Hydrothermal Treatment of Alumina Powders to Alter the Low-Temperature Binding of Chemically Bonded Phosphate Ceramic Composites Via Infrared Irradiation

This paper explores the hydrothermal treatment of α-Al 2 O 3 powders to improve the binding in ceramic parts produced via a chemical bonding of ceramic powders combining a phosphate binder and infrared (IR) curing. Alumina powders are pre-treated in hydrothermal conditions with both water and phosphoric acid solution before slurry preparation and infrared irradiation. The effects of hydrothermal treatments on the powder’s chemical reactions and the composite’s final microstructure are assessed upon IR irradiation. While the water treatment does not induce any chemical changes, the presence of phosphoric acid leads to the appearance of phosphate phases Al(PO 3 ) 3 and AlPO 4 . Hydrothermal treatments in water and in phosphoric acid solution are found to drive faster and more intense phosphate condensation reactions at lower temperatures, with a stronger effect in the presence of H 3 PO 4 (condensation temperature of 150 °C for the H 3 PO 4 -hydrothermal treated powder compared to 165 °C for the untreated powder). This improved reactivity of hydrothermal treatment α-Al 2 O 3 powders, especially in the presence of H 3 PO 4 , leads to a general decrease of porosity for 3D printed parts compared to untreated alumina (for both top surfaces and cross-sections). While further optimization is needed to reduce the final porosity, it is possible to rapidly print 3D parts using this method, demonstrating a possible pathway to a single-step additive manufacturing (AM) process for ceramics.