Flow-Spurt Transition under Shear Deformation of Concentrated Suspensions

Abstract

The rheological properties of concentrated suspensions containing 55 and 60 vol % aluminum particles are studied. This concentration range corresponds to the transition from the viscoelastic to the elastoplastic behavior of the suspensions. Moreover, this range is most favorable for powder injection molding of metals and ceramics. The measurements are performed in a rheometer at different shear stresses. Elastoplastic deformations take place at low stresses, while separation, i.e., the rupture of the adhesion contact between the bulk suspension and the boundary of a solid wall of the rheometer, is observed with an increase in the stress. The rupture of the adhesion contact occurs with time, and this process is characterized by a dependence of the lifetime (durability) of the contact on stress. This dependence is of an exponential character. The replacement of smooth working surfaces of the rheometer by rough ones leads to some increase in the stresses. This reflects the replacement of the adhesion separation by the formation of discontinuities in the bulk suspension at the asperities of the rough surface. The observed behavior of the concentrated suspensions in the aforementioned range of compositions is represented by a mechanical model, into which a new element has been introduced. It is represented by a friction pair with variable length (and, accordingly, variable friction force). The spurt arises when a threshold force that induces displacement over the friction surface is reached.