FRICTIONAL STUDY OF MICROMOTOR BEARINGS

Static and kinetic coefficients of friction have been determined in ai r for a range of microbearing designs suitable for use in micromotors. Both dry sliding and rolling friction using microspheres have been in vestigated using forces, contact areas and contact pressures typical o f those expected in a particular design of double-stator axial-drive m icromotor. Micromachined test specimens coated with polysilicon have b een slid on a variety of substrate materials. It is found that the coe fficients of friction for these small areas and loads are not constant and decrease with surface pressure for all ceramics except silicon di oxide. The coefficient of friction on aluminium remains constant throu gh all the variations studied. Sliding of polysilicon on diamond-like carbon and single-crystal silicon exhibits the lowest static coefficie nts of friction of 0.42 and 0.35, respectively. The use of glass micro spheres of diameter 40 mu m for the rolling tests reveals effective st atic and kinetic coefficients of friction of 0.04 and 0.02, respective ly. The electrostatic torques oi the micromotor for applied stator vol tages of 100 and 150 V determined using 3D finite-element analysis are compared with the friction torques for the bearings studied. The resu ltant motive torques suggest that a bushing design is the preferred op tion for this motor, since it results in both lower coefficients of fr iction and reduced electrostatic contact forces. (C) 1997 Elsevier Sci ence S.A.