FACTORS INFLUENCING THE CRYSTALLIZATION OF ULTRAFINE PLASMA-SYNTHESIZED SILICON-NITRIDE AS A SINGLE POWDER AND IN COMPOSITE SIC-SI3N4 POWDER

A study of various parameters affecting the crystallization process of amorphous silicon nitride produced by plasma gas-phase reaction was undertaken to determine the conditions under which whiskers are formed. This process is influenced by the ammonium chloride content of the starting powder and the presence of nitrogen in the furnace atmosphere. This last parameter is also influential on the alpha/beta phase ratio, along with other factors like the silica content, temperature and duration of the thermal treatment. Heat treatment at 1500-degrees-C for 30 min under argon produced well-defined alpha-Si3N4 crystals with a hexagonal cross section, a mean length around 0.8-mu-m, and no sign of agglomeration. Under the same conditions, crystallization of silicon nitride in SiC-Si3N4 composite did not give crystals, but Si3N4 whiskers. Therefore silicon carbide plays a major role in their formation.