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Significance of contact resistance in boundary lubrication
49
Wear, 156 (1992) 49-55
Significance of contact resistance Etsuo Marui” and Hiroki
in boundary lubrication
Endob
“Department of Mechanical Engineering, Faculty of Engineering, Gifi University, l-l Yanagido, Gifu-shi 501-11 (Japan) bFirst Research and Development Department, OKUMA Corporation, Oguchi-cho, Niwa-gun, Aichi-ken 480-01 (Japan) (Received
August 28, 1991; revised and accepted
November
19, 1991)
Abstract The mean contact pressure of a machine tool slideway is relatively low, and its sliding velocity is also low. Many surfaces in such a contact are in a boundary lubrication state. In this paper, the contact resistance between smooth model surfaces under boundary lubrication is precisely measured. The result is compared with the static friction measured on real rough surfaces lubricated by various lubricants. It is shown that contact resistance and static friction correlate strongly, and that the boundary lubrication ability of lubricants (the oiliness) is predicted by the contact resistance between smooth model surfaces.
1. Introduction
The examination of surface contact mechanisms by electric resistance at a contact, which was developed by Holm [l], is applicable not only to the research field of electrical contacts, but also to tribology, i.e. to the relative movement between contact surfaces. Bowden and Tabor [2] estimated the real area of contact and the number of contact points by contact resistance measurement, and they also suggested that the measurement of contact resistance is an effective means of surface analysis. In fact, by examining the influences of surface roughness, surface film [3, 41 and load [5] on contact resistance, the contact mechanism in the dry state becomes clear. This method is also applied to the boundary lubrication state [6] and the effectiveness of additives in a lubricant [7j is verified. The mean contact pressure of a machine tool slideway is relatively low, and its sliding velocity is also low. Such surface contacts are often in the boundary lubrication state. It is important [8] in boundary lubrication to clarify the characteristics of the adsorbed lubricant film on the metal surface. Smith and Cameron [9] measured the thickness of an adsorbed lubricant film on surfaces of mild steel and chromium by capacitance measurement. Studies of the relation between static friction and applied electric current through contact surfaces [lo], and of the chemical reactivity between metal surfaces and lubricant molecules [ll] were carried out. In the present research work, the relation between the contact resistance of smooth model metal surfaces in boundary lubrication and the static friction characteristics of real rough surfaces is studied. The results yield interesting data that are useful for the complete understanding of boundary lubrication.
0043-1648/92/%5.00
0 1992 - Elsevier Seauoia. All riehts reserved
50
2. Measurement
of contact
resistance
2.1. ~qui~rne~ and procedure Figure 1 shows the outline
of the apparatus used to measure contact resistance. The main components of the apparatus are supported by two rigid foundations F1, F2 and four pillars R. Model contact surfaces are made of two cylindrical test pieces T of 25 mm diameter, and a hard steel ball bearing S. S is sandwiched between the flat surfaces of the two test pieces T. They are lifted with a hydraulic jack J against foundation F1, and a normal load FN is applied. The loading time is 300 s. Plates P are mica and insulate the test pieces T and the steel ball S from other parts of the apparatus. The magnitude of the normal load is measured by strain gauges, attached to dynamometer D. The lubricant to be characterized is stored in an acrylic resin vessel V. After applying a preselected load to the contact surfaces, a constant electric current (100 mA) flows through the whole circuit, including the contact surfaces in boundary lubrication state. By measuring the voltage drop in the whole circuit with a multimeter, the electric resistance is obtained. The experiments are carried out in a constant-temperature room at 23 “C. The test pieces T are finished by surface grinding and buffing. The maximum surface roughness of T and S is less than 0.05 pm. The Vickers hardness number of T is about 180; thus, it is much lower than that of S, which has a Vickers hardness number of about 900. A list of lubricants is given in Table 1. Castor oil (vegetable fatty oil), liquid paraffin (mixture of normal paraffins), normal decane (paraffin-based oil) and normal decane containing three kinds of fatty acids, are tested in contact resistance measurements. These additives are good friction reducing agents [12, 131. The concentration of all the additives is determined as 20 mol mmv3 by considering the minimum solubility of stearic acid at room temperature (23 “C). 2.2. Result of contact resistance measurement in the dry state Figure 2 shows an example of the measured relation between the total electrical resistance R and the normal load FN. The total resistance R is the sum of the contact resistance and the internal resistance of the circuit. Generally, the contact resistance is composed of the film resistance and the constriction resistance. In the figure, two series of measurements are shown. It turns out that the reproducibili~of the experimental data is good. The total resistance R decreases with increasing normal load, presumably
Fig. 1. Experimental apparatus for contact resistance measurement.
51 TABLE
1
Lubricants Lubricant
Molecular
Castor oil Liquid paraffin n-decane n-decane + Caproic acid n-decane + Caprinic acid n-decane + Stearic acid
-
formula
CH,(CW,CH3 CH3(CHJ&OOH CH,(CH,),COOH CH,(CH,),,COOH
Kinematic viscosity at 23 “C (mm* s-‘)
Concentration (mol m-‘)
838.1 197.1 1.186 1.181 1.184 1.186
-
20 20 20
IO ’
IO ’
10-40
5
IO FN
15 2025
2
Fig. 2. Total resistance Fig. 3. Contact resistance
3456i FN
kN
R in dry state as a function
kN
of FN.
Rd in dry state as a function of FN with ball diameter D as parameter.
because the metallic load bearing area increases. It can be seen that R gradually approaches a constant low value. This value is probably the internal resistance of the circuit. Thus, the electrical resistance at FN = 20 kN is regarded as the internal resistance R. of the circuit. As there are two contact surfaces in series, the contact resistance Rd in the dry state is calculated by the following equation R
d=
R-Ro
(1)
2
Figure 3 shows the relation between for different diameters D of the hard steel diameters [14]. It can be seen that Rd and increasing FN. The relation between Rd and FN can
Rd=aFNmb
Rd and FN plotted
on a logarithmic scale ball S. Contact resistance is high for large the slope of the Rd/FN curve increase with be written
as follows (2)
where the factors a and b are constants determined by contact surface configuration and surface topography. The values of a and b obtained from Fig. 3 are plotted in Fig. 4. It is seen that the larger the ball diameter D, the larger the values of a and b.
52
I(:
’ _
:
D-IOmm n Decane
+Stearlc
I0 0
5i
Acid
Dry 4~
C Dz
..J
I6 D
0
Fig. 5. Total resistance
5
IO
I5
FN
kN
20
Fig. 6. Nondimensional function of FN. Fig. 7. Contact diameter).
IO FN
Fig. 4. Factors a and b of experimental
0
5
mm
R in lubricated
relation,
eqn. (2) as a function
state as a function
25
of ball diameter
D.
of FN.
I o-4
25
FN
difference
resistance
15 20 kN
R,
between
in lubricated
contact resistances state as a function
kN
in dry and lubricated of FN (influence
state as a
of steel ball
2.3. Result of contact resistance measurement in the lubricated state Figure 5 shows an example of the measured relation between the total electric resistance R and the normal load FN, in the case of lubrication with normal decane, blended with stearic acid. In the figure, two series of results are given. Again, a good
reproducibility of the experimental data is found. To discuss the difference between contact resistances in dry and lubricated states, the ratio AR* of the contact resistance difference to the contact resistance in dry state is shown in Fig. 6 as a function of FN. It can be seen that at the lower FN values, the nondimensional difference AR* is quite pronounced. To be of practical significance AR* should be rather large. Effective lubrication is realized at the lower FN values. As in eqn. (l), the contact resistance R, in the lubricated state is obtained by subtracting the internal resistance RO in the circuit from the total electric resistance R as follows
53
IO ’
IO ’
100
IO0
CIO-'
CliO-'
2
2 I o-2
I o-2
10-s
IO-3
I o-4
I o-4
2
3456810 FN
2
3456810 FN
kN
kN
Fig. 8. Contact resistance R, in lubricated state as a function of FN (influence of lubricant). The dashed curve represents the results obtained dry (from Fig. 3). Fig. 9. Contact resistance R, in lubricated state as a function of FN (influence of additive). The dashed curve represents the results obtained with pure n-decane (from Fig. 8).
R,=
R-R0 2
The film resistance ratio is larger in the lubricated state than in the dry state. Figure 7 illustrates lubricated ion with normal decane, with the diameter of the steel ball as a parameter. The slopes of R,,,-F, lines are almost the same as those in the dry state (Fig. 3). Figure 8 shows the effect of the applied lubricants, in the case of ball diameter D= 10 mm. It can be seen that in all cases R, clearly exceeds R,,, the contact resistance in the dry condition. With castor oil (vegetable fatty oil) R, is fairly high in comparison with the R, values of the surfaces lubricated by other lubricants. Slightly higher contact resistance is obtained with surfaces lubricated by normal decane, than with surfaces lubricated by liquid paraffin (mixture of normal paraffins). Figure 9 shows that the type of fatty acid (stearic acid, caprinic acid or caproic acid) does not affect R,. However, the contact resistance of the surfaces lubricated with these blended lubricants is a little larger than the resistance of the surfaces lubricated by normal decane only. Thus it is clear that the additives have some effect. Summarizing, it can be concluded that the variation of the contact resistance with normal load is dominated by the steel ball diameter D, that is the geometry of the contact surfaces. Its value is also related to the type of lubricant.
3. Confirmation
by friction
test
Owing to the build-up of adsorbed lubricant film on contact surfaces and the reduction of real metallic contact area, the contact resistance increases in the lubricated state. From this, it can be suggested that the effective lubrication and the lower friction in boundary lubrication are realized when lubricated by a lubricant of higher contact resistance. In the previous section, the contact resistance of smooth model surfaces was measured to clarify the inherent characteristic of lubricants. In this chapter, the static
54
friction coefficient of real rough surfaces is measured to examine how the contact resistance of smooth model surfaces is reflected in the friction characteristic of real rough surfaces. 3.1. Ekperimental apparatus and procedure Figure 10 indicates the outline of the friction testing device. Both the slideway G and the slider S are made of mild steel. Mean contact pressure is low, about 40 kPa. The slider S is driven very slowly by the square thread T via a leaf spring K. The driving speed (v) is 0.00235 mm s-l. The contact surfaces are lubricated by the same lubricants used in the contact resistance measurements described in the previous section. The room temperature is 23 “C. The maximum static friction force F is detected by the strain gauges E applied to the leaf spring K. The maximum static friction coefficient ps is found as the ratio of the maximum static friction force and the gravity force acting on the slider. The contact surfaces are finished by surface grinding. The maximum surface roughness, measured in a direction perpendicular to the direction of grinding, is given in Table 2.
0.5
0.4 I
~~
0.3 I
-~
O. I 3 Stemc 0 t
0
Acid
Castor Oil
0. I
0.2
RW .Q
Fig. 10. Experimental
apparatus
for static friction
test.
Fig. 11. Correlation between maximum static friction coefficient and contact resistance at different values of surface roughness (see Table 2).
TABLE
2
Maximum surface roughness
of surfaces
finished by surface grinding and buffing
Surface
Maximum surface roughness,
Sl s2 s3 s4
4.5 2.7 1.6 0.95
R,,
(pm)
55
3.2. Experimental
result and comparison
with contact resistance
Figure 11 shows an example of the correlation between the maximum static friction coefficient p, of rough surfaces and the contact resistance R, of smooth surfaces at the normal load FN= 2 kN, taken from Figs. 8 and 9. The curves in the figure show an average tendency towards the experimental correlation. The results of blended ndecane are all for the surface S2. It is quite clear that CL,decreases with decreasing surface roughness. This reduction of the static friction coefficient is induced by the effect of surface topography on the static friction characteristic [8]. Further, at each roughness level, the maximum static friction coefficient ps decreases with increasing values of R,. So, it is shown that there is a strong correlation between the static friction characteristic and the contact resistance. The result of Fig. 11 can be explained as follows: An adsorbed lubricant film thickness on real rough surfaces is also thick, and the static friction coefficient in boundary lubrication state decreases, corresponding to the reduction of metallic contact area, when lubricated by the lubricant having a large contact resistance and a thick adsorbed lubricant film on smooth surfaces (see ref. 8, Chap. 2). As a result, the contact resistance measurement in smooth model surfaces is an effective and simple means of assessing the ability of lubricants to provide boundary lubrication (oiliness) in real rough surfaces.
4. Conclusions
The correlation between the contact resistance of smooth model contact surfaces and the static friction of real rough surfaces is investigated experimentally. Some characteristics of the contact resistance are clarified. The variation of the contact resistance with normal load is dominated mainly by the geometry of the contact surfaces, while its value is related to the properties of the applied lubricant. It is also shown that there is a strong correlation between contact resistance and static friction. Thus, contact resistance measurement is an effective means of assessing the ability of lubricants to provide boundary lubrication.
References
1 R. Holm, Electric Contacts, Almquist and Wiksells Akademiska Handbocker Hugo Gebers Forlag, 1946. 2 F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, 1950, p. 25. 3 R. W. Wilson, POX. Phys. Sot. B, 68 (9) (1955) Oxford, 625. 4 M. cocks, ti. Phys. Sot. B, 67 (3) (1954) 238. 5 D. V. Keller, Jr., J. tic. Sci. Tech., 9 (1) (1972) 133. 6 M. J. Furey, ASLE Trans., 4 (1) (1961) 1. 7 C. Czichos, W. Grimmer and H.-U. Mittmann, Wear, 40 (1976) 265. 8 S. Kato, E. Mann, A. Kobayashi and S. Senda, ASME. J. Ttibol., 107 (2) (1985) 188. 9 A. J. Smith and A. Cameron, PIVC.R Sot. Lond. A, 328 (1972) 541. 10 J. R. Brallsford, Wear, 25 (1973) 85. 11 S. S. Wang, S. P. Maheswari and Y. M. Wang, ASLE Trans.., 30 (3) (1987) 394. 12 S. Jahanmir and M. Beltzer, ASME J. T&Z., 108 (1) (1986) 109. 13 S. Jahanmir and M. Beltzer, ASLE Tmns., 29 (3) (1986) 423. 14 T. Hisakado, J. JSLE, 21 (3) (1976) 824.
Wear, 156 (1992) 49-55
Significance of contact resistance Etsuo Marui” and Hiroki
in boundary lubrication
Endob
“Department of Mechanical Engineering, Faculty of Engineering, Gifi University, l-l Yanagido, Gifu-shi 501-11 (Japan) bFirst Research and Development Department, OKUMA Corporation, Oguchi-cho, Niwa-gun, Aichi-ken 480-01 (Japan) (Received
August 28, 1991; revised and accepted
November
19, 1991)
Abstract The mean contact pressure of a machine tool slideway is relatively low, and its sliding velocity is also low. Many surfaces in such a contact are in a boundary lubrication state. In this paper, the contact resistance between smooth model surfaces under boundary lubrication is precisely measured. The result is compared with the static friction measured on real rough surfaces lubricated by various lubricants. It is shown that contact resistance and static friction correlate strongly, and that the boundary lubrication ability of lubricants (the oiliness) is predicted by the contact resistance between smooth model surfaces.
1. Introduction
The examination of surface contact mechanisms by electric resistance at a contact, which was developed by Holm [l], is applicable not only to the research field of electrical contacts, but also to tribology, i.e. to the relative movement between contact surfaces. Bowden and Tabor [2] estimated the real area of contact and the number of contact points by contact resistance measurement, and they also suggested that the measurement of contact resistance is an effective means of surface analysis. In fact, by examining the influences of surface roughness, surface film [3, 41 and load [5] on contact resistance, the contact mechanism in the dry state becomes clear. This method is also applied to the boundary lubrication state [6] and the effectiveness of additives in a lubricant [7j is verified. The mean contact pressure of a machine tool slideway is relatively low, and its sliding velocity is also low. Such surface contacts are often in the boundary lubrication state. It is important [8] in boundary lubrication to clarify the characteristics of the adsorbed lubricant film on the metal surface. Smith and Cameron [9] measured the thickness of an adsorbed lubricant film on surfaces of mild steel and chromium by capacitance measurement. Studies of the relation between static friction and applied electric current through contact surfaces [lo], and of the chemical reactivity between metal surfaces and lubricant molecules [ll] were carried out. In the present research work, the relation between the contact resistance of smooth model metal surfaces in boundary lubrication and the static friction characteristics of real rough surfaces is studied. The results yield interesting data that are useful for the complete understanding of boundary lubrication.
0043-1648/92/%5.00
0 1992 - Elsevier Seauoia. All riehts reserved
50
2. Measurement
of contact
resistance
2.1. ~qui~rne~ and procedure Figure 1 shows the outline
of the apparatus used to measure contact resistance. The main components of the apparatus are supported by two rigid foundations F1, F2 and four pillars R. Model contact surfaces are made of two cylindrical test pieces T of 25 mm diameter, and a hard steel ball bearing S. S is sandwiched between the flat surfaces of the two test pieces T. They are lifted with a hydraulic jack J against foundation F1, and a normal load FN is applied. The loading time is 300 s. Plates P are mica and insulate the test pieces T and the steel ball S from other parts of the apparatus. The magnitude of the normal load is measured by strain gauges, attached to dynamometer D. The lubricant to be characterized is stored in an acrylic resin vessel V. After applying a preselected load to the contact surfaces, a constant electric current (100 mA) flows through the whole circuit, including the contact surfaces in boundary lubrication state. By measuring the voltage drop in the whole circuit with a multimeter, the electric resistance is obtained. The experiments are carried out in a constant-temperature room at 23 “C. The test pieces T are finished by surface grinding and buffing. The maximum surface roughness of T and S is less than 0.05 pm. The Vickers hardness number of T is about 180; thus, it is much lower than that of S, which has a Vickers hardness number of about 900. A list of lubricants is given in Table 1. Castor oil (vegetable fatty oil), liquid paraffin (mixture of normal paraffins), normal decane (paraffin-based oil) and normal decane containing three kinds of fatty acids, are tested in contact resistance measurements. These additives are good friction reducing agents [12, 131. The concentration of all the additives is determined as 20 mol mmv3 by considering the minimum solubility of stearic acid at room temperature (23 “C). 2.2. Result of contact resistance measurement in the dry state Figure 2 shows an example of the measured relation between the total electrical resistance R and the normal load FN. The total resistance R is the sum of the contact resistance and the internal resistance of the circuit. Generally, the contact resistance is composed of the film resistance and the constriction resistance. In the figure, two series of measurements are shown. It turns out that the reproducibili~of the experimental data is good. The total resistance R decreases with increasing normal load, presumably
Fig. 1. Experimental apparatus for contact resistance measurement.
51 TABLE
1
Lubricants Lubricant
Molecular
Castor oil Liquid paraffin n-decane n-decane + Caproic acid n-decane + Caprinic acid n-decane + Stearic acid
-
formula
CH,(CW,CH3 CH3(CHJ&OOH CH,(CH,),COOH CH,(CH,),,COOH
Kinematic viscosity at 23 “C (mm* s-‘)
Concentration (mol m-‘)
838.1 197.1 1.186 1.181 1.184 1.186
-
20 20 20
IO ’
IO ’
10-40
5
IO FN
15 2025
2
Fig. 2. Total resistance Fig. 3. Contact resistance
3456i FN
kN
R in dry state as a function
kN
of FN.
Rd in dry state as a function of FN with ball diameter D as parameter.
because the metallic load bearing area increases. It can be seen that R gradually approaches a constant low value. This value is probably the internal resistance of the circuit. Thus, the electrical resistance at FN = 20 kN is regarded as the internal resistance R. of the circuit. As there are two contact surfaces in series, the contact resistance Rd in the dry state is calculated by the following equation R
d=
R-Ro
(1)
2
Figure 3 shows the relation between for different diameters D of the hard steel diameters [14]. It can be seen that Rd and increasing FN. The relation between Rd and FN can
Rd=aFNmb
Rd and FN plotted
on a logarithmic scale ball S. Contact resistance is high for large the slope of the Rd/FN curve increase with be written
as follows (2)
where the factors a and b are constants determined by contact surface configuration and surface topography. The values of a and b obtained from Fig. 3 are plotted in Fig. 4. It is seen that the larger the ball diameter D, the larger the values of a and b.
52
I(:
’ _
:
D-IOmm n Decane
+Stearlc
I0 0
5i
Acid
Dry 4~
C Dz
..J
I6 D
0
Fig. 5. Total resistance
5
IO
I5
FN
kN
20
Fig. 6. Nondimensional function of FN. Fig. 7. Contact diameter).
IO FN
Fig. 4. Factors a and b of experimental
0
5
mm
R in lubricated
relation,
eqn. (2) as a function
state as a function
25
of ball diameter
D.
of FN.
I o-4
25
FN
difference
resistance
15 20 kN
R,
between
in lubricated
contact resistances state as a function
kN
in dry and lubricated of FN (influence
state as a
of steel ball
2.3. Result of contact resistance measurement in the lubricated state Figure 5 shows an example of the measured relation between the total electric resistance R and the normal load FN, in the case of lubrication with normal decane, blended with stearic acid. In the figure, two series of results are given. Again, a good
reproducibility of the experimental data is found. To discuss the difference between contact resistances in dry and lubricated states, the ratio AR* of the contact resistance difference to the contact resistance in dry state is shown in Fig. 6 as a function of FN. It can be seen that at the lower FN values, the nondimensional difference AR* is quite pronounced. To be of practical significance AR* should be rather large. Effective lubrication is realized at the lower FN values. As in eqn. (l), the contact resistance R, in the lubricated state is obtained by subtracting the internal resistance RO in the circuit from the total electric resistance R as follows
53
IO ’
IO ’
100
IO0
CIO-'
CliO-'
2
2 I o-2
I o-2
10-s
IO-3
I o-4
I o-4
2
3456810 FN
2
3456810 FN
kN
kN
Fig. 8. Contact resistance R, in lubricated state as a function of FN (influence of lubricant). The dashed curve represents the results obtained dry (from Fig. 3). Fig. 9. Contact resistance R, in lubricated state as a function of FN (influence of additive). The dashed curve represents the results obtained with pure n-decane (from Fig. 8).
R,=
R-R0 2
The film resistance ratio is larger in the lubricated state than in the dry state. Figure 7 illustrates lubricated ion with normal decane, with the diameter of the steel ball as a parameter. The slopes of R,,,-F, lines are almost the same as those in the dry state (Fig. 3). Figure 8 shows the effect of the applied lubricants, in the case of ball diameter D= 10 mm. It can be seen that in all cases R, clearly exceeds R,,, the contact resistance in the dry condition. With castor oil (vegetable fatty oil) R, is fairly high in comparison with the R, values of the surfaces lubricated by other lubricants. Slightly higher contact resistance is obtained with surfaces lubricated by normal decane, than with surfaces lubricated by liquid paraffin (mixture of normal paraffins). Figure 9 shows that the type of fatty acid (stearic acid, caprinic acid or caproic acid) does not affect R,. However, the contact resistance of the surfaces lubricated with these blended lubricants is a little larger than the resistance of the surfaces lubricated by normal decane only. Thus it is clear that the additives have some effect. Summarizing, it can be concluded that the variation of the contact resistance with normal load is dominated by the steel ball diameter D, that is the geometry of the contact surfaces. Its value is also related to the type of lubricant.
3. Confirmation
by friction
test
Owing to the build-up of adsorbed lubricant film on contact surfaces and the reduction of real metallic contact area, the contact resistance increases in the lubricated state. From this, it can be suggested that the effective lubrication and the lower friction in boundary lubrication are realized when lubricated by a lubricant of higher contact resistance. In the previous section, the contact resistance of smooth model surfaces was measured to clarify the inherent characteristic of lubricants. In this chapter, the static
54
friction coefficient of real rough surfaces is measured to examine how the contact resistance of smooth model surfaces is reflected in the friction characteristic of real rough surfaces. 3.1. Ekperimental apparatus and procedure Figure 10 indicates the outline of the friction testing device. Both the slideway G and the slider S are made of mild steel. Mean contact pressure is low, about 40 kPa. The slider S is driven very slowly by the square thread T via a leaf spring K. The driving speed (v) is 0.00235 mm s-l. The contact surfaces are lubricated by the same lubricants used in the contact resistance measurements described in the previous section. The room temperature is 23 “C. The maximum static friction force F is detected by the strain gauges E applied to the leaf spring K. The maximum static friction coefficient ps is found as the ratio of the maximum static friction force and the gravity force acting on the slider. The contact surfaces are finished by surface grinding. The maximum surface roughness, measured in a direction perpendicular to the direction of grinding, is given in Table 2.
0.5
0.4 I
~~
0.3 I
-~
O. I 3 Stemc 0 t
0
Acid
Castor Oil
0. I
0.2
RW .Q
Fig. 10. Experimental
apparatus
for static friction
test.
Fig. 11. Correlation between maximum static friction coefficient and contact resistance at different values of surface roughness (see Table 2).
TABLE
2
Maximum surface roughness
of surfaces
finished by surface grinding and buffing
Surface
Maximum surface roughness,
Sl s2 s3 s4
4.5 2.7 1.6 0.95
R,,
(pm)
55
3.2. Experimental
result and comparison
with contact resistance
Figure 11 shows an example of the correlation between the maximum static friction coefficient p, of rough surfaces and the contact resistance R, of smooth surfaces at the normal load FN= 2 kN, taken from Figs. 8 and 9. The curves in the figure show an average tendency towards the experimental correlation. The results of blended ndecane are all for the surface S2. It is quite clear that CL,decreases with decreasing surface roughness. This reduction of the static friction coefficient is induced by the effect of surface topography on the static friction characteristic [8]. Further, at each roughness level, the maximum static friction coefficient ps decreases with increasing values of R,. So, it is shown that there is a strong correlation between the static friction characteristic and the contact resistance. The result of Fig. 11 can be explained as follows: An adsorbed lubricant film thickness on real rough surfaces is also thick, and the static friction coefficient in boundary lubrication state decreases, corresponding to the reduction of metallic contact area, when lubricated by the lubricant having a large contact resistance and a thick adsorbed lubricant film on smooth surfaces (see ref. 8, Chap. 2). As a result, the contact resistance measurement in smooth model surfaces is an effective and simple means of assessing the ability of lubricants to provide boundary lubrication (oiliness) in real rough surfaces.
4. Conclusions
The correlation between the contact resistance of smooth model contact surfaces and the static friction of real rough surfaces is investigated experimentally. Some characteristics of the contact resistance are clarified. The variation of the contact resistance with normal load is dominated mainly by the geometry of the contact surfaces, while its value is related to the properties of the applied lubricant. It is also shown that there is a strong correlation between contact resistance and static friction. Thus, contact resistance measurement is an effective means of assessing the ability of lubricants to provide boundary lubrication.
References
1 R. Holm, Electric Contacts, Almquist and Wiksells Akademiska Handbocker Hugo Gebers Forlag, 1946. 2 F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, 1950, p. 25. 3 R. W. Wilson, POX. Phys. Sot. B, 68 (9) (1955) Oxford, 625. 4 M. cocks, ti. Phys. Sot. B, 67 (3) (1954) 238. 5 D. V. Keller, Jr., J. tic. Sci. Tech., 9 (1) (1972) 133. 6 M. J. Furey, ASLE Trans., 4 (1) (1961) 1. 7 C. Czichos, W. Grimmer and H.-U. Mittmann, Wear, 40 (1976) 265. 8 S. Kato, E. Mann, A. Kobayashi and S. Senda, ASME. J. Ttibol., 107 (2) (1985) 188. 9 A. J. Smith and A. Cameron, PIVC.R Sot. Lond. A, 328 (1972) 541. 10 J. R. Brallsford, Wear, 25 (1973) 85. 11 S. S. Wang, S. P. Maheswari and Y. M. Wang, ASLE Trans.., 30 (3) (1987) 394. 12 S. Jahanmir and M. Beltzer, ASME J. T&Z., 108 (1) (1986) 109. 13 S. Jahanmir and M. Beltzer, ASLE Tmns., 29 (3) (1986) 423. 14 T. Hisakado, J. JSLE, 21 (3) (1976) 824.