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

Abstract

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% to 66% theoretical value. When nitrided, these compacts yielded nitride-bonded Si3N4 products having a density of 2.06 to 2.28 g cm−3, Si3N4 bonding phase of 20–60%, and 3-point flexural strength values in the range of 50 to 150 MPa. The microstructure consisted of very fine particles as well as fibrous or whiskerlike α–Si3N4 binding phase enveloping the matrix Si3N4, in total consisting of 90% α–Si3N4 and the rest being β–Si3N4 phase.