Nitride-bonded silicon nitride from slip-cast Si + Si3N4 compacts

The dispersability of Si and Si3N4 powders in aqueous media was monitored by particle-size distribution, sedimentation behavior, viscosity/rheological studies, and electrokinetic behavior (zeta potential (ZP) analysis] as a function of pH of their slips. The pH values of 4 and 8 for Si and 10 for Si3N4 resulted in optimum dispersion, characterized by minimum in sedimentation height, minimum in viscosity, and maximum in ZP. The optimum slips of Si + Si3N4 mixtures conditioned in the pH range 8 to 10 were slip cast to obtain green compacts having a density in the range of 59-66 percent theoretical value. When nitrided, these compacts yielded nitride-bonded Si3N4 products having a density of 2.06-2.28 g/cu cm, Si3N4 bonding phase of 20-60 percent, and 3-point flexural strength values in the range of 50-150 MPa. The microstructure consisted of very fine particles as well as fibrous or whiskerlike alpha-Si3N4 binding phase enveloping the matrix Si3N4, in total consisting of 90 percent alpha-Si3N4 and the rest being beta-Si3N4 phase.