[tt] kurzweill: nature: pentanol-spiked argon functions as good lubricant for MEMS

[Alejandro Dubrovsky]

(
http://www.nature.com/news/2008/080407/full/news.2008.740.html
)

Published online 7 April 2008 | Nature | doi:10.1038/news.2008.740

News
Helping a micromachine to work

Vapour may solve lubrication problem in tiny devices.

Rachel Courtland
micromachinesThe range of uses for tiny micromachines could now
grow.Sandia National Laboratories, SUMMiTTM Technologies,
www.mems.sandia.gov

A dilute gas may soon become the lubricant of choice for microscopic
devices. The approach could pave the way for a new range of tiny
machines, such as miniature turbines, that are finally capable of having
sliding parts.

Limited types of microelectromechanical systems, or MEMS, devices are
already widely used in inkjet printers, as accelerometers to trigger
airbag deployment, and to modulate light in high-definition televisions
and projectors.

But MEMS devices with moving parts have largely been off-limits — add
parts that slide over each other, such as a gear rotating around a shaft
or a shuttle moving back and forth, and the thin lubricant coating on
the machine will rub off. Sliding components usually fail within
minutes.

Now researchers say they have found a fix. By saturating devices with
argon gas containing a small amount of alcohol vapour, they can make
microscopic machines run at least 100,000 times longer without failing.

“We’ve found a chemical that really works,” says Seong Kim of
Pennsylvania State University in University Park, who has been working
in a collaboration with Michael Dugger of Sandia National Laboratories
in Albuquerque, New Mexico. He reported their latest results on 6 April
at a meeting of the American Chemical Society in New Orleans, Louisiana.
Slippery slope

Micromachined silicon surfaces can easily stick together through
capillary forces, chemical bonding and other interactions. To prevent
lock-up, engineers spray MEMS with fluorine-bearing silane to create a
single-layer coating. But any contact between machine parts can easily
rub this thin layer off and cause parts to stick.

Such devices could theoretically have moving parts if they could be
immersed in lubricant, as there would then be a constant supply of
slippery material to keep parts sliding.

But liquid lubricants will not work with MEMS. The components are so
tiny that ordinary oils would create a lot of drag, Kim says, rather
like asking a person to swim in a pool of honey or waltz in a swamp.

Kim thought that a gas might accomplish continuous lubrication without
imposing a lot of drag. To investigate this, his research team pumped
1-pentanol vapour around a MEMS containing a part that slid back and
forth over another surface. The gas seemed to eliminate wear entirely,
allowing the machine to move for extended times with little resistance1.

"I really think this is very significant," says Nicholas Spencer of ETH
Zurich in Switzerland. "As far as I know, the numbers they're getting
are when the operator gets bored and stops the experiment, not when the
device is failing."
Strange chains

Exactly how the vapour lubricates the system is unknown. Like solid
lubricants, the pentanol seems to adhere to silicon MEMS surfaces,
creating a one-molecule-thick coating. Measurements indicate that
molecules from the surrounding gas replace those that get knocked off
the surface. The time it takes to replace a molecule is on the order of
100 nanoseconds, says Kim.

Mysteriously, the sliding motion creates longer hydrocarbon molecules
from the pentanol2. These bigger molecules seem to make the surfaces
even more slippery.

Since the team published its results, the researchers have been trying
to figure out just how much these bigger molecules contribute to the
lubrication. Recent calculations indicate that the pentanol itself may
play the larger role, by raising the energy needed to knock molecules
off the surface.


Although some chemical details need to be worked out, Kim says, the only
real obstacle to creating MEMS with moving parts is finding a way to
enclose the device and to pump the gas in as needed.

 *
 References
 1. Asay, D. B., Dugger, M. T. & Kim, S. H. Tribol. Lett. 29, 67-74
(2008).
 2. Asay, D. B., Dugger, M. T., Ohlhausen, J. A. & Kim, S. H. Langmuir
24, 155-159 (2008).