SI3N4-BN COMPOSITES OBTAINED FROM AMINOBORANES AS BN PRECURSORS AND SINTERING AIDS

The Si3N4-BN composite ceramic has been elaborated by combining Si3N4 fine powder with boron nitride provided by the thermolysis of a molecu lar precursor. The properties of this new type of sample has been comp ared with those of composite ceramics obtained by the classical hot-pr essing method using Si3N4-BN powders and sintering aids (Y2O3, Al2O3). Giving the best ceramic yield, tris(methylamino) borane (TMB) has bee n used as BN precursor and sintering aid. Boron nitride formed from TM B thermolysis was poorly crystallized when the hot-pressing was run up to a temperature lower than 1800 degrees C. The density of the compos ite ceramic samples obtained from molecular precursors was higher than expected, this could be related to the sintering activation propertie s of the precursor. Moreover the problems due to the BN platelets orie ntations were removed. The Vickers hardness was clearly improved for s amples without BN platelets, however the bending strength was not incr eased. The precursor improved some properties of the composite and cou ld be considered as a sintering activator avoiding oxide addition as s intering aid. (C) 1997 Elsevier Science Limited.