Synthesis of TiC, TiC-Cu Composites, and TiC-Cu Functionally Graded Materials by Electrothermal Combustion

Titanium carbide and composites and functionally graded materials (FGMs) of TiC– x Cu were synthesized by an electrothermal combustion (ETC) method. TiC synthesized by ETC showed small amounts of porosity relative to those synthesized by ignition using radiative heating. Composites and FGMs with higher copper content can be synthesized by ETC. In the FGM products a nearly linear change in composition in the graded region was observed in samples with 0 ≤ x (wt%) ≤ 75.     

Glass-alumina functionally graded materials: their preparation and compositional profile evaluation

This work was focused on glass-alumina functionally graded materials (FGMs). For the glass phase, a proper composition was chosen belonging to the ternary system CaO–ZrO₂–SiO₂ and the substrate was made up of a sintered, high-purity polycrystalline alumina. Both of the ingredient materials were carefully characterized. The fabricated functionally graded materials were analysed in detail, by observing them under a scanning electron microscope (SEM) coupled with an X-ray energy dispersive spectrometer (X-EDS). The depth of penetration of the glass and the compositional profile were evaluated by means of a SEM-image elaboration. Moreover, this work applied an analytical model to predict the depth of penetration as a function of time and fabricating parameters such as temperature.     

Fabrication of Functionally Graded Materials Via Inkjet Color Printing

A new method for fabricating functionally graded materials (FGMs) via inkjet color printing is reported in this paper. Al₂O₃ and ZrO₂ aqueous suspensions were stabilized electrostatically and placed in different color reservoirs in inkjet cartridges. The volume and composition of the suspensions printed in droplets at a small area were controlled by the inkjet cyan–magenta–yellow–black color printing principle. The analysis of energy-dispersive spectrometry shows that with multi-layer printing, the composition profile of the printed FGM is consistent with the designed profile. The new method shows the potential for fabricating FGMs with arbitrarily designed three-dimensional composition profiles.     

Electrical properties of barium titanate stannate functionally graded materials

Barium titanate stannate (BTS) functionally graded materials (FGMs) with different tin/titanium concentration gradient were prepared by the powder-stacking method and uniaxially pressing process, followed by sintering. Impedance spectroscopy (IS) was used to determine the electrical characteristics of FGMs and ingredient BTS ceramics, as well as to distinguish the grain-interior and grain boundary resistivity of the ceramics. Activation energies of FGMs and ingredients were calculated. It has been established that for BTS ceramics the activation energy deduced from grain-interior conductivity (0.73–0.75 eV) is defined by chemical composition, while activation energy for grain boundary conductivity (1.07–1.25 eV) is influenced by microstructural development (density and average grain size). Furthermore, for FGMs, activation energy for grain-interior conductivity kept the intrinsic properties (0.74–0.78 eV) and did not depend on tin/titanium concentration gradient, while activation energy (1.03–1.29 eV) for grain boundary was determined by the microstructural gradient. No point dissipation was observed by IS, accordingly, no insulator interfaces (cracks and/or delamination) between graded layers were detected.     

Preparation and experimental characterization of glass–alumina functionally graded materials

This work aims at investigating the effects of the processing conditions on the final microstructure of glass–alumina functionally graded materials (FGMs). The ingredient materials, i.e. a polycrystalline sintered alumina and a CaO–ZrO₂–SiO₂ glass, were accurately characterized, since their mechanical and thermal properties may deeply influence the fabricating process and the overall FGM behaviour. The functionally graded materials were obtained by means of percolation of the molten glass into the alumina substrate. Two types of samples were considered—the “Bulk” FGMs, produced starting from a glass bulk, and the “Powder” FGMs, produced starting from a glass powder; in both cases four different heating cycles were attempted. The functionally graded materials were analysed using a SEM-EDS and a X-ray diffractometer. Great attention was devoted to the resulting microstructure; moreover the depth of penetration was measured and related to the fabricating parameters, such as time and temperature.     

Functionally graded laminated composites fabricated from MAX-phase filled preceramic papers: Microstructure,mechanical properties and oxidation resistance

This paper describes the fabrication and characterization of novel preceramic paper-derived functionally graded materials (FGMs) based on Ti₃(Si,Al)C₂ MAX phase. The FGMs with different architecture were fabricated via spark plasma sintering of stacked preceramic papers at 1250 °C for 5 min. Microstructure, phase composition and elemental distribution were analyzed by scanning electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy, respectively. Oxidation tests were performed in air at 1300 °C for 5 h. FGMs containing Al- and Si-enriched MAX-phase layers were formed. The fabricated materials exhibit high flexural strength (over 600 MPa), which are dependent on microstructure and composition of individual layers as well as the architecture of composites. It was found that texturing of MAX phase grains during SPS results in anisotropic hardness of the composite. The difference in the composition of the individual layers also provides a hardness gradient in the composite. It was shown that the formation of the outer layer from the Al-enriched Ti₃Al(Si)C₂ MAX phase increases the corrosion resistance of Ti₃SiC₂-based composites. The high corrosion resistance of FGMs is due to the growth of a continuous and dense Al₂O₃ oxide layer.     

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al₂O₃ in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.     

流延法制备ZrO2/不锈钢功能梯度材料

采用流延法制备了ZrO2/316L不锈钢功能梯度材料,利用成分分布函数设计梯度层厚度来减小各层间的残余应力,通过改变黏合剂添加量得到各梯度层中ZrO2的体积分数(φ)和黏合剂与混合粉体积比(M)的关系:M=1.029φ 1.652,并研究了烧结工艺对材料组织形貌的影响.结果表明:在真空无压条件下,1 350 ℃烧结可得到成分呈梯度变化,各梯度层界面结合良好的ZrO2/316L功能梯度材料.     

Development of functionally graded vapor‐grown carbon‐fiber/polymer materials

We successfully produced vapor‐grown carbon‐fiber (VGCF)‐incorporated polymer‐based functionally graded materials (FGMs) using a centrifugal method. Gradual VGCF incorporation within an epoxy resin effectively produced depth gradients in the fiber distribution, microstructure, mechanical, and electrical conductivities and microwave absorbing properties. This VGCF‐grading capability indicated that it is possible to tailor desired gradient filler content distributions by careful selection of the processing parameters to control variations in the property and microstructure precisely. The results confirmed that the volume content of VGCF in the epoxy substrate increased as a function of the normalized thickness along the centrifugal force direction, which caused a gradient. A uniform VGCF gradient in the composite can also be observed using field‐emission scanning electron microscopy. In the case of the electrical properties, for example, the volume resistance exhibited a depth‐graded distribution in the matrix as the electrical conductivity of the FGM nicely followed the grading direction; this is considered to be ideal for applications demanding an electrically conductive surface and an insulating core for FGMs. The results of microwave absorption behavior of FGMs indicate that the grading structure can lead to a graded absorption ability, which could be a better design for microwave absorption materials. The concept of FGMs bridges conventional materials and nanocomposites and will be effective for wider material applications. POLYM. COMPOS., 34:1774–1781, 2013. © 2013 Society of Plastics Engineers     

Electrical conductivity and electromagnetic shielding properties of Ti3SiC2/SiC functionally graded materials prepared by positioning impregnation

Ti₃SiC₂/SiC functionally graded materials (FGMs) were prepared by hot-pressing/positioning impregnation. Positioning impregnation is a novel local impregnation technique targeted at layers exhibiting poor sintering behaviour, resulting in uniform densification across the different layers and minimizing the deviation from the designed composition distribution. With the increased Ti₃SiC₂ content in the layer, the resistivity decreased significantly firstly and then decreased slowly and finally tended to be a constant. The resistivity of the layers with SiC content of 80–100 vol.% decreased significantly via positioning impregnation and further decreased by introducing γ-Al₂O₃. However, introducing γ-Al₂O₃ facilitated the decomposition of Ti₃SiC₂ into TiC. The FGMs had a total shielding effectiveness over X band of 43–52 dB and is suitable for potential applications in excellent electromagnetic shielding materials. The dominating shielding mechanism of the FGMs is reflection, and the FGMs had high intrinsic absorption capabilities.     

Formation of compositional gradient in Al/SiC FGMs fabricated under huge centrifugal forces using solid-particle and mixed-powder methods

Formations of graded distribution of SiC ceramic particles within the hollow cylindrical shaped Al/SiC functionally graded materials (FGMs) fabricated by centrifugal solid-particle method (CSPM) and centrifugal mixed-powder method (CMPM) under huge centrifugal force are experimentally and theoretically investigated. The movement of SiC ceramic particles in viscous liquid under centrifugal force is explained based on Stoke's law. The effect of compositional gradient of particles on viscosity is taken into account. Also, the effect of temperature distribution on viscosity and density are considered. A computer code to simulate the formation of compositional gradient in the Al/SiC FGMs fabricated by CSPM and CMPM is developed. From the results, it is found that the volume fraction of SiC ceramic particles can be graded from the inner to the outer surface of hollow cylindrical shaped Al/SiC FGMs by CSPM. Meanwhile by CMPM, the SiC ceramic particles can be dispersed on the outer surface of hollow cylindrical shaped Al/SiC FGMs. The graded distribution in Al/SiC FGMs under huge centrifugal force is found to be significantly affected by the mold temperature but less affected by the temperature of molten Al and casting atmosphere.     

A novel approach for developing boron carbide (B4C)/cyanate ester (CE) co-continuous Functionally Graded Materials (FGMs) with eliminated abrupt interfaces

Although traditional functionally graded materials (FGMs) have weakened a certain extent abrupt changes along interfaces between layers, eliminating residual abrupt interfaces which results in great inter laminar stresses remains a major challenge. Herein, ceramic scaffolds with novel continuously graded channels have been firstly prepared through combining layer-by-layer casting with gelation-freeze-dry. Followed by infiltrating soft phase, the co-continuous FGMs without delamination and abrupt changes between layers can be successfully fabricated. The wet layering of suspensions and continuous ice crystals across the interface are the main reasons for the eliminated interfaces. More importantly, the performance of FGMs can be optimized to meet the practical requirement attributed to the varied porosity and pore size distribution of freeze-cast ceramic scaffolds. The novel process for co-continuous ceramics/ FGMs without abrupt interfaces can be extended to other polymers or even metals.     

Spark plasma sintered step graded Al2O3–NbC composites

Functionally graded materials (FGM) present enormous potential to combine materials with distinct mechanical and thermal properties in a component via a compositional gradient throughout the body. In the present work, step graded Al2O3/NbC composites were assessed in order to obtain a FGM with high hardness and good fracture toughness. Step graded uniaxially pressed cylinders were consolidated by Spark plasma sintering (SPS) into fully dense (>99% TD) crack-free graded bodies. The designed gradient was successfully maintained after sintering and, due to the high density of FGMs, it was possible to obtain a high hardness up to 25.1 GPa and a good fracture toughness of ~5 MPa m^1/2.     

Microwave-assisted combustion synthesis and compaction of intermetallic-based functionally graded materials: Numerical simulation and experimental results

The use of microwave (MW) energy as an ignition source for combustion synthesis and compaction of intermetallics-based functionally graded materials (FGMs) is suggested. Numerical simulation was used to investigate the temperature distribution in reacting powders before, during, and after combustion synthesis, showing that microwaves are capable of continuing to convey energy to the reactants and products, despite an adverse temperature gradient. Examples of the application of Microwave-Assisted Combustion Synthesis (MACS) are reported, involving principally neat intermetallic-based FGMs belonging to the CoAl-NiAl system, both as free-standing samples and as coatings on titanium Grade 5 alloy. Some other preliminary results on the reliability of the here proposed experimental approach to the synthesis of free-standing samples based on intermetallic-matrix ceramic particles reinforced FGMs are presented.     

The master sintering curves for BaTi0.975Sn0.025O3/BaTi0.85Sn0.15O3 functionally graded materials

The most important aim in the design and processing of functionally graded materials (FGMs) is to produce devices free from any deformation. Smart choices of different combination of graded layers, as well as the heating rate during sintering, are important for the fabrication of high-quality FGMs. In this study, BaTi_0.975Sn_0.025O₃/BaTi_0.85Sn_0.15O₃ (noted as BTS2.5 and BTS15, respectively) FGM was used as a model system for the construction of master sintering curves (MSCs) and estimation of the effective activation energies of sintering for different BTS graded layers. The MSCs were constructed, for BTS2.5 and BTS15 graded layers in FGMs, using shrinkage data obtained by a heating microscope during sintering at four constant heating rates, 2, 5, 10 and 20 °/min. The effective activation energies were determined using the concept of MSC; values of 359.5 and 340.5 kJ/mol were obtained for graded layers BTS2.5 and BTS15, respectively. A small difference of the effective activation energies of chosen powders made it possible for us to prepare high-quality FGMs, without delamination, distortion or other forms of defects.     

Improved functional response of spark plasma sintered hydroxyapatite based functionally graded materials: An impedance spectroscopy perspective

Owing to the inherent polarization regulated functionality of living bone, the present study aims to enhance the polarizability of hydroxyapatite (HA) without affecting its’ surface chemistry i.e., excellent biocompatibility. This concept has been materialized by the development of functionally graded materials (FGMs) using perovskites BaTiO₃ and CaTiO₃ as intermediary layers. These perovskites were sandwiched between HA layers via buffer interlayers to provide the structural integrity to the FGMs. FGMs were optimally processed using spark plasma sintering route at 1100°C for 10?min. Microstructural analyses of fractured surfaces revealed no sign of delamination between HA/BaTiO₃ or HA/CaTiO₃ layers, despite of differences in their coefficients of thermal expansions. Further, detailed impedance spectroscopic analysis was performed over a wide range of temperature (35–500°C) and frequency (1?Hz - 1?MHz). Almost two times increase in polarizability has been observed in both types of FGMs as compared to HA. Overall, the developed FGMs can be suggested as potential materials for polarized bone applications.     

Effect of Silicon Carbide Weight Percentage and Number of Layers on Microstructural and Mechanical Properties of Al7075/SiC Functionally Graded Material

Silicon - In this research, four different types of Al7075/SiC functionally graded materials (FGMs) are produced by varying the number of layers and weight percentages of silicon carbide(SiC). Each...     

Design and mechanical properties of particle-reinforced polymer-matrix functionally graded materials applied on elastic polishing pad

Aiming at the functionally graded materials (FGMs) formed by mixing the abrasive particles and rubber with different kinds and mass ratios, the numerical analysis and experimental verification of its mechanical properties were carried out. The FGMs was applied on the elastic polishing pad for obtaining ultra-smooth surfaces of hard and brittle materials, such as glass ceramics and silicon wafers. Its structure and properties changed in the radial direction quasi-continuously. Regarding to the design of the FGMs, the maximum standard deviation of Young's modulus was 1.09 after 6 repeated tests, when the SiC abrasive particles and chloroprene rubber (CR) were mixed with mass ratio less than 50 phr. Furthermore, there are three types of mechanical properties, named edge stress, boundary stress and central stress, when the workpiece was applied to the elastic polishing pad mainly made of the FGMs mentioned above. If using matching module method, the abrupt phenomenon of edge stress was basically eliminated. Increasing the gradient rings could reduce the abrupt change of boundary stress, and the contact stress in the single gradient ring was basically kept constant when the FGMs was up to 8 gradient distribution. Meanwhile, due to the viscoelastic property of rubber, there was a difference of 25% between the dynamic and static stress values of the elastic polishing pad with acrylate rubber (ACM) as the matrix, and the maximum difference was less than 4.7% when using CR as the matrix. Besides, the dynamic mechanical analysis (DMA) showed that the loss factor tanδ was less than 0.1 in the processing temperature range of 20 °C–50 °C. Therefore, the FGMs with SiC or Al₂O₃ as particle-reinforcement, and CR as polymer-matrix could largely solve the inconsistent problem of central stress under dynamic and static conditions.     

Preparation and mechanical properties of Ti3SiC2/SiC functionally graded materials

Ti₃SiC₂/SiC functionally graded materials (FGMs) were prepared via hot-pressing sintering followed by positioning impregnation. Positioning impregnation is a novel technique for local impregnation targeted at graded layers that exhibit poor sintering behaviour. The positioning impregnation process significantly densified layers with SiC volume fractions of more than 70% while only slightly affecting the densities of the other layers and preserving sufficiently weak interfaces between layers. FGMs that were hot pressed at 1600 and 1700 °C and then subjected to impregnation showed not only high flexural strengths but also zigzag load-displacement behaviour. The flexural strengths of these FGMs were 436 and 485 MPa, respectively; in comparison, the values for the FGMs without impregnation that were hot pressed at 1600, 1700 and 1800 °C were 235, 268 and 328 MPa, respectively. Moreover, the fracture toughnesses of these FGMs were 8.23 and 7.15 MPa m^1/2, respectively; in comparison, the values for the FGMs without impregnation that were hot pressed at 1600, 1700 and 1800 °C were 6.77, 7.05 and 4.65 MPa m^1/2, respectively.     

Temperature and Stress Distributions in a Hollow Cylinder of Functionally Graded Material: The Case of Temperature-Independent Material Properties

We have presented a numerical technique for analyzing one-dimensional transient temperature distributions in a circular hollow cylinder that was composed of functionally graded ceramic–metal-based materials, without considering the temperature-dependent material properties. The functionally graded material (FGM) cylinder was assumed to be initially in a steady state of gradient temperature; the ceramic inner surface was exposed to high temperature, and the metallic outer surface, which was associated with its in-service performance, was exposed to low temperature. Then, the FGM cylinder was cooled rapidly on the ceramic surface of the cylinder, using a cold medium. The transient temperature and related thermal stresses in the FGM cylinder were analyzed numerically for a model of the mullite–molybdenum FGM system. The technique for analyzing the temperature distribution is quite simple and widely applicable for various boundary conditions of FGMs, in comparison with methods that have been proposed recently by other researchers.