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Langmuir 1990,6, 1141-1145
Triboattraction: Friction under Negative Load? Bo Brezoczky and H. Seki' IBM Research Division, Almaden Research Center, 650 Harry Rd., San Jose, California 95120-6099 Received February 15, 1989 When a square single-crystal diamond slider with a smooth flat surface is placed in contact with a rotating smooth surface of amorphous carbon, we observe the development of an attractive force. The force is such that even when the load applied to the slider is made negative (i.e., the slider is pulled away from the surface), it still remains in physical contact with the rotating surface, and its frictional force can be measured. This attractive force is maintained so long as the slider is rubbing against the rotating disk but ceases when the rotation is stopped. Experiments to investigate this triboattractive force indicate that it is due to electric charge generated by triboelectricity. The present observation is compared to previous reports of friction under negative load. Introduction The close interrelation between the friction and adhesion phenomena is well rec0gnized.l Proper consideration of the same interatomic and intermolecular forces is involved in t h e understanding of both of these phenomena. In the process of such considerations, Derjaguin2was the first to conclude that Amonton's law of friction, which states that the frictional force between two sliding bodies is proportional to the applied load by a coefficient of friction which is independent of the apparent area of contact and the relative speed, should be modified to include a second term. Derjaguin proposed the relation
F = pAS + p L
(1) where p is the true coefficient of friction, A is the true area of contact, S is the specific force of molecular interaction, and L is the external load. In most cases, physical contact is made only at the asperities, and the true area of contact is much smaller than the apparent contact area. The true area of contact is generally a function of the load through the elastic and plastic properties of these contact regions. To highlight the two-term aspect of the expression for friction, Lazarev3 carried out experiments which were designed to make the true area of contact coincide with the apparent area of contact. This was achieved by sandwiching a boundary layer of polar lubricant molecules between friction pairs consisting of a hard and soft material. Under these conditions, the first term in eq 1 is independent of the load and
F = pLo + p L (2) The implication of this equation is that, due to an attractive force given by the first term, friction will occur a t zero external load or even for negative external loads, + Presented at the 196th National Meeting of the American Chemical Society, Los Angeles, CA, Sept 26, 1988. (1)See, for example: (a) Akhmatov, A. S. Molecular Physics of Boundary Friction; Nauka: Moscow, 1963; Translation by N. Kaner, Israel
Program for Scientific Translations, Jerusalem, 1966. (b) Rabinowicz, E.Friction and Wear ofMaterials; Wiley: New York, 1966. (c) Adamson, A. W. Physical Chemistry of Surfaces;Wiley: New York, 1976. (d) Bowden, F. P.; Tabor, D. Friction An Introduction to Tribology; Robert Krieger: Malabar, FL, 1982. (2) (a) Derjaguin, B. V. 2. Physik 1934,88, 661. (b) Derjaguin, B. V. Zhurn, Phis. Khim. 1934,5, 1163. (3) Derjaguin, B.V.; Lazarev, V. P. Kolloidn Zh. 1935, I, 293.
0743-7463/90/2406-1141$02.50/0
i.e., tensile loads. This relation was verified by frictional measurements for Wood's alloy on glass and for paraffin on glass lubricated by a boundary layer of oleic acid. Similar results were obtained by Denisovlb with two highly polished steel flats sandwiching a boundary layer of myristic acid or vaseline oil. More recently, Skinner and Gane4 demonstrated that even without boundary layer lubrication it is possible to observe the effect of a constant second term in the friction expression. The experiment involved the sliding of a sharp stylus of gold or lead on a highly polished diamond surface and was carried out in a specially constructed SEM so that the surfaces could be examined in situ. Key to the observation was the flattening of the stylus tip under positive load prior to measurement. On the basis of the experimental results, Skinner and Gane presented an explanation for the observation in terms of the persistence of nanometric asperities after plastic deformation and in terms of van der Waals forces between most of the surface. We have recently carried out some frictional measurements of diamond sliders on hard carbon thin films and found that eq 2 closely approximates our observation. The attractive interaction takes place only when there is sliding motion; therefore, we call this the triboattractive phenomenon. As will be discussed in greater detail, we have concluded that the attractive force is due to electrostatic attraction from triboelectric charge generated between the diamond slider and the amorphous carbon surface. Experimental Section The primary experimental apparatus used for this work is shown schematically in Figure 1. A commercial spin coater was modified so that 3.5-in. disks could be mounted to spin at speeds up to 8000 rpm. The disks were of smooth aluminum, about 3 mm thick, on which hard amorphous carbon films had been sputtered. The film thickness was about 300-400 A. A mount with r,z movement (radial and vertical) was located at the side of the disk with strain gauges to monitor the friction and load forces. The diamond slider was glued onto an IBM 3380 disk drive suspension, which is shown enlarged in Figure 1, so that the slider could roll and pitch. The sliders were of industrial grade, type I1 diamond with the (111) face as the sliding surface. Measurements were made using three square sliders with 2 X 2 X 0.5 "3, or 3 X 3 X 0.5 dimensions of 2 X 2 X 1 "3, (4) Skinner, J.; Gane,
N. J . Phys. D 1972,5, 2087.
0 1990 American Chemical Society
1142 Langmuir, Vol. 6, No. 6, 1990
Brezoczky and Seki
Vertlcal and Hor>zontai I-Beams for
Diamond Slider
n
Spot Welded to Suspension Arm
Adhesive
Diamond Slider 11111 Face
Figure 1. Experimental apparatus. The spindle mounting the disk can be rotated in either direction at speeds up to 8000 rpm. The mount for the diamond slider can be moved radially and vertically with respect to the disk. mm3. An inductive potentiometer6 which measures the elec-
tric surface potential was mounted in line with the slider arm so that the voltage of the bottom slider surface could be measured by placing it above the probe opening. Triboattractive Phenomenon The relative speed was of the order of lo+ m/s or less, and the distance traversed was of the order of 0.1 mm or less for the friction measurments a t negative loads described in the Introduction. Compared to this, the triboattractive phenomenon occurs at high relative speeds, greater than 4 m/s, and is easily observed continuously for any length of time. The triboattractive phenomenon can best be described by the experiment illustrated in Figure 2. In Figure 2a, the disk is stationary, and, if the slider is lowered onto the disk surface and lifted back up, the slider readily separates from the disk after contact. However, if the disk is rotating as in Figure 2b, then the slider is attracted to the disk surface once the two make contact. The slider stays in contact with the disk, and raising the slider arm strains the spring suspension as shown in Figure 2c. If the rotational speed is lowered with the slider under tensile load, there is a point at which the triboattractive force is no longer able to keep the slider on the disk surface and the slider snaps away from the surface. Experimental Results Direct measurement of the triboattractive force by measuring the pull necessary to separate the slider from the disk resulted in considerable data scatter. This is attributed to the rotating disk and to the difficulty of pulling on the slider without introducing some torque. However, measurement of the friction as a function of a positive load indicates a linear relation. Examples of the friction measurements are shown in Figure 3 for a 2 X 2 X 0.5 mm3 slider for three rotational speeds, corresponding to a linear speed of 4, 12, and 20 m/s. It is significant that lines of essentially the same slope can be drawn through the experimental points for all three speeds. The lines intersect the friction axis at a finite value, and extrapolation to 0 friction gives the triboattractive force operating at the experimental points lying along this line. In the speed range 4-20 m/s, the lines are close to each other, ( 5 ) Model 244, Monroe Electronics, Inc., Lyndville, NY.
n
/
Figure 2. Triboattractive phenomenon. (a) When the disk is stationary there is no triboattraction. (b) Triboattraction develops upon contact when the disk is in rotation. (c) The attractive force is sufficient to keep the slider in contact continuously, even when the slider is being pulled away. (d) Under tension, the slider separates from the disk when the rotation drops below some critical value.
:
Friction
RPM 0 1000
A 3000
, -2
0 0
0
1.o
0 5000
-3
(gm)
/ '
'A 0
A/&* ,
I
-1
0
1 2 3 External Load (gm)
Figure 3. Measurement of frictional force (ordinate)vs external load (abscissa) for different rotational speeds. The slider is located about 3.5 cm from the disk center for these measurements.
having a slight tendency of the triboattractive force to increase with speed. A t speeds below 4 m/s, the frictionload line starts to move down indicating, that the triboattractive (TA) force weakens with decreasing speed. The slope of the line remains the same, and the TA force is essentially zero at speeds of about 1 m/s. In the speed range 4-15 m/s, the friction-load relation is almost independent of the speed. The results obtained in this speed range with three different diamond sliders are summaried in Figure 4. The coefficient of friction to a first order is considered similar for the different sliders, but the total triboattractive force is proportional to the apparent area of the slider with the force per unit area being about 65 g/cm2 or 6150 N/m2. The electrical nature of the triboattractive force is seen in the measurement of the surface potential of the slider shortly after sliding. A simple analysis of the expected potential can be made in terms of a parallel capacitor model shown in Figure 5. A similar analysis is given by
Langnuir, Vol. 6, No. 6, 1990 1143
Triboattraction: Friction under Negative Load
t
volts
/
2000
1500
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,
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Figure 6. Surface potential measurement of the bottom surface of the slider after rubbing at various relative speeds.
I
Figure 4. Summary of friction measurements. The data for the 2 X 2 X 0.5 and the 2 X 2 X 1 mm3 sliders are essentially the same. Model
10
(gm)
I 5 9 gm
5
Load
Surface Charge Density vs. Spacing
Equivalent Circuit
Figure 5. Parallel capacitance model for the disk and diamond slider.
Harpers on Medley's experiments on contact charging of polymers and mercury.' Diamond is a good insulator, and the slider itself constitutes one capacitor while the bottom surface of the slider and the disk surface can be considered to be a second capacitor. A diamond of thickness d has a capacitance per unit area of CD = t o t / d , and the corresponding capacitance per unit area of the bottom surface with respect to the disk separated by z is C,(z) = Q / Z , where t is the specific dielectric constant of the material (diamond in this case) and €0 is the vacuum dielectric constant. The potential of the bottom surface of the diamond can be considered to be across the two capacitors in parallel, as shown in Figure 5, and is then given as
w
I 10-3
10-2
1 z(cm)
10-1
Figure 7. Surface charge density on the slider corresponding to the breakdown voltage in air as a function of the separation from the disk surface.
an attractive force of 1g/mm2 or -lo4 N/m2 implies an average surface charge density of 4.15 X 10-8 C cm-2 (-2.5 x 1011 electronic charge/cm2) and an average field of 4.7 X 105 V/cm. Note that this field is independent of disk slider separation when it is much smaller than the prod-
uct of the dielectric constant and thickness of the diamond; the voltage at the sliding surface, assuming a 50-nm average spacing to the disk, is only 2.35 V. When the slider is lifted off the disk, the voltage of the bottom of the slider varies as eq 3, and when it is placed over the surface potentiometer, the voltage can be well approximated by eq 3 with z m. The value is then proportional to the thickness or inversely proportional to the the capacitance of the slider. For the charge density corresponding to a force per area of 1g/mm2 on a 0.5-mm-thick slider, mentioned above, this corresponds to -4300 V. Measurements of the surface potential of the slider after rubbing at different speeds are given in Figure 6 for 0.5and 1.0-mm-thick sliders. The diamond surface is charged positively, and the voltage is independent of the relative speed. The latter fact appears to be consistent with the triboattractive force extrapolated from the friction measurement being independent of the relative speed. The surface voltage is roughly proportional to the thickness, and this is consistent with eq 3. However, the observed magnitude of the voltage is lower than expected. This is, for the most part, due to discharge by atmospheric breakdown.6,8 It has been shown that the breakdown voltage in gas depends primarily on the product of the pressure and distance, a relation known as Paschen's law.9 In Figure 7, the surface charge density on the diamond slider has been calculated by using eq 3 from measurements of the Paschen law in air. (This curve can vary depending on the condition of the atmosphere, presence of radiation, etc.) The maximum surface charge density that can be supported in air when the slider is moved away from the surface is determined
(6) Harper, W. R. Contact and Frictional Electrification; Clarendon Press: Oxford, 1967. (7) Medley, J. A. Br. J . Appl. Phys. 1953, Suppl. 2, 528.
(8) Lowell, J.; Rose-Innes, A. C. Adu. Phys. 1980, 29, 947. (9) Knoll, M.; Ollendorff,F.; Romp, R. Gasentlodungstabellen; Springer-Verlag OHG; Berlin, 1935.
u zd V ( z )= -(3) tod + tz The surface of the disk and the slider has a root mean square roughness of about several tens of nanometers. It is therefore reasonable to assume an average separation of that order for C,(z). Assuming z = 50 nm, C, 2X 10-8 F cm-2 is much larger than CD = 9.68 X F cm-2 for 0.5-mm-thick diamond, and most of the field lines from the surface charge are directed to the disk when the diamond is sliding on it. Since the force between the capacitor plate is related to the electric charge and field as
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Brezoczky and Seki
1144 Langmuir, Vol. 6, No. 6, 1990
by the minimum of the curve. According to this curve, the discharge takes place at a separation of about 0.1 mm, and the average surface charge density becomes 1.8 X 10-8 C/cm2 for a 0.5-mm-thick diamond. Thus, this analysis shows that, for experiments carried out in air, the charge density derived from the voltage observed by the surface potentiometer is less than the charge density present when the diamond is actually sliding on the disk surface. The measurement gives the proper sign of the charge on the surface. The triboattractive phenomenon has also been observed to take place in a vacuum system at pressures of 10+ Torr. This clearly indicates that effects due to air flow are not involved in the slider remaining in contact during the disk rotation. So far we have not yet made any surface potential measurements of the slider in vacuum since to do this would require major changes in the apparatus.
Discussion The essential features of the triboattractive force reported here, between single-crystal diamond sliders and a flat amorphous carbon film coated A1 disk, can be summarized as follows: (a) The triboattractive force is independent of the speed above a speed of about 4 m/s but decreases below this speed. (b) The trioattractive force is proportional to the slider area. (c) The force per unit area is about lo4 N/m2, which corresponds to a charge density of 4 X C/cm2. Feature a implies that there is a regulating mechanism which limits the buildup of surface charge, and this limit is reached at speeds less than 4 m/s. This is based only on the triboattractive force obtained from the extrapolated friction curve. The constancy of the surface potential measurements merely reflects the limiting charge density allowed by the discharge through the air and only indicates that the surface charge density during sliding is larger than what is read afterwards. Since diamond is insulating and most of the electric field lines are directed to the disk when the slider is in contact, the regulating mechanism is discharge back to the disk surface. The mean free path of molecules in air can be obtained from the kinetic theory of of gases and is about 65 nm for air.1° The average spacing between the bottom of the slider and the disk is of the order of a few tens of nanometers, comparable or smaller than the mean free path in air, and as the Paschen curves indicate? the breakdown fields are essentially those of vacuum. Thus the main discharge process is not dielectric breakdown but tunneling at asperities whose tips come within tunneling range." As reviewed in ref 6, field emission from a metal surface and tunneling between two metal surfaces has been well studied. Here the surfaces are of diamond and amorphous carbon, and more detailed information is needed on the nature of the surface charge carrier as well as the electronic surface states for a proper analysis. It is reasonable to expect that the tribocharging takes place at the points of true contact whose area is a small fraction of the apparent area of the slider. Nevertheless, feature b implies that the charge spreads over the entire slider surface due to the mobility of surface charge pulled by the countercharge on the disk which speeds by. The average charge density estimated on the rub(IO) See, for example: OHanlon, J. F. A User's Guide t o Vacuum Technology, Wiley: New York, 1980. (11) Derjaguin, B. V.; Smilga, K. Adhesion of Solids; Consultants Bureau, 1978.
Table I . Comparison of van der Waals and Triboattractive Forces sliding motion relative speed, m / s force/area, N/m2 separation dependence average separation, nm
van der Waals
triboattraction
not necessary 7 X 1W7 107-10*
necessary >4 104 independent 30
z - ~
