Evolution of Wear Characteristics and Frictional Behavior in MEMS Devices

A surface-micromachined nanotractor device has been used to investigate the tribological behavior of MEMS devices made of polycrystalline silicon. An accelerated wear test, spanning several hundreds of thousands of cycles, was developed to monitor the evolution of wear characteristics and frictional behavior during its operational lifetime. Postmortem microscopic observations of the wear surfaces revealed features that can be categorized into two regimes of wear: (i) adhesion-dominated wear and (ii) third-body wear. The former was characterized by asperity blunting, plastic deformation of asperity peaks, and smearing of fine wear debris into a thin-surface film. With an increased number of wear cycles, the wear mechanism transitioned to the latter regime which consisted of debris agglomeration and material removal through scratches induced by these agglomerates. Finally, it was theorized that one of the agglomerates grows to a large size, adheres to one of the contact surfaces and causes severe wear in a localized region on the counter surface to lock the two surfaces and cause device failure.