An Integrated Force Probe and Quartz Crystal Microbalance for High-Speed Microtribology

We have developed a technique for measuring frictional forces and contact areas, over a wide range of applied loads, at microscopic contacts reaching high sliding speeds near 1 m/s. Our approach is based on integrating two stand-alone methods: nanoindentation and quartz crystal microbalance (QCM). Energy dissipation and lateral contact stiffness are monitored by a transverse shear quartz resonator, while a spherical indenter probe is loaded onto its surface. Variations in these two quantities as functions of shear amplitude, with the normal load held fixed, reveal a transition from partial to full slip at a critical amplitude. Average values of both the threshold force for full slip and the kinetic friction during sliding are determined from these trends, and the contact area is inferred from the lateral stiffness at low shear amplitudes. Measurements are performed at loads ranging from 5 µN to 8 mN using an electrostatically actuated indenter probe. For the materials chosen in this study, we find that the full slip threshold force is about a factor of two larger than kinetic friction. The forces increase sublinearly with load in close correspondence with the contact area, and the shear strengths are found to be relatively insensitive to pressure. The threshold shear amplitude scales in proportion to the contact radius. These results demonstrate that the probe–QCM technique is a versatile and full-featured platform for microtribology in the speed range relevant to practical applications.