Rheological and related colloidal aspects of aqueous processing that affect the development of microstructure.

摘要

Shear flow in (alpha)-Al(sub 2)O(sub 3) suspensions having a volume fraction of solids ((phi)) in the range between 0.17 and 0.50 was investigated between pH 4 and 12. It is Newtonian if the magnitude of the zeta potential exceeds a critical value which depends on (phi); its value is 39 mV if (phi) = 0.40 and 74 mV if (phi) = 0.50. If this potential is less than the critical value, shear flow is pseudoplastic; its yield value markedly changes (e.g., 0 to (gt)100 Pa) in a slightly (phi)-dependent, narrow pH range ((lt)0.5 units). If a second oxide, t-ZrO(sub 2), is present, its pH-dependent colloidal behavior governs the overall rheology, though its concentration may be only 11 % that of (alpha)-Al(sub 2)O(sub 3). Scanning electron microscopy of composite pieces indicates that a detrimental, rheologically detectable interaction between (alpha)-Al(sub 2)O(sub 3) and t-ZrO(sub 2) can be avoided and the distribution of t-ZrO(sub 2) can be optimized during pressure casting by control of pH to ensure Newtonian flow or a very low yield value when the flow is pseudoplastic.