Sliding friction and wear behaviour of polytetrafluoroethylene and its composites under dry conditions

Sliding friction and wear behaviour of polytetrafluoroethylene and its composites under dry conditions

Materials & Design Materials and Design 25 (2004) 239–245 www.elsevier.com/locate/matdes Technical report Sliding friction and wear behaviour of pol...

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Materials & Design Materials and Design 25 (2004) 239–245 www.elsevier.com/locate/matdes

Technical report

Sliding friction and wear behaviour of polytetrafluoroethylene and its composites under dry conditions H. Unal a, A. Mimaroglu a

b,*

, U. Kadıoglu a, H. Ekiz

a

Faculty of Technical Education, University of Sakarya, Esentepe Kampusu, Adapazari, Turkey b Faculty of Engineering, University of Sakarya, Esentepe Kampusu, Adapazari, Turkey Received 22 April 2003; accepted 6 October 2003

Abstract In this paper, we studied and explored the influence of test speed and load values on the friction and wear behaviour of pure polytetrafluoroethylene (PTFE), glass fibre reinforced (GFR) and bronze and carbon (C) filled PTFE polymers. Friction and wear experiments were run under ambient conditions in a pin-on-disc arrangement. Tests were carried out at sliding speed of 0.32-, 0.64-, 0.96- and 1.28-m s1 and under a nominal load of 5-, 10-, 20- and 30-N. The results showed that, for pure PTFE and its composites used in this investigated, the friction coefficient decrease with the increase in load. The maximum reductions in wear rate and friction coefficient were obtained by reinforced PTFE + 17% glass fiber. The wear rate for pure PTFE was in the order of 107 mm2 /N, while the wear rate values for PTFE composites were in the order of 108 and 109 mm2 /N. Adding glass fiber, bronze and carbon fillers to PTFE were found effective in reducing the wear rate of the PTFE composite. In addition, for the range of load and speeds used in this investigation, the wear rate showed very little sensitivity to test speed and large sensitivity to the applied load, particularly at high load values. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: PTFE; Filled polymer; Friction; Wear; Tribology

1. Introduction Many polymers and polymer based composites are widely used for sliding couples against metals, polymers and other materials. However, where the contact is there, there is the problem of friction and wear. The friction between polymers can be attributed to two main mechanisms, deformation and adhesion. In this case, the deformation mechanism involves complete dissipation of energy in the contact area while the adhesion component is responsible for the friction of polymer and is a result of breaking of weak bonding forces between polymer chains in the bulk of the material [1–3]. In fact, tribologists often classify thermoplastic polymeric materials into three distinct groups according to their friction and wear behaviour. These are: the normal polymers such as low-density polyethylene (LDPE), *

Corresponding author. Tel.: +90-264-3460353x317; fax: +90-2643460351. E-mail address: [email protected] (A. Mimaroglu). 0261-3069/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2003.10.009

polypropylene (PP); the amorphous polymers such as polyvinyl chloride (PVC), polymethylmethacrylate (PMMA); and the Ôsmooth molecular profileÕ polymers such as polytetrafluoroethylene (PTFE) and ultra high molecular weight polyethylene (UHMWPE). Among them, the better frictional performance of the smooth molecular profile polymers can be explained by the easiness with which the long chain molecules shear across each other [4,5]. PTFE is a high performance engineering plastics which is widely used in industry due to its properties of self-lubrication, low friction coefficient, high temperature stability and chemically resistant. In fact, PTFE exhibits poor wear and abrasion resistance, leading to early failure and leakage problem in the machine parts. To minimise this problem, various suitable fillers added to PTFE. Generally, reinforcements such as glass fibres, carbon fibers and solid lubricants are added internally or incorporated into the PTFE. In this field many investigations [6–9] report that the coefficient of friction can, generally, be

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reduced and the wear resistance improved when the polymers are reinforced with glass, carbon and aramid fibres. In fact, the tribological behaviour of polymers is affected by environmental and operating conditions and by the type, size, amount, shape and orientation of the fibres [10]. A relationship between the wear of the polymers and operating parameters is desirable to obtain the better understanding on the wear behaviour. There have been numerous investigations exploring the influence of test conditions, contact geometry and environment on the friction and wear behaviour of polymers. Yamaguchi [11], Hooke et al. [12] and Lawrence and Stolarski [13] report that the friction coefficient can, generally, be reduced and the wear resistance increased by selecting the right material combinations. Santner and Czichos [14], Brentnall and Lancaster [15], Tevruz [16,17] and Clerico [18] and Anderson [19] observed that the friction coefficient of polymers rubbing against metals decreases with the increase in load while Unal and Mimaroglu [20,21], Stuart [22] and Yamaguchi [11] showed that its value increases with the increase in load. Ludema and Tabor [23] showed good correlation between rolling coefficient friction and damping loss factor of polymeric materials in function of testing temperature. Watanabe [24], Tanaka [25] and Bahadur and Tabor [26] has reported that the tribological behaviour of polyamide, HDPE and their composites is affected greatly by normal load, sliding speed and temperature. Moreover, Wang and Li [5] has investigated the sliding wear behaviour of UHMWPE. He reported that the sliding velocity shows greater influence on the wear loss than load. In this paper, we studied and explored the influence of test speed and load values on the friction and wear behaviour of pure PTFE, glass fibre reinforced PTFE, bronze and carbon filled PTFE polymers. Friction and wear tests vs. AISI 440C stainless steel disc were carried out on a pin-on-disc arrangement and at a dry conditions. Tribological tests were at room temperature, under 5-,10-, 20-, 30-N loads and at 0.32-, 0.64-, 0.96- and 1.28-m s1 speeds.

Dead Weight

Pin

Disc

Motor

Fig. 1. Schematic diagram of wear test rig.

cylindrical pin specimens of size 6 mm diameter and 50 mm length were tested against AISI 440C stainless steel disc. Table 1 shows chemical composition of AISI 440C stainless steel disc used in this study. The surface roughness of the disc was 26.1 lm Ra. Fig. 1 represents a schematic diagram of the pin-on-disc wear test rig that was designed and used for this work. As shown in this figure, the rig consists of a stainless steel table which is mounted on a turntable, a variable speed motor which provide the unidirectional motion to the turntable, hence to the disc sample and a pin sample holder which is rigidly attached to a pivoted loading arm. This loading arm is supported in bearing arrangements to allow loads to be applied to the specimen. During the test, friction force was measured by a transducer mounted on the loading arm. The friction force readings were taken as the average of 100 readings every 40 s for a period of a 1-h test time. For this purpose a microprocessor controlled data acquisition system was used. The physical, mechanical properties and the specific test conditions of the experimental samples are given in Table 2. Sliding wear data reported here is the average of at least three runs. The average mass loss was used to calculate the specific wear rate (K0 ) as

2. Experimental details K0 ¼ Dm=L  F  q

ðmm3 N1 mm1 Þ;

2.1. Friction and wear tests Wear tests were carried out on a pin-on-disc wear test rig at room temperature under dry conditions. The

where Dm is average mass loss (g), L is sliding distance (mm), F is the applied load (N) and q is density of the materials (g mm3 ).

Table 1 Chemical composition of AISI 440C stainless steel disc used in this study %C

% Si

% Mn

%P

%S

% Cr

% Mo

% Ni

0.98

1.2

0.8

0.040

0.030

17

0.70

0.80

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Table 2 The physical, mechanical properties and the specific test conditions of the experimental samples Materials

Colour

Density (g cm3 )

Test temperature (°C)

Load (N)

Speed (m s1 )

Humidity (%)

PTFE

White

2.14

21  2

5 10 20 30

0.32 0.64 0.96 1.28

59

PTFE + 17% GFR

White

2.39

18  1

5 10 20 30

0.32 0.64 0.96 1.28

65

PTFE + 25% bronze

Brown

3.03

23  1

5 10 20 30

0.32 0.64 0.96 1.28

58

PTFE + 35% C

Black

2.15

23  1

5 10 20 30

0.32 0.64 0.96 1.28

64

GFR, glass fiber reinforced; C, carbon.

3. Results and discussions Table 3 presents friction coefficient values for PTFE and its composites tested at room temperature, at dry wear conditions and at 5-, 10-, 20- and 30-N loads and at 0.32-, 0.64-, 0.96- and 1.28-m s1 speeds, respectively. Fig. 2 shows the variation of friction coefficients of PTFE and its composites tested at 0.32 m s1 speeds and 5-, 10-, 20- and 30-N loads. For PTFE and its composites, the friction coefficient decrease with the increase in load. It is known that polymers are a visco-elastic materials their deformation under load is viscoelastic.

Therefore, the variation of friction coefficient with load follows the equation l ¼ kN ðn1Þ [27] where l is the coefficient of friction, N is the load, k constant and n is also a constant, its value 2=3 < n < 1. According to this equation, the coefficient of friction decreases with the load increase. But when the load increase to the limit load values of the polymer, the friction and wear will increase due to the critical surface energy of the polymer. Furthermore, this is explained as the frictional heat raised the temperature of the friction surfaces, which lead to relaxation of polymer molecule chains. In the meantime, molecules of polymer surfaces were pressed,

Table 3 Coefficient of friction values for PTFE and its composites tested at different load and speed values Materials

Load (N)

Speed (m s1 ) 1.28

1.28

1.28

1.28

Coefficient of friction, l PTFE

5 10 20 30

0.76 0.38 0.19 0.127

0.775 0.385 0.19 0.127

0.78 0.38 0.19 0.13

0.79 0.395 0.20 0.13

PTFE + 17% GFR

5 10 20 30

0.645 0.32 0.158 0.107

0.65 0.32 0.16 0.10

0.66 0.32 0.16 0.109

0.66 0.33 0.165 0.11

PTFE + 25% bronze

5 10 20 30

0.68 0.34 0.16 0.107

0.69 0.34 0.16 0.11

0.695 0.34 0.17 0.113

0.71 0.35 0.174 0.115

PTFE + 35% C

5 10 20 30

0.69 0.34 0.175 0.11

0.70 0.35 0.175 0.115

0.69 0.34 0.17 0.11

0.705 0.34 0.175 0.11

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Fig. 2. Coefficient of friction of the PTFE and its composites against stainless steel (sliding speed ¼ 0.32 m s1 ).

drawn and sheared. Highly active radicals could react with unbroken chains, giving rise to a series of new chain. Fig. 3 illustrates the variation of friction coefficient of pure PTFE and its composites at different sliding speeds and under 20-N load. In this figure, for all materials there is an average 50% decrease in friction coefficient value for a 100% increase in speed. Table 4 presents the mass loss in PTFE and its composites tested under 5-, 10-, 20- and 30-N load and at 0.32-, 0.64-, 0.96- and 1.28-m s1 speeds. Table 5 shows the specific wear rate values calculated from mass

Fig. 3. Coefficient of friction of the PTFE and its composites against stainless steel (load ¼ 20 N).

loss data of Table 4. Figs. 4–6 illustrate the variation of specific wear rate with test speed and load. It is clear from Fig. 4 that the variation in sliding speed has little influence on wear rate of PTFE and its composites. On the other hand, Figs. 5 and 6 show that apart from pure PTFE the specific wear rate for PTFE composites decrease with the increase in load value. The lowest wear rate is for PTFE + 17% glass fiber with a value of 8  109 mm2 /N. The wear rate for pure PTFE was in the order of 107 mm2 /N, while the wear rate values for PTFE composites were in the order of 108 and 109

Table 4 Mass loss values for PTFE and its composites tested at different load and speed values Materials

Load (N)

Speed (m s1 ) 0.32

0.64

0.96

1.28

Mass loss (g) PTFE

5 10 20 30

0.0092 0.0183 0.0465 0.0560

0.0134 0.0283 0.0470 0.0830

0.0250 0.0468 0.0790 0.1423

0.0380 0.0700 0.1123 0.2020

PTFE + 17% GFR

5 10 20 30

0.0005 0.0008 0.0012 0.0015

0.0007 0.0012 0.0015 0.0016

0.0010 0.0013 0.0014 0.0018

0.0014 0.0017 0.0018 0.0020

PTFE + 25% bronze

5 10 20 30

0.0017 0.0023 0.0025 0.0034

0.0025 0.0030 0.0037 0.0047

0.0046 0.0050 0.0060 0.0066

0.0070 0.0080 0.0085 0.0098

PTFE + 35% C

5 10 20 30

0.0016 0.0021 0.0025 0.0029

0.0024 0.0030 0.0039 0.0045

0.0042 0.0049 0.0057 0.0064

0.0071 0.0077 0.0086 0.0096

GFR, glass fiber reinforced; C, carbon.

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Table 5 Specific wear rates for PTFE and its composites tested at different load and speed values Materials

Load (N)

Speed (m s1 ) 0.32

0.64

0.96

1.28

2

Specific wear rate, K0 (mm /N) PTFE

5 10 20 30

7.46E ) 7 7.42E ) 7 9.43E ) 7 7.57E ) 7

5.43E ) 7 5.73E ) 7 4.76E ) 7 5.61E ) 7

7.57E ) 7 6.32E ) 7 5.34E ) 7 6.41E ) 7

7.70E ) 7 7.09E ) 7 5.69E ) 7 6.82E ) 7

PTFE + 17% GFR

5 10 20 30

3.63E ) 8 2.90E ) 7 2.18E ) 8 1.84E ) 8

2.54E ) 8 2.17E ) 8 1.36E ) 8 9.68E ) 9

2.42E ) 8 1.69E ) 8 8.00E ) 9 7.20E ) 9

2.54E ) 8 1.54E ) 8 8.10E ) 9 6.00E ) 9

PTFE + 25% bronze

5 10 20 30

1.37E ) 7 7.73E ) 8 4.72E ) 8 3.24E ) 8

7.16E ) 8 6.30E ) 8 5.44E ) 8 4.67E ) 8

5.92E ) 8 5.06E ) 8 2.62E ) 8 3.31E ) 8

7.30E ) 8 5.51E ) 8 3.76E ) 8 3.22E ) 8

PTFE + 35% C

5 10 20 30

1.29E ) 7 8.47E ) 8 5.04E ) 8 3.90E ) 8

9.68E ) 8 6.05E ) 8 3.93E ) 8 3.02E ) 8

1.13E ) 7 6.59E ) 8 3.83E ) 8 2.87E ) 8

1.43E ) 7 7.77E ) 8 4.34E ) 8 3.23E ) 8

Load (N) 5N

10N

20N

30N

1,00E-04

K0 (mm2/N)

1,00E-05

Fig. 4. Average wear rate (log scale) of the PTFE and its composites against stainless steel (load ¼ 20 N).

PTFE PTFE+17%GFR PTFE+25%bronze PTFE+35%C

1,00E-06

1,00E-07

1,00E-08

1,00E-09

Fig. 6. Variation of specific wear rate (log scale) with load for PTFE and its composites (sliding speed ¼ 0.96 m s1 ).

Fig. 5. Average wear rate (log scale) of the PTFE and its composites against stainless steel (sliding speed ¼ 0.96 m s1 ).

mm2 /N. The highest wear rates are for PTFE + 25% bronze and PTFE + 35% C with a values of 3  108 followed by PTFE with a value of 5  107 mm2 /N. The

average wear rates for PTFE + 17% GFR, PTFE + 25% bronze and PTFE + 35% C are 98% and 91%, 90% lower than pure PTFE wear rate, respectively. It is well known that wear process involve fracture, tribochemical effects and plastic flow. Transitions between regions dominated by each of these commonly give rise to changes in wear rate with load. Furthermore, this result is closely related to structure characteristics, and chemical effects occurred in frictional processes as well as transfer film formation on the counterface. For all materials tested in this investigation and within the speed range of 0.32– 1.28 m s1 , the speed value have not shown any significant influence on the specific wear rate of the PTFE and its composites.

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Fig. 7. Microscopy of discs worn surfaces of at 5 N load and 0.64 m s1 : (a) PTFE, 200, (b) PTFE + 17% GFR, 200; (c) PTFE + 25% bronze 200 and (d) PTFE + 35% C, 200.

Fig. 7 present the microscopy examination of discs worn surfaces of PTFE and its composites at 5-N load and 0.64 m s1 speed. In the case of pure PTFE and PTFE + 17% glass fiber Figs. 7(a) and (b), it is seen that the PTFE with no filler and with glass fiber reinforcement form a good thin and uniform transfer film. In the case of PTFE with bronze and carbon filler Figs. 7(c) and (d), there appears to be some disruption of transfer film for bronze and carbon which have affected the wear rate performance.

4. Conclusions The following conclusions can be drawn from the present study. 1. Wear studies against AISI 440C stainless steel disc counterface under various loads and sliding speeds, materials used in this study were ranked as follows for their wear performance. PTFE + 17% GFR > PTFE + 25% bronze > PTFE + 35% C > pure PTFE. PTFE + 17% GFR exhibited best wear performance (K0 in the order of 108 mm2 /N) and can be consid-

ered as a very good tribo-material between materials used in this study. 2. The friction coefficient of pure PTFE and its composites decreases when applied load increases. 3. Pure PTFE is characterised by high wear because of its small mechanical properties. Therefore, the reinforcement PTFE with glass fibres improve the load carrying capability that lowers the wear rate of the PTFE. 4. For the specific range of load and speed explored in this study, the load has stronger affect on the wear behaviour of PTFE and its composites than the sliding velocity. References [1] Lancaster JK. Wear 1990;141:159–83. [2] Hutchings IM. Tribology friction and wear of engineering materials. London: Edward Arnold; 1992. p. 51. [3] Tewari US, Sharma SK, Vasudevan P. Rev Macromol Chem Phys C 1989;29(1):1–38. [4] Amuzu JKA, Briscoe BJ, Tabor D. Polymers as bearings and lubricants; aspects and fundamental research: advances in tribology. London: Institute of Mechanical Engineering; 1978. p. 59–62.

H. Unal et al. / Materials and Design 25 (2004) 239–245 [5] Wang YQ, Li J. Sliding wear behaviour and mechanism of ultrahigh molecular weight polyethylene. Mater Sci Eng A 1999;266: 155–60. [6] Bahadur S, Zheng Y. Mechanical and tribological behaviour of polyester reinforced with short glass fibers. Wear 1990;137: 251–66. [7] Sung NH, Suh HP. Effect of fibre orientation on friction and wear of fibre reinforced polymer composites. Wear 1979;53: 129–41. [8] Cirino M, Friedric K, Pipes RB. The effect of fibre orientation on the abrasive wear behaviour of polymer composite materials. Wear 1988;121:127–41. [9] Friedrich K, Lu Z, Mager AM. Recent advances in polymer composites. Tribology 1995;1990:139–44. [10] Kukureka SN, Chen YK, Hooke CJ, Liao P. The wear mechanisms of acetal in unlubricated rolling-sliding contact. Wear 1995;185:1–8. [11] Yamaguchi Y. Tribology of plastic materials: their characteristics and applications to sliding components. Amsterdam: Elsevier; 1990. [12] Hooke CJ, Kukureka SN, Liao P, Rao M, Chen YK. The friction and wear of polymers in non-conformal contacts. Wear 1996;200:83–94. [13] Lawrence CC, Stolarski TA. Rolling contact wear of polymers: a preliminary study. Wear 1989;132:83–91. [14] Santner E, Czichos H. Tribol Int 1989;22(2):103–9. [15] Brentnall AB, Lancaster JK. Proceedings of the Wear of Materials Congress, ASME, 1989, p. 596–603.

245

[16] Tevruz T. Tribological behaviours of bronze-filled polytetrafluoroethylene dry journal bearings. Wear 1999;230:61–9. [17] Tevruz T. Tribological behaviours of carbon-filled polytetrafluoroethylene dry journal bearings. Wear 1998;221:61–8. [18] Clerico M. Wear 1969;13:183–97. [19] Anderson JC. The wear and friction of commercial polymers and composites. In: Friction and wear and polymer composites.Friedrich K, editor. Composite materials series, vol. 1. Amsterdam: Elsevier; 1986. p. 329–62. [20] Unal H, Mimaroglu A. Friction and wear behaviour of unfilled engineering thermoplastics. Mater Design 2003;24: 183–7. [21] Unal H, Mimaroglu A. Influence of test conditions on the tribological properties of polymers. J Ind Lubric Tribol 2003; 55(4):178–83. [22] Stuart BH. Tribological studies of poly(ether ether ketone) blends. Tribol Int 1998;31(11):647–51. [23] Ludema KC, Tabor D. Wear 1966;9:329–48. [24] Watanabe M. The friction and wear properties of nylon. Wear 1968;110:379–88. [25] Tanaka K. Transfer of semicrystalline polymers sliding against smooth steel surface. Wear 1982;27:183. [26] Bahadur S, Tabor D. Role of fillers in the friction and wear behaviour of high-density polyethylene. In: Lee LH, editor. Polymer wear and its control. ACS Symposium Series, vol. 287. Washington DC; 1985. p. 253–68. [27] Stuart BH. Surface plasticisation of poly(ether ether ketone) by chloroform. Polym Test 1997;16:49–57.