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Exploiting friction can make MEMS work

Robert W. Carpick

Robert W. Carpick (large image)

Envision the topography of a packed-gravel road. While not "smooth" like the bottom of a brand-new leather-soled shoe or even an asphalt street, the road probably appears relatively flat and functional. Yet the varied shapes of the small stones that comprise it create millions of tiny points of contact between it and a vehicle's tires. These contacts determine the amount of friction between the tire and the road.

The gravel road offers a life-sized picture of what materials surfaces look like under a microscope. "Surfaces are always rough," says Assistant Professor of Engineering Physics Rob Carpick.

"Topography" of a silicon MEMS device (large image)

And at the nanoscale, where there is a greater surface-to-volume ratio, friction, adhesion and bonding can be such strong factors that micromachines — currently used in applications ranging from airbag triggers in cars to computer projection systems — won't work at all.

Building on seed projects with both Sandia and Argonne National Laboratories, Carpick, Professor of Engineering Physics Mike Plesha and research associate Anirudha Sumant are using a three-year, $525,000 grant from the Department of Energy Nanoscale Science Engineering and Technology program to study friction at the nanoscale and how it applies to microelectromechanical systems (MEMS).

"People have been making all sorts of elaborate and incredible micromachine devices for many years, but there are no commercial MEMS devices on the market that involve surfaces that contact and slide," says Carpick.

The project's goal is to enable researchers to design robust, reliable MEMS devices. And the trio's results could open a whole new market for micromachines such as motors, pumps, actuators, communications devices and miniature mechanical combination locks with sliding parts — all of which have been nearly impossible.

"There's really no theory of friction at the micro- and nanoscale that would be relevant for these MEMS devices," says Carpick. "So what we're trying to do is measure friction at the level of a single point of contact."

The group is using Carpick's atomic-force microscope to characterize friction at that contact point, called an asperity, based on variables such as pressure, sliding speed, type of material and environmental conditions. Once the three have established data for friction at a single point, they will develop models with multiple contact points, ultimately using a finite-element approach, or approximation method, to analyze and design MEMS devices.

Diagram of an atomic-force microscope

The atomic-force microscope — diagrammed here — can "see" MEMS' tiny surfaces. (large image)

Plesha, who is developing the models, brings a wealth of experience solving engineering contact and friction problems in geological systems at considerably larger sizes — up to the kilometer scale. "But we think that, in certain specific ways, a lot of the ideas are actually applicable," says Carpick of the larger scale. "And that's one of the things I think is very cool, very fascinating, about this project: that we might make some connections to friction at the earthquake-geological fault level."

Wouldn't it make more sense just to grind and polish "rough" surfaces smooth? You'd think, says Carpick, because if they're rough, you'd imagine gouging and scratching. "But if they're too smooth, what happens is they touch everywhere and then you get a very high friction force because the area of contact is too large," he says.

And in fact, scientists at Sandia and elsewhere can control how rough or smooth they make a surface. Right now, however, they don't know which works best. "Ultimately, our goal is to be able to tell them, 'If you want a micromachine that's going to last, this is how you should design your surface,' " says Carpick.