(161e) Effect of Fines and Material Micro-hardness on Unconfined Yield Strength of Powders

Handling, storage and transport of fine powders is known to offer considerable flow problems due to inter-particulate adhesive-cohesive forces resulting in poor flowability, difficult aeration/deaeration characteristics. In present paper effects of inter particle cohesive forces and particle hardness on flow behavior of powders are examined. Studies on flowability are carried out using various well defined mixtures of ultrafine and coarser particles of Alumina (Moh's hardness index = 9) as well as Talc (Moh's hardness index = 1) using Jenike shear tester. Cohesivity of the samples is progressively increased in steps by adding cohesive fine fraction (d₅₀ ≈ 3.5 μm) to coarser fraction (d₅₀ ≈ 75 μm) followed by flow characterisation. Individual linearised yield loci of samples are then analyzed for increase in inter-particle adhesive forces as a function of normal forces applied during consolidation for understanding of micro processes occurring at inter particulate contacts.

The flowability of the sample mixtures is then characterized in the light of elastic-plastic particle contact deformation theory [Tomas, 2000] based on the concept of steady state yield locus. Finally, Unconfined Yield Strengths (σ_c) of the given samples are evaluated solely on the basis of material characteristic parameters namely angle of internal friction (φ_i), stationary angle of friction (φ_st), iso-static tensile strength of an unconsolidated powder (σ₀) along with characteristic pre-consolidation influence (σ_M,st) and subsequently predicted flow functions (ff_c) are compared with those evaluated experimentally. It is observed that addition of fines to coarser particles start affecting flow properties (φ_e-φ_i), φ_i and σ₀ only after there are sufficient inter particle deformable contacts between fine particles depending upon the material hardness as shown in following figure. Effect of fines becomes negligible after coarser particles get completely embedded by fine particles.

Ref.: Tomas, J., (2000), KONA Powder & Particle, no.18, pp.157.