Tiny machines built as part of silicon chips are all around us, and their need for lubrication is the same as large machines such as automobile engines, but conventional lubricants, like oils, are too heavy for these micro electromechanical systems (MEMS), so Penn State researchers are looking to gases to provide thin films of slippery coating.
MEMS today are mostly found in automobile air bags as the sensor that marks sudden deceleration and triggers airbag use. They can also take the form of tiny motors that move mirrors to focus a beam of light, or tiny nozzles that provide minute droplets of ink in ink jet printers.
"Traditionally, the lubrication industry uses viscose liquids to lubricate – oils or oils and additives – to reduce friction and increase efficiency," says Dr. Seong H. Kim, assistant professor of chemical engineering. "However, oil-based lubricant use in MEMS causes a power dissipation that is unacceptable."
Because MEMS are so small, with parts about the width of a human hair, and exert so little force, from almost none to the equivalent of the Earth's gravity on a thousand red blood cells, conventional lubricants simply do not work. Oil molecules are usually large and relatively heavy. They not only stop the MEMS dead in their tracks, but also cannot infiltrate the microscopic cracks and crevices of the machines.
The current trend in MEMS is to use solid lubricants, thin-film coatings of diamond-like carbon or self-assembling monolayers of methylated or fluorocarbon compounds. While solids provide a thin enough layer, they do not always coat the entire mechanism. They are also subject to wear because of their thinness and are not self-healing or replenishing.
Kim and Dr. Kenneth Strawhecker, postdoctoral fellow in chemical engineering, investigated delivering a thin coating of liquid lubricant by condensing a gas onto the surface of the MEMS. The researchers investigated alcohols including ethanol, propanol, butanol and pentanol.
The researchers chose alcohols because they are both hydrophilic and hydrophobic, easily combining with water on one end and combining with other compounds on the other. At the incredibly low forces encountered in MEMS, alcohols, which are not generally considered good lubricants, work.
Solubility in water is an important characteristic in lubricating MEMS. Water is always present in the air as humidity and the water does condense on surfaces. For some devices, like the air bag sensor, water is why these MEMS are used only once. These sensors have two tiny strips of material that come into contact upon rapid deceleration. Any water on the strip surfaces causes the strips to stick in the closed mode. Surface tension of the water holds the material together in the same way two panes of glass with water between become stuck. However, alcohol as a lubricant would prevent water from causing the strips to attach.
"It might also be possible to use a gas delivered liquid thin film that would regenerate the sensors allowing recycling of the air bag mechanisms," says Kim.
The researchers tested the gas lubricants at various vapour pressures and find that they produce a thin film across a wide range. The small size of the alcohol molecules allows them to coat fine details of the tiny machines and the presence of gas around the MEMS makes the system self-repairing. As the thin layer wears away, more lubricant condenses to heal the area. The thin films do not interfere with either mechanical or electrical operation.
"The next research issue we have is how to encapsulate the MEMS so we can entrap the gas," says Kim. "A variety of delivery methods exist including possibly using a polymer that emits the alcohol as temperatures increase."
The researchers also want to look at other alcohols and other compounds as potential MEMS lubricants.