Characterization of particle and bulk level cohesion reduction of surface modified fine aluminum powders

In this work, characterization of cohesion reduction at the particle as well as bulk scales is addressed, including the investigation of analytical models to relate the properties at two scales. Two techniques, surface silanization and dry particle coating, are utilized to reduce the cohesiveness and improve the flowability of fine cohesive aluminum powders. Cohesiveness of these surface modified powders is evaluated at the particle scale through the particle properties such as surface roughness and surface energy, and at the bulk scale in both consolidated and aerated state using a Sevilla Powder Tester through unconfined tensile strength, solid fraction, settling and bubbling velocity. In addition, bulk-scale characterization using standard Angle of Repose (AoR) method is carried out. An analytical model is proposed, which allows for calculating interparticle pull-off force where the representative surface morphology is randomly generated and the plastic deformation of asperities in contact is taken into account. Experimental results indicate that surface silanization and dry particle coating can dramatically improve the flowability, and make otherwise unfluidizable powder fluidizable. The reduction of cohesiveness is largely ascribed to the reduction of surface free energy, which becomes less than 1/4 of the original value. Bond number is computed for all samples from the interparticle pull-off force and measured particle size. Surface modification drastically reduces the Bond number by approximately 2 orders of magnitude indicating the significant improvement of flowability. Representation of particle cohesiveness by Bond number has good qualitative agreement with the bulk-scale cohesiveness characterized by the Angle of Repose as well as the bubbling velocity which is measured using the Sevilla Powder Tester.