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Material transfer in polymer-polymer sliding
177
Wear, 46 (1978) 177 - 188 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
MATERIAL TRANSFER IN POLYMER-POLYMER
SLIDING*
V. K. JAIN and S. BAHADUR Mechanical Engineering Department University, Ames, Iowa (U.S.A.)
and Engineering
Research Institute, Iowa State
(Received July 5, 1977)
Summary The transfer of material in sliding between polyethylene films and polytetrafluoroethylene, polyvinyl chloride, polypropylene and polymethyl methacrylate discs and between Mylar films and polyethylene and polyvinyl chloride discs was investigated. Infrared spectroscopy and differential thermal analysis were used to detect the direction of material transfer. The transfer process was studied as a function of sliding time, speed and load. It was found that the transfer occurs under all conditions of rubbing, and invariably occurs from a material of low cohesive energy density to one of higher cohesive energy density. The thickness of the layer of material transferred has been estimated as a function of the above sliding variables by quantitative infrared spectroscopy. It is found that the layer thickness increases with sliding speed and time but decreases with load. The effect of this material transfer on the coefficient of friction has also been studied for one combination.
Introduction The phenomenon of material transfer during sliding between metal and metal, polymer and metal and polymer and polymer is important from both scientific and practical considerations. It offers a potential explanation of the frictional and wear behavior of the sliding combinations. The transferred material may deteriorate or improve the service characteristics of a system by directly taking part in a sliding operation. There are three things that contribute to material transfer: the deformation of surface asperities under load, the fracture of material in the substrate and the transfer of this material to the other surface.
*Paper presented at the International Conference on the Wear of Materials, St. Louis, MO., U.S.A., 26 - 28 April, 1977.
178
A number of investigators have studied the transfer of material during sliding from one surface to another. For example, Lancaster [l] reported the transfer of brass to hardened steel and the transition from mild to severe wear with varying conditions of sliding velocity, load and temperature. Brown and Armarego [2] found that there was a transfer from soft steel flats to hard steel riders, followed by back transfer to the flats and accompanied by a reduction in the friction and wear of the rider. Using riders and rings of the same metal, Cocks [3] found that there was a transfer from the larger to the smaller surface, with subsequent wear from the latter. There are numerous other cases reported in the literature verifying the transfer of material in sliding between similar and dissimilar metallic pairs [4-S]. The phenomenon of film transfer during sliding of polymers against glass and metal surfaces has been reported by a number of workers. Makinson and Tabor [9] reported a massive transfer of polytetrafluoroethylene (PTFE) to clean glass surfaces. Pooley and Tabor [lo] found that lumps of PTFE and high density polyethylene were transferred to glass and polished metal surfaces. Bowers et al. [ 111 found that there was a transfer of a thin PTFE film to steel surfaces and showed by electron diffraction that the film was oriented in the direction of sliding. The transfer of PTFE film to glass surfaces was explained by Tanaka et al. [12] and Makinson and Tabor [9] as being due to the slippage between crystalline slices of the banded structure of PTFE. The transfer of material during sliding between polymers has not received much attention. The only study reported in the literature is that of Sviridyonok et al. [ 131 . Using infrared spectroscopy, they found that the transfer of PTFE to polyethylene and polyethylene terephthalate and of polymethyl methacrylate (PMMA) to polycaproamide takes place under severe sliding conditions. In the present study the material transfer between polymers under varying rubbing conditions was investigated. The variation in the thickness of the layer of transferred material with changes in the rubbing parameters was studied using quantitative infrared spectroscopy. The interaction of material transfer with the frictional behavior of the sliding interfaces was also investigated. The direction of material transfer is related to a material property such as the cohesive energy density.
Experimental An experimental
set-up of the pin and disc type, described elsewhere polymers. Here the pin holder was replaced by a film holder as the sliding was to be performed between a disc and a film. Owing to the difficulties experienced in handling, the films were cut to a size of 0.075 m X 0.075 m and the edges were glued to a sheet of plain paper. The polymer film which was backed by paper was held firmly in the [ 141, was used for sliding between
179
film holder. A speed reducer with a reduction of 9:l was used to obtain low sliding speeds. The disc was moun~d either directly on the motor shaft or on the output shaft of the speed reducer. The friction force was measured using strain gauges mounted on a cantilever arm carrying the film holder. The output of the gauges was amplified and recorded. The sliding was performed between polyethylene films (100 pm thick) and polyvinyl chloride (PVC), polypropylene, PMMA and PTFE discs and between Mylar films (2.5 pm thick) and polyethylene and PVC discs. Thin films were used because the infrared spectroscopy technique was utilized to detect the material transfer. The sliding pairs were chosen so that the characteristic absorption peak(s) of the sliding polymers did not overlap. The latter was considered necessary in order to resolve the material transfer unambiguously. The parameters changed in the sliding process were load, speed and time of sliding. Since thin films were used, severe sliding conditions could not be used. The adsorbed layers from the surfaces of polymer films and the discs used in the rubbing operation were partially removed by washing them with propanol and storing them overnight in a desiccator. A Beckman IR-4250 double beam spectrophotometer was used for the infrared spectroscopy work. The spectra of the films before and after the sliding operation were obtained and compared to detect the material transfer. In cases where infrared spectroscopy could not reveal the material transfer because the transfer occurred to the disc material instead of the film material, the particles transferred to the disc surface were identified by differential thermal analysis using a Rigaku differential thermal analyzer.
Quantitative
infrared
spectroscopy
This technique was used to determine the thickness of the transferred material under a particular sliding condition. The amount of transferred material, termed the absorbance A, is given by Beer’s law [ 151 as = abc
(1)
where I0 is the mount of incident radiation, f the amount of ~~srnit~d radiation, b the internal cell length, c the concentration of absorbing molecules and a a proportionality constant. Since a and b are material constants, the higher the concentration of the absorbing molecules, the larger will be the absorbance. The base-line method [ 161 was used to determine the absorbance. It was preferred over other methods because here the cell absorptions and other absorptions not related to the band are minimized. In this method two wavenumber locations, X1 and Xa (Fig. l), are selected at each side of the band of interest as far removed as possible and free of absorption by all components of the sample. A straight line drawn tangentially to the
YAVE NUWER,
Fig. 1. Illustration
cm
-1
of the absorbance
measured
by the base-line
method.
spectrum curves locates the points corresponding to Xi and ha and provides the magnitudes of I, and I marked in Fig. 1. Thus the estimate of absorbance, or the amount of material transferred, is obtained from eqn. (1).
Results and discussion Sliding between the following polymer combinations was performed under varying conditions. The infrared spectroscopy and differential thermal analysis results relevant to particular combinations are described below. Polyethylene
film and PTFE
disc
This combination was selected because PTFE gives a very strong absorption band in the wavenumber range 1100 - 1300 cm-r which does not overlap the absorption peaks of polyethylene. A portion of the infrared spectra of polyethylene films that were rubbed against PTFE at varying speeds and loads and different times is shown in Fig. 2 together with the spectrum of unrubbed polyethylene. The transfer of PTFE to polyethylene is indicated by the change in the infrared spectrum of the polyethylene film in the wavenumber range 1100 - 1300 cm-‘. An absorption peak starts showing up at a wavenumber of about 1150 cm-‘. It appears to be due to the C-F stretching mode which is characteristic for PTFE. It should be noted that the intensity of the absorption peak increases with increasing speed and sliding time but decreases with increasing load. The thickness of the transferred PTFE layer was calculated by the baseline method using Fig. 2 and is plotted against the rubbing speed, load and time in Figs. 3, 4 and 5, respectively. The thickness initially increases with increasing sliding speed (Fig. 3) and time (Fig. 5) but finally levels off. This would indicate that there is an optimum thickness of the transferred PTFE material that stays on the polyethylene film surface. Sviridyonok et al. [ 131 have reported a similar variation in thickness with the time of sliding for the PTFE-polyethylene combination. Contrary to the above observed variation in thickness with sliding speed and time, the thickness of the transferred layer tends to decrease with increasing loads in this investigation.
181
0.26
(4
-
0.26
-
0.3
Y f
-0.3451 2 - 0.4
13m
12m
urn
WRVE I1oILR,aI-1 WI
Fig. 2. (For caption see overleaf.)
182
1200
1300
1100
WAVE NLMER,
1000
cm-'
Cc)
Fig. 2. Infrared spectra of polyethylene films rubbed against PTFE showing the intensity of the characteristic PTFE peak with varying (a) sliding speed, (b) load and (c) sliding time.
‘.i‘:“t_:
~~.:~~
0
a.025 0.05 0.075
0.1
0.125 0.15
0
Fig. 3. Variation ethylene.
with sliding speed
Fig. 4. Variation
with load
5w
loo0
1500
2000
25w
3ooa
Loho.
SLIDING SPEED, m/see
in the thickness
of the PTFE layer transferred
to poly-
in the thickness of the PTFE fayer transferred to polyethylene.
The variation in coefficient of friction with load and sliding time was also studied in order to investigate the effect of material transfer on the frictional behavior. It was found (Fig. 6) that the coefficient of friction decreases with sliding time but increases with load. The friction behavior reported here seems to be influenced by the material transfer process. As sliding commences, the transferred film of PTFE builds up with time which results in the sliding later occurring more and more between similar materials. Thus the coefficient of friction decreases rapidly with time but levels off
r
0.10,
5 ; BO.3
k
~0. ’ E 0.1 L
P10816 SPEED O.OZS m/see
2 e b
o.o.3 0.0
s " 2
0.04
__
183
LOAD 0 3000 A1500 02500 01000 0 2000 0 500 SLIDING SPEEB 0.025 M/SW
Y
ao0-I
SLIDING TIME, min
Fig. 5. Variation ethylene.
with sliding time in the thickness
of the PTFE
Fig. 6. Effect of load and sliding time on the coefficient rubbing against PTFE.
layer transferred
of friction
of polyethylene
to poly-
film
after a stable thickness of the film is deposited. At small loads the thickness of the PTFE layer on polyethylene is large which results in a small coefficient of friction. With higher loads the thickness of the film decreases and so the friction goes up. From the above it is noted that the transfer occurs under all conditions of sliding from the PTFE surface to the polyethylene surface, and the layer of transferred material affects the friction behavior of the sliding pair. Mylar film and polyethylene
disc
Whereas polyethylene has a very strong infrared absorption band in the wavenumber range 2600 - 3100 cm- l, Mylar shows hardly any absorption in this range. When the two were rubbed together, a new absorption peak appeared in the Mylar infrared spectrum at a wavenumber of about 2850 on the Mylar. As can be cm- ’ which showed the presence of polyethylene seen from Fig. 7, the intensity of absorption increases with sliding speed and
Fig. 7. Infrared spectra of Mylar films rubbed against polyethylene showing istic polyethylene peak with varying (a) sliding speed and (b) sliding time.
the character.
184
time. No specific trend in the change in absorption with varying loads was discernible. Since the vibration due to the CHz stretching mode occurs in the wavenumber range 1843 - 2863 cm-’ [ 171, the absorption peak at 2850 cm- ’ must be due to the transfer of polyethylene to Mylar. It can be seen from Figs. 8 and 9 that the thickness of the polyethylene layer transferred to the Mylar film increased with sliding time and speed, as was the case for the PTFE-polyethylene combination. However, the curves in the present case do not tend to level off. This would indicate the transfer of more and more material with increasing speed and time. Severe sliding conditions to verify this trend could not be used because the Mylar film was damaged at higher sliding speeds and time. Owing to the scatter in the data, it is not possible to say anything specific about the variation in thickness of the transferred layer with load (Fig. 10). Slight film damage, as observed in some tests at large loads, may be partly or wholly responsible for the large scatter.
O,Oil
0.7 0.75
SLIDING SPEED, mlsec
SLIDING TIME, mln
Fig. 8. Variation to Mylar film.
with sliding time in the thickness
Fig. 9. Variation Mylar film.
with sliding speed in the thickness
of the polyethylene
of the polyethylene
layer transferred
layer transferred
to
1 0.048 kO.O3-
z 2 0.02 a $O.O,_
8
E?
0
TIWE 30 nin SLIDING SPEED 0.408 m/WC
0.0 lOGa
I
I
zoo0
Fig. 10. Variation film.
I
I
2500 3000 LOAD, g
1
3500
I
4Ooa
with load in the thickness
of the polyethylene
layer transferred
to Mylar
It is further noted that the thickness of the transferred layer of polyethylene is much smaller than that of PTFE even though the sliding conditions used were more severe. This is probably because the wear rate of polyethylene is smaller than that of PTFE, which indicates that in the combination used saturation conditions were never reached.
185 Mylar film and PVC disc The ch~ac~r~tic infrared absorption peak due to the C-Cl stretching mode vibration at a wavenumber of about 690 cm-l for PVC appears on the Mylar spectrum (Fig. 11) when the two are rubbed together. This indicates the transfer of PVC to Mylar. From the spectra of Mylar films rubbed for different times, the absorption intensity was found to increase with sliding time. Using quantitative infrared spectroscopy, it was concluded that the thickness of the transferred layer of PVC also increased with time (Fig. 12). The change in the thickness of the layer of transferred PVC with varying speed and load conditions could not be studied for this combination because the Mylar film was damaged at severe speeds and loads. -0.1
‘5-
24
-0.
155
-0.
187
‘0 -
t
60-
--L+-15
ain
-.----20 _..-.._25
mln m,"
---30
min
LDAD 2oOg SLIOING SPEED 0,425 m/see -
I
750
700 UAVE NUIBER.
0. 225
ti50 cm-'
Fig. 11. Infrared spectra of Mylar films rubbed against PVC showing characteristic peak of PVC for different sliding times.
the intensity
of the
So.03 k t0.02 -I 2 60.01 ~!5tN6SPE~D
f
0.
SLIDING TIRE, mln
Fig. 12. Variation film.
with sliding time in the thickness
of the PVC layer transferred
to Mylar
186
Polyethylene
film and PMMA, PVC and polypropylene
discs
These combinations were selected because the characteristic infrared absorption peaks for PMMA (1123 and 1724 cm-‘), PVC (690 cm I) and polypropylene (820 - 1175 cm ‘) [ 171 do not overlap the absorption peaks of polyethylene. Sliding between polyethylene film and a PMMA disc was performed with varying sliding speeds in the range 0.375 - 0.55 m s-l and with loads of 80 - 800 g. The spectrum of rubbed polyethylene film did not show any of the characteristic peaks of PMMA under any sliding conditions. Instead, the intensity of some of the polyethylene absorption peaks decreased, which on investigation was found to be due to film damage. Similar experiments were performed with polyethylenePVC and polyethylene-polypropylene combinations and the results were the same. Thus it was concluded that none of the disc materials transferred to the polyethylene film surface. A close examination of the rubbing disc surfaces revealed the presence of some wear particles. In order to ascertain whether these particles were of the film or the disc material, differential thermal analysis of these particles was carried out; the melting points are given in Table 1. Since the melting point of the particles collected in each case was equal to the melting point of the unrubbed polyethylene film, the transfer of polyethylene to the disc materials was confirmed. TABLE
1
Melting
points
Material
of materials
and particles
involved
in sliding Melting point
and condition
(“C) Polyethylene, unrubbed PMMA, unrubbed* PVC, unrubbed* Polypropylene, unrubbed* Particles from rubbing between PMMA PVC Polypropylene
114 200 212 176 polyethylene
and 114 114 114
*The melting points of the materials marked with an asterisk were obtained from ref. 18 and those in the remaining cases were determined by differential thermal analysis.
Material transfer in sliding The above observations indicate that material transfer occurs from PTFE to polyethylene, from PVC and polyethylene to Mylar and from polyethylene to PVC, polypropylene and PMMA under all conditions. The direction of material transfer seemed to be associated with the properties of the materials involved in a sliding combination. An examination of the material
187
properties led to the conclusion that the transfer always occurs from a polymer with a low cohesive energy density to one of higher cohesive energy density (Fig. 13).
I
SO
II 60
Ill
7b
80
II
90
I
1M)
II 110
L 121
CM(ESIVE ElKR6V OE1(51l'V C.l,h3
Fig. 13. Material transfer direction for various combinations from material cohesive energy density to material of higher cohesive energy density.
of low
In the case of sliding between polymer films and smooth polymer discs, the process of material transfer must be largely due to adhesion between the contacting surfaces. The rubbing surfaces were found to heat up which is likely to result in increased adhesion. Thus any condition that promotes increased heating at the interface should result in greater adhesive wear. This does not mean that in this case the thickness of the transferred material layer will also be greater, because with increased contact pressure there will be a likelihood of more material being detached from the surface. Thus the increase in the thickness of the layer of deposited material with increasing sliding speed and time and the decrease in thickness with increasing load seem to agree with our understanding of the wear process. In the case of sliding, both adhesion and subsequent fracture are necessary, and fracture will always occur at the weaker of the two probable sites, the substrate and the interface. In the case of fracture at the interface, there would be hardly any perceptible material transfer. Thus the fact that material transfer occurs in sliding between polymers indicates that fracture occurs in the weaker substrate. This is well supported by the evidence that the transfer takes place from a material of low cohesive energy density to one of higher cohesive energy density.
Conclusions On the basis of the studies made, the following conclusions may be drawn. (1) The transfer of one polymer to another takes place under all conditions of sliding. (2) The material transfer takes place from a polymer of low cohesive energy density to one of higher cohesive energy density.
188
(3) The thickness of the layer of transferred material increases with increasing sliding speed and time, but decreases with increasing load.
Acknowledgments The authors gratefully acknowledge the generous support of this work by Grant GH-41427 from the National Science Foundation. The work was also supported by the Engineering Research Institute of Iowa State University.
References 1
J. K. Lancaster, The formation of surface films at the transition between mild and severe metallic wear, Proc. R. Sot. London, Ser. A, 273 (1355) (1963) 466 - 483. 2 R. H. Brown and E. J. A. Armarego, Some observations on the friction and wear of a hardened rider, Wear, 6 (2) (1963) 106 - 117. 3 M. Cocks, Interaction of sliding metal surfaces, J. Appl. Phys., 33 (7) (1962) 2152 2161. 4 M. Kerridge and J. K. Lancaster, The stages in a process of severe metallic wear, Proc. R. Sot. London, Ser. A, 236 (1205) (1956) 250 - 264. 5 M. Antler, Wear, friction and electrical noise phenomena in severe sliding systems, ASLE Trans., 5 (2) (1962) 297 - 307. 6 A. W. J. De Gee and J. H. Zaat, Wear of copper alloys against steel in oxygen and argon, Wear, 5 (4) (1962) 257 - 274. 7 M. Antler, Metal transfer and the wedge forming mechanism, J. Appl. Phys., 34 (2) (1963) 438 - 439. 8 M. Kerridge, Metal transfer and the wear process, Proc. Phys. Sot. London, Sect. B, 68 (427B) (1955) 400 - 407. 9 K. R. Makinson and D. Tabor, The friction and transfer of polytetrafluoroethylene, Proc. R. Sot. London, Ser. A, 281 (1384) (1964) 49 - 61. 10 C. M. Pooley and D. Tabor, Friction and molecular structure: the behavior of some thermoplastics, Proc. R. Sot. London, Ser. A, 329 (1578) (1972) 251 - 274. 11 R. C. Bowers, W. C. Clinton and W. A. Zisman, Frictional properties of plastics, Mod. Plast., 31(6) (1954) 131 - 144,210 - 213,220 - 225. 12 K. Tanaka, Y. Uchiyama and S. Toyooka, The mechanism of wear of polytetrafluoroethylene, Wear, 23 (2) (1973) 153 - 172. 13 A. I. Sviridyonok, V. A. Bely, V. A. Smurugov and V. G. Savkin, A study of transfer in frictional interaction of polymers, Wear, 25 (3) (1973) 301 - 308. 14 M. K. Kar and S. Bahadur, The wear equation for unfilled and filled polyoxymethylene, Wear, 30 (3) (1974) 337 - 348. 15 R. T. Conley, Infrared Spectroscopy, 2nd edn., Allyn and Bacon, Boston, 1972, pp. 221 - 238. 16 Proposed recommended practices for general techniques of infrared quantitative analysis, Am. Sot. Test. Mater. Proc., 59 (1959) 610 - 621. 17 A. R. H. Cole, Applications of infrared spectroscopy. In K. W. Benteley (ed.), Elucidation of Structures by Physical and Chemical Methods, Wiley-Interscienee, New York, 1963, Part 1, p. 143. 18 L. E. Nielsen, Mechanical Properties of Polymers, 1st edn., Reinhold, New York, 1962, pp. 33 - 35.
Wear, 46 (1978) 177 - 188 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
MATERIAL TRANSFER IN POLYMER-POLYMER
SLIDING*
V. K. JAIN and S. BAHADUR Mechanical Engineering Department University, Ames, Iowa (U.S.A.)
and Engineering
Research Institute, Iowa State
(Received July 5, 1977)
Summary The transfer of material in sliding between polyethylene films and polytetrafluoroethylene, polyvinyl chloride, polypropylene and polymethyl methacrylate discs and between Mylar films and polyethylene and polyvinyl chloride discs was investigated. Infrared spectroscopy and differential thermal analysis were used to detect the direction of material transfer. The transfer process was studied as a function of sliding time, speed and load. It was found that the transfer occurs under all conditions of rubbing, and invariably occurs from a material of low cohesive energy density to one of higher cohesive energy density. The thickness of the layer of material transferred has been estimated as a function of the above sliding variables by quantitative infrared spectroscopy. It is found that the layer thickness increases with sliding speed and time but decreases with load. The effect of this material transfer on the coefficient of friction has also been studied for one combination.
Introduction The phenomenon of material transfer during sliding between metal and metal, polymer and metal and polymer and polymer is important from both scientific and practical considerations. It offers a potential explanation of the frictional and wear behavior of the sliding combinations. The transferred material may deteriorate or improve the service characteristics of a system by directly taking part in a sliding operation. There are three things that contribute to material transfer: the deformation of surface asperities under load, the fracture of material in the substrate and the transfer of this material to the other surface.
*Paper presented at the International Conference on the Wear of Materials, St. Louis, MO., U.S.A., 26 - 28 April, 1977.
178
A number of investigators have studied the transfer of material during sliding from one surface to another. For example, Lancaster [l] reported the transfer of brass to hardened steel and the transition from mild to severe wear with varying conditions of sliding velocity, load and temperature. Brown and Armarego [2] found that there was a transfer from soft steel flats to hard steel riders, followed by back transfer to the flats and accompanied by a reduction in the friction and wear of the rider. Using riders and rings of the same metal, Cocks [3] found that there was a transfer from the larger to the smaller surface, with subsequent wear from the latter. There are numerous other cases reported in the literature verifying the transfer of material in sliding between similar and dissimilar metallic pairs [4-S]. The phenomenon of film transfer during sliding of polymers against glass and metal surfaces has been reported by a number of workers. Makinson and Tabor [9] reported a massive transfer of polytetrafluoroethylene (PTFE) to clean glass surfaces. Pooley and Tabor [lo] found that lumps of PTFE and high density polyethylene were transferred to glass and polished metal surfaces. Bowers et al. [ 111 found that there was a transfer of a thin PTFE film to steel surfaces and showed by electron diffraction that the film was oriented in the direction of sliding. The transfer of PTFE film to glass surfaces was explained by Tanaka et al. [12] and Makinson and Tabor [9] as being due to the slippage between crystalline slices of the banded structure of PTFE. The transfer of material during sliding between polymers has not received much attention. The only study reported in the literature is that of Sviridyonok et al. [ 131 . Using infrared spectroscopy, they found that the transfer of PTFE to polyethylene and polyethylene terephthalate and of polymethyl methacrylate (PMMA) to polycaproamide takes place under severe sliding conditions. In the present study the material transfer between polymers under varying rubbing conditions was investigated. The variation in the thickness of the layer of transferred material with changes in the rubbing parameters was studied using quantitative infrared spectroscopy. The interaction of material transfer with the frictional behavior of the sliding interfaces was also investigated. The direction of material transfer is related to a material property such as the cohesive energy density.
Experimental An experimental
set-up of the pin and disc type, described elsewhere polymers. Here the pin holder was replaced by a film holder as the sliding was to be performed between a disc and a film. Owing to the difficulties experienced in handling, the films were cut to a size of 0.075 m X 0.075 m and the edges were glued to a sheet of plain paper. The polymer film which was backed by paper was held firmly in the [ 141, was used for sliding between
179
film holder. A speed reducer with a reduction of 9:l was used to obtain low sliding speeds. The disc was moun~d either directly on the motor shaft or on the output shaft of the speed reducer. The friction force was measured using strain gauges mounted on a cantilever arm carrying the film holder. The output of the gauges was amplified and recorded. The sliding was performed between polyethylene films (100 pm thick) and polyvinyl chloride (PVC), polypropylene, PMMA and PTFE discs and between Mylar films (2.5 pm thick) and polyethylene and PVC discs. Thin films were used because the infrared spectroscopy technique was utilized to detect the material transfer. The sliding pairs were chosen so that the characteristic absorption peak(s) of the sliding polymers did not overlap. The latter was considered necessary in order to resolve the material transfer unambiguously. The parameters changed in the sliding process were load, speed and time of sliding. Since thin films were used, severe sliding conditions could not be used. The adsorbed layers from the surfaces of polymer films and the discs used in the rubbing operation were partially removed by washing them with propanol and storing them overnight in a desiccator. A Beckman IR-4250 double beam spectrophotometer was used for the infrared spectroscopy work. The spectra of the films before and after the sliding operation were obtained and compared to detect the material transfer. In cases where infrared spectroscopy could not reveal the material transfer because the transfer occurred to the disc material instead of the film material, the particles transferred to the disc surface were identified by differential thermal analysis using a Rigaku differential thermal analyzer.
Quantitative
infrared
spectroscopy
This technique was used to determine the thickness of the transferred material under a particular sliding condition. The amount of transferred material, termed the absorbance A, is given by Beer’s law [ 151 as = abc
(1)
where I0 is the mount of incident radiation, f the amount of ~~srnit~d radiation, b the internal cell length, c the concentration of absorbing molecules and a a proportionality constant. Since a and b are material constants, the higher the concentration of the absorbing molecules, the larger will be the absorbance. The base-line method [ 161 was used to determine the absorbance. It was preferred over other methods because here the cell absorptions and other absorptions not related to the band are minimized. In this method two wavenumber locations, X1 and Xa (Fig. l), are selected at each side of the band of interest as far removed as possible and free of absorption by all components of the sample. A straight line drawn tangentially to the
YAVE NUWER,
Fig. 1. Illustration
cm
-1
of the absorbance
measured
by the base-line
method.
spectrum curves locates the points corresponding to Xi and ha and provides the magnitudes of I, and I marked in Fig. 1. Thus the estimate of absorbance, or the amount of material transferred, is obtained from eqn. (1).
Results and discussion Sliding between the following polymer combinations was performed under varying conditions. The infrared spectroscopy and differential thermal analysis results relevant to particular combinations are described below. Polyethylene
film and PTFE
disc
This combination was selected because PTFE gives a very strong absorption band in the wavenumber range 1100 - 1300 cm-r which does not overlap the absorption peaks of polyethylene. A portion of the infrared spectra of polyethylene films that were rubbed against PTFE at varying speeds and loads and different times is shown in Fig. 2 together with the spectrum of unrubbed polyethylene. The transfer of PTFE to polyethylene is indicated by the change in the infrared spectrum of the polyethylene film in the wavenumber range 1100 - 1300 cm-‘. An absorption peak starts showing up at a wavenumber of about 1150 cm-‘. It appears to be due to the C-F stretching mode which is characteristic for PTFE. It should be noted that the intensity of the absorption peak increases with increasing speed and sliding time but decreases with increasing load. The thickness of the transferred PTFE layer was calculated by the baseline method using Fig. 2 and is plotted against the rubbing speed, load and time in Figs. 3, 4 and 5, respectively. The thickness initially increases with increasing sliding speed (Fig. 3) and time (Fig. 5) but finally levels off. This would indicate that there is an optimum thickness of the transferred PTFE material that stays on the polyethylene film surface. Sviridyonok et al. [ 131 have reported a similar variation in thickness with the time of sliding for the PTFE-polyethylene combination. Contrary to the above observed variation in thickness with sliding speed and time, the thickness of the transferred layer tends to decrease with increasing loads in this investigation.
181
0.26
(4
-
0.26
-
0.3
Y f
-0.3451 2 - 0.4
13m
12m
urn
WRVE I1oILR,aI-1 WI
Fig. 2. (For caption see overleaf.)
182
1200
1300
1100
WAVE NLMER,
1000
cm-'
Cc)
Fig. 2. Infrared spectra of polyethylene films rubbed against PTFE showing the intensity of the characteristic PTFE peak with varying (a) sliding speed, (b) load and (c) sliding time.
‘.i‘:“t_:
~~.:~~
0
a.025 0.05 0.075
0.1
0.125 0.15
0
Fig. 3. Variation ethylene.
with sliding speed
Fig. 4. Variation
with load
5w
loo0
1500
2000
25w
3ooa
Loho.
SLIDING SPEED, m/see
in the thickness
of the PTFE layer transferred
to poly-
in the thickness of the PTFE fayer transferred to polyethylene.
The variation in coefficient of friction with load and sliding time was also studied in order to investigate the effect of material transfer on the frictional behavior. It was found (Fig. 6) that the coefficient of friction decreases with sliding time but increases with load. The friction behavior reported here seems to be influenced by the material transfer process. As sliding commences, the transferred film of PTFE builds up with time which results in the sliding later occurring more and more between similar materials. Thus the coefficient of friction decreases rapidly with time but levels off
r
0.10,
5 ; BO.3
k
~0. ’ E 0.1 L
P10816 SPEED O.OZS m/see
2 e b
o.o.3 0.0
s " 2
0.04
__
183
LOAD 0 3000 A1500 02500 01000 0 2000 0 500 SLIDING SPEEB 0.025 M/SW
Y
ao0-I
SLIDING TIME, min
Fig. 5. Variation ethylene.
with sliding time in the thickness
of the PTFE
Fig. 6. Effect of load and sliding time on the coefficient rubbing against PTFE.
layer transferred
of friction
of polyethylene
to poly-
film
after a stable thickness of the film is deposited. At small loads the thickness of the PTFE layer on polyethylene is large which results in a small coefficient of friction. With higher loads the thickness of the film decreases and so the friction goes up. From the above it is noted that the transfer occurs under all conditions of sliding from the PTFE surface to the polyethylene surface, and the layer of transferred material affects the friction behavior of the sliding pair. Mylar film and polyethylene
disc
Whereas polyethylene has a very strong infrared absorption band in the wavenumber range 2600 - 3100 cm- l, Mylar shows hardly any absorption in this range. When the two were rubbed together, a new absorption peak appeared in the Mylar infrared spectrum at a wavenumber of about 2850 on the Mylar. As can be cm- ’ which showed the presence of polyethylene seen from Fig. 7, the intensity of absorption increases with sliding speed and
Fig. 7. Infrared spectra of Mylar films rubbed against polyethylene showing istic polyethylene peak with varying (a) sliding speed and (b) sliding time.
the character.
184
time. No specific trend in the change in absorption with varying loads was discernible. Since the vibration due to the CHz stretching mode occurs in the wavenumber range 1843 - 2863 cm-’ [ 171, the absorption peak at 2850 cm- ’ must be due to the transfer of polyethylene to Mylar. It can be seen from Figs. 8 and 9 that the thickness of the polyethylene layer transferred to the Mylar film increased with sliding time and speed, as was the case for the PTFE-polyethylene combination. However, the curves in the present case do not tend to level off. This would indicate the transfer of more and more material with increasing speed and time. Severe sliding conditions to verify this trend could not be used because the Mylar film was damaged at higher sliding speeds and time. Owing to the scatter in the data, it is not possible to say anything specific about the variation in thickness of the transferred layer with load (Fig. 10). Slight film damage, as observed in some tests at large loads, may be partly or wholly responsible for the large scatter.
O,Oil
0.7 0.75
SLIDING SPEED, mlsec
SLIDING TIME, mln
Fig. 8. Variation to Mylar film.
with sliding time in the thickness
Fig. 9. Variation Mylar film.
with sliding speed in the thickness
of the polyethylene
of the polyethylene
layer transferred
layer transferred
to
1 0.048 kO.O3-
z 2 0.02 a $O.O,_
8
E?
0
TIWE 30 nin SLIDING SPEED 0.408 m/WC
0.0 lOGa
I
I
zoo0
Fig. 10. Variation film.
I
I
2500 3000 LOAD, g
1
3500
I
4Ooa
with load in the thickness
of the polyethylene
layer transferred
to Mylar
It is further noted that the thickness of the transferred layer of polyethylene is much smaller than that of PTFE even though the sliding conditions used were more severe. This is probably because the wear rate of polyethylene is smaller than that of PTFE, which indicates that in the combination used saturation conditions were never reached.
185 Mylar film and PVC disc The ch~ac~r~tic infrared absorption peak due to the C-Cl stretching mode vibration at a wavenumber of about 690 cm-l for PVC appears on the Mylar spectrum (Fig. 11) when the two are rubbed together. This indicates the transfer of PVC to Mylar. From the spectra of Mylar films rubbed for different times, the absorption intensity was found to increase with sliding time. Using quantitative infrared spectroscopy, it was concluded that the thickness of the transferred layer of PVC also increased with time (Fig. 12). The change in the thickness of the layer of transferred PVC with varying speed and load conditions could not be studied for this combination because the Mylar film was damaged at severe speeds and loads. -0.1
‘5-
24
-0.
155
-0.
187
‘0 -
t
60-
--L+-15
ain
-.----20 _..-.._25
mln m,"
---30
min
LDAD 2oOg SLIOING SPEED 0,425 m/see -
I
750
700 UAVE NUIBER.
0. 225
ti50 cm-'
Fig. 11. Infrared spectra of Mylar films rubbed against PVC showing characteristic peak of PVC for different sliding times.
the intensity
of the
So.03 k t0.02 -I 2 60.01 ~!5tN6SPE~D
f
0.
SLIDING TIRE, mln
Fig. 12. Variation film.
with sliding time in the thickness
of the PVC layer transferred
to Mylar
186
Polyethylene
film and PMMA, PVC and polypropylene
discs
These combinations were selected because the characteristic infrared absorption peaks for PMMA (1123 and 1724 cm-‘), PVC (690 cm I) and polypropylene (820 - 1175 cm ‘) [ 171 do not overlap the absorption peaks of polyethylene. Sliding between polyethylene film and a PMMA disc was performed with varying sliding speeds in the range 0.375 - 0.55 m s-l and with loads of 80 - 800 g. The spectrum of rubbed polyethylene film did not show any of the characteristic peaks of PMMA under any sliding conditions. Instead, the intensity of some of the polyethylene absorption peaks decreased, which on investigation was found to be due to film damage. Similar experiments were performed with polyethylenePVC and polyethylene-polypropylene combinations and the results were the same. Thus it was concluded that none of the disc materials transferred to the polyethylene film surface. A close examination of the rubbing disc surfaces revealed the presence of some wear particles. In order to ascertain whether these particles were of the film or the disc material, differential thermal analysis of these particles was carried out; the melting points are given in Table 1. Since the melting point of the particles collected in each case was equal to the melting point of the unrubbed polyethylene film, the transfer of polyethylene to the disc materials was confirmed. TABLE
1
Melting
points
Material
of materials
and particles
involved
in sliding Melting point
and condition
(“C) Polyethylene, unrubbed PMMA, unrubbed* PVC, unrubbed* Polypropylene, unrubbed* Particles from rubbing between PMMA PVC Polypropylene
114 200 212 176 polyethylene
and 114 114 114
*The melting points of the materials marked with an asterisk were obtained from ref. 18 and those in the remaining cases were determined by differential thermal analysis.
Material transfer in sliding The above observations indicate that material transfer occurs from PTFE to polyethylene, from PVC and polyethylene to Mylar and from polyethylene to PVC, polypropylene and PMMA under all conditions. The direction of material transfer seemed to be associated with the properties of the materials involved in a sliding combination. An examination of the material
187
properties led to the conclusion that the transfer always occurs from a polymer with a low cohesive energy density to one of higher cohesive energy density (Fig. 13).
I
SO
II 60
Ill
7b
80
II
90
I
1M)
II 110
L 121
CM(ESIVE ElKR6V OE1(51l'V C.l,h3
Fig. 13. Material transfer direction for various combinations from material cohesive energy density to material of higher cohesive energy density.
of low
In the case of sliding between polymer films and smooth polymer discs, the process of material transfer must be largely due to adhesion between the contacting surfaces. The rubbing surfaces were found to heat up which is likely to result in increased adhesion. Thus any condition that promotes increased heating at the interface should result in greater adhesive wear. This does not mean that in this case the thickness of the transferred material layer will also be greater, because with increased contact pressure there will be a likelihood of more material being detached from the surface. Thus the increase in the thickness of the layer of deposited material with increasing sliding speed and time and the decrease in thickness with increasing load seem to agree with our understanding of the wear process. In the case of sliding, both adhesion and subsequent fracture are necessary, and fracture will always occur at the weaker of the two probable sites, the substrate and the interface. In the case of fracture at the interface, there would be hardly any perceptible material transfer. Thus the fact that material transfer occurs in sliding between polymers indicates that fracture occurs in the weaker substrate. This is well supported by the evidence that the transfer takes place from a material of low cohesive energy density to one of higher cohesive energy density.
Conclusions On the basis of the studies made, the following conclusions may be drawn. (1) The transfer of one polymer to another takes place under all conditions of sliding. (2) The material transfer takes place from a polymer of low cohesive energy density to one of higher cohesive energy density.
188
(3) The thickness of the layer of transferred material increases with increasing sliding speed and time, but decreases with increasing load.
Acknowledgments The authors gratefully acknowledge the generous support of this work by Grant GH-41427 from the National Science Foundation. The work was also supported by the Engineering Research Institute of Iowa State University.
References 1
J. K. Lancaster, The formation of surface films at the transition between mild and severe metallic wear, Proc. R. Sot. London, Ser. A, 273 (1355) (1963) 466 - 483. 2 R. H. Brown and E. J. A. Armarego, Some observations on the friction and wear of a hardened rider, Wear, 6 (2) (1963) 106 - 117. 3 M. Cocks, Interaction of sliding metal surfaces, J. Appl. Phys., 33 (7) (1962) 2152 2161. 4 M. Kerridge and J. K. Lancaster, The stages in a process of severe metallic wear, Proc. R. Sot. London, Ser. A, 236 (1205) (1956) 250 - 264. 5 M. Antler, Wear, friction and electrical noise phenomena in severe sliding systems, ASLE Trans., 5 (2) (1962) 297 - 307. 6 A. W. J. De Gee and J. H. Zaat, Wear of copper alloys against steel in oxygen and argon, Wear, 5 (4) (1962) 257 - 274. 7 M. Antler, Metal transfer and the wedge forming mechanism, J. Appl. Phys., 34 (2) (1963) 438 - 439. 8 M. Kerridge, Metal transfer and the wear process, Proc. Phys. Sot. London, Sect. B, 68 (427B) (1955) 400 - 407. 9 K. R. Makinson and D. Tabor, The friction and transfer of polytetrafluoroethylene, Proc. R. Sot. London, Ser. A, 281 (1384) (1964) 49 - 61. 10 C. M. Pooley and D. Tabor, Friction and molecular structure: the behavior of some thermoplastics, Proc. R. Sot. London, Ser. A, 329 (1578) (1972) 251 - 274. 11 R. C. Bowers, W. C. Clinton and W. A. Zisman, Frictional properties of plastics, Mod. Plast., 31(6) (1954) 131 - 144,210 - 213,220 - 225. 12 K. Tanaka, Y. Uchiyama and S. Toyooka, The mechanism of wear of polytetrafluoroethylene, Wear, 23 (2) (1973) 153 - 172. 13 A. I. Sviridyonok, V. A. Bely, V. A. Smurugov and V. G. Savkin, A study of transfer in frictional interaction of polymers, Wear, 25 (3) (1973) 301 - 308. 14 M. K. Kar and S. Bahadur, The wear equation for unfilled and filled polyoxymethylene, Wear, 30 (3) (1974) 337 - 348. 15 R. T. Conley, Infrared Spectroscopy, 2nd edn., Allyn and Bacon, Boston, 1972, pp. 221 - 238. 16 Proposed recommended practices for general techniques of infrared quantitative analysis, Am. Sot. Test. Mater. Proc., 59 (1959) 610 - 621. 17 A. R. H. Cole, Applications of infrared spectroscopy. In K. W. Benteley (ed.), Elucidation of Structures by Physical and Chemical Methods, Wiley-Interscienee, New York, 1963, Part 1, p. 143. 18 L. E. Nielsen, Mechanical Properties of Polymers, 1st edn., Reinhold, New York, 1962, pp. 33 - 35.