Factors influencing the crystallization and the densification of ultrafine Si/N/C powders

The employment of nanocomposite Si/N/C laser-formed powders to produce high performance ceramics results in several technological problems related to their nanometer particle size and to their high affinity for oxygen, which influence phase composition and densification. This study has focused on several aspects: the improvement of experimental methodologies for processing of ultrafine powders in a compact green body and for the addition of sintering aids; the evaluation of the starting composition of the Si/N/C ultrafine amorphous powders and of the thermal treatment conditions (temperature, time, atmosphere) on phase composition, thermal stability, grain size, specific surface area and crystallite size; and the production and characterization of dense Si,N,/SiC composites. Above 1400 C the amorphous Si/N/C powders crystallize in (Y- and /3-Si,N,, SiC and Si2N,0, their relative amounts and grain sizes depending on processing conditions. The phenomena are discussed on the basis of a series of reactions involving the formation of intermediates in the system Si-C-N-O. After densification by hot pressing, a very fine microstructure ( = 100 nm) was observed in the dense Si,N,/SiC composites. High values of hardness (Hv > 21GPa) and good values of fracture toughness (K,, = 4.8 MPa m0.5) were measured.