Friction drive of an SAW motor. Part IV: Physics of contact

A procedure for modeling the frictional heating and electricity of a surface acoustic wave (SAW) motor is proposed. The frictional heat is developed during friction drive when sliding occurs at the frictional interface; the heat is conducted into the solids, resulting in an increase in temperature. The spatial distribution of the heat source was associated with the contact pressure distribution, and the heat conduction from the heat source was formulated. Owing to the piezoelectricity and pyroelectricity of the stator used in the present study, the elastic deformation and temperature increase produce the electric fields. The electric fields in the stator were determined with respect to each cause. Electric discontinuity at the boundary between the stator and the slider, moreover, produces electrostatic force, which was calculated using a Maxwell stress tensor. All the analyses revealed the underlying physical fields in addition to the mechanical fields of the SAW motor. By the use of those analytical methods, the frictional properties of the SAW motor were discussed. We pointed out that another physical phenomenon-contact electrification-could arise at the contact interface. The electrostatic force due to contact electrification had sufficient strength to change the friction property, which corresponded to the variation of the friction coefficient front 0.1 to 1.