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Evaluation of tribological behaviour of PEEK and glass fibre reinforced PEEK composite under dry sliding and water lubricated conditions
Wear 265 (2008) 1061–1065
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Evaluation of tribological behaviour of PEEK and glass fibre reinforced PEEK composite under dry sliding and water lubricated conditions M. Sumer b , H. Unal a,1 , A. Mimaroglu b,∗ a b
University of Sakarya, Faculty of Technical Education, Esentepe Kampusu, Adapazari, Turkey University of Sakarya, Faculty of Engineering, Esentepe Kampusu, Adapazari, Turkey
a r t i c l e
i n f o
Article history: Received 29 May 2007 Received in revised form 23 January 2008 Accepted 19 February 2008 Available online 2 April 2008 Keywords: PEEK Glass fibre Tribology Water lubrication
a b s t r a c t In this study, the tribological performance of pure polyetheretherketone (PEEK) and 30 wt% fibre glass (GFR) reinforced PEEK composite were studied at dry sliding and water lubricated conditions. Wear tests were carried out with configuration of a polymer pin on a rotating AISI D2 disc. Test conditions were atmospheric conditions, 1.77, 3.54, 5.30 MPa pressures and 0.80, 1.60 m/s sliding speeds. The results show that the coefficient of friction and specific wear rates for pure PEEK and PEEK + 30 wt% GFR composite slightly in increase with the increase in applied pressure values. On the other hand the coefficient of friction is in decrease while the specific wear is in increase with the increase in sliding speed values. Moreover, for the range of pressure and sliding speed of this study, the coefficient of friction and specific wear rates using water lubricant registered lower values than that of the dry condition. The influence of GFR fibre on the coefficient of friction and wear of the composite is more pronounced at dry wear test condition. Finally the specific wear rates for pure PEEK and PEEK + 30 wt% GFR under water lubricated condition were in the order of 10−15 m2 /N while under dry sliding condition this value is in the order of 10−14 m2 /N. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polymers such as PTFE and PEEK are important engineering materials therefore, in recent years, PEEK as a typical high performance semicrystalline thermoplastic polymer, has received significant attention. This is due to its high mechanical strength and elastic modulus, high melting temperature, chemical inertness, high toughness, easy processing and wear resistance. On the other hand, in tribological applications, because of the corrosive problems of the metals in water applications, polyetheretherketone (PEEK) and polyetheretherketone composites are preferred as rubbing materials. So, PEEK polymer material plays more important role as a bearing and slider material especially under water environment [1,2]. However, it is known that the friction and wear behaviours of polymers in fluid environments differ greatly from those in the dry friction condition. The absorption of water and plasticization of polymer surfaces influence the friction and wear of the polymer. Lancaster [3] reported that fluids such as water and other solutions inhibit the formation of transfer films of carbon/polymer debris on the counter-face and the wear rates are
∗ Corresponding author. Tel.: +90 264 2955845; fax: +90 264 3460351. E-mail addresses: [email protected] (H. Unal), [email protected] (A. Mimaroglu). 1 Tel.: +90 264 3460265; fax: +90 264 3460262. 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.008
greater than those obtained in dry conditions. Absorption of water can lead to reduction in strength, modulus of elasticity, increase in the elongation and swelling of the surface layer [4,5]. In past, vast number of investigations related to the tribological properties of PEEK has been reported [6–12]. Having said that, most investigations published were related to the friction and wear of polymers sliding against steels at dry conditions. In addition, few of scholarship have reported on the friction and wear of polymers in water lubrication conditions [3–5]. Tanaka [13] and Evans [5] reported that the introduction of water into a polymer/metal sliding combination generally reduces the coefficient of friction, but may increase the wear rate of the polymer. Unal and Mimaroglu [14] investigated the water lubricated tribological performance of carbon reinforced PEEK composite and reported that the coefficient of friction under water lubricated condition is lower than that the dry condition. Having said that, friction and wear phenomena also lead to a loss of mechanical efficiency. Therefore the accurate knowledge of the influence of sliding speed and load value on the wear and friction is extremely important [15]. It is seen from the literature that the majority of the investigations confined their discussions to dry sliding condition. Therefore there is a need to investigation and clarify the tribological characteristic of PEEK and glass fibre reinforced PEEK composite sliding against steel under dry sliding and water lubricated conditions. Friction and wear tests against AISI D2 steel were carried out on a pin-on-disc arrangement. Tribological tests were at room temperature under 1.77, 3.54 and 5.3 MPa pressures
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Table 1 Mechanical properties of extruded PEEK and PEEK + 30 wt% GFR composite Materials
Supplier
Ultimate tensile strength (MPa)
Elongation at break (%)
Tensile modulus (MPa)
Hardness (HRM )
Remarks
Extruded PEEK
Quadrant EPP Ketron® PEEK 1000 Quadrant EPP Ketron® PEEK 30% GFR
110
40
4350
100
97
2
6900
103
Unreinforced, extruded Ketron PEEK offers good wear resistance and can be used continuously to 250 ◦ C The addition of glass fibers significantly reduces the expansion rate and increases the flexural modulus of PEEK. This grade is ideal for structural applications that require improved strength, stiffness or stability, especially at temperatures above 300 ◦ C
Extruded PEEK + 30 wt% GFR
Fig. 1. Schematic diagram of wear test apparatus.
and at 0.80 and 1.6 m/s sliding speeds. The specific wear rates were realised from mass loss and reported. The wear mechanism was examined at dry and water lubricated conditions. 2. Experimental details Fig. 1 shows a schematic diagram of the pin-on-disc wear test apparatus 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 disk 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. For the tests, flat-ended pins with 6 mm in diameter from PEEK and PEEK + 30 wt% GFR polymer composite were prepared. All composite percentages are by weight values. The AISI D2 steel discs were machined to 10 mm thickness and 100 mm diameter and ground to a surface roughness of 0.28–0.35 m and a hardness value of 57 HRC. All PEEK and PEEK composites are Extruded type and were turned down to sample dimensions. For materials suppliers and detail properties see, Table 1. All samples were conducted to wear test at condition (i.e. ambient temperature, pressure, speed and humidity) see, Table 2. Lubrication was applied on wear truck using 20 drops of water every minute which present a drop of water every 3 s. Before each test the flat-ended polymer pins and AISI D2 steel discs were cleaned with alcohol and acetone and then installed in
Fig. 2. The relationship between coefficient of friction and applied pressure values of pure PEEK polymer under dry and water lubricated conditions.
the pin-on-disc apparatus. The friction and wear tests were performed at room temperature under 1.77, 3.54, 5.3 MPa pressures and sliding speed of 0.80 and 1.6 m/s for dry sliding and distilled water lubricated conditions. The specific wear rates were calculated from mass loss. Each test was repeated three times and the close results were considered and their average values were presented. 3. Results and discussions Figs. 2 and 3 present the variation of friction coefficient for pure PEEK and PEEK + 30 wt% GFR with the change in applied pressure and test speed under dry and water lubrication conditions. It is clear
Table 2 Materials and test conditions Materials
Test temperature (◦ C)
Load (MPa)
Speed (m/s)
Humidity (%)
Pure PEEK PEEK + 30 wt% GFR composite
19 18
1.77 3.54 5.30
0.8 1.6
54 52
Fig. 3. The relationship between coefficient of friction and applied pressure values of PEEK + 30 wt% GFR polymer composite under dry and water lubricated conditions.
M. Sumer et al. / Wear 265 (2008) 1061–1065
Fig. 4. The relationship between specific wear rate and applied pressure values of pure PEEK polymer under dry and water lubricated conditions.
from these figures that a slight increase in coefficient of friction is observed for the increase in applied pressure and the decrease in sliding speed values. A large change is observed between dry and lubricated conditions. For PEEK the coefficient of friction under water lubricated condition is an average of 63% lower than that of dry condition value. On the other hand for PEEK + 30 wt% GFR composite the coefficient of friction under water lubricated condition is an average of 51% lower than that of dry condition value. The mean overall average difference is about 57%. As it is known that PEEK polymer is a visco-elastic material; PEEK’s deformation under load is visco-elastic. So, the variation of friction coefficient with the load (which represent pressure as well) follows the equation = K N(n−1) [16], where is the coefficient of friction, N is the applied load, K is a constant and n is also a constant, its value between 2/3 and 1. According to this equation, the friction coefficient decreases with the load increase. But when the load increases to the critical load value of the pure PEEK polymer, the friction will increase. This behaviour is explained as the frictional power NV or PV (specific energy input) increases the sliding temperature of the friction surfaces, which lead to relaxation of polymer molecule chains, where V: sliding speed, P: pressure. Generally, the friction coefficients for pure PEEK and PEEK + 30 wt% GFR polymers are lower under water lubricated conditions than under dry sliding. This can be explained that water as a lubricant might function to significantly hinder to friction-induced thermal effect. Figs. 4 and 5 present the variation of specific wear rate for PEEK and PEEK composite with applied pressure, test speed under
Fig. 5. The relationship between specific wear rate and applied pressure values of PEEK + 30 wt% GFR polymer composite under dry and water lubricated conditions.
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Fig. 6. The relationship between friction coefficient and specific wear rate of pure PEEK with PV factor value for dry sliding condition and under water lubricated condition.
dry and water lubricated conditions. These figures show that the specific wear rate is slightly influenced by the change in applied pressure. The higher the sliding speed the higher is the specific wear rate. The specific wear rate values at dry and water lubricated condition are close to each other. This is explained by removal of the film layer under water lubricated condition. In fact water inhibit the formation of transfer films of fiber glass/polymer debris on the counter-face and the wear rates are close to those obtained in dry conditions. Generally, the specific wear rate for pure PEEK under dry sliding conditions is in the order of 10−14 m2 /N while pure PEEK under water lubricated conditions is in the order of 10−15 m2 /N. The highest wear rate is for pure PEEK under dry sliding conditions with a value of 2 × 10−14 m2 /N at 1.6 m/s sliding speed and under 3.54 MPa pressure. The lowest wear rate is 4 × 10−15 m2 /N for PEEK + 30 wt% GFR composite under water lubricated conditions at 0.80 m/s sliding speed and 1.77 MPa pressure. Figs. 6 and 7 present the combined influence of pressure and speed (PV factor) on the coefficient of friction and the specific wear rate of PEEK and PEEK composite under dry and water lubricated conditions. Although the tribological performance showed slight change with pressure and speed individually, this influence almost diminished by considering the combined effect using PV factor. It can be deduced that PEEK and PEEK composite is much suitable for bearing applications under varying load and sliding speed conditions. The optical microscopy examination of worn surfaces of pure PEEK and PEEK + 30 wt% GFR composite pins against AISI D2
Fig. 7. The relationship between friction coefficient and specific wear rate of PEEK + 30 wt% GFR with PV factor value for dry sliding condition and under water lubricated condition.
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Fig. 8. The worn surfaces of steel disc: (a) pure PEEK polymer, under dry sliding conditions; (b) PEEK + 30 wt%GFR polymer composite, under dry sliding conditions; (c) pure PEEK polymer under water lubricated conditions; (d) PEEK + 30 wt% GFR polymer composite under water lubricated conditions (applied pressure: 3.54 MPa, sliding speed: 0.8 m/s), magnification 100×.
steel discs both dry and water lubricated conditions under 3.54 MPa pressure and at 0.80 m/s sliding speed are given in Figs. 8(a–d) and 9. The disc worn surfaces for pure PEEK pin showed some patches of PEEK with wide and deep grooves. These patches
almost disappeared during water lubricated condition, see Fig. 8a and c. For PEEK + 30 wt% GFR pin case, the counterface disc showed less PEEK patches and slight scuffing. In addition, PEEK polymer transfer is not observed on the steel counterface in water lubri-
Fig. 9. The worn surfaces of pin polymers: (a) pure PEEK polymer, under dry sliding conditions; (b) pure PEEK polymer under water lubricated conditions; (c) PEEK + 30%GFR polymer composite, under dry sliding conditions; (d) PEEK + 30%GFR polymer composite under water lubricated conditions (applied pressure 3.54 MPa, sliding speed: 0.8 m/s), magnification 100×.
M. Sumer et al. / Wear 265 (2008) 1061–1065
cated conditions see Fig. 8b and d. In another words, water inhibited the transfer of PEEK and PEEK + 30 wt% GFR polymer onto the steel counterface. Fig. 9 shows pin surfaces for pure PEEK and its composite under dry and water lubricated conditions. A smoother surface is clear under water lubricated conditions, see Fig. 9 b and d. This is explained as the sliding process generates heat at the polymer pin and the steel counterface contact area, the PEEK polymer surface is softening, and result, to increase in specific wear rate. The cooling effect of water hindered the melting of PEEK and PEEK + 30 wt% GFR surface layer and inhibited the transfer and generation of wear debris on the sliding counterpart steel surface. Thus, a smoother worn surface and a slight low wear rate of PEEK and PEEK + 30 wt% GFR are observed under water lubrication, as compared with under dry sliding. 4. Conclusions 1. The friction coefficients under water lubricated conditions for pure PEEK and PEEK + 30 wt% GFR composite are lower than that of dry sliding conditions. 2. The specific wear rates for pure PEEK and PEEK + 30 wt% GFR composite under water lubricated conditions were in the order of 10−15 m2 /N while for pure PEEK and PEEK + 30 wt% GFR composite under dry sliding conditions is in the order of 10−14 m2 /N. 3. The highest wear rate is for pure PEEK under dry sliding conditions with a value of 2 × 10−14 m2 /N at 1.6 m/s sliding speed and under 3.4 MPa applied pressure. The lowest wear rate is 4 × 10−15 m2 /N for pure PEEK under water lubricated conditions at 0.80 m/s sliding speed and under 1.77 pressure values. 4. In general, the specific wear rate is little influenced by the change in applied pressure but is more influenced by the environmental condition from dry to water lubricated.
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5. The tribological behaviour for the materials and within the range of test conditions of this investigation is not influence by the change in combined pressure and speed factor (PV factor). References [1] D.J. Blundell, B.N. Osborn, The morphology of poly (aryletheretherketone), Polymer 25 (1983) 953–958. [2] D.P. Jones, D.C. Leach, D.R. Moore, Mechanical properties of polyetheretherketone for Engineering applications, Polymer 26 (1985) 1385–1393. [3] J.K. Lancaster, Lubrication of carbon fiber-reinforced polymers. Part 1. Water and aqueous solution, Wear 20 (1972) 315–333. [4] M.D. Lutton, T.A. Stolarski, The effect of water lubrication on polymer wear under rolling contact condition, J. Appl. Polym. Sci. 54 (1994) 771–782. [5] D.C. Evans, Polymer fluid interaction in relation to wear, in: Proceeding of the Third Leeds-Lyon Symposium on Tribology, The wear of Non-Metallic Materials, Mechanical Engineering Publication Ltd., 1978, pp. 47–55. [6] D.P. Jones, D.C. Leach, D.R. Moore, Mechanical properties of poly(ether–ether–ketone) for engineering applications, Polymer 26 (1985) 1385–1393. [7] S. Bahadur, D. Gong, The role of copper compounds as fillers in the transfer and wear behaviour of polyetheretherketone, Wear 154 (1992) 151–165. [8] T.A. Stolarski, Tribology of polyetheretherketone, Wear 158 (1992) 71–78. [9] M. Cirino, K. Friedrich, R.B. Pipes, Composites 19 (5) (1988) 383–392. [10] Z.P. Lu, K. Friedrich, on sliding friction and wear of PEEK and its composites, Wear 181–183 (1995) 624–631. [11] T.C. Ovaert, H.S. Cheng, The unlubricated sliding wear behaviour of polyetheretherketone against smooth mild-steel counterfaces, ASME J. Tribol. 113 (1991) 150–157. [12] T.C. Ovaert, H.S. Cheng, Counterface topographical effects on the wear of polyetheretherketone and a polyetheretherketone–carbon fiber composite, Wear 150 (1991) 275–287. [13] K. Tanaka, Friction and wear of semicrystalline polymers sliding against steel under water lubrication, Trans. ASME J. Lubric. Technol. 102 (4) (1980) 526–533. [14] H. Unal, A. Mimaroglu, Friction and wear characteristics of PEEK and its composites under water lubrication, J. Reinforced Plast. Compos. 16 (2006) 1659–1667. [15] C.J. Hooke, S.N. Kukureka, P. Liao, M. Rao, Y.K. Chen, The friction and wear of polymers in non-conformal contacts, Wear 200 (1996) 83–94. ¨ [16] H. Uetz, J. Wiedemeyer, Tribologie der Polymere, Carl Hanser Verlag, MunchenVienna, 1985.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Evaluation of tribological behaviour of PEEK and glass fibre reinforced PEEK composite under dry sliding and water lubricated conditions M. Sumer b , H. Unal a,1 , A. Mimaroglu b,∗ a b
University of Sakarya, Faculty of Technical Education, Esentepe Kampusu, Adapazari, Turkey University of Sakarya, Faculty of Engineering, Esentepe Kampusu, Adapazari, Turkey
a r t i c l e
i n f o
Article history: Received 29 May 2007 Received in revised form 23 January 2008 Accepted 19 February 2008 Available online 2 April 2008 Keywords: PEEK Glass fibre Tribology Water lubrication
a b s t r a c t In this study, the tribological performance of pure polyetheretherketone (PEEK) and 30 wt% fibre glass (GFR) reinforced PEEK composite were studied at dry sliding and water lubricated conditions. Wear tests were carried out with configuration of a polymer pin on a rotating AISI D2 disc. Test conditions were atmospheric conditions, 1.77, 3.54, 5.30 MPa pressures and 0.80, 1.60 m/s sliding speeds. The results show that the coefficient of friction and specific wear rates for pure PEEK and PEEK + 30 wt% GFR composite slightly in increase with the increase in applied pressure values. On the other hand the coefficient of friction is in decrease while the specific wear is in increase with the increase in sliding speed values. Moreover, for the range of pressure and sliding speed of this study, the coefficient of friction and specific wear rates using water lubricant registered lower values than that of the dry condition. The influence of GFR fibre on the coefficient of friction and wear of the composite is more pronounced at dry wear test condition. Finally the specific wear rates for pure PEEK and PEEK + 30 wt% GFR under water lubricated condition were in the order of 10−15 m2 /N while under dry sliding condition this value is in the order of 10−14 m2 /N. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polymers such as PTFE and PEEK are important engineering materials therefore, in recent years, PEEK as a typical high performance semicrystalline thermoplastic polymer, has received significant attention. This is due to its high mechanical strength and elastic modulus, high melting temperature, chemical inertness, high toughness, easy processing and wear resistance. On the other hand, in tribological applications, because of the corrosive problems of the metals in water applications, polyetheretherketone (PEEK) and polyetheretherketone composites are preferred as rubbing materials. So, PEEK polymer material plays more important role as a bearing and slider material especially under water environment [1,2]. However, it is known that the friction and wear behaviours of polymers in fluid environments differ greatly from those in the dry friction condition. The absorption of water and plasticization of polymer surfaces influence the friction and wear of the polymer. Lancaster [3] reported that fluids such as water and other solutions inhibit the formation of transfer films of carbon/polymer debris on the counter-face and the wear rates are
∗ Corresponding author. Tel.: +90 264 2955845; fax: +90 264 3460351. E-mail addresses: [email protected] (H. Unal), [email protected] (A. Mimaroglu). 1 Tel.: +90 264 3460265; fax: +90 264 3460262. 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.008
greater than those obtained in dry conditions. Absorption of water can lead to reduction in strength, modulus of elasticity, increase in the elongation and swelling of the surface layer [4,5]. In past, vast number of investigations related to the tribological properties of PEEK has been reported [6–12]. Having said that, most investigations published were related to the friction and wear of polymers sliding against steels at dry conditions. In addition, few of scholarship have reported on the friction and wear of polymers in water lubrication conditions [3–5]. Tanaka [13] and Evans [5] reported that the introduction of water into a polymer/metal sliding combination generally reduces the coefficient of friction, but may increase the wear rate of the polymer. Unal and Mimaroglu [14] investigated the water lubricated tribological performance of carbon reinforced PEEK composite and reported that the coefficient of friction under water lubricated condition is lower than that the dry condition. Having said that, friction and wear phenomena also lead to a loss of mechanical efficiency. Therefore the accurate knowledge of the influence of sliding speed and load value on the wear and friction is extremely important [15]. It is seen from the literature that the majority of the investigations confined their discussions to dry sliding condition. Therefore there is a need to investigation and clarify the tribological characteristic of PEEK and glass fibre reinforced PEEK composite sliding against steel under dry sliding and water lubricated conditions. Friction and wear tests against AISI D2 steel were carried out on a pin-on-disc arrangement. Tribological tests were at room temperature under 1.77, 3.54 and 5.3 MPa pressures
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Table 1 Mechanical properties of extruded PEEK and PEEK + 30 wt% GFR composite Materials
Supplier
Ultimate tensile strength (MPa)
Elongation at break (%)
Tensile modulus (MPa)
Hardness (HRM )
Remarks
Extruded PEEK
Quadrant EPP Ketron® PEEK 1000 Quadrant EPP Ketron® PEEK 30% GFR
110
40
4350
100
97
2
6900
103
Unreinforced, extruded Ketron PEEK offers good wear resistance and can be used continuously to 250 ◦ C The addition of glass fibers significantly reduces the expansion rate and increases the flexural modulus of PEEK. This grade is ideal for structural applications that require improved strength, stiffness or stability, especially at temperatures above 300 ◦ C
Extruded PEEK + 30 wt% GFR
Fig. 1. Schematic diagram of wear test apparatus.
and at 0.80 and 1.6 m/s sliding speeds. The specific wear rates were realised from mass loss and reported. The wear mechanism was examined at dry and water lubricated conditions. 2. Experimental details Fig. 1 shows a schematic diagram of the pin-on-disc wear test apparatus 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 disk 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. For the tests, flat-ended pins with 6 mm in diameter from PEEK and PEEK + 30 wt% GFR polymer composite were prepared. All composite percentages are by weight values. The AISI D2 steel discs were machined to 10 mm thickness and 100 mm diameter and ground to a surface roughness of 0.28–0.35 m and a hardness value of 57 HRC. All PEEK and PEEK composites are Extruded type and were turned down to sample dimensions. For materials suppliers and detail properties see, Table 1. All samples were conducted to wear test at condition (i.e. ambient temperature, pressure, speed and humidity) see, Table 2. Lubrication was applied on wear truck using 20 drops of water every minute which present a drop of water every 3 s. Before each test the flat-ended polymer pins and AISI D2 steel discs were cleaned with alcohol and acetone and then installed in
Fig. 2. The relationship between coefficient of friction and applied pressure values of pure PEEK polymer under dry and water lubricated conditions.
the pin-on-disc apparatus. The friction and wear tests were performed at room temperature under 1.77, 3.54, 5.3 MPa pressures and sliding speed of 0.80 and 1.6 m/s for dry sliding and distilled water lubricated conditions. The specific wear rates were calculated from mass loss. Each test was repeated three times and the close results were considered and their average values were presented. 3. Results and discussions Figs. 2 and 3 present the variation of friction coefficient for pure PEEK and PEEK + 30 wt% GFR with the change in applied pressure and test speed under dry and water lubrication conditions. It is clear
Table 2 Materials and test conditions Materials
Test temperature (◦ C)
Load (MPa)
Speed (m/s)
Humidity (%)
Pure PEEK PEEK + 30 wt% GFR composite
19 18
1.77 3.54 5.30
0.8 1.6
54 52
Fig. 3. The relationship between coefficient of friction and applied pressure values of PEEK + 30 wt% GFR polymer composite under dry and water lubricated conditions.
M. Sumer et al. / Wear 265 (2008) 1061–1065
Fig. 4. The relationship between specific wear rate and applied pressure values of pure PEEK polymer under dry and water lubricated conditions.
from these figures that a slight increase in coefficient of friction is observed for the increase in applied pressure and the decrease in sliding speed values. A large change is observed between dry and lubricated conditions. For PEEK the coefficient of friction under water lubricated condition is an average of 63% lower than that of dry condition value. On the other hand for PEEK + 30 wt% GFR composite the coefficient of friction under water lubricated condition is an average of 51% lower than that of dry condition value. The mean overall average difference is about 57%. As it is known that PEEK polymer is a visco-elastic material; PEEK’s deformation under load is visco-elastic. So, the variation of friction coefficient with the load (which represent pressure as well) follows the equation = K N(n−1) [16], where is the coefficient of friction, N is the applied load, K is a constant and n is also a constant, its value between 2/3 and 1. According to this equation, the friction coefficient decreases with the load increase. But when the load increases to the critical load value of the pure PEEK polymer, the friction will increase. This behaviour is explained as the frictional power NV or PV (specific energy input) increases the sliding temperature of the friction surfaces, which lead to relaxation of polymer molecule chains, where V: sliding speed, P: pressure. Generally, the friction coefficients for pure PEEK and PEEK + 30 wt% GFR polymers are lower under water lubricated conditions than under dry sliding. This can be explained that water as a lubricant might function to significantly hinder to friction-induced thermal effect. Figs. 4 and 5 present the variation of specific wear rate for PEEK and PEEK composite with applied pressure, test speed under
Fig. 5. The relationship between specific wear rate and applied pressure values of PEEK + 30 wt% GFR polymer composite under dry and water lubricated conditions.
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Fig. 6. The relationship between friction coefficient and specific wear rate of pure PEEK with PV factor value for dry sliding condition and under water lubricated condition.
dry and water lubricated conditions. These figures show that the specific wear rate is slightly influenced by the change in applied pressure. The higher the sliding speed the higher is the specific wear rate. The specific wear rate values at dry and water lubricated condition are close to each other. This is explained by removal of the film layer under water lubricated condition. In fact water inhibit the formation of transfer films of fiber glass/polymer debris on the counter-face and the wear rates are close to those obtained in dry conditions. Generally, the specific wear rate for pure PEEK under dry sliding conditions is in the order of 10−14 m2 /N while pure PEEK under water lubricated conditions is in the order of 10−15 m2 /N. The highest wear rate is for pure PEEK under dry sliding conditions with a value of 2 × 10−14 m2 /N at 1.6 m/s sliding speed and under 3.54 MPa pressure. The lowest wear rate is 4 × 10−15 m2 /N for PEEK + 30 wt% GFR composite under water lubricated conditions at 0.80 m/s sliding speed and 1.77 MPa pressure. Figs. 6 and 7 present the combined influence of pressure and speed (PV factor) on the coefficient of friction and the specific wear rate of PEEK and PEEK composite under dry and water lubricated conditions. Although the tribological performance showed slight change with pressure and speed individually, this influence almost diminished by considering the combined effect using PV factor. It can be deduced that PEEK and PEEK composite is much suitable for bearing applications under varying load and sliding speed conditions. The optical microscopy examination of worn surfaces of pure PEEK and PEEK + 30 wt% GFR composite pins against AISI D2
Fig. 7. The relationship between friction coefficient and specific wear rate of PEEK + 30 wt% GFR with PV factor value for dry sliding condition and under water lubricated condition.
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M. Sumer et al. / Wear 265 (2008) 1061–1065
Fig. 8. The worn surfaces of steel disc: (a) pure PEEK polymer, under dry sliding conditions; (b) PEEK + 30 wt%GFR polymer composite, under dry sliding conditions; (c) pure PEEK polymer under water lubricated conditions; (d) PEEK + 30 wt% GFR polymer composite under water lubricated conditions (applied pressure: 3.54 MPa, sliding speed: 0.8 m/s), magnification 100×.
steel discs both dry and water lubricated conditions under 3.54 MPa pressure and at 0.80 m/s sliding speed are given in Figs. 8(a–d) and 9. The disc worn surfaces for pure PEEK pin showed some patches of PEEK with wide and deep grooves. These patches
almost disappeared during water lubricated condition, see Fig. 8a and c. For PEEK + 30 wt% GFR pin case, the counterface disc showed less PEEK patches and slight scuffing. In addition, PEEK polymer transfer is not observed on the steel counterface in water lubri-
Fig. 9. The worn surfaces of pin polymers: (a) pure PEEK polymer, under dry sliding conditions; (b) pure PEEK polymer under water lubricated conditions; (c) PEEK + 30%GFR polymer composite, under dry sliding conditions; (d) PEEK + 30%GFR polymer composite under water lubricated conditions (applied pressure 3.54 MPa, sliding speed: 0.8 m/s), magnification 100×.
M. Sumer et al. / Wear 265 (2008) 1061–1065
cated conditions see Fig. 8b and d. In another words, water inhibited the transfer of PEEK and PEEK + 30 wt% GFR polymer onto the steel counterface. Fig. 9 shows pin surfaces for pure PEEK and its composite under dry and water lubricated conditions. A smoother surface is clear under water lubricated conditions, see Fig. 9 b and d. This is explained as the sliding process generates heat at the polymer pin and the steel counterface contact area, the PEEK polymer surface is softening, and result, to increase in specific wear rate. The cooling effect of water hindered the melting of PEEK and PEEK + 30 wt% GFR surface layer and inhibited the transfer and generation of wear debris on the sliding counterpart steel surface. Thus, a smoother worn surface and a slight low wear rate of PEEK and PEEK + 30 wt% GFR are observed under water lubrication, as compared with under dry sliding. 4. Conclusions 1. The friction coefficients under water lubricated conditions for pure PEEK and PEEK + 30 wt% GFR composite are lower than that of dry sliding conditions. 2. The specific wear rates for pure PEEK and PEEK + 30 wt% GFR composite under water lubricated conditions were in the order of 10−15 m2 /N while for pure PEEK and PEEK + 30 wt% GFR composite under dry sliding conditions is in the order of 10−14 m2 /N. 3. The highest wear rate is for pure PEEK under dry sliding conditions with a value of 2 × 10−14 m2 /N at 1.6 m/s sliding speed and under 3.4 MPa applied pressure. The lowest wear rate is 4 × 10−15 m2 /N for pure PEEK under water lubricated conditions at 0.80 m/s sliding speed and under 1.77 pressure values. 4. In general, the specific wear rate is little influenced by the change in applied pressure but is more influenced by the environmental condition from dry to water lubricated.
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