| Acoustics and contact mechanics typically are viewed as separate fields. Studies of adhesion and friction mostly occur at low frequencies and large stress. However, time-dependent processes are certainly known to be of influence. Fast processes are, for instance, known to be very important in tire friction. Historically, a link between contact mechanics and acoustics was first formulated by Mindlin, who was concerned with the acoustic properties of an interface between two rough surfaces. He finds that Hertzian contacts under lateral stress behave nonlinearly even at minute amounts of stress due to the concentration of stress at the rim of the contact area. The talk is concerned with the application of acoustic resonators to contact mechanics problems. Acoustic resonators are characterized by small amplitudes (< 100 nm, typically below 1 nm) and high frequency (many MHz). The elastic stiffness of the contact is straight-forwardly inferred from the (positive) frequency shift of the resonator induced by the contact. At small amplitudes, contacts between multi-asperity interfaces (MCIs) behave like Hookean springs. The spring constant much increases after exposure of the sample to humid air, which is related to the sandcastle effect. When the amplitudes are increased to above 1 nm, the force/displacement relations turn nonlinear. The shift of the resonance parameters can still be predicted on the basis of a perturbation analysis. There is an extended range of amplitudes, where the data are well matched by the Mindlin theory of partial slip. Finally, a transition to gross slip is observed at the highest accessible amplitudes. At these levels of stress, one finds strong memory effects, mostly in the sense that shear sound induces contact renewal. A second class of experiments concerns the combination of acoustic resonators with the atomic force microscope and the colloidal probe. The high-frequency lateral movement decreases the force needed for detachment. There is a logarithmic dependence of the rupture force on the amplitude of oscillation. For an AFM tip in steady contact with quartz crystal, the oscillation much reduces the sliding friction and - on slightly rough surfaces - creates a static repulsive force, pushing the tip away from the substrate. |
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