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

The adhesion and friction between two orthogonally arranged carbon fibers has been measured in undersaturated vapor pressures of decane, n-propanol, and water. An analysis, which is described, of the frictional data allowed the normal adhesive force under sliding conditions to be deduced. Contact angle measurements and adsorption studies showed that both decane and n-propanol wetted the fibers and also their vapors exhibited typical BET adsorption 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. Equilibrium thermodynamic theory predicts that the two wetting fluids should significantly attenuate the autoadhesion. The converse was observed and is ascribed to the actions of a combination of two factors. First, it is argued that the high contact pressures (ca. 10(9) Pa) at the carbon interface, which were developed even under the adhesive loads alone, resulted in the adsorbates being excluded or displaced from the contact region. Second, the crack propagation velocity during the interfacial separation process was very fast 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 cracking at high crack propagation velocities. An increase in the adhesion at high relative vapor pressures of the n-propanol and water is considered to correspond to the formation of capillary bridges, The rate at which this process occurs appears to be enhanced under sliding conditions due to an accumulation of the adsorbates in the moving contact. The capillary bridges formed in saturated decane vapor were highly unstable which may be related to the relatively weak adsorption characteristics.