Fig. 4
Friction between calcite and an AFM tip in a dry nitrogen atmosphere. Friction force as a function of a velocity and b applied load. Tip radius = 100 nm. Under dry conditions, the friction force—higher than in wet experiments—still scales with the logarithm of the sliding velocity at contact stresses as high as 620 MPa. For comparison purposes, friction is described as a shear-promoted thermally activated slip that accounts for the rupture of the adhesive bonds between the surface atoms on calcite and tip17. Stress-activation lengths λ obtained by fitting Eq. (2) to the velocity-dependent friction force are shown in a for each load
