Nanofretting behaviours of ultrathin DLC coating on Si(1 0 0) substrate

Due to its excellent mechanical properties and stable chemical behaviours, the diamond-like carbon (DLC) coating has been extensively used as a low-frictional and wear-resistant protective coating in MEMS. However, its efficacy on resisting nanofretting damage of silicon substrate needs to be verified. With an atomic force microscopy, the nanofretting behaviours of ultrathin DLC coating and its Si(1 0 0) substrate against SiO2 microsphere were investigated under vacuum and air conditions. The applied normal load Fn was 2 μN and the displacement amplitudes ranged from 0.5 nm to 100 nm. The experimental results indicated that the DLC coating revealed a strong effect on the nanofretting regime, adhesion force, friction force and nanofretting damage of Si(1 0 0)/SiO2 pairs. Either in vacuum or in air condition, the coating could expand the slip regime of Si(1 0 0)/SiO2 pairs into a lower value of displacement amplitude. Owing to the absence of oxygen and water molecule in vacuum, both the adhesion and friction forces on DLC coating were similar as those on Si(1 0 0) substrate. However, while the nanofretting ran in air, the DLC coating could significantly reduce the adhesion and friction forces of Si(1 0 0) substrate. Even though the coating was only 2 nm in thickness, it could effectively protect the silicon substrate from nanofretting damage. In vacuum, the ultrathin DLC coating can prevent the formation of hillock on Si(1 0 0) substrate. In air, the wear depth on DLC coating was <5% of that on Si(1 0 0) substrate under the same nanofretting conditions. These results can help us to develop the surface engineering technology to minimize the nanofretting damage of silicon contact surfaces in MEMS.