Interior-collapsing mechanism by hydrothermal process of the MgAl₂O₄/MgO porous ceramic

Ceramic core is a critical component in the super-alloy turbine blade casting. In our previous work, a novel multi-phase MgAl₂O₄/MgO porous ceramic was prepared for this purpose. The most important property was that it crumbled completely after hydrothermal treatment in just pure water, due to the hydration of MgO. In this work, the hydration process of the MgO embedded in the inert matrix was investigated in detail. The collapse behaved as an interior destruction without any bulk expansion of the sample. The hydration percentage was the only factor related to the water-collapsibility. The morphology of hydration product indicated that the reaction advanced in particular direction. Based on the finite element analysis for the expansion effect on the porous structure, the interior-collapsing mechanism was proposed. During the hydration process, the MgO grains exerted pressure to the surrounding matrix and induced the collapse in the adjacent structure. This process took place throughout the matrix. Finally, the sample crumbled completely to the powders. No bulk dilatation was detected before the powdering, indicating that the collapse process would not exert pressure outward. Thus the alloy blade would not be damaged during the removal of the ceramic core. It was also predicted that the decrease in the MgO grain size was beneficial to the water-collapsibility.