Evaluation of Friction Behavior and Its Contact-Area Dependence at the Micro- and Nano-Scales

The friction behavior of two different materials, mica and ultra-high molecular weight polyethylene (UHMWPE), was evaluated at the nanoscale with an atomic force microscope and with a custom-built ball-on-flat microtribometer at the microscale. The same counterface (Si₃N₄ probe), environmental conditions (25 °C, RH < 10%), and similar load ranges were maintained for all experiments. The friction-force data obtained were analyzed for contact-area dependence. Friction force between silicon nitride and mica at the nanoscale showed initial non-linearity with normal load up to a certain load, beyond which surface damage was observed resulting in a linear dependence of friction force on normal load. At the microscale, the friction force of the mica–silicon nitride interface exhibited linear dependence on normal load. Friction force between silicon nitride and UHMWPE exhibited non-linearity with normal load at both the length scales, for the applied load ranges of our experiment. An appropriate contact mechanics theory was applied to calculate an interfacial shear strength value for the material pair at both the scales. The values at both the scales were similar, when the conditions were carefully maintained to be the same across scales.