EFFECT OF SUBSTRATE ON SHEARING PROPERTIES OF ULTRATHIN POLYMER-FILMS

We have investigated the frictional behavior of molecularly smooth solid surfaces lubricated with a linear perfluoropolyether. The substrates used were mica and thin films (10 nm) of zirconia and alumina sputtered onto mica. The surface topography was determined by atomic force microscopy, and the relative surface energies were estimated from contact angle measurements. Sliding experiments were performed with a modified surface forces apparatus under both dry and humid conditions. In a dry environment, an ultrathin (< 5.0 nm) polymer film acted as a good lubricant for all three surfaces and the friction coefficients were small (mu < 0.01). In a humid environment, there was a marked difference in the shearing behavior and a decrease in the ability of the lubricant to protect the surface. With mica surfaces, water rapidly displaced the polymer from the surfaces with the result that the friction increased dramatically and the surfaces were subsequently damaged. Displacement of the lubricant was not observed with the zirconia and alumina surfaces, although the friction increased and sliding occurred in an irregular stick-slip manner. The results emphasize the importance of the surface free energy and interfacial adhesion on shearing properties.