Nanotribological properties and scratch resistance of MoS₂ bilayer on a SiO₂/Si substrate

The tribological properties and scratch resistance of MoS₂ bilayer deposited on SiO₂/Si substrates prepared via chemical vapor deposition are investigated. Friction force microscopy (FFM) is employed to investigate the friction and wear properties of the MoS₂ bilayer at the nanoscale by applying a normal load ranging from 200 to 1,000 nN. Scratch resistance is measured using the scratch mode in FFM based on a linearly increasing load from 100 to 1,000 nN. Kelvin probe force microscopy (KPFM) is performed to locally measure the surface potential in the tested surface to qualitatively measure the wear/removal of MoS₂ layers and identify critical loads associated with the individual failures of the top and bottom layers. The analysis of the contact potential difference values as well as that of KPFM, friction, and height images show that the wear/removal of the top and bottom layers in the MoS₂ bilayer system occurred consecutively. The FFM and KPFM results show that the top MoS₂ layer begins to degrade at the end of the low friction stage, followed by the bottom layer, thereby resulting in a transitional friction stage owing to the direct contact between the diamond tip and SiO₂ substrate. In the stable third stage, the transfer of lubricious MoS₂ debris to the tip apex results in contact between the MoS₂-transferred tip and SiO₂. Nanoscratch test results show two ranges of critical loads, which correspond to the sequential removal of the top and bottom layers.