- Email: [email protected]
The effect of voids on the sliding friction of polyimide film
Wear, 37 (1976) 15 - 20 0 Elsevier Sequoia S.A., Lausanne
THE EFFECT FILM
R. G. BAYER
15 - Printed
in the Netherlands
OF VOIDS ON THE SLIDING
FRICTION
OF POLYIMIDE
and E. SACHER
IBM Corporation,
System Products Division, P. 0. Box 6, Endicott, N. Y. 13760
(Received
6, 197 5)
August
(U.S.A.)
Summary Voids 1.5 - 2 pm in diameter have been &found randomly distributed just under one surface of commercial polyimide film. Their presence leads to a significant reduction in friction and wear. The data do not suggest the source of the reduced friction.
Introduction We wish to report a phenomenon observed during a study of the sliding friction of selected polymer films: voids just under the loaded surface of polyimide film markedly alter the type of friction, lowering both the frictional coefficient and the wear. These voids (Fig. 1) are about 1.5 - 2 pm in diameter and randomly distributed. Their continued presence beneath the steel slider led to sample distortion (plastic flow) without measurable wear; on the other hand, their removal during sliding was signalled by an increased frictional coefficient in that area of the stroke, and its spreading could be followed as each subsequent stroke removed more of the voids. Experimental The sliding experiments were performed on a modified Bowden-Leben apparatus [l] , using a 1.27 cm diameter spherical slider of 52100 steel of V2 roughness. Samples of 5 mil DuPont Kapton type H polyimide film were peripherally secured on a support of 4140 alloy steel having a V16 finish. A constant 1 kg load was used and average velocities, held constant during a run, ranged from 0.046 - 12.3 cm s-l. All experiments were carried out at 24 ‘C, 35% RH. Results In most cases the maximum frictional coefficient pmax was in the range 0.29 - 0.32 and occurred in a regular stick-slip mode (Fig. 2(a)),
16
Fig. 1. SEM of the voids in Kapton (500
X ).
(a)
Fig. 2. (a) Regular stick-slip friction, proceeding from left to right at an average vetocity of 0.4 cm s-l. (b) The start of degradation, proceeding from left to right at an average velocity of 0.047 cm s-l I coated Kapton, proceeding from left to right at an average velocity of 0.04; 12 Y
gradually increasing to 0.38 - 0.44 after some 3500 strokes. In a few cases pmax began as low as 0.15 - 0.20 in a regular smooth mode before eventually degrading to the stick-slip mode previously found. When it did degrade, this occurred at one point in the wear path during the stroke, eventually spreading across the whole path as the sliding progessed (Fig. 2(b)).
Fig. 3. Optical micrograph of the scar over an area containing voids. Illumination through the film clearly shows the voids (63 x ). Fig. 4. SEM of the same scar as Fig. 3 (70
x ).
Fig. 5. A profilometer trace of the wear scar on polyimide whose voids were not worn away after 7 200 strokes. The trace is across the scar.
It was noted that, whenever the low kmax manifested itself, voids were invariably found beneath that surface. As the frictional coefficient increased with the number of strokes, microscope examination indicated that the surface had been worn through. Visual inspection indicated that the wearthrough matched that portion of the stroke, in Fig. 2(b), in which pmax was increasing. In most cases the voids were quickly worn through but, in the rare case when they were not, both friction and wear remained low. Figure 3 is an optical micrograph of a wear scar over an area containing voids and Fig. 4 is an SEM of the same surface, both after some 7200 strokes. The coefficient of friction remained smooth and regular, with a value of about 0.20, and a profilometer trace indicated the expected distortion with immeasurable wear (<0.5 pm), as seen in Fig. 5*. There was no wear on the steel slider. By comparison, both friction and wear were substantially higher in the absence of voids, at the same sliding velocity. The coefficient of friction “Measurable wear is defined [ 1 ] as that amount exceeding half the peak-to-peak surface roughness.
18
Fig. 6. A profilometer trace of the wear scar on polyimide without voids. Note the convex curvature, compared with the concave curvature of Fig. 5.
exhibited a regular stick-slip mode, with pmrnax= 0.38. After some 7200 strokes the wear scar was 780 - 960 pm wide and 12.5 - 16.5 I.rm deep (Fig. 6). In addition, the slider also exhibited considerable wear, of the order of a few microns (Fig. 7). Discussion The results indicate that the presence of voids 1.5 - 2 pm in diameter just below the loaded surface of polyimide film leads, under the experimental conditions employed here, to low friction and immeasurable wear of that surface. The voids appear randomly distributed and vary randomly in density. In most cases they are quickly worn through, suggesting that they lie very close to that surface. In the rare instance, however, they remained throughout the experiment, suggesting that they lay farther below that surface. The largest distance actually measured between void and surface was approximately 1.5 p m. Although these voids appear to arise during the manufacturing process, a knowledge of that process [2 - 51 does not suggest why only some areas of the film have voids or why they lie closer to one surface. Although the voids are somehow connected with low friction and wear, it is not clear how they function. For example, one would expect areas containing voids to deform more easily, leading to an increase in the coefficient of friction. Indeed, this has been found to be the case in our laboratory for rigid polyurethane foams. Two proprietary foams, differing only in density (0.48 and 0.72 g ml-l), were tested in the same apparatus at loads up to 1000 g. Three sets of samples were used: base foam, base foam lubricated with a light machine oil, and foam with its non-porous molding skin in place. In no case did the less dense foam give lower coefficients of friction; it always exhibited the same or higher values. It may be, however, that because the voids exist only in one layer they are not functioning as voids do in a foam. Perhaps, rather than fix on the voids in polyimide film, one should fix on what they do to the layer between them and the surface. A recent paper [6] considers the effect of a soft metal surface layer, which is continuously deformable under a slider, on a steel substrate. At some optimum thickness, it causes a significant decrease in both friction and wear. The optimum thickness is determined as
19
Fig. 7. Optical micrograph of the wear on the steel slider used with the polyimide in Fig. 6 (45 x ).
a surface layer thick enough not to be easily worn off and thin enough for dislocations formed at the interface to escape to the surface. In the present study, it is suggested that a minimum thickness between voids and surface is required. This is evidenced by the eventual wearing away of voids except in the rare case. As in the case of metals, the continued presence of the layer maintains both low friction and low wear. Since there is evidence for subsurface wear in this material [7], it may be that the presence of this layer permits a similar type of dislocation escape mechanism as that which occurs in the case of metals. The possibility also exists that the voids might be filled with some exudable material which might reduce the coefficient of friction, although a review of the manufacturing process [Z - 51 does not suggest this to be likely. To test this, some light machine oil was placed on a sample having eventually giving P = 0.1, but, I-1max = 0.44. It began to drop immediately, as seen in Fig. 2(c), it did not appear the same as the unlubricated material with voids. It is also possible that the voids per se do not influence the friction and wear but are associated with a structural or chemical change of the polymer in that vicinity. The improvement in friction and wear might then be associated with the modified polymer. However, IR evaluation of the film with voids does not give any indication of this nor does a review of the manufacturing process and chemistry indicate this as likely. It appears that, as with the presence and distribution of these voids, their action in,reducing both friction and wear is unknown at present. However, since low friction usually results in low wear, it is probable that the voids primarily affect the friction mechanism rather than the wear mechanism.
20
Conclusions Voids with diameters of 1.5 - 2 pm, randomly distributed just under one surface of polyimide film, have been shown to significantly reduce both sliding friction and wear. Acknowledgment Thanks are due to P. A. Engel for advice on thermal modes of wear.
and mechanical
References 1 ‘R. G. Bayer, W. C. Clinton, C. W. Nelson and R. A. Schumacher, Engineering model for wear, Wear, 5 (1962) 378 - 391. 2 C. E. Sroog, A. L. Endrey, S. V. Abramo, C. E. Barr, W. M. Edwards and K. L. Olivier, Aromatic polypyromellitimides from aromatic polyamic acids, J. Polym. Sci., Part A, 3 (1965) 1373 - 1390. 3 J. E. Kreuz, A. L. Endrey, F. P. Gay and C. E. Sroog, Studies of thermal cyclizations of polyamic acids and tertiary amine salts, J. Polym. Sci., Part A-l, 4 (1966) 2607 - 2616. 4 H. Lee, D. Stoffey and K. Neville, New Linear Polymers, McGraw-Hill, New York, 1967, Chap. 8. 5 N. A. Adrova, M. I. Bessonov, L. A. Laius and A. P. Rudakov, Polyimides, A New Class of Thermally Stable Polymers, Technomic, Stamford, Conn., 1970, Chap. 1. 6 S. Jahanmir, N. P. Suh and E. P. Abrahamson II, The delamination theory of wear and the wear of a composite surface, Wear, 32 (1975) 33 - 49. 7 R. G. Bayer, P. A. Engel and E. Sacher, Impact wear phenomena in thin polymer films, Wear, 32 (1975) 181 - 194.
THE EFFECT FILM
R. G. BAYER
15 - Printed
in the Netherlands
OF VOIDS ON THE SLIDING
FRICTION
OF POLYIMIDE
and E. SACHER
IBM Corporation,
System Products Division, P. 0. Box 6, Endicott, N. Y. 13760
(Received
6, 197 5)
August
(U.S.A.)
Summary Voids 1.5 - 2 pm in diameter have been &found randomly distributed just under one surface of commercial polyimide film. Their presence leads to a significant reduction in friction and wear. The data do not suggest the source of the reduced friction.
Introduction We wish to report a phenomenon observed during a study of the sliding friction of selected polymer films: voids just under the loaded surface of polyimide film markedly alter the type of friction, lowering both the frictional coefficient and the wear. These voids (Fig. 1) are about 1.5 - 2 pm in diameter and randomly distributed. Their continued presence beneath the steel slider led to sample distortion (plastic flow) without measurable wear; on the other hand, their removal during sliding was signalled by an increased frictional coefficient in that area of the stroke, and its spreading could be followed as each subsequent stroke removed more of the voids. Experimental The sliding experiments were performed on a modified Bowden-Leben apparatus [l] , using a 1.27 cm diameter spherical slider of 52100 steel of V2 roughness. Samples of 5 mil DuPont Kapton type H polyimide film were peripherally secured on a support of 4140 alloy steel having a V16 finish. A constant 1 kg load was used and average velocities, held constant during a run, ranged from 0.046 - 12.3 cm s-l. All experiments were carried out at 24 ‘C, 35% RH. Results In most cases the maximum frictional coefficient pmax was in the range 0.29 - 0.32 and occurred in a regular stick-slip mode (Fig. 2(a)),
16
Fig. 1. SEM of the voids in Kapton (500
X ).
(a)
Fig. 2. (a) Regular stick-slip friction, proceeding from left to right at an average vetocity of 0.4 cm s-l. (b) The start of degradation, proceeding from left to right at an average velocity of 0.047 cm s-l I coated Kapton, proceeding from left to right at an average velocity of 0.04; 12 Y
gradually increasing to 0.38 - 0.44 after some 3500 strokes. In a few cases pmax began as low as 0.15 - 0.20 in a regular smooth mode before eventually degrading to the stick-slip mode previously found. When it did degrade, this occurred at one point in the wear path during the stroke, eventually spreading across the whole path as the sliding progessed (Fig. 2(b)).
Fig. 3. Optical micrograph of the scar over an area containing voids. Illumination through the film clearly shows the voids (63 x ). Fig. 4. SEM of the same scar as Fig. 3 (70
x ).
Fig. 5. A profilometer trace of the wear scar on polyimide whose voids were not worn away after 7 200 strokes. The trace is across the scar.
It was noted that, whenever the low kmax manifested itself, voids were invariably found beneath that surface. As the frictional coefficient increased with the number of strokes, microscope examination indicated that the surface had been worn through. Visual inspection indicated that the wearthrough matched that portion of the stroke, in Fig. 2(b), in which pmax was increasing. In most cases the voids were quickly worn through but, in the rare case when they were not, both friction and wear remained low. Figure 3 is an optical micrograph of a wear scar over an area containing voids and Fig. 4 is an SEM of the same surface, both after some 7200 strokes. The coefficient of friction remained smooth and regular, with a value of about 0.20, and a profilometer trace indicated the expected distortion with immeasurable wear (<0.5 pm), as seen in Fig. 5*. There was no wear on the steel slider. By comparison, both friction and wear were substantially higher in the absence of voids, at the same sliding velocity. The coefficient of friction “Measurable wear is defined [ 1 ] as that amount exceeding half the peak-to-peak surface roughness.
18
Fig. 6. A profilometer trace of the wear scar on polyimide without voids. Note the convex curvature, compared with the concave curvature of Fig. 5.
exhibited a regular stick-slip mode, with pmrnax= 0.38. After some 7200 strokes the wear scar was 780 - 960 pm wide and 12.5 - 16.5 I.rm deep (Fig. 6). In addition, the slider also exhibited considerable wear, of the order of a few microns (Fig. 7). Discussion The results indicate that the presence of voids 1.5 - 2 pm in diameter just below the loaded surface of polyimide film leads, under the experimental conditions employed here, to low friction and immeasurable wear of that surface. The voids appear randomly distributed and vary randomly in density. In most cases they are quickly worn through, suggesting that they lie very close to that surface. In the rare instance, however, they remained throughout the experiment, suggesting that they lay farther below that surface. The largest distance actually measured between void and surface was approximately 1.5 p m. Although these voids appear to arise during the manufacturing process, a knowledge of that process [2 - 51 does not suggest why only some areas of the film have voids or why they lie closer to one surface. Although the voids are somehow connected with low friction and wear, it is not clear how they function. For example, one would expect areas containing voids to deform more easily, leading to an increase in the coefficient of friction. Indeed, this has been found to be the case in our laboratory for rigid polyurethane foams. Two proprietary foams, differing only in density (0.48 and 0.72 g ml-l), were tested in the same apparatus at loads up to 1000 g. Three sets of samples were used: base foam, base foam lubricated with a light machine oil, and foam with its non-porous molding skin in place. In no case did the less dense foam give lower coefficients of friction; it always exhibited the same or higher values. It may be, however, that because the voids exist only in one layer they are not functioning as voids do in a foam. Perhaps, rather than fix on the voids in polyimide film, one should fix on what they do to the layer between them and the surface. A recent paper [6] considers the effect of a soft metal surface layer, which is continuously deformable under a slider, on a steel substrate. At some optimum thickness, it causes a significant decrease in both friction and wear. The optimum thickness is determined as
19
Fig. 7. Optical micrograph of the wear on the steel slider used with the polyimide in Fig. 6 (45 x ).
a surface layer thick enough not to be easily worn off and thin enough for dislocations formed at the interface to escape to the surface. In the present study, it is suggested that a minimum thickness between voids and surface is required. This is evidenced by the eventual wearing away of voids except in the rare case. As in the case of metals, the continued presence of the layer maintains both low friction and low wear. Since there is evidence for subsurface wear in this material [7], it may be that the presence of this layer permits a similar type of dislocation escape mechanism as that which occurs in the case of metals. The possibility also exists that the voids might be filled with some exudable material which might reduce the coefficient of friction, although a review of the manufacturing process [Z - 51 does not suggest this to be likely. To test this, some light machine oil was placed on a sample having eventually giving P = 0.1, but, I-1max = 0.44. It began to drop immediately, as seen in Fig. 2(c), it did not appear the same as the unlubricated material with voids. It is also possible that the voids per se do not influence the friction and wear but are associated with a structural or chemical change of the polymer in that vicinity. The improvement in friction and wear might then be associated with the modified polymer. However, IR evaluation of the film with voids does not give any indication of this nor does a review of the manufacturing process and chemistry indicate this as likely. It appears that, as with the presence and distribution of these voids, their action in,reducing both friction and wear is unknown at present. However, since low friction usually results in low wear, it is probable that the voids primarily affect the friction mechanism rather than the wear mechanism.
20
Conclusions Voids with diameters of 1.5 - 2 pm, randomly distributed just under one surface of polyimide film, have been shown to significantly reduce both sliding friction and wear. Acknowledgment Thanks are due to P. A. Engel for advice on thermal modes of wear.
and mechanical
References 1 ‘R. G. Bayer, W. C. Clinton, C. W. Nelson and R. A. Schumacher, Engineering model for wear, Wear, 5 (1962) 378 - 391. 2 C. E. Sroog, A. L. Endrey, S. V. Abramo, C. E. Barr, W. M. Edwards and K. L. Olivier, Aromatic polypyromellitimides from aromatic polyamic acids, J. Polym. Sci., Part A, 3 (1965) 1373 - 1390. 3 J. E. Kreuz, A. L. Endrey, F. P. Gay and C. E. Sroog, Studies of thermal cyclizations of polyamic acids and tertiary amine salts, J. Polym. Sci., Part A-l, 4 (1966) 2607 - 2616. 4 H. Lee, D. Stoffey and K. Neville, New Linear Polymers, McGraw-Hill, New York, 1967, Chap. 8. 5 N. A. Adrova, M. I. Bessonov, L. A. Laius and A. P. Rudakov, Polyimides, A New Class of Thermally Stable Polymers, Technomic, Stamford, Conn., 1970, Chap. 1. 6 S. Jahanmir, N. P. Suh and E. P. Abrahamson II, The delamination theory of wear and the wear of a composite surface, Wear, 32 (1975) 33 - 49. 7 R. G. Bayer, P. A. Engel and E. Sacher, Impact wear phenomena in thin polymer films, Wear, 32 (1975) 181 - 194.