Processing of Yttria-Alumina Coated Silicon Nitride Slips by Slip Casting

ISSN： 1064-7562

Source： Journal of Materials Synthesis and Processing, Vol.10, Iss.4, 2002-07, pp. : 211-218

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Abstract

Si₃N₄ powders coated with 6 wt% Y₂O₃ and 4 wt% Al₂O₃ were prepared by coprecipitation. The resulting powders were dispersed in water at different pH values and with addition of various amounts of ammonium polyacrylate (NH₄PA) to produce 32 vol% slips. The influence of the amount of NH₄PA solution added and pH on the rheological properties of 32 vol% coated Si₃N₄ slips were studied. In addition, the sintered density of cast samples was determined and related to the degree of slip dispersion. The adsorption of the NH₄PA on the coated particle surface was rather high and the surface became saturated near 0.86 mg/m² at pH 9.2. High NH₄PA concentrations (1.73 wt%) were necessary to obtain well dispersed 32 vol% coated Si₃N₄ slips at pH 9.2. The best stabilization was obtained with the addition of 2.3 wt% NH₄PA; in this condition, the viscosity reached a minimum value of 35 mPa.s at 100 s^-1. The slip viscosity increased with increasing pH from 9.2 to 10.2. Slips with low viscosities gave a more dense packing of cast samples and consequently higher sintered density values.