Effect of ageing time on chemical and rheological evolution in γ-Al2O3 slurries for dip-coating

The evolution during ageing time of a suspension of a submicronic γ-Al2O3 powder with HNO3/Al2O3 = 2.16 mmol/g and H2O/Al2O3 = 3.2 ml/g has been studied. The physico-chemical and rheological evolution has been followed up to 48 h of ageing. Surface charging and dissolution reactions are fast and proceed with comparable rates in the first 5 h to reach a stable pH 3.5 that corresponds to maximum surface charging and maximum Al3+. Surface charging is responsible for the formation of the colloidal particles that aggregates to form the physic-type gel phase (weak gel). Gel formation is a slow process and is completed within 20 h of ageing. The suspensions present a non-Newtonian pseudoplastic behaviour; at shear = 10 s−1 viscosities between 0.03 and 0.5 Pa s are measured. Viscosity is strongly time dependent and shows a maximum about 27 h of ageing. This behaviour has been related to the presence of modification of the suspension composition during ageing: viscosity increases due to the increasing amounts of gel, the increasing of gel strength and flocks formation, while the viscosity decreases due to both decrease of gel strength and flocks re-dispersion. Well adherent coating layers characterised by loadings between 1.1–2.2 mg/cm2 and thickness of 10–20 μm have been obtained upon dip-coating deposition.