesive mixture containing the filler. The immersion of the Nomex fabric in the mixed adhesive and the successive drying of the coated Nomex fabric around 60 C were repeated to allow the generation of the Nomex fabric composites saturated with the mixed adhesive resin and about 400 lm thick. Finally, the filled Nomex fabric (abridged as NFC) with proper mass fraction particles were affixed on the AISI1045 steel surface using the pure phenolic resin adhesive and curing at 180 C for 2 h under 0.15 MPa. The unfilled Nomex fabric composite was prepared in the same manner except that no particles fillers were introduced into the phenolic resin as the binder. The cross-section picture of the pure and filled-Nomex fabric composites were analyzed with JSM-5600LV scanning electron micrograph.
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2.3. Adhesion and tensile strength test The tensile strength of the single strip of 120 mm · 15 mm Nomex fabric composite was determined on a DY35 universal materials test machine at a constant speed of 10 mm/min. Thus a single strip Nomex fabric composite was clamped and fixed vertically and then pulled until it was broken. The force at the breakage point was cited as the pulled out force FP that was determined from the recorded load–displacement curve. The accurate thickness D and width B of the Nomex fabric composites were measured with a vernier caliper, and the tensile strength s of the Nomex fabric composites was calculated as s = FP/B Æ D. Before carrying out the adhesion test, the Nomex fabric composites was cut into strips of 20 mm · 12.5 mm, with a thickness of about 400 lm. Then the strip was affixed between two AISI-1045 steel plates using phenolic resin adhesive and cured at 180 C for 2 h under 0.15 MPa. The adhesion strength between the Nomex fabric composite, adhesive resin and the AISI-1045 steel was also determined on the universal materials test machine. The resulting two stainless steel plates containing the strip Nomex fabric composites were clamped and fixed vertically and pulled at a constant speed of 50 mm/min, until the two stainless steel plates containing the strip Nomex fabric composites were pulled apart. The force at this point was cited as the pulled out force P that was determined from the recorded load–displacement curve. The exact length L and width B of the Nomex fabric composites were measured on a vernier caliper. The adhesion strength sA between the Nomex fabric composites and the adhesive resin on the stainless steel plates was calculated as sA = P/L Æ B. Five replica tests were carried out for each specimen and the averaged results of the five replica tests were reported. (The relative errors to measure the adhesion and tensile strength are ±5%.) 2.4. Friction and wear test The friction and wear behaviors of the Nomex fabric composites adhered on the stainless steel disc sliding against AISI-1045 steel pin of a diameter 3 mm were evaluated on an Xuanwu-III high temperature friction and wear tester. Fig. 1 shows the schematic diagram of the test rig. Prior to the tests, the pin was successively mechanically polished with 350, 700, and 900 grade waterproof abrasive paper, to a surface roughness Ra = 0.15 lm, and then cleaned with acetone. The sliding was performed under ambient condition at a sliding velocity of 0.256 m/s, and a normal load within 235.2–600.0 N, a temperature of 20–240 C and over a period of 2 h except for the otherwise indication. At the end of each test, the disc was cleaned and dried, then its wear volume loss (V) was obtained by measuring the wear scar and wear depth on a micrometer (±0.001 mm). The wear rate was obtained from dividing the wear volume loss by the normal load and sliding distance, and it was still calculated using the same formula
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Fig. 1. Schematic diagram of the pin-on-disc (pin:u 3.0 mm and disc: u 44.0 mm) friction and wear tester: P, applied load; 1, counterpart pin; 2, NFC coated AISI-1045 steel disc; 3, electric furnace; and 4, thermocouple.
(w = V/p Æ L) even the tested composites sample was unable to endure sliding for 2 h. The friction coefficients can obtain directly from the computer with the friction-measured software that has translated the frictional torque to friction coefficient accurately. The contact temperature of the worn surface was monitored by a thermocouple position on the edge of the counterpart pin. The environmental temperature of frictional condition was controlled with the electric furnace and was monitored with a thermocouple in the furnace. Each experiment was carried out three times and average value was considered. (The relative errors to measure the friction coefficient and wear volume loss are ±5%.) The morphology of the worn composite surfaces was analyzed on a JSM-5600LV scanning electron microscope (SEM).
in composites greatly decrease the friction coefficient and wear rate of the Nomex fabric composite. However nanoSiO2 filler with a mass fraction of 4% only slightly changes the friction coefficient as compared with the unfilled Nomex fabric composite, but exhibit the optimum improvement of the anti-wear abilities. And it is clearly observed that PFW as a filler is better than nano-SiO2 in obtaining the best tribological property of Nomex fabric composite. The comparison of the maximal loading abilities (a series of friction and wear tests at a fixed velocity of 0.256 m/s with various loads conducted under dry sliding for a test duration up to 120 min, the largest normal load at which the composites can endure is cited as the maximal loading ability) of the unfilled and particle filler-filled Nomex fabric composites is shown in Fig. 2. It is clearly seen that the composites filled with 4% of nano-SiO2 and 20% of PFW show higher maximal loading abilities than the unfilled one. The latter can endure a load as high as 525.0 N, which is much higher than that of the unfilled one (302.5 N). The effects of the PFW content on friction coefficient and wear rate of Nomex fabric composites at 470.4 N and room temperature is shown in Table 2. The unfilled Nomex fabric composite cannot endure sliding for over 5 min under the load of 470.4 N but the Nomex fabric composites filled with different contents of PFW can endure sliding for over 2 h under the same condition, which indicates that the PFW as a filler can greatly enhance the
3. Results
Table 1 Comparison of the friction coefficient, wear rate and the contact temperature of unfilled and filled Nomex fabric composites at 274.4 N under room temperature
3.1. Friction and wear behaviors
Composite
Table 1 shows the coefficient of friction, wear rate and the contact temperature of the unfilled and filled-Nomex fabric composites at 274.4 N under room temperature. As seen in Table 1, PFW filler with a mass fraction of 20%
Friction coefficient (l)
Wear rate/10 14 m3 (N m) 1
Contact temperature (C)
Unfilled NFC 20%PFW/NFC 4%nano-SiO2/NFC
0.136 0.057 0.128
3.59 0.75 1.96
62 40 52
0
10
NFC+4% nano-SiO2
NFC+20% PFW
Unfilled NFC
0
100
200
300
400
500
The Maximal loading ability /N Fig. 2. Comparison of maximal loading ability of unfilled and filled Nomex fabric composites at room temperature.
F.-H. Su et al. / Composites Science and Technology 67 (2007) 102–110 Table 2 Effect of PFW contents on friction coefficient and wear rate of Nomex fabric composite filled with PFW at 470.4 N under room temperature Composites
Friction coefficient (l)
Wear rate/10
Unfilled NFC 10%PFW/NFC 20%PFW/NFC 30%PFW/NFC 40%PFW/NFC
– 0.051 0.044 0.042 0.040
– 1.72 1.11 2.24 4.35
14
m3 (N m)
1
105
Table 3 Effect of nano-SiO2 contents on the friction coefficient and wear rate of Nomex fabric composites filled with nano-SiO2 at 274.4.4 N and room temperature 14
m3 (N m)
1
Composites
Friction coefficient (l)
Wear rate/10
‘‘–’’ represents t the composites cannot slide for over 5 min under this condition.
Unfilled NFC 2%Nano-SiO2/NFC 4%Nano-SiO2/NFC 6%Nano-SiO2/NFC 8% nano-SiO2/NFC
0.136 0.130 0.128 0.127 0.133
3.59 2.75 1.96 3.01 3.93
friction and wear behaviors of Nomex fabric composites. From Table 2, we could see that the friction coefficient of the composites decrease with the increasing PFW content, but the corresponding wear rate decreases initially with increasing content of PFW up to 20%, and then, gradually increases as PFW content further increased above 20%. Table 3 shows the effect of nano-SiO2 content on the friction coefficient and wear rate of nano-SiO2 filled-Nomex fabric composites at 274.4 N under room temperature. It is seen that the friction coefficient of the nano-SiO2 filledNomex fabric composite change very slightly as compared with the pure Nomex fabric composite. However the
inclusion of nano-SiO2 leads to a decrease in the wear rate of the composites and the smallest wear rate is obtained with a filler mass fraction of 4% for the nano-SiO2-filled Nomex fabric composites. The composite filled with 8% nano-SiO2 even have a wear rate larger than the unfilled one. In order to illustrate the friction and wear mechanism of the unfilled and filled-Nomex fabric composites, the typical variations in friction coefficient of the composites with sliding time at typical loads of 274.4 and 313.6 N are shown in Fig. 3. We can find that the frictional process contains the severe friction process and the smooth friction process, and the friction coefficient at the severe friction coefficient
Fig. 3. Typical friction curves with sliding time of the unfilled and filled-Nomex fabric composites at typical loads of 274.4 and 313.6 N (room temperature).
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is much higher than that at the smooth friction process. At the load of 274.4 N for the unfilled and 4%nano-SiO2 filled Nomex fabric composites, the severe-smooth friction process transition completes rapidly within 10 min (see Fig. 3(a)). However at the load of 313.6 N the friction process of the unfilled Nomex fabric composites is at the persistent severe friction, but the severe-smooth friction process transition also completes rapidly within 20 min of 4%nano-SiO2-filled Nomex fabric composites (see Fig. 3(b)), which correspond to the improved anti-wear abilities of the 4%nano-SiO2 filled Nomex fabric composites. In addition, the 20%PFW filled-Nomex fabric composites exhibited constant smooth friction process at 274.4 and 313.6 N, which indicates that PFW as filler can obviously reduce the friction force and at last obtain the lowest friction coefficient and wear rate. Variation of friction coefficient and wear rate of the unfilled and filled-Nomex fabric composites with load under room temperature are shown in Fig. 4. It is seen that the unfilled Nomex fabric composites show a slightly increased friction coefficient and wear rate with increasing load up to 274.4 N and then show a rapidly increased friction coefficient and wear rate from 274.4 to 313.6 N, which is related with the friction and wear mechanism of the composites. But the friction coefficient of the 4%nano-SiO2 and
Unfilled NFC 4%nano-SiO2 /NFC 20% PFW/NFC
0.20 0.16 0.12 0.08 0.04 235.2
313.6
392.0
Wear rate /10 -14m3(N.m)-1
*
0.24
Friction coefficient µ
20%PFW filled Nomex fabric composites decrease with the increase of load. However the wear rate of 4%nano-SiO2 filled-Nomex fabric composites increase gradually with increasing load and the wear rate of 20%PFW filled Nomex fabric composites do not show obviously change with increasing load. The Nomex fabric composites filled with 20% PFW exhibited the lowest friction coefficient and wear rate, followed by 4%nano-SiO2 under the identical wear condition. Unfilled and 20%PFW filled-Nomex fabric composites are selected as example to investigate the effect of the environmental temperature on the friction and wear properties of composites. Fig. 5 shows the effect of the environmental temperature on the friction coefficient and wears rate of the unfilled and 20%PFW filled-Nomex fabric composites at 274.4 N. It can be obviously seen that the friction coefficient and wear rate of the 20%PFW-filled Nomex fabric composites is much lower than that of the unfilled Nomex fabric composites under different temperature, which indicate that PFW as filler can improve the wear-resistance, friction-reducing and thermal stability of the composites. The friction coefficient of the unfilled and 20%PFW-filled Nomex fabric composites decreased initially with the increasing temperature up to 180 C, and then rapidly increased from 180 to 240 C. However, the wear rate of
18 16
* Unfilled NFC 4%nano-SiO2 /NFC 20%PFW/NFC
4 3 2 1 0 235.2
470.4
313.6
Apllied load /N
392.0
470.4
548.8
Apllied load /N
Fig. 4. Variation in friction coefficient and wear rate of unfilled and filled Nomex fabric composites with load at room temperature. (The data marked with * represents the composites cannot slide for 120 min under this condition.)
unfilled NFC 20% PFW/NFC
Friction coefficient
0.16 0.14
*
0.12 0.10 0.08 0.06
*
0.04 0
60
120
180
240
Environmental temperature /°C
Wear rate /10-14m3(N.m)-1
0.18
30 25 20 15
*
Unfilled NFC 20% PFW/NFC
*
4 3 2 1 0 0
60
120
180
240
Environmental temperature /°C
Fig. 5. Effect of the environmental temperature on the friction coefficient and wear rate of unfilled and 20%PFW-filled NFC at 274.4 N. (The data marked with * represent that the composites cannot slide for 120 min under this condition.)
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the unfilled and 20%PFW-filled composites increased gradually with the increase of the environmental temperature up to 180 C, and then rapidly increased from 180 to 240 C. The rapidly increase of the friction coefficient and wear rate from 180 to 240 C are owing to the catastrophic destruction of the composites through wearing process at evaluated high temperature [12,13]. 4. Discussion 4.1. Tensile and adhesion strength Tensile strength indirectly represents the bonding strength between the adhesive, fabric-fiber and filler particles, which directly affect the structural integrity of the fabric composites and finally affect the friction and wear properties of the composites. And the adhesion strength between the Nomex fabric composite, adhesive resin and the AISI-1045 steel directly influence on the friction and wear properties of fabric composites because many fabric composites with the low adhesion strength might be peeled off from the steel disc through the wearing process. Table 4 shows the tensile and adhesion strength of the unfilled and filled Nomex fabric composites. Table 4 Adhesion and tensile strength of the unfilled and filled Nomex fabric composites Composite
Mechanical properties (MPa) Tensile strength
Adhesion strength
Unfilled NFC 20%PFW/NFC 40%PFW/NFC 4%nano-SiO2/NFC 8%nano-SiO2/NFC
41.1 69.6 57.0 65.1 54.7
10.3 15.9 13.2 13.7 11.1
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It is seen that different contents of PFW and nano-SiO2, as the fillers, can enhance the tensile and adhesion strength of the composites. And the Nomex fabric composites filled with 20% PFW can obtain the largest adhesion and tensile strength, which correspond to the best anti-wear and friction-reducing abilities. Our previous studies on the structure of PFW with IR have found that some active groups such as mC@O and mO–H generated during the curing of the PFW in fabric composites at 180 C [12]. Because these active groups generated, the proper content of PFW as filler reinforcing Nomex fabric composites help to increase the bonding strength among Nomex fabric and the adhesive resin. However, when the composites were filled with much excessive content of the PFW, the particles tended to conglomerate. The agglomeration of excessive content of PFW would affect the structure of the Nomex fabric composites, as a result the tensile and adhesion strength of the Nomex fabric composites filled with 40% PFW is obvious lower than the 20%PFW-filled Nomex fabric composites (see Fig. 6). Moreover, the Nomex fabric composites filled with 4%nano-SiO2 also shows the proper enhancement of tensile and adhesion strength. This corresponds to the better wearresistance of the filled Nomex fabric composites than that of the unfilled one. It is supposed that the enhanced adhesion and tensile strength is related to the large surface energy and activity of nano-particles, which contribute to increase the interfacial boding among the fillers, the Nomex fabric matrix and the adhesive resin, and accordingly improve the anti-wear abilities of the Nomex fabric composites. However, when the nano-SiO2 particulates were incorporated in the fabric composite at a much excessive content, the nano-particulates would also tend to conglomerate, and subsequently the corresponding structural integrity of the composites would be affected (see Fig. 6). Because of the destroyed structure of Nomex fabric
Fig. 6. SEM pictures of the cross section of: (a) unfilled NFC, (b) 20%PFW/NFC, (c) 40%PFW/NFC, (d) 4%nano-SiO2/NFC and (e) 8%Nano-SiO2/ NFC.
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composites filled with excessive nano-SiO2, the composites would have poor friction-reducing and anti-wear properties (see Table 3). 4.2. SEM analysis of the structure In order to verify the difference of the tribological properties, the adhesion strength and the tensile strength of the unfilled and different particles filled-Nomex fabric composites. Fig. 6 shows the SEM micrograph of the cross-section of these composites, which directly reflect structural integrity of the composites, namely, the interfacial bonding strength between the Nomex fabric, the adhesive resin doped with filler particles. It is obviously observed that some holes and cracks exist in the pure Nomex fabric composites, and the Nomex fiber cannot compactly bond with the adhesive (see Fig. 6(a)), which correspond to the poor tribological and mechanical properties of the unfilled composites. However, the Nomex fabrics bond well with the adhesive resin doped with 20% PFW (see Fig. 6(b)) and 4%nano-SiO2 (see Fig. 6(c)) respectively. Surprisingly, the interface between Nomex fabric and adhesive resin of the 20%PFW filled-Nomex fabric composites become blurry (see Fig. 6(b)), which indicate that the proper content PFW does contribute to the increase of bonding strength between the Nomex fabric and the adhesive resin. This could be rational to understand why 20%PFW-filled Nomex fabric composites obtain the best tribological properties, largest adhesion and tensile strength
as compared with the unfilled and nano-SiO2 filled-Nomex fabric composites. But when the content of PFW and nanoSiO2 fillers are 40% and 8%, respectively, some holes and cracks are obviously observed, and the Nomex fiber cannot compactly compact with the adhesive doped with excessive filler (see Figs. 6(d) and (e)), particularly doped with the 8%nano-SiO2. As a result, the structure of the composites filled with excessive PFW and nano-SiO2 would deteriorate, and then result in the lower adhesion and tensile strength and hence the poor tribological performance. 4.3. SEM analysis of the worn surfaces Fig. 7 shows the SEM pictures of the worn surfaces of the unfilled, 4%nano-SiO2 and 20%PFW filled Nomex fabric composites at 313.6 N and room temperature. It is seen that most Nomex fibers are pulled out and cut from the composites matrix on the worn surface of unfilled Nomex fabric composites after sliding for 40 min (see Fig. 7(a)), which indicate that the unfilled Nomex fabric composites experienced severe peeling off as it slide against the steel. However, the worn surface of 4%nano-SiO2 filled Nomex fabric composites are characterized with a little pulledout and cut fibers (see Fig. 7(b)), which agree well with the friction and wear behaviors of the composites filled with 4%nano-SiO2. Contrary to the above, the worn surface of the 20%PFW-filled Nomex fabric composites is very smooth and the pulling-out and exposure of the Nomex fiber are nearly invisible (see Fig. 7(c)), which indicate that
Fig. 7. SEM pictures of the worn surface of: (a) Unfilled NFC at 40 min; (b) 4%nano-SiO2/NFC at 120 min; and (c) 20%PFW/NFC at 120 min (313.6 N and room temperature).
Fig. 8. SEM morphologies of the worn steel surfaces sliding against: (a) Unfilled NFC; (b) 4%nano-SiO2/NFC; and (c) 20%PFW/NFC (274.4 N, room temperature, 2 h).
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the PFW particles effectively incorporated in the Nomex fabric composites and improved the friction and wear properties of the Nomex fabric composites. The SEM morphologies of the worn surfaces of the counterpart pin sliding against the unfilled, 4%nanoSiO2 and 20%PFW filled Nomex fabric composites at 274.4 N and room temperature are shown in Fig. 8, respectively. We can find that the transfer film on the pin against the unfilled Nomex fabric composites is characterized with thick, rough and discontinuous film, and the transfer film was easy scaled off during wear process (see Fig. 8(a)), which corresponds to the poor mechanical strength and the poor wear-resistance of the unfilled Nomex fabric composites. However the transfer film on the counterpart pin surface sliding 4%nano-SiO2 filled Nomex fabric composites is thin and uniform but show some signs of scuffing (see Fig. 8(b)), which agree well with the increased anti-wear abilities and maximal loading ability of 4%nano-SiO2 filled Nomex fabric composites as compared with the unfilled one. Contrary to the above, the transfer film on the counterpart pin sliding against 20%PFW filled-Nomex fabric composites is much smooth and uniform and showed no signs of scuffing (see Fig. 8(c)), which conform to the best mechanical and tribological properties and the largest adhesion and tensile strength of this composites. 5. Summary The incorporation of PFW with different contents in the Nomex fabric composites can significantly enhance the anti-wear and friction-reducing abilities and increase the adhesion and tensile strength of the composites. Nano-SiO2 at optimum content can enhance the anti-wear abilities, the maximal loading abilities and increase the adhesion and tensile strength of the Nomex fabric composites. The Nomex fabric composites filled with 20%PFW exhibits the lowest friction coefficient and wear rate, largest adhesion and tensile strength and largest maximal loading abilities. PFW, as a filler with melting point 112 C, can homogeneously disperse in Nomex fabric composites because some active groups generate during the curing of the PFW-filled Nomex fabric composites at 180 C. These active groups contribute to the increase of the bonding strength between the Nomex fabric and the adhesive, and hence to increase the adhesion strength and tensile strength and finally improve the tribological properties. At the same time, the self-lubricity of PFW can significantly reduce the friction and shear force, and also contribute to the increase of friction-reducing and anti-wear abilities of PFW-filled Nomex fabric composites. Moreover, Nomex fabric composites filled with a proper content of nano-SiO2 exhibit larger adhesion and tensile strength and better anti-wear abilities than that of the unfilled Nomex fabric composites owing to specific properties of nano-SiO2 such as high surface activity and
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energy and small size effect. The difference in the characters of the transfer film on the surfaces of counterpart pin sliding against unfilled and different particles filled-Nomex fabric composites also account for the differences in the friction and wear behaviors of the unfilled, 4% of nanoSiO2 and 20% of PFW filled-Nomex fabric composites. In a word, the character of the transfer film and the difference in composites structure, coupled with the difference in the properties of the particles themselves, account for the difference in the wear resistance and friction-reduction of Nomex fabric composites filled with PFW and nano-SiO2. Acknowledgements We thank Prof. Jiazheng Zhao for carrying out SEM analysis. We acknowledged financial support from the National Nature Science Foundation of China (50421502). References [1] Friedrich K, Zhang Z, Schlarb AK. Effects of various fillers on the sliding wear of polymer composites. Compos Sci Technol 2005;65(15):2329–43. [2] Jia JH, Chen JM, Zhou HD, Hu LT, Chen L. Comparative investigation on the wear and transfer behaviors of carbon fiber reinforced polymer composites under dry sliding and water lubrication. Compos Sci Technol 2005;65(7):1139–47. [3] Zhang H, Zhang Z. Comparison of short carbon fibre surface treatments on epoxy composites: II. Enhancement of the wear resistance. Compos Sci Technol 2004;64(13):2031–8. [4] Luo RY. Fabrication of carbon/carbon composites by electrified perform beating CVI method. Carbon 2002;40(11):2693–701. [5] Huang BZ, Hu XZh, Liu J. Modelling of inter-laminar toughening from chopped Kevlar fibers. Compos Sci Technol 2004;64(13): 2165–75. [6] Park BY, Kim SC. A study of the interlaminar fracture toughness of a carbon-fiber/epoxy composite containing surface-modified short kevlar fibers. Compos Sci Technol 1998;58(3):389–400. [7] Pihtili H, Tosum NH. Effect of load and speed on the wear behaviour of woven glass cloth and aramid fiber-reinforced composites. Wear 2002;252(11):979–84. [8] Lancaster JK, Bay RG. Selecting and use of wear tests for coatings. ASTM, STP 1982;769:92–9. [9] Blanchet TA, Kennedy FE. Sliding wear mechanism of polytetrafluoroethylene (PTFE) and PTFE composites. Wear 1992;153(1):229–43. [10] Craig WD. PTFE bearings. Lubricat Eng 1964;20(12):456–62. [11] Su FH, Zhang ZZ, Wang K, Jiang W, Liu WM. Tribological and mechanical properties of the composites made of carbon fabrics modified with various methods. Compos Part A 2005;36(12): 1601–17. [12] Zhang ZZ, Su FH, Wang K, Jiang W, Men XH, Liu WM. Study on the friction and wear properties of carbon fabric composites reinforced with micro- and nano-particles. Mater Sci Eng A 2005;404(1):251–8. [13] Su FH, Zhang ZZ, Liu WM. Study on the friction and wear properties of glass fabric composites filled with nano- and microparticles under different conditions. Mater Sci Eng A 2005;392(1): 359–65. [14] Ren ZH, Wang QH, Wu ZD. Research on tribological properties of wear-resistant polytetrafluoroethylene fabric. Tribology (In Chinese) 2002;22:193–8.
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