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.     

SiC dispersed polysulphide epoxy resin based functionally graded material

Gravity and centrifugal castings are considered the most economical and attractive processes for making functionally graded materials (FGMs). The control on property change in radial direction in aforesaid methods is mainly based on the initial concentration, viscosity, time, centrifugal force, particle size, and density difference in dispersed and dispersion medium as per Stoke's law. The modified Stoke's law in the form of HD model that takes into account the intrinsic changes in viscosity those occur during the polymerization of resins, was used for determining changes in concentration with time at different locations of test samples for SiC dispersed poly sulphide epoxy resin FGM. The experiments as well as simulation were conducted under gravity as well as centrifugal force. The simulated results are in good agreement with those ofexperiments. The process parameters, initial concentration, etc., can be selected for a desired concentration profile of FGM. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

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.     

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 of a C/SiC functionally graded coating for the oxidation protection of C/C composites

To reduce the residual thermal stress between the carbon fiber-reinforced carbon (C/C) composites and the SiC coating layer, functionally graded materials (FGM) consisting of a C/SiC compositionally graded layer (C/SiC interlayer) were adopted. After designing the compositional distribution of the C/SiC interlayer which can relieve the thermal stress effectively, the deposition conditions of the entire compositional range of the C/SiC composites were determined using a thermodynamic calculation. According to the design and calculation the C/SiC interlayer and the SiC outer layer were deposited on the C/C composites by a low pressure chemical vapor deposition (LPCVD) method at deposition temperatures of 1100 and 1300 °C. The stress calculation and the experimental results suggested that the SiC-rich compositional profile in the FGM layer is the most effective for relieving the thermal stress and increasing the oxidation resistance.     

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.     

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.     

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     

New Al2O3–Cu–Ni functionally graded composites manufactured using the centrifugal slip casting

The aim of this paper was to establish the general suitability of the centrifugal slip casting technique for fabrication of graded material from the Al₂O₃–Cu–Ni system. The present investigation is a preliminary one and the obtained results may be regarded as a point for acquiring the new knowledge in the ternary ceramic metal system fabricated using centrifugal slip casting. The rheological study provided the evidence that the suspension used to fabricate composites is a non-Newtonian slurry. Moreover, the results demonstrated that the prepared slurry is stable over time. The obtained composites has a hollow cylindrical shape with relative density of > 95%. The XRD study results revealed three phases: Al₂O₃, Ni, and CuNi. The microscopic investigations revealed a nonhomogeneous distribution of the metallic phase in the composite. It has been shown that the content of the metallic phase decreased with the increased distance from the outside to the inner edge of the ceramic-metal composite. The study results allow to conclude that the interface between the phases is more continuous when sintering was carried out in a solid phase. However, it was found that when sintering was carried out in a liquid phase separated metal particles with spherical phase are formed.     

纳米SiC陶瓷的超高压烧结研究(英文)

以纳米SiC为原料,用两面项压机在不同工艺条件下(1 000～1 300℃,4.0～4.5 GPa,15～35 min)实现了40(质量分数,下同)Al2O3烧结助剂添加的SiC陶瓷体的烧结.研究了烧结工艺对SiC陶瓷性能的影响.用X射线衍射、扫描电镜、显微硬度测试仪等对SiC高压烧结体进行了表征.结果表明:Al2O3是有效的低温烧结助剂,在超高压工艺下添加4%Al2O3即可实现SiC陶瓷全致密化烧结;烧结体晶粒长大得到抑制,维持在纳米级,晶格常数收缩了约0.45%;烧结体显微硬度和密度随烧结温度、烧结压力的升高或保温时间的延长而提高.     

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.     

Fabrication of a graded-index polymer optical fiber preform by using a centrifugal force

We have investigated a fabricating method for a graded index polymer optical fiber preform using a centrifugal force. When two monomers with different densities and refractive indices are polymerized under a centrifugal force, a concentration gradient is generated due to their density difference. Therefore, a graded refractive index can be obtained according to the concentration gradient. When a monomer is polymerized under a centrifugal force, a preform with a hollow is obtained because of volume shrinkage. To compensate for this practically, additional monomer should be filled into the hollow. Monomer should be fed to obtain a continuous gradient of refractive index at the interface before the first polymerized product is perfectly glassified. Two different types of feeding additional monomer were experimented with: monomer-monomer pair (case I) and monomer-polymer pair (case II). The graded index profile with a proper δn (about 0.01) was successfully obtained in either case.     

Estimation of Concentration of Particles in Polymerizing Fluid during Centrifugal Casting of Functionally Graded Polymer Composites

The concentration of uniformly distributed particles in a fluid changes with time in the direction of gravitational or centrifugal force to form a concentration gradient. The change in the concentration is an outcome of velocity variation of particles in a fluid. A modified equation for terminal velocity, v _m of particles in polymerizing-fluid under centrifugal force is proposed to estimate the changes in the volume fraction of particles in the graded composites. The proposed equation introduces the effect of cure kinetics of polymer and its effect on particle movement in the model that was based on the modified Stoke’s law, considering the parameters related to particle hindrance, centrifugal force, particle dimensions, viscosity variation etc. The model predictions of concentration changes at the different locations of samples were compared with calcium carbonate filled polysulphide-modified-epoxy graded composites prepared by centrifugal casting.. The effect of particle size, delayed curing rate of matrix were explored. The simulated results are in good agreement with those of experiments.     

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.     

Design,fabrication and properties of layered SiC/TiC ceramic with graded thermal residual stress

The finite element method (FEM) was used to design a symmetrical layered SiC/TiC ceramic with gradual thermal residual stress distribution. In the final model ceramic, the sequence of layers, from surface to inside, was SiC, SiC+2 wt.% TiC (S2T), SiC+4 wt.% TiC (S4T), SiC+6 wt.% TiC (S6T), SiC+8 wt.% TiC (S8T), and SiC+10 wt.% TiC (S10T); the thickness ratio of SiC:S2T:S4T:S6T:S8T:S10T was 1:1:1:1:1:10. After the model ceramic had been cooled from assumed sintering temperature 1850–20 °C in FEM calculation, gradual thermal residual stress, varying from surface compressive stress to inner tensile stress, was introduced. The designed ceramic then was fabricated by aqueous tape casting, stacking and hot-press sintering at 1850 °C, under 35 MPa pressure, for 30 min. The surface stress conditions of the sintered ceramic were tested by X-ray stress analysis, and those results were very close to the results from the FEM calculations. Compared with pure SiC and S10T ceramics fabricated by the same process, the designed ceramic showed excellent mechanical properties. The tested strength was close to the theoretical value. The strengthening and toughening mechanisms of the ceramic were ascribed to surface compressive residual stress.     

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.     

Simulation of compositional profile in functionally graded materials

The development of a polymer based functionally graded material (FGM) of desired composition profile by the centrifugation technique requires control on centrifugation, size, shape, and concentration of suspended particles, time, viscosity variation of polymerizing fluid, etc. A simulation was conducted to observe the compositional variations with time at different places of FGM, using a modified terminal velocity equation for particle movement in polymerizing fluid. It was further modified for the particles having different sizes. The simulation demonstrated two graded‐composition profiles each one in low concentration region from where particles were moved to the other part of sample and second high concentration profile in which particles entered to increase the concentration. The third region situated between the two composition profiles was observed as that of uniform distribution of particles and the length of this region can be controlled by adjusting the size of the centrifuged sample. The simulation was compared with the experimental results of FGM having SiC particles in polysulphide epoxy resin. POLYM. ENG. SCI. 46:1660–1666, 2006. © 2006 Society of Plastics Engineers     

高温金属熔液在旋转多孔介质内的渗流传热过程

针对转动坐标系中铝熔液在SiC多孔介质内的流动传热现象 ,考虑离心力对渗流传热过程的影响 ,采用局部非热平衡假设建立多孔介质渗流传热数理模型 ,研究不同工况下流体的流速、压力损失及铝熔液和多孔介质的温度变化 .计算结果表明 :在渗透区域 ,液固两相存在温差 ,且液固温差随渗透界面的移动而减小 ;在非渗透区域 ,固体的温度曲线基本不变 .离心转速或孔隙率的增加都使渗透前沿区域液固两相温差增大 .孔隙率对流场和压力损失有较大影响.     

Microstructure and thermal conductivity of fully ceramic microencapsulated fuel fabricated by spark plasma sintering

Fully ceramic microencapsulated pellet (FCM), consisting of tristructural isotropic (TRISO) particles embedded in silicon carbide (SiC) matrix, was fabricated using spark plasma sintering. The parameters affecting the densification of SiC matrix were first investigated, and then FCM pellets were prepared using TRISO particles with/without outer pyrolytic carbon (OPyC) layer. Effects of thermal exposure on the TRISO particles during SPS were evaluated. In addition, the thermal condcutvitities of FCM pellet, as well as the SiC matrix, were measured using laser flash. It was revealed that the TRISO particles with OPyC layers significantly lower the thermal conductivity of FCM pellet. Based on Maxwell‐Eucken model, the predicted effective thermal conductivities of TRISO particles with/without OPyC layers were 14.4 W/m K and 25.2 W/m K, respectively. Finite elements simulation indicated that the SiC layer in TRISO particle plays a dominant role on the thermal conductivity of FCM. The presence of OPyC layers would generate gaps/porous SiC near the interface and resist the heat flows, leading to a lower thermal conductivity of FCM.     

Air-sintered silicon (Si)-bonded silicon carbide (SiC) hollow fiber membranes for oil/water separation

In this work we evaluated the effect of adding Si as sintering additive into SiC for producing air-sintered hollow fiber membranes. According to crystallographic analyses, SiC and Si were converted to SiO₂ after sintering at 1350 °C. The addition of 30 wt% of Si into SiC ceramic material promoted the binding of SiC particles and improved the membrane mechanical resistance to 42.25 ± 3.39 MPa after air sintering at 1350 °C. The produced asymmetric ceramic membrane presented a packed pore-network and micro-voids with pore sizes of 1.73 and 5.29 μm, respectively. The filtration of an oil/water emulsion enabled oil retention 98.75 ± 0.95 %. Cake formation was the main fouling occurrence and membrane regeneration with equivalent oil retention and similar steady sate flux was achieved after water cleaning under ultrasound irradiation. Thus, the use of Si as air-sintering aid was favorable for producing Si-bonded SiC hollow fiber membranes with suitable application for oil/water separation.