Optimum design of graded powder compacts with microstructure through prediction of crack formation

采用实验和数值分析相结合的方法研究了烧结性能、梯度组织和显微组织对氧化铝/不锈钢粉末层合板烧结过程中裂纹形成的影响。通过实验和分析结果，提出了分级粉末压坯的设计方法，并通过实施进行了验证。显微组织、烧结行为和结块形成的研究结果表明：在烧结初期，即使有轻微的内应力，也会在烧结过程中产生100%氧化铝层的表面开裂，在低温条件下，粉末颗粒之间的结合仍然不够。为了抑制裂纹的形成，必须避免在弱陶瓷层中产生拉应力，但仅通过改变层的烧结性能很难控制应力的产生。在烧结过程中单轴加压，通过调节烧结性能，使陶瓷层产生反弯曲，有效地将张力转化为压缩。同时考虑冷却过程中的热应力对梯度多层材料的优化设计也很重要。分级粉末压实机设计中烧结分析方法的发展采用有限元法和板理论对分级多层材料在烧结和冷却过程中的内应力进行了分析。阐明了具有不同烧结性能（粘度、烧结应力）和热性能（弹性、热膨胀系数）的梯度结构与内应力之间的关系。理论上发现，在烧结和冷却过程中，对某些层进行减薄可以有效地降低内应力。优化后的试样在1200℃下实际烧制，未观察到表面开裂现象。

Influence of sintering properties, graded structures and microstructures on crack formation in firing process of alumina/stainless teel powder laminates was examined by experiment and numerical analysis. Through the experimental and analytical results, methodology for design of the graded powder compacts without cracking during firing was proposed, and verified by implementation.1.Examination of microstructure, sintering behavior and crock formationSurface cracking in alumina 100% layer during firing occurred even by a slight internal stress at the early stage of sintering, in which the bonding among powder particles was still not enough in the low temperature. To suppress crack formation, the generation of tensile stress in weak ceramic layer must be avoided, but it was difficult to control stress generation only by modifying sintering properties of the layers. Uniaxial pressing by weights during sintering, with adjusting sintering properties, was effective to convert tension into compression in the ceramic layer by giving counter bending. However, taking thermal stress during cooling into consideration simultaneously was also important for the optimum design of graded multilayers.2.Development of method of sintering analysis for design of graded powder compactsThe internal stresses in graded multilayers during sintering and cooling processes were analyzed by using the finite element method as well as plate theory. The relationship between the internal stress and the graded structures with different sintering properties (viscosity, sintering stress) and thermal properties (elasticity, thermal expansion coefficient) was clarified. Thinning some layers was theoretically found to be effective for the reduction in internal stresses during both sintering and cooling. The optimally designed specimen was fired at 1200 actually, and no surface cracking was observed.