Fabrication of Multi-Functional Sintered Materials with Graded Compositional Distribution

根据这两年制定的设计方案，对金属和陶瓷的分级试样进行了激光束差温加热试验和氢氧燃烧器火焰加热试验。采用声发射监测方法检测试样在加热和冷却过程中的裂纹形成情况。在试验的冷却循环过程中，观察到垂直裂纹的形成。纵向裂纹在进一步加热和冷却的过程中不断扩展，并最终合并，导致陶瓷层从试样表面剥落。利用测得的温度分布对试样的应力状态进行了有限元分析，结果表明，在加热过程中，陶瓷表面产生了较大的压应力，导致陶瓷层发生非弹性变形。非弹性变形会在冷却过程中产生拉应力，拉应力会超过陶瓷的固有强度，导致垂直裂纹的形成。通过对金属-陶瓷相复合材料裂纹扩展的显微组织观察发现，提高富陶瓷区的延性是保持这类功能梯度材料耐热性的必要条件。通过工艺优化控制富陶瓷区金属相的连通性来实现这种改进。隔热性能是通过材料的热阻来评价的，热阻的取值与垂直裂缝形成的加热温度一起，是功能梯度材料的热力学性能的特征参数。梯度材料的微观结构控制的概念，特别是关于所涉及的相的连通性，将应用于其他类型的功能梯度材料，如其他

A laser beam differential temperature heating test and a hydrogen/oxygen burner flame heating test were made on the graded specimens of metal and ceramics, fabricated under the design scheme established in the forgoing two years. An acoustic emission monitoring was used to detect the crack formation in the specimen during heating and cooling cycle. Vertical cracks were observed to be formed during cooling cycle of testings. The vertical cracks propagated by further heating and cooling, and they eventually coalesce to result in the spallation of the ceramic layr from specimen surface. Finite element analysis on the stress state in the specimen was done using the measured temperature distribution, and it has shown that a large compressive stress is generated during heating period at the ceramic surface to cause non-elastic deformation of the ceramic layr. The non-elastic deformation will cause during cooling peiod tensile stress, which will exceed the intrinsic strength of the ceramics and result in the formation of the vertical cracks. From the microstructural observations of the crack propagation in the composite structures consisted of metal and ceramic phase the improvement of the ductility of the ceramic-rich regions has been found to be indispensible for the retention of the heat resisrtance of this kind of functionally gradient materials. Such an improvement will be realized by an control of the connectivity of the metal phase in the ceramic-rich region through a process optimization. The heat insulating performance is evaluated by thermal resistance of the material, and its value is a characteristic parameter, along with the heating temperature of the vertical crack formation, for the thermomechanical performance of functionally gradient materials. The concept of the microstructual control of graded materials, particulary with respect to the connectivity of the phases involved, will be applied to the other kind of functionally gradient materials, such as ther