Friction in Micromachines
Figure 4: St. Olaf’s integrated nanoindenter-quartz microbalance apparatus positioned on a negative-stiffness vibration isolator. Image courtesy St. Olaf College.
Figure 2: Scanning electron micrograph of enlarged 910×. Image courtesy St. Olaf College.
aluminum oxide microsphere
quartz crystal oscillates at 5 million times per second, it is eas- ily able to achieve speeds and differentials comparable to the fastest micromachines [2,3].
Vibration Isolation Tis level of micro-research requires extreme stability for
the nanoindenter-quartz microbalance instrumentation. Since 2001, the lab has used negative-stiffness vibration isolation
exclusively for all of its micro/nanotribology work, including for support on its main test rig and AFM system. Negative- stiffness vibration isolators (Minus K Technology, Inglewood, CA) are compact and do not require electricity or compressed air, which enables sensitive instruments to be located wherever a production facility, laboratory, or observatory needs to be set up, whether that be in a basement or on a building’s vibration-com- promised upper floors. Tere are no motors, pumps, or cham- bers, nor any maintenance because there is nothing to wear out. Tey operate purely in a passive mechanical mode (Figure 4). A primary advantage of negative-stiffness isolators is that
they achieve a high level of isolation in multiple directions. Tese isolators have the flexibility of custom tailoring reso- nant frequencies to 0.5 Hz vertically and horizontally (with some versions at 1.5 Hz horizontally). For an isolation system with a 0.5 Hz natural frequency, isolation begins at 0.7 Hz and improves with increase in the vibration frequency. Te natu- ral frequency is more commonly used to describe the system performance. Vertical motion isolation is provided by a stiff spring that supports a weight load, combined with a negative- stiffness mechanism. Te net vertical stiffness is made very low without affecting the static load-supporting capability of the spring. Beam-columns connected in series with the vertical- motion isolator provide horizontal-motion isolation. A beam- column behaves as a spring combined with a negative-stiffness mechanism. Te result is a compact passive isolator capable of very low vertical and horizontal natural frequencies and very high internal structural frequencies. Negative-stiffness isolators deliver very high performance,
Figure 3: Friction test with the indenter probe loaded onto a surface, which oscillates laterally back and forth under the probe’s tip at speeds of 5 million times per second. Image courtesy St. Olaf College.
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as measured by a transmissibility curve. Vibration transmissi- bility is a measure of the vibrations that are transmitted through the isolator relative to the input vibrations. Negative-stiffness isolators, when adjusted to 0.5 Hz, achieve approximately 93%
www.microscopy-today.com • 2020 November
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