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Sliding behaviour of pure polyester and polyester-PTFE filled bulk composites in overload conditions
Polymer Testing 24 (2005) 588–603 www.elsevier.com/locate/polytest
Test Method
Sliding behaviour of pure polyester and polyester-PTFE filled bulk composites in overload conditions Pieter Samyn*, Jan Quintelier, Wouter Ost, Patrick De Baets, Gustaaf Schoukens Department Mechanical Construction and Production, Laboratory Soete, Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium Received 14 January 2005; accepted 25 February 2005
Abstract Polymers are frequently used in tribological applications because of their self-lubricating ability and loadability. However, most research on their friction and wear mechanisms is performed on small-scale test samples under relatively low normal loads. The present paper presents test results that are obtained on large-scale test samples under extremely high loads (190– 3380 kN, corresponding to 8–150 MPa), providing more accurate design information for construction of e.g. sea-lock or crane guides. Performance and failure modes in overload conditions are investigated. Pure polyethyleneterephtalate (PET) shows high friction and unstable wear due to stick-slip. Filled with polytetrafluouroethylene (PTFE), friction is reduced and becomes stable even when loaded above the yield strength. The plastic deformation of the sliding surface under overload conditions allows for the formation of a transfer film on the counterface, resulting in a decrease of specific wear rate at higher loads. A model is presented for correction of wear measurements that contains both real material loss and deformation due to creep and thermal expansion. For the latter, temperatures as measured from frictional heating are corrected and evaluated according to known temperature models. Compared to small-scale cylinder-on-plate and bloc-on-ring tests, friction coefficients measured on large-scale tests are much lower due to large polymer transfer, while wear rates are much higher. The availability of PTFE and favourable plastication of the sliding surface on large-scale tests result in the build up of a coherent lubrication transfer film. q 2005 Elsevier Ltd All rights reserved. Keywords: Polymers; Wear; Adhesion; Additives
1. Introduction Polyethylenetherephtalate, or PET, is a semi-crystalline thermoplastic with high dimensional stability and chemical resistance. Nowadays, it is most widely used as biaxially oriented film in soda bottles or as thermally sprayed coating for surface protection against corrosion and wear. The wear behaviour of unlubricated PET coatings in three conditions (as molded, thermally sprayed and quenched after thermal spraying) has been investigated by Branco et al. [1] using a small-scale pin-on-disc tester. Beake et al. [2] studied * Corresponding author. Tel.: C32 9 264 33 08; fax: C32 9 264 32 95. E-mail address: [email protected] (P. Samyn).
0142-9418/$ - see front matter q 2005 Elsevier Ltd All rights reserved. doi:10.1016/j.polymertesting.2005.02.012
the frictional and adhesive properties of commercial films. For a better understanding of the molecular processes leading to surface degradation of PET films, they used scanning force microscopy experiments for describing the surface deformation during a tip-induced wear process. As polymers generally posses good self-lubricating abilities through the formation of a polymer transfer film or ‘third body’, they are also used in industry as sliding materials in, e.g. gears, slides and bearings. While polyamides or polyethylene are most commonly used, they should be replaced by polyesters for obtaining higher temperature and fatigue resistance because of the stiffening action of the aromatic phenylene group. However, the sensitivity to brittle fracture due to notch and stress concentrations can restrict its applicability [3]. The favourable use of PET in
P. Samyn et al. / Polymer Testing 24 (2005) 588–603
dry sliding was recently demonstrated by Neogi et al. [4] on small-scale tests, who found gradually decreasing wear rates and lower friction when PET was added to polypropylene, while the limit of moderate wear increased towards higher contact stresses. For design purposes of machine elements, not only the surface characteristics but also the integrity of the polymer bulk properties become important, mainly determined by its load carrying capacity. As situations of dry sliding frequently occur under highly loaded conditions (e.g., storm barriers or sea-locks), creep and overloads can cause weakening and degradation in bulk properties with consequent alterations in friction and wear mechanisms. However, little information on the tribological behaviour of pure and filled PET bulk materials are available in literature and if so, all test results are obtained from traditional smallscale tests such as pin-on-disc or cylinder-on-plate tests, not taking into account overload conditions [5,6]. These methods are, however, only useful for a general classification of various materials, resulting in important errors when extrapolating towards the real working environment as, e.g. reported by [7]. Real criteria for material selection in a given application require the tribological characterisation of polymer parts under conditions that are closely related to its practical functionality. Mainly for tribotesting of polymer materials under high loads, the influence of creep and deformation and the moveability of wear debris into the contact zone should be simulated effectively. Presently, friction and wear of extruded PET bulk materials are investigated on large-scale sliding samples with an apparent contact surface of 22,500 mm2 under loads ranging from 8 to 150 MPa. Therefore, this work is believed to provide primarily information about the practical use and failure of polyesters in tribology. The friction and wear behaviour observed is further discussed in relation to deformation and thermal effects. As the formation and aspect of a molten or plasticized polymer transfer film strongly governs the tribological performance, the sliding surfaces are microscopically analysed. Surface depositions on the counterface result in lower roughness with reduced friction and polymer wear, therefore, only smooth transfer layers are favourable. Additions of internal solid lubricants, as e.g. PTFE with its lamellar structure, improve both friction and wear characteristics through the formation of a uniform film with proper thickness, although they weaken the creep resistance and compressive strength of the bulk polymer. Research towards optimum filler content indicated a working range for PTFE in PET of between 10 and 20 vol% [8].
2. Experimental large-scale testing 2.1. Test equipment The tribotester for high-load testing of large specimen is shown in Fig. 1, where two sliding couples (each consisting
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of polymer test specimens (3) and counterface plates (2)) are tested in a flat-on-flat reciprocating motion. The polymer wear samples are 150!150!20 mm3 and the steel counterfaces size 410!200!20 mm3. Polymer specimens are mounted in two rectangular specimen holders (4), inserted on top and on the bottom of the machine into a rigid horizontal structure. The counterface plates are positioned on both sides of the central sliding block (1) that is pushed by two horizontal jacks at the left and right. The vertical load is exerted by the vertical jack in the column. Both horizontal and vertical forces are measured with dynamometers at positions (5) and (6). The sliding velocity is controlled by a hydraulic circuit and is almost constant over the sliding stroke, except near the edges where the sliding direction reverses. The coefficient of friction m is calculated by formula (1) with Fl the horizontal force at the left and Fr the horizontal force at the right of the central sliding block, Fn the normal force and the factor 1/2 resulting from the two sliding pairs on top and on the bottom of the test machine. As such, m represents an average friction coefficient between two test pairs. The modulus results from the negative forces occurring on reversal of the sliding direction. 1 Fl K Fr mZ (1) 2 Fn The vertical displacement of the top and bottom polymer specimens towards their counterface is continuously measured by two displacement transducers. As measurements are influenced by deformation under creep (enhancing the wear signal) or thermal expansion (counteracting the wear signal), the real wear (i.e., material loss) is determined by weighing the polymer sample before and after testing and is compared to thickness reduction. The sliding temperatures of the counterfaces are continuously measured at 20 mm beneath the contact surface by means of a K-type thermocouple. 2.2. Test materials Pure and polytetrafluoroethylene (PTFE) filled PET samples were machined from commercial extruded plates (Quadrant EPP) and have a surface finish of milling grooves perpendicular to the sliding direction. Pure PET has a white colour, while its PTFE-filled variant is grey coloured, consisting of a homogeneous dispersion of PTFE powder as an internal solid lubricant into the polymer bulk. Mechanical properties are listed in Table 1 [8]. The limiting pressurevelocity for pure PET is 0.10 MPa m/s, but filled with PTFE it increases towards 0.21 MPa m/s. The beneficial effect of PTFE on low friction has generally been reported by Tanaka et al. [9], Briscoe [10], Lancaster [11] and Gong [12], and results from its lamellar structure with low shear strength leading to easy transfer behaviour. However, pure PTFE shows extremely high wear rates and should be blended into
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Fig. 1. Schematical view on the large-scale tribotester. (1) Central sliding bloc; (2) steel counterface plate; (3) PET or PET/PTFE polymer specimen; (4) fixed specimen holder.
a matrix providing higher strength. PET/PTFE is industrially well-known for its good dimensional stability. The counterface plates consist of cold rolled steel 42 CrMo4 (DIN 1.7225) with hardness 330 HV, yield strength ReZ765 N/mm2, tensile strength RmZ980–1180 N/mm2 and chemical composition (in wt%): CZ0.38–0.45, Si!0.40, MnZ0.60–0.90, P!0.035, S!0.030, CrZ0.90–1.20, MoZ0.15–0.30. Before each test, the surfaces were roughly ground to an average surface roughness RaZ4 mm with machining grooves parallel to the sliding direction. Afterwards, the surfaces were ground to a lower roughness RaZ0.20 mm measured parallel to the sliding direction and RaZ0.60 mm perpendicular to the sliding direction. Roughness was measured on a 2 dimensional Perthen 5 SP according to DIN 4768. Prior to each test, polymer and steel surfaces were cleaned with acetone.
the sliding interface arises only from frictional heating. With a fixed sliding velocity of 5 mm/s, five different normal loads between 190 and 3380 kN were applied, corresponding to 8, 16, 25, 55 and 150 MPa. The pv-parameters range between 0.04 and 0.75 MPa m/s, which is above the generally accepted limit [8]. One single sliding stroke comprises 230 mm. Two sliding strokes are called one sliding cycle. Frictional stability during running-in was investigated by short-time sliding tests over 10 sliding cycles, corresponding to 4.6 m total sliding distance on counterfaces with RaZ4 and 0.2 mm. Long-time sliding tests (approx. 4 km or 168 h testing time depending on the observed wear) were performed on low roughness counterfaces for determination Table 1 Test materials
2.3. Large-scale test procedure The test conditions are given in Table 2. Large-scale sliding tests were performed under relative humidity RHZ50G5% and ambient temperature of 23G2 8C, while the counterface temperature was controlled by internal cooling of the central sliding block with a constant water flow at 15 8C. The rise in contact temperature at
Density (g/cm3) Tensile yield strength (MPa) Tensile strain at break (%) Tensile modulus (MPa) Heat deflection temperature (8C) Melting temperature (8C) Shear strength (MPa)
PET
PET/PTFE
1.39 90 15 3700 75 255 55
1.44 76 7 3450 75 255 59
P. Samyn et al. / Polymer Testing 24 (2005) 588–603 Table 2 Large-scale test conditions Test parameter
Large-scale test
Dimensions polymer sample Dimensions counterface plate Counterface roughness Ra Single sliding stroke Sliding velocity Normal load Contact pressure Ambient humidity/temperature
150!150!20 mm3 410!200!20 mm3 4, 0.2 mm 230 mm 0.005 m/s 190, 380, 560, 1260, 3380 kN 8, 16, 25, 55 150 MPa 60%/23 8C Short-time Long-time tests tests 4.6 m approx. 4 km
Total sliding distance
of the material’s wear lifetime. Test results of friction, wear and bulk temperatures on large-scale were calculated as average values of two separate test runs for each contact pressure.
3. Results and discussion 3.1. Creep and deformation under static loading Prior to dynamic sliding tests, the dimensional stability of PET and PET/PTFE mounted on top and bottom was determined specifically for the present test layout under static loading against steel with RaZ0.2 mm. Samples were loaded at 10 MPa/min, continuously measuring the vertical indentation as displacement between the polymer samples and the steel counterfaces. Immediately after the required load level was attained, the initial elastic indentation was recorded and compared to the theoretical normal deformation of PET/PTFE calculated from the modulus of elasticity (Table 3): for a contact pressure p, the indentation DlZ(p/E!20 mm). Creep was measured after 24 and 168 h constant loading.
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For the first three loading steps (190, 380 and 600 kN) the measured elastic indentation is in good agreement with calculated values for both PET and PET/PTFE (error 6%). The higher modulus of elasticity for pure PET results in lower elastic indentation. At loads of 1260 and 3380 kN, the experimental normal deformation is smaller than the calculated normal deformation, due to clamping of the polymer samples into their holders. The retaining action of the sample holder restricts the lateral deformation of the entire polymer bulk, concentrating the deformation of the polymer samples in the free neck 5 mm outside the holder. The in situ stiffness of the polymer is higher than estimated from the E modulus measured under free deformation: between 1260 and 3380 kN the bulk modulus attains 5750 MPa for PET and 5000 MPa for PET/PTFE. The total creep after 168 h loading is very small for PET/PTFE, reflecting its good dimensional stability. The average creep rate under steady-state equals 0.024 mm/h at 1260 kN and 0.070 mm/h at 3380 kN, or 1% compressive strain occurs after 8000 h loading under 1260 kN and after 2000 h loading under 3380 kN. Pure PET has significantly higher creep rates (0.028 mm/h at 1260 kN and 0.083 mm/h at 3380 kN) than PET/PTFE. Through the lower elastic deformation of PET during initially loading, the lateral expansion of the polymer in its holder is not as large as for PET/PTFE and proceeds over longer loading times. Therefore, creep is less restricted by the retaining action of the holder. The difference in deformation between top and bottom specimens is attributed to the machining tolerances on the sample width (150G0.5 mm). 3.2. Friction and wear under running-in conditions The selection of a proper bearing material is not only based on low coefficients of friction, but also stable friction is required under high stresses. Especially when short sliding distances and several stop/restart conditions are considered, stick-slip and high static friction should be
Table 3 Deformation of large-scale PET and PET/PTFE samples under static loading Polymer
PET
PET/PTFE
Normal load (kN)
190 380 600 1260 3380 190 380 600 1260 3380
Normal deformation (mm)
Creep (mm)
Top
Bottom
Calculated
47 89 127 228 558 57 92 155 282 666
42 85 121 237 581 45 79 144 281 691
45 86 135 297 811 48 97 145 319 870
Top
Bottom
24 h
168 h
24 h
168 h
1 3 6 15 65 1 2 4 13 60
1 1 3 4 12 1 1 2 3 10
1 1 7 17 67 0 1 4 11 63
0 1 2 6 11 0 0 4 4 10
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Fig. 2. Friction-displacement characteristics for (a) PET and (b) PET/PTFE sliding against steel under 190 kN.
avoided [13]. The frictional behaviour of PET and PET/ PTFE at the start and at reversal of the sliding direction is, therefore, studied in more detail, revealing frictional instabilities caused by difference in static and dynamic friction and/or vibrations. Recently, Rymuza et al. [14]
reported on the static friction and adhesion in polymerpolymer contact, but little data is known on the static friction of PET/steel contacts (only Vaziri et al. [15]). The coefficient of friction is plotted as a function of the sliding distance of the central sliding bloc in Fig. 2(a) for PET and in Fig. 2(b) for PET/PTFE under 190 kN. The sliding block is initially at the position xZ0 and starts sliding towards positive x-values. At the end of the sliding stroke (xZC108 mm) the direction reverses towards negative x-values until xZK122 mm. Static friction is measured at the initial start (ms1) and at the reversals of the sliding motion (ms,min and ms,max), while the dynamic friction md,max or md,min is measured at the center of the sliding stroke, as summarized in Table 4 for different normal loads and counterface roughness. Pure PET shows unstable friction, continuously increasing with incremental sliding distance. The horizontal pistons vibrate because of heavy stick-slip observed as irregularities on the friction–displacement characteristics. Loud noise during sliding forced the tests to be stopped prematurely under high contact pressures (1260 kN only contains one sliding cycle). The static friction at the reversals of the sliding stroke is significantly larger than the dynamic friction. Other polymers, e.g. polyamides, are also prone to high static friction and stick-slip in dry sliding [16]. Friction exceeds the capacity of the test rig at 3380 kN normal load, which is above the material’s yield strength. The observed tendency of increasing friction is in contrast with the generally accepted model of decreasing friction at higher normal loads [17]. Under 1260 kN, the horizontal surface stress equals 15 MPa, which is 27% of the shear strength. Filled with PTFE, friction is reduced and becomes stable during the test: after the first sliding cycle it attains a fixed value that lowers with increase in contact pressure. PET/PTFE even performs well when loaded above its yield strength, allowing for plastic deformation of the sliding
Table 4 Static and dynamic coefficients of friction during running-in Polymer
PET
PET/PTFE
Normal load (kN) 600 1260 190 380 600 1260 3380 600 1260 190 380 600 1260 3380
Static friction ms1
Dynamic friction msn,min
0.11 0.12 0.12 0.15 0.12 0.13 0.17 0.18 0.21 0.22 0.21 0.23 Overload (stick-slip) 0.11 0.11 0.09 0.09 0.12 0.10 0.11 0.09 0.09 0.08 0.07 0.05 0.04 0.04
msn,max
md,min
md,max
0.29 0.35 0.16 0.21 0.24 0.26
0.16 0.18 0.11 0.16 0.19 0.20
0.25 0.28 0.14 0.18 0.21 0.23
0.12 0.10 0.12 0.10 0.09 0.06 0.04
0.10 0.09 0.10 0.09 0.07 0.05 0.03
0.11 0.10 0.11 0.09 0.08 0.06 0.03
Running-in wear rate (mm/km)
Counter face
9.4 17.7 1.7 4.8 6.9 7.6 21 2.9 4.6 0.9 2.2 2.2 2.4 4.1
RaZ4 mm RaZ0.20 mm
RaZ4 mm RaZ0.20 mm
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surface and extremely low friction. Under 1260 kN the horizontal stress equals 3 MPa and under 3380 kN it is 6 MPa, which is, respectively, 5 and 10% of the PET/PTFE shear strength. The shear strength of PTFE is much lower (5 MPa) allowing for low friction and easy shear when transferred onto the counterface. The static friction becomes equal to dynamic friction in the presence of PTFE, excluding stick-slip. When the counterface roughness increases, friction of pure PET is further destabilised with worse increase in friction coefficient during the first sliding cycles. Friction for PET/PTFE is also higher when sliding against higher roughness counterface. As it is generally accepted that friction contains both adhesion and deformation, the latter becoming more important in sliding against rough surfaces due to the ploughing action of the counterface asperities. Linear wear rates, including both real material loss and deformation at running-in, indicate that PTFE additions reduce running-in wear and cause the amount of wear nearly independent of the normal load when loaded below its yield strength. The wear rates at 3380 kN mainly represents deformation of the sliding surface. Higher counterface roughness reveals higher wear rates with consequent generation of PTFE debris, although they are not favourable for lower friction. This contradiction indicates that under the present conditions of high contact pressures an optimum film thickness is established as generally described by [18], although not able to resist the ploughing interaction of counterface asperities with consequently high friction and wear. 3.3. Friction and temperature under steady-state conditions PET/PTFE samples were slid for 168 h (1 week) against steel counterfaces with average roughness RaZ0.20 mm, investigating the evolution in friction as shown in Fig. 3. The coefficient of friction after 5 m of sliding corresponds to the previously described friction under running-in conditions. Steady-state conditions are markably different:
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under contact pressures of 190–600 kN there is an increase in friction towards steady-state, while for contact pressures of 1260 and 3380 kN it stabilises more frequently or decreases towards a steady-state. These trends were also observed on preliminary small-scale tests performed by Zsidai et al. [19]: under 100 N normal load, the coefficient of friction rises from 0.2 towards 0.3 on smooth surfaces while it frequently stabilises at 0.2 on rough surfaces. Stable friction is attributed to the formation of a thin transfer film on the counterface, as shown in Section 3.5. Literature models suggest that with increasing normal load FN the coefficient of friction decreases according to mZK. FKn N with K constant and the parameter depending on the amount of plastic or elastic deformation. In the case of full contact through plastic deformation, nZ1, while nZ0.25 for local contact at the asperities [20]. Under the present high-load conditions it is calculated that nZ0.75 (R2Z0.98), indicating the huge influence of plastic deformation at the sliding surface which is favourable for low friction. Temperatures at 20 mm beneath the sliding surface are given in Table 5. Due to the measuring depth s those values should be corrected with an amount DT calculated from a linear temperature model for conduction. As the heating source moves with low sliding velocity, it can be approximately assumed stationary in the centre of the sliding stroke. The amount of generated heat qZmp v, depends on the coefficient of friction m, the contact pressure p and the sliding velocity v. Therefore, DTZmpvs/k with the thermal conductivity of the steel counterface kZ 33 W/(mK). The theoretical maximum and mean rise in bulk temperature at the sliding interface is calculated according to Loewen and Shaw (analytical model [21]) and is evaluated in Table 5. Flash temperatures are evaluated from Jaeger’s model (experimental model [22]). It is concluded that bulk temperatures for PET/PTFE remain below the heat deflection temperature from Table 1. Measured and corrected temperatures represent an average value over the entire sliding stroke under steadystate, while the fluctuation in temperature as a function of the sliding distance is demonstrated in Fig. 4. Under running-in conditions the temperature increases gradually, with a discontinuity at the borders of the sliding stroke Table 5 Evaluation of measured, corrected and calculated steady-state temperatures for sliding of PET/PTFE
Fig. 3. Steady-state friction for PET/PTFE sliding against steel as a function of the sliding distance for different contact pressures.
Normal load (kN)
Measured temperatures (8C)
Modelled temperatures (8C)
T
DT
Tcorr
qf
q
qmax
190 380 600 1260 3380
21 23 26 26 35
4 6 6 7 9
25 29 32 34 44
26 28 29 30 35
33 36 38 40 51
36 39 41 44 57
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3.4. Wear and deformation under steady-state conditions
Fig. 4. Evolution of the contact temperature during (a) running-in and (b) steady-state for sliding of PET/PTFE under 3380 kN.
due to the stop/restart motion. The variation in steady-state temperature is limited to 1 8C at 20 mm beneath the sliding surface, while it fluctuates over 4 8C at the sliding surface. The increase in temperature towards the edges of the sliding stroke occurs because of small drop in sliding velocity upon reversal of the sliding direction and increase in friction.
Linear wear rates (mm/km) and volumetric wear rates (mm3/m) are given in Table 6, calculated from both weight and thickness reduction. For each normal load, the wear rates from thickness reduction are larger than from weight loss. The relative part of real material loss in the total thickness reduction gradually decreases with higher normal loads, indicating influences of additional thickness reduction by creep. This effect is more important for the bottom samples than for the top samples due to the difference in relative position of the polymer sample and counterface, and the additional load of the central sliding block on the polymer sample. Both the volumetric wear rates from thickness and weight measurements increase linearly for the top specimen, while a transition in wear rates is observed for the bottom samples towards less than linear increase at high loads. The specific wear rates (mm3/Nm) are not constant, but are 7.5!10K7 and 6.2!10K7 mm3/Nm on average for the respective top and bottom samples. The specific wear rates decrease with higher loads due to the transfer film formation as shown in Fig. 9. The wear characteristics, continuously measured by the vertical displacement transducers, are shown in Fig. 5(a) for the top specimen and Fig. 5(b) for the bottom specimen. The initial decrease towards negative wear values is attributed to thermal expansion of the test samples, caused by frictional heating. It can be demonstrated that the minimum value in vertical displacement occurs after a running-in sliding distance needed for stabilisation in temperature. The evolution of wear with sliding distance is different for the top and bottom specimen at 3880 kN. The initial plateau in vertical displacement measured for the top specimen is due to the accumulation of wear debris on the counterface and easy transfer film formation. The relative position between polymer sample and counterface is reversed for the bottom
Table 6 Wear rates for PET/PTFE under steady-state conditions Normal load (kN)
Top specimen 190 380 600 1260 3380 Bottom specimen 190 380 600 1260 3380
Weight Dg (g)
Dimensional
mm/km
mm3/m
Part of material loss in total thickness reduction (%)
0.02 0.06 0.12 0.19 0.53
0.007 0.013 0.033 0.049 0.129
0.153 0.292 0.750 1.102 2.912
92 86 80 83 78
0.02(5) 0.08(5) 0.10 0.17 0.27
0.009 0.017 0.028 0.041 0.066
0.191 0.413 0.625 0.985 1.484
76 76 65 74 56
Dh (mm)
Wear rate mm/km
mm3/m
mm3/Nm
0.60 1.68 3.10 5.12 13.38
0.006 0.011 0.026 0.041 0.100
0.141 0.252 0.598 0.916 2.269
7.4!10K7 6.6!10K7 9.9!10K7 7.3!10K7 6.2!10K7
0.62 2.08 2.12 4.07 4.87
0.007 0.015 0.018 0.032 0.037
0.146 0.313 0.409 0.728 0.826
7.7!10K7 8.2!10K7 6.8!10K7 5.8!10K7 2.3!10K7
Wear rate
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Fig. 5. Vertical displacement (wear characteristic) for PET/PTFE sliding against steel under different contact pressures. (a) Top specimen; (b) bottom specimen.
specimen with wear debris falling off the steel counterface. The wear characteristics (vertical displacement) are corrected for thermal expansion and creep as shown in Table 7 to be in accordance with post-mortem measurements of thickness reduction Dh. Thermal expansion is calculated with a coefficient aZ65!10K6 m/(mK) for PET/PTFE and a temperature rise between 15 8C and the corrected bulk temperature. As there is very little running-in wear under 190 kN for the top specimen, the minimum value in vertical displacement equals the entire thermal expansion. The minimum value for the bottom specimens correlate with the entire thermal expansion up to 1260 kN normal loads, indicating no severe running-in wear due to early transfer film formation. It is demonstrated that for each of the wear characteristics, the vertical displacement at
maximumZDhCcreep–thermal expansion (e.g., at 380 kN: 60 mm (Dh)C3 mm (creep) K18 mm (aDT)Z45 mm compared to 47 mm maximum vertical displacement, i.e. 96% correspondence). The immediate wear rates as calculated from the wear characteristics are shown in Fig. 6. For the top samples the curve initially decreases towards extremely low (zero) wear rates for 190, 380 and 600 kN normal loads, while the decrease to low or zero initial wear rates appears up the 1260 kN normal load for the bottom samples. This means that under those conditions the running-in wear is very low, in agreement with previous paragraph where it was mentioned that a minimum value in the vertical displacement curve occurs due to thermal expansion. No minimum displacement value is found for the top specimen under
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Table 7 Corrections of the wear characteristics (vertical displacement) for deformation Normal load (kN)
Top specimen
Bottom specimen
Thermal expansion (mm)
Vertical displacement at minimum (mm)
Vertical displacement at maximum (mm)
Creep 168 h (mm)
Thermal expansion (mm)
Vertical displacement at minimum (mm)
Vertical displacement at maximum (mm)
Creep 168 h (mm)
190 380 600 1260 3380
13 18 23 25 38
K13 K6 K4 No No
14 47 109 193 487
2 3 6 16 70
13 18 23 25 38
K13 K19 K23 K26 No
18 49 84 154 318
0 1 8 15 73
1260 and 3880 kN and for the bottom specimen under 3380 kN, due to the high running-in wear as indicated by the peak values in Fig. 6. 3.5. Transfer films The worn PET surfaces in Fig. 7 show huge wear as grooves parallel to the sliding direction (abrasive action), surface pits (tearing action) and shearing bands perpendicular to the sliding direction. On small-scale pin-on-disc tests, Branco et al. [1] found that abrasion and plastic deformation was the main failure mode of pure PET. The PET surfaces have higher toughness than PET/PTFE, represented by its high elongation at break and tensile yield strength. Therefore, they show rather ductile failure and possess higher resistance to shear than PET/PTFE, observed as surface deformation rather than fracture. Under low loads, the polymer surface is smooth with some abrasive wear grooves. Chevron marks perpendicular to the sliding direction are observed on the polymer surfaces from 1260 kN on, indicative of a stick-slip process [23]. The surface pits under 3380 kN indicate severe cohesive wear. Wear process in polymers is a combination of adhesive and cohesive failures leading to debris detachment. When the latter is prevailing, the interfacial bond between the polymer and the transferred film is stronger than the strength within the polymer bulk, as frequently observed [24]. The PET depositions are non-uniformly distributed over the sliding area as flake-like particles. Transfer films are always thick and discontinuous (lumpy transfer) with debris particles progressively growing around a fixed nucleus or containing agglomerated plasticized polymer, reducing the performance through vibrations and torsion within the sliding interface, contributing to stick-slip. Fig. 8 shows micrographics of the worn PET/PTFE surfaces at different normal loads. Deformation of the polymer bulk and free neck (5 mm above the specimen holder) is restricted compared to other polymers as e.g. polyacetal or polyamides. The brown deposits result from PTFE fillers that are worn out of the polymer bulk as wear debris. Deposited onto both the polymer sliding surface
and steel counterface, they are sheared under sliding. However, the type of deposits depends strongly on the applied normal load. Under low loads (190 kN) they consist of separate particles that are only locally sheared into relatively thick bands parallel to the sliding direction. The lack of surface plastication does not allow for favourable thin film formation spread over the entire polymer surface. From 380 kN on the polymer surface progressively plasticizes (observed by the glossy appearance with irregular shapes on the polymer surface) resulting in tearing
Fig. 6. Linear wear rates for (a) top specimen; (b) bottom specimen under (A) 190 kN, (B) 380 kN, (C) 600 kN, (D) 1260 kN, (E) 3380 kN.
P. Samyn et al. / Polymer Testing 24 (2005) 588–603
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Fig. 7. Microscopy of PET surfaces and steel counterfaces after sliding under different normal loads.
of separate polymer flakes out of the bulk and the formation of a more continuous transfer layer. Under 600 and 1260 kN the PTFE deposits become thinner and spread over a broader range of the sliding surface. Under 3380 kN this PTFE layer is completely molten and looses its load-carrying capacity with local destruction. Through progressive softening of the polymer surface, the indentation of the counterface asperities also increases, observed as grooves parallel to
the sliding direction. The roughness profiles of the polymer surfaces indicate wear grooves as a combination of abrasive wear and plastic deformation. The surface deformation increases under high loads, observed as more grooves with larger depths. Surface deformation manifests as polymer extrusion next to the grooves. As observed under 600 kN, the PTFE deposits are not preferentially into the roughness grooves, but occur on the tops of the roughness profile.
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Fig. 8. Microscopy and roughness profile of PET/PTFE surfaces after sliding under different contact pressures.
Deposits on the steel counterfaces are shown for different normal loads in Fig. 9 with an indication of the different components (AZPTFE, BZPET, CZsteel). The transfer film generally consists of a smooth overall-PTFE film
(brown colour), locally mixed with softened PET particles (white colour). Under 190 kN it only develops locally and is not homogeneous, with lumpy transfer of PTFE flakes. A huge amount of PET is observed at 600 kN, although not
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Fig. 9. Microscopy and roughness profile of steel surfaces after sliding against PET/PTFE under different contact pressures (AZPTFE, BZPET, CZsteel).
600
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adequately plasticized for the formation of a smooth film. This explains the high specific wear rate (9.9 10K7 mm3/ Nm) under 600 kN in Table 6. Under higher loads, both PTFE and PET are deposited on the entire steel surface and smoothen through plastication. This type of transfer is favourable for lower specific wear rates and low friction. The average steel counterface roughness is shown in Fig. 9. It uniformly decreases after sliding under low loads, representing the formation of a film that is mainly deposited into the original roughness valleys. As the roughness peaks are not covered, they still provide local polymer/steel contact with high friction. A slight increase in roughness is observed under 1260 and 3880 kN sliding, as polymer transfer becomes more important. Due to a higher amount of deposits, not only the valleys but also the peaks of the roughness profiles become covered with a transfer film, altering the adhesion mechanisms. The latter implies that the PET/PTFE film on the steel counterface is in contact with the polymer sample over the entire sliding area, corresponding to frequent stabilisation in friction (Fig. 4) and an initial plateau in vertical displacement (Fig. 6). It is after 2000 m sliding distance that the load carrying capacity of the film fails under 3380 kN and leads to sharp increase in vertical displacement for the top specimen.
4. Relation between large-scale tests and small-scale tests 4.1. Small-scale test methods As large-scale tests are expensive and require much preparation time, it is verified whether previous test results
under high load conditions can be obtained on small-scale test devices. Therefore, friction and wear properties of PET/PTFE are evaluated on two standardised test rigs (cylinder-on-plate ‘COP’ and block-on-ring ‘BOR’) providing, respectively, a reciprocating linear motion and a continuous rotating motion as shown in Fig. 10. They are frequently used in reference work as [25]. As the loading capacity of small-scale tribotesters is limited compared to the large-scale tester, an initial line contact is chosen for obtaining high Hertzian contact stresses under 100 and 200 N normal load and sliding velocity 0.3 m/s over 3 km sliding distance. Due to increase in contact area, the contact pressure drops to steady-state values, as calculated in Table 8. 4.2. Small-scale friction and wear results Results in Fig. 11a show the friction and temperatures under steady-state, indicating higher friction than obtained on large-scale tests. Variations in friction between different small-scale test geometries have already been discussed by Play et al. [26] and are mainly due to differences in contact area and wear debris circulation. For COP tests, debris removal towards the outer edges of the sliding stroke is more likely and prevents the formation of a continuous polymer film. In unidirectional sliding on BOR tests, the close fitting between the polymer sample and the steel ring leads to squeezing of wear particles and development of a transfer film. The role of PTFE added to PET was previously investigated by e.g. Kozma [27] on small-scale test samples and showed no essential improvement of the friction characteristics compared to pure PET, in contrast to the
Fig. 10. Small-scale tribotesters for comparative standard testing (dimensions in mm).
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Table 8 Small-scale contact conditions before and after testing Normal load (N)
COP 100 200 BOR 100 200
Test conditions Initial hertz contact
Steady-state
Elastic indentation (mm)
Contact area (mm2)
pmax (MPa)
pmean (MPa)
6 11
2.13 3.01
30 42
25 33
6 11
5.5 7.8
11 16
9 13
remarkable improvement of internal lubricated PET presently demonstrated on large-scale tests. Linear wear rates in Fig. 11(b) show lower values than obtained on large-scale tests. The less production of wear
Contact area (mm2) 9.27 8.46 18.1 23.2
p (MPa)
Temperature (8C)
11 24
33 35
5.5 8.6
37 38
debris under small-scale sliding causes less PTFE at the sliding interface as there was hardly detected any transfer film on small-scale counterfaces. The lubricating effects of internal additives is less favourably demonstrated on
Fig. 11. Small-scale test results for PET/PTFE on COP (cylinder-on-plate) and BOR (bloc-on-ring) tests. (a) Friction; (b) wear.
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small-scale than on large-scale sliding. The specific wear rates are constant in COP tests, while they slightly increase under higher loads in the case of BOR tests. It is only the latter trend that is in correlation with the general tendencies on large-scale tests. 4.3. Friction, wear and energy input Although, the test conformation has huge implications on the observed tribological phenomena, small-scale and large-scale test results can be compared in relation to the energy input. Fig. 12(a) shows the coefficients of friction and wear rates as a function of the mechanical energy input, given by FNv. Friction measured on small-scale under the highest normal load show agreement with the friction measured on large-scale tests under the mildest normal load. Although, the further decrease in large-scale friction shows
stronger dependency on the FNv parameter than estimated from small-scale tests. This is caused by plastic deformation of the sliding surface and the different amount of PTFE at the sliding interface. The large-scale wear rates under 8 MPa can be estimated from small-scale wear rates, although the further evolution of wear rates has a lower slope than extrapolated from BOR or COP tests. The total energy input (both mechanical and thermal) as represented by mFNv (friction power) or mpv (specific energy input) depends on the coefficient of friction (m), normal load (FN) or contact pressure (p) and sliding velocity (v). This energy results partially in an increase of the sliding temperature and partially in the creation of wear debris through fracture, both influencing the polymer wear rate. The plot in Fig. 12(b) shows that small-scale wear rates can be linearly extrapolated towards large-scale wear rates, although only indicating a rough estimation. Local transitions in the transfer
Fig. 12. Extrapolation from small-scale test results towards large-scale tests based on (a) mechanical energy input (FNv-factor); (b) thermal energy input (mFNv-factor) (% friction, B wear on small-scale and large-scale top samples, 6 wear on large-scale bottom samples, - specific wear rate).
P. Samyn et al. / Polymer Testing 24 (2005) 588–603
behaviour of large-scale tests cannot be estimated from small-scale tests, through plastication effects at the surface.
5. Conclusions Large-scale tribotests show high friction for pure polyethyleneterephtalate (PET), continuously increasing with higher loads. Sliding becomes unstable when the material is loaded above its yield strength, resulting in high wear rates that are characterised by ductile deformation (abrasive and pitting) and stick-slip (chevron lines). Therefore, flake-like polymer transfer is observed. Filled with polytetrafluoroethylene (PET/PTFE) friction and wear is much lower and more stable through the formation of a transfer film on both the polymer surface and the steel counterface. Under low loads, it consists of mixed PET/PTFE as separate particles. At 600 kN (55 MPa) and 3380 kN (150 MPa), the film covers the entire sliding area and causes an increase in surface roughness. The latter type of transfer causes immediate stabilisation in friction and an initial plateau-value in wear depth. As modelled interface temperatures remain below any PET transition temperature, sliding properties are mainly favoured by plastication of the polymer surface under overload rather than thermal softening. The vertical displacement as continuously measured can be successfully corrected for thermal expansion and creep to be in agreement with the final thickness reduction and material loss. Correlations with small-scale cylinder-onplate or bloc-on-ring friction and wear tests are difficult to make. The large-scale coefficients of friction are lower and wear rates are larger due to different transfer mechanisms and surface plastication effects. It is demonstrated that largescale testing of polymers is a useful tool for design of highly loaded sliding applications, providing unique information.
Acknowledgements The authors wish to thank Mr F. De Bruyne (Quadrant Engineering Plastic Products, Tielt, Belgium) who kindly supplied us with test materials. Mr R. Gillis assisted us in the technical realisation of this project. Present research is financially sponsored by Fund for Scientific Research of the Flemish Community (FWO) and the Ghent University Research Board (BOF). Thanks to the Belgian Welding Institute (BIL) for using their optical microscope.
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[3] N.G. McCrum, C.P. Buckley, C.B. Bucknall, Principles of Polymer Engineering, Oxford Science Publication, Oxford, 1988. [4] S. Neogi, S.A.R. Hashmi, N. Chand, Role of PET in improving wear properties of PP in dry sliding condition, Bull. Mater. Sci. 26 (2003) 579. [5] S.E. Franklin, Wear experiments with selected engineering polymers and polymer composites under dry reciprocating sliding conditions, Wear 251 (2001) 1591. [6] L.C. Seabra, A.M. Baptista, Tribological behaviour of food grade polymers against stainless steel in dry sliding and with sugar, Wear 253 (2002) 394. [7] M. Vermeulen, Wear research on large-scale test specimen, Wear 132 (1989) 287. [8] Quadrant EPP, General Purpose Plastic Products, Manual, 2002. [9] K. Tanaka, Y. Uchiyama, S. Tokooya, The mechanism of wear of polytetrafluoroethylene, Wear 23 (1973) 153. [10] B.J. Briscoe, L.H. Yao, in: K.C. Ludema (Ed.), Wear of Materials, ASME, New York, 1985. [11] J. Lancaster, R. Bramham, D. Play, R. Waghorne, Effects of amplitude on the wear of dry bearings containing PTFE, J. Lubr. Technol. 104 (1982) 559. [12] D. Gong, Q. Xue, H. Wang, Physical models of adhesive wear of polytetrafluoroethylene and its composites, Wear 147 (1991) 9. [13] C. Gao, D. Kuhlmann-Wilsdorf, D.D. Makel, Fundamentals of stick-slip, Wear 162–164 (1993) 1139. [14] Z. Rymuza, Z. Kusznierewicz, T. Solarski, M. Kwacz, S.A. Chizhik, A.V. Goldade, Static friction and adhesion in polymer–polymer microbearings, Wear 238 (2000) 56. [15] M. Vaziri, F.H. Scott, R.T. Spurr, Studies of the friction of polymeric materials, Wear 122 (1988) 313. [16] F. Van de Velde, P. De Baets, The friction and wear behaviour of polyamide 6 sliding against steel at low velocity under very high contact pressures, Wear 209 (1997) 106. [17] G.M. Bartenev, V.V. Lavrentev, Friction and Wear of Polymers, Elsevier, Amsterdam, 1981. [18] R.L. Fusaro, Self-lubricating polymer composites and polymer transfer film lubrication for space applications, Tribiol. Int. 23 (2) (1990) 105. [19] L. Zsidai, P. De Baets, P. Samyn, G. Kalacska, A.P. Van Peteghem, F. Van Parys, The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens, Wear 253 (2002) 673. [20] Y. Yamaguchi, Tribology of Plastic Materials, Elsevier, Amsterdam, 1990. [21] B. Bhushan, Principles and Applications of Tribology, WileyInterscience, New York, 1999. [22] C.J. Jaeger, Moving sources of heat and the temperature at sliding contacts, Proc. R. Soc. London 76 (1942) 1107. [23] A.I.G. Lloyd, R.E.J. Noel, The effect of counterface surface roughness on the wear of UHMWPE in water and oil-in-water emulsion, Tribiol. Int. 21 (2) (1988) 83. [24] I. Hutchings, Tribology: Friction and Wear of Engineering Materials, CRC Press, London, 1992. [25] Plastics Design Library, Fatigue and Tribological Properties of Plastics and Elastomers (PDL, 1995). [26] D.F. Play, B. Pruvost, Comparative dry wear of PTFE liner bearing in cylindrical and spherical geometry, J. Tribiol. 106 (1984) 185. [27] M. Kozma, in: Proceedings First Conference Mechanical Engineering, Gepeszet, 1998, pp. 157
Test Method
Sliding behaviour of pure polyester and polyester-PTFE filled bulk composites in overload conditions Pieter Samyn*, Jan Quintelier, Wouter Ost, Patrick De Baets, Gustaaf Schoukens Department Mechanical Construction and Production, Laboratory Soete, Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium Received 14 January 2005; accepted 25 February 2005
Abstract Polymers are frequently used in tribological applications because of their self-lubricating ability and loadability. However, most research on their friction and wear mechanisms is performed on small-scale test samples under relatively low normal loads. The present paper presents test results that are obtained on large-scale test samples under extremely high loads (190– 3380 kN, corresponding to 8–150 MPa), providing more accurate design information for construction of e.g. sea-lock or crane guides. Performance and failure modes in overload conditions are investigated. Pure polyethyleneterephtalate (PET) shows high friction and unstable wear due to stick-slip. Filled with polytetrafluouroethylene (PTFE), friction is reduced and becomes stable even when loaded above the yield strength. The plastic deformation of the sliding surface under overload conditions allows for the formation of a transfer film on the counterface, resulting in a decrease of specific wear rate at higher loads. A model is presented for correction of wear measurements that contains both real material loss and deformation due to creep and thermal expansion. For the latter, temperatures as measured from frictional heating are corrected and evaluated according to known temperature models. Compared to small-scale cylinder-on-plate and bloc-on-ring tests, friction coefficients measured on large-scale tests are much lower due to large polymer transfer, while wear rates are much higher. The availability of PTFE and favourable plastication of the sliding surface on large-scale tests result in the build up of a coherent lubrication transfer film. q 2005 Elsevier Ltd All rights reserved. Keywords: Polymers; Wear; Adhesion; Additives
1. Introduction Polyethylenetherephtalate, or PET, is a semi-crystalline thermoplastic with high dimensional stability and chemical resistance. Nowadays, it is most widely used as biaxially oriented film in soda bottles or as thermally sprayed coating for surface protection against corrosion and wear. The wear behaviour of unlubricated PET coatings in three conditions (as molded, thermally sprayed and quenched after thermal spraying) has been investigated by Branco et al. [1] using a small-scale pin-on-disc tester. Beake et al. [2] studied * Corresponding author. Tel.: C32 9 264 33 08; fax: C32 9 264 32 95. E-mail address: [email protected] (P. Samyn).
0142-9418/$ - see front matter q 2005 Elsevier Ltd All rights reserved. doi:10.1016/j.polymertesting.2005.02.012
the frictional and adhesive properties of commercial films. For a better understanding of the molecular processes leading to surface degradation of PET films, they used scanning force microscopy experiments for describing the surface deformation during a tip-induced wear process. As polymers generally posses good self-lubricating abilities through the formation of a polymer transfer film or ‘third body’, they are also used in industry as sliding materials in, e.g. gears, slides and bearings. While polyamides or polyethylene are most commonly used, they should be replaced by polyesters for obtaining higher temperature and fatigue resistance because of the stiffening action of the aromatic phenylene group. However, the sensitivity to brittle fracture due to notch and stress concentrations can restrict its applicability [3]. The favourable use of PET in
P. Samyn et al. / Polymer Testing 24 (2005) 588–603
dry sliding was recently demonstrated by Neogi et al. [4] on small-scale tests, who found gradually decreasing wear rates and lower friction when PET was added to polypropylene, while the limit of moderate wear increased towards higher contact stresses. For design purposes of machine elements, not only the surface characteristics but also the integrity of the polymer bulk properties become important, mainly determined by its load carrying capacity. As situations of dry sliding frequently occur under highly loaded conditions (e.g., storm barriers or sea-locks), creep and overloads can cause weakening and degradation in bulk properties with consequent alterations in friction and wear mechanisms. However, little information on the tribological behaviour of pure and filled PET bulk materials are available in literature and if so, all test results are obtained from traditional smallscale tests such as pin-on-disc or cylinder-on-plate tests, not taking into account overload conditions [5,6]. These methods are, however, only useful for a general classification of various materials, resulting in important errors when extrapolating towards the real working environment as, e.g. reported by [7]. Real criteria for material selection in a given application require the tribological characterisation of polymer parts under conditions that are closely related to its practical functionality. Mainly for tribotesting of polymer materials under high loads, the influence of creep and deformation and the moveability of wear debris into the contact zone should be simulated effectively. Presently, friction and wear of extruded PET bulk materials are investigated on large-scale sliding samples with an apparent contact surface of 22,500 mm2 under loads ranging from 8 to 150 MPa. Therefore, this work is believed to provide primarily information about the practical use and failure of polyesters in tribology. The friction and wear behaviour observed is further discussed in relation to deformation and thermal effects. As the formation and aspect of a molten or plasticized polymer transfer film strongly governs the tribological performance, the sliding surfaces are microscopically analysed. Surface depositions on the counterface result in lower roughness with reduced friction and polymer wear, therefore, only smooth transfer layers are favourable. Additions of internal solid lubricants, as e.g. PTFE with its lamellar structure, improve both friction and wear characteristics through the formation of a uniform film with proper thickness, although they weaken the creep resistance and compressive strength of the bulk polymer. Research towards optimum filler content indicated a working range for PTFE in PET of between 10 and 20 vol% [8].
2. Experimental large-scale testing 2.1. Test equipment The tribotester for high-load testing of large specimen is shown in Fig. 1, where two sliding couples (each consisting
589
of polymer test specimens (3) and counterface plates (2)) are tested in a flat-on-flat reciprocating motion. The polymer wear samples are 150!150!20 mm3 and the steel counterfaces size 410!200!20 mm3. Polymer specimens are mounted in two rectangular specimen holders (4), inserted on top and on the bottom of the machine into a rigid horizontal structure. The counterface plates are positioned on both sides of the central sliding block (1) that is pushed by two horizontal jacks at the left and right. The vertical load is exerted by the vertical jack in the column. Both horizontal and vertical forces are measured with dynamometers at positions (5) and (6). The sliding velocity is controlled by a hydraulic circuit and is almost constant over the sliding stroke, except near the edges where the sliding direction reverses. The coefficient of friction m is calculated by formula (1) with Fl the horizontal force at the left and Fr the horizontal force at the right of the central sliding block, Fn the normal force and the factor 1/2 resulting from the two sliding pairs on top and on the bottom of the test machine. As such, m represents an average friction coefficient between two test pairs. The modulus results from the negative forces occurring on reversal of the sliding direction. 1 Fl K Fr mZ (1) 2 Fn The vertical displacement of the top and bottom polymer specimens towards their counterface is continuously measured by two displacement transducers. As measurements are influenced by deformation under creep (enhancing the wear signal) or thermal expansion (counteracting the wear signal), the real wear (i.e., material loss) is determined by weighing the polymer sample before and after testing and is compared to thickness reduction. The sliding temperatures of the counterfaces are continuously measured at 20 mm beneath the contact surface by means of a K-type thermocouple. 2.2. Test materials Pure and polytetrafluoroethylene (PTFE) filled PET samples were machined from commercial extruded plates (Quadrant EPP) and have a surface finish of milling grooves perpendicular to the sliding direction. Pure PET has a white colour, while its PTFE-filled variant is grey coloured, consisting of a homogeneous dispersion of PTFE powder as an internal solid lubricant into the polymer bulk. Mechanical properties are listed in Table 1 [8]. The limiting pressurevelocity for pure PET is 0.10 MPa m/s, but filled with PTFE it increases towards 0.21 MPa m/s. The beneficial effect of PTFE on low friction has generally been reported by Tanaka et al. [9], Briscoe [10], Lancaster [11] and Gong [12], and results from its lamellar structure with low shear strength leading to easy transfer behaviour. However, pure PTFE shows extremely high wear rates and should be blended into
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Fig. 1. Schematical view on the large-scale tribotester. (1) Central sliding bloc; (2) steel counterface plate; (3) PET or PET/PTFE polymer specimen; (4) fixed specimen holder.
a matrix providing higher strength. PET/PTFE is industrially well-known for its good dimensional stability. The counterface plates consist of cold rolled steel 42 CrMo4 (DIN 1.7225) with hardness 330 HV, yield strength ReZ765 N/mm2, tensile strength RmZ980–1180 N/mm2 and chemical composition (in wt%): CZ0.38–0.45, Si!0.40, MnZ0.60–0.90, P!0.035, S!0.030, CrZ0.90–1.20, MoZ0.15–0.30. Before each test, the surfaces were roughly ground to an average surface roughness RaZ4 mm with machining grooves parallel to the sliding direction. Afterwards, the surfaces were ground to a lower roughness RaZ0.20 mm measured parallel to the sliding direction and RaZ0.60 mm perpendicular to the sliding direction. Roughness was measured on a 2 dimensional Perthen 5 SP according to DIN 4768. Prior to each test, polymer and steel surfaces were cleaned with acetone.
the sliding interface arises only from frictional heating. With a fixed sliding velocity of 5 mm/s, five different normal loads between 190 and 3380 kN were applied, corresponding to 8, 16, 25, 55 and 150 MPa. The pv-parameters range between 0.04 and 0.75 MPa m/s, which is above the generally accepted limit [8]. One single sliding stroke comprises 230 mm. Two sliding strokes are called one sliding cycle. Frictional stability during running-in was investigated by short-time sliding tests over 10 sliding cycles, corresponding to 4.6 m total sliding distance on counterfaces with RaZ4 and 0.2 mm. Long-time sliding tests (approx. 4 km or 168 h testing time depending on the observed wear) were performed on low roughness counterfaces for determination Table 1 Test materials
2.3. Large-scale test procedure The test conditions are given in Table 2. Large-scale sliding tests were performed under relative humidity RHZ50G5% and ambient temperature of 23G2 8C, while the counterface temperature was controlled by internal cooling of the central sliding block with a constant water flow at 15 8C. The rise in contact temperature at
Density (g/cm3) Tensile yield strength (MPa) Tensile strain at break (%) Tensile modulus (MPa) Heat deflection temperature (8C) Melting temperature (8C) Shear strength (MPa)
PET
PET/PTFE
1.39 90 15 3700 75 255 55
1.44 76 7 3450 75 255 59
P. Samyn et al. / Polymer Testing 24 (2005) 588–603 Table 2 Large-scale test conditions Test parameter
Large-scale test
Dimensions polymer sample Dimensions counterface plate Counterface roughness Ra Single sliding stroke Sliding velocity Normal load Contact pressure Ambient humidity/temperature
150!150!20 mm3 410!200!20 mm3 4, 0.2 mm 230 mm 0.005 m/s 190, 380, 560, 1260, 3380 kN 8, 16, 25, 55 150 MPa 60%/23 8C Short-time Long-time tests tests 4.6 m approx. 4 km
Total sliding distance
of the material’s wear lifetime. Test results of friction, wear and bulk temperatures on large-scale were calculated as average values of two separate test runs for each contact pressure.
3. Results and discussion 3.1. Creep and deformation under static loading Prior to dynamic sliding tests, the dimensional stability of PET and PET/PTFE mounted on top and bottom was determined specifically for the present test layout under static loading against steel with RaZ0.2 mm. Samples were loaded at 10 MPa/min, continuously measuring the vertical indentation as displacement between the polymer samples and the steel counterfaces. Immediately after the required load level was attained, the initial elastic indentation was recorded and compared to the theoretical normal deformation of PET/PTFE calculated from the modulus of elasticity (Table 3): for a contact pressure p, the indentation DlZ(p/E!20 mm). Creep was measured after 24 and 168 h constant loading.
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For the first three loading steps (190, 380 and 600 kN) the measured elastic indentation is in good agreement with calculated values for both PET and PET/PTFE (error 6%). The higher modulus of elasticity for pure PET results in lower elastic indentation. At loads of 1260 and 3380 kN, the experimental normal deformation is smaller than the calculated normal deformation, due to clamping of the polymer samples into their holders. The retaining action of the sample holder restricts the lateral deformation of the entire polymer bulk, concentrating the deformation of the polymer samples in the free neck 5 mm outside the holder. The in situ stiffness of the polymer is higher than estimated from the E modulus measured under free deformation: between 1260 and 3380 kN the bulk modulus attains 5750 MPa for PET and 5000 MPa for PET/PTFE. The total creep after 168 h loading is very small for PET/PTFE, reflecting its good dimensional stability. The average creep rate under steady-state equals 0.024 mm/h at 1260 kN and 0.070 mm/h at 3380 kN, or 1% compressive strain occurs after 8000 h loading under 1260 kN and after 2000 h loading under 3380 kN. Pure PET has significantly higher creep rates (0.028 mm/h at 1260 kN and 0.083 mm/h at 3380 kN) than PET/PTFE. Through the lower elastic deformation of PET during initially loading, the lateral expansion of the polymer in its holder is not as large as for PET/PTFE and proceeds over longer loading times. Therefore, creep is less restricted by the retaining action of the holder. The difference in deformation between top and bottom specimens is attributed to the machining tolerances on the sample width (150G0.5 mm). 3.2. Friction and wear under running-in conditions The selection of a proper bearing material is not only based on low coefficients of friction, but also stable friction is required under high stresses. Especially when short sliding distances and several stop/restart conditions are considered, stick-slip and high static friction should be
Table 3 Deformation of large-scale PET and PET/PTFE samples under static loading Polymer
PET
PET/PTFE
Normal load (kN)
190 380 600 1260 3380 190 380 600 1260 3380
Normal deformation (mm)
Creep (mm)
Top
Bottom
Calculated
47 89 127 228 558 57 92 155 282 666
42 85 121 237 581 45 79 144 281 691
45 86 135 297 811 48 97 145 319 870
Top
Bottom
24 h
168 h
24 h
168 h
1 3 6 15 65 1 2 4 13 60
1 1 3 4 12 1 1 2 3 10
1 1 7 17 67 0 1 4 11 63
0 1 2 6 11 0 0 4 4 10
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Fig. 2. Friction-displacement characteristics for (a) PET and (b) PET/PTFE sliding against steel under 190 kN.
avoided [13]. The frictional behaviour of PET and PET/ PTFE at the start and at reversal of the sliding direction is, therefore, studied in more detail, revealing frictional instabilities caused by difference in static and dynamic friction and/or vibrations. Recently, Rymuza et al. [14]
reported on the static friction and adhesion in polymerpolymer contact, but little data is known on the static friction of PET/steel contacts (only Vaziri et al. [15]). The coefficient of friction is plotted as a function of the sliding distance of the central sliding bloc in Fig. 2(a) for PET and in Fig. 2(b) for PET/PTFE under 190 kN. The sliding block is initially at the position xZ0 and starts sliding towards positive x-values. At the end of the sliding stroke (xZC108 mm) the direction reverses towards negative x-values until xZK122 mm. Static friction is measured at the initial start (ms1) and at the reversals of the sliding motion (ms,min and ms,max), while the dynamic friction md,max or md,min is measured at the center of the sliding stroke, as summarized in Table 4 for different normal loads and counterface roughness. Pure PET shows unstable friction, continuously increasing with incremental sliding distance. The horizontal pistons vibrate because of heavy stick-slip observed as irregularities on the friction–displacement characteristics. Loud noise during sliding forced the tests to be stopped prematurely under high contact pressures (1260 kN only contains one sliding cycle). The static friction at the reversals of the sliding stroke is significantly larger than the dynamic friction. Other polymers, e.g. polyamides, are also prone to high static friction and stick-slip in dry sliding [16]. Friction exceeds the capacity of the test rig at 3380 kN normal load, which is above the material’s yield strength. The observed tendency of increasing friction is in contrast with the generally accepted model of decreasing friction at higher normal loads [17]. Under 1260 kN, the horizontal surface stress equals 15 MPa, which is 27% of the shear strength. Filled with PTFE, friction is reduced and becomes stable during the test: after the first sliding cycle it attains a fixed value that lowers with increase in contact pressure. PET/PTFE even performs well when loaded above its yield strength, allowing for plastic deformation of the sliding
Table 4 Static and dynamic coefficients of friction during running-in Polymer
PET
PET/PTFE
Normal load (kN) 600 1260 190 380 600 1260 3380 600 1260 190 380 600 1260 3380
Static friction ms1
Dynamic friction msn,min
0.11 0.12 0.12 0.15 0.12 0.13 0.17 0.18 0.21 0.22 0.21 0.23 Overload (stick-slip) 0.11 0.11 0.09 0.09 0.12 0.10 0.11 0.09 0.09 0.08 0.07 0.05 0.04 0.04
msn,max
md,min
md,max
0.29 0.35 0.16 0.21 0.24 0.26
0.16 0.18 0.11 0.16 0.19 0.20
0.25 0.28 0.14 0.18 0.21 0.23
0.12 0.10 0.12 0.10 0.09 0.06 0.04
0.10 0.09 0.10 0.09 0.07 0.05 0.03
0.11 0.10 0.11 0.09 0.08 0.06 0.03
Running-in wear rate (mm/km)
Counter face
9.4 17.7 1.7 4.8 6.9 7.6 21 2.9 4.6 0.9 2.2 2.2 2.4 4.1
RaZ4 mm RaZ0.20 mm
RaZ4 mm RaZ0.20 mm
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surface and extremely low friction. Under 1260 kN the horizontal stress equals 3 MPa and under 3380 kN it is 6 MPa, which is, respectively, 5 and 10% of the PET/PTFE shear strength. The shear strength of PTFE is much lower (5 MPa) allowing for low friction and easy shear when transferred onto the counterface. The static friction becomes equal to dynamic friction in the presence of PTFE, excluding stick-slip. When the counterface roughness increases, friction of pure PET is further destabilised with worse increase in friction coefficient during the first sliding cycles. Friction for PET/PTFE is also higher when sliding against higher roughness counterface. As it is generally accepted that friction contains both adhesion and deformation, the latter becoming more important in sliding against rough surfaces due to the ploughing action of the counterface asperities. Linear wear rates, including both real material loss and deformation at running-in, indicate that PTFE additions reduce running-in wear and cause the amount of wear nearly independent of the normal load when loaded below its yield strength. The wear rates at 3380 kN mainly represents deformation of the sliding surface. Higher counterface roughness reveals higher wear rates with consequent generation of PTFE debris, although they are not favourable for lower friction. This contradiction indicates that under the present conditions of high contact pressures an optimum film thickness is established as generally described by [18], although not able to resist the ploughing interaction of counterface asperities with consequently high friction and wear. 3.3. Friction and temperature under steady-state conditions PET/PTFE samples were slid for 168 h (1 week) against steel counterfaces with average roughness RaZ0.20 mm, investigating the evolution in friction as shown in Fig. 3. The coefficient of friction after 5 m of sliding corresponds to the previously described friction under running-in conditions. Steady-state conditions are markably different:
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under contact pressures of 190–600 kN there is an increase in friction towards steady-state, while for contact pressures of 1260 and 3380 kN it stabilises more frequently or decreases towards a steady-state. These trends were also observed on preliminary small-scale tests performed by Zsidai et al. [19]: under 100 N normal load, the coefficient of friction rises from 0.2 towards 0.3 on smooth surfaces while it frequently stabilises at 0.2 on rough surfaces. Stable friction is attributed to the formation of a thin transfer film on the counterface, as shown in Section 3.5. Literature models suggest that with increasing normal load FN the coefficient of friction decreases according to mZK. FKn N with K constant and the parameter depending on the amount of plastic or elastic deformation. In the case of full contact through plastic deformation, nZ1, while nZ0.25 for local contact at the asperities [20]. Under the present high-load conditions it is calculated that nZ0.75 (R2Z0.98), indicating the huge influence of plastic deformation at the sliding surface which is favourable for low friction. Temperatures at 20 mm beneath the sliding surface are given in Table 5. Due to the measuring depth s those values should be corrected with an amount DT calculated from a linear temperature model for conduction. As the heating source moves with low sliding velocity, it can be approximately assumed stationary in the centre of the sliding stroke. The amount of generated heat qZmp v, depends on the coefficient of friction m, the contact pressure p and the sliding velocity v. Therefore, DTZmpvs/k with the thermal conductivity of the steel counterface kZ 33 W/(mK). The theoretical maximum and mean rise in bulk temperature at the sliding interface is calculated according to Loewen and Shaw (analytical model [21]) and is evaluated in Table 5. Flash temperatures are evaluated from Jaeger’s model (experimental model [22]). It is concluded that bulk temperatures for PET/PTFE remain below the heat deflection temperature from Table 1. Measured and corrected temperatures represent an average value over the entire sliding stroke under steadystate, while the fluctuation in temperature as a function of the sliding distance is demonstrated in Fig. 4. Under running-in conditions the temperature increases gradually, with a discontinuity at the borders of the sliding stroke Table 5 Evaluation of measured, corrected and calculated steady-state temperatures for sliding of PET/PTFE
Fig. 3. Steady-state friction for PET/PTFE sliding against steel as a function of the sliding distance for different contact pressures.
Normal load (kN)
Measured temperatures (8C)
Modelled temperatures (8C)
T
DT
Tcorr
qf
q
qmax
190 380 600 1260 3380
21 23 26 26 35
4 6 6 7 9
25 29 32 34 44
26 28 29 30 35
33 36 38 40 51
36 39 41 44 57
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3.4. Wear and deformation under steady-state conditions
Fig. 4. Evolution of the contact temperature during (a) running-in and (b) steady-state for sliding of PET/PTFE under 3380 kN.
due to the stop/restart motion. The variation in steady-state temperature is limited to 1 8C at 20 mm beneath the sliding surface, while it fluctuates over 4 8C at the sliding surface. The increase in temperature towards the edges of the sliding stroke occurs because of small drop in sliding velocity upon reversal of the sliding direction and increase in friction.
Linear wear rates (mm/km) and volumetric wear rates (mm3/m) are given in Table 6, calculated from both weight and thickness reduction. For each normal load, the wear rates from thickness reduction are larger than from weight loss. The relative part of real material loss in the total thickness reduction gradually decreases with higher normal loads, indicating influences of additional thickness reduction by creep. This effect is more important for the bottom samples than for the top samples due to the difference in relative position of the polymer sample and counterface, and the additional load of the central sliding block on the polymer sample. Both the volumetric wear rates from thickness and weight measurements increase linearly for the top specimen, while a transition in wear rates is observed for the bottom samples towards less than linear increase at high loads. The specific wear rates (mm3/Nm) are not constant, but are 7.5!10K7 and 6.2!10K7 mm3/Nm on average for the respective top and bottom samples. The specific wear rates decrease with higher loads due to the transfer film formation as shown in Fig. 9. The wear characteristics, continuously measured by the vertical displacement transducers, are shown in Fig. 5(a) for the top specimen and Fig. 5(b) for the bottom specimen. The initial decrease towards negative wear values is attributed to thermal expansion of the test samples, caused by frictional heating. It can be demonstrated that the minimum value in vertical displacement occurs after a running-in sliding distance needed for stabilisation in temperature. The evolution of wear with sliding distance is different for the top and bottom specimen at 3880 kN. The initial plateau in vertical displacement measured for the top specimen is due to the accumulation of wear debris on the counterface and easy transfer film formation. The relative position between polymer sample and counterface is reversed for the bottom
Table 6 Wear rates for PET/PTFE under steady-state conditions Normal load (kN)
Top specimen 190 380 600 1260 3380 Bottom specimen 190 380 600 1260 3380
Weight Dg (g)
Dimensional
mm/km
mm3/m
Part of material loss in total thickness reduction (%)
0.02 0.06 0.12 0.19 0.53
0.007 0.013 0.033 0.049 0.129
0.153 0.292 0.750 1.102 2.912
92 86 80 83 78
0.02(5) 0.08(5) 0.10 0.17 0.27
0.009 0.017 0.028 0.041 0.066
0.191 0.413 0.625 0.985 1.484
76 76 65 74 56
Dh (mm)
Wear rate mm/km
mm3/m
mm3/Nm
0.60 1.68 3.10 5.12 13.38
0.006 0.011 0.026 0.041 0.100
0.141 0.252 0.598 0.916 2.269
7.4!10K7 6.6!10K7 9.9!10K7 7.3!10K7 6.2!10K7
0.62 2.08 2.12 4.07 4.87
0.007 0.015 0.018 0.032 0.037
0.146 0.313 0.409 0.728 0.826
7.7!10K7 8.2!10K7 6.8!10K7 5.8!10K7 2.3!10K7
Wear rate
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Fig. 5. Vertical displacement (wear characteristic) for PET/PTFE sliding against steel under different contact pressures. (a) Top specimen; (b) bottom specimen.
specimen with wear debris falling off the steel counterface. The wear characteristics (vertical displacement) are corrected for thermal expansion and creep as shown in Table 7 to be in accordance with post-mortem measurements of thickness reduction Dh. Thermal expansion is calculated with a coefficient aZ65!10K6 m/(mK) for PET/PTFE and a temperature rise between 15 8C and the corrected bulk temperature. As there is very little running-in wear under 190 kN for the top specimen, the minimum value in vertical displacement equals the entire thermal expansion. The minimum value for the bottom specimens correlate with the entire thermal expansion up to 1260 kN normal loads, indicating no severe running-in wear due to early transfer film formation. It is demonstrated that for each of the wear characteristics, the vertical displacement at
maximumZDhCcreep–thermal expansion (e.g., at 380 kN: 60 mm (Dh)C3 mm (creep) K18 mm (aDT)Z45 mm compared to 47 mm maximum vertical displacement, i.e. 96% correspondence). The immediate wear rates as calculated from the wear characteristics are shown in Fig. 6. For the top samples the curve initially decreases towards extremely low (zero) wear rates for 190, 380 and 600 kN normal loads, while the decrease to low or zero initial wear rates appears up the 1260 kN normal load for the bottom samples. This means that under those conditions the running-in wear is very low, in agreement with previous paragraph where it was mentioned that a minimum value in the vertical displacement curve occurs due to thermal expansion. No minimum displacement value is found for the top specimen under
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Table 7 Corrections of the wear characteristics (vertical displacement) for deformation Normal load (kN)
Top specimen
Bottom specimen
Thermal expansion (mm)
Vertical displacement at minimum (mm)
Vertical displacement at maximum (mm)
Creep 168 h (mm)
Thermal expansion (mm)
Vertical displacement at minimum (mm)
Vertical displacement at maximum (mm)
Creep 168 h (mm)
190 380 600 1260 3380
13 18 23 25 38
K13 K6 K4 No No
14 47 109 193 487
2 3 6 16 70
13 18 23 25 38
K13 K19 K23 K26 No
18 49 84 154 318
0 1 8 15 73
1260 and 3880 kN and for the bottom specimen under 3380 kN, due to the high running-in wear as indicated by the peak values in Fig. 6. 3.5. Transfer films The worn PET surfaces in Fig. 7 show huge wear as grooves parallel to the sliding direction (abrasive action), surface pits (tearing action) and shearing bands perpendicular to the sliding direction. On small-scale pin-on-disc tests, Branco et al. [1] found that abrasion and plastic deformation was the main failure mode of pure PET. The PET surfaces have higher toughness than PET/PTFE, represented by its high elongation at break and tensile yield strength. Therefore, they show rather ductile failure and possess higher resistance to shear than PET/PTFE, observed as surface deformation rather than fracture. Under low loads, the polymer surface is smooth with some abrasive wear grooves. Chevron marks perpendicular to the sliding direction are observed on the polymer surfaces from 1260 kN on, indicative of a stick-slip process [23]. The surface pits under 3380 kN indicate severe cohesive wear. Wear process in polymers is a combination of adhesive and cohesive failures leading to debris detachment. When the latter is prevailing, the interfacial bond between the polymer and the transferred film is stronger than the strength within the polymer bulk, as frequently observed [24]. The PET depositions are non-uniformly distributed over the sliding area as flake-like particles. Transfer films are always thick and discontinuous (lumpy transfer) with debris particles progressively growing around a fixed nucleus or containing agglomerated plasticized polymer, reducing the performance through vibrations and torsion within the sliding interface, contributing to stick-slip. Fig. 8 shows micrographics of the worn PET/PTFE surfaces at different normal loads. Deformation of the polymer bulk and free neck (5 mm above the specimen holder) is restricted compared to other polymers as e.g. polyacetal or polyamides. The brown deposits result from PTFE fillers that are worn out of the polymer bulk as wear debris. Deposited onto both the polymer sliding surface
and steel counterface, they are sheared under sliding. However, the type of deposits depends strongly on the applied normal load. Under low loads (190 kN) they consist of separate particles that are only locally sheared into relatively thick bands parallel to the sliding direction. The lack of surface plastication does not allow for favourable thin film formation spread over the entire polymer surface. From 380 kN on the polymer surface progressively plasticizes (observed by the glossy appearance with irregular shapes on the polymer surface) resulting in tearing
Fig. 6. Linear wear rates for (a) top specimen; (b) bottom specimen under (A) 190 kN, (B) 380 kN, (C) 600 kN, (D) 1260 kN, (E) 3380 kN.
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Fig. 7. Microscopy of PET surfaces and steel counterfaces after sliding under different normal loads.
of separate polymer flakes out of the bulk and the formation of a more continuous transfer layer. Under 600 and 1260 kN the PTFE deposits become thinner and spread over a broader range of the sliding surface. Under 3380 kN this PTFE layer is completely molten and looses its load-carrying capacity with local destruction. Through progressive softening of the polymer surface, the indentation of the counterface asperities also increases, observed as grooves parallel to
the sliding direction. The roughness profiles of the polymer surfaces indicate wear grooves as a combination of abrasive wear and plastic deformation. The surface deformation increases under high loads, observed as more grooves with larger depths. Surface deformation manifests as polymer extrusion next to the grooves. As observed under 600 kN, the PTFE deposits are not preferentially into the roughness grooves, but occur on the tops of the roughness profile.
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Fig. 8. Microscopy and roughness profile of PET/PTFE surfaces after sliding under different contact pressures.
Deposits on the steel counterfaces are shown for different normal loads in Fig. 9 with an indication of the different components (AZPTFE, BZPET, CZsteel). The transfer film generally consists of a smooth overall-PTFE film
(brown colour), locally mixed with softened PET particles (white colour). Under 190 kN it only develops locally and is not homogeneous, with lumpy transfer of PTFE flakes. A huge amount of PET is observed at 600 kN, although not
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Fig. 9. Microscopy and roughness profile of steel surfaces after sliding against PET/PTFE under different contact pressures (AZPTFE, BZPET, CZsteel).
600
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adequately plasticized for the formation of a smooth film. This explains the high specific wear rate (9.9 10K7 mm3/ Nm) under 600 kN in Table 6. Under higher loads, both PTFE and PET are deposited on the entire steel surface and smoothen through plastication. This type of transfer is favourable for lower specific wear rates and low friction. The average steel counterface roughness is shown in Fig. 9. It uniformly decreases after sliding under low loads, representing the formation of a film that is mainly deposited into the original roughness valleys. As the roughness peaks are not covered, they still provide local polymer/steel contact with high friction. A slight increase in roughness is observed under 1260 and 3880 kN sliding, as polymer transfer becomes more important. Due to a higher amount of deposits, not only the valleys but also the peaks of the roughness profiles become covered with a transfer film, altering the adhesion mechanisms. The latter implies that the PET/PTFE film on the steel counterface is in contact with the polymer sample over the entire sliding area, corresponding to frequent stabilisation in friction (Fig. 4) and an initial plateau in vertical displacement (Fig. 6). It is after 2000 m sliding distance that the load carrying capacity of the film fails under 3380 kN and leads to sharp increase in vertical displacement for the top specimen.
4. Relation between large-scale tests and small-scale tests 4.1. Small-scale test methods As large-scale tests are expensive and require much preparation time, it is verified whether previous test results
under high load conditions can be obtained on small-scale test devices. Therefore, friction and wear properties of PET/PTFE are evaluated on two standardised test rigs (cylinder-on-plate ‘COP’ and block-on-ring ‘BOR’) providing, respectively, a reciprocating linear motion and a continuous rotating motion as shown in Fig. 10. They are frequently used in reference work as [25]. As the loading capacity of small-scale tribotesters is limited compared to the large-scale tester, an initial line contact is chosen for obtaining high Hertzian contact stresses under 100 and 200 N normal load and sliding velocity 0.3 m/s over 3 km sliding distance. Due to increase in contact area, the contact pressure drops to steady-state values, as calculated in Table 8. 4.2. Small-scale friction and wear results Results in Fig. 11a show the friction and temperatures under steady-state, indicating higher friction than obtained on large-scale tests. Variations in friction between different small-scale test geometries have already been discussed by Play et al. [26] and are mainly due to differences in contact area and wear debris circulation. For COP tests, debris removal towards the outer edges of the sliding stroke is more likely and prevents the formation of a continuous polymer film. In unidirectional sliding on BOR tests, the close fitting between the polymer sample and the steel ring leads to squeezing of wear particles and development of a transfer film. The role of PTFE added to PET was previously investigated by e.g. Kozma [27] on small-scale test samples and showed no essential improvement of the friction characteristics compared to pure PET, in contrast to the
Fig. 10. Small-scale tribotesters for comparative standard testing (dimensions in mm).
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Table 8 Small-scale contact conditions before and after testing Normal load (N)
COP 100 200 BOR 100 200
Test conditions Initial hertz contact
Steady-state
Elastic indentation (mm)
Contact area (mm2)
pmax (MPa)
pmean (MPa)
6 11
2.13 3.01
30 42
25 33
6 11
5.5 7.8
11 16
9 13
remarkable improvement of internal lubricated PET presently demonstrated on large-scale tests. Linear wear rates in Fig. 11(b) show lower values than obtained on large-scale tests. The less production of wear
Contact area (mm2) 9.27 8.46 18.1 23.2
p (MPa)
Temperature (8C)
11 24
33 35
5.5 8.6
37 38
debris under small-scale sliding causes less PTFE at the sliding interface as there was hardly detected any transfer film on small-scale counterfaces. The lubricating effects of internal additives is less favourably demonstrated on
Fig. 11. Small-scale test results for PET/PTFE on COP (cylinder-on-plate) and BOR (bloc-on-ring) tests. (a) Friction; (b) wear.
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small-scale than on large-scale sliding. The specific wear rates are constant in COP tests, while they slightly increase under higher loads in the case of BOR tests. It is only the latter trend that is in correlation with the general tendencies on large-scale tests. 4.3. Friction, wear and energy input Although, the test conformation has huge implications on the observed tribological phenomena, small-scale and large-scale test results can be compared in relation to the energy input. Fig. 12(a) shows the coefficients of friction and wear rates as a function of the mechanical energy input, given by FNv. Friction measured on small-scale under the highest normal load show agreement with the friction measured on large-scale tests under the mildest normal load. Although, the further decrease in large-scale friction shows
stronger dependency on the FNv parameter than estimated from small-scale tests. This is caused by plastic deformation of the sliding surface and the different amount of PTFE at the sliding interface. The large-scale wear rates under 8 MPa can be estimated from small-scale wear rates, although the further evolution of wear rates has a lower slope than extrapolated from BOR or COP tests. The total energy input (both mechanical and thermal) as represented by mFNv (friction power) or mpv (specific energy input) depends on the coefficient of friction (m), normal load (FN) or contact pressure (p) and sliding velocity (v). This energy results partially in an increase of the sliding temperature and partially in the creation of wear debris through fracture, both influencing the polymer wear rate. The plot in Fig. 12(b) shows that small-scale wear rates can be linearly extrapolated towards large-scale wear rates, although only indicating a rough estimation. Local transitions in the transfer
Fig. 12. Extrapolation from small-scale test results towards large-scale tests based on (a) mechanical energy input (FNv-factor); (b) thermal energy input (mFNv-factor) (% friction, B wear on small-scale and large-scale top samples, 6 wear on large-scale bottom samples, - specific wear rate).
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behaviour of large-scale tests cannot be estimated from small-scale tests, through plastication effects at the surface.
5. Conclusions Large-scale tribotests show high friction for pure polyethyleneterephtalate (PET), continuously increasing with higher loads. Sliding becomes unstable when the material is loaded above its yield strength, resulting in high wear rates that are characterised by ductile deformation (abrasive and pitting) and stick-slip (chevron lines). Therefore, flake-like polymer transfer is observed. Filled with polytetrafluoroethylene (PET/PTFE) friction and wear is much lower and more stable through the formation of a transfer film on both the polymer surface and the steel counterface. Under low loads, it consists of mixed PET/PTFE as separate particles. At 600 kN (55 MPa) and 3380 kN (150 MPa), the film covers the entire sliding area and causes an increase in surface roughness. The latter type of transfer causes immediate stabilisation in friction and an initial plateau-value in wear depth. As modelled interface temperatures remain below any PET transition temperature, sliding properties are mainly favoured by plastication of the polymer surface under overload rather than thermal softening. The vertical displacement as continuously measured can be successfully corrected for thermal expansion and creep to be in agreement with the final thickness reduction and material loss. Correlations with small-scale cylinder-onplate or bloc-on-ring friction and wear tests are difficult to make. The large-scale coefficients of friction are lower and wear rates are larger due to different transfer mechanisms and surface plastication effects. It is demonstrated that largescale testing of polymers is a useful tool for design of highly loaded sliding applications, providing unique information.
Acknowledgements The authors wish to thank Mr F. De Bruyne (Quadrant Engineering Plastic Products, Tielt, Belgium) who kindly supplied us with test materials. Mr R. Gillis assisted us in the technical realisation of this project. Present research is financially sponsored by Fund for Scientific Research of the Flemish Community (FWO) and the Ghent University Research Board (BOF). Thanks to the Belgian Welding Institute (BIL) for using their optical microscope.
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