Frictional study of micromotor bearings

Static and kinetic coefficients of friction have been determined in air for a range of microbearing designs suitable for use in micromotors. Both dry sliding and rolling friction using microspheres have been investigated using forces, contact areas and contact pressures typical of those expected in a particular design of double-stator axial-drive micromotor. Micromachined test specimens coated with polysilicon have been slid on a variety of substrate materials. It is found that the coefficients of friction for these small areas and loads are not constant and decrease with surface pressure for all ceramics except silicon dioxide. The coefficient of friction on aluminium remains constant through all the variations studied. Sliding of polysilicon on diamond-like carbon and single-crystal silicon exhibits the lowest static coefficients of friction of 0.42 and 0.35, respectively. The use of glass microspheres of diameter 40 mu m for the rolling tests reveals effective static and kinetic coefficients of friction of 0.04 and 0.02, respectively. The electrostatic torques of the micromotor for applied stator voltages of 100 and 150 V determined using 3D finite-element analysis are compared with the friction torques for the bearings studied. The resultant motive torques suggest that a bushing design is the preferred option for this motor, since it results in both lower coefficients of friction and reduced electrostatic contact forces. (C) 1997 Elsevier Science S.A.