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 slurri es. The first method involves the precipitation of an aluminum hydroxy carbonate from dissolved Al(NO3)(3) during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with alumi num tri-sec-butoxide in hexane at room temperature. Both methods produ ce a coated powder in which the electrophoretic and theological proper ties 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 produc es a weakly attractive particle network similar to that previously rep orted for Al2O3 powder. Although electrophoretic and theological data showed that the coated powder behaved like Al2O3, consolidation data i ndicated that slurries of coated powder with added salt did not pack t o high density. In addition, these bodies were not plastic as found fo r bodies consolidated from dispersed and salt-added Al2O3 slurries.