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Experimental investigation on dry frictional behavior of the two self-lubricating composites under heavy loading conditions
Materials Letters 59 (2005) 2352 – 2356 www.elsevier.com/locate/matlet
Experimental investigation on dry frictional behavior of the two self-lubricating composites under heavy loading conditions Dinghan Xiang*, Zhengjun Yao, Jianping Wen College of Materials science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People’s Republic of China Received 4 December 2004; accepted 8 February 2005 Available online 1 April 2005
Abstract Two novel types of self-lubricating composites such as polytetrafluoroethylene (PTFE) composite and metal – plastic transverse section (SBP) composite were developed. The friction behavior of these composites, reciprocating sliding against stainless steel track in dry friction under heavy loading conditions, was investigated. Experimental results showed that the two self-lubricating composites exhibited low and stable coefficient of dry sliding friction of about 0.05. The time dependent of static friction of PTFE composite was strong, while that of SBP composite weak. Because the actual contact area of PTFE composite great increases with rest time, while that of SBP composite little increases resulting from supporting of thick metal backing under heavy loading conditions. This work is believed to be helpful for understanding of the time-dependence of dry sliding friction between different self-lubricating composites against stainless steel track under heavy loading conditions. D 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Wear; Polytetrafluoroethylene; Time dependence; Friction behavior
1. Introduction The present research deals with dry frictional behaviors between the two novel self-lubricating composites used as sliders of the sluice gate and stainless steel track. The sluice gate is a large steel plate, and sliders of supporting the gates slides on stainless steel track in the sides of the dam. Usually the lifting-machine drives the sluice gate up or down. Raising a sluice gate allows water to flow under it. If no flood, the sluice gates are fully lowered. On the other hand, in the period of flood, if the sluice gate cannot be lifted due to overload of its lifting-machine, water sometimes spills over the top of dam, consequently catastrophe of destroying dam will perhaps take place. Generally, the overload of the lifting-machine results in an increase in friction between the sliders of sluice gate and stainless steel tracks of the dam with rest time. Without flood, the steel
* Corresponding author. Tel.: +86 25 84892952; fax: +86 25 84892951. E-mail address: [email protected] (D. Xiang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.078
gate will not move in one year or several years. Therefore, the time dependent of static friction of dry interfaces occurs under the intermittent operating conditions, results in the lifting-machine having difficulty to lift sluice gate. It is worth emphasizing that the sliders are installed on the back of sluice gates, so relative motion between sliders and stainless steel track remains dry sliding. Since Coulomb (1785) reported that after four days of rest time, the coefficient of static friction for an oak slider on an iron bed grew by a factor of about 2.4 [1], the experimental investigations of the time dependent of static friction of dry surfaces were performed under different operating conditions, and the research workers had presented the different theoretical explanations [1– 3]. The data show that coefficient of static friction l s approach to a maximum value after some time (Kragelskii, 1965) [1]. Recently, A. Panait et al. [2] observed the time-dependent dry frictional behavior of glass and aluminium, after 1 h contact, the coefficient of static friction in dry contact between glass and aluminium under a given condition increases about 50%. After 10 h of constant normal load, it
D. Xiang et al. / Materials Letters 59 (2005) 2352 – 2356
represents an increase of 65% with respect to l 0, and discussed the time-dependent dry friction between brittle material and metal. The objective of this work is to solve some safety problems resulted from raising friction of commonly used self-lubricating sliders of supporting sluice gates. This work is believed to be helpful for understanding of the timedependence of dry friction between different self-lubricating composites against stainless steel under heavy loading conditions.
2. Experimental details 2.1. Materials The two self-lubricating composites were developed in the experiment, such as PTFE composite and metal – plastic transverse section (SBP) composite. The PTFE powders with a grit size of about 50 Am, tin (Sn) powder of 55 Am, graphite powder of 35 Am, nano-Al2O3 with a diameter of about 75 nm, ball of bronze with a diameter of about 0.8 mm, and steel backing with a thickness of 9.1 mm, cut from AISI 1045 steel plate. The PTFE powders, tin powders, nano-Al2O3 powders and graphite powders were fully mixed ultrasonically, PTFE composite (PTFE + 15 wt.% nano-Al2O3 + 10 wt.% Sn + 5 wt.% graphite) was prepared by compression molding, in which the mixture was heated at a rate of 10 -C/min to 380 -C, held 2 h, then cooled to room temperature with the sintering apparatus. The SBP composites was composed of a steel backing, a sintered porous bronze middle layer and a surface layer of PTFE composite filled with 15 wt.% nano-Al2O3 and 5 wt.% graphite. Firstly, a middle layer of porous bronze (about 0.8 mm) on a steel plate was sintered at 800 -C for 2.5 h, then a surface layer at the about 0.2 mm of PTFE, was heated to 380 -C for 1.0 h. 2.2. Measurements of load-carrying capacity In this work, the contact of the pair between cylindrical surface of stainless steel track and flat slider is a linear contact. Therefore, a parameter q is now introduced such that the load-carrying capacity ( q kN/m) is defined as the normal load per length under the linear contact condition.
Fig. 1. Schematic representation of the part related to loading of the apparatus.
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Fig. 2. Schematic representation of the part related to loading of the apparatus.
Measuring load-carrying capacity of the two selflubricating composites was conducted on the 10 mm thick specimens as GB7314-87 Materials—Compression Testing Standards. The specimens of the two composites were 80.0 mm in length, 40.0 mm in width and 10.0 mm in thickness. The cylindrical surface of stainless steel track was 300 mm in diameter. Schematic representation of the part related to loading of the apparatus is shown in Fig. 1. 2.3. Simulation friction tests The static friction tests were carried out in laboratory air using a computer controlled reciprocating sliding tribometer. The coefficient of friction force was measured with the aid of a linear variable strain gauges and was recorded automatically throughout the tests connected to a PC Fig. 2. A 1Cr18Ni9Ti stainless steel (C < 0.12 wt.%, Cr 16 –20 wt.%, Ni 8 –11 wt.%, Ti about 0.8 wt.%) bar, 300 mm in diameter was used as a track material. The cylindrical surface of stainless steel track was 300 mm in diameter and 400 mm in length and 40 mm in maximum value of thickness as shown in Fig. 2. Surface roughness R a of cylindrical surface of stainless steel track is 0.1 Am. The specimens of the two composites were 80.0 mm in length, 40.0 mm in width and 10.0 mm in thickness. Surface roughness R a of the specimens is 0.2 Am. Sliding friction tests were conducted at room temperature in ambient atmosphere (65 T 2% RH ) under dry friction conditions, with a sliding speed of 2.5 m/min in agreement with the actual operating conditions of sluice gates and loads from 6 to 48 kN/cm. The reciprocating sliding distance is 80 mm. Then the specimens and stainless steel track were cleaned with acetone followed by drying. At the end each test, the specimens and stainless steel track were cleaned with acetone followed by drying. At the start of sliding, during the so-called run-in period, the two composites exhibited lower coefficient of friction, followed by steady-state sliding motion until coefficient of friction remained constant. In our work, coefficient of friction was reported during steady-state sliding period. Two replicate friction tests were performed in order to enhance accuracy of data, and the average values of two replicate test results were reported. The
D. Xiang et al. / Materials Letters 59 (2005) 2352 – 2356
variation in the data of the replicate friction test was within 10% of the average value. The time-dependent experiments of static friction were performed under the same loadcarrying capacity of 30 kN/cm and a sliding speed of 2.5 m/min conditions, and the rest time were 0, 0.5, 2.0, 4.0, 12.0, 24.0 and 48.0 h, respectively.
3. Results and discussion
coefficient of static friction
2354
0.12
SBP
0.09
PTFE 0.06 0.03 0 6
12
18 24
30 36 42
48
load-carrying capacity (kN/cm)
3.1. Load-carrying capacity Fig. 4. The variation of l s with q of the two composites.
Fig. 3 shows the comparative curves of load-carrying capacity ( q) between different composites and cylindrical stainless steel track. It can be seen that the inflexions of compression curves, such as A and B, were regarded as yield points, as well as the maximum value of load-carrying capacity ( q m) of two composites. The q m of PTFE composite was 30.5 kN/cm. The SBP composite exhibited heavier q m of 51.5 kN/cm. A considerable improvement in q m of SBP composite was observed. Pure PTFE commonly reveals super low friction and good corrosion resistance, but poor q. The PTFE composite exhibited high q due to the presence of nano-Al2O3 and tin. Because nano-Al2O3 exhibits high strength, which can be used to promote mechanical properties such as q, consequently to improve wear resistance [3– 11]. In addition, tin was also helpful for an increase of q. SBP composite exhibits heavier q resulting from thin surface of PTFE composite and thick metal backing. Metals such as plain carbon steel and bronze show much higher strength than engineering plastics. Developing novel SBP composite aims to combine high strength of metals with low friction of plastics, that is, to remain low friction of plastics and enhance q of novel self-lubricating composite. Metal backing plays a significant role in improving q of composite.
3.2. Friction behavior 3.2.1. Static friction The variation of l s with load-carrying capacity ( q) of the two composites is shown in Fig. 4. It is observed that coefficient of static friction (l s) for two composites tested varied in the range of 0.04 < l s < 0.10, l s decreased with q, then almost remained constant. Under same q conditions, PTFE and SBP composites exhibited low l s less than 0.1. The measurement of l s for PTFE composite was only until the q of 30 kN/cm due to q m of 30.5 kN/cm. SBP composite revealed heavier q resulting from effect of metal backing. When q of SBP composite was greater than 18 kN/cm, l s almost remained constant at 0.048. It is also a well-accepted fact that PTFE composite has a high ability to form the transfer film on the steel surface. Thus the two composites exhibited much low coefficient due to PTFE transfer films on the mating surface. The curves between l and sliding time for the two composites are seen in Fig. 5. It has been found experimentally that the curves of friction coefficient show the maximum points such as A and B. According to the classical theory of friction, initiating maximum points such
0.06 SBP
70 60 50
Coefficient of friction
Load-carrying capacity (kN/cm)
80
A
40
B
30
PTFE
20
A
0.04
B
SBP
PTFE 0.03 0.02 0.01
10 0 0.0
0.05
0.4
0.8
1.2
1.6
Deformation (mm) Fig. 3. The compressive curves of load-carrying capacity of various composites.
0.00 0.0
0.2
0.4
0.6
0.8
1.0
Sliding time (s) Fig. 5. The curves between friction coefficient of the two composites and sliding time.
D. Xiang et al. / Materials Letters 59 (2005) 2352 – 2356
0.09 SBP
0.08
PTFE
µs
0.07 0.06 0.05 0.04 0.03 0
2
4 8 12 Rest time (h)
24
48
Fig. 6. Time-dependent of static friction for the two composites with rest time.
as A and B should be generally regarded as l s of the two composites. However, PTFE composite is a visco –elastic material, after sliding about 0.5 s, l is greater than classical l s as much as 10 –20%. Perhaps, the phenomenon is responsible for creep of PTFE composite at high loads. Indeed, it needs discussion whether there exists static friction for visco – elastic materials or not. In our work, l s of PTFE composite reported were the maximum values after sliding about 0.5 s. Jaydeep Khekar et al. [9] reported that the wear in the composites depends mainly on three factor: thermal stability, thermal conductivity, and the characteristics of the filler materials. In our work, the addition of Sn powder for PTFE composite was in order to dissipate frictional heating at dry sliding under heavy loads conditions, and improving load-carrying capacity. SBP composite consisting of thick metal backing and relatively thin plastics surface layer, exhibited excellent thermal stability and thermal conductivity under heavy loading conditions. It is also observed that the two self-lubricating composites decreased in coefficient of friction with load-carrying capacity (see Fig. 5) in agreement with classic work of Moore [12]. 3.2.2. Time-dependence of static friction The experimental results of time-dependent static friction for the two composites with rest time are shown in Fig. 6. This was in agreement with the experimental results of Bhushan [1], coefficient of static friction l s approach to a maximum value after some time. It can be seen that, for PTFE composite after 2 h of contact, the frictional coefficient’s increase with respect to friction for zero rest time l 0 is on the order of 33%, the maximum value of l s of 0.070 was recorded after 12 h of rest time under the load-carrying load of 30 kN/cm condition, it revealed an increase of about 75%. However, under the same conditions as PTFE composite, SBP exhibited the maximum value of l s of 0.061 after 2 h of rest time, l s only increased in about 15% with respect to l 0. Comparing to the experimental results of the two composites shows that SBP composite exhibits weaker time-dependence of static friction. Thus the friction stability of SBP composite is excellent.
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Bhushan [1] summarized that the time dependent of static friction of dry surfaces is believed to increase because of plastic flow and creep of interface materials and the degree of interaction of the atoms on the mating surfaces under load [1]. The authors reported in an earlier study (1999) that the time-dependence of static friction between visco –elastic materials against a mating metal was strong, whereas that of metal to metal or brittle material to metal was weak. A theoretical explanation was given by using the classical theory of adhesion [3]. According to Bowden and Tabor theory, friction force is proportional to the product of real contact area and effective shear strength. The real area of contact increases with rest time. For soft materials such as polymers, the real area of contact is effectively equal to the apparent area of contact [1]. PTFE composites against a metal cylinder under dry sliding conditions exhibited strong time-dependent of static friction due to the creep of PTFE composite insulting in significantly great real area of contact. Because of thick metal backing, the creep of SBP composite was restrained resulting in little increase in real area of contact. This is major cause that SBP composite revealed weak time-dependent of static friction.
4. Conclusions SBP composite is suitable as bearing material at dry sliding under heavy loading conditions as it resists significantly heavy contact pressure and exhibits low and stable coefficient of friction, outstanding thermal conductivity. PTFE composite is suitable as a bearing material at dry sliding under medium loading conditions as it resists only medium contact pressure and exhibits low coefficient of friction. The time dependent of static friction PTFE composite exhibited strong, while that of SBP composite weak, due to heavy loads caused a great increase in actual contact area of PTFE composite, a little increase in actual contact area of SBP composite.
Acknowledgement The authors would like to acknowledge the financial support of the Aeronautics Science Foundation of China (No. 04G52044).
References [1] Bharat Bhushan, Principles and Applications of Tribology, Wiley, New York, 1999, pp. 344 – 382. [2] A. Panait, Q.-C. He, R. Ami Saada, B. Bary, Wear 257 (2004) 271 – 278. [3] D.H. Xiang, G.M. Li, Tribology 19 (1999) 275 – 277 (in Chinese). [4] M. Kurokawa, Y. Uchiyama, J. Tribol. 125 (2003) 661 – 669. [5] X.H. Cheng, Y. Xue, C.Y. Xie, Mater. Lett. 57 (2003) 2553 – 2557.
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[6] L. Zsidai, P. De Baets, P. Samyn, G. Kalacska, Wear 253 (2002) 673 – 688. [7] M. Palabiyik, S. Bahadur, Wear 253 (2002) 369 – 376. [8] Q.H. Wang, Q.J. Xue, W.M. Liu, J.M. Chen, Wear 243 (2000) 140 – 146.
[9] [10] [11] [12]
J. Khekar, I. Negulescu, E.I. Meletis, Wear 252 (2002) 361 – 369. F. Li, K.A. Hu, J.L. Li, B.Y. Zhao, Wear 249 (2002) 877 – 882. H. Unal, U. Sen, A. Mimaroglu, Tribol. Int. 37 (2004) 727 – 737. D.F. Moore, Principles and Applications of Tribology, Pergamon, Oxford, 1975, pp. 39 – 73.
Experimental investigation on dry frictional behavior of the two self-lubricating composites under heavy loading conditions Dinghan Xiang*, Zhengjun Yao, Jianping Wen College of Materials science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People’s Republic of China Received 4 December 2004; accepted 8 February 2005 Available online 1 April 2005
Abstract Two novel types of self-lubricating composites such as polytetrafluoroethylene (PTFE) composite and metal – plastic transverse section (SBP) composite were developed. The friction behavior of these composites, reciprocating sliding against stainless steel track in dry friction under heavy loading conditions, was investigated. Experimental results showed that the two self-lubricating composites exhibited low and stable coefficient of dry sliding friction of about 0.05. The time dependent of static friction of PTFE composite was strong, while that of SBP composite weak. Because the actual contact area of PTFE composite great increases with rest time, while that of SBP composite little increases resulting from supporting of thick metal backing under heavy loading conditions. This work is believed to be helpful for understanding of the time-dependence of dry sliding friction between different self-lubricating composites against stainless steel track under heavy loading conditions. D 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Wear; Polytetrafluoroethylene; Time dependence; Friction behavior
1. Introduction The present research deals with dry frictional behaviors between the two novel self-lubricating composites used as sliders of the sluice gate and stainless steel track. The sluice gate is a large steel plate, and sliders of supporting the gates slides on stainless steel track in the sides of the dam. Usually the lifting-machine drives the sluice gate up or down. Raising a sluice gate allows water to flow under it. If no flood, the sluice gates are fully lowered. On the other hand, in the period of flood, if the sluice gate cannot be lifted due to overload of its lifting-machine, water sometimes spills over the top of dam, consequently catastrophe of destroying dam will perhaps take place. Generally, the overload of the lifting-machine results in an increase in friction between the sliders of sluice gate and stainless steel tracks of the dam with rest time. Without flood, the steel
* Corresponding author. Tel.: +86 25 84892952; fax: +86 25 84892951. E-mail address: [email protected] (D. Xiang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.078
gate will not move in one year or several years. Therefore, the time dependent of static friction of dry interfaces occurs under the intermittent operating conditions, results in the lifting-machine having difficulty to lift sluice gate. It is worth emphasizing that the sliders are installed on the back of sluice gates, so relative motion between sliders and stainless steel track remains dry sliding. Since Coulomb (1785) reported that after four days of rest time, the coefficient of static friction for an oak slider on an iron bed grew by a factor of about 2.4 [1], the experimental investigations of the time dependent of static friction of dry surfaces were performed under different operating conditions, and the research workers had presented the different theoretical explanations [1– 3]. The data show that coefficient of static friction l s approach to a maximum value after some time (Kragelskii, 1965) [1]. Recently, A. Panait et al. [2] observed the time-dependent dry frictional behavior of glass and aluminium, after 1 h contact, the coefficient of static friction in dry contact between glass and aluminium under a given condition increases about 50%. After 10 h of constant normal load, it
D. Xiang et al. / Materials Letters 59 (2005) 2352 – 2356
represents an increase of 65% with respect to l 0, and discussed the time-dependent dry friction between brittle material and metal. The objective of this work is to solve some safety problems resulted from raising friction of commonly used self-lubricating sliders of supporting sluice gates. This work is believed to be helpful for understanding of the timedependence of dry friction between different self-lubricating composites against stainless steel under heavy loading conditions.
2. Experimental details 2.1. Materials The two self-lubricating composites were developed in the experiment, such as PTFE composite and metal – plastic transverse section (SBP) composite. The PTFE powders with a grit size of about 50 Am, tin (Sn) powder of 55 Am, graphite powder of 35 Am, nano-Al2O3 with a diameter of about 75 nm, ball of bronze with a diameter of about 0.8 mm, and steel backing with a thickness of 9.1 mm, cut from AISI 1045 steel plate. The PTFE powders, tin powders, nano-Al2O3 powders and graphite powders were fully mixed ultrasonically, PTFE composite (PTFE + 15 wt.% nano-Al2O3 + 10 wt.% Sn + 5 wt.% graphite) was prepared by compression molding, in which the mixture was heated at a rate of 10 -C/min to 380 -C, held 2 h, then cooled to room temperature with the sintering apparatus. The SBP composites was composed of a steel backing, a sintered porous bronze middle layer and a surface layer of PTFE composite filled with 15 wt.% nano-Al2O3 and 5 wt.% graphite. Firstly, a middle layer of porous bronze (about 0.8 mm) on a steel plate was sintered at 800 -C for 2.5 h, then a surface layer at the about 0.2 mm of PTFE, was heated to 380 -C for 1.0 h. 2.2. Measurements of load-carrying capacity In this work, the contact of the pair between cylindrical surface of stainless steel track and flat slider is a linear contact. Therefore, a parameter q is now introduced such that the load-carrying capacity ( q kN/m) is defined as the normal load per length under the linear contact condition.
Fig. 1. Schematic representation of the part related to loading of the apparatus.
2353
Fig. 2. Schematic representation of the part related to loading of the apparatus.
Measuring load-carrying capacity of the two selflubricating composites was conducted on the 10 mm thick specimens as GB7314-87 Materials—Compression Testing Standards. The specimens of the two composites were 80.0 mm in length, 40.0 mm in width and 10.0 mm in thickness. The cylindrical surface of stainless steel track was 300 mm in diameter. Schematic representation of the part related to loading of the apparatus is shown in Fig. 1. 2.3. Simulation friction tests The static friction tests were carried out in laboratory air using a computer controlled reciprocating sliding tribometer. The coefficient of friction force was measured with the aid of a linear variable strain gauges and was recorded automatically throughout the tests connected to a PC Fig. 2. A 1Cr18Ni9Ti stainless steel (C < 0.12 wt.%, Cr 16 –20 wt.%, Ni 8 –11 wt.%, Ti about 0.8 wt.%) bar, 300 mm in diameter was used as a track material. The cylindrical surface of stainless steel track was 300 mm in diameter and 400 mm in length and 40 mm in maximum value of thickness as shown in Fig. 2. Surface roughness R a of cylindrical surface of stainless steel track is 0.1 Am. The specimens of the two composites were 80.0 mm in length, 40.0 mm in width and 10.0 mm in thickness. Surface roughness R a of the specimens is 0.2 Am. Sliding friction tests were conducted at room temperature in ambient atmosphere (65 T 2% RH ) under dry friction conditions, with a sliding speed of 2.5 m/min in agreement with the actual operating conditions of sluice gates and loads from 6 to 48 kN/cm. The reciprocating sliding distance is 80 mm. Then the specimens and stainless steel track were cleaned with acetone followed by drying. At the end each test, the specimens and stainless steel track were cleaned with acetone followed by drying. At the start of sliding, during the so-called run-in period, the two composites exhibited lower coefficient of friction, followed by steady-state sliding motion until coefficient of friction remained constant. In our work, coefficient of friction was reported during steady-state sliding period. Two replicate friction tests were performed in order to enhance accuracy of data, and the average values of two replicate test results were reported. The
D. Xiang et al. / Materials Letters 59 (2005) 2352 – 2356
variation in the data of the replicate friction test was within 10% of the average value. The time-dependent experiments of static friction were performed under the same loadcarrying capacity of 30 kN/cm and a sliding speed of 2.5 m/min conditions, and the rest time were 0, 0.5, 2.0, 4.0, 12.0, 24.0 and 48.0 h, respectively.
3. Results and discussion
coefficient of static friction
2354
0.12
SBP
0.09
PTFE 0.06 0.03 0 6
12
18 24
30 36 42
48
load-carrying capacity (kN/cm)
3.1. Load-carrying capacity Fig. 4. The variation of l s with q of the two composites.
Fig. 3 shows the comparative curves of load-carrying capacity ( q) between different composites and cylindrical stainless steel track. It can be seen that the inflexions of compression curves, such as A and B, were regarded as yield points, as well as the maximum value of load-carrying capacity ( q m) of two composites. The q m of PTFE composite was 30.5 kN/cm. The SBP composite exhibited heavier q m of 51.5 kN/cm. A considerable improvement in q m of SBP composite was observed. Pure PTFE commonly reveals super low friction and good corrosion resistance, but poor q. The PTFE composite exhibited high q due to the presence of nano-Al2O3 and tin. Because nano-Al2O3 exhibits high strength, which can be used to promote mechanical properties such as q, consequently to improve wear resistance [3– 11]. In addition, tin was also helpful for an increase of q. SBP composite exhibits heavier q resulting from thin surface of PTFE composite and thick metal backing. Metals such as plain carbon steel and bronze show much higher strength than engineering plastics. Developing novel SBP composite aims to combine high strength of metals with low friction of plastics, that is, to remain low friction of plastics and enhance q of novel self-lubricating composite. Metal backing plays a significant role in improving q of composite.
3.2. Friction behavior 3.2.1. Static friction The variation of l s with load-carrying capacity ( q) of the two composites is shown in Fig. 4. It is observed that coefficient of static friction (l s) for two composites tested varied in the range of 0.04 < l s < 0.10, l s decreased with q, then almost remained constant. Under same q conditions, PTFE and SBP composites exhibited low l s less than 0.1. The measurement of l s for PTFE composite was only until the q of 30 kN/cm due to q m of 30.5 kN/cm. SBP composite revealed heavier q resulting from effect of metal backing. When q of SBP composite was greater than 18 kN/cm, l s almost remained constant at 0.048. It is also a well-accepted fact that PTFE composite has a high ability to form the transfer film on the steel surface. Thus the two composites exhibited much low coefficient due to PTFE transfer films on the mating surface. The curves between l and sliding time for the two composites are seen in Fig. 5. It has been found experimentally that the curves of friction coefficient show the maximum points such as A and B. According to the classical theory of friction, initiating maximum points such
0.06 SBP
70 60 50
Coefficient of friction
Load-carrying capacity (kN/cm)
80
A
40
B
30
PTFE
20
A
0.04
B
SBP
PTFE 0.03 0.02 0.01
10 0 0.0
0.05
0.4
0.8
1.2
1.6
Deformation (mm) Fig. 3. The compressive curves of load-carrying capacity of various composites.
0.00 0.0
0.2
0.4
0.6
0.8
1.0
Sliding time (s) Fig. 5. The curves between friction coefficient of the two composites and sliding time.
D. Xiang et al. / Materials Letters 59 (2005) 2352 – 2356
0.09 SBP
0.08
PTFE
µs
0.07 0.06 0.05 0.04 0.03 0
2
4 8 12 Rest time (h)
24
48
Fig. 6. Time-dependent of static friction for the two composites with rest time.
as A and B should be generally regarded as l s of the two composites. However, PTFE composite is a visco –elastic material, after sliding about 0.5 s, l is greater than classical l s as much as 10 –20%. Perhaps, the phenomenon is responsible for creep of PTFE composite at high loads. Indeed, it needs discussion whether there exists static friction for visco – elastic materials or not. In our work, l s of PTFE composite reported were the maximum values after sliding about 0.5 s. Jaydeep Khekar et al. [9] reported that the wear in the composites depends mainly on three factor: thermal stability, thermal conductivity, and the characteristics of the filler materials. In our work, the addition of Sn powder for PTFE composite was in order to dissipate frictional heating at dry sliding under heavy loads conditions, and improving load-carrying capacity. SBP composite consisting of thick metal backing and relatively thin plastics surface layer, exhibited excellent thermal stability and thermal conductivity under heavy loading conditions. It is also observed that the two self-lubricating composites decreased in coefficient of friction with load-carrying capacity (see Fig. 5) in agreement with classic work of Moore [12]. 3.2.2. Time-dependence of static friction The experimental results of time-dependent static friction for the two composites with rest time are shown in Fig. 6. This was in agreement with the experimental results of Bhushan [1], coefficient of static friction l s approach to a maximum value after some time. It can be seen that, for PTFE composite after 2 h of contact, the frictional coefficient’s increase with respect to friction for zero rest time l 0 is on the order of 33%, the maximum value of l s of 0.070 was recorded after 12 h of rest time under the load-carrying load of 30 kN/cm condition, it revealed an increase of about 75%. However, under the same conditions as PTFE composite, SBP exhibited the maximum value of l s of 0.061 after 2 h of rest time, l s only increased in about 15% with respect to l 0. Comparing to the experimental results of the two composites shows that SBP composite exhibits weaker time-dependence of static friction. Thus the friction stability of SBP composite is excellent.
2355
Bhushan [1] summarized that the time dependent of static friction of dry surfaces is believed to increase because of plastic flow and creep of interface materials and the degree of interaction of the atoms on the mating surfaces under load [1]. The authors reported in an earlier study (1999) that the time-dependence of static friction between visco –elastic materials against a mating metal was strong, whereas that of metal to metal or brittle material to metal was weak. A theoretical explanation was given by using the classical theory of adhesion [3]. According to Bowden and Tabor theory, friction force is proportional to the product of real contact area and effective shear strength. The real area of contact increases with rest time. For soft materials such as polymers, the real area of contact is effectively equal to the apparent area of contact [1]. PTFE composites against a metal cylinder under dry sliding conditions exhibited strong time-dependent of static friction due to the creep of PTFE composite insulting in significantly great real area of contact. Because of thick metal backing, the creep of SBP composite was restrained resulting in little increase in real area of contact. This is major cause that SBP composite revealed weak time-dependent of static friction.
4. Conclusions SBP composite is suitable as bearing material at dry sliding under heavy loading conditions as it resists significantly heavy contact pressure and exhibits low and stable coefficient of friction, outstanding thermal conductivity. PTFE composite is suitable as a bearing material at dry sliding under medium loading conditions as it resists only medium contact pressure and exhibits low coefficient of friction. The time dependent of static friction PTFE composite exhibited strong, while that of SBP composite weak, due to heavy loads caused a great increase in actual contact area of PTFE composite, a little increase in actual contact area of SBP composite.
Acknowledgement The authors would like to acknowledge the financial support of the Aeronautics Science Foundation of China (No. 04G52044).
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