improved method for correcting nano- and micro-scale
friction measurements has been developed by researchers
at the National Institute of Standards and Technology
(NIST). The new technique should help designers produce
more durable micro- and nano-devices with moving parts,
such as tiny motors, positioning devices or encoders.
Friction measurements made at the micro- and nano-scale
can differ substantially due to changes in applied
load. In a series of experiments described by nanotribologist
Stephen Hsu at a technical meeting held May 17-20
in Toronto,* NIST scientists confirmed that many of
the measured differences appear to be caused by unintended
scratching of the surface by the sharp tips used in
making the measurements themselves.
The NIST team used a specially designed friction tester
developed jointly by NIST and Hysitron Inc. of Minneapolis.
A carefully calibrated force was applied to diamond
tips having a range of sizes. Friction forces were
then measured as each tip was slid across a very smooth
surface of silicon. Friction at the macroscopic scale
is usually straightforward--doubling the force between
two objects produces twice the friction. However,
work at NIST and elsewhere has shown that friction
at the microscale does not always obey this scaling
rule. Forces greater than about 2 milliNewton** produced
substantially greater friction values than expected.
Images of the test surface made with an atomic force
microscope confirmed that unintentional scratching
produced the extra friction. To correct for this effect,
NIST researchers developed a way to measure precisely
the size, shape and orientation of the diamond tips
so that friction forces caused by "plowing"
can be subtracted to produce a more accurate final
*The work was presented at the Society of Tribology
and Lubrication Engineers annual meeting.
** For comparison, a penny held against Earth's gravity
produces a force of about 25 milliNewtons