陶瓷/金属功能梯度热障材料研究进展

梯度层中金属相在高温环境下的氧化是导致陶瓷／金属功能梯度热障材料失效的主要原因,限制了功能梯度热障材料的应用,叙述了陶瓷／金属功能梯度热障材料的研究进展,并讨论了其发展前景。     

航空发动机用热障涂层陶瓷材料的发展现状及展望

热障涂层具有低热导率、高稳定性、高热膨胀系数等特点,是航空发动机高温部件热防护的重要手段,因此对热障涂层隔热性能和使用寿命的研究一直是工程应用领域的一个重要内容.国内外学者对热障涂层材料的结构、材料体系以及失效和损伤行为进行了广泛的研究.本文综述了热障涂层材料的国内外研究现状,并对其材料结构、材料体系及失效和损伤行为三个方面在不同影响因素下的差异进行了归纳和总结,同时对其今后的发展趋势进行了展望.     

航天器热防护材料研究现状与发展趋势

热防护系统中所采用的多层复合热防护材料的层间界面结合和小块材料之间的连接对航天器的可靠性有很大影响,目前二者都存在一定的缺陷.依据功能梯度材料和C/C复合材料的理论,将高导热率碳泡沫和低导热率碳微球设计成密度和热导率功能梯度热防护碳泡沫材料,使其具备组分之间无层间界面和小块材料间易于连接等特点.     

稀土氧化物掺杂对YSZ热障涂层热物理性能影响的研究进展

随着航空发动机和燃气轮机(简称“两机”)服役温度的升高,目前,在两机热端部件表面防护方面应用最为广泛的热障涂层(Thermal barrier coatings, TBCs)存在陶瓷层材料氧化钇稳定氧化锆(Yttria-stabilized zirconia, YSZ)在高温下会发生相转变、热膨胀系数与金属基底不匹配以及烧结导致涂层的热导率升高等问题,严重影响TBCs的服役寿命。新一代TBCs陶瓷面层材料分为以下几类：(1)稀土氧化物稳定YSZ;(2)钙钛矿结构陶瓷材料;(3)稀土六铝酸盐或稀土钽酸盐;(4)烧绿石或萤石结构稀土锆酸盐。其中,稀土氧化物掺杂可有效降低YSZ热障涂层的热导率,提高其热膨胀系数、高温相稳定性及耐烧结性能,被认为是提高YSZ热障涂层高温稳定性的有效方法。基于此,本文重点阐述了单元或多元稀土氧化物掺杂YSZ热障涂层材料的研究进展,讨论了稀土氧化物掺杂对YSZ陶瓷面层高温相稳定性、热导率和热膨胀系数的影响机理。基于耦合作用机理为未来稀土氧化物掺杂YSZ热障涂层的研发提供一定的借鉴。     

陶瓷热障涂层的热导率和热扩散率测量

提出了应用3ω法进行等离子喷涂热障涂层材料的热导率和热扩散率测量的方法。测试了室温下2种典型的热障涂层材料Y2SiO5和La2Zr2O7的热导率和热扩散率,测试结果与文献中的结果吻合良好。实验中对不同孔隙率的样品的热导率在室温附近的温度区间内进行测试,结果表明,孔隙率的变化对热导率有明显的影响。另外,孔隙率对热扩散率有双向的影响,即存在某一孔隙率值使得涂层样品的热扩散率最大。     

热障涂层热导率的研究进展

简要回顾了热障涂层体系的发展,讨论了氧化锆陶瓷材料的传热规律,包括涂层微观结构、陶瓷成分等因素的影响.同时指出了改进陶瓷涂层热导率的方法和开发适用于更高温度下的陶瓷涂层材料的指导原则,并详细介绍了改善热障涂层热导率的研究现状.     

应用于航空发动机涡轮叶片的热障涂层材料研究

热障涂层材料(TBC)能够应用于航空发动机涡轮叶片等许多尖端领域,这种材料是一种能够缓冲外界热量进入表面合金的低热导率材料。研究表明,空气是一种近乎最低的低热导率材料,所以在热障涂层材料中加入空气能够有效提高热量缓冲作用。将采用感应耦合等离子体(ICP)深沟刻蚀硅的表面,在金属钛表面电解氧化形成多孔的氧化层薄膜以及泡沫铜、泡沫镍的多孔结构来系统阐述这一理论的可行性。使用了扫描电镜(SEM)、原子力显微镜(AFM)和热导率测量仪对微观形貌和热导率进行了表征测量,并利用计算机仿真对多孔结构进行了进一步的热传导性能分析。     

热障涂层的研究进展与发展趋势

热障涂层一般由金属粘结层和具有低热导率的陶瓷顶层组成,应用于涡轮发动机的热端部件可显著提高其使用温度,延长部件的使用寿命,提高发动机的效率.综述了热障涂层的成分选择、制备方法及等离子喷涂和电子束物理气相沉积2种热障涂层的典型结构,分析了热障涂层的剥落失效机理,并简单介绍了热障涂层的寿命预测模型和隔热特性的研究.     

稀土钽酸盐陶瓷热障涂层的研究进展

热障涂层材料是高效燃气轮机稳定工作最重要的材料之一。在燃气轮机使用中不仅能达到抗腐蚀、提高工作温度的目的,还可以减少燃油消耗,延长发动机使用寿命等。目前广泛使用的热障涂层材料是氧化钇稳定氧化锆(YSZ),其具有一定局限性,在高温下YSZ发生相变体积变化致使涂层失效,因此使用温度在1200℃以下,发展新型热障涂层材料势在必行。现有的其他类型热障涂层材料虽然在热导率或热膨胀系数等方面优于氧化钇稳定氧化锆,但后者具有优异的力学性能,这是由其铁弹性决定的。而稀土钽酸盐作为一种新的铁弹体陶瓷热障涂层材料,除了其使用温度可达1600℃外,兼具有优异的高温相稳定性及力学性能、相变前后体积变化小等优点,更重要的是其热导率低于YSZ及其它的热障涂层候选材料,这使得稀土钽酸盐陶瓷成为一种新型的热障涂层候选材料。     

面向21世纪的热障涂层结构设计

为适应涡轮发动机不断增长的热效率需求,迫切希望改进热障涂层系统在苛刻使用条件下的结构完整性和工作寿命,这种改进可借助于热障涂层的结构设计来民世纪热障涂层结构设计的需求方向,在分析和综合材料科学与热障涂层发展的相互关系的基础上,归纳了当今热障涂层结构设计的４种基本型式：（１）定向凝固高温合金型;（２）颗粒增强复合材料型;（３）梯度功能材料型;（４）多层膜材料型。     

Effective thermal and elastic properties of [+θ/−θ]s laminates

This paper presents a set of explicit analytical formulae, some of which have already appeared in the literature, that enable many of the effective elastic, thermal expansion and thermal conductivity properties for unidirectional composites to be calculated in terms of the fibre and matrix properties, and the fibre volume fraction. These properties are required as input values for another set of explicit analytical formulae that are presented, enabling the estimation of the effective elastic, thermal expansion and thermal conductivity properties of balanced symmetric angle-ply laminates. Effective through-thickness properties are calculated in addition to the in-plane properties so that a set of consistent values can be applied to finite element and boundary element analyses of structural elements involving the use of angle-ply laminates, provided that the ply layers in a structure are replaced by the calculated homogenised properties.Examples are given of the application of the various formulae to the prediction of the effective elastic and thermal properties of both CFRP and GRP angle ply laminates for various ply angles, using prescribed representative values of fibre and matrix properties. Zero-expansion laminates and the properties of [±45]_s laminates are considered as special cases of interest.     

PROCESSING ASPECTS OF SHAPING ADVANCED MATERIALS BY ELECTRICAL DISCHARGE MACHINING

The principles of applying electrical discharge in die-sinking and wire-cutting machines are reviewed. This technique causes material to be eroded by switching electrical current on and off between the tool and the work-piece through a dielectric fluid. For the case of metals and many ceramics and ceramic composites, the mechanism for this erosion is melting or perhaps evaporation/condensation. For refractory materials, a new mechanism for erosion, thermal spalling, was observed. The role of the dielectric fluid, its purity, and the proper nitration system are discussed. The effect of various operating conditions, including the current, pulse time duration, and wire feed rate, on the material removal rate and the surface quality is also reviewed. It was seen that EDM can be applied to machining advanced materials, including single phases and composites of ceramic/ceramic and ceramic/metal, if a minimum electrical conductivity is met.     

Mg-Si-Sn基热电器件电极材料的优化选择与连接工艺

研究了采用不同放电等离子烧结(SPS)工艺获得的单质金属(Ni、Cu、Ag、Al)电极与Mg-Si-Sn基热电材料结合界面的微观形貌和成分分布特征, 测试了合金(Ni-Al、Cu-Al)、金属/合金复合电极材料的热膨胀系数、电导率和热导率等物性参数。实验结果表明: 通过SPS烧结可以有效实现电极材料与Mg-Si-Sn基材料的连接, 复合电极材料Ni-Al/Al(60:40)和Cu-Al/Cu(45:55)具有高的电导率和热导率, 并且热膨胀系数与Mg-Si-Sn基热电材料相匹配, 有可能成为Mg-Si-Sn基材料的较理想电极材料。     

Scattering of Thermal Waves and Non-steady Effective Thermal Conductivity of Unidirectional Fibrous Composites with Functionally Graded Interface

In this paper, a thermal wave method is applied to investigate the non-steady effective thermal conductivity of unidirectional fibrous composites with a functionally graded interface, and the analytical solution of the problem is obtained. The Fourier heat conduction law is applied to analyze the propagation of thermal waves in the fibrous composite. The scattering and refraction of thermal waves by a cylindrical fiber with an inhomogeneous interface layer in the matrix are analyzed, and the results of the single scattering problem are applied to the composite medium. The wave fields in different material layers are expressed by using the wave function expansion method, and the expanded mode coefficients are determined by satisfying the boundary conditions of the layers. The theory of Waterman and Truell is employed to obtain the effective propagating wave number and the non-steady effective thermal conductivity of composites. As an example, the effects of a graded interface on the effective thermal conductivity of composites are graphically illustrated and analyzed. Analysis shows that the non-steady effective thermal conductivity under higher frequencies is quite different from the steady thermal conductivity. In the region of intermediate and high frequencies, the effect of the properties of the interface on the effective thermal conductivity is greater. Comparisons with the steady thermal conductivity obtained from other methods are also presented.     

Comparative analysis of steady state heat transfer in a TBC and functionally graded air cooled gas turbine blade

Internal cooling passages and thermal barrier coatings (TBCs) are presently used to control metal temperatures in gas turbine blades. Functionally graded materials (FGMs), which are typically mixtures of ceramic and metal, have been proposed for use in turbine blades because they possess smooth property gradients thereby rendering them more durable under thermal loads. In the present work, a functionally graded model of an air-cooled turbine blade with airfoil geometry conforming to the NACA0012 is developed which is then used in a finite element algorithm to obtain a non-linear steady state solution to the heat equation for the blade under convection and radiation boundary conditions. The effects of external gas temperature, coolant temperature, surface emissivity changes and different average ceramic/metal content of the blade on the temperature distributions are examined. Simulations are also carried out to compare cooling effectiveness of functionally graded blades with that of blades having TBC. The results highlight the effect of including radiation in the simulation and also indicate that external gas temperature influences the blade heat transfer more strongly. It is also seen that graded blades with about 70% ceramic content can deliver better cooling effectiveness than conventional blades with TBC.     

Investigation of elastic stresses in an adhesively bonded single lap joint with functionally graded adherends in tension

In this study three-dimensional elastic stress state of an adhesively bonded single lap joint with functionally graded adherends in tension was investigated. The adherends compose of a functionally gradient layer between a pure ceramic (Al₂O₃) layer and a pure metal (Ni) layer. Stress concentrations are observed along the free edges of the adhesive layer and through the corresponding zones in the upper and lower adherends. The adhesive layer experiences stress concentrations along the left and right free edges in the horizontal plane, and the normal stresses and the shear stress σ_xy are critical. Whereas the middle overlap region has a uniform low stress distribution the zones in the upper adherend corresponding to the left free edge of the adhesive layer and the zones in the lower adherend corresponding to the right free edge of the adhesive layer are subjected to higher stresses. The normal stress σ_xx among the normal stresses and the shear stress σ_xy among the shear stresses are dominant in both upper and lower adherends. The normal stress σ_xx changes uniformly from compression in the ceramic layer to tension in the metal layer through the upper plate-thickness and from tension in the ceramic layer to compression in the metal layer through the lower plate-thickness. In the adhesive layer, the normal stress σ_yy becomes peak at the left free edge of the upper adherend–adhesive interface and at the right free edge of the lower adherend–adhesive interface and then decreases uniformly across the adhesive layer towards the other adherend–adhesive interface. The functionally gradient region across the adherend thickness was modelled using the layers with the mechanical properties calculated based on the power law. However, a layer number larger than 20 has a minor effect on the through-thickness profiles and magnitudes of von Mises and normal stresses in both the adherends and the adhesive. In addition, increasing the ceramic phase in the material composition (compositional gradient exponent n) of the functionally gradient region does not affect the through-thickness profiles of von Mises and normal stresses in the adherends and adhesive whereas their magnitudes in the ceramic rich layer of both adherends and along the adherend–adhesive interfaces increase considerably. On the contrary, the layer number and compositional gradient exponent have an evident effect on the through-thickness profiles and magnitudes of the critical stress components in the adherends and adhesive layer of the functionally graded adhesively bonded joints.     

纳米氮化硼增强金属基复合材料的研究进展

在金属中添加陶瓷增强相是调控和改善金属材料结构和性能的重要途径。传统硬质陶瓷增强相难以满足金属材料日益严苛的应用需求。以氮化硼纳米片（boron nitride nanosheet,BNNS）和氮化硼纳米管（boron nitridenanotube,BNNT）为代表的纳米氮化硼具有极大的比表面积和优异的力学性能、热稳定性、化学稳定性等,是制备性能优异的金属基复合材料的理想增强相。系统总结了纳米氮化硼的种类和特征,综述了纳米氮化硼增强金属基复合材料的制备方法,归纳了纳米氮化硼增强Cu、Al、Ti复合材料的研究成果,总结了纳米氮化硼/金属复合材料的力学和摩擦学性能,并揭示了复合材料性能改善的机理。最后,展望了纳米氮化硼/金属复合材料的发展趋势。     

组份成分分布规律对功能梯度防护装甲动态响应的影响

利用数值模拟方法研究了在冲击载荷作用下组份成分对称分布的功能梯度板的动态响应。梯度板材料为陶瓷颗粒增强的铝基复合材料(MMC) 。增强相体积分数随厚度服从指数定律连续分布, 在对称分布条件下增强相体积分数分别在梯度板的前后表面达到最大值。结果显示, 在这种功能梯度装甲板中, 应力波的传播非常复杂, 弹性和粘塑性波耦合在一起, 反射拉伸波和卸载波的大小依赖于组份成分沿厚度的分布; 等效塑性应变的幅值、动能、弹性应变能及耗散能随时间的变化规律与功能梯度材料组份成分沿厚度的变化密切相关。这些因素对强冲击载荷作用下功能梯度板的优化设计非常重要。      

基于孔尺度的泡沫金属强化相变储热材料传热性能数值模拟

导热系数低是影响相变储热材料应用的主要难题之一,而泡沫金属具有高热导率、高孔隙率以及高比表面积等特性,在相变材料中添加泡沫金属可实现强化传热。该文基于泡沫金属基3D微观结构W-P模型,重点分析了泡沫金属基复合相变材料有效导热系数与泡沫金属孔隙率以及孔径的关系,采用数值模拟方法利用该模型预测并验证了泡沫铝6101添加空气与水的有效导热系数,研究结果表明该模型能够精确预测泡沫金属材料有效导热系数,在此基础上预测了石蜡中添加泡沫铜的有效导热系数,结果表明,泡沫金属可以显著提高相变材料的导热系数,当泡沫铜的孔隙率为97.57%时,复合相变材料的导热系数与纯石蜡相比提高了13倍。研究结果对于相变储热材料的热物性强化研究具有一定参考价值。