Fabrication of alumina/zirconia functionally graded material: From optimization of processing parameters to phenomenological constitutive models

Cracks and camber are prevalent when co-sintering multi-layered functionally graded material (FGM) due to the residual stress caused by the mismatches in thermal expansion and sintering between successive layers. This paper describes a novel processing technique to relieve the residual stress in such co-sintering process. The effectiveness of the proposed technique was demonstrated by fabricating a three-layered Al2O3/ZrO2 FGM. Various processing parameters, such as the powder characteristics, compaction load, interface profile, and temperature ramp/soak profiles, were varied in order to determine the optimum parameters for eliminating cracks and decreasing camber. The optimum processing parameters include low compaction load, higher ramp rate, a proper mixing of multiple powders with distinct powder characteristics and the modification of interface profile in producing undamaged and undistorted samples. In addition, the thermomechanical analysis (TMA) on various powder mixtures with the determined optimized parameters was carried out, which indicates: (1) the shrinkage rate–sintering temperature curves of each layer are almost identical and (2) the shape of the densification rate–relative density curves are very similar, yielding a common term in the phenomenological constitutive models of all three layers.