Investigating ultra-thin lubricant layers using lateral atomic force acoustic microscopy

The ultrasonic friction mode of an atomic force microscope is a scanning probe technique allowing one to analyze the load and velocity dependence of friction. The technique is based on the evaluation of the resonance behavior of an AFM cantilever when in contact with a vibrating sample surface. The effect of load and lateral displacement of the sample surface on the shape of the torsional resonance spectra of the AFM cantilever is evaluated under dry and lubricated sliding conditions. A characteristic flattening of the torsional resonance curve has been observed at large surface displacements, resulting from the onset of sliding friction in the contact of the AFM cantilever tip with the sample surface. This is confirmed by an analytical model describing torsional cantilever vibrations in case of a Hertzian, i.e. purely elastic, contact and by numerical simulations of the cantilever vibrations.