Aqueous powder coating methods for preparation of grain boundary engineered ceramics

Coated powders provide a mechanism for uniform incorporation of inorganic dopants into ceramics, resulting in improved control of microstructures, grain boundary phases and properties. Incorporation of inorganic dopants into polymeric additive systems is a novel method of coating powders in aqueous suspension. Controlled adsorption of polymer–cation complexes can be used to coat powder surfaces in a suspension, which can be subsequently spray-dried without dopant segregation. This method streamlines the coating process, making it economical for large scale manufacturing. Two alumina formulations with controlled grain boundary chemistry were used as model systems to develop these methods. Alumina with 2.5% magnesium aluminosilicate glass grain boundary phase was prepared and sintered. The conventional mixed powder method was compared with sol–gel coated powder prepared in an alcohol–water solvent, and powder spray-dried from a suspension containing silicone emulsion with added magnesium stearate. The sol–gel coated powder densified at a temperature 100°C below that required for the conventional mixed oxide powder system. Poor mixedness from the emulsion method hindered microstructure development. High purity alumina with Mg-rich grain boundaries was prepared by coating particles with a Mg–polyacrylic acid chelate adsorbed from aqueous solution. Uniform distribution of magnesium at grain boundaries resulted in microstructure with uniform grain size for MgO additions as low as 100 ppm, while 400 ppm MgO was required to obtain similar microstructure using the mixed powder method.