Production of Si3N4/Si3N4 and Si3N4/Al2O3 composites by CVD coating of fine particles with ultrafine powder

Si3N4 fine particles covered with deposited Si3N4 ultrafine powder, denoted as Si3N4/Si3N4 composites, with excellent sinterability were prepared usin g a new conceptual CVD reactor. The effects of reactant gas flow rate ratio , N/Si and reaction temperature on NH3 conversion, N content in the deposit ed ultrafine powder and N selectivity were investigated. The CVD reactor wa s designed to accelerate homogeneous nucleation reaction. The CVD condition s were optimized to form stoichiometric ultrafine powder of Si,N, on the co re fine particles. XPS results of ultrafine powder predict the synthesis of stoichiometric Si3N4. The required conditions for the sintering of produce d Si3N4/Si3N4 composites were drastically milder than those for conventiona l sintering process for commercial Si3N4 fine particles. When Al2O3 was use d as fine core particles instead of Si3N4, Si1.8Al2O1.2N1.8, one of complic ated SIALON ceramics, was formed by the sintering on the core particles. Th ese results indicate the feasibility of our CVD reactor to apply the prepar ation of other functional composite materials with easy sinterability. (C) 1999 Elsevier Science Ltd. All rights reserved.