Frictional Responses of Octadecyltrichlorosilane (OTS) and 1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane (FOTS) Monolayers Self-assembled on Aluminium over Six Orders of Contact Length Scale

We have measured the frictional responses of two silane molecules (Octadecyltrichlorosilane; OTS and 1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane; FOTS) self-assembled on an aluminium substrate in their non heat-treated and heat-treated states. We varied the contact length scale by performing the experiments in atomic force microscope, nanotribometer and macro scale pin-on-disc machine. We have found that conformational disorder and molecular stiffness have major influences on friction in strongly distinguishing the response of OTS from that of the FOTS molecules. The frictional identity between the two molecules before and after heat treatment, though somewhat contrary to that suggested from purely surface energy considerations, is found to hold over six orders of contact length scales, ten orders of normal load and in three vastly different contact conditions; (a) the probe penetrates about 25% of the molecular backbone of a SAM; no substrate deformation, (b) the contact of the probe with the SAM is conformal and the substrate is plastically deformed and (c) the molecule is dispersed in 5% (v/v) concentration in n-hexadecane to lubricate a pin-on-disc contact.