Microstructure and Thermal Conductivity of Al–Graphene Composites Fabricated by Powder Metallurgy and Hot Rolling Techniques

The objective of this research is to improve the thermal conductivity and mechanical properties of Al/GNPs(graphene nanoplatelets) nanocomposites produced by classical powder metallurgy and hot rolling techniques. The microstructural evaluation confirmed the uniform dispersion of GNPs at low content and agglomeration at higher contents of GNPs. The structure of graphene was studied before and after the mixing and the Raman spectrum proofs that the wet mixing has a great potential to be used as a dispersion method. There was no significant peak corresponding to the Al_4C_3 formation in both the differential scanning calorimetry curves and X-ray diffraction patterns. The microstructural observation in both fabrication techniques showed grain refinement as a function of the GNPs content. Moreover, the introduction of the GNPs not only improved the Vickers hardness of the composites but also decreased their density. The thermal conductivity investigations showed that in both the press-sintered and hot-rolled samples, although the thermal conductivity of composites was improved at low GNPs contents, it was negatively affected at high GNPs contents.     

Development of electrically conductive plasma sprayed coatings on glass ceramic substrates

The production of functional coatings on glass or glass ceramic substrates is of outstanding interest in modern product development due to the specific thermophysical properties of glasses, like low or even negative CTE, low heat conductivity and high dimensional stability. Atmospheric plasma spraying (APS) is an adequate technology for the deposition of a wide variety of materials on glasses and opens new application fields for thermal spraying technology in engineering and consumer industries.Metals are the frequent solution to produce electrically conductive layers in thermal spraying operations. Concerning applications with glass ceramic as a substrate, an intermediate oxide ceramic coating is applied before depositing the metallic layer, so that the distribution of residual stresses in the composite caused during and after the deposition process due to the mismatch in the materials thermophysical properties is minimized. However, the electrical properties required for the developed coatings presented in this paper can be fulfilled using other spraying materials, like mixed phases of oxide ceramics and metal powders, or pure ceramic materials. In this way, mono-layer electrically conductive systems which ensure the required stability and adhesion of the coating can be developed, reducing as well production time and costs.In the proposed approach, the three systems, metal oxide layer-composites, ceramic-metal mixed layers and ceramic mono-layers as conductive coatings on glass ceramics were thermally sprayed with APS. The coatings were characterized in terms of residual stress distribution and electrical conductivity. The influence of the process parameters on the coating electrical and mechanical properties was analyzed using the design of experiments (DOE) methodology.     

Ag-GNPs 新型电接触材料的制备及其电接触行为

为阐明Ag-GNPs新型电接触材料的电弧侵蚀行为,采用粉末冶金技术制备了石墨烯纳米片质量分数为0.5%~2.0%的Ag-GNPs新型电接触材料,研究了Ag-GNPs材料的微观结构、密度、电导率,分析了材料电弧侵蚀后的质量损耗、表面形貌,探讨了材料的电弧侵蚀机制。实验结果表明,高含量的GNPs降低Ag-GNPs材料的密度和导电率,但可显著增加其力学性能。GNPs的密度与含量对熔池表面元素再分布有重要影响。高含量的GNPs更容易团聚和降低Ag-GNPs材料的电接触性能,特别是当Ag-GNPs材料中GNPs含量超过1.5%。GNPs含量为1.5%的Ag-GNPs电接触材料具有最佳的抗电弧侵蚀性能,DC 25/15A条件下电弧侵蚀后其质量损耗最低、侵蚀坑最浅。     

Influence of the Surface State on the Adherence of the Coating: Case of an Alumina Coating Plasma Sprayed on SiC Composites

In thermal spraying, adherence between the coating and the substrate appears as the fundamental point. To favor a good interaction between both, it is often necessary to clean and prepare the substrate surface. Conventionally, solvents and sand blasting are applied to remove the contaminants and increase the surface roughness for a mechanical anchorage. However, according to the substrate nature (ceramic) or the substrate morphology, it can be prejudicial to apply a mechanical treatment because of peeling of the surface or a decrease in the global properties. Then, to obtain an appropriate preparation, several techniques can be investigated, such as water jet, ice blasting, and heat treatment; as well, laser ablation can be an interesting technology to prepare the substrate surface. The aim of this work was to study the modifications induced by 10 ns single or cumulative pulses of a Q-switched Nd:YAG near-infrared laser and its influence on the interface adhesion. The case of an alumina coating sprayed on a ceramic matrix composite (CMC) was studied. In these conditions, the laser treatment seems favorable from the adherence viewpoint according to the mechanical effect (induced by a conelike structure) and the chemical effect.     

Intermetallic Al-, Fe-, Co- and Ni-Based Thermal Barrier Coatings Prepared by Cold Spray for Applications on Low Heat Rejection Diesel Engines

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bond coat and a ceramic heat insulating topcoat. They possess the desired low thermal conductivity, but at the same time they are very brittle and sensitive to thermal shock and thermal cycling due to the inherently low coefficient of thermal expansion. Recent research activities are focused on the developing of multilayer TBC structures obtained using cold spraying and following annealing. Aluminum intermetallics have demonstrated thermal and mechanical properties that allow them to be used as the alternative TBC materials, while the intermetallic layers can be additionally optimized to achieve superior thermal physical properties. One example is the six layer TBC structure in which cold sprayed Al-based intermetallics are synthesized by annealing in nitrogen atmosphere. These multilayer coating systems demonstrated an improved thermal fatigue capability as compared to conventional ceramic TBC. The microstructures and properties of the coatings were characterized by SEM, EDS and mechanical tests to define the TBC material properties and intermetallic formation mechanisms.     

压力烧结制备二氧化钛包覆的石墨烯增强铝基复合材料

使用压力烧结方法制备了石墨烯纳米片（GNP）增强的7075铝基纳米复合材料,提出了一种通过在GNP的表面涂覆二氧化钛（TiO₂）来优化界面结合的新工艺,并比对了原石墨烯及具有包覆层石墨烯对铝基纳米复合材料的力学性能和微观结构的影响。结果表明,与添加纯GNP相比,添加具有TiO₂涂层的GNP的纳米复合材料的力学性能提高。相比于基体,TiO₂包覆GNP增强的纳米复合材料的屈服强度、抗拉强度和显微硬度分别增加了38.9%、34.4%和20.1%。性能的进一步改善是由于TiO₂涂层优化了增强相与基体之间的界面结合,从而提高了载荷传递的有效性。     

Experimental study and simulation of plastic deformation of zirconia-based ceramics in a pulsed electric current apparatus

Plastic deformation by compression of cylindrically shaped zirconia (ZrO₂)-based ceramics in a pulsed electrical current apparatus was studied using a combined experimental and theoretical approach. Both fully dense electrically insulating 3Y–ZrO₂ and electrically conductive 3Y–ZrO₂–TiCN 60/40 (vol.%) ceramics were subjected to a compressive load at temperatures above 1200 °C. Deformed non-conductive 3Y–ZrO₂ samples were concave shaped, whereas the composite samples exhibited a different behaviour depending on the electrical current path within the set-up. A convex shape was obtained when the current was freely flowing through them, while they started to become concave shaped when the samples were separated from the graphite pressing punches by relatively low conductive silicon carbide disks. The secondary titanium carbonitride (TiCN) phase in the composite materials exhibited a grain boundary pinning effect, which limited coarsening of their microstructure. The influence of current flow on the shape of the deformed ceramic samples was interpreted in terms of the temperature distribution generated during hot deformation. Finite-element simulations, coupling thermal, electrical and mechanical fields, were used to explain the deformation behaviour of the different samples. A subsequently coupled thermal–electrical and mechanical analysis procedure was developed for this aim. Special attention was paid to the materials and interactions properties used during modelling. The modelling results are in good agreement with the experimental data, so that the developed finite-element approach and code can be used for the analysis of near net shaping of ceramic parts assisted by an electrical field.     

Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials,Processes, and Properties

The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic–extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined.     

Optimizing Electroless Plating of Ni-Mo-P Coatings Toward Functional Ceramics

Ni-Mo-P coatings are obtained at the surface of ceramic substrate by electroless deposition using palladium as a surface catalyst.The influence of catalyst activation conditions on coatings' properties was assessed by structural,morphological,electrical,mechanical measurements and adhesion strength by using X-ray diffraction,field emission scanning electron microscopy,atomic force microscopy,Hall effect and nanoindentation techniques,respectively.The results indicated the formation of dense,continuous and uniform coatings consisting mainly of Ni.The coatings obtained at 300℃for 12 h exhibited the best electrical properties,namely resistivity of 9.32 μΩ cm,smaller roughness(R_a 0.090 μm) and average mechanical properties.The adhesion tests showed a firm adherence of the Ni-Mo-P coatings to the ceramic surface.The results of this study could offer an approach for obtaining conducting ceramic substrates in order to be employed in photovoltaic applications.The performance of demonstrative heterojunction solar cell obtained with such metallized ceramic is indicative of the high potential of the Ni-Mo-P electroless coatings for functional ceramics.     

Material properties and machining performance of hybrid Ti2AlN bulk material for micro electrical discharge machining

Mn+1AXn(MAX) phases are a family of nanolaminated compounds that possess unique combination of typical ceramic properties and typical metallic properties.As a member of MAX phase,Ti2 AlN bulk materials are attractive for some high-temperature applications.The synthesis,characteristics and machining performance of hybrid Ti2 AlN bulk materials were focused on in this work.The bulk samples mainly consisting of Ti2 AlN MAX phase with density close to theoretic one were synthesized by a spark plasma sintering method.Scanning electron microscopy results indicate homogenous distribution of Ti2 AlN grains in the samples.Micro-hardness values are almost constant under different loads (6-6.5 GPa).A machining test was carried out to compare the effect of material properties on micro-electrical discharge machining (micro-EDM) performance for Ti2 AlN bulk samples and Ti6242 alloy.The machining performance of the Ti2 AlN sample is better than that of the Ti6242 alloy.The inherent mechanism was discussed by considering their electrical and thermal conductivity.`     

基于热喷涂界面微纳结构设计的涂层力学性能研究进展

热喷涂陶瓷涂层在航空航天、交通运输等众多领域具有广阔的应用前景,常见的热喷涂陶瓷涂层体系包括陶瓷层、金属/合金粘结层和金属基体.由于陶瓷层与粘结层具有较大的物化性能差异,使界面成为热喷涂陶瓷涂层易发生失效的区域,极大降低了涂层的服役寿命,遏制了热喷涂陶瓷涂层更为广泛的应用.以热喷涂界面的微观和宏观结构设计为出发点,综述了微观界面和扩散对界面力学性能的影响,总结了微米-纳米颗粒的界面结构、成分连续梯度变化的涂层结构和涂层缺陷对性能的影响.同时总结了三点弯曲、显微硬度和纳米压痕对界面力学性能的系统表征方法,并结合不同测试方法的特性,给出了对应的多尺度界面力学性能的计算公式.上述结果对设计和制备高性能复合涂层具有重要的理论意义,对延长涂层服役寿命具有实际的应用价值.     

石墨烯纳米片增强铝基复合材料的制备与性能

采用高能球磨、放电等离子烧结以及热挤压工艺制备含量为5.0％(体积分数)的石墨烯增强铝基复合材料.分别采用X射线光电子能谱、透射电镜及拉伸试验研究挤压态复合材料的显微组织与力学性能,发现5.0％(体积分数)的石墨烯分散在铝晶界上,并且未与铝基体发生界面反应.最终,挤压态复合材料的屈服强度和抗拉强度高达462 MPa和4...     

Ti_3SiC_2弥散强化Cu:一种新的弥散强化铜合金

选用具有高导电、高导热性能的新型陶瓷Ti3SiC2做为弥散强化相,通过与Cu粉末高能球磨混合后,热压成一种新型弥散强化Cu材料机械性能测试表明,随着Ti3SiC2体积分数的提高,弥散强化,Cu的屈服强度和维氏硬度线性上升分析表明Ti3SiC2相的晶粒细化和位错塞积是主要强化机制,当颗粒粗化和团聚后Ti3SiC2的强化效果将明显减弱     

The activated sintering of WCu composites through spark plasma sintering

Tungsten-copper composites of network structure are increasingly being studied because of their unique mechanical, thermal and electrical properties. Chemical plating and mechanical alloying were used to prepare tungsten-copper alloys with different compositions in this investigation. Samples obtained through mechanical alloying method and sintered by spark plasma sintering (SPS) technique at 1030 °C, 60 MPa exhibited excellent performance. Ni element and Cr element were added to the composite as activating elements to improve the interface wettability of tungsten‑copper. The experimental results showed the Cr element contributes more to the densities, tensile strength, flexural strength and thermal properties of the composites than Ni element. Besides, the effect of two different ball milling ways on the properties of the composites was also studied. The sample W20Cu (15Cr), which was prepared by Cu(Cr) ball milling, showing a network structure, has the best performance.     

Ti3SiC2陶瓷颗粒增强铜基复合材料的组织和性能

为了考察Al,Sn,Zr,Mo合金元素对α钛合金在室温和77 K低温（液氮）下的缺口冲击韧性（冲击值Ak）的影响,采用示波冲击试验机测试了Ti-2Al,Ti-2Sn,Ti-2Zr和Ti-1Mo 4种α钛合金在室温和77K下的Ak值,并计算了表征其冲击韧性的弹性变形功、塑性变形功和裂纹扩展撕裂功.用扫描电镜观察了4种合金冲击试样断口的形貌.计算数据和显微组织表明,4种合金均显示韧性特征,4种合金元素对冲击韧性贡献的顺序为：Mo＞Zr＞Sn＞Al.     

The concept of a strong interface applied to SiC/SiC composites with a BN interphase

Strong interfaces have been shown to allow improvement of the mechanical properties of ceramic matrix composites (CMC). The concept of a strong interface has been established in SiC/SiC composites with pyrocarbon (PyC) or multilayered (PyC/SiC) fiber coatings (also referred to as interphases). The present paper reports an attempt directed at applying the concept of a strong interface to SiC/SiC composites with a BN coating (referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests performed on microcomposites. The stress–strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface.     

EXTREME PROPERTIES OF FIBER COMPOSITES AND THEIR CRYOGENIC APPLICATIONS

1.IntroductionEachclassofmaterialshasitsspecificmerits.Metalsaret0ugh,whilefibercompositesarebrittle,especiaJlyatlowtemperatures.0nthe0therhandtheelaJsticlimitismuchlowerformetalsthanf0rfibercomposites.Carbonfibercomp0sitesf0rexampleexhibitaveryhightensilestrength,whileshear-andcompressivestrengtharelesspr0nounced.Agreatadvantage,however,offiberc0mpositesistheirl0wweightwhichyieldsexcellelltspecificproperties(e.g.stiffnessorstrengthperdensity).Severalfeaturesfav0rtheapplicati0noffiberc0mposi…     

Vapor-grown carbon-fiber composites: Processing and electrostatic dissipative applications

In recent years, the demand for composite materials has grown in many directions. Although polymer composites have long been classified as structural materials for purposes such as mechanical enhancement and weight savings, the need for conducting polymer composites is growing. Vapor-grown carbon nanofibers (VGCFs), used as reinforcements for thermoplastic matrices, have potential application as conducting polymers, enhancing both stiffness and thermal stability.^1–4 This article discusses the processing of VGCFs from the perspective of their electrical and thermal properties, availability, and application in the electrostatic dissipative market. For more information, contact K. Lozano, University of Texas Pan American, Department of Engineering, Edinburg, Texas, 78539 USA; (956) 316-7020; fax (956) 381-3527; e-mail lozanok@panam.edu.     

Finite Element Prediction of the Thermal Conductivity of GNP/Al Composites

A 3D multi-scale finite element model was developed to predict the effective thermal conductivity of graphene nanoplatelet (GNP)/Al composites. The factors influencing the effective thermal conductivity of the GNP/Al composites were investigated, including the orientation, shape, aspect ratio, configuration and volume fraction of GNPs. The results show that GNPs shape has a little influence on the thermal conductivity of GNP/Al composites, and composites with elliptic GNPs have the highest thermal conductivity. In addition, with increasing the aspect ratio of GNPs, the thermal conductivity of GNP/Al composites increases and finally tends to be stable. The GNPs configuration strongly influences the thermal conductivity of GNP/Al composites, and the thermal conductivity of the composites with layered GNPs is the highest among the five configurations. The effective thermal conductivity is sensitive to volume fraction of GNPs. Ideally, when the volume fraction of layered GNPs reaches 1.54%, the thermal conductivity of GNP/Al composites is as high as 400 W/m K. The findings of this study could provide a good theoretical basis for designing high thermal conductivity GNP/Al composites.     

先进陶瓷涂层结构调控及其在固体氧化物燃料电池中的应用

等离子喷涂技术可以对陶瓷涂层的微观结构进行调控设计,因此在制备固体氧化物燃料电池方面具有独特的优势。基于等离子喷涂方法,可以直接制备或经过后处理获得致密的电解质涂层。采用等离子喷涂技术也可以制备高性能的多孔阳极和阴极,并可对钙钛矿结构阴极材料的成分和晶体结构进行调控。文中介绍了目前国内外采用涂层制备电池的方法,主要探讨了热喷涂方法制备电解质涂层的特点,对存在的问题和可行思路进行了讨论,并探讨了基于提高三相反应界面长度来制备高性能电极的方法。由于固体氧化物燃料全电池各功能层都有可能通过热喷涂方法制备,因此该方法在固体氧化物燃料电池结构设计具有巨大的潜力。