43.
Si
3N
4powders coated with 6 wt% Y
2O
3and 4 wt% Al
2O
3were prepared by coprecipitation. The resulting powders were dispersed in water at different pH values and with addition of various amounts of ammonium polyacrylate (NH
4PA) to produce 32 vol% slips. The influence of the amount of NH
4PA solution added and pH on the rheological properties of 32 vol% coated Si
3N
4slips were studied. In addition, the sintered density of cast samples was determined and related to the degree of slip dispersion. The adsorption of the NH
4PA on the coated particle surface was rather high and the surface became saturated near 0.86 mg/m
2at pH 9.2. High NH
4PA concentrations (1.7–3 wt%) were necessary to obtain well dispersed 32 vol% coated Si
3N
4slips at pH 9.2. The best stabilization was obtained with the addition of 2.3 wt% NH
4PA; 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.
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