Microtribological properties of silicon and silicon coated with diamond like carbon, octadecyltrichlorosilane and stearic acid cadmium salt films: A comparative study

A comparative study of the microtribological properties of native oxide covered Si(100) and Si(100) coated with chemical vapor deposition grown diamond like carbon (DLC) films, stearic acid cadmium salts (STCd) Langmuir-Blodgett films and octadecyl-trichlorosilane (OTS) self-assembled monolayers is presented. The frictional properties between these samples and a sapphire ball were studied using a micro friction tester as a function of the normal force, sliding speed and relative humidity. While all coatings significantly reduce friction, OTS and STCd coatings show the largest reduction. The bare silicon surface exhibits hydrophilic surface properties with a significant jump-into-contact at zero normal load. All the coated Si samples show negligible dependence on humidity, which is in agreement with their hydrophobic nature, while friction on the oxide covered Si surface rapidly increases with a rise in humidity. The friction vs, normal load curves of oxide covered silicon can be described by the Johnson-Kendall-Roberts model with a pressure-dependent shear strength. The monolayer samples and DLC coated samples appear to possess contact area vs. load characteristics that are approximated by the Hertz contact theory. While the oxide covered Si surface shows a large reduction in friction with sliding speed, all the other samples show a relatively weak dependence. (C) 2001 Elsevier Science B.V. All rights reserved.