الفهرس 
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Abstract
The use of metals and their alloys has always been limited due to their poor strength at evaluated temperatures. This restriction necessitated the use of advanced materials like functionally graded materials (FGMs), which are made from a mixture of ceramic and metal. The ceramic part has a good thermal resisting properties and the metal has superior fracture toughness. Therefore, ceramic/metal FGMs are promising in thermal and structural applications. A large part of the research work on FGMs has been dedicated to processing and a large variety of production methods have been developed for the manufacturing of FGM. It is noted that powder metallurgy method is one of the most commonly employed techniques due to its wide range control on composition and microstructure and shape forming capability.
In the present work, The mechanical properties of ZrO2/Ni functionally graded materials fabricated by powder metallurgy technique were investigated experimentally. The relative density, linear shrinkage and electrical conductivity of NGC and FGMs were also measured. Microscopic examination exhibits that the material composition and microstructure of the FGMs vary gradually. The distributions of mechanical properties in the FGMs were obtained from the mechanical testing of homogeneous composite samples with different contents of Ni.
· The fracture behavior of FGMs and NGC (50%Ni/ZrO2) was investigated experimentally by utilizing the three point bending fixture (3PBF) under the mixed mode loading. The mixed-mode loadings were generated by offsetting the crack line relative to the loading point.
The phase analysis of the selected non-graded composites was determined by the X-ray diffractometer (XRD). The microstructure and the fracture surfaces of the prepared ZrO2/Ni FGMs samples were examined by using the scanning electron microscope (SEM). The elemental analysis of the NGC and the FGM specimens were also examined by the Energy-dispersive X-ray spectroscopy (EDX) chart.
Finite element modeling (Cosmos/M 2.6) was applied to predict crack-tip stress fields, displacement fields, stress intensity factor and crack propagation paths of FGM-A (crack situated on the ceramic side of the specimen), FGM-B (crack situated on the compliant side of the specimen) and NGC (50%Ni/ZrO2) in two dimensions under Generated by Foxit PDF Creator © Foxit Software
http://www.foxitsoftware.com For evaluation only. plane stress conditions. In addition and according to the maximum principle stress criterion (MPS), the crack propagation angle was predicted by analytical method and finite element modeling and it was also measured experimentally.
The results show that relatively high density (in the range of 0.86–0.98 of the theoretical density) for the NGC and 94.80%, 99% by presureless sintering (PLS) and spark plasma sintering (SPS), respectively for the FGMs. Also, the linear shrinkage of the NGC was reduce with nickel content. The electrical conductivity of the YSZ/Ni was strongly depended on its nickel content. The electrical conductivity as a function of nickel content had a typical ‘S’ shape curve increasing from 0.06 Ù.cm-1 to 781.79 Ù.cm-1.
The elastic modulus was decreased from 190 GPa to 70 GPa and the bending strength was reduced from 350 MPa to 80 MPa for the NGCs with increasing nickel content, this is mainly affected by the weakly bonded ceramic/metal interface.
The fracture toughness for NGCs increased with an increase in nickel content. Also, it was found that the fracture toughness and the elastic modulus for the FGMs were higher than that of the NGCs. Vickers’s hardness of ZrO2/Ni was lower than that of pure ceramic ZrO2 and was reduced by decreasing the relative density of the layer of ZrO2/Ni FGM, which was attributed to the variation of the matrix phase from the ceramic to the metallic.
None broken metal particles were observed in the fracture surfaces, also there were more dimples in the microstructure resulted from agglomerations of nickel metalnleaved from place in 50% NGCs.
The values of the stress intensity factors, SIFs (KI, KII) for the FGM A and FGM B were less than their corresponding values for NGC under the effect of mixed mode loading. Also, it was found that the SIF for the FGM-A was greater than those for the FGM-B. This is believed to be related to the increase in nickel content from the stiffness side of specimen. This indicates that the stress intensity factors (KI, KII) are mainly affected by the elastic gradient (change in elastic modulus) ahead of the cracktip and the local fracture toughness at the crack-tip. There were some local perturbations in the crack propagation paths of the FGM-A, FGM-B and NGCs specimens and most of local perturbations were exhibited in the layers with high Ni content such as the layers with 30%, 40% and 50% Ni, Generated by Foxit PDF Creator For evaluation only respectively. The local perturbations are believed to be caused by the local heterogeneity of the microstructure.
In the numerical molding, the initial propagation angle was approximately predicted as (7-16 °) for NGCs (50%Ni/ZrO2) and FGM, respectively with a good agreement with the estimated propagation angle of the experimental results.