THE MECHANISM OF WEAR OF ... - Science Direct

Wear, 23 (1973) 153-172 0 Elsevier Sequoia %A., Lausanne - Printed in The Netherlands

THE MECHANISM

KYUICHIRO

153

OF WEAR OF POLYTETRAFLUOROETHYLENE

TANAKA, YOSHITAKA UCHIYAMA and SATORU TOYOOKA*

Faculty of Technology, Kanazawa University, Kanazawa (Japan) (Received June 20, 1972)

SUMMARY

Experiments to study the effects of heat treatment, speed and temperature on the friction and wear properties of polytetrafluoroethylene (PTFE) have been carried out. The mechanism of wear is discussed on the basis of the results of these experiments and electron microscopy of the friction surfaces. The wear rate is affected by the width of bands in the fine structure rather than the crystallinity, while the friction is little affected by both the factors. The effect of speed and temperature on the wear rate is expressed by a master curve. From the temperature dependence of the shift factor, the activation energy of the slippage between the crystalline slices in bands is estimated to be about 7 kcal/mol. As the result of easy slipping of crystalline slices, the destruction of banded structure occurs easily without any melting of the sliding surface and a film of about 300 A in thickness is produced on the surface. Since the film detaches easily from the surface, PTFE reveals a very high rate of wear. A mechanism of formation of the film is also proposed on the basis of electron microscopy of the worn surfaces.

INTRODUCTION

It is well known that polytetrafluoroethylene (PTFE) exhibits very low friction. On the other hand, PTFE is the polymer that reveals the highest wear among the crystalline polymers. According to our data obtained in wear tests of various polymers rubbed against a glass plate, the wear rate of PTFE is of the order of lo-’ cm/cm and those of the other crystalline polymers (polyethylene, nylon and polypropylene, for instance) are of the order of 10m9 cm/cm or lower. No explanation has been given to the origin of high wear of PTFE. In electron micrographs of PTFE structure, the structure appears as long bands with striations perpendicular to the length of the bands’. This structure is in marked contrast to the spherulitic structure of other crystalline polymers. Therefore, the high wear of PTFE seems to be attributable to the unusual fine structure. Makinson and Tabor’ discussed the friction and transfer of PTFE on the basis of the banded structure. However, they did not observe the band in the friction * Present address: Faculty of Science and Engineering, Saitama University, Urawa (Japan).

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K. TANAKA, Y. UCHIYAMA, S. TOYOOKA

surface of PTFE and transferred lumps. Steijn3 examined with the electron microscope the sliding tracks that were made in the simple sliding experiments with hardened steel sliders on PTFE and indicated that the sliding against PTFE involved drawing thin highly oriented films. An investigation of morphological structures in PTFE fibers produced by abrading the surface has been made by O’Leary and Gei14, who observed that the fibers were composed of microtibers or librils. In all the studies mentioned, no measurements of wear rate have been made and the deformation and destruction of banded structure in the wear process have not been observed. It is therefore desirable to carry out morphological investigations to determine the mechanism of wear of PTFE. The purposes of this work are, firstly, to obtain the effect of band width on wear properties, secondly, to find the effects of sliding speed and temperature on wear rate, and thirdly, to clarify the mechanism of wear from the data of wear experiments under various conditions and the electron microscopic observations of worn surfaces of PTFE. EXPERIMENTAL

Wear testing machines and experimental procedures

Two types of the pin-plate wear testing machines were used in this work. In both machines, the measurements of friction and amount of wear can be made continuously during the wear process. One (type I) can be used for experiments in vacuum as well as in air and the details of this type of machine have been reported previously’. By the use of a type I machine, the effects of heat treatment of PTFE specimen on wear properties were studied at about 22°C in a vacuum of about 4 x lo-’ torr to avoid disturbance due to the humidity in room air. The experiments

Fig. 1. Wear testing machine (type II). (a) PTFE pin, (b) glass plate, (c) base plate, (d) arm, (c) linear differential transformer, (f) ring-shaped plate spring, (g) strain gauges, (h) heating wire, (i) slip rings.

MECHANISM OF WEAR OF PTFE

155

in vacuum were made at a sliding speed of 30 cm/s and under a load of 1.5 kg. The second machine (type II), shown in Fig. 1, was used to study the effects of sliding speed and temperature on the wear properties under a load of 1 kg. In all experiments, the flat ends of PTFE rods of 3 mm diameter were rubbed against the glass plates. The diameters of frictional tracks on the glass plates in type I and type II machines are 4.5 cm and 5.0 cm, respectively. In type II machine, the temperature of the glass plate was usually measured by means of a copper-constantan thermocouple with the electromotive force led to a meter through the slip rings. In addition, the temperature was also measured by using an infrared radiation thermometer over a range of high sliding speeds. A linear differential transformer was used to measure the displacement of the arm which varied with the wear of PTFE. From the displacement the wear depth of the PTFE pm was determined. The frictional force was also measured by strain gauges attached to the ring-shaped spring plate shown in Fig. I. After the PTFE pin specimen was mounted to the specimen holder, the pin was initially rubbed against a 6/O grade emery paper placed on the glass plate. This pre-rubbing was useful for making a good contact between the PTFE pin and glass plate. Before the start of the wear test, the PTFE pin and glass surfaces were made as clean as possible by rubbing with an alcohol moistened cloth. In the wear depth versus sliding distance curve, there is a stationary stage, where the wear depth increases linearly with the increase of distance. The wear rate described in the following results is obtained during the stationary stage. The frictional surfaces were examined with an electron microscope as well as an optical microscope. A replica for the electron microscopy was made in the following procedure. The surfaces were replicated by pressing them to on a softened thin polymethylmethacrylate sheet (at about 1lOC). The sheet was shadowed with chromium and backed with carbon in a vacuum chamber, and the polymethylmethacrylate was dissolved away, leaving a carbon replica. Specimens The PTFE pins were shaped by turning the small blocks cut from a commercial Teflon plate of 5 mm thickness. In the experiments for the study of the effects of speed and temperature, the commercial Teflon was used without any heat treatment. On the other hand, live different heat-treated specimens were used in the experiments to study the effects of heat treatment. After the commercial Teflon blocks were heated for 60 min at 380°C they were cooled at two different rates. The slow-cooled specimens were prepared at a cooling rate of about 7.4”C/h and the rapid-cooled specimens were prepared by quenching in ice water, The specimens were also prepared by annealing the slow and rapid cooled Teflon as well as the commercial Teflon. The annealing was done by heating for 96 hours at 3OO“C.The specimens used in this work are listed in Table I with the values of band widths and degrees of crystallinity. The band widths were measured from the electron micrographs of the fracture surfaces obtained by impacting the specimens at liquid nitrogen temperature. Each specimen has bands with a range of widths, as seen in Table I. The degrees of crystallinity were determined by measuring the densities. A photographic plate glass was used as the glass plate rubbed with the PTFE pin.

156

K. TANAKA,

TABLE

Y. UCHIYAMA,

S. TOYOOKA

I

IDENTIFICATION Specimen

OF PTFE

SPECIMENS

Heat treatment

Degree of crystallinity

Width of bands

Cooling

Anneal

(%I

(pm)

A-l A-2 B-l B-2

untreated untreated slow (7.4”C/hr) slow (7.4”C/hr)

3OO”C, 96h 3OO”C, 96h

52 54 56 57

0.2-0.3 0.2-0.3 0.34.4 0.3-0.4

C-l

quenched

-

47

0.14.2

3OO”C, 96h

54

0.1-0.2

into

iced water c-2

quenched into iced water

EXPERIMENTAL

RESULTS

The effects of heat treatment on the friction and wear The relations between the wear depths of various specimens and the sliding distance are shown in Figs. 2 and 3. The coefficients of friction are also shown in the figures. It is seen that the wear depths increase linearly with increase in distance over a wide range of distances. On the other hand, the coefficients of friction have approximately constant values after a gradual decrease with distance following the initial higher values. Such a gradual decrease in friction is considered to be due partly to the transfer of PTFE film to the glass plate and partly to the gradual increase in temperature of the glass plate during the wear process. From polarized

0

2

4 sliding

6 distance

Fig. 2. Variations of wear depth 30 cm/s, in vacuum).

( 104cm )

and coefficient

of friction

with sliding

distance.

(Load

1.5 kg, speed

157


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0

+

A-2

A I

0-2

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I

2

4

sliding

I

I

6

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8

distance

loo 1

(lO+cm

Fin. 3. Variations of wear denth and coefficient of friction with sliding distance. (Load 1.5 kg, 3Ocm/s, in vacuum). -

0.3-

8

.

1.

6 Om3- a-J+-+go.2 -

.! &.2$ * o*,

8 :GgO-lI

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vz15-

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0

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F u15-

.

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.*.a

;10-

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L

cl 5g F 5.,,,,

y/-, $5

‘;O

O 0.1 0.2 03 0.4 b.n~.vjdth(#n) (-o-A-l,

0

.

O 45 50 55 6c degree ~fbfryslallinity(%)

+A-2,+8-1.&B-2,-0-C-l

.-s-C-2

)

Fig. 4. Plots of wear rate and coefficient of friction against band width and degree of crystallinity.

microscopic observations of the friction tracks on glass plates, it was found that the transfer of PTFE is mainly in the form of a fibrous thin film. In measuring the thickness of transferred film, aluminium was evaporated on the glass plates in vacuum after the wear tests and the friction tracks were examined with an interference microscope. As the result, it was deduced that the thickness was of the order of 300 A.

158

K. TANAKA, Y. IJCHIYAMA, S. TOYOOKA

The plots of the wear rates and coefficients of friction against the band width and the degree of crysbllinity are shown in Fig. 4(a) and (b), respectively. Although the crystallinity of the specimen C-l is appreciably different from that of the specimen C-2, the differences between these specimens are relatively small. In addition, it is noticed that the wear rates of both specimens are remarkably low in comparison with those of other specimens. The band width of the specimen C-2 is smaller than that of the specimen A-2, while both specimens have the same degree of crystallinity. Making a comparison between the specimen A-2 and C-2, it is also noticed that the wear rate of the specimen C-2 is markedly lower than that of the specimen A-2. Lontz and Kumnick6 obtained data showing that the wear of PTFE decreases as the crystallinity decreases. However, they took no account of the band width. The results shown in Fig. 4(a) and (b) indicate that the wear of PTFE is apparently affected by the band width rather than the crystallinity. Figure 4 also shows that the effects of the band width and crystallinity on friction are relatively small. The effects of sliding speed and temperature on friction and wear

In the experiments at various speeds and temperatures of glass plates, it was found that there are two types of wear depth versus sliding distance curves. The two types of the curves obtained at 50°C are illustrated in Fig. 5. It is seen that the curve at 20 cm/s is a straight line, while the curve at 5 cm/s has a stationary stage having a lower rate of wear following the initial transition stage having a higher rate of wear. In the following, we shall call the straight wear curve type 1 and the curve having an initial transition stage type 2. The wear rates in type 1 curves are nearly the same as the initial wear rates in type 2 curves, provided that both type

P

12

x3 8 6 4

2 I

1

I

5

10 stiding

I

15

distance

I

20

I

25

( ldcrn)

Fig. 5. Example of two types of the wear depth OS.sliding distance curves. (S~imen WC, load 1 kg).

A-1, temperature

159


sliding

speed

v(cm/s)

Fig. 6, Variations of wear rate and coefficient of friction with speed at temperatures (Specimen A-l, load 1 kg).

below 100°C.

+lZO’c *15or

+200%

sliding

speed

Vtcmls)

Fig. 7. Variations of wear rate and coefficient of friction with speed at temperatures (Specimen A-f, load 1 kg).

above 1CKYC.

curves are obtained at a given temperature. In addition, relatively high wear rates are generally obtained and the transfer of PTFE to glass plate is very small, if the type 1 curves are obtained. The relations between the wear rates and sliding speed are given in Figs. 6 and 7, where the temperature of the glass plate is taken as a parameter. Over temperatures up to about lOWC, it is noticed that the wear rate versus speed curves have peaks of wear rates and the peaks shift to higher speeds as the temperature is increased. On the other hand, the wear rate uer-sus speed curves over a higher temperature range seem to shift to lower speeds with increase in temperature.

160


l/T

( !n-3 'K-l')

Fig. 8. Master curves derived from Fig. 6. Fig. 9. Temperature dependence of the shift factor.

The coefficients of friction become smaller with increasing temperature and increase with increasing speed. Over a range of speeds below about 100 cm/s, parallel curves are obtained in the relation between the coefficient of friction p and the speed v in Fig. 6. The coefficient of friction follows the relation ~KZJ”,where the index n takes the value of 0.26 for a range of temperatures below 100°C. A master curve is derived from the experimental results in Fig. 6 by translating the curve obtained at a temperature T horizontally by an amount log ar to fit the reference curve at temperature To. The composite curve thus derived is shown in Fig. 8, with T,=23”C. It is well known that a time-temperature reduced law holds in the viscoelastic properties of polymers and a master curve is derived. The fact that the master curve is also obtained for friction has been found by Grosch7, Ludema and Tabor in rubbers and by Bahadur and Ludema’ in polymers. However, it is noticed that the master curve is also derived in the wear of PTFE. Figure 9 shows the temperature dependence of the shift factor ur obtained by deriving the master curve shown in Fig. 8. The temperature dependence of log aT can not be expressed by the Williams-Landel-Ferry equation lo , but by the Arrhenius form expressed as follows: log ur = AH/R( l/T-

l/T,)

(1)

where T is the absolute temperature, AH is the activaton energy and R is the universal gas constant. The value of activation energy of about 7 kcal/mol is deduced from Fig. 9 by using eqn. (1). It is interesting to note that this value is approximately equal to the value of activation energy obtained by Steijn” in the experiment on the friction of PTFE. The reason for the wear behaviour changes in the neighbourhood of 100°C is not clear. However, this may be related to the existence of the transition at 127°C found by McCrum”. Although the thickness of the transferred film is smaller than 0.1 pm in the neighbourhood of the speed at which the maximum rate of wear appears, thicker

MECHANISM

OF WEAR OF PTFE

161

films (1 N 2 pm) adhere on the glass plate under the condition of log olfr being approximately zero. When the value of urn is about 0.1 and 2.0, the f4ms have a thickness of about 0.1 p or smaller. It is obvious that the temperature of the friction surface is higher than the temperature of the glass plate indicated with the thermometer because of frictional heating. To obtain information about the temperature of the friction surface, a thermocouple embedded in a PTFE pin was rubbed against the glass plate. In this experiment, copper and constantan wires of 0.1 mm in diameter were used as the the~ocouple. The junction of the the~~ouple was initially embedded below the friction surface and it was made to rub against the glass plate as the wear of the pm proceeded. The thermocouple rubbing against glass plate indicated a temperature rise of 7°C at a sliding speed of 70 cm/s. This suggests that the effects of frictional heating on the results obtained in this work are relatively small. In our experience, melting occurs at the frictional surface with a speed of 70 cm/s and a load of 1 kg, if other polymers are rubbed against glass plate. The fact that the temperature rise due to frictional heating is relatively small may be attributed to the fact that the wear rate of PTFE is much higher and the film removed from the PTFE pin surface plays a role in heat dissipation.

Detailed electron microscopic examinations were made of the worn surfaces of PTFE pins rubbed under various conditions. However, the remarkable effects of sliding speed and temperature were not found in the features of the worn surfaces. This indicates that the microscopic wear mechanism of PTFE is similar under all conditions. In contrast, some differences were found in the features of worn surfaces by means of optical microscopy. The worn surfaces revealed characteristic features corresponding to the values of log ur u in the master curve shown in Fig. 8. However, useful information on the mechanism of wear could not be obtained from optical microscopy. Therefore, we shall describe mainly the results obtained by electron microscopic examinations of the surfmes rubbed in a vacuum at 30 cm/s and about 22°C. The chara~te~stics of worn surfaces of PTFE are that there exist a number of very long films and fibers. Based on our studies on other polymers, such a film and fiber appear only on PTFE worn surfaces. Figure 10 shows the most instructive electron micrograph, where the banded structure appears very clearly and the long films and fibers are laid down on the bands. On the basis of the shadowing angle used in the preparation of replica, the thickness of fAms is estimated at about 300 A. This vaIue is similar to the thickness of very thin ftims adhering on the glass plate. The feature in the area of the film marked by the arrow in Fig. 10 seems to suggest that the fiber is produced by the deformation and destruction of banded structure. The area where the bands are clearly exposed must correspond to the places where the films have been removed by transfer to the glass plate and have not been rubbed after the transfer. Since the band width is similar to that observed on the fracture surface of the PTFE block, it is clear that no melting occurs at the friction surface. The reason is that smaller widths of bands must be observed because of a rapid cooling of the surface at the termination of the wear test, provided melting occurs at the surface.

162


Fig. 10. Electron micrograph of the worn surface of PTFE. [Specimen A-l, load 1.5 kg, speed 30 cm/s, in vacuum).


163

Fig. 11. Electron micrograph of the worn surface of PTFE. (Specimen C-2, load 1.5 kg, speed 30 cm/s, in vacuum).

Figure 11 also suggests that the film originates from the bands. The ends of films appear in Fig. If and in the neighbourhood of the places indicated by the arrows the striations in adjacent bands have somewhat ordered arrangements along the length of the films. The feature usually observed in the worn surfaces is illustrated in Fig. 12, where the film covers over the banded structure. Places where the fnm is seen to be very smooth may correspond to places where they have been rubbed against the glass plate for a long period. A part of the film sometimes adheres to the replica which suggests that the film detaches somewhat easily from the frictional surface of PTFE. The films adhering to the replica are shown in Fig. 13. The selected area electron diffraction pattern of the film is also shown in the figure and this indicates that the PTFE chain molecules orient along the length of film. Figure 14 is an example of electron micrograph of the surface rubbed at 100 cm/s and 120°C. It is seen that the very thin film shown by the arrow remains after the relatively thick film has been separated. Therefore, this micrograph shows that the separation of film occurs not only just over the banded structure but also inside the film. In all the electron micrographs of lilms, there are

164

K. TANAKA, Y. U~~IYAMA, S. TOYOOKA

Fig. 12. Electron micrograph of the worn surface of PTFE. (Specimen A-l, load 1.5 kg, speed 30 cm/s, in vacuum).

many striations along the length of films and the distance between successive striations is relatively uniform. This suggests that the fnm is produced by the lateral connation of adjacent fibers. Figure 15 shows the electron micrograph of the friction track in the case of very small amounts of transfer. It is seen that the features are very different from that of the worn surface, while similar fdms as seen in the PTFE pin surface exist in a part of the photograph. The back-transferred films were sometimes observed in electron micrographs of PTFE pin surfaces. This is illustrated in Fig. 16, where the relatively large and small back-transferred films appear with the differences in the sliding direction. The fiber adhering to the replica is also shown in Fig. 16. This fiber does not have a uniform width but a bead-like shape. A similar fiber is the microtiber in the paper of O’Leary and Geil, who reported that it is composed of platelet-like units 60 A thick oriented normal to the fiber axis. The fibers protruding from the PTFE surface during replica formation were also observed by Schimmeli3. At the present time, the relation between the microfiber and the banded structure


165

Fig. 13. Electron micrograph and diffraction pattern of PTFE film adhered to the replica. (Specimen B-l, load 1.5 kg, speed 30 cm/s, in vacuum).

is not quite clear. When the sliding speed is extremely high, the melting of surface layer occurs over a considerable depth and the wear rate is very high. With such conditions, a film does not exist on the worn surface. Figure 17 shows an example of the electron micrograph of the worn surface produced by rubbing the PTFE pin against a cast iron disk with 8 kg/cm2 of contact pressure and a speed of 1,900 cm/s using the brake testing machine in our laboratory. It is seen that the surface is relatively rough and the bands have considerably smaller widths as a result of the rapid cooling of the surface when rubbing is ceased. A number of bead-like fibers also exist in Fig. 17. THE MECHANISM OF WEAR OF PTFE

In our experience, the film and fiber as observed at the worn surface of PTFE have not been observed in other polymers. Figure 18 shows an example of an

166


Fig. 14. Electron micrograph of the worn surkce of FWE. (Specimen A-l, temperature 12o”C, load 1 kg, speed 100 cm/s).

electron micrograph of the polypropylene worn surface rubbed against the glass plate in a vacuum with a 1.5 kg load at 30 cm/s. It is seen that there exists a flat smooth portion and a relatively rough portion. The existence of a very smooth portion suggests that melting of the friction surface occurs. The coeff&ient of friction between the polypropylene pin and the glass plate was about 0.8 and the wear rate was of the order of lo-’ cm/cm, in the case of Fig. 18. According to our studies of the wear of polymers with spherulitic structure, it has been observed that the destruction of spherulite does not occur by rubbing but that melting of a thin surface layer occurs. Therefore, the high rate of wear of PTFE must originate in the fact that no melting occurs on the friction surface under the ordinary conditions of wear tests and the film produced by the destruction of banded structure due to rubbing detaches easil from the friction surface. Since the thickness of the fdm is of the order of 300 gl the wear of PTFE proceeds discretely on a microscopic scale with the removal ii a unit thickness of about 300 A. The fact that the very thin film

MECHANISM

OF WEAR OF PTFE

Fig. 15. Electron micrograph of the friction track on glass plate. (Specimen A-l, load 1.5 kg, 30 cm/s, in vacuum).

167

speed

is transferred to the glass plate indicates that the transferred film adheres weakly to the glass plate and is removed continuously from the friction track in the wear process by the front edge of the PTFE pin. However, Fig. 15 shows that the transferred film detaches at the PTFE having an undefined structure transferred initially on the glass plate. Although the details of banded structure have not yet been completely explained, it has been ‘proposed that the’bands are composed of crystalline slices about 200 A thick, oriented normal to the length of band, and each slice is separated from its neighbour by an extremely thin amorphous region2’14. Speerschneider and Li14 consider that shearing occurs mainly by slippage within the amorphous region between the individual slices. Makinson and Tabor seem to believe that the thin film originates from the slippage within the amorphous region and a relatively massive fragment is produced by the fracture at the boundaries between the bands. We also believe a similar origin applies to the formation of the film. However, the existence of an inter-band fracture is quite doubtful, since our electron microscopic observations do not give any evidence for the existence of

168


Fig. 16. Electron micrograph of the worn surface of PTFE having the back-transferred films. (Specimen A-l, temperature 23”C, load 1 kg, speed 1 cm/s).

bands in the friction surfaces. Lump-like wear particles are frequently visible on the friction surfaces. It seems that they are probably produced by the pile up or wrapping of the thin films. Applying the model of banded structure mentioned above, it is considered that the fiber is produced by the serial connection of crystalline slices and the film is formed by the lateral connation of adjacent fibers. This concept of production of film is consistent with the facts that the films transferred to the glass plate and adhering to the replica have a molecular orientation in the direction of the length of the film. Since the film is extremely long, as shown in the electron micrographs, the view can be discarded that a fiber is composed of the connection of crystalline slices slipping in one band. A fiber is probably produced by the serial connection of ~rys~lline slices slipping in many adjacent bands. Figure 19 demonstrates our ideas on the mechanism of film formation. It is natural to consider. that the amorphous region between crystalline slices has a viscoelastic nature. Therefore, it is not suprising that the sliding speed

MECHANISM

OF WEAR

169

OF PTFE

Fig. 17. Electron micrograph of the worn surface of PTFE rubbed against 300°C. (Specimen A-l, contact pressure 8 kg/cm2, speed 1,900 cm/s).

the cast iron disk heated

at

dependencies of friction and ‘wear at various temperatures can be expressed by the master curves shown in Fig. 8, provided that the elemental mechanism of wear of PTFE is due to slip in the amorphous region. In other words, it is reasonable to consider the master curve reflects the viscoelastic nature of shear deformation in the amorphous region between crystalline slices. The small activation energy of 7 kcal/mol obtained from Fig. 9 indicates that the shear deformation or rupture of the amorphous region occurs very easily. From the fact that the wear rate of PTFE specimens with small width of bands is markedly small, it is also suggested that the slippage of crystalline slices corresponds to the elemental mechanism of wear. If the band width is smaller, a longer period may be necessary to produce a long fiber in comparison with the case of larger width bands. This seems to explain that PTFE specimens with small band widths have a smaller rate of wear. Although the films and fibers exist on the surfaces when rubbed against metal as well as glass, they are not observed when rubbed against an abrasive paper. Under conditions of abrasive wear, the mutual slippage of crystalline slices does


170

Fig. 18. An example of electron micrograph of the worn surface of pol ypropylene. (Load 1.5 kg, speed 30 cm/s, in vacuum).

a* J

mo!ecu le

cf ystalline siice

-------_-----

undefamed bands

---------F-v-

slid&

-Film

c---w

p--m_-----------------------------

Fig. 19. Proposed m~hanism structure.

Ir

unckfamed bands

of the formation of PTFE film due to the destruction of the b,anded

MECHANISM

OF WEAR

171

OF PTFE

not occur gently, but the destruction of the banded structure appears to be a severe random fracture process. Therefore, the fact that the films are not observed in abrasive wear supports the conclusion that the film originates from the slippages between crystalline slices. CONCLUSIONS

The wear rate of PTFE is affected by the band width rather than the degree of crystallinity. The PTFE quenched into iced water from a melt has bands of a range of widths (0.1 to 0.2 pm) and reveals a markedly low wear rate in comparison with commercial PTFE and slow-cooled PTFE. On the other hand, friction is little affected by the band width and crystallinity. The wear rate uersus sliding speed curve reveals a wear rate maximum, provided the temperature of the glass plate rubbed by PTFE is lower than about 100°C. A master cuve is derived by translating the wear rate versus speed curve obtained at a given temperature by an amount along the speed axis. A master curve seems also to be obtained in a coefficient of friction uersus speed curve, while a peak is not observed for the sliding conditions used in this work. In the temperature range above about lOO”C, a master curve could not be obtained. The mechanism of wear of PTFE can be explained by applying the model of banded structure proposed by Speerschneider and Li. From the temperature dependence of the shift factor in obtaining a master curve, it is estimated that the activation energy of the slippage between crystalline slices in bands is about 7 kcal/mol. Because of the low activation energy, the destruction of bands occurs easily without any melting of the frictional surface under ordinary wear conditions, and a film of about 300 A in thickness is produced on the friction surface of PTFE. Since the film detaches easily from the friction surface, the wear proceeds discretely by the removal of a unit thickness of about 300 A. Owing to the mechanism of wear mentioned above, the wear rate of PTFE must have a very high value in comparison with the other crystalline polymers having a spherulitic structure, because the destruction of spherulite does not occur but melting of a thin surface layer does. It is considered that the long fiber is produced by the serial connection of crystalline slices slipping in many adjacent bands during the wear process and the long film is produced by the lateral connection of adjacent fibers. ACKNOWLEDGEMENTS

We wish to express our thanks to Mr. H. Kato and Mr. S. Ueda for their assistance in making the wear testing machine and in measuring the friction and wear. We also wish to thank Dr. S. Iwayanagi of the Institute of Physical and Chemical Research for helpful discussion. REFERENCES 1 C. W. Bunn, A. J. Cobbold and R. P. Palmer, J. Polymer Sci., 38 (1958) 365. 2 K. Rachel Makinson and D. Tabor, Proc. Roy. Sm. (London), Ser. A, 281(1964)

49.

172 3 4 5 6 7 8

9 10 11 12 13 14


R. P. Steijn, Wear, 12 (1968) 193. K. O’Leary and P. H. Geil, J. Appl. Phys., 38 (1967) 4169. K. Tanaka, Y. Uchiyama and S. Toyooka, J. Japan Sot. Lub. Engrs., 12 (1967) 31 (in Japanese). J. F. Lontz and M. C. Kumnick, ASLE Trans., 6 (1963) 276. K. A. Grosch, Proc. Roy. Sot. (London), Ser. A, 274 (1963) 21. K. C. Ludema and D. Tabor, Wear, 9 (1966) 329. S. Bahadur and K. C. Ludema, Wear, 18 (1971) 109. J. D. Ferry, Viscoelastic Properties of Polymers, Wiley-Interscience, New York, 1961. R. P. Steijn, ASLE Trans., 11 (1968) 235. N. G. McCrum, J. Polymer Sci., 34 (1954) 355. G. Schimmel, J. Polymer Sci., 36 (1956) 522. C. J. Speerschneider an4 C. H. Li, J. Appl. Phys., 33 (1962) 1871.