Shear of thin polytetrafluoroethylene films on Si substrate determined by laser interferometer

A setup based on a laser interferometer has been developed to measure the shear response of thin polytetrafluoroethylenethe-like fluorocarbon (FC) films. By using a smooth Si sphere in a ball-on-flat configuration, the shear force was determined for FC films on Si for a thickness of 300, 600 and 1200 nm, respectively. Results indicate that the shear behaviour of FC films is mainly determined by the contact area and film thickness. For the films with thickness less than 600 nm, the shear force increases with normal load in a similar manner as predicted by the Hertz contact model for Si/Si point contacts. For the film with thickness of 1200 nm, the increase of contact stiffness, and thus the contact area with normal load leads to a much higher shear force. A slight velocity weakening of the shear force for FC films was also observed.