Influence of vapor condensation on the adhesion and friction of carbon-carbon nanocontacts

The adhesion and friction between two orthogonally arranged carbon fibers h as been measured in undersaturated vapor pressures of decane, n-propanol, a nd water. An analysis, which is described, of the frictional data allowed t he normal adhesive force under sliding conditions to be deduced. Contact an gle measurements and adsorption studies showed that both decane and n-propa nol wetted the fibers and also their vapors exhibited typical BET adsorptio n isotherms. It was also found that water did not wet these fibers and that the adsorption isotherm could not be described by the BET equation. Equili brium thermodynamic theory predicts that the two wetting fluids should sign ificantly attenuate the autoadhesion. The converse was observed and is ascr ibed to the actions of a combination of two factors. First, it is argued th at the high contact pressures (ca. 10(9) Pa) at the carbon interface, which were developed even under the adhesive loads alone, resulted in the adsorb ates being excluded or displaced from the contact region. Second, the crack propagation velocity during the interfacial separation process was very fa st relative to the rate of vapor transport and hence the rate of adsorption at the crack tip. A similar effect is observed with environmental stress c racking at high crack propagation velocities. An increase in the adhesion a t high relative vapor pressures of the n-propanol and water is considered t o correspond to the formation of capillary bridges, The rate at which this process occurs appears to be enhanced under sliding conditions due to an ac cumulation of the adsorbates in the moving contact. The capillary bridges f ormed in saturated decane vapor were highly unstable which may be related t o the relatively weak adsorption characteristics.