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.     

Thermomechanical Analysis of Functionally Graded Thermal Barrier Coatings with Different Microstructural Scales

The recently developed two-dimensional version of the higher-order theory for functionally graded materials (denoted as HOTFGM-2D in previous communications) has been used to investigate the effects of microstructural architectures in graded thermal barrier coatings (TBCs) on stress distributions in the presence of a through-thickness temperature gradient. In particular, the response of TBCs with different levels of functionally graded microstructural refinement and different arrangements has been investigated, and the results for the through-thickness stress distributions are compared with those based on the standard micromechanical homogenization scheme. The examples presented illustrate the shortcomings of the standard micromechanics-based approach that is applied to the analysis of functionally graded TBCs, particularly if the presence of creep effects is included in the analysis.     

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.     

Oxidation-Based Model for Thermal Barrier Coating Life

A procedure is described for modeling the lives of thermal barrier coatings subjected to high-temperature environments. The model is used to calculate cycles-to-failure as a function of heating cycle duration. It is based on the presumption that oxidation is the single important time-dependent factor which limits the life of these coatings, and that oxidation-induced strains combine with cyclic strains to promote slow crack growth in the ceramic layer. Good agreement is obtained between calculated and experimental lives for specimens tested in a furnace. This shows that an oxidation-based approach is promising. The importance of reproducible specimen preparation is also discussed.     

Nickel–Alumina Functionally Graded Materials by Electrophoretic Deposition

Electrophoretic deposition has been used to synthesize nickel–alumina, functionally graded materials from NiO and alumina suspensions in ethanol. Suspension stability and the kinetics of deposition were studied to determine optimum conditions. Deposition starts with an alumina suspension into which a stream of NiO suspension is injected at various flow rates to obtain the desired profiles. The latter were controlled by varying the deposition current density and component flow rate. The green bodies obtained were sintered in Ar/H₂ atmosphere to reduce the NiO to nickel. Various gradation profiles were obtained illustrating the facility of EPD to synthesize continuously graded materials. NiO was used as the precursor for nickel to alleviate settling and rough columnar deposit problems.     

Functionally Graded Zircon–Molybdenum Materials without Residual Stresses

A layered, zircon–molybdenum functionally graded material was obtained by starting from commercial powders of molybdenum (median particle size ( d ₅₀) of ∼3 μm) and zircon ( d ₅₀∼ 0.8 μm). Conventional processing led to a material that was free of internal residual stress. The sintering behavior of green compacts with compositions that corresponded to the different layers was studied via dynamic sintering. The thermal expansion coefficients of each layer were measured using conventional dilatometry. The presence of residual stresses was determined using Vickers indentations. Both thermal expansion mismatch and differential shrinkage between the layers were negligible.     

Production of Functionally Graded Materials from Electrochemically Modified Carbon Preforms

A new process based on carbon as a precursor material has been applied successfully to the production of graded Al₂O₃/Al composites. For this purpose, distinct porosity gradients were introduced into carbon materials via anodic oxidation. The graded structure of the carbon preforms was replicated using two subsequent infiltration steps to yield fully dense Al₂O₃/Al functionally graded materials with an interpenetrating network microstructure and a continuous variation of metal content.     

Noncontact Methods for Measuring Thermal Barrier Coating Temperatures

Three noncontact, optical methods for measuring temperature are reviewed with an emphasis on their application to the measurement of temperatures of thermal barrier coatings (TBCs). The methods are: infrared pyrometry, Raman spectroscopy, and photo-stimulated luminescence from lanthanide-doped coatings. Although each has the capability of measuring temperatures pertinent to monitoring TBCs, the finite thickness of typical coatings together with the optical properties of zirconia place severe restrictions on the depth from which the temperature sensing can be obtained. Some of these limitations can be circumvented using photo-stimulated luminescence with coatings containing dopants at specific locations. To illustrate this, it is demonstrated that by depositing coatings with a lanthanide dopant, such as Eu³⁺, at specific locations, for instance in contact with the metallic alloy, temperature sensing can be performed with much higher spatial resolution.     

Ti/Al2O3系梯度功能材料研究动态

介绍了目前用于Ti/Al2O3系梯度功能材料制备的几种主要工艺,提出了利用共沉降法制备Ti/Al2O3系梯度功能材料的设想,简述了Ti/Al2O3复合材料制备技术的基础理论及其研究现状,并指出了其不足和未来的发展方向.     

Advanced Porous Coating for Low-Density Ceramic Insulation Materials

The need for improved coatings on low-density reusable surface insulation (RSI) materials used on the space shuttle has stimulated research into developing tougher coatings. The processing of a new porous composite "coating" for RSI called toughened unipiece fibrous insulation is diseassed. Characteristics including performance in a simulated high-speed atmospheric entry, morphological structure before and after this exposure, resistance to impact, and thermal response to a typical heat pulse are described. It is shown that this coating has improved impact resistance while maintaining optical and thermal properties comparable to the previously available reaction-cured glass coating.     

Development of Advanced Low Conductivity Thermal Barrier Coatings

Advanced multi-component, low-conductivity oxide thermal barrier coatings have been developed using an approach that emphasizes real-time monitoring of thermal conductivity under conditions that are engine-like in terms of temperatures and heat fluxes. This is in contrast to the traditional approach where coatings are initially optimized in terms of furnace and burner rig durability with subsequent measurement in the as-processed or furnace-sintered condition. The present work establishes a laser high-heat-flux test as the basis for evaluating advanced plasma-sprayed and electron beam-physical vapor deposited (EBPVD) thermal barrier coatings under the NASA Ultra-Efficient Engine Technology (UEET) Program. The candidate coating materials for this program are novel thermal barrier coatings that are found to have significantly reduced thermal conductivities and improved thermal stability due to an oxide defect-cluster design. Critical issues for designing advanced low-conductivity coatings with improved coating durability are also discussed.     

颗粒共沉降方法消除梯度材料内部界面的可能性分析

本文旨在寻求一种结构控制手段,实现梯度材料在组成成份上连续分布.从颗粒共沉降的立场出发,建立了A/B系FGM沉降模型,并从理论上对用共沉降法制备A/B系FGM时,材料内部可能出现的界面情况进行了分析,提出了解决方案,为用共沉降法制备组成成份连续或准连续分布的FGM提供理论依据.     

水性隔热保温节能涂料的制备研究

以丙烯酸弹性乳液为基料,空心玻璃微珠、金红石型钛白粉、云母及空心硼硅酸盐等为颜填料制备隔热保温涂料,该涂料性能稳定、表面光滑、涂层薄,耐沾污、热反射效率高,隔热性能良好,适应范围广。     

Modelling of Functionally Graded Adhesive Joints

Nowadays, there is a strong trend towards the use of functionally graded materials, with particular importance for the functionally graded joints. The main objective of this work was to study a functionally modified adhesive in order to have mechanical properties that vary gradually along the overlap of a joint, allowing a uniform stress distribution along the overlap. This allows for a stronger and more efficient adhesive joint and would permit to work with much smaller areas, reducing considerably the weight of the structure which is a key factor in the transport industry. In the proposed joint, the adhesive stiffness varies along the overlap, being maximum in the middle and minimum at the ends of the overlap. The functionally graded joint was found to have a higher joint strength compared to the cases where the adhesive has homogeneous properties along the overlap. A simple analytical model to study the performance of the functionally graded joints was developed. The differential equation of this model was solved by a power series. Numerical modelling by finite element analysis was performed to validate the analytical model developed.     

应用于功能性热流体的相变微胶囊的制备与表征

相变材料微胶囊以其特有的蓄热性能在热能存储领域引起了人们的广泛关注。以异佛尔酮二异氰酸酯和四乙撑五胺为壁材原料,以相变点为20℃的石蜡为芯材,以非离子表面活性剂span60和tween60为乳化剂,利用界面聚合法制备了应用于功能热流体的相变微胶囊,分别用相差显微镜和差示扫描量热分析仪测定了微胶囊的形貌和热性能,结果表明所得微胶囊粒径分布均匀,其直径约为1.7μm。芯材含量约为92.4%,颗粒均匀,相变热124.47 J/g。相变微胶囊热稳定性高,具有好的致密性和不溶胀性。     

Observation of Subcritical Spall Propagation of a Thermal Barrier Coating

Observations are reported of the room-temperature propagation of a spalling failure mode of a thermal barrier coating (TBC) from its bond coat after oxidation. The coating is a Y₂O₃-stabilized ZrO₂ coating formed by electron-beam deposition on a Ni-Co-Cr-Al-Y bond coat. The spall shape evolution and stress redistribution as the spall propagates are reported. The failure propagates primarily as an interface crack between the bond coat and the thermally grown aluminum oxide (TGO) formed on the underside of the TBC during oxidation. The observations are consistent with subcritical propagation of an interface crack between the TGO and bond coat assisted by the presence of moisture. An estimate of 9 J/m² is made of the fracture resistance in air of the interface.