AFM at the Macroscale: Methods to Fabricate and Calibrate Probes for Millinewton Force Measurements

The difficulty in detecting and controlling forces in the gap between the nanoscale and macroscale tribometry regimes has so far limited the application of fundamental atomic-scale insights to practical friction and wear control. This paper describes methods to achieve and quantify millinewton forces measured by atomic force microscopy (AFM) using existing experimental tools. We mounted colloidal microspheres at different points along the span of commercial AFM cantilevers to reduce their effective flexural length from 125 µm to between 21 and 107 µm. The resulting spring constants, based on direct calibration, varied from 100 to 10,000 N/m. Within a commercial AFM (Dimension 3100), these cantilevers produced normal force calibration constants between 0.006 and 0.430 mN/V; i.e., increasing the spring constant by 100 × caused a corresponding increase in the calibration constant but only a negligible increase in V/m sensitivity. We demonstrate these new capabilities by measuring friction between the colloids and single-crystal MoS ₂ at applied normal forces up to 3.4 mN, which is in the range of existing tribometers and well above the forces typically used in AFM-based measurements. These methods, which make use of well-established procedures and only require a modified AFM cantilever, are intended for use by other researchers as a platform for bridging the gap between nanoscale and macroscale tribometry.