Rheological properties of Si3N4, pseudoboehmite and bayerite coated Si3N4 suspensions with ammonium polyacrylate dispersant

Si3N4 powders coated with pseudoboehmite and bayerite were prepared by precipitation using AlCl3 solution and Al13 polymer solution, respectively. The resulting powders were dispersed in water with ammonium polyacrylate (NH4PA) at pH 9.4 to produce 18 vol% slips. The influence of the pseudoboehmite and bayerite coatings on the rheological properties of 18 vol% aqueous Si3N4 slips with NH4PA were studied. In addition, the effect of the amount of NH4PA solution added on the stabilization of the uncoated and coated Si3N4 slips were determined. Lower viscosity values and greater stability of the coated Si3N4 slips with respect to the uncoated one were found due to the greater NH4PA adsorption of the coated powders. However, the best stabilization was obtained with the bayerite coated Si3N4 powder. This was attributed to the greater NH4PA adsorption and lower specific surface area of the bayerite coated powder. The Si3N4 slips had a pseudoplastic behavior. The addition of NH4PA resulted in slips with high apparent yield stress and viscosity values due to the appreciable amount of NH4PA remaining in solution. The viscosity increased from 98 mPa s without NH4PA to about 155 mPa s for NH4PA concentration ≥0.28 wt.%. The flow curves of the suspensions followed the Herschel–Bulkley model. The variation of τ0, k and n constants of the model were used to represent the rheological properties of the slips. The minimum viscosity of the bayerite coated slips (3 mPa s) was obtained with the addition of 0.14 wt.% NH4PA. In this condition, the slips exhibited a Newtonian behavior (τ0=0, n=1). The pseudoboehmite coated slips were stabilized with 0.42 wt.% NH4PA, having a viscosity value of 6.6 mPa s. The τ0, k and n constants of the pseudoboehmite coated slips had an intermediate value between the Si3N4 and the bayerite coated one.