Influence of powder pre-treatments on dispersion ability of aqueous silicon nitride-based suspensions

The rheological properties and the dispersion stability of highly concentrated aqueous silicon nitride powder suspensions in the presence of an anionic polyelectrolyte dispersant have been investigated. Silicon nitride-based suspensions with solids loading as high as 60 vol.% could be prepared following a slow process of increasing the solids volume fraction in steps of 5 vol.%, starting from 45 vol.%. After each increment of solids, ball milling was conducted for periods of 24 h, which then increased up to 48 h with solids loading increasing. Slips were prepared either from the as received or the pre-treated powders. Zeta potential measurements were conducted on Si3N4, Al2O3 and Y2O3 powders submitted to different pre-treatments, in absence and in the presence of dispersant, to evaluate the influence of these experimental variables on surface-charge properties of the particles. It was concluded that both aqueous ball milling and calcining enhance the repulsive interaction forces between particles, like the presence of the dispersant. On the other hand, rheological and particle size analysis revealed that ball milling and the ageing time enhance the dispersing ability of the powders and decrease the viscosity (for a given solids loading) and the shear-thinning character of the suspensions.