ABSTRACT
The discovery of new techniques has permitted molecular level investigations of chemisorption bonds, which lead to strong adhesion and physisorption, and of van der Waals bonds, that lead to weak adhesion and control motion at the interface (molecular tribology). The atomic force and scanning tunneling microscopes (AFM and STM), the surface force apparatus (SFA), and vibrational spectroscopic studies using sum frequency generation (SFG) by nonlinear laser optics have permitted in situ investigations of buried interfaces: solid–liquid and solid–solid. Strong adhesion leads to adsorbate-induced restructuring of the substrate, where the exothermic heat of chemisorption produces weakening of metal-metal bonds (an endothermic process) that is needed to induce the relocation of the substrate atoms. The work of adhesion and the coefficient of friction were measured by AFM in the elastic deformation regime (nanonewton forces) for hard carbon films with variable hydrogen content. Both the work of adhesion and the friction coefficient increase with increasing hydrogen content of the carbon film that is smooth on the atomic scale. Friction can be markedly decreased by a thin perfluoroether film. SFG studies permit detection of the restructuring of adsorbed molecules at the buried interfaces, under applied load.
