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Influence of counterface roughness on friction properties of engineering plastics for bearing applications
Materials and Design 30 (2009) 1650–1658
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Influence of counterface roughness on friction properties of engineering plastics for bearing applications Virginio Quaglini *, Paolo Dubini, Daniela Ferroni, Carlo Poggi Materials Testing Laboratory, Department of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 27 September 2007 Accepted 15 July 2008 Available online 25 July 2008 Keywords: A. Polymers: thermoplastics E. Mechanical properties of materials H. Selection for materials properties I. Adhesion
a b s t r a c t The study investigates the effects of the roughness of the metal counterface (mirror finished or polished) on the coefficient of dry friction for some of the most common engineering plastics used in current bearing technology. The results show that an optimal roughness for minimum friction is likely to exist for any polymer, and it depends on the bulk properties of the polymer itself. ‘‘Soft” plastics characterized by a low modulus of elasticity exhibit better sliding behaviour on very smooth, mirror finished surfaces, whereas for high-modulus plastics lower friction is measured in combination with rougher, polished counterfaces. The influence of the contact pressure and sliding velocity are also investigated and found to depend on the layout of the tribological system. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Today thermoplastic polymers are widely employed in mechanical engineering in a large variety of applications like bearings, gears and cams, running under dry sliding, where their self-lubricating properties are exploited to avoid the need for external lubrication with oil or grease, with the attendant problems of contamination and needing of periodic maintenance. In most of bearing designs, the self-lubricating polymer is combined with a hard metallic counterface, usually made of steel or aluminium, since polymers have been proved to be more effective, as concerns friction and wear performances, against a metallic counterface than when sliding against themselves [1]. In a polymer/metal tribological system, two mechanisms are generally acknowledged to contribute to the friction force: shearing of the junctions formed by adhesion of the asperities of the two mating surfaces at the contact points, and dissipation of mechanical energy due to the deformation taking place when the asperities come into contact with each other [1–5]. In addition, the asperities of the harder surface are assumed to plough the softer one, and the relevant ploughing resistance further contributes to the frictional force [6]. The adhesion and deformation components of friction influence each other (for example, the extension of the junctions depends on the deformation of the asperities), and usually they both occur during sliding. Their relative contribution to the overall friction force depends on the normal load as well as on the chemical, mechanical and geometrical properties of both surfaces. In particular, on rough metal surfaces the polymer deformation component increases, * Corresponding author. Tel.: +39 02 2399 4248; fax: +39 02 2399 4369. E-mail address: [email protected] (V. Quaglini). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.025
while on smooth surfaces the adhesion becomes more important [7]. A feature of polymer/metal tribology is the creation of a transfer film of the polymer against the harder metal counterface. Since for most polymers the strength of the adhesive junctions formed between the polymer and the metal is comparable with the bulk strength of the polymer itself, shearing is accompanied by the detachment of fragments of polymer, large on atomic scale, that stick on the metal counterface. The creation and growth of this transfer film is usually followed by a moderate decrease of friction, once the sharp asperities of the hard counterface cease to abrade after being covered by the fragments of polymer [8–10]. However, some semicrystalline polymers characterized by a very smooth molecular profile, like polytetrafluoroethylene (PTFE), exhibit a very large drop of friction after a very few cycles of sliding against the metal counterface: the coefficient of dynamic friction becomes as low as 0.01–0.02, less than half of the value at the breakaway. This behaviour is a consequence of the orientation of the molecular chains within both the transfer film and the polymer slider, as the low adhesion force represents the effect of very easy sliding between the PTFE chains once they have been aligned. Other semicrystalline polymers which exhibit a similar feature are high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE) [10]. A considerable number of studies dealing with the tribological behaviour of engineering plastics have been published up to date, but a small number of them investigated in detail the influence of the roughness of the metal counterface on friction and wear, and the relevant data are available for few polymers only. In a cornerstone paper by Pooley and Tabor [10], the authors demonstrated that the coefficient of friction of PTFE is poorly
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affected by the nature of the counterface, as long as smooth counterfaces are used, but when the polymer is slid on rough surfaces friction and transfer change considerably. As the roughness increases, the asperities of the hard metal counterface digs deeply into the softer polymer, and the resistance to sliding becomes significantly larger. For UHMWPE for velocities between 1 and 5 m/s there seems to exist an optimal value of surface roughness which minimizes friction, whereas increase of friction occurs either on rough counterfaces, where the deformation contribution increases due to the ploughing action of the counterface asperities, and on extremely smooth surfaces, where the lack of sharp edged grooves prevents the formation of the transfer film, and abraded polymer debris is expelled as wear debris [11,12]. Nevertheless, up to now the terms ‘‘smooth” and ‘‘rough” have been used only in a qualitative sense, and the corresponding values have not been defined. In current bearing manufacturing, smooth metal counterfaces are commonly classified as mirror-finished (Ra = 0.02–0.1 lm), polished (Ra = 0.1–0.6 lm), and machined (Ra = 0.6–2 lm). However experimental studies proved that even a small change of roughness within the manufacturing tolerances may significantly affect the coefficient of friction. Zsidai and co-workers [13] studied the tribological behaviour of different engineering plastics in combination with two steel counterfaces, the first one characterized by a roughness of Ra = 0.05–0.10 lm (called by the authors as ‘‘smooth” surface), and the second one with roughness Ra = 0.10–0.30 lm (called as ‘‘rough” surface). All the plastics exhibited lower coefficient of friction when mating ‘‘rough” surfaces rather than ‘‘smooth” ones, although for some plastics the influence of the roughness was larger at low contact pressures, and reduced at high loads. In opposite, the friction of PTFE- and polyoxymethylene (POM)based composites is generally little dependent on the mating surface roughness, even though the relevant wear rates have a sudden rise as the roughness increases beyond a certain critical value characteristic of each one of these composites. Such roughness-dependency of the wear rate has been proved to be markedly affected by the transfer of polymer during sliding [14]. Within this framework, the present study aims at giving an insight into the dependence of the friction coefficient of some engineering plastics used in bearing technology on the metal counterface roughness in dry sliding conditions, investigating the influence of service conditions like the contact pressure and the velocity of sliding. The tests are carried out on a selection of the most common self-lubricating plastics, and the specimens are slid on either polished or mirror-finished steel plates, with surface finishing according to the current practice of bearing manufacturing. The results are used to generate friction maps under different values of roughness of the counterface for different sliding velocities and normal loads, and a relationship between the sliding behaviour and the mechanical properties of the polymers is suggested.
2. Experimental details 2.1. Tested materials The tests were performed on a selection of engineering plastics currently used in bearing technology. The selection includes: polyoxymethylene homopolymer (POM-H), polyethylene terephtalate filled with solid polytetrafluoroethylene as internal lubricant (PETP + PTFE), cast polyamide, type 6, filled with wax lubricant (PA6 + wax), cast polyamide, type 66, filled with molybdenum disulphide (PA66 + MoS2), ultra-high-molecular-weight-polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE). PTFE is characterized by the lowest friction among all engineering plastics, but its wear resistance is generally low; UHMWPE
Table 1 Mechanical and physical properties of the tested engineering plastics (manufacturer’s data) Polymer
POM-H PETP + PTFE PA6 + wax PA66 + MoS2 UHMWPE PTFE
Melting temperature
Density
Elastic modulus
Hardness (H132/60)
(g/cm3)
(MPa)
(N/mm2)
(°C)
1.41 1.44 1.10 1.14 0.95 2.15
3100 2600 2700 4000 770 600
155 160 120 180 32 24
165 255 220 260 135 327
exhibits slightly higher friction values, but larger wear endurance than PTFE; both plastics have a moderate compressive strength and low elastic modulus. PA6, PA66, POM and PETP show superior load carrying capacity, higher surface hardness and good wear resistance, but usually higher friction coefficient, and for this reason they are commonly filled with solid lubricants when used in dry sliding conditions. The nominal values of some mechanical and physical properties of the tested materials are shown in Table 1. These values of elastic modulus and hardness hold as long as the polymer is in its solid state, and reduce as it approaches its melting temperature. 2.2. Apparatus The experiments were conducted on a test model of flat bearing (Fig. 1a) consisting of: – a specimen of thermoplastic polymer (65 mm diameter; 6.7 mm thickness), recessed for 70% of its thickness into a steel backing plate; – a plate of austenitic steel, type 1.4404 + 2B according to EN 10088-2, with 180 HV hardness, used as the mating counterface. Two different methods for preparing the steel surface were used. The first consisted of electro-polishing the surface to a roughness Ra = 0.10–0.20 lm; when a lower roughness was required, the steel surface was mirror-finished to a Ra-value of 0.02–0.08 lm (roughness measured perpendicularly to the sliding direction). The tests were performed using a custom bench available at the Materials Testing Laboratory, Politecnico of Milan, (Fig. 1b) consisting of: – a four-column frame with high axial and transversal stiffness; – a servohydraulic jack rated 200 kN, capable of applying a constant normal force to the sliding surfaces to generate the desired contact pressure; – a sliding plate wherein the polymer specimen is recessed, moving on a set of roller bearings with a coefficient of dynamic friction less than 0.001. The movement is produced by a double-effect servohydraulic jack with a maximum stroke of 50 mm; – a support for the austenitic steel plate, holding the steel plate fixed when the polymer is slid against the mating surface; – a thermal chamber able to control the temperature of the austenitic steel plate of the bearing test model from 50 °C to +50 °C with an accuracy of 0.1 °C; – an instrumentation system which measures and records the normal force, the horizontal force (friction force), the horizontal movement and the temperature of the austenitic steel plate throughout the test period with an error <1%.
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a
1
2.5 mm
6.7 mm
2
3
= 65 mm
b 4
7 5
6
Fig. 1. (a) Schematic drawing of the test model of flat bearing (dimensions in mm): (1) polymer specimen; (2) austenitic steel plate; (3) backing plate with recess for the polymer specimen. b) Test bench: (4) vertical jack applying the normal load to the specimen; (5) horizontal jack producing the movement of the polymer specimen recessed into the (6) sliding plate; (7) thermal chamber enclosing the steel plate and he polymer specimen.
The accuracy of the test bench was assessed to become worse than 1% for friction forces higher than 15 kN. This was probably due to the horizontal deflection of the piston rod of the compression jack applying the normal load. So, during the tests, when the friction force exceeded the limiting value of 15 kN, the test was stopped and the friction measurement discharged as affected by low accuracy. However, since this limit directly concerns the friction force, the maximum coefficient of friction that could be measured in each test depended on the actual normal load. 2.3. Test parameters The coefficient of friction of the engineering plastics was measured in combination with either the mirror finished or the polished counterface under different combinations of contact pressure and sliding velocity. After the final assembling of the bearing test model on the friction test bench, the procedure consisted of the following steps: 1. application of the normal force to give the desired contact pressure, and hold for a preload period of 1 h; 2. execution of movements of sinusoidal waveform (stroke 8 mm), at different frequencies adjusted to obtain the following peak velocities of the polymer specimen: 2.5–12.5–22–42 mm/s; the movements were performed until a steady value of the friction force was attained; 3. continuous record of the normal and horizontal forces and determination at each cycle of movement of the horizontal force vs. horizontal displacement diagram. The horizontal force is the friction force produced by the relative sliding of the polymer against the steel counterface.
A sinusoidal movement was applied to the specimen, and the sliding velocity varied accordingly during the cycle: it smoothly increases from a zero value at the beginning of the stroke, reaches its peak value at half of the stroke, and then decreases again, becoming zero at the end of the stroke. For each tested material, the three steps were repeated for the different combinations of counterface roughness (either mirrorfinished or polished) and contact pressure (20–30–35 MPa). The temperature of the austenitic steel counterface in the bearing test model was controlled by the thermal chamber of the test bench and kept constant at 21 ± 2 °C throughout the whole tests. 3. Results 3.1. General An example of the typical horizontal force versus displacement diagrams measured in the tests is illustrated in Fig. 2. The curves in Fig. 2a represent the measurements taken at the first cycles of movement: a sudden increase of the horizontal force occurs as the motion begins at the end of the preload period, and the maximum value, Fs1, or static friction force, is reached at the breakaway; then the force lowers to a dynamic value. The force–displacement curves initially do not overlap, and the friction force decreases as the slide path increases, due to the running-in of the surfaces, resulting in a minimization of the energy dissipation (proportional to the area enclosed by the force–displacement diagram). However for each specimen after the initial running-in a steady-state regime condition of friction is attained, as shown in Fig. 2b where the force–displacement curves taken at the last five cycles of the test are perfectly overlapped.
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b
1.5
Horizontal force [kN]
0.0 -4
-3
-2
-1
0
1
2
3
F dyn, +
1.0
0.5
-5
1.5
F s, 1
1.0
4
5
-0.5
Horizontal force [kN]
a
0.5 0.0 -5
-4
-3
-2
-1
0
1
2
3
4
5
-0.5 -1.0
-1.0
F dyn,
—
-1.5
-1.5
displacement [mm]
displacement [mm]
Fig. 2. Typical horizontal force–displacement curves measured during a test (specimen: PTFE; contact pressure 20 MPa; peak velocity 2.5 mm/s): (a) at the beginning of the test, after 1 h preload the diagram shows the effect of the resistance to movement at breakaway (Fs,1 represents the static friction force) and of the initial running-in of the surfaces (decrease of the friction force from one cycle to the subsequent one). (b) At the end of the test the steady-state regime condition is attained and the friction curves perfectly overlap.
Two parameters were calculated in each test to characterize the sliding behaviour of the engineering plastic specimen: – the coefficient of static friction at breakaway (ls,1), evaluated at the very first movement
ls;1 ¼
F s;1 N
where Fs,1 is the static friction force at the breakaway (Fig. 2a) and N is the normal load applied to the polymer–steel surfaces; – the coefficient of dynamic friction at the peak sliding velocity (ldyn), evaluated in the steady-state regime condition as the mean value in the cycle:
ldyn ¼
1 jF dyn;þ j jF dyn; j þ 2 N N
where Fdyn,+ and Fdyn, are the values of the friction force measured at the middle point of each stroke during a cycle (Fig. 2b).
3.2. Dynamic coefficient of friction All the results concerning the values of ldyn are summarized in Table 2, where for each material the reported values are averaged from three repetitions of each test run with the same test parameters. The data of Table 2 are used to generate friction maps for each tested engineering plastic (Fig. 3). Each map shows the dependence of the coefficient of friction on the roughness of the counterface for different sliding velocities and normal loads. Among the tested materials, two different trends are clearly disclosed: some polymers (PTFE, UHMWPE and PETP+PTFE) show higher values of the coefficient of friction in combination with the rougher counterface, while for other polymers (PA66 + MoS2 and POM-H) the behaviour is the opposite. These trends are not affected by the magnitude of the normal load, with the exception of PA6 + wax, for which the effect of the counterface roughness is significant only at the highest loads, while at 20 MPa close values of friction on both counterfaces are measured. By matching the experimental results reported in Fig. 3 with the material properties reported in Table 1, a correlation between the
Table 2 Coefficient of dynamic friction in the steady state regime condition, ldyn , in combination with either polished or mirror finished austenitic steel counterface Polymer
PA6 + wax
PA66 + MoS2
POM-H
PETP + PTFE
PTFE
UHMWPE
Contact pressure (MPa)
20 30 35 20 30 35 20 30 35 20 30 35 20 30 35 20 30 35
Polished counterface Ra = 0.10–0.20 lm
Mirror finished counterface Ra = 0.02–0.08 lm
2.5 (mm/s)
12.5 (mm/s)
22 (mm/s)
42 (mm/s)
2.5 (mm/s)
12.5 (mm/s)
22 (mm/s)
42 (mm/s)
0.088 0.077 0.087 0.067 0.070 0.095 0.053 0.062 0.070 0.090 0.100 NT 0.041 0.023 0.022 0.052 0.043 0.039
0.102 0.090 0.090 0.071 0.079 0.135 0.064 0.064 0.077 0.087 0.101 NT 0.045 0.028 0.029 0.055 0.049 0.040
0.090 0.070 0.069 0.076 0.085 OVL 0.062 0.067 0.074 0.089 0.096 NT 0.045 0.038 0.035 0.060 0.052 0.042
0.065 0.050 0.045 0.068 0.074 OVL 0.054 0.057 0.062 0.087 0.088 NT 0.053 0.047 0.044 0.066 0.055 0.047
0.093 0.092 0.087 0.170 OVL OVL 0.103 0.098 OVL 0.042 0.043 0.048 0.011 0.009 0.008 0.035 0.023 0.013
0.103 0.093 0.090 0.197 OVL OVL 0.123 OVL OVL 0.048 0.051 0.055 0.013 0.011 0.011 0.034 0.027 0.017
0.090 0.070 0.070 OVL OVL OVL 0.111 OVL OVL 0.053 0.052 0.055 0.016 0.012 0.013 0.037 0.030 0.022
0.062 0.049 0.048 OVL OVL OVL 0.091 OVL OVL 0.061 0.055 0.054 0.021 0.017 0.020 0.046 0.040 0.034
OVL = overload: friction force exceeding 15 kN. NT = not tested.
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0.10
PTFE
b
0.10
UHMWPE
0.08
0.08
0.06
0.06
μdyn [-]
μdyn [-]
a
0.04
0.04
0.02
0.02
0.00
0.00 0
10
20
30
0
40
10
sliding velocity [mm/s]
c
0.20
PETP+PTFE
d
0.10
0.05
40
0.20
PA6 + wax
0.10
0.05
0.00
0.00
0
10
20
30
40
0
10
sliding velocity [mm/s]
20
30
40
sliding velocity [mm/s]
0.20
POM-H
f
0.20
PA66 + MoS2 0.15
μdyn [-]
0.15
μdyn [-]
30
0.15
μdyn [-]
μdyn [-]
0.15
e
20
sliding velocity [mm/s]
0.10
0.05
0.10
0.05
0.00
0.00 0
10
20
30
40
sliding velocity [mm/s]
0
10
20
30
40
sliding velocity [mm/s]
Fig. 3. Friction maps reporting the steady-state coefficient of dynamic friction of the tested self-lubricating plastics in combination with either mirror finished (Ra = 0.02– 0.08 lm) and polished (Ra = 0.10–0.20 lm) austenitic steel counterfaces for different values of normal load and sliding velocities . Mirror finished counterface: dotted line, contact pressure : 20 MPa (}), 30 MPa (h), 35 MPa (N). Polished surface: solid line, contact pressure : 20 MPa (), 30 MPa (j), 35 MPa (N).
sliding properties and the modulus of elasticity was disclosed for the tested plastics. The tested engineering plastics can be classified into three groups according to their elastic modulus: (a) PTFE and UHMWPE, which are the plastics with the lowest values of the modulus of elasticity (less than 1000 MPa), represent the ‘‘soft” materials; (b) PA66 + MoS2 and POM-H are the materials with the highest modulus (more than 3000 MPa), and represent the ‘‘stiff” materials; (c) PA6 + wax and PET + PTFE with a modulus of elasticity between 1000 MPa and 3000 MPa represent the materials with intermediate stiffness behaviour.
The ‘‘soft” materials with low modulus of elasticity exhibit the lowest values of friction among the tested engineering plastics in combination with both mirror finished and polished surfaces. Their frictional behaviour as enlightened by the experiments is characterized by common features: (1) the coefficient of friction is affected by the roughness of the counterface, with lower values on the smoother surface, even though the difference between mirror finished and polished surfaces tends to reduce at high velocities of sliding, especially for PTFE; (2) on both counterfaces the coefficient of friction increases with the sliding velocity; (3) friction is likely to decrease at increasing normal load
V. Quaglini et al. / Materials and Design 30 (2009) 1650–1658
PET + PTFE and PA6 + wax, which have intermediate values of the elastic modulus among the tested plastics, show higher coefficient of friction than ‘‘soft” materials. PET + PTFE has again a lower coefficient of friction on the mirror finished surface, whereas for PA6 + wax the coefficient of friction is nearly the same on both surfaces. The influence of sliding velocity and pressure on the friction behaviour of PET + PTFE is very small, whereas for PA6 + wax the coefficient of friction attains a maximum value at moderate velocities and then reduces at higher velocities, and this behaviour is the same on both mirror finished and polished surfaces. In addition, for PA6 + wax friction is affected by the normal load, reducing at increasing pressure. ‘‘Stiff” plastics have the highest values of the coefficient of friction among the tested materials; in opposition to the behaviour of ‘‘soft” plastics, friction is more than two times larger on the mirror finished surface than on the polished one. The dependence of friction on the sliding velocity is similar to the one exhibited by PA6 + wax: increasing the sliding velocity promotes an initial increase of friction, followed by a steep decrease at high velocities (42 mm/s in the present experiments). The dependence of friction on the contact pressure is opposite to the behaviour exhibited by the ‘‘soft” plastics, with higher values of friction at higher normal loads. In particular, the polymer with the highest bulk modulus (PA66 + MoS2) exhibits the highest values of friction among the tested materials, and most of the tests could not be performed due to overload, that is friction force exceeding the limits for accuracy of the measuring system better than 1%.
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3.3. Static friction at the breakaway For simplicity, only the results of the tests carried out at 20 and 30 MPa contact pressure are reported and compared in form of column charts in Fig. 4. The tendencies disclosed in these tests are followed also at the highest normal load, corresponding to 35 MPa contact pressure. The trends relevant to the effect of the counterface roughness on the dynamic friction coefficient are confirmed also for the breakaway friction, as the ‘‘stiff” plastics exhibit higher friction on the mirror finished counterface, in opposition to the ‘‘soft” plastics. Only for UHMWPE and PE + PTFE the influence of the counterface roughness seems to be not significant. By comparing the values of the coefficient of static friction at breakaway and of the dynamic friction after running-in, a further distinction can be made between the behaviours of ‘‘soft” plastics PTFE and UHMWPE on one side, and of ‘‘stiff” plastics on the other: for PTFE and UHMWPE, the value of friction at the breakaway is several times larger than the dynamic value, and this difference is larger on the mirror finished counterface (the ratio of ls,1 by ldyn is about 4) than on the polished one (ratio ls,1:ldyn about 2.5). On the contrary, the other plastics exhibit a steady-state coefficient of dynamic friction closer to the value at breakaway, and the relevant ratio of ls,1 by ldyn spans from 1.05 to 1.3.
4. Discussion 4.1. Rationale of the tests
a
0.175
20 MPa
polished
0.150
mirror finished
Overload
μs,1 [ - ]
0.125 0.100 0.075 0.050 0.025
b
PE
0.175
PE W M U
TP
H
+P
PT FE
TF
E
-H PO
M
PA 66 +M oS 2
PA
6+
w
ax
0.000
30 MPa
polished
0.150
mirror finished
Overload
μs,1 [ - ]
0.125 0.100 0.075 0.050 0.025
M W PE U H
+P T TP PE
PT FE
FE
-H M PO
+M oS 66 PA
PA
6+ w ax
2
0.000
Fig. 4. Coefficient of static friction at breakaway for different engineering plastics: (a) after 1 h preload at 20 MPa contact pressure; (b) after 1 h preload at 30 MPa contact pressure. Roughness of the steel counterface: Ra = 0.02–0.08 lm (mirror finished surface), Ra = 0.10–0.20 lm (polished surface). Data non available for PA66 + MoS2 in combination with polished steel counterface (overload).
The tests performed in the present study aim at replicating typical in-service conditions for most of flat bearing applications employing self-lubricating engineering plastics. In the experiments the influence of the roughness of the metal counterface on the coefficient of dynamic friction was investigated by using two different steel surfaces: a polished surface (Ra = 0.10– 0.20 lm), and a mirror-finished surface (Ra = 0.02–0.08 lm). The surface finishing and the relevant values of roughness are in accordance with the current practice of bearing manufacturing. The materials included in the experimentation were chosen among the most common polymers used in bearing technology. All the engineering plastics listed in Table 1 are characterized by medium or high compressive strength, and in condition of confined compression (as replicated in the bearing test model), contact pressures from 20 to 35 MPa fall within the typical service range. The sliding velocities applied in the tests are also typical for many sliding bearings applications. Table 1 lists as well two mechanical properties of the plastics, the modulus of elasticity and the hardness, that have been reported to be indirectly related to the friction behaviour of the materials. The elastic modulus can be used to characterize the adhesion friction component, since it is correlated to the flexibility of the polymer molecular chain and to the easy deposition and orientation of the transfer film on the counterface [8]. The hardness is proportional to the bulk strength of the polymer and it is related both to the adhesion and to the deformation components, characterizing in this latter case the resistance provided by the weaker polymer surface to the ploughing action of the asperities of the harder surface. The friction and wear performances of sliding bearings, although dependent on the physical and chemical properties of the mating surfaces, are also strongly affected by the layout of the tribological system, as concern contact geometry, normal load, temperature and sliding velocity. The experimental set-up was designed in order to allow the full control of the main test conditions including the temperature of
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the austenitic steel counterface and the actual value of the applied pressure: the flat-on-flat configuration (Fig. 1a) was chosen over the most common cylinder-on-plate [13], pin-on-disc [15–18] and ring-on-plate [19] tribometer layouts since the contact occurring between two flat surfaces prevents significant change of the area of contact between the polymer and the harder counterface due to wear and creep. This way variations of the contact pressure throughout the test were minimized. The size of the polymer specimen (65 mm diameter) was larger than the size of the specimens used in conventional tribometers; this allowed to reduce the edge effects induced by small scale tests, including unrealistic heat balance or non-uniform distribution of contact pressure at the interface. 4.2. Influence of surface roughness The lowest values of the coefficient of dynamic friction after running-in are given by PTFE and UHMWPE, which are the polymers with the lowest elastic modulus within the test group (Table 1). On the contrary, the polymers with high modulus show the highest coefficients of friction, especially when sliding against the smoother, mirror finished counterface. The effects of the roughness of the steel counterface can be categorized into two main classes. The polymers with low elastic modulus (PTFE, UHMWPE, PETP + PTFE) exhibit a decrease of the coefficient of friction when used in combination with the mirror finished surface; the opposite tendency characterizes the polymers with high modulus (POM-H and PA66 + MoS2), which show a remarkable increase of the friction coefficient on the mirror-finished counterface. For PA66 + MoS2 most of friction measures on the smooth counterface could not be performed due to overload, i.e. friction force exceeding 15 kN. The friction coefficient of PA6 + wax, whose elastic modulus is intermediate among the tested materials’, is characterized by a negligible influence of the mating surface roughness, and it exhibits close values on both the counterfaces. The existence of a correlation between the elastic modulus of the polymer, the roughness of the counterface and the coefficient of dynamic friction is well illustrated by the friction maps reported in Fig. 5 that collects the results of all the tests at different velocities carried out at the contact pressure of 20 MPa, even though such trends were found to be not affected by the magnitude of the normal load, at least within the test range. According to the literature [7,14], a theoretical physical interpretation of this behaviour can be proposed based on the mechanism of formation of the polymer transfer film. For ‘‘soft”, 0.200
polished mirror finished
μdyn [-]
0.150
PA+wax
UHMWPE
0.100
PETP+ PTFE
PTFE
PA66+ MoS2
POM-H
0.000 1000
2000
4.3. Influence of contact pressure When sliding on the smoother counterface, all the tested engineering plastics show a decrease of the coefficient of dynamic friction as the normal load is increased, with the only exception of PETP + PTFE at low velocities, from 2.5 to 12.5 mm/s. On the polished counterface, PTFE, UHMWPE and PA6 + wax exhibit again lower values of the coefficient of dynamic friction at higher pressures, whichever the velocity; only for PA6 + wax some disturbances affect the friction coefficient at low velocity. An opposite behaviour is shown by ‘‘stiff” plastics, like PETP + PTFE, POM-H and PA66 + MoS2, with the tendency to increase friction as the contact pressure is increased too. Increasing of normal load influences both the deformation and the adhesion components of friction. As a consequence of the deformation of asperities, the true contact area becomes larger, with an increase of the friction forces, but also the strength of the adhesions can change. For most of polymers the overall effect consists in a decrease of friction with increasing of the normal load [20]. However in some cases the balance of adhesion and deformation contributions can produce an increase of friction: in previous tests, the friction coefficient of PA6, POM and UHMWPE was shown to increase after a corresponding increase of the normal load beyond a threshold value depending on the surface energy of the polymer [17]. As a general conclusion, for ‘‘soft” engineering plastics (like PTFE and UHMWPE in the present study), the tendency of the coefficient of dynamic friction to decrease with increasing of pressure is generally fulfilled in most of in-service conditions; nevertheless for most of filled engineering plastics the actual effect of the normal load on the coefficient of friction depends on many factors, including the roughness and the chemical composition of the counterface, and should be assessed case by case. 4.4. Influence of sliding velocity
0.050
0
low-modulus bulk engineering plastics the height of the asperities of the smooth mirror finished counterface is enough to allow the formation of an effective transfer film, with further improvement of the sliding behaviour of the polymer. On the polished counterface, the hard asperities of the steel surface dig deeply into the softer polymer and the deformation contribution to friction considerably increases, resulting in higher value of the coefficient of friction. On the contrary, for hard and high-modulus bulk polymers, characterized by a lower mobility of the molecular chains, the height of the asperities of the mirror finished counterface is not enough to promote the detachment and the adhesion of polymer fragments, and only on the rougher, polished surface the formation of a rudimental transfer film can occur. These findings suggest the existence of an optimal surface roughness for minimal friction for each engineering plastic, and that the relevant range of tolerance depends on the bulk properties of the polymers itself. A similar conclusion was also reported by other authors [13] in a study on PA, POM-H and PETP + PTFE.
3000
4000
5000
Elastic modulus [MPa] Fig. 5. Steady-state coefficient of dynamic friction of different engineering plastics versus elastic modulus at 20 MPa contact pressure and different peak velocities (2.5–12.5–22–42 mm/s); surface roughness: Ra = 0.02–0.08 lm (mirror finished surface), Ra = 0.10–0.20 lm (polished surface).
The influence of the sliding velocity on the steady-state regime value of the coefficient of friction is contrasting, as illustrated by the example given in Fig. 6 which shows the dependence of dynamic friction on sliding velocity exhibited by some among the tested self-lubricating plastics at 20 MPa pressure. For PTFE and UHMWPE (Fig. 6a), which are the polymers exhibiting the lowest values of friction, the coefficient of dynamic friction shows a monotonic increase with the velocity of sliding, whichever the value of contact pressure and the counterface roughness. ‘‘Stiffer” polymers (Fig. 6b) exhibit an initial increase of the coefficient of friction as velocity rises from low to moderate values, but at higher
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a
b
0.08
UHMWPE rough
PA6+wax sm ooth
POM H smooth
0.12
μdyn [-]
0.06
μdyn [-]
0.16
PTFE rough
0.04
UHMWPE smooth
PA6+wax rough
0.08
0.04
0.02
POM H rough
PTFE smooth 0.00
0.00 0
10
20
30
40
50
0
10
20
30
40
50
sliding velocity [mm/s]
sliding velocity [mm/s]
Fig. 6. Influence of the sliding velocity on the coefficient of dynamic friction of different engineering plastics in combination with either polished or mirror finished counterface. The specimens were loaded at 20 MPa contact pressure and tested at peak velocities of 2.5–12.5–22–42 mm/s.
velocities the coefficient of friction decreases; this behaviour is observed on both mirror finished and polished counterfaces. Almost all polymers exhibit visco-elasticity under certain conditions of load and strain rate. The tangential or frictional forces between asperities of polymer and steel counterface become a direct function of the sliding speed, and consequently the coefficient of dynamic friction is expected to increase with the velocity due to the increase of the deformation component. This behaviour was actually followed by all the tested polymers when the sliding speed was increased from low to moderate values; however for some polymers a decrease of friction occurred at higher velocities. It should be noted that the sliding speed also affects the coefficient of friction through its effect on the frictional heating. Most polymers melt at relatively low temperatures, and this characteristic, combined with the low thermal conductivity of both the polymer and the austenitic steel mating surfaces, ensures that frictional heat temperatures can easily reach the melting point of the polymer and cause its surface to melt. When the polymer melts, its friction tends to decrease, according to a mechanism of ‘‘thermal control of friction” [21]. The latent heat of melting imposes a temperature limit on the frictional temperatures in a polymer–counterface sliding contact: when the melting temperature of the polymer is reached any additional frictional heat released in the contact tends to melt additional polymer rather than cause the temperature of the already molten polymer to rise. As a result, the friction coefficient varies with sliding speed so that the temperature within the contact remains constant at the melting point. Good agreement with the model of limiting frictional heat was shown in a previous study [21] for the friction coefficients of polypropylene, polyamide and low density polyethylene. In particular, for polyamide the friction coefficient rises until a maximum value is achieved. At this point the friction coefficient determined by the ‘thermal control model’ equals the friction coefficient dictated by ‘solid state friction’. After reaching its maximum value the friction coefficient declines to a low level which may be of benefit in bearing design. It should be noted that the test bench used in the present study allowed to control the temperature of the bulk of the steel counterface, but due to the low thermal conductivity of austenitic steel the actual temperature at the sliding interface was more dictated by the frictional heat than from the bulk temperature. The development of frictional heat and the rise of temperature up to the melting point are enhanced by high speeds and high values of the coefficient of friction, so the range of sliding velocities considered in the tests was not enough to attain this effect for the polymers exhibiting the lowest values of coefficient of friction, like PTFE and UHMWPE.
4.5. Static coefficient of friction The opposite influence of the counterface roughness on the coefficient of dynamic friction of ‘‘soft” and ‘‘stiff” plastics is confirmed also for the static friction: replacing the mirror finished steel counterface by the polished one produces an increase of the breakaway friction for PTFE and PA6 + wax, and a decrease for POM-H and PA66 + MoS2. A further difference between the behaviour of ‘‘soft” and ‘‘stiff” plastics is revealed by comparing the values of ls,1 and ldyn. Usually the typical behaviour of most polymers is represented by a steady-state value of dynamic friction close to the value at breakaway; however some semicrystalline polymers characterized by a smooth molecular profile, like PTFE and UHMWPE, exhibit a different behaviour, consisting of a drop of more than 50% of friction after the first cycles of running-in and a very low dynamic value at the steady-state condition. This behaviour is a consequence of the creation of a continuous transfer film on the metal counterface and of the orientation of the molecular chains of the polymer within both the transfer film and the polymer slider during running-in, which is typical of the tribological behaviour of ‘‘soft” low-modulus polymers, which strongly reduces the resistance to relative motion [10].
5. Conclusions Although the roughness of the metal counterface is known to have considerable influence on the friction of self-lubricated thermoplastic polymers used in dry friction bearings, a systematic study combining the effects of such parameter with contact pressure and velocity for the most common engineering plastics is lacking. The available data for different plastics from the literature are not enough to build a consistent database, since friction and wear properties are not intrinsic material characteristics, but strongly depend on the layout of the tribological system. In this study the influence of the counterface roughness on the steady-state coefficient of dynamic friction of some of the most common engineering plastics has been investigated by means of a flat-on-flat bearing test model. The data obtained from the experiments allowed to generate friction maps combining the surface roughness with the effects of sliding velocity and normal loads as well as with the elastic properties (modulus of elasticity) of self-lubricating plastics currently used in bearing manufacturing. The following general conclusions can be drawn form the experimental data:
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1. An optimal surface roughness for minimum friction of the different engineering plastics seems to exist, and the relevant value depends on the bulk properties of the polymer itself. For low-modulus bulk polymers, among which PTFE and UHMWPE, the coefficient of dynamic friction has a lower value on very smooth counterfaces, as mirror finished ones (Ra = 0.02–0.08 lm), while for high-modulus plastics minimization of friction is attained on rougher surfaces (Ra = 0.10– 0.20 lm). This effect of controlled friction by surface asperities is different for each plastic depending on the relevant bulk stiffness and hardness. 2. Further, the same trend has been observed also for the static friction at the breakaway. 3. The dependence of the coefficient of friction on the contact pressure is affected by the properties of the metal counterface, including the chemical affinity with the polymer and the surface roughness. For most of engineering plastics increasing the normal load promotes a reduction of friction, but the actual behaviour of each polymer–counterface pair should be assessed case by case. 4. The sliding velocity influences the value of the coefficient of friction through two mechanisms: visco-elasticity and frictional heat. When the melting temperature of the polymer is reached at the sliding interface, friction stops increasing and declines to a lower value. The velocity corresponding to the inversion point is lower when the polymer is used in combination with the counterface roughness upon which the higher coefficient of friction is produced. Acknowledgement The authors wish to acknowledge Sara Mezzadri, MsC, for her contribution and assistance to the experiments. References [1] Stachowiak GW, Batchelor AW. Engineering tribology. Elsevier Science; 2005. [2] Hutchings IM. Tribology, friction and wear of engineering materials. London: Edward Arnold; 1992.
[3] Tewari US, Sharma SK, Vasudevan P. Polymer tribology. Rev Macromol Chem Phys 1989;C29:1–38. [4] Pascoe MW, Tabor D. The friction and deformation of polymers. Proc Royal Soc London 1956;A235:210–24. [5] Myshkin NK, Petrokovets MI, Kovalev AV. Tribology of polymers: adhesion, friction, wear, and mass-transfer. Tribol Int 2005;38:910–21. [6] Bely VA, Sviridenok AI, Petrokovets MI, Savkin VG. Friction and wear in polymer-based materials. Oxford: Pergamon Press; 1982. p. 416. [7] Samyn P, Quintelier J, Ost W, De Baets P, Schoukens G. Sliding behaviour of pure polyester and polyester-PTFE filled bulk composites in overload conditions. Polym Test 2005;24:588–603. [8] Bahadur S. The development of transfer layers and their role in polymer tribology. Wear 2000;245:92–9. [9] Gao J, Mao S, Liu J, Feng D. Tribochemical effects of some polymers/stainless steel. Wear 1997;212:238–43. [10] Pooley CM, Tabor D. Friction and molecular structure: the behaviour of some thermoplastics. Proc Royal Soc London 1972;A329:251–74. [11] Barrett TS, Stachowiak GW, Batchelor AW. Effect of roughness and sliding speed on the wear and friction of ultra-high molecular weight polyethylene. Wear 1992;153:331–50. [12] Dowson D, Challen JM, Atkinson JR. The influence of counterface roughness on the wear rate of polyethylene. In: Dowson D, Godet D, Taylor CM, editors. Proc third Leeds–Lyon symposium on tribology. Wear of non-metallic materials. London: Inst Mech Engrs Publ; 1978. p. 99–102. [13] Zsidai L, De Baets P, Samyn P, Kalacska G, Van Peteghem AP, Van Parys F. The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens. Wear 2002;253:673–88. [14] Tanaka K, Yamada Y. Influence of counterface roughness on the friction and wear of polytetrafluoroethylene- and polyacetal-based composites. J Syn Lubr 2006;5:115–31. [15] Liu CZ, Wu JQ, Li JQ, Renb LQ, Tong J, Arnell AD. Tribological behaviours of PA/ UHMWPE blend under dry and lubricating condition. Wear 2006;260:109–15. [16] Palabiyik M, Bahadur S. Tribological studies of polyamide 6 and high-density polyethylene blends filled with PTFE and copper oxide and reinforced with short glass fibres. Wear 2002;253:369–76. [17] Unal H, Mimaroglu A. Friction and wear behaviour of unfilled engineering thermoplastics. Mater Des 2003;24:183–7. [18] Unal H, Mimaroglu A, Kadıoglu U, Ekiz H. Sliding friction and wear behavior of polytetrafluoroethylene and its composites under dry conditions. Mater Des 2004;25:239–45. [19] Wang HG, Jian LQ, Pan BL, Zhang JY, Yang SR, Wang HG. Mechanical and tribological behaviours of polyamide 66/ultra high molecular weight polyethylene blends. Polym Eng Sci 2007. doi:10.1002/pen.20748. [20] Bowden FP, Tabor D. The friction and lubrication of solids. Oxford: Clarendon Press; 1954. [21] Ettles CMMcC. Polymer and elastomer friction in the thermal control regime. ASLE Trans 1987;30:149–59.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Influence of counterface roughness on friction properties of engineering plastics for bearing applications Virginio Quaglini *, Paolo Dubini, Daniela Ferroni, Carlo Poggi Materials Testing Laboratory, Department of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 27 September 2007 Accepted 15 July 2008 Available online 25 July 2008 Keywords: A. Polymers: thermoplastics E. Mechanical properties of materials H. Selection for materials properties I. Adhesion
a b s t r a c t The study investigates the effects of the roughness of the metal counterface (mirror finished or polished) on the coefficient of dry friction for some of the most common engineering plastics used in current bearing technology. The results show that an optimal roughness for minimum friction is likely to exist for any polymer, and it depends on the bulk properties of the polymer itself. ‘‘Soft” plastics characterized by a low modulus of elasticity exhibit better sliding behaviour on very smooth, mirror finished surfaces, whereas for high-modulus plastics lower friction is measured in combination with rougher, polished counterfaces. The influence of the contact pressure and sliding velocity are also investigated and found to depend on the layout of the tribological system. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Today thermoplastic polymers are widely employed in mechanical engineering in a large variety of applications like bearings, gears and cams, running under dry sliding, where their self-lubricating properties are exploited to avoid the need for external lubrication with oil or grease, with the attendant problems of contamination and needing of periodic maintenance. In most of bearing designs, the self-lubricating polymer is combined with a hard metallic counterface, usually made of steel or aluminium, since polymers have been proved to be more effective, as concerns friction and wear performances, against a metallic counterface than when sliding against themselves [1]. In a polymer/metal tribological system, two mechanisms are generally acknowledged to contribute to the friction force: shearing of the junctions formed by adhesion of the asperities of the two mating surfaces at the contact points, and dissipation of mechanical energy due to the deformation taking place when the asperities come into contact with each other [1–5]. In addition, the asperities of the harder surface are assumed to plough the softer one, and the relevant ploughing resistance further contributes to the frictional force [6]. The adhesion and deformation components of friction influence each other (for example, the extension of the junctions depends on the deformation of the asperities), and usually they both occur during sliding. Their relative contribution to the overall friction force depends on the normal load as well as on the chemical, mechanical and geometrical properties of both surfaces. In particular, on rough metal surfaces the polymer deformation component increases, * Corresponding author. Tel.: +39 02 2399 4248; fax: +39 02 2399 4369. E-mail address: [email protected] (V. Quaglini). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.025
while on smooth surfaces the adhesion becomes more important [7]. A feature of polymer/metal tribology is the creation of a transfer film of the polymer against the harder metal counterface. Since for most polymers the strength of the adhesive junctions formed between the polymer and the metal is comparable with the bulk strength of the polymer itself, shearing is accompanied by the detachment of fragments of polymer, large on atomic scale, that stick on the metal counterface. The creation and growth of this transfer film is usually followed by a moderate decrease of friction, once the sharp asperities of the hard counterface cease to abrade after being covered by the fragments of polymer [8–10]. However, some semicrystalline polymers characterized by a very smooth molecular profile, like polytetrafluoroethylene (PTFE), exhibit a very large drop of friction after a very few cycles of sliding against the metal counterface: the coefficient of dynamic friction becomes as low as 0.01–0.02, less than half of the value at the breakaway. This behaviour is a consequence of the orientation of the molecular chains within both the transfer film and the polymer slider, as the low adhesion force represents the effect of very easy sliding between the PTFE chains once they have been aligned. Other semicrystalline polymers which exhibit a similar feature are high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE) [10]. A considerable number of studies dealing with the tribological behaviour of engineering plastics have been published up to date, but a small number of them investigated in detail the influence of the roughness of the metal counterface on friction and wear, and the relevant data are available for few polymers only. In a cornerstone paper by Pooley and Tabor [10], the authors demonstrated that the coefficient of friction of PTFE is poorly
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affected by the nature of the counterface, as long as smooth counterfaces are used, but when the polymer is slid on rough surfaces friction and transfer change considerably. As the roughness increases, the asperities of the hard metal counterface digs deeply into the softer polymer, and the resistance to sliding becomes significantly larger. For UHMWPE for velocities between 1 and 5 m/s there seems to exist an optimal value of surface roughness which minimizes friction, whereas increase of friction occurs either on rough counterfaces, where the deformation contribution increases due to the ploughing action of the counterface asperities, and on extremely smooth surfaces, where the lack of sharp edged grooves prevents the formation of the transfer film, and abraded polymer debris is expelled as wear debris [11,12]. Nevertheless, up to now the terms ‘‘smooth” and ‘‘rough” have been used only in a qualitative sense, and the corresponding values have not been defined. In current bearing manufacturing, smooth metal counterfaces are commonly classified as mirror-finished (Ra = 0.02–0.1 lm), polished (Ra = 0.1–0.6 lm), and machined (Ra = 0.6–2 lm). However experimental studies proved that even a small change of roughness within the manufacturing tolerances may significantly affect the coefficient of friction. Zsidai and co-workers [13] studied the tribological behaviour of different engineering plastics in combination with two steel counterfaces, the first one characterized by a roughness of Ra = 0.05–0.10 lm (called by the authors as ‘‘smooth” surface), and the second one with roughness Ra = 0.10–0.30 lm (called as ‘‘rough” surface). All the plastics exhibited lower coefficient of friction when mating ‘‘rough” surfaces rather than ‘‘smooth” ones, although for some plastics the influence of the roughness was larger at low contact pressures, and reduced at high loads. In opposite, the friction of PTFE- and polyoxymethylene (POM)based composites is generally little dependent on the mating surface roughness, even though the relevant wear rates have a sudden rise as the roughness increases beyond a certain critical value characteristic of each one of these composites. Such roughness-dependency of the wear rate has been proved to be markedly affected by the transfer of polymer during sliding [14]. Within this framework, the present study aims at giving an insight into the dependence of the friction coefficient of some engineering plastics used in bearing technology on the metal counterface roughness in dry sliding conditions, investigating the influence of service conditions like the contact pressure and the velocity of sliding. The tests are carried out on a selection of the most common self-lubricating plastics, and the specimens are slid on either polished or mirror-finished steel plates, with surface finishing according to the current practice of bearing manufacturing. The results are used to generate friction maps under different values of roughness of the counterface for different sliding velocities and normal loads, and a relationship between the sliding behaviour and the mechanical properties of the polymers is suggested.
2. Experimental details 2.1. Tested materials The tests were performed on a selection of engineering plastics currently used in bearing technology. The selection includes: polyoxymethylene homopolymer (POM-H), polyethylene terephtalate filled with solid polytetrafluoroethylene as internal lubricant (PETP + PTFE), cast polyamide, type 6, filled with wax lubricant (PA6 + wax), cast polyamide, type 66, filled with molybdenum disulphide (PA66 + MoS2), ultra-high-molecular-weight-polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE). PTFE is characterized by the lowest friction among all engineering plastics, but its wear resistance is generally low; UHMWPE
Table 1 Mechanical and physical properties of the tested engineering plastics (manufacturer’s data) Polymer
POM-H PETP + PTFE PA6 + wax PA66 + MoS2 UHMWPE PTFE
Melting temperature
Density
Elastic modulus
Hardness (H132/60)
(g/cm3)
(MPa)
(N/mm2)
(°C)
1.41 1.44 1.10 1.14 0.95 2.15
3100 2600 2700 4000 770 600
155 160 120 180 32 24
165 255 220 260 135 327
exhibits slightly higher friction values, but larger wear endurance than PTFE; both plastics have a moderate compressive strength and low elastic modulus. PA6, PA66, POM and PETP show superior load carrying capacity, higher surface hardness and good wear resistance, but usually higher friction coefficient, and for this reason they are commonly filled with solid lubricants when used in dry sliding conditions. The nominal values of some mechanical and physical properties of the tested materials are shown in Table 1. These values of elastic modulus and hardness hold as long as the polymer is in its solid state, and reduce as it approaches its melting temperature. 2.2. Apparatus The experiments were conducted on a test model of flat bearing (Fig. 1a) consisting of: – a specimen of thermoplastic polymer (65 mm diameter; 6.7 mm thickness), recessed for 70% of its thickness into a steel backing plate; – a plate of austenitic steel, type 1.4404 + 2B according to EN 10088-2, with 180 HV hardness, used as the mating counterface. Two different methods for preparing the steel surface were used. The first consisted of electro-polishing the surface to a roughness Ra = 0.10–0.20 lm; when a lower roughness was required, the steel surface was mirror-finished to a Ra-value of 0.02–0.08 lm (roughness measured perpendicularly to the sliding direction). The tests were performed using a custom bench available at the Materials Testing Laboratory, Politecnico of Milan, (Fig. 1b) consisting of: – a four-column frame with high axial and transversal stiffness; – a servohydraulic jack rated 200 kN, capable of applying a constant normal force to the sliding surfaces to generate the desired contact pressure; – a sliding plate wherein the polymer specimen is recessed, moving on a set of roller bearings with a coefficient of dynamic friction less than 0.001. The movement is produced by a double-effect servohydraulic jack with a maximum stroke of 50 mm; – a support for the austenitic steel plate, holding the steel plate fixed when the polymer is slid against the mating surface; – a thermal chamber able to control the temperature of the austenitic steel plate of the bearing test model from 50 °C to +50 °C with an accuracy of 0.1 °C; – an instrumentation system which measures and records the normal force, the horizontal force (friction force), the horizontal movement and the temperature of the austenitic steel plate throughout the test period with an error <1%.
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a
1
2.5 mm
6.7 mm
2
3
= 65 mm
b 4
7 5
6
Fig. 1. (a) Schematic drawing of the test model of flat bearing (dimensions in mm): (1) polymer specimen; (2) austenitic steel plate; (3) backing plate with recess for the polymer specimen. b) Test bench: (4) vertical jack applying the normal load to the specimen; (5) horizontal jack producing the movement of the polymer specimen recessed into the (6) sliding plate; (7) thermal chamber enclosing the steel plate and he polymer specimen.
The accuracy of the test bench was assessed to become worse than 1% for friction forces higher than 15 kN. This was probably due to the horizontal deflection of the piston rod of the compression jack applying the normal load. So, during the tests, when the friction force exceeded the limiting value of 15 kN, the test was stopped and the friction measurement discharged as affected by low accuracy. However, since this limit directly concerns the friction force, the maximum coefficient of friction that could be measured in each test depended on the actual normal load. 2.3. Test parameters The coefficient of friction of the engineering plastics was measured in combination with either the mirror finished or the polished counterface under different combinations of contact pressure and sliding velocity. After the final assembling of the bearing test model on the friction test bench, the procedure consisted of the following steps: 1. application of the normal force to give the desired contact pressure, and hold for a preload period of 1 h; 2. execution of movements of sinusoidal waveform (stroke 8 mm), at different frequencies adjusted to obtain the following peak velocities of the polymer specimen: 2.5–12.5–22–42 mm/s; the movements were performed until a steady value of the friction force was attained; 3. continuous record of the normal and horizontal forces and determination at each cycle of movement of the horizontal force vs. horizontal displacement diagram. The horizontal force is the friction force produced by the relative sliding of the polymer against the steel counterface.
A sinusoidal movement was applied to the specimen, and the sliding velocity varied accordingly during the cycle: it smoothly increases from a zero value at the beginning of the stroke, reaches its peak value at half of the stroke, and then decreases again, becoming zero at the end of the stroke. For each tested material, the three steps were repeated for the different combinations of counterface roughness (either mirrorfinished or polished) and contact pressure (20–30–35 MPa). The temperature of the austenitic steel counterface in the bearing test model was controlled by the thermal chamber of the test bench and kept constant at 21 ± 2 °C throughout the whole tests. 3. Results 3.1. General An example of the typical horizontal force versus displacement diagrams measured in the tests is illustrated in Fig. 2. The curves in Fig. 2a represent the measurements taken at the first cycles of movement: a sudden increase of the horizontal force occurs as the motion begins at the end of the preload period, and the maximum value, Fs1, or static friction force, is reached at the breakaway; then the force lowers to a dynamic value. The force–displacement curves initially do not overlap, and the friction force decreases as the slide path increases, due to the running-in of the surfaces, resulting in a minimization of the energy dissipation (proportional to the area enclosed by the force–displacement diagram). However for each specimen after the initial running-in a steady-state regime condition of friction is attained, as shown in Fig. 2b where the force–displacement curves taken at the last five cycles of the test are perfectly overlapped.
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b
1.5
Horizontal force [kN]
0.0 -4
-3
-2
-1
0
1
2
3
F dyn, +
1.0
0.5
-5
1.5
F s, 1
1.0
4
5
-0.5
Horizontal force [kN]
a
0.5 0.0 -5
-4
-3
-2
-1
0
1
2
3
4
5
-0.5 -1.0
-1.0
F dyn,
—
-1.5
-1.5
displacement [mm]
displacement [mm]
Fig. 2. Typical horizontal force–displacement curves measured during a test (specimen: PTFE; contact pressure 20 MPa; peak velocity 2.5 mm/s): (a) at the beginning of the test, after 1 h preload the diagram shows the effect of the resistance to movement at breakaway (Fs,1 represents the static friction force) and of the initial running-in of the surfaces (decrease of the friction force from one cycle to the subsequent one). (b) At the end of the test the steady-state regime condition is attained and the friction curves perfectly overlap.
Two parameters were calculated in each test to characterize the sliding behaviour of the engineering plastic specimen: – the coefficient of static friction at breakaway (ls,1), evaluated at the very first movement
ls;1 ¼
F s;1 N
where Fs,1 is the static friction force at the breakaway (Fig. 2a) and N is the normal load applied to the polymer–steel surfaces; – the coefficient of dynamic friction at the peak sliding velocity (ldyn), evaluated in the steady-state regime condition as the mean value in the cycle:
ldyn ¼
1 jF dyn;þ j jF dyn; j þ 2 N N
where Fdyn,+ and Fdyn, are the values of the friction force measured at the middle point of each stroke during a cycle (Fig. 2b).
3.2. Dynamic coefficient of friction All the results concerning the values of ldyn are summarized in Table 2, where for each material the reported values are averaged from three repetitions of each test run with the same test parameters. The data of Table 2 are used to generate friction maps for each tested engineering plastic (Fig. 3). Each map shows the dependence of the coefficient of friction on the roughness of the counterface for different sliding velocities and normal loads. Among the tested materials, two different trends are clearly disclosed: some polymers (PTFE, UHMWPE and PETP+PTFE) show higher values of the coefficient of friction in combination with the rougher counterface, while for other polymers (PA66 + MoS2 and POM-H) the behaviour is the opposite. These trends are not affected by the magnitude of the normal load, with the exception of PA6 + wax, for which the effect of the counterface roughness is significant only at the highest loads, while at 20 MPa close values of friction on both counterfaces are measured. By matching the experimental results reported in Fig. 3 with the material properties reported in Table 1, a correlation between the
Table 2 Coefficient of dynamic friction in the steady state regime condition, ldyn , in combination with either polished or mirror finished austenitic steel counterface Polymer
PA6 + wax
PA66 + MoS2
POM-H
PETP + PTFE
PTFE
UHMWPE
Contact pressure (MPa)
20 30 35 20 30 35 20 30 35 20 30 35 20 30 35 20 30 35
Polished counterface Ra = 0.10–0.20 lm
Mirror finished counterface Ra = 0.02–0.08 lm
2.5 (mm/s)
12.5 (mm/s)
22 (mm/s)
42 (mm/s)
2.5 (mm/s)
12.5 (mm/s)
22 (mm/s)
42 (mm/s)
0.088 0.077 0.087 0.067 0.070 0.095 0.053 0.062 0.070 0.090 0.100 NT 0.041 0.023 0.022 0.052 0.043 0.039
0.102 0.090 0.090 0.071 0.079 0.135 0.064 0.064 0.077 0.087 0.101 NT 0.045 0.028 0.029 0.055 0.049 0.040
0.090 0.070 0.069 0.076 0.085 OVL 0.062 0.067 0.074 0.089 0.096 NT 0.045 0.038 0.035 0.060 0.052 0.042
0.065 0.050 0.045 0.068 0.074 OVL 0.054 0.057 0.062 0.087 0.088 NT 0.053 0.047 0.044 0.066 0.055 0.047
0.093 0.092 0.087 0.170 OVL OVL 0.103 0.098 OVL 0.042 0.043 0.048 0.011 0.009 0.008 0.035 0.023 0.013
0.103 0.093 0.090 0.197 OVL OVL 0.123 OVL OVL 0.048 0.051 0.055 0.013 0.011 0.011 0.034 0.027 0.017
0.090 0.070 0.070 OVL OVL OVL 0.111 OVL OVL 0.053 0.052 0.055 0.016 0.012 0.013 0.037 0.030 0.022
0.062 0.049 0.048 OVL OVL OVL 0.091 OVL OVL 0.061 0.055 0.054 0.021 0.017 0.020 0.046 0.040 0.034
OVL = overload: friction force exceeding 15 kN. NT = not tested.
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0.10
PTFE
b
0.10
UHMWPE
0.08
0.08
0.06
0.06
μdyn [-]
μdyn [-]
a
0.04
0.04
0.02
0.02
0.00
0.00 0
10
20
30
0
40
10
sliding velocity [mm/s]
c
0.20
PETP+PTFE
d
0.10
0.05
40
0.20
PA6 + wax
0.10
0.05
0.00
0.00
0
10
20
30
40
0
10
sliding velocity [mm/s]
20
30
40
sliding velocity [mm/s]
0.20
POM-H
f
0.20
PA66 + MoS2 0.15
μdyn [-]
0.15
μdyn [-]
30
0.15
μdyn [-]
μdyn [-]
0.15
e
20
sliding velocity [mm/s]
0.10
0.05
0.10
0.05
0.00
0.00 0
10
20
30
40
sliding velocity [mm/s]
0
10
20
30
40
sliding velocity [mm/s]
Fig. 3. Friction maps reporting the steady-state coefficient of dynamic friction of the tested self-lubricating plastics in combination with either mirror finished (Ra = 0.02– 0.08 lm) and polished (Ra = 0.10–0.20 lm) austenitic steel counterfaces for different values of normal load and sliding velocities . Mirror finished counterface: dotted line, contact pressure : 20 MPa (}), 30 MPa (h), 35 MPa (N). Polished surface: solid line, contact pressure : 20 MPa (), 30 MPa (j), 35 MPa (N).
sliding properties and the modulus of elasticity was disclosed for the tested plastics. The tested engineering plastics can be classified into three groups according to their elastic modulus: (a) PTFE and UHMWPE, which are the plastics with the lowest values of the modulus of elasticity (less than 1000 MPa), represent the ‘‘soft” materials; (b) PA66 + MoS2 and POM-H are the materials with the highest modulus (more than 3000 MPa), and represent the ‘‘stiff” materials; (c) PA6 + wax and PET + PTFE with a modulus of elasticity between 1000 MPa and 3000 MPa represent the materials with intermediate stiffness behaviour.
The ‘‘soft” materials with low modulus of elasticity exhibit the lowest values of friction among the tested engineering plastics in combination with both mirror finished and polished surfaces. Their frictional behaviour as enlightened by the experiments is characterized by common features: (1) the coefficient of friction is affected by the roughness of the counterface, with lower values on the smoother surface, even though the difference between mirror finished and polished surfaces tends to reduce at high velocities of sliding, especially for PTFE; (2) on both counterfaces the coefficient of friction increases with the sliding velocity; (3) friction is likely to decrease at increasing normal load
V. Quaglini et al. / Materials and Design 30 (2009) 1650–1658
PET + PTFE and PA6 + wax, which have intermediate values of the elastic modulus among the tested plastics, show higher coefficient of friction than ‘‘soft” materials. PET + PTFE has again a lower coefficient of friction on the mirror finished surface, whereas for PA6 + wax the coefficient of friction is nearly the same on both surfaces. The influence of sliding velocity and pressure on the friction behaviour of PET + PTFE is very small, whereas for PA6 + wax the coefficient of friction attains a maximum value at moderate velocities and then reduces at higher velocities, and this behaviour is the same on both mirror finished and polished surfaces. In addition, for PA6 + wax friction is affected by the normal load, reducing at increasing pressure. ‘‘Stiff” plastics have the highest values of the coefficient of friction among the tested materials; in opposition to the behaviour of ‘‘soft” plastics, friction is more than two times larger on the mirror finished surface than on the polished one. The dependence of friction on the sliding velocity is similar to the one exhibited by PA6 + wax: increasing the sliding velocity promotes an initial increase of friction, followed by a steep decrease at high velocities (42 mm/s in the present experiments). The dependence of friction on the contact pressure is opposite to the behaviour exhibited by the ‘‘soft” plastics, with higher values of friction at higher normal loads. In particular, the polymer with the highest bulk modulus (PA66 + MoS2) exhibits the highest values of friction among the tested materials, and most of the tests could not be performed due to overload, that is friction force exceeding the limits for accuracy of the measuring system better than 1%.
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3.3. Static friction at the breakaway For simplicity, only the results of the tests carried out at 20 and 30 MPa contact pressure are reported and compared in form of column charts in Fig. 4. The tendencies disclosed in these tests are followed also at the highest normal load, corresponding to 35 MPa contact pressure. The trends relevant to the effect of the counterface roughness on the dynamic friction coefficient are confirmed also for the breakaway friction, as the ‘‘stiff” plastics exhibit higher friction on the mirror finished counterface, in opposition to the ‘‘soft” plastics. Only for UHMWPE and PE + PTFE the influence of the counterface roughness seems to be not significant. By comparing the values of the coefficient of static friction at breakaway and of the dynamic friction after running-in, a further distinction can be made between the behaviours of ‘‘soft” plastics PTFE and UHMWPE on one side, and of ‘‘stiff” plastics on the other: for PTFE and UHMWPE, the value of friction at the breakaway is several times larger than the dynamic value, and this difference is larger on the mirror finished counterface (the ratio of ls,1 by ldyn is about 4) than on the polished one (ratio ls,1:ldyn about 2.5). On the contrary, the other plastics exhibit a steady-state coefficient of dynamic friction closer to the value at breakaway, and the relevant ratio of ls,1 by ldyn spans from 1.05 to 1.3.
4. Discussion 4.1. Rationale of the tests
a
0.175
20 MPa
polished
0.150
mirror finished
Overload
μs,1 [ - ]
0.125 0.100 0.075 0.050 0.025
b
PE
0.175
PE W M U
TP
H
+P
PT FE
TF
E
-H PO
M
PA 66 +M oS 2
PA
6+
w
ax
0.000
30 MPa
polished
0.150
mirror finished
Overload
μs,1 [ - ]
0.125 0.100 0.075 0.050 0.025
M W PE U H
+P T TP PE
PT FE
FE
-H M PO
+M oS 66 PA
PA
6+ w ax
2
0.000
Fig. 4. Coefficient of static friction at breakaway for different engineering plastics: (a) after 1 h preload at 20 MPa contact pressure; (b) after 1 h preload at 30 MPa contact pressure. Roughness of the steel counterface: Ra = 0.02–0.08 lm (mirror finished surface), Ra = 0.10–0.20 lm (polished surface). Data non available for PA66 + MoS2 in combination with polished steel counterface (overload).
The tests performed in the present study aim at replicating typical in-service conditions for most of flat bearing applications employing self-lubricating engineering plastics. In the experiments the influence of the roughness of the metal counterface on the coefficient of dynamic friction was investigated by using two different steel surfaces: a polished surface (Ra = 0.10– 0.20 lm), and a mirror-finished surface (Ra = 0.02–0.08 lm). The surface finishing and the relevant values of roughness are in accordance with the current practice of bearing manufacturing. The materials included in the experimentation were chosen among the most common polymers used in bearing technology. All the engineering plastics listed in Table 1 are characterized by medium or high compressive strength, and in condition of confined compression (as replicated in the bearing test model), contact pressures from 20 to 35 MPa fall within the typical service range. The sliding velocities applied in the tests are also typical for many sliding bearings applications. Table 1 lists as well two mechanical properties of the plastics, the modulus of elasticity and the hardness, that have been reported to be indirectly related to the friction behaviour of the materials. The elastic modulus can be used to characterize the adhesion friction component, since it is correlated to the flexibility of the polymer molecular chain and to the easy deposition and orientation of the transfer film on the counterface [8]. The hardness is proportional to the bulk strength of the polymer and it is related both to the adhesion and to the deformation components, characterizing in this latter case the resistance provided by the weaker polymer surface to the ploughing action of the asperities of the harder surface. The friction and wear performances of sliding bearings, although dependent on the physical and chemical properties of the mating surfaces, are also strongly affected by the layout of the tribological system, as concern contact geometry, normal load, temperature and sliding velocity. The experimental set-up was designed in order to allow the full control of the main test conditions including the temperature of
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the austenitic steel counterface and the actual value of the applied pressure: the flat-on-flat configuration (Fig. 1a) was chosen over the most common cylinder-on-plate [13], pin-on-disc [15–18] and ring-on-plate [19] tribometer layouts since the contact occurring between two flat surfaces prevents significant change of the area of contact between the polymer and the harder counterface due to wear and creep. This way variations of the contact pressure throughout the test were minimized. The size of the polymer specimen (65 mm diameter) was larger than the size of the specimens used in conventional tribometers; this allowed to reduce the edge effects induced by small scale tests, including unrealistic heat balance or non-uniform distribution of contact pressure at the interface. 4.2. Influence of surface roughness The lowest values of the coefficient of dynamic friction after running-in are given by PTFE and UHMWPE, which are the polymers with the lowest elastic modulus within the test group (Table 1). On the contrary, the polymers with high modulus show the highest coefficients of friction, especially when sliding against the smoother, mirror finished counterface. The effects of the roughness of the steel counterface can be categorized into two main classes. The polymers with low elastic modulus (PTFE, UHMWPE, PETP + PTFE) exhibit a decrease of the coefficient of friction when used in combination with the mirror finished surface; the opposite tendency characterizes the polymers with high modulus (POM-H and PA66 + MoS2), which show a remarkable increase of the friction coefficient on the mirror-finished counterface. For PA66 + MoS2 most of friction measures on the smooth counterface could not be performed due to overload, i.e. friction force exceeding 15 kN. The friction coefficient of PA6 + wax, whose elastic modulus is intermediate among the tested materials’, is characterized by a negligible influence of the mating surface roughness, and it exhibits close values on both the counterfaces. The existence of a correlation between the elastic modulus of the polymer, the roughness of the counterface and the coefficient of dynamic friction is well illustrated by the friction maps reported in Fig. 5 that collects the results of all the tests at different velocities carried out at the contact pressure of 20 MPa, even though such trends were found to be not affected by the magnitude of the normal load, at least within the test range. According to the literature [7,14], a theoretical physical interpretation of this behaviour can be proposed based on the mechanism of formation of the polymer transfer film. For ‘‘soft”, 0.200
polished mirror finished
μdyn [-]
0.150
PA+wax
UHMWPE
0.100
PETP+ PTFE
PTFE
PA66+ MoS2
POM-H
0.000 1000
2000
4.3. Influence of contact pressure When sliding on the smoother counterface, all the tested engineering plastics show a decrease of the coefficient of dynamic friction as the normal load is increased, with the only exception of PETP + PTFE at low velocities, from 2.5 to 12.5 mm/s. On the polished counterface, PTFE, UHMWPE and PA6 + wax exhibit again lower values of the coefficient of dynamic friction at higher pressures, whichever the velocity; only for PA6 + wax some disturbances affect the friction coefficient at low velocity. An opposite behaviour is shown by ‘‘stiff” plastics, like PETP + PTFE, POM-H and PA66 + MoS2, with the tendency to increase friction as the contact pressure is increased too. Increasing of normal load influences both the deformation and the adhesion components of friction. As a consequence of the deformation of asperities, the true contact area becomes larger, with an increase of the friction forces, but also the strength of the adhesions can change. For most of polymers the overall effect consists in a decrease of friction with increasing of the normal load [20]. However in some cases the balance of adhesion and deformation contributions can produce an increase of friction: in previous tests, the friction coefficient of PA6, POM and UHMWPE was shown to increase after a corresponding increase of the normal load beyond a threshold value depending on the surface energy of the polymer [17]. As a general conclusion, for ‘‘soft” engineering plastics (like PTFE and UHMWPE in the present study), the tendency of the coefficient of dynamic friction to decrease with increasing of pressure is generally fulfilled in most of in-service conditions; nevertheless for most of filled engineering plastics the actual effect of the normal load on the coefficient of friction depends on many factors, including the roughness and the chemical composition of the counterface, and should be assessed case by case. 4.4. Influence of sliding velocity
0.050
0
low-modulus bulk engineering plastics the height of the asperities of the smooth mirror finished counterface is enough to allow the formation of an effective transfer film, with further improvement of the sliding behaviour of the polymer. On the polished counterface, the hard asperities of the steel surface dig deeply into the softer polymer and the deformation contribution to friction considerably increases, resulting in higher value of the coefficient of friction. On the contrary, for hard and high-modulus bulk polymers, characterized by a lower mobility of the molecular chains, the height of the asperities of the mirror finished counterface is not enough to promote the detachment and the adhesion of polymer fragments, and only on the rougher, polished surface the formation of a rudimental transfer film can occur. These findings suggest the existence of an optimal surface roughness for minimal friction for each engineering plastic, and that the relevant range of tolerance depends on the bulk properties of the polymers itself. A similar conclusion was also reported by other authors [13] in a study on PA, POM-H and PETP + PTFE.
3000
4000
5000
Elastic modulus [MPa] Fig. 5. Steady-state coefficient of dynamic friction of different engineering plastics versus elastic modulus at 20 MPa contact pressure and different peak velocities (2.5–12.5–22–42 mm/s); surface roughness: Ra = 0.02–0.08 lm (mirror finished surface), Ra = 0.10–0.20 lm (polished surface).
The influence of the sliding velocity on the steady-state regime value of the coefficient of friction is contrasting, as illustrated by the example given in Fig. 6 which shows the dependence of dynamic friction on sliding velocity exhibited by some among the tested self-lubricating plastics at 20 MPa pressure. For PTFE and UHMWPE (Fig. 6a), which are the polymers exhibiting the lowest values of friction, the coefficient of dynamic friction shows a monotonic increase with the velocity of sliding, whichever the value of contact pressure and the counterface roughness. ‘‘Stiffer” polymers (Fig. 6b) exhibit an initial increase of the coefficient of friction as velocity rises from low to moderate values, but at higher
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a
b
0.08
UHMWPE rough
PA6+wax sm ooth
POM H smooth
0.12
μdyn [-]
0.06
μdyn [-]
0.16
PTFE rough
0.04
UHMWPE smooth
PA6+wax rough
0.08
0.04
0.02
POM H rough
PTFE smooth 0.00
0.00 0
10
20
30
40
50
0
10
20
30
40
50
sliding velocity [mm/s]
sliding velocity [mm/s]
Fig. 6. Influence of the sliding velocity on the coefficient of dynamic friction of different engineering plastics in combination with either polished or mirror finished counterface. The specimens were loaded at 20 MPa contact pressure and tested at peak velocities of 2.5–12.5–22–42 mm/s.
velocities the coefficient of friction decreases; this behaviour is observed on both mirror finished and polished counterfaces. Almost all polymers exhibit visco-elasticity under certain conditions of load and strain rate. The tangential or frictional forces between asperities of polymer and steel counterface become a direct function of the sliding speed, and consequently the coefficient of dynamic friction is expected to increase with the velocity due to the increase of the deformation component. This behaviour was actually followed by all the tested polymers when the sliding speed was increased from low to moderate values; however for some polymers a decrease of friction occurred at higher velocities. It should be noted that the sliding speed also affects the coefficient of friction through its effect on the frictional heating. Most polymers melt at relatively low temperatures, and this characteristic, combined with the low thermal conductivity of both the polymer and the austenitic steel mating surfaces, ensures that frictional heat temperatures can easily reach the melting point of the polymer and cause its surface to melt. When the polymer melts, its friction tends to decrease, according to a mechanism of ‘‘thermal control of friction” [21]. The latent heat of melting imposes a temperature limit on the frictional temperatures in a polymer–counterface sliding contact: when the melting temperature of the polymer is reached any additional frictional heat released in the contact tends to melt additional polymer rather than cause the temperature of the already molten polymer to rise. As a result, the friction coefficient varies with sliding speed so that the temperature within the contact remains constant at the melting point. Good agreement with the model of limiting frictional heat was shown in a previous study [21] for the friction coefficients of polypropylene, polyamide and low density polyethylene. In particular, for polyamide the friction coefficient rises until a maximum value is achieved. At this point the friction coefficient determined by the ‘thermal control model’ equals the friction coefficient dictated by ‘solid state friction’. After reaching its maximum value the friction coefficient declines to a low level which may be of benefit in bearing design. It should be noted that the test bench used in the present study allowed to control the temperature of the bulk of the steel counterface, but due to the low thermal conductivity of austenitic steel the actual temperature at the sliding interface was more dictated by the frictional heat than from the bulk temperature. The development of frictional heat and the rise of temperature up to the melting point are enhanced by high speeds and high values of the coefficient of friction, so the range of sliding velocities considered in the tests was not enough to attain this effect for the polymers exhibiting the lowest values of coefficient of friction, like PTFE and UHMWPE.
4.5. Static coefficient of friction The opposite influence of the counterface roughness on the coefficient of dynamic friction of ‘‘soft” and ‘‘stiff” plastics is confirmed also for the static friction: replacing the mirror finished steel counterface by the polished one produces an increase of the breakaway friction for PTFE and PA6 + wax, and a decrease for POM-H and PA66 + MoS2. A further difference between the behaviour of ‘‘soft” and ‘‘stiff” plastics is revealed by comparing the values of ls,1 and ldyn. Usually the typical behaviour of most polymers is represented by a steady-state value of dynamic friction close to the value at breakaway; however some semicrystalline polymers characterized by a smooth molecular profile, like PTFE and UHMWPE, exhibit a different behaviour, consisting of a drop of more than 50% of friction after the first cycles of running-in and a very low dynamic value at the steady-state condition. This behaviour is a consequence of the creation of a continuous transfer film on the metal counterface and of the orientation of the molecular chains of the polymer within both the transfer film and the polymer slider during running-in, which is typical of the tribological behaviour of ‘‘soft” low-modulus polymers, which strongly reduces the resistance to relative motion [10].
5. Conclusions Although the roughness of the metal counterface is known to have considerable influence on the friction of self-lubricated thermoplastic polymers used in dry friction bearings, a systematic study combining the effects of such parameter with contact pressure and velocity for the most common engineering plastics is lacking. The available data for different plastics from the literature are not enough to build a consistent database, since friction and wear properties are not intrinsic material characteristics, but strongly depend on the layout of the tribological system. In this study the influence of the counterface roughness on the steady-state coefficient of dynamic friction of some of the most common engineering plastics has been investigated by means of a flat-on-flat bearing test model. The data obtained from the experiments allowed to generate friction maps combining the surface roughness with the effects of sliding velocity and normal loads as well as with the elastic properties (modulus of elasticity) of self-lubricating plastics currently used in bearing manufacturing. The following general conclusions can be drawn form the experimental data:
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1. An optimal surface roughness for minimum friction of the different engineering plastics seems to exist, and the relevant value depends on the bulk properties of the polymer itself. For low-modulus bulk polymers, among which PTFE and UHMWPE, the coefficient of dynamic friction has a lower value on very smooth counterfaces, as mirror finished ones (Ra = 0.02–0.08 lm), while for high-modulus plastics minimization of friction is attained on rougher surfaces (Ra = 0.10– 0.20 lm). This effect of controlled friction by surface asperities is different for each plastic depending on the relevant bulk stiffness and hardness. 2. Further, the same trend has been observed also for the static friction at the breakaway. 3. The dependence of the coefficient of friction on the contact pressure is affected by the properties of the metal counterface, including the chemical affinity with the polymer and the surface roughness. For most of engineering plastics increasing the normal load promotes a reduction of friction, but the actual behaviour of each polymer–counterface pair should be assessed case by case. 4. The sliding velocity influences the value of the coefficient of friction through two mechanisms: visco-elasticity and frictional heat. When the melting temperature of the polymer is reached at the sliding interface, friction stops increasing and declines to a lower value. The velocity corresponding to the inversion point is lower when the polymer is used in combination with the counterface roughness upon which the higher coefficient of friction is produced. Acknowledgement The authors wish to acknowledge Sara Mezzadri, MsC, for her contribution and assistance to the experiments. References [1] Stachowiak GW, Batchelor AW. Engineering tribology. Elsevier Science; 2005. [2] Hutchings IM. Tribology, friction and wear of engineering materials. London: Edward Arnold; 1992.
[3] Tewari US, Sharma SK, Vasudevan P. Polymer tribology. Rev Macromol Chem Phys 1989;C29:1–38. [4] Pascoe MW, Tabor D. The friction and deformation of polymers. Proc Royal Soc London 1956;A235:210–24. [5] Myshkin NK, Petrokovets MI, Kovalev AV. Tribology of polymers: adhesion, friction, wear, and mass-transfer. Tribol Int 2005;38:910–21. [6] Bely VA, Sviridenok AI, Petrokovets MI, Savkin VG. Friction and wear in polymer-based materials. Oxford: Pergamon Press; 1982. p. 416. [7] Samyn P, Quintelier J, Ost W, De Baets P, Schoukens G. Sliding behaviour of pure polyester and polyester-PTFE filled bulk composites in overload conditions. Polym Test 2005;24:588–603. [8] Bahadur S. The development of transfer layers and their role in polymer tribology. Wear 2000;245:92–9. [9] Gao J, Mao S, Liu J, Feng D. Tribochemical effects of some polymers/stainless steel. Wear 1997;212:238–43. [10] Pooley CM, Tabor D. Friction and molecular structure: the behaviour of some thermoplastics. Proc Royal Soc London 1972;A329:251–74. [11] Barrett TS, Stachowiak GW, Batchelor AW. Effect of roughness and sliding speed on the wear and friction of ultra-high molecular weight polyethylene. Wear 1992;153:331–50. [12] Dowson D, Challen JM, Atkinson JR. The influence of counterface roughness on the wear rate of polyethylene. In: Dowson D, Godet D, Taylor CM, editors. Proc third Leeds–Lyon symposium on tribology. Wear of non-metallic materials. London: Inst Mech Engrs Publ; 1978. p. 99–102. [13] Zsidai L, De Baets P, Samyn P, Kalacska G, Van Peteghem AP, Van Parys F. The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens. Wear 2002;253:673–88. [14] Tanaka K, Yamada Y. Influence of counterface roughness on the friction and wear of polytetrafluoroethylene- and polyacetal-based composites. J Syn Lubr 2006;5:115–31. [15] Liu CZ, Wu JQ, Li JQ, Renb LQ, Tong J, Arnell AD. Tribological behaviours of PA/ UHMWPE blend under dry and lubricating condition. Wear 2006;260:109–15. [16] Palabiyik M, Bahadur S. Tribological studies of polyamide 6 and high-density polyethylene blends filled with PTFE and copper oxide and reinforced with short glass fibres. Wear 2002;253:369–76. [17] Unal H, Mimaroglu A. Friction and wear behaviour of unfilled engineering thermoplastics. Mater Des 2003;24:183–7. [18] Unal H, Mimaroglu A, Kadıoglu U, Ekiz H. Sliding friction and wear behavior of polytetrafluoroethylene and its composites under dry conditions. Mater Des 2004;25:239–45. [19] Wang HG, Jian LQ, Pan BL, Zhang JY, Yang SR, Wang HG. Mechanical and tribological behaviours of polyamide 66/ultra high molecular weight polyethylene blends. Polym Eng Sci 2007. doi:10.1002/pen.20748. [20] Bowden FP, Tabor D. The friction and lubrication of solids. Oxford: Clarendon Press; 1954. [21] Ettles CMMcC. Polymer and elastomer friction in the thermal control regime. ASLE Trans 1987;30:149–59.