Observation of a carbon-based protective layer on the sidewalls of boron doped ultrananocrystalline diamond-based MEMS during in situ tribotests

We have fabricated dedicated MEMS tribotesters made from boron doped ultrananocrystalline diamond (B-UNCD) as the structural material, and carried out comprehensive nano-tribological measurements when two B-UNCD sidewall surfaces underwent sliding interaction in a micro-electromechanical systems (MEMS) in a humid and dry atmosphere. We have investigated the evolution of tribological contacts during sliding interactions and corresponding surface modification under repeated cyclic sliding conditions, while measuring displacement and lateral force with 4 nm and 64 nN resolution, respectively. We have observed the formation of carbon-based ultra-thin protective layer at the sliding interface as characterized by Raman spectroscopy and scanning electron microscopy. Interestingly, the formation of this protective layer occurs in both a dry and wet atmosphere, albeit at different rates when the energy dissipated due to friction reaches a plateau, starting from 200 000 and 400 000 cycles, respectively. Once this layer is formed, we do not observe any measurable wear indicating stable operation for an extended time period. Our results demonstrate that B-UNCD is a very promising material to overcome the wear-related reliability problems in MEMS.