凝胶注模成型氧化铝基陶瓷坯体的冷冻干燥缺陷控制

Freeze-Drying Defect Control of Alumina-Based Ceramic Body Prepared with Gel Casting Technology

采用凝胶注模成型技术制造复杂精铸件陶瓷铸型的过程中，铸型坯体会产生内部裂纹等缺陷，严重影响铸型质量。为此，采用三维CT检测技术，通过实验跟踪内部裂纹产生的过程，发现在冷冻干燥失水率（质量比）为20％～60％时，坯体容易产生裂纹。基于冷冻干燥理论，对裂纹产生的机理进行了分析，结果表明：在坯体冷冻干燥过程中，若冻结层温度高于其共晶点温度（-3．6℃），将使冻结层融化，致使其强度急剧下降，从而导致裂纹产生。为了降低坯体冻结层温度，将干燥供热板温度从15℃降低为-5℃，并在坯体内部设计直径为0．8mm的通孔结构，结果表明：采用这2项措施后，当坯体失水率为20％～60％时，可保证冻结层温度低于-3．6℃，有效避免裂纹的产生。

In producing precision ceramic casting mould of complex components by gel casting technology, the internal cracks generated in the freeze-drying process affect the mould quality seriously. In this study, computer tomography （CT） was employed to track the crack generation. The results show that internal cracks occur when water loss rate is within 20 %-60 %. According to the freeze-drying theory, the internal crack generation mechanism in gel casting ceramic body is analyzed. When the temperature of the subsurface gets higher than the eutectic temperature of the green body （--3.6 ℃） in the rapid ice sublimation stage （in which the water loss rate is about 20%-60%）, the frozen layer melts to result in a dramatic drop in strength and an increase in drying stress, leading to cracks generation. Lowering the drying temperature （from 15 ℃ to --5 ℃） and setting through holes of 0.8 mm in diameter in the green body can keep the subsurface temperature lower than the eutectic temperature in the rapid ice sublimation stage to prevent the crack generation effectively.