TRIBOLOGICAL DESIGN FOR MICRO SLIDING BEARINGS

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In movable parts of microsystems, increase in friction and wear caused by surface forces is one of the most serious issues, because the surface forces become dominant compared with volume forces in microsized systems. In this research, a sliding bearing on the basis of a new lubrication principle utilizing surface tension, which is advantageous in microsized systems, has been developed. Two moving surfaces of the bearing are coated with hydrophobic films and water droplets are confined between them. The bearing supports normal load by the repulsive force generated between the water drops and the hydrophobic films, and exhibits very low friction because of the low flow resistance of the water drops on the hydrophobic filmsLoad carrying capacity and flow resistance of a water droplet on two fluoride films with different roughness have been measured. The results show that the rougher surface exhibits lower friction and higher load carrying capacity. It is found that the major factor contributing to the friction force at low shearing velocity is dynamic surface tension caused by deformation of the water droplet and a component of viscous force increases gradually as shearing velocity increases. The coefficient of friction is less than 0.001 at low shearing velocity and the water droplet operates as a bearing at sliding velocities up to 1.5 m/s.Additionally, creation of micropatterned hydrophobic surfaces using photolithography and etching techniques and SAM (Self Assembled Monlayer) formation has been carried out. On various surfaces obtained by this method, sliding angles and shearing forces of a water droplet have been measured. It is found that the surfaces with bumps exhibit lower friction than those with dents, and that shearing force decreases significantly when the surface roughness is high enough to trap air between the water droplet and the hydrophobic surface.