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。     

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.     

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.     

Calculation of the Composition Profile of a Functionally Graded Material Produced by Centrifugal Casting

Functionally graded materials have designed inhomogeneous distributions of different components on the scale of the material. They can be made by suspension processing, in which particles are stacked in a controlled manner. Segregation effects can be used to obtain the required gradient if the particles in suspension have different velocities. A model is derived for the composition profiles that develop as a result of these effects. The model can be used to determine the most suitable process conditions. The outcome of the model shows fair agreement with experiments found in the literature in which an Al₂O₃/ZrO₂ composite has been produced by uniaxial centrifugal casting.     

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。     

Fabrication and Properties of Tape-Cast Laminated and Functionally Gradient Alumina–Titanium Carbide Materials

A tape-casting process was used to prepare various Al₂O₃–TiC green tapes, from which laminated and functionally gradient Al₂O₃–TiC materials (FGM Al₂O₃–TiC) were produced by cutting, stacking, and laminating the material, removing the binder, and hot-pressing the green bodies. The bending strength of the FGM Al₂O₃–TiC composites was not much less than that of the laminated Al₂O₃–TiC composites. However, the fracture toughness was >50% higher; the fracture toughness of the FGM composite sintered at 1700°C was 9.43 MPa·m^1/2. This increased toughness was attributed to the stress distribution that was caused by variations in the composition of the FGM composite layers. The present results demonstrate that the FGM design is a useful method for modifying the mechanical properties of ceramic composites.     

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.     

功能梯度材料及其复合电镀制备现状与进展

综述了功能梯度材料的概念、性能、研究动态及最新进展。总结了功能梯度材料的各种制备方法。重点探讨了采用复合电镀技术制备功能梯度材料的方法。该方法成本低,易于操作,所得镀层孔隙率低,结合力好,耐磨,耐蚀性好,具有广阔的应用前景。     

对称型梯度功能陶瓷材料的抗热震参数

采用摄动法推导出第三类边界条件下无限大对称型梯度功能材料(functionally gradient materials，FGM)平板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，通过求解无限大对称型梯度功能陶瓷平板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，并以此作为对称型梯度功能陶瓷的抗热震参数。通过计算实例并与均质陶瓷材料对比，分析了材料的热—物理性能分布规律对其抗热震性的影响。对高抗热震性复合陶瓷材料的研究开发具有实际意义。     

Drying of Ceramic Layers with a Graded Pore Structure

Thin self-supporting ceramic TiO2-layers with a graded pore structure were prepared by using centrifugal deposition of powders and sols with different particle size distributions from mixed, diluted suspensions. During the evaporation drying step the layers have a strong tendency to warping and crack-formation because of the resulting difference in the capillary pressure in the upper and the bottom side pores. Four drying methods were investigated concerning their suitability for diminishing or eliminating capillary forces and for the production of planar, crack-free dried specimens. The drying techniques used in the experiments are briefly introduced. It should be emphasized that the most successful drying method for the layers mentioned above is a combination of microwave drying and subsequent critical point drying.     

Thermal Shock Behavior of Isotropic and Anisotropic Porous Silicon Nitride

The thermal shock behavior of isotropic and anisotropic porous Si₃N₄ was evaluated using the water-quenching technique. The critical temperature difference for crack initiation was found to be strongly dependent on the ratio of fracture strength to elastic modulus. Because of a very high strain-to-failure, anisotropic porous Si₃N₄ showed no macroscopic cracks and was able to retain its strength even at a quenching-temperature difference of ∼1400°C.     

Thermal Shock Behavior of Iron-Particle-Toughened Alumina

The thermal shock behavior of an alumina monolith and two alumina–iron ceramic-matrix composites has been investigated by superimposing the measured K _R-curves of the materials onto the theoretically generated curves of the thermally induced stress intensity factor. Predictions of the critical-temperature differentials and retained strengths after quenching are in good agreement with the experimental data. The inclusion of metallic particles into an alumina matrix improves the thermal shock resistance, although the increase in toughness is not solely responsible for this improvement. There is a decrease in thermal stress-intensity factor that is generated for the composites; this decrease is due to a reduction in the Young's modulus and/or Biot modulus. However, the increased toughness for large crack lengths may offer increased damage resistance for severe thermal shock treatments.     

Investigation of Thermal Shock in a High-Temperature Refractory Ceramic: A Fracture Mechanics Approach

The results of a study on the thermal shock behavior of a high-temperature refractory ceramic that is used as a furnace liner in the melting of steels are presented in this paper. The experimental studies show that thermal shock damage initiates by edge cracking after the first shock cycle. Subsequent subcritical crack growth occurs by the incremental extension of dominant cracks until catastrophic failure occurs. The observations of the crack profiles also reveal the formation of viscoelastic bridges that promote crack-tip shielding/toughening via crack bridging. Following a brief discussion of the respective mechanisms of fracture and thermal shock damage at different temperatures and temperature ranges, the implications of the results are discussed for refractory ceramics that are toughened by viscoelastic crack bridging.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Characterization of anisotropic gas permeability and thermomechanical properties of highly textured porous alumina

Porous alumina with a highly textured microstructure was fabricated by pulse electric current sintering (PECS) using alumina platelets. Highly oriented porous alumina with a porosity of 3%–50% was obtained by a pressure-controlled method of PECS. The properties of the highly textured porous alumina were measured in two directions. The nitrogen gas permeance and thermal conductivity at room temperature were higher in the direction along the platelet length due to the higher continuity of pores and the connectivity of alumina platelets, respectively. The anisotropy of the thermal conductivity at room temperature was investigated and explained by the effect of grain size of platelets as well as morphology and orientation of pores. The bending strength was higher with the loading direction along the platelet thickness. The thermal shock strength was clearly different in the two directions. The difference in the thermal shock strength was investigated by the measurement of properties and thermal stress analysis.     

Synthesis and thermal shock evaluation of porous SiC ceramic foams for solar thermal applications

Reticulated porous ceramics with structural features spanning across multiple length scales are emerging as the primary media in a variety of demanding mass and heat transfer applications, most notably solar-assisted synthetic fuel processing. In this study, we focus on engineering of the open pore silicon carbide (SiC)-based foams in such catalytic applications. We evaluate the mechanical integrity and thermal stability of these porous structures. X-ray tomography analyses of the 3D structures reveal the presence of dual pore size distribution different by up to an order of magnitude in length scale. We further study the effect of thermal shock—induced via water quenching—on the SiC structures and we conclude that the mechanical properties of the ceramic foams are significantly reduced after thermal stress. Comparison of SEM micrographs—before and after thermal shock—reveals that needle-like features appear inside the foam matrix. These elongated defects may be responsible for structural and mechanical weakening.     

Influence of a Dispersion of Aluminum Titanate Particles of Controlled Size on the Thermal Shock Resistance of Alumina

The possibility of developing fine-grained (∼0.5–3 μm) and dense (≥0.98ρ_th) alumina (90 vol%)–aluminum titanate (10 vol%) composites with improved thermal shock resistance and maintained strength is investigated. One alumina material and one composite with similar microstructures (porosity and grain-size distribution) were fabricated to investigate the effect of Al₂TiO₅ on thermal shock behavior. The size of the Al₂TiO₅ particles was kept under 2.2 μm to avoid spontaneous microcracking. The mechanical and thermal properties of the materials involved in their response to thermal shock and the results for the evolution of indentation cracks of equal initial crack length with increasing Δ T in samples quenched in glycerine are described. The combination of thermal and mechanical properties—thermal conductivity, thermal expansion coefficient, Young's modulus, and toughness—improve the thermal shock resistance of the alumina–aluminum titanate composite in terms of critical temperature increment (>30%). The suitable structural properties of alumina—hardness and strength—are maintained.     

The Stress Intensity Factor in Bonded Quarter Planes after a Change in Temperature

High stresses can occur in bonded dissimilar materials after a change in temperature in the vicinity of the intersection of the interface and the free edge. These stresses depend on the thermal expansion and on the elastic constants of the two materials. In bonded quarter planes the stresses near the intersection of the interface and the free edge can be described by the sum of one singular term and one regular term which is independent of the distance to the singular point. With the exception of the stress intensity factor of the singular term, all parameters can be calculated analytically. The stress intensity factor was evaluated numerically using the finite element method. Joints with different ratios of height to length and various material combinations were investigated. An empirical relationship between the stress intensity factor, the elastic constants and the ratios of height to length of the joint is given by exponential and polynomial equations.     

Thermal shock resistance of the porous boron nitride/silicon oxynitride ceramic composites

The thermal shock resistance of the porous boron nitride/silicon oxynitride (BN/Si₂N₂O) ceramic composites were tested by the quenching‐strength method with temperature differences of 600‐1400°C. The residual flexural strength of the composites decreased with increasing temperature difference from 600°C to 900°C. This weakening in flexural strength was attributed to the formation of microcracks in the matrix caused by thermal stress damage. Afterward, as the formation of a dense oxidized layer sealed the surface and hindered further oxidation, the residual flexural strength increased with the further increase of temperature difference from 900°C to 1100°C. Finally, when the temperature differences were above 1100°C, the residual flexural strength gradually decreased with increasing temperature difference, which was attributed to the further oxidation and large thermal stress damage. And the thermal shock resistance of the porous BN/Si₂N₂O ceramic can be improved by the introduction of high contents of sintering aids and h‐BN.