ELSEVIER Wear 194 (1996) 1X5-189

WEAR

Tribocharging and friction in insulators in ambient air

Keiji Nakayama Mechanica1 Engiruw-ing Luborcrrory, Numiki 1-2, Tsukuba, Iburuki 305, Japan

Received 10 July 1995; accepted 19 October 1995

Abstract

Surface potential and friction coefficient were measured simultaneously during scratching of insulating materials made of ceramics and polymers with a diamond stylus in ambient air. The insulating solids used were A1?03, Si,N,, glass and polytetrafluoroethylene (PTFE). It was found that the surface potential and friction coefficient have a positive correlation.

Keywordst Tribocharging; Triboelectrification; Friction; Surface potential; Ceramics; Polymer

1. Introduction

It is known that when two solid surfaces are rubbed together, opposite surface potentials are generated on the mating surfaces. This phenomenon is called tribocharging or triboelectrification. Several mechanisms have been proposed to explain the tribocharging mechanism, such as the work function differente for metals [ 11, asymmetrie rubbing [ 21, charging by protons or hydroxide transfer [ 31, pyroelectricity and piezoelectricity [ 41, and wearing down of the neutralized surface layer to expose an intrinsic charged surface [ 51. How- ever, there is no unified theory that explains the tribocharging phenomenon. This suggests that tribocharging may occur as a result of the complex effects of these cause factors.

On the other hand, when two bodies are slid against each other, frictional resistance takes place along with surface damage. The development of friction-reducing techniques is an important task in the field of tribology, since energy-saving and machine efficiency are concerned with friction [ 61.

The purpose of the present study is to clarify the relation between the tribocharging phenomenon and the friction resis- tante in a system of an insulating solid sliding on an insulating solid. Experimental findings on the correlation between the intensity of the surface potential caused by tribocharging and the friction coefficient are presented.

2. Experimental details

Fig. 1 shows the tribocharging and friction coefficient measuring system for scratching of a disk specimen surface by a diamond stylus in ambient air. The friction coefficient, f, was detected using strain gauges. The surface potential, VS, generated by the tribocharging phenomenon was measured using a non-contacting type surface potential measuring apparatus (Trek Japan, Electrostatic voltmeter model 344) at a position 180” opposite the place where the surface was scratched, i.e. where the friction coefficient was measured. The apparatus has unique characteristics in that the surface

Potential

Friction

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Fig. 1. Black diagram of tribocharging and friction coefficient measuring system.

0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06840-6

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-50 0 20 40 60 80 100

Time (s)

6.01 1 Frictik I I / I

w 4.0

2.0

'0 20 40 60 80 100 Time (s)

0 20 40 60 80 100 Number of rubbing (cycle)

Fig. 2. Dependence of maximum and minimum surface potential, VS, and friction coefficient,f, on number of rubbing cycles in scratching of Alz03: t, = 200 ms, n = 100 points.

potentials measures are not affected by a change in the dis- tante between the detector and specimen surface; the surface potential is accurately measured even when there is mechan- ical up-and-down vertical motion of the disk surface at the measuring point during rotation. The two signals of the fric- tion coefficient and the surface potential were simultaneously sampled using a common trigger unit, processed on a PC, and then recorded.

The abscissa of al1 the figures displayed on the CRT were divided into 500 points. So the time that the signals were collected at every point, f,, can be expressed as t, = r,/500, where t, is the total measurement time. The signals of surface potential and friction coefficient were sampled every 2 ms ( = 0.002 s). Then at each point of the figure, the sampling number, n, can be expressed as n = tJO.002. In the figures, however, only the maximum and the minimum surface poten- tials were plotted. For example, the maximum and the mini- mum surface potentials of those measured at 100 points are plotted in Fig. 2 giving separate curves, while in Fig. 3, the surface potential was measured at only one point (n = l), so that the maximum and the minimum surface potentials cor- respond, producing a single line. On the other hand, only the maximum friction coefficient was measured for al1 n points. It is described simply as the friction coefficient in the text below.

The diamond stylus used had an included angle of 90” and a tip radius of 100 Pm. Al,O, (normal sintered alumina, Hu = 20 GPa, RMS = 0.62 Pm) and Si3N4 (normal sintered silicone nitride, Hu = 15 GPa, Rh4S = 0.08 km), glass (soda lime glass, Hu = 5.7 GPa, RMS = 0.11 km), and PTFE (pol- ytetrafluoroethylene, Hu = 40 MPa, RMS = 0.81 Pm) were used as the disk specimens, where Hv is the Vickers hardness and RMS is the roughness expressed as the root mean square. The surface of each disk specimen was finished with #400 corundum paper, washed with petroleum benzine and acetone

E 1: 0

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3.5 3.7 3.9 4.1 4.3 4.5 Time (s)

2.0

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0 3.0 3.2 3.4 3.6 3.8 4.0

Time (s) Fig. 3. Time dependence of surface potential. VS, and friction coefficient,f; dwing scratching of A1203: I, = 2 ms, II = 1 point.

in an ultrasonic bath, and then dried with a warm flow of air.

The scratching experiments were conducted under a normal force of F, =OS N with a sliding velocity of V= 6 cm s- ’ (rotating velocity: 1 rps) in ambient air having relative humidity of 66-78%.

3. Results and discussion

Fig. 2 shows the maximum and minimum surface poten- tials and the friction coefficient during scratching of Al,O, in ambient air under a normal force F,, = 0.5 N with sliding velocity V=6 cm s-‘. The maximum and minimum surface potentials were measured at 100 points every 200 ms. The maximum surface potential increased first from initial zero and then decreased after peaking, while the minimum surface potential first decreased and then increased after reaching a minimum. That is, the differente between the values of the two curves, i.e. the width of the two curves, first increased and then decreased after reaching a maximum. The friction coefficient also increased with repeated rubbing, peaked, and then gradually decreased. The correlation between the surface potential and the friction coefficient, however, is not clear in the curves shown in Fig. 2.

Many smal1 spikes are seen on both the surface potential and the friction coefficient curves. To determine whether there is a microscopic correlation of the surface potential and the friction coefficient, the abscissa of the curves in Fig. 2 were enlarged and divided into four periods: (1) the very early period of the first rubbing cycle (Fig. 3), (2) the 1 st to the 10th rubbing cycles (Fig. 4), (3) the 37th to the 47th cycles (mid-range, Fig. S), (4) the 77th to the 87th cycles (final, Fig. 6).

Fig. 3 shows the surface potential and the friction coeffi- cient during scratching of Al,O, in the first rubbing cycle obtained by enlarging the abscissa of Fig. 2. In this figure the measuring point, n every ta= 2 ms. was one point, so the maximum and the minimum coincide to produce a single line. The surface potential on the wear track surface was not uniform, having the negative charges shown in Fig. 3. The

K. Nakuyatna/ Wear 194 (1996) 185-189 187

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0 40 42 44 46 48 50

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Number of rubbing (cycle) Number of rubbing (cycle) Fig. 4. Dependence of surface potential, V,, and friction coefficient, A on Fig. 5. Dependence of surface potential, L’,, and friction coefficient, f, on number of rubbing cycles in scratching of AlzO+ t, = 20 ms, n = 10 points. number of rubbing cycles in scratching of AlzO-<: f, = 20 ms, n = 10 points.

friction coefficient also varies as it gradually increases with scratching. NO correlation between the surface potential and the friction coefficient, however, was observed in the first scratching.

Fig. 4 shows the maximum and minimum surface poten- tials for 10 measuring points every 20 ms and the friction coefficient in the first 10 rubbing cycles of Al,O,. The max- imum and the minimum surface potential curves are separate. However, they are too close and fluctuate too often to be distinguished clearly. This may be because the surface poten- tial varies on a microscopic scale. Therefore, it is not clear whether there are periodic waves of the surface potential on the sliding distance. The width of the fluctuations of the surface potential increased with the number of rubbing cycles. On the other hand, periodic waves of the friction coefficient can be seen from the second rubbing cycle, and the value of the friction coefficient increased with the number of rubbing cycles. The period of the friction is consistent with that of the rotation of the disk (1 rps), so it is certain that the friction coefficient depends on the position of the wear track surface. The dependency of the friction coefficient on the sliding distance continued even after wear caused by repeated scratching.

Fig. 5 shows the surface potentia and the friction coeffi- cient in the middle period, the 37th to the 47th rubbing cycle, also obtained by enlarging the abscissa of Fig. 2. The maxi- mum and minimum surface potential curves in n = 10 and r, = 20 ms approach each other to show distinctly the exis- tence of periodic waves. It can also be seen that the surface is charged negatively. The periodicity of the friction coeffi- cient can also be observed.

Fig. 6 shows the maximum and minimum surface potential and the friction coefficient for n = 10 and t, = 20 ms in the final scratching period, from the 77th to the 87th rubbing cycle in Fig. 2. As shown in Fig. 6, the microscopic heavy fluctuation of the surface potential decreased to become mac- roscopic and the maximum and minimum surface potentials coincided to produce a single line. On the other hand, the

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Fig. 6. Dependence of surface potential, VS, and friction coefficient, f, on number of rubbing cycles in scratching of AIZO.l: t, = 20 ms, n = 10 points.

periodic waves of the friction coefficient are stil1 present. It should be noted that the positions of the maximum and the minimum of the surface potential curve coincide with those of the friction coefficient curves. It can also be seen that the values of the friction coefficient decrease steadily, corre- sponding to the decrease of the surface potential with repeated scratching.

To ascertain whether this correlation is a universal law, the surface potential was measured simultaneously with the fric- tion coefficient for another test specimen of Al,O, and spec- imens of other materials. Fig. 7 shows the surface potential and the friction coefficient for the 77th to the 87th cycle with another Al,O, test specimen. A similar positive correlation between the surface potential and the friction coefficient can be observed.

Fig. 8 shows the surface potential and the friction coeffi- cient for one rubbing cycle from 89.5 s to 90.5 s rubbing time, obtained by enlarging the abscissa of Fig. 7. The friction coefficient is clearly highest in the center portion of the rise of the surface potential curve.

188 K. Nakuyumu/ Weur 194 (1996) 185-189

Time (s)

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77 79

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Fig. 7. Dependence of surface potential, V,. and friction coefficient. f; on number of rubbing cycles in scratching of AlzO,: t,=20 ms. n = 10 points.

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90.0 90.2 90.4 90.6 90.8 91.0 Time (s)

89.5 1 89.7 l 89.9 I 90.1 / 90.3 / 90.5 I

Time (s) Fig. 8. Time dependence of surface potential, V,, and friction coefficient,f; in scratching of AIZ03: f, = 2 ms, II = 1 point.

80.5 82.5 84.5 86.5 88.5 90.5 Time (s)

80 82 84 86 88 90 Time (s)

77 79 il 83 85 Si Number of rubbing (cycle)

Fig. 9. Dependence of surface potential, Vs. and friction coeftîcient, f, on number of rubbing cycles in scratching of Si,N,: r, = 20 ms, n = 10 points.

Fig. 9 and Fig. 10 show the surface potentials and the fric- tton coefficients during scratching of Si,N, and glass for the 77th to the 87th rubbing cycles. Periodic waves of the surface potential with negative charge and the friction coefficient can

1 / I / I I 80.5 82.5 84.5 86.5 88.5 90.5

Time (s)

80 82 84 86 Time (s)

88 90

77 79 81 83 85 87 Number of rubbing (cycle)

Fig. 10. Dependence of surface potential. V,, and friction coefficient,f, on number of rubbing cycles in scratching of glass: t, = 20 ms. n = 10 points.

be observed in both Si,N, and glass. Positive correlations of the surface potential and the friction coefficient are also def- initely observed with Si,N, and glass.

The observed periodic waves of the surface potential and the friction coefficient must be caused by surface non-uni- formity, because the materials used in this study were not uniform single crystals but engineering materials. The non- uniformity of the materials may produce local variations of the surface charges and the friction coefficient. Non-uniform engineering materials normally have deficiencies, the types and densities of which are closely connected with the gen- eration and intensity of tribocharging [ 51. The deficiencies may also be produced by repeated scratching. Before the scratching test, the surfaces were covered with adsorbed films from the molecules of the ambient air that neutralizes the original surface charges [ 51. The neutralized surface layers may be scratched off to reveal the underlying intrinsic surface charges. The character of the non-homogeneity of the mate- rial continued to be evident down to layers deep beneath the surface. The character of the layers does not vary greatly on repeated scratching. Therefore, the periodic surface potential and the friction coefficient were maintained even after repeated sliding.

On the other hand, friction is not a material property but a phenomenon caused by the interaction between the surface and the surrounding gas molecules [ 61. That is, when a sur- face is covered with a strongly chemisorptive and weakly physisorptive film, the friction coeffìcient was low and high, respectively. There are two possibilities for the interaction between the solid surface and surrounding gas molecules producing a low surface potential and low friction coefficient: ( 1) the solid surface itself is charged negatively by some of the reported tribocharging mechanisms, such as formation of defects or asymmetrical rubbing [ 1.2.4,5] and neutral mol- ecules adsorb to the negatively charged surfaces, or (2) the solid surface is neutral and adsorption of ions such as OH-

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60.5 62.5 64.5 66.5 68.5 70.5 Time (s)

1 Friction I / I

I l I I I I 60 62 64 66 68 70

Time (s)

57~ . 59 61 63 6’5 67 Number of rubbing (cycle)

Fig. ll. Dependence of surface potential, V,, and friction c0efficient.f. on number of rubbing cycles in scratching of PTFE: t, = 20 ms, n = 10 points.

or (H,O) ~ from the ambient air produce the charges on the scratched surfaces. These ions may be formed by molecules having atoms with high electronegativity, such as H,O catch- ing formed electrons, e.g. triboemission [ 7-101 due to dis- charge of the ambient air molecules. One or both of these possibilities may contribute to the simultaneous reduction of both surface potential and friction coefficient. To determining this, however, further work is needed and this is now under way.

Fig. 11 shows the surface potential and the friction coef- fìcient during scratching PTFE from the 57th to the 67th rubbing cycles. PTFE is known to be chemically stable and relatively inactive with respect to the adsorption of molecules. As shown in Fig. 11, a surface potential of high intensity was generated over the scale in PTFE, so the overall shape of the surface potential curve could not be obtained. However, only the minimum portion of the measured curve was shown to have periodicity. On the other hand, the friction coefficient fluctuates with the sliding distance, producing no clear peri- odic waves. It is thus difficult to find a correlation between the surface potential and the friction coefficient. However, it

can be seen that where the surface potential was at a mini- mum, the friction coefficient was also locally minimized. These results on PTFE, an inactive material, also suggest that the interaction between the solid surface and the ambient gas molecules has an important role in the observed correlation between the surface potential and the friction coefficient.

4. Conclusions

Surface potentials and friction coefficients were measured simultaneously during scratching of insulating materials of ceramics and polymers by a diamond stylus in ambient air and the results led to the following important conclusion.

The surface potential caused by tribocharging and the fric- tion coefficient have a positive correlation in insulating solids except PTFE. That is, where the surface potential is low (high), the friction coefficient is correspondingly low (high).

References

[ 1] W.R. Harper, Conract and Fricrional Elecrrijicariorl. Clarendon Press, Oxford, 1967, pp. 223-246.

[2] J. Latham, Electrification produced by the asymmetrie rubbing of ice on ice, Brit. J. Appl. Phys., 14 ( 1963) 488-490.

[ 31 T. Oguchi and M. Tamatani, Contact electrification in inorganic binary compounds, J. E[ectrochemical Sec.. 133 ( 1986) 841-847.

[4] A.J.P. Martin, Triboelectricity in wool and hair, Proc. Phys. SIX., 53 (1941) 186-189.

[5] M.I. Komfeld, Frictional electrification, J. Phys. D.: App. Phys.. 9 (1976) 1183-1192.

[6] H. Czichos, Tribology: A Sysrems Approach to the Science and Technology of Friction, Lubricariun and Wear, Elsevier, New York, 1978, pp. 97-175,.

[7] K. Nakayama and H. Hashimoto, Triboemission from various materials in atmosphere, Wear, 147 (1991) 335-343.

[ 81 K. Nakayama, N. Suzuki and H. Hashimoto, Triboemission of charged particles and photons from solid surfaces during frictional damage, J. Phys. D: Appl. Phys.. 25 (1992) 303-308.

[9] K. Nakayama and H. Hashimoto, Triboemission of charged particles and photons from wearing ceramic surfaces in various gases, Tribology Trans., 35 (1992) 643450.

[ 101 K. Nakayama and H. Hashimoto, Effect of surrounding gas pressure on triboemission of charged particles and photons from wearing ceramic surfaces, Tribology Trans., 3R ( 1995) 3542.