(38a) Adhesion, Friction and Wear of Particle Contacts

The bulk properties of a powder in the plastic-solid and the collisional regimes are the result of the response of all the contacts between its constituent particles. Numerical studies by the Discrete Element Method (DEM) rely on this assumption to predict the behaviour of a particle assembly by calculating the forces transmitted through the network of contacts. Although considerable progress has been made with DEM models, little attention has been paid to the experimental study of the contact between real particles, which is the key to the success of those simulations. The knowledge gained with such a study may provide more realistic models for computer simulations and also useful hints to help in the production of powders with desirable properties. The Atomic Force Microscope (AFM) is a useful tool to test particle contact mechanics because it is capable of measuring the adhesion between a particle and a surface and the friction force when the particle and the substrate are in relative movement. Although the procedure to measure the adhesion is well established and data on it are now abundant, measurements of the microscopic friction between particles and substrates are still scarce. Mostly this is due to the lack of a good established calibration procedure to convert the lateral deflection signal of the AFM into units of force. In this paper we present AFM measurements of friction forces between a variety of substrates and several types of particles (glass beads, UO₂ and UO₃). Glass beads are a model system often used in scientific work, while uranic particles are an example of industrially relevant materials with morphology much more complicated than the glass beads. Apart from the friction, AFM reverse images of the particles and the evolution of the adhesion of the particle to the substrate during the experiment are used to evaluate the importance of wear. The microscopic friction coefficient determined using a calibration procedure developed by the authors is compared with the internal friction of the bulk powder measured in a Schulze Shear Tester.