Frictional forces between cohesive powder particles studied by AFM

A range of commercially important powders (hydrated alumina, limestone, titania and zeolite) and glass ballotini were attached to atomic force microscope cantilevers, and inter-particle friction forces studied in air using lateral force microscopy (LFM). The in situ calibration procedure for friction forces is described. LF images, line profiles, LF histograms, surface roughness, pull-off forces, and the load dependence of friction in the range 0–25 nN were studied for both particle–particle and particle–wall (steel) contacts. The single-particle friction results are discussed in terms of contact mechanics theory. Particle–particle contacts showed load-dependent friction, involving single asperity contacts (non-linear behaviour) or multi-asperity contacts (linear behaviour). Particle–wall contacts usually showed little load dependence and were more adhesive. The results are also related to shear stress–normal stress data (yield loci) for the same materials from bulk shear testers.