梯度功能陶瓷圆球的抗热震性

推导和提出了第三类边界条件下梯度功能材料(functionally gradient materials,FGM)圆球的瞬态温度场及瞬态热应力场表达式.基于陶瓷材料的临界应力断裂判据,通过求解梯度功能陶瓷圆球表面达到其局部断裂强度的时间,建立了引起其表面临界热应力的临界温差△Tc的表达式,并以此作为梯度功能陶瓷圆球的抗热震参数.通过计算实例并与均质陶瓷圆球对比,分析了材料的热-物理性能分布规律对其抗热震性的影响,并提出了高抗热震性FGM陶瓷圆球的设计原则:线膨胀系数和热扩散率应由表及里增大,而弹性模量应由表及里减小.     

Influence of the Compositional Profile of Functionally Graded Material on the Crack Path under Thermal Shock

Thermal cracking under a transient-temperature field in a ceramic/metal functionally graded plate is discussed. When the functionally graded plate is cooled from high-temperature, curved or straight crack paths often occur on the ceramic surface. It is shown that the crack paths are influenced by the compositional profile of the functionally graded plate. Transient-thermal stresses are treated as a linear quasi-static thermoelastic problem for a plane strain state. The crack paths are obtained using finite element method with Mode I and Mode II stress intensity factors.     

陶瓷圆柱体的抗热震参数

通过求解第三类边界条件下无限长圆柱体的瞬态温度场及瞬态热应力场，研究了陶瓷圆柱体的抗热冲击行为，建立了引起无限长陶瓷圆柱体表面临界热应力的临界温差表达式，并以此作为陶瓷圆柱体的抗热震参数。计算结果表明，无限长陶瓷圆柱体的临界温差大于具有相同Biot模数的无限大陶瓷平板的临界温差，但其表面达到临界热应力的时间小于无限大平板的数值。     

Functionally Graded Materials Under Severe Thermal Environments

Thermal shock strengths of a plate of a functionally graded material (FGM) are analyzed when the plate is suddenly exposed to an environmental medium of different temperature. A finite element/mode superposition method is proposed to solve the time-dependent temperature field. The admissible temperature jump that the material can sustain is studied using the stress-based and fracture mechanics-based criteria. The critical parameters governing the level of the transient thermal stress in the medium are identified. The strength of FGMs under transient thermal stresses is analyzed using both maximum local tensile stress and maximum stress intensity factor criteria.     

Strengthening the ZrC-SiC ceramic and TC4 alloy brazed joint using laser additive manufactured functionally graded material layers

In order to improve the ZrC-SiC and TC4 brazed joint property, functionally graded material (FGM) layers (two SiC particles reinforced TC4-based composite layers) were designed to relieve the residual stress in the ZrC-SiC and TC4 brazed joint. The FGM layers were fabricated on the TC4 surface using laser additive manufacturing technology before the brazing. Then the TC4 coated with the FGM layers and ZrC-SiC ceramic were brazed using Ti-15Cu-15Ni (wt%) filler. According to the SEM and TEM results, the volume fractions of SiC particles in the FGM layers could reach 20% and 39% respectively. Ti from the braze filler and TC4 reacted with the ZrC-SiC ceramic to form TiC and Ti₅Si₃ adjacent to the ZrC-SiC ceramic. The shear test results indicate that the adoption of the FGM layers and the brazing temperature both affected the joint property significantly. The FGM layers could benefit the mitigation of coefficient of thermal expansion (CTE) mismatch between the ZrC-SiC and TC4, so that the residual stress caused by the CTE mismatch in the joint was relieved and the joint strength increased. The brazing temperature would affect the microstructure of the brazing seam and then control the joint strength. When the ZrC-SiC ceramic and TC4 coated with the FGM layers were brazed at 970?°C for 10?min, the maximum shear strength could reach 91?MPa, and cracks propagated in the ZrC-SiC ceramic substrate during the shear test.     

High-Intensity Discharge Lamp with Mo–SiO2 Functionally Graded Material

We show the invention of the new type of hermetically sealed high-intensity discharge lamps, made of Mo–SiO₂ functionally graded material (FGM) as an electrode and a sealing component. In the case of high-intensity discharge lamps with Mo–SiO₂ FGM (FGM lamp), the thermal expansion coefficient between Mo and SiO₂ is functionally graded so that it tolerates a large number of heating cycles, with no cooling system required. Furthermore, lamps survive without breakage. The W electrode is totally separated from the lamp envelope by the FGM, so that no leakage of the luminous elements or gases takes place, when a large gas pressure exists inside a lamp.     

Photo-Thermal-Elastic Interaction in a Functionally Graded Material (FGM) and Magnetic Field

Silicon - In this work, we aim to investigate the photo-thermal-elastic waves interaction in a nano-composite semiconductor, elastic and functionally graded material (FGM). The governing equations...     

Centrifugally cast functionally graded materials: Fabrication and challenges for probable automotive cylinder liner application

In response to stringent environmental rules and rising public awareness, internal combustion (IC) engines have undergone fast improvement to reduce friction and wear in recent decades. Liner is a sacrificial engine component that protects and provides a smooth reciprocation surface to the engine block. High hardness aluminum alloys are used to produce cylinder liners. When reinforced with suitable ceramic particles, the strength, hardness, stiffness, thermal stability, and wear resistance of these alloys are improved. Here, the underlying challenges in liners are discussed, along with various solutions. The detailed fabrication process of centrifugally cast functionally graded composite materials (FGMs) for prospective use as liners is emphasized. Various parameters and their effect on mechanical and tribological properties are discussed in depth, and a comparison is made with existing Aluminum liners. A general framework for optimal material selection, parameter selection, and processing procedure is proposed to develop the FGM liner. In addition, challenges, research opportunities, and possibilities for the development of this field are presented.     

Development of functionally graded ZrB2–B4C composites for lightweight ultrahigh-temperature aerospace applications

In the present work, a lightweight three-layer ZrB₂–B₄C functionally graded composite material has been developed by spark plasma sintering route. The functionally graded material (FGM) is free from interlayer defects and displays a smooth transition between the individual layers. The composition of each layer was designed to reduce the overall density without sacrificing the functionality of the material. The density of the FGM is almost 40% lower than monolithic ZrB₂ and 24% lower than ZrB₂–30B₄C rendering it potentially very attractive for high-temperature aerospace applications. A detailed structural characterization of the FGM was carried out to evaluate the elemental distribution in the graded composite, as well as determine the spatial distribution of the crystalline phases. Vickers hardness was measured within each layer and in the interlayer regions to further evaluate the gradient structure and interlayer transitions. Longitudinal elastic constants of the FGM along the thickness and across the layers measured using ultrasound phase spectroscopy showed that despite the gradient structure, the FGM can be treated as a quasi-isotropic solid.     

对称型梯度功能陶瓷材料抗热震性的研究

通过水淬实验并结合热冲击过程瞬态热应力场的计算，研究了Al2O3/W(W,Ti）C系列对型梯度功能陶瓷材料的抗热震性能。结果表明，采用合理的梯度组成分布规律可提高陶瓷材料的抗热震性。     

Continuous functionally graded material to improve the thermoelectric properties of ZnO

Functionally graded material (FGM) in terms of grain size gradation is fabricated from ZnO with a combination of modified Spark Plasma Sintering (SPS) graphite tooling, water sintering enhancements through transient liquid phase surface transport, and strategic SPS mechanical loading. The grain size gradation of the ZnO FGM spans from 180 nm grains to 1.2 micrometers in a fully dense material. This is the first semiconductor or ceramic to be graded microstructurally to this extent. Predictions of the microstructure with a Master Sintering Curve (MSC) approach were done with a series of isothermal experiments on two different FGM conditions revealing a slight offset due to a constrained mechanism. The mechanical properties were tested with Vickers micro hardness across the sample, showing a gradient in hardness from 2.6 GPa to 4.2 GPa. In addition, the thermoelectric properties of the FGM were measured and show a zT of 2 × 10⁻⁵ at 100 °C compared to uniform small- and large-grained samples of 1 × 10⁻⁶. This is an order of magnitude difference making a new path for improvements of bulk thermoelectric material.     

ECBC边界下夹FGM金属/陶瓷复合板稳态热应力

采用有限元法研究ECBC边界下中间夹FGM金属/陶瓷复合板的稳态热应力问题,检验了研究方法的正确性,给出了该材料复合板的稳态热应力场分布(Ta=500K,Tb=1800K)。结果表明,FGM层厚度的增加对ECBC复合板的稳态热应力影响不明显。当M=1时,热应力曲线平缓而光滑;当M=0.1和10时,热应力曲线出现明显的转折点;随着FGM层孔隙率的增大,三层板的衔接界面处,热应力变化增大,曲线出现钝角;与金属/陶瓷二层复合板界面处热应力出现突变相比,夹FGM金属/陶瓷复合板的热应力非常缓和。此结果为该复合板的设计和应用提供了准确的理论计算依据。     

Synthesis and characterization of in-situ reinforced Fe-TiC steel FGMs

This paper presents a novel technique to synthesize TiC-reinforced iron base (Fe-TiC) functionally graded materials (FGM) using SHS reaction followed by centrifugal casting. The in-situ reinforced Fe-TiC steel composites were prepared by aluminothermic SHS reactions using metal oxides and elemental powders. Ferritic as well as austenitic matrix Fe-TiC composites were obtained by judicious thermite charge composition. FGM microstructure was characterized in terms of TiC area fraction, particle size, and chemical analysis using optical and analytical electron microscopy. The results reveal a gradient of TiC particles across the thickness in the inner face and a particle-free region in the outer surface. Hardness profiles have shown an increasing trend from outer surface to TiC-rich inner surface. Wear studies indicated better wear performance of TiC-rich inner face as compared to particle-free outer surface of ferritic or austenitic steel matrices.      

自蔓延高温合成/快速加压法制备(TiB2+Fe)/Fe梯度材料

通过对不同Fe含量的Ti B Fe体系绝热曙度计算了试样温度有限元计算，确定了（TiB2 Fe)/Fe梯度材料各层合理的厚度，并采用SHS／QP技术制备了（TiB2 Fe)/Fe梯度材料。电子探针分析表明（TiB2 Fe)/Fe梯度试样中各层间没有了明显的界面，沿厚度方向由富陶瓷相向富金属相连续转变。背散射电子像分析表明从富陶瓷侧到富金属侧，材料的显微结构也是梯度变化的。SEM分析表明，从富陶瓷层到富金属层，TiB2晶粒减小。     

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     

Extrusion-based additive manufacturing of functionally graded ceramics

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.     

The effect of residual stresses in functionally graded alumina–ZTA composites on their wear and friction behaviour

In this work we have evaluated the effect of compressive stress levels in functionally graded alumina–ZTA composites on their wear and friction behaviour during sliding in water. Neutron diffraction, X-ray diffraction and scanning electron microscopy were employed to analyze the samples and assess the acting tribological mechanisms. The results, which are compared to results from homogeneous alumina, show that with increasing residual compressive stresses in the samples of functionally graded material (FGM) both the wear and the friction are reduced. As a consequence of reduced crack formation and debris detachment from the surface (due to increased residual compressive stress) the tribochemical layer became thinner, with fewer topographical irregularities at the surface. This increases the role of the tribochemical actions compared to the mechanical wear, which beneficially affects the tribological performance in water.     

Unique performance of thermal barrier coatings made of yttria-stabilized zirconia at extreme temperatures (>1500°C)

Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200°C. Above this temperature the deposited metastable tetragonal (t´) phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500°C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates.     

Thermal shock and thermal fatigue resistance of Al2O3/(W,Ti)C/TiN/Mo/Ni multidimensional graded ceramics

Al₂O₃/(W, Ti)C/TiN/Mo/Ni multidimensional graded ceramics and homogeneous reference ceramic were prepared by two step hot press sintering. The thermal shock and thermal fatigue resistance of the multidimensional graded ceramics were tested using the water quenching method. Scanning electron microscopy (SEM) and optical microscope were used to investigate microscopic failure mechanism of ceramics. The results showed that the retained flexural strength of two-dimensional and one-dimensional graded ceramics was almost same, but higher than that of the homogeneous ceramic. The crack growth (∆c) of homogeneous ceramic increased rapidly, while that of two-dimensional graded ceramics is the lowest. Hence, thermal fatigue resistance of the two-dimensional graded ceramics was highest. The residual compressive stress in the first layer induced by the optimal graded structure played an important role. In addition, the increasing toughness on the crack propagation path by adding different amounts of metals was also a contributing factor.     

分形论在无机材料研究中的应用

介绍了分形论在无机材料表面研究、无机薄膜材料、梯度功能材料及无机材料断裂研究等方面的应用情况,并展望了其在玻搪材料中的应用前景.