Charge Development during Sliding Friction
between Diamond and Quartz or Sapphire
Department of Physics, Gakushuin University
*Eri HOSOBUCHI, Takashi MIURA, and Ichiro ARAKAWA
07141016@gakushuin.ac.jp
3. Experimental results
Charge development with sliding time for various
pressures of N2 is shown in Fig.1. In vacuum (6.8×
10-4 Pa), the amount of charge increased in a linear
manner with the sliding time (in other words, sliding
distance) during first rotation (0s<time<50s). The
charge density calculated from this result was ~5×10-3
C/m2, which was the same order as the value reported
by Lowell et al. [1]. In N2 gas, the rate of charge
accumulation on the pin decreased with an increase in
gas pressure and the charge continued to increase after
the first rotation completed. This is because a fraction
of charge generated by friction was removed by gas
discharge. Almost all charge was removed when N2
pressure was higher than 103 Pa.
2
-8
Charge [10 C]
2. Experimental method
The friction experiment was performed by the pin
on disk method; pin-shaped diamond with the radius of
curvature of 300μm or 500μm and disk-shaped quartz
or sapphire with the radius of 25mm were settled in a
vacuum chamber. The surface of the diamond pin,
except the contact area, was coated with gold thin film
and connected to an electrometer, which makes it
possible to measure the amount of charge transferred
from the quartz or sapphire to the diamond during
friction. The diameter of the contact area of the pin, i.e.,
the friction track width, was about 10μm. We have
investigated how charge develops depending on a gas
pressure, a sliding velocity, and a normal load of
friction. The measurement was done for Ne and N2 gas
at pressures between 10-3 and 105 Pa, sliding velocities
between 10-2 and 140 mm/s, and normal loads between
0 and 300 mN.
We have examined the triboelectric charge per unit
length of the sliding track as a function of the sliding
velocity over four orders of magnitude in vacuum
(<10-3Pa). It has been believed that the frictional
electrification solely depends on the contact area and
therefore not on the sliding velocity. However, we
found that the charge increased linearly with the
logarithm of velocity as shown in Fig.2. Although
there are no satisfactory explanations for this
dependence at this time, one acceptable explanation
may be back flow of the charge immediately after the
separation of the contact.
-4
6.8×10 Pa
1.3 Pa
1.2×10 Pa
3.9×10 Pa
1.3×104 Pa
1
0
0
100
Time [s]
200
300
Fig. 1 Charge development on a gold-coated diamond
pin during friction with quartz disk. Rotational speed:
0.02rps, radius of the friction track: 22.4mm, radius of
curvature of the pin: 300μm, load: ~100mN.
1
Charge [10-10C/mm]
1. Introduction
Triboelectricity caused by contact or friction of
insulating materials has long been studied but the
mechanism is not fully understood yet. To examine
behavior of electronic transfer between insulators by
friction, we have observed charge development and
light emission during sliding between the spherical
diamond and the flat quartz or sapphire.
0.8
Diamond/Quartz
Diamond/Sapphire
0.6
0.4
0.2
0
0.01
0.1
1
10
Sliding speed [mm/s]
100
Fig. 2 Triboelectric charge per unit length of sliding
track as a function of the sliding velocity in vacuum
(<10-3Pa). The radius of curvature of the pin: 300μm,
load: ~50mN.
Reference
1) J.Lowell and A.C.Rose-Innes: Adv. Phy., 29 (1980)
947-1023.