ALUMINA SURFACE MODIFICATION OF SILICON-NITRIDE FOR COLLOIDAL PROCESSING

Two different methods are used to coat silicon nitride particles with an alumina precursor to make Si3N4 behave like Al2O3 in aqueous slurries. The first method involves the precipitation of an aluminum hydroxycarbonate from dissolved Al(NO3)(3) during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with aluminum tri-sec-butoxide in hexane at room temperature. Both methods produce a coated powder in which the electrophoretic and theological properties of aqueous slurries mimic those of alumina. When salt is added to slurries consisting of coated Si3N4 powder, all theological evidence suggests the presence of a short-range repulsive potential that produces a weakly attractive particle network similar to that previously reported for Al2O3 powder. Although electrophoretic and theological data showed that the coated powder behaved like Al2O3, consolidation data indicated that slurries of coated powder with added salt did not pack to high density. In addition, these bodies were not plastic as found for bodies consolidated from dispersed and salt-added Al2O3 slurries.