Processing of ZrW₂O₈-ZrO₂ Continuous Functionally Graded Materials by Co-Sintering ZrO₂ and ZrO₂+WO₃ Multi-Layer Compacts

A powder mixture of ZrO₂+WO₃ and ZrO₂ powder were stacked, co-compacted and co-sintered in the processing steps commonly used to fabricate multi-layer materials. However, the observation of the cross-sectional microstructures as well as the measurement of the radial thermal expansion provided the evidence that the sintered samples are continuous Functionally Graded Materials (FGMs) made of ZrW₂O₈ and ZrO₂, Because of the discrepancy in the sintering potentials between two materials, the sintered samples do not retain the original cylindrical shapes of the green compacts. This problem has been resolved by choosing the appropriate powder mixture for each layer of the compacts. The formation of the continuous FGM structure is due to three factors: 1) the diffusion of WO₃, 2) the sublimation of WO₃ and 3) the reaction between ZrO₂ and WO₃. The continuous variation in the radial coefficient of thermal expansion can be utilized to reduce the thermal stress induced from a thermal gradient loading within a material system. This study shows that the processing steps typically used in processing stepwise FGMs can also be used to create continuous FGMs for some special powder mixtures.