Creating a tool small enough to measure friction on a microelectromechanical systems (MEMS) device is not an easy task. The tool has to be about the width of a human hair.
Yet, researchers at the at the National Nuclear Security Administration’s Sandia National Laboratories have developed a new “inchworm” actuator instrument that provides detailed information about friction at the microscale.
The main objective of the project was to study the validity of Amonton’s Law at the microscale. This law, first stated 300 years ago, says friction force is proportional to normal force (normal means perpendicular to the surfaces). Although it remains a good description of friction today, there are interesting deviations from Amonton’s Law, especially at low normal forces, where adhesion between the two surfaces is thought to contribute an extra force. Because of the large surface-to-volume ratio at the microscale, these adhesive forces could cause a strong deviation from Amonton’s Law.
“We must accomplish several different functions in one device to study the laws of friction at the microscale,” said Sandia researcher and inchworm creator Maarten de Boer. “We need to build an actuator that controllably and accurately generates both very low and very high forces.”
These forces need to be applied both perpendicularly and tangentially to test the validity of Amonton’s Law over a wide force range, he said. Another goal is to see if the friction measurements can describe the actual operation of the device.
Parallel plates are easily fabricated in MEMS and can be used to obtain large normal forces. However, conventional MEMS tangential force actuators, known as comb drives, provide only 10 micronewtons of tangential force. De Boer wanted to achieve millinewtons of force and resorted to a mechanical amplification scheme.
“We can bend plates to convert normal force to tangential force,” he said. The inchworm has a force-amplifying plate that spans two frictional clamps. While “inching” along, its step size is very small, about 40 nanometers, but its stepping cycle can be repeated over and over.
De Boer and Sandia postdoctoral researcher Alex Corwin developed a measurement methodology to confirm that the inchworm operates at up to 80,000 cycles a second, with a velocity of up to 3 millimeters per second.
The force the inchworm develops is measured by attaching it to a load spring that has a special nonlinear characteristic — for each step the inchworm walks, the force increases more than it did in the previous step. A maximum tangential force of 2.5 millinewtons is achieved when the inchworm stalls out, about 250 times more force than a comb drive.
So far this describes the inchworm’s ability to move and provide forces. But de Boer asks the question: How does the inchworm give information on friction?
To determine the coefficient of static friction, the inchworm is walked out a long distance of about 20 micrometers against the nonlinear load spring. Then, a large voltage on one clamp holds it in place. This voltage is then gradually reduced, which decreases the frictional force it can sustain. At a certain point, the load spring overcomes that frictional force, and the inchworm slides forward a short distance (several micrometers) until the load spring force decreases enough that it stops again. This measurement is now repeated and the next measurement is at a much lower load due to the load spring’s nonlinearity. Now, Amonton’s Law can be studied over a wide force range.
Working with Bob Ashurst, University of California, Berkeley, de Boer found that the friction force depends on the normal force from 1 millinewton down to normal loads as small as 50 micronewtons, in accordance with Amonton’s Law. Corwin further decreased the load and found that the coefficient of static friction began to increase. This was the deviation from Amonton’s Law due to adhesive forces, de Boer said.
De Boer and Corwin also mapped out a way to measure dynamic friction, accounting for the friction forces in the presence of inertial forces, air damping forces, and the spring force. The result was that dynamic friction is about 80 percent of static friction.