Frictional forces between a diamond stylus and specimens at low load

Abstract

Instrumentation design and characterization and the initial results of an investigation into the frictional behaviour of a diamond stylus at loads in the region 0.5–5 mN are presented. Lapped specimens of mild steel, copper, brass, aluminium and polished silicon were traversed by a stylus-based profilometer that had been modified to allow continuous variation of the tracking force. A high-compliance force actuator was used for this purpose. System testing, by loading up to 6 mN and then relaxing, resulted in permanent deformation of the metal specimens from 80 nm for steel up to 340 nm for aluminium. These values are consistent with other known microhardness results for a blunt punch of approximately 2 μm diameter. Horizontal drag forces were measured by fixing the specimens to a linear spring with its displacement axis parallel to the traverse axis of the stylus. Drag forces during traverse cause submicrometre deflections of the spring platform which are measured by an inductive transducer. The overall precision of the force measurement is approximately 10 μN for a 10 mN range. Typical friction coefficients were found to vary from 0.07 for silicon to 0.25 for aluminium. These coefficients vary little with the load and show a slight tendency to decrease with increasing traverse speed. Variations of the drag force about a mean value were observed to be dependent upon the surface finish. Consistency with a simple model implying that the variations should depend on the local surface slope is demonstrated by a strong correlation between the frictional drag and the first derivative of the surface profile.