Binder jetting of spinel-based refractory materials – processing, microstructure and properties

Spinel-based refractories were produced by binder jetting using a novel MgO-citric acid binder system and water as an activator of the acid–base reaction. Additionally to different amounts of binder and saturation levels, the addition of small amounts of PVA was investigated. Powder characteristics such as particle size distribution, particle shape and flowability as well as thermal behavior of the binder system were evaluated. Phase analysis by XRD conducted on dried and sintered samples indicated in situ spinel formation. Sintered samples exhibited low shrinkage ($<$  4%), but rather high apparent porosity ($>$  50 vol.%) and median pore size ($>25\mu m$). Compressive strength of dried and sintered samples was measured both parallel and perpendicular to the printed layers with values up to 8.5 MPa in the sintered state. After thermal shock with water, microcracks were formed and the residual strength was about 1.9 MPa. Selected sintered samples were analyzed with microfocused X-ray computed tomography, revealing the orientation of larger angular-shaped particles along the printed layers. Finally, a small crucible was successfully printed and sintered.