Rheology and consolidation of colloidal alumina-coated silicon nitride suspensions

In an attempt to improve the colloidal processing of Si3N4 ceramics, we studied the rheology and consolidation of colloidal suspensions of Si3N4 particles (average particle size 0.7 mu m) with small Al2O3 particles (average particle size 20 nm). It was found that at pH <7, the viscosity of the mixtures increased and then decreased with an increasing concentration of Al2O3. zeta-potential measurements, optical micrographs, and visible light absorptance measurement suggest that such viscosity behavior is due to clustering of Si3N4 particles bridged by the small Al2O3 particles. This is also supported by the Derjaquin-Landau-Verwey-Overbeek (DLVO) potential calculations that show the barrier height in the DLVO potential between Al2O3 and Si3N4 is small. The small barrier height under current experimental conditions stems from the small size of the Al2O3 particles. The small barrier height allows the thermal motion of the two kinds of particles to overcome the barrier and attach to each other. The adsorption of small Al2O3 particles on Si3N4 can occur even when both Al2O3 and Si3N4 carry the same sign of charges. The adsorption of Al2O3 on Si3N4 also increases the density of consolidated compacts.