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.

ZrB₂-SiBCN ceramics with ZrO₂ additive are hot-pressed under a constant applied pressure. The densification behavior of the composites is studied in a view of creep deformation by means of the Bernard-Granger and Guizard model. With determination of the stress exponent (n) and the apparent activation energy (Q_d), the specific deformation mechanisms controlling densification are supposed. Within lower temperature ranges of 1300-1400 ℃, the operative mechanism is considered to be grain boundary sliding accommodated by atom diffusion of the polymer-derived SiBCN (n = 1, Q_d = 123±5 kJ/mol) and by viscous flow of the amorphous SiBCN (n = 2, Q_d = 249±5 kJ/mol). At higher temperatures, the controlling mechanism transforms to lattice or intra-granular diffusion creep (n = 3-5) due to gradual consumption of the amorphous phase. It is suggested that diffusion of oxygen ions inside ZrO₂ into the amorphous SiBCN decreases the viscosity, modifies the fluidity, and contributes to the grain boundary mobility.

Three dimensional (3D) printing technology by direct ink writing (DIW) is an innovative complex shaping technology, possessing advantages of flexibility in fabrication, high efficiency, low cost, and environmental-friendliness. Herein, 3D printing of complex alumina ceramic parts via DIW using thermally induced solidification with carrageenan swelling was investigated. The rheological properties of the slurry under different thermally-induced modes were systematically studied. The solidification properties of thermally-induced pastes with varying contents of carrageenan were optimized. The experimental results showed that the optimized paste consisting of 0.4 wt% carrageenan could be rapidly solidified at about 55 ℃, which could print inclined-plane more than 60° in vertical without support, resulting in better homogeneity of the green body. A nearly pore-free structure was obtained after sintering at 1600 ℃ for 2 h.