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526 CAM DESIGN HANDBOOK

major failure mechanism for microscale devices is not fatigue failure but wear between
contacting surfaces. A detailed study conducted by Sandia National Laboratories on poly-
crystalline silicon micromechanical transmissions concluded that the major failure mech-
anism for operating MEMS was the wear of the rubbing surfaces.
Friction and wear can be most effectively minimized or avoided if elastically flexible
structures are used. This is probably the reason for the widespread use of simple elastic
structures in microtransducers. However, intermittent surface contacts are unavoidable
when applications such as the ones described in the previous section involve sophisticated
mechanical movement. Cams and gears suffer from the problem of friction and wear but
provide the designer with numerous possibilities. As the tribological issues at the micro-
and nanoscale are better understood, cams are likely to play an increasing role in MEMS
devices. Some past and current studies on friction and wear pertinent to the successful
operation of cams are described next.
Two early studies on in situ friction measurement on electrostatically actuated
micromotors reported friction coefficients in the range of 0.21 to 0.38 for polysilicon on
silicon nitride and 0.25 to 0.35 for polysilicon on silicon. In the latter study, the micro-
motors stopped running after 0.75 to 1 million revolutions and this was attributed to wear.
Another study used a polysilicon wobble micromotor in which the wear between the rotor
and the central hub changes the gear ratio of the motor. This was used to quantify wear.
A very detailed three-year study (Sandia 2000) considered the effects of humidity, tem-
perature, shock, vibration, and storage. The device on which this study focused was called
a microengine, which as described earlier has many polysilicon parts that rubbed against
each other. This study outlined the observed failure modes in operating and nonoperating
conditions and recommended some design rules to avoid such failure. Even though the
study was based entirely on a particular type of polysilicon microdevice, their findings
could be generalized to any MEMS device with rubbing surfaces or surfaces in close
contact. Wear was observed to be the major failure mechanism. The debris and the asper-
ities caused by wear lead to momentary and intermittent sticking and eventually perma-
nent adhesion and seizure. The three-body wear in which debris gets caught between
the rubbing surfaces was found to be a major contributor to wear. It was also found that
humidity helps to mitigate wear by acting as a lubricant. Therefore, relative humidity of
30 percent to 60 percent is recommended for operation at room temperature. Reduction
of three-body wear by removing the initial debris is recommend but it may not always be
practical. Minimizing rubbing surfaces by design and minimizing the impact forces at the
rubbing surfaces by controlled actuation were recommended to reduce wear. All of these
are relevant when cams are used in MEMS because rubbing surfaces are inevitable in
cams.
Several studies have been conducted to study friction at the microscale. In one such
study (Bhushan, 1999), a silicon nitride (Si 3N 4) probe whose tip had a radius of 50nm (50
-9
¥ 10 m) was scanned over a 1 ¥ 1mm sample area at a scanning speed of 5mm/s in the
load range of 10 to 150nN. Several samples of single-crystalline, polycrystalline, and
oxide-coated silicon were used. It was found that the coefficient of friction was between
0.02 and 0.04 for all the samples. This is considerably smaller than the coefficient of fric-
tion at the macroscale, which was found to be about 0.18 for the same samples but using
a Si 3 N 4 ball of radius 3mm. Two reasons are cited for this. First, the indentation hardness
and elastic modulus are higher at the microscale and this reduces wear. Second, there is a
smaller apparent area of contact, which contributes less to plowing. Even though the coef-
ficient of friction is an order of magnitude smaller than the macroscale value, it should be
remembered that friction forces at the microscale are still dominant because of the scaling
effect mentioned earlier.