Concept of Functionally Graded Materials for Advanced Thermal Barrier Coating Applications

This feature article explores the concept of creating functionally graded metal-ceramic composite microstructures for thermal barrier coatings used in gas-turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal-ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal-ceramic microstructures for realistic application environments.     

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.     

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.     

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.     

Structural Integrity in Severe Thermal Environments

The theory of structural stability of ceramics in severe thermal environments was explored via the use of precracking. It was shown that structural stability should be possible when an array of precracks is used, but that the precracks required for this purpose are relatively large. The approach is thus limited to systems which are not required to sustain a significant mechanical stress. Preliminary quenching experiments on short cylinders confirmed that stabilization can be achieved by precracking.     

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.     

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.     

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.     

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.     

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.     

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.     

Preparation of Polymethylmethacrylate-Reinforced Functionally Graded Hydroxyapatite Composites

A functionally graded hybridization approach has been used in the formation of polymer-ceramic composites of polymethylmethacrylate (PMMA) and hydroxyapatite (HAp). HAp was successfully reinforced by sedimentary HAp distributions on a PMMA matrix using a centrifuge to avoid stress convergence on the interface. The stress-strain curves of the functionally graded PMMA-HAp composite showed sufficient mechanical strength with reduced brittleness. Scanning electron micrographs also showed evidence of exposed HAp on the surface of the composites.     

Functionally Graded Polyurethane/Cellulose Nanocrystal Composites

The preparation and investigation of functionally graded polymer nanocomposites, which have a concentration gradient of cellulose nanocrystals (CNCs) along one direction, is reported here. As a test bed, a series of nanocomposites consisting of a thermoplastic polyurethane (PU) and 0–15% w/w CNCs is prepared via solvent casting and the mechanical properties of films of these materials are characterized by dynamic mechanical analyses and tensile tests. The formation of graded materials is accomplished by lamination of films with varying CNC content. The processing conditions are optimized to achieve intimate fusion of the individual layers. The elimination of internal interfaces is evidenced by an elongation at break of up to 500%. In order to explore potential applications of graded PU/CNC nanocomposites, structure‐dependent actuation in response to water is demonstrated in a bioinspired architecture. In addition, the damping behavior of cylindrical shaped composites is investigated by way of compression tests. The results show that functionally graded PU/CNC composites show good damping behavior over a much larger range of forces than the neat PU or the homogeneous nanocomposites.     

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.     

Thermal Residual Stresses in Adhesively Bonded In-plane Functionally Graded Clamped Plates Subjected to an Edge Heat Flux

In this study we have carried out the thermal residual stress analyses of adhesively bonded functionally graded clamped plates for different edge heat fluxes. The material properties of the functionally graded plates were assumed to vary with a power law along an in-plane direction not through the plate thickness direction. The transient heat conduction and Navier equations describing the two-dimensional thermo-elastic problem were discretized using the finite-difference method, and the set of linear equations was solved using the pseudo singular value method. The plate material properties near the interfaces played an important role in the interfacial adhesive stresses. The compositional gradient affected considerably both in-plane temperature distributions and heat transfer periods. The type of in-plane heat flux had only a minor effect on the temperature profiles but affected both the temperature levels and heat transfer period. Both plates undergo considerable compressive normal strains and stresses, but shear strains were more effective. Peak equivalent strains were observed for a constant heat flux and plates with a metal-rich composition. The compositional gradient and direction played important role in the profiles and levels of normal, shear and equivalent stresses as well as strains. The equivalent stress and strains concentrated along the free edges of the adhesive layer. The adhesive layer experienced a considerable distortional deformation rather than volumetric deformation. The equivalent stress exhibited small changes through the adhesive thickness and along the overlap length. The equivalent stress remained uniform in a large region of the overlap length and increased to a peak level around the free edges of the first plate–adhesive interface, whereas it increased to a peak level in a large region of the overlap length from a minimum level around the free edges of the second plate–adhesive interface. The strains and equivalent strains were higher for a metal-rich material composition. The direction of the material composition of the plates affected both stress and strain levels; thus, the CM–CM and CM–MC plates exhibited lower strain and stress levels than those in the MC–CM and MC–MC plates. However, only the adhesively bonded CM–MC plate configuration could achieve the lowest deformations and stresses in both plates and adhesive layer.     

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。     

Ni／Ni3Al—TiC系梯度功能材料的热应力缓和特性设计

本文选择Ｎｉ／ＮｉＡｌ－ＴｉＣ体系的ＦＧＭ，对其在制备过程中的残余絷应力进行了计算机有限元模拟。在综合考虑热应力最小，应力强度比值最小以及纯ＴｉＣ侧应力状态等因素的基础上，完成了ＦＧＭ体系的热应力缓和特性设计，得到组成分布指数Ｐ＝１．６的最佳设计结果。     

Fabrication of Functionally Graded SiAlON Ceramics by Tape Casting

Functionally graded SiAlON ceramics (FG-SiAlONs) were successfully prepared by the tape casting and lamination approach. Non-aqueous SiAlON slurries with five different α to β-SiAlON ratios (85α:15β, 70α:30β, 55α:45β, 40α:60β, and 25α:75β) were prepared by a 66 methyl ethyl ketone/34 ethanol (vol%) mixture. Phase and microstructure analyses incorporation with hardness measurements clearly show that the FG-SiAlONs, prepared by tape casting, exhibit a continuous and gradual change in composition and hardness. Thus, the tape casting approach is a viable method to produce FG-SiAlONs with a precisely controlled composition, and subsequently properties, as a function of position.