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Functionally graded materials have composition gradients from one end to the other as the result of a gradual transition of the properties of different materials. The residual stress caused by the difference of coefficient of thermal expansion can be minimized using functionally graded material. Therefore, the gradient of the coefficient of thermal expansion should vary according to the compositional gradient. In this study, the coefficient of thermal expansion of each compositional layer of Ni-Al 2 O 3 functionally graded material was measured using a dilatometer. These measurements provided the material properties required to calculate the residual stress, using three-dimensional modeling for accurately predicting crack positions, since it is difficult to measure residual stress experimentally. The measurement results showed the gradual increase of the coefficient of thermal expansion from Al 2 O 3 -rich composition to Ni-rich composition. Finally, the results of calculating residual stresses using the measured coefficient of thermal expansion showed that the crack positions were predicted more accurately than those using the coefficient of thermal expansion calculated by the linear rule of mixtures. This was because the measured values include the effect of porosity of the composite, whereas the linear rule of mixtures cannot account for the porosity of each layer.

With the recent emphasis on the importance of successfully joining materials, researchers have tried to join metals and ceramics with different coefficients of thermal expansion (CTEs) by using the functionally graded material (FGM) method. This involves inserting interlayers with composition gradients that range from one material to the other, thereby minimizing the stress caused by differences in CTE values. In this study, the FGM that included 10 layers of Ni-Al 2 O 3 with eight inter-layers was studied. Previous studies have focused on controlling the composition of inter-layers and optimizing the dispersion process to prevent cracks. Thermal stress was reduced by varying the weights of the inter-layers and increasing the green-body density by using several powder sizes. The powders were well-dispersed during fabrication by using simultaneous dispersion and dry processes followed by a cold isostatic press (CIP) and pressure-less sintering in an inert atmosphere. As a result, a crack-free Ni-Al 2 O 3 FGM joint was obtained. The residual stress in each layer was calculated to predict cracks using ANSYS simulation and maximum principal stress criterion; experimental values matched simulation results. In addition, an oriented Vickers indentation test was used to assess the quality of the joint. Crack-paths were not deflected across the interface, indicating good bond strength between interfaces. Sample density was measured using the Archimedes method; the sintered joint was less dense than its theoretical density but was denser than the results obtained by using previous methods.

Numerical analysis method was used to reduce the number of functionally graded material (FGM) layers for joining Si3N4-Al2O3 using polytypoid interlayer by estimating the position of crack. In the past, hot press sintering of multi-layered FGMs with 20 layers of thickness 500 mm each have been fabricated successfully. In this paper, thermal residual stresses were calculated using finite element method (FEM) to find the optimized number of layers and its thicknesses of FGM joint. The number of layers for FGM was reduced to 15 layers from 20 layers. Thicknesses were varied to minimize residual stresses within the layers while reducing the number of FGM layers. The damage caused by thermal residual stress was estimated using maximum principal stress theory and maximum tensile stress theory. The calculated maximum stress was found to be axial stress of 430 MPa around 90% 12H/10% Al2O3 area. For each case, calculated strength of each FGM layer by linear rule of mixture was compared with computed thermal residual stresses. Thermal analysis results correctly predicted the position of crack, and this position agreed well with fabricated joints. Therefore, this numerical analysis method can be applied to reduced FGM layers of crack free joint. Finally, new composition profile of crack free joint was proposed using FGM method. [doi:10.2320/matertrans.MRA2007319]