Colloidal filtration of silicon nitride aqueous slips. Part I: Optimization of the slip parameters

The slip parameters for the manufacture of dense, pressureless sintered silicon nitride by colloidal forming methods, such as slip or pressure casting, are studied. This work focuses the optimization of the rheological behaviour of the slip considering different kinds and concentrations of dispersants and it has been stated that the electrostatic stabilizing mechanism is essential. The best rheological conditions are achieved with tetramethylammoniun hydroxide (TMAH) or any strong base (as NaOH), but the use of TMAH limits the progressive oxidation of the particles suspended in water by action of a partially oxidized protective screen. The study has been made with Al2O3 and Y2O3 as sintering aids, in concentrations of 3 and 6 wt% each. An optimum pH for dispersing has been located at 11.3 +/- 0.2, where neither yield point nor thixotropy are found for 65 wt% slips, Different homogenizing techniques are used and compared, such as strong mechanical agitation, centrifugal milling or mixing in a rotatory bottle with nylon balls, Since the starting silicon nitride powder is submicronic, Mo milling is required, but a long homogenization time is convenient to assure an equilibrium state at the particles surface. (C) 1998 Published by Elsevier Science Limited.