Frictional Characteristics of Sub-100- $\mu \text{m}$ Borosilicate Glass Balls for Actuator Applications

The frictional characteristics of ~62-μm diameter borosilicate glass balls used as rolling elements for a miniature electromagnetic linear actuator were assessed. The glass balls were placed between two silicon plates of the actuator with linear bearing tracks fabricated in the shape of V-grooves on the silicon surface. Rolling friction experiments were conducted using the electromagnetic linear actuator as well as a tribotester. It was shown that the glass balls were effective in generating a low friction coefficient of ~0.01 over 11 million cycles under a load of 6.6 mN. For the durability test using the tribotester, a relatively high load of 245 mN was applied. In this case, evidence of silicon surface damage could be found. The feasibility of using Au as a solid lubricating film on the silicon surface was investigated. It was found that the friction coefficient and surface damage depended on the thickness of the film, which was varied from 7~20 nm. It was postulated that the low shear strength of Au aided in reducing the friction coefficient from ~0.017 to ~0.013. With improved adhesion of the Au coating on silicon, the use of sub-100-μm borosilicate glass balls as rolling elements for miniature actuators seems promising.