Development of alumina primary coat for single crystal investment casting ceramic mould

The use of an alumina primary coat to eliminate the interaction between aluminium in NiAl single crystal alloy and silica in the ceramic investment casting mould has been examined in this study. The properties of the alumina slurry were characterised and the main factors controlling stability identified. It was found that the pH of the slurry must be carefully controlled to avoid polycondensation reactions. The most stable slurry was produced using filler with lower mean particle size and alkalinity. Alumina primary coat produced with low filler loading does not survive completely during wax removal. The surface condition can be improved by increasing the plate weight, which corresponds to a higher filler loading in the slurry. Moulds with a consistent alumina primary coat exhibit slightly higher green and sintered strength than moulds with the standard zircon-silica primary coat.