Formation Mechanism of Cracks during the Freeze Drying of Gelcast Ceramic Parts

The near-net shaping of gelcast ceramic parts can be achieved using freeze-drying technology. It was discovered that the cracks probably occurred inside the part during freeze drying due to the formation of liquid water. To explain the formation mechanism of cracks, the electrical resistance method was employed to measure the part's eutectic temperature which can be used to determine the freezing and melting state of the part. Affecting factors on cracking were investigated by testing the temperature and water loss of the part during the drying stage, and the cracks inside the part were detected through computed tomography (CT). It was found that when the temperature of the part was higher than the eutectic temperature, the ice crystals at the sublimation front would melt, resulting in the formation of wet green body. The cracks will occur inside the part because great capillary forces are formed in the wet green body. Cracks could be controlled through reducing heat supply by lowing shelf temperature, decreasing pressure of the drying chamber or lowering the dried layer resistance against water vapor by improving the freezing temperature or reducing the solid loading of the part.