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Microtribology |
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Microtribology |
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Many studies on the manufacture of micro machines and MEMS (micro
electromechanical systems) have been conducted recently . For micro
machines, as the dimensions of components decrease, the surface area
decreases in proportion to the square of the linear decrease while
the volume decreases in proportion to the cube power of the linear
decrease (Fig. 1). Therefore, the influence of the surface
force increases dramatically, and the tribological characteristics
become more important.
We do research on microtribology in order to clarify the tribological
characteristics under an extremely low normal load where the effect
of the adhesion forces is not negligible, and to develop suitable
lubrication methods for micromachine.
Fig. 1 Relation between surface area and volume when dimensions
decrease
Fig. 2 shows the friction force and the pull-off force
between a copper test piece and a silicon plate, which were measured
under constant normal load. the pull-off force is the average
measured before and after each friction force measurement. The
friction force is about 30 mN even when
the applied normal load is zero.
The data in Fig. 2 are arranged in Fig. 3. In this
figure, ● (light blue) shows the
friction coefficient calculated from only the normal load, while
● (red) shows the friction coefficient
calculated from the friction force divided by the sum of the normal
load and the pull-off force. In Fig. 3, assuming that the
pull-off force acts as a type of normal load, the friction
coefficients become constant. This indicates that the adhesion force
as same as the mean pull-off force measured before and after friction
also acts during friction and influences the friction force just as
the normal load does.
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When the applied normal load is very low as found in MEMS, the
friction force can be reduced by reducing the adhesion force. The
adhesion force caused by the Van der Waals force and surface tension
of adsorbed water is greatly affected by contact area. Some studies
reported that slight roughness decreases the adhesion force
significantly. Therefore, creating asperity is considered to be an
effective way to reduce the adhesion and friction forces.
We examined the adhesion (pull-off) and friction forces on silicon
surfaces having periodic asperity (two-dimensional array of mounds)
First, we used a focused ion beam (FIB) to create two-dimensional
asperity arrays as shown in Fig. 4 by milling grooves in two
orthogonal directions. We then used an atomic force microscope (AFM)
to measure these forces between the silicon surface and the flat AFM
probe. Figure 5 shows the relation among the friction and
pull-off forces and groove depth. The friction and pull-off forces
are proportional to the radius of curvature of asperity peak.
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At the micro-scale level, the adhesion force dominates the friction force when the normal load approaches zero. For determining the effects of micro wear on the adhesion (pull-off) force, the wear-induced changes in surface topography of asperities and the pull-off force between the asperities and leaf springs were determined. First, single asperities were formed on a single-crystal gold plate and the asperities were rubbed with a silicon leaf spring attached to an AFM. A focused ion beam (FIB) system was used to form gold pyramid-shaped asperities on the surface of a single crystal gold plate. The FIB was also used to create the single crystal silicon parallel leaf spring. The pull-off force increased monotonically with sliding distance, showing a more rapid increase at the higher normal load. The worn area of the asperity peak (measured by using an ordinary AFM probe) was proportional to the pull-off force.
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->Microdevices
for micro/nano science
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last update October.10.
2001 |
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Primal Scientist: Yasuhisa ANDO |
