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

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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.