- Email: [email protected]
Fluid film lubrication of rubber—an interferometric study
Wear -
Elsevier Sequoia S. A., Lausanne - Printed in the Netherlands
163
Short Communication Fluid film lubrication of rubber-an
interferometric
study
Inte~eromet~~ methods have been used to study the thickness and profile of fluid films between polymersl, between steel and glasse~s,and between rubber and gla&. During the last few years we have been studying the extrusion of liquid films between rubber and a hard transparent solid such as glass or quartz. The earlier work showed that conventional rubber surfaces are rough and poorly reflecting so that interferometric observations were unsatisfactory. This difficulty was solved by Blok by covering the rubber surface with a thin sheet of smooth plastic aluminised on its outer face. In our preliminary studies we were able to obtain good interference patterns by gilding the rubber surface itself. Recently with guidance from N.R.P.R.A. and with the assistance of the Avon Rubber Company we have been able to prepare rubber surfaces of spherical, cylindrical and flat form which are optically smooth and, without gilding, give excellent interference patterns against glass even when very large elastic deformations occur, The interference between a flat glass plate and a hemisphere or hemicylinder of smooth rubber has now been examined by reflected monochromatic light(1=5460 A) through the glass plate. In the absence of liquid films the interference is strong: in the presence of a liquid film of intermediate refractive index the interference is weaker and the contrast between the interference pattern and back scattered light is greatly reduced with a subsequent loss in fringe visibility. This deficiency has been largely overcome by depositing, on the back surface of the glass, an anti-reflection coating. The mbber was held in a fixed position with its convex surface uppermost and in the focal plane of a low power microscope. The glass plate was held in a horizontal position above the rubber surface and loaded on to it. The plate could be drawn across the rubber and the frictional drag on it measured. The liquid used in the results described here was a silicone fluid of viscosity IOOO cSt and refractive index 1.40.The refractive indices of the glass and rubber were almost identical cu. I.51 *o.oz. Rzlbber hemisphere Interferometric results for a rubber hemisphere of radius of curvature 2.3 cm are shown in Fig. I (a), (b), (c), and deduced profiles in Fig. 2. Figure I (a) shows static contact in the presence of liquid 5 set after the normal load has been applied: the trapped “bell” of liquid is clearly recognisable and its contour is plotted as line a in Fig. 2. Figure r(b) shows contact after 5 min when most of the liquid has been squeezed out, and the corresponding contour is shown as line b in Fig. 2. The results are similar to those described by DOWSONAND JONES5 for steel on glass and in a much earlier investigation by RABINOWICZ~. After about 15 h the interference fringes disppeared leaving a uniform dark tint over the whole contact area. At this stage the film is less than a few hundred Angstroms thick. In these experiments the mean contact pressure is of the order of 300 g cm-z. Wear,
II
(1968)
SHORT COMMUNICATIONS
164
Fig. I. Interferograms formed between curved rubber surfaces and a flat glass plate in the presence of a silicone fluid. (a) Spherical surface, static contact under ro g load after 5 set loading, showing trapped bell of fluid. (x 21). (b) Spherical surface, after 5 min loading. (x 21). (c) Spherical surface, sliding at a speed of 0.10 cm set-i showing wedge-shaped contour at the fluid entrance (on the left) and horse-shoe shaped protuberance at the exit. ( x 21). (d) Cylindrical surface sliding at o. 15 cm set-i. ( x go). The interference is between surfaces without reflection-coatings.
at a (left This steel
Figure I(C) shows the results obtained when the glass surface was set sliding speed of 0.10 cmjsec. It shows a wedge shaped contour at the fluid entrance hand side of the figure) and a horse-shoe shaped protruberance at the exit. closely resembles the results previously published by CAMERON AND GOHAR~ for on glass. The corresponding contour along the dotted line is drawn in Fig. 2(c).
Rubber hemicylinder Similar experiments again entrapped when the a second or two. When the interference pattern shown Weav, II (1968)
were carried out with a rubber hemicylinder. Liquid was surfaces were first placed in contact but escaped within glass plate was set sliding at a speed of 0.15 cmjsec the in Figure r(d) was observed: the corresponding contour
165
SHORT COMMUNICATIONS
is shown in Fig. 3. This shows a relatively linear portion BC and a typical constriction at the exit region D. It is interesting to consider this as a simpKetilted pad bearing surface, the mean slope being given by BC, and the minimum film thickness by the average of CF and DG. Using the standard hydrodynamic treatment in SHAWAND BEACKS~ this gives a calculated coefficient of friction of ,H=0.03 ; the measured value was 0.04. h
Film Thickness
b
16
1
2
3
4
5
t
.
I
0
6.
7
8
x
9
Diameter Fig. 2. Profiles of spherical rubber surfaces across the diameter of contact as deduced from the interferograms. (a) Profile corresponding to Fig. ~(a) ; (b) corresponding to Fig. 1(b) ; (c) corresponding to Fig. I(C) across the dotted line. (Along the x-axis 3.5 divisions = I mm and along the h axis I division=zooo A.)
Some of the results quoted here have already been described by other workers, employing different materials and different arrangements. The main contribution of the present work is to show that by using optically smooth rubber and carefully designed interferometric techniques it is possible to study, in considerable detail, the contact between surfaces subjected to large elastic deformations under relatively small stresses both in the absence and in the presence of liquid films. The low modulus of rubber implies that the contact pressures are low and
166
SHORTCOMMUNICATIONS
that the material can be easily deformed to provide a convenient shape for effective hydrodynamic lubrication. The results also suggest more generally that the trapping of liquid films between deformable surfaces may play an important part in preventing contact between stationary or slowly moving surfaces. h
Film Thickness
Diameter
Fig. 3. Profile of cylindrical rubber surface corresponding to interferogram the x-axis I division = 0.1 mm and along the L axis I division = zooo A).
in Fig. r(d). (Along
We wish to thank the ministry of Technology for general support, TricoFolberth for a research grant to one of us (A.D.R.) and the Avon Rubber Company for a grant for equipment. .%--ace
Pdiysics, C~v~~dish L~bor~o~~, of Cambridge (Gt. Britais,l
U~i~eysit~
A. D. ROBERTS D. TABOR
I M. T. KIRK, Nature, 194 (1962) 965. J. F. ARCHARXI AND M. T. KIRK, I*tst. Me&. Engvs. Lubrication and Wear Conventaost, rp63. 2 A. CAMERONAND R. GOHAR, Nature, aoo (1963) 458; PYOC.Roy. Sot. (London), A291 (1966) 520. 3 A. CAMERON AND F. J. WESTLAKE, Natuve, a14 (1967) 633. 4 H. BLOK AND H. J. KOENS, Inst. Mech. Engrs. Elasta~ydro~~am~c Lubricatk Sym$osizlm,
1965. 5 D. DOWSON
AND D. A. JONES, Xatwe, 214 (rg67) 947. 6 E. RABINOWICZ, Pvoc. Phys. Sot., 365 (1952) 630. F; P. BOWDEN AND I). TABOR, Fvictb and L&r&a&m of SoEa’ds,Part I, Clarendon Press, Oxford, 1950, Chap. XIII. 7 M. C. SHAW AND 3%. F. MACKS, Analysis and Lufwkatiolt of Bea&zgs, McGraw-Hill, New York,
r949.
Received Wear,
II
October (1968)
I, x967
Elsevier Sequoia S. A., Lausanne - Printed in the Netherlands
163
Short Communication Fluid film lubrication of rubber-an
interferometric
study
Inte~eromet~~ methods have been used to study the thickness and profile of fluid films between polymersl, between steel and glasse~s,and between rubber and gla&. During the last few years we have been studying the extrusion of liquid films between rubber and a hard transparent solid such as glass or quartz. The earlier work showed that conventional rubber surfaces are rough and poorly reflecting so that interferometric observations were unsatisfactory. This difficulty was solved by Blok by covering the rubber surface with a thin sheet of smooth plastic aluminised on its outer face. In our preliminary studies we were able to obtain good interference patterns by gilding the rubber surface itself. Recently with guidance from N.R.P.R.A. and with the assistance of the Avon Rubber Company we have been able to prepare rubber surfaces of spherical, cylindrical and flat form which are optically smooth and, without gilding, give excellent interference patterns against glass even when very large elastic deformations occur, The interference between a flat glass plate and a hemisphere or hemicylinder of smooth rubber has now been examined by reflected monochromatic light(1=5460 A) through the glass plate. In the absence of liquid films the interference is strong: in the presence of a liquid film of intermediate refractive index the interference is weaker and the contrast between the interference pattern and back scattered light is greatly reduced with a subsequent loss in fringe visibility. This deficiency has been largely overcome by depositing, on the back surface of the glass, an anti-reflection coating. The mbber was held in a fixed position with its convex surface uppermost and in the focal plane of a low power microscope. The glass plate was held in a horizontal position above the rubber surface and loaded on to it. The plate could be drawn across the rubber and the frictional drag on it measured. The liquid used in the results described here was a silicone fluid of viscosity IOOO cSt and refractive index 1.40.The refractive indices of the glass and rubber were almost identical cu. I.51 *o.oz. Rzlbber hemisphere Interferometric results for a rubber hemisphere of radius of curvature 2.3 cm are shown in Fig. I (a), (b), (c), and deduced profiles in Fig. 2. Figure I (a) shows static contact in the presence of liquid 5 set after the normal load has been applied: the trapped “bell” of liquid is clearly recognisable and its contour is plotted as line a in Fig. 2. Figure r(b) shows contact after 5 min when most of the liquid has been squeezed out, and the corresponding contour is shown as line b in Fig. 2. The results are similar to those described by DOWSONAND JONES5 for steel on glass and in a much earlier investigation by RABINOWICZ~. After about 15 h the interference fringes disppeared leaving a uniform dark tint over the whole contact area. At this stage the film is less than a few hundred Angstroms thick. In these experiments the mean contact pressure is of the order of 300 g cm-z. Wear,
II
(1968)
SHORT COMMUNICATIONS
164
Fig. I. Interferograms formed between curved rubber surfaces and a flat glass plate in the presence of a silicone fluid. (a) Spherical surface, static contact under ro g load after 5 set loading, showing trapped bell of fluid. (x 21). (b) Spherical surface, after 5 min loading. (x 21). (c) Spherical surface, sliding at a speed of 0.10 cm set-i showing wedge-shaped contour at the fluid entrance (on the left) and horse-shoe shaped protuberance at the exit. ( x 21). (d) Cylindrical surface sliding at o. 15 cm set-i. ( x go). The interference is between surfaces without reflection-coatings.
at a (left This steel
Figure I(C) shows the results obtained when the glass surface was set sliding speed of 0.10 cmjsec. It shows a wedge shaped contour at the fluid entrance hand side of the figure) and a horse-shoe shaped protruberance at the exit. closely resembles the results previously published by CAMERON AND GOHAR~ for on glass. The corresponding contour along the dotted line is drawn in Fig. 2(c).
Rubber hemicylinder Similar experiments again entrapped when the a second or two. When the interference pattern shown Weav, II (1968)
were carried out with a rubber hemicylinder. Liquid was surfaces were first placed in contact but escaped within glass plate was set sliding at a speed of 0.15 cmjsec the in Figure r(d) was observed: the corresponding contour
165
SHORT COMMUNICATIONS
is shown in Fig. 3. This shows a relatively linear portion BC and a typical constriction at the exit region D. It is interesting to consider this as a simpKetilted pad bearing surface, the mean slope being given by BC, and the minimum film thickness by the average of CF and DG. Using the standard hydrodynamic treatment in SHAWAND BEACKS~ this gives a calculated coefficient of friction of ,H=0.03 ; the measured value was 0.04. h
Film Thickness
b
16
1
2
3
4
5
t
.
I
0
6.
7
8
x
9
Diameter Fig. 2. Profiles of spherical rubber surfaces across the diameter of contact as deduced from the interferograms. (a) Profile corresponding to Fig. ~(a) ; (b) corresponding to Fig. 1(b) ; (c) corresponding to Fig. I(C) across the dotted line. (Along the x-axis 3.5 divisions = I mm and along the h axis I division=zooo A.)
Some of the results quoted here have already been described by other workers, employing different materials and different arrangements. The main contribution of the present work is to show that by using optically smooth rubber and carefully designed interferometric techniques it is possible to study, in considerable detail, the contact between surfaces subjected to large elastic deformations under relatively small stresses both in the absence and in the presence of liquid films. The low modulus of rubber implies that the contact pressures are low and
166
SHORTCOMMUNICATIONS
that the material can be easily deformed to provide a convenient shape for effective hydrodynamic lubrication. The results also suggest more generally that the trapping of liquid films between deformable surfaces may play an important part in preventing contact between stationary or slowly moving surfaces. h
Film Thickness
Diameter
Fig. 3. Profile of cylindrical rubber surface corresponding to interferogram the x-axis I division = 0.1 mm and along the L axis I division = zooo A).
in Fig. r(d). (Along
We wish to thank the ministry of Technology for general support, TricoFolberth for a research grant to one of us (A.D.R.) and the Avon Rubber Company for a grant for equipment. .%--ace
Pdiysics, C~v~~dish L~bor~o~~, of Cambridge (Gt. Britais,l
U~i~eysit~
A. D. ROBERTS D. TABOR
I M. T. KIRK, Nature, 194 (1962) 965. J. F. ARCHARXI AND M. T. KIRK, I*tst. Me&. Engvs. Lubrication and Wear Conventaost, rp63. 2 A. CAMERONAND R. GOHAR, Nature, aoo (1963) 458; PYOC.Roy. Sot. (London), A291 (1966) 520. 3 A. CAMERON AND F. J. WESTLAKE, Natuve, a14 (1967) 633. 4 H. BLOK AND H. J. KOENS, Inst. Mech. Engrs. Elasta~ydro~~am~c Lubricatk Sym$osizlm,
1965. 5 D. DOWSON
AND D. A. JONES, Xatwe, 214 (rg67) 947. 6 E. RABINOWICZ, Pvoc. Phys. Sot., 365 (1952) 630. F; P. BOWDEN AND I). TABOR, Fvictb and L&r&a&m of SoEa’ds,Part I, Clarendon Press, Oxford, 1950, Chap. XIII. 7 M. C. SHAW AND 3%. F. MACKS, Analysis and Lufwkatiolt of Bea&zgs, McGraw-Hill, New York,
r949.
Received Wear,
II
October (1968)
I, x967