Operational Wear and Friction in MEMS Devices

Friction and wear are extremely important issues in micromachined surfaces in design applications that allow rubbing. A polysilicon surface-micromachined inchworm device has been developed to obtain detailed in-situ information on these properties under well-controlled loading conditions. Here, we investigate the inchworm operational wear and the evolution of friction coefficient as a function of the number of imposed wear cycles. A test procedure was developed to monitor various functional parameters such as the travel distance of the inchworm under an imposed drag force for a fixed number of steps and the friction coefficient. While subject to this drag force, the travel distance decreased gradually until the foot of the device became permanently lodged in the grooves created by the wear-track. Meanwhile, it was found that the friction coefficient increased from 0.2 on a virgin surface to 3 when the accumulated number of wear cycles reached around 200,000. The friction test itself was found to interact with the wear processes. By minimizing the number of friction tests performed during the wear test, the operational life of the device was extended well beyond 700,000 cycles. Microscopic observation of the wear surfaces revealed that the early wear is characterized by the blunting of the sharp peaks on the poly silicon grains and then flattening of this fine wear debris on the surface. Evidence of plastic deformation was inferred by the spread of the wear debris over several grains. With increased number of wear cycles, material removal through scratches induced by the wear debris was observed. The device failure occurred due to a large volume of material removal (severe wear) in localized regions.