Effects of sintering temperature of whisker preform on the microstructures and tensile properties of Bi(OH)3-coated Al18B4O33 whisker-reinforced aluminum matrix composites

A alumina borate whisker with Bi(OH)₃ coating was prepared by a chemical method. The coated whiskers were sintered at various temperatures. The coated whisker-reinforced pure aluminum matrix composite was fabricated by squeeze casting method. The microstructures of the coated whiskers and coated composites with the different sintering temperature of whisker preform were studied, and the tensile properties of the coated composites at room temperature were also investigated. It can be found that the microstructures of coatings on the whisker surfaces and at the interface in the coated composites are strongly dependent on the sintering temperature of whisker preform. The ultimate tensile strength and elongation to fracture of the coated composites increased with the increasing of sintering temperature of the whisker preform.     

Cellular arrays of alumina fibres

In conventional short fibre reinforced metal matrix composites, the quest is for a method of processing that will provide a homogeneous and preferably random arrangement of fibres. In contrast, recently developed contiguity models for multiphase composites on the one hand, and finite element modelling of structures on the other, independently predict that the modulus enhancement provided by short-fibre reinforcement can be improved if the fibres are arranged in a cellular structure. Furthermore, provided the metallic phase is continuous, the toughness of the composite may also thereby be enhanced. This paper, which is part of an attempt to explore the question of reinforcement arrangements, presents a method for making ceramic preforms for MMCs in which a polymeric foam is used to position the fibres in cellular array. The polymer is then removed by pyrolysis and the preform of fibres is strengthened by sintering. During high temperature sintering, phase changes and grain growth degraded the fibre. Methods of increasing the compressive strength of the preform by incorporation of alumina particles and by subsequent infiltration are described and compared.     

Fabrication and characterization of machinable Si3N4/h-BN functionally graded materials

A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si₃N₄ ceramics. Silicon nitride/hexagonal boron nitride (h-BN) functionally graded materials (FGMs) were fabricated by hot pressing at 1750°C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si₃N₄/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. Such design can improve the machinability of composite, and at the same time can make the mechanical properties of Si₃N₄ ceramic not to be sacrificed too much. The texture of h-BN and β-Si₃N₄ was observed during hot pressing sintering.     

(SiC,TiB2)/B4C复合材料的烧结机理

研究了在热压条件下制备 (SiC, TiB₂)/ B₄C复合材料的烧结机理。认为烧结助剂的加入使本体系成为液相烧结,同时粉料的微细颗粒对复合材料的烧结致密也有重要贡献。分析和测量了制取的复合材料的相组成、显微结构和力学性能。结果表明,采用B₄C与Si₃N₄和少量SiC、TiC为原料,Al₂O₃＋Y₂O₃为烧结助剂,在烧结温度1800~1880℃,压力30 MPa的热压条件下烧结反应生成了SiC、TiB₂和少量的BN,制取了(SiC, TiB₂)/B₄C复合材料。所形成的晶体显微结构为层片状。制得的试样的硬度、抗弯强度和断裂韧性分别可达HRA88.6、540 MPa和5.6 MPa·m1/2。      

Complete determination of elastic moduli of interpenetrating metal/ceramic composites using ultrasonic techniques and micromechanical modelling

The complete stiffness matrices of several metal/ceramic composites were analysed using the complementary ultrasonic spectroscopic techniques ultrasound phase spectroscopy (UPS) and resonant ultrasound spectroscopy (RUS). Three different aluminum/alumina composites having complex interpenetrating architectures were studied: a composite based on freeze-cast ceramic preform, a composite based on open porous ceramic preform obtained by pyrolysis of cellulose fibres, and a composite based on discontinuous fibre preform. Six of the nine independent elastic constants describing orthotropic elastic anisotropy were pre-determined by ultrasound phase spectroscopy and used as initial guess input for resonant ultrasound spectroscopy analysis, making the final results of all nine elastic constants more reliable. In all cases, consistent and reproducible results are obtained. Finally the experimental results were compared with effective elastic constants calculated using micromechanical modelling and a good correspondence between both is obtained.     

B4C包覆ZTA颗粒增强铁基复合材料制备与性能

ZTA (ZrO₂增韧Al₂O₃)陶瓷颗粒表面包覆B₄C微粉,将其制备成蜂窝状结构陶瓷预制体。采用传统重力浇注工艺将陶瓷预制体与熔融的高铬铸铁(HCCI)金属溶液进行复合,获得ZTA陶瓷颗粒增强高铬铸铁基复合材料。对复合材料中ZTA陶瓷颗粒增强相与高铬铸铁基体之间的界面及复合材料的耐磨料磨损性能进行了研究。结果表明,ZTA陶瓷颗粒与高铬铸铁界面结合处形成了明显的过渡区域,界面过渡区域的存在提高了陶瓷颗粒与金属基体的结合,从而提升了复合材料的整体稳定性能。同时,三体磨料磨损试验表明该复合材料的耐磨料磨损性能是高铬铸铁的3.5倍左右。     

无压烧结Ti3SiC2/铝基复合材料组织与性能

MAX相具有独特的层状晶体结构,不但具备常用铝基复合材料外加陶瓷颗粒的性能特征,同时具有可与石墨媲美的摩擦性能.本文以Al粉、Si粉和典型MAX相Ti_3SiC_2为原料,采用冷压成型-无压烧结方法制备了Ti_3SiC_2/Al-Si复合材料,并通过金相显微镜、X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)等分析手段,研究了烧结温度、Si元素含量对复合材料组织与性能的影响.研究表明:随着烧结温度从500℃提高到700℃,复合材料致密度先上升后下降,摩擦系数先降低后上升,硬度逐渐增大至最大值并基本保持稳定;随着Si质量分数从0增加到20.7%,复合材料的致密度逐渐降低,硬度逐渐增大,摩擦系数先降低后增大,晶粒尺寸随之下降,12.5%Si晶粒最为细小;烧结温度为650℃,Si元素质量分数为12.5%的铝基复合材料具有最低的摩擦系数0.18,相应的硬度为62 HV,致密度为92.12%.XRD物相和扫描电镜组织分析表明,复合材料的主要相组成为Al、Ti_3SiC_2,及由界面反应产生的Al_4C_3和Al的氧化产物Al_2O_3.     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

Fibrous monoliths of mullite-AlPO4 and alumina/YAG-alumina platelets

Two-layer, fibrous monolithic composites consisting of mullite-aluminum phosphate (AlPO₄) and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet interphase components, were fabricated by a co-extrusion technique. The four powders were characterized for particle size, specific surface area, and SEM analysis. The mixing formulations for extruding the powders were developed using ethylene vinylacetate copolymer as a binder. The variation in the mixing torque, in a Brabender mixer, as a function of temperature was measured. The binder removal behavior of the mullite-AlPO₄ fibrous monolithic composite was studied by thermogravimetric analysis (TGA). The AlPO₄ and alumina platelet interphase layers formed a porous and less porous interphase region, respectively, after sintering. The sintered mullite-AlPO₄ two-layer fibrous monolithic showed non-brittle fracture behavior. Its 3-point bend strength and work of fracture were 76 ± 5 MPa and 0.45 ± 0.02 kJ/m², respectively.     

Investigation of the physical and mechanical properties of hot-pressed machinable Si3N4/h-BN composites and FGM

A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si₃N₄ ceramics. Silicon nitride/hexagonal boron nitride (h-BN) ceramic composites and functionally graded materials were fabricated by hot pressing at 1750 °C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si₃N₄/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. The texture of h-BN and β-Si₃N₄ was observed during hot pressing sintering. Physical and mechanical properties of Si₃N₄/h-BN with different content of h-BN were investigated, such as bulk density, Vickers's hardness, flexural strength, and elastic modulus. All of these properties are important for the design of the machinable Si₃N₄/h-BN FGM (Functionally Graded Materials).     

Hydroxyapatite/Al2O3/diopside ceramic composites and their behaviour in simulated body fluid

Abstract

In the present work, alumina and diopside were introduced in hydroxyapatite matrix, and hydroxyapatite/Al₂O₃/diopside ceramic composites with good mechanical properties were fabricated by uniaxial hot pressing. The behaviours of hydroxyapatite/Al₂O₃/diopside ceramic composites in simulated body fluid were studied by SEM, Fourier transform infrared spectroscopy and electron probe microanalyser. Scanning electron microscopy images showed an obvious mineral layer formed on the soaked composite surface, which indicated that the introduction of Al₂O₃ and diopside in hydroxyapatite matrix could not only improve the strength and toughness of the composites but also maintain its ability to precipitate an apatite layer.     

原位气相生长法制备碳纳米管/Al2O3陶瓷复合材料

以Al₂O₃陶瓷成型体为基体,通过化学气相反应在陶瓷体内原位生长碳纳米管（CNTs）,制备出CNTs/Al₂O₃陶瓷复合材料。结果表明,Al₂O₃陶瓷体中均匀分布有可观量的多壁CNTs,碳管根部嵌于Al₂O₃晶粒间并从晶粒表面生长出。在Al₂O₃陶瓷成型体中原位生长CNTs需严格控制生长条件,尤其是生长温度（850℃）,温度过高和过低都难以长出CNTs,此外造孔剂、碳源和催化剂也影响CNTs的原位生长。对原位生长的CNTs/Al₂O₃复合体进一步高温烧结获得致密化的复合材料,其导电率达3.7 S/m,较纯Al₂O₃提高13个数量级。在陶瓷成型体中原位生长CNTs是一步法制备CNTs/陶瓷复合材料的新方法,可用于发展高性能的结构陶瓷和具有导电导热等多功能特性的新型陶瓷复合材料。     

Electrical properties of thick-film NTC thermistor composed of Ni0.8Co0.2Mn2O4 ceramic: Effect of inorganic oxide binder

The thick-film NTC thermistors were prepared by screen printing Ni_0.8Co_0.2Mn₂O₄ ceramic on the alumina. The influence of inorganic oxide binder composition and thickness of thermistor layer on the thermistor constant and initial resistivity are studied. The relation between the resistivity (ρ) and the absolute temperature for the prepared thick-film thermistor comply with Arrhenius equation. The room temperature sheet resistivities of the thick films were in the range 0.56-7.45 MΩ cm and temperature sensitivity index in the range 1492-3335 K. Binder composition dependent agglomeration of microcrystallites is observed in the microstructure of the thick-film Ni_0.8Co_0.2Mn₂O₄ ceramic. The spinel ceramic was prepared by oxalate co-precipitation and sintering.     

Effect of tungsten addition on thermal conductivity of graphite/copper composites

Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%.     

Artificially controlling of inner structure to porous hydroxyapatite ceramic by using solidified coated fibers

Porous hydroxyapatite (HA) ceramic was fabricated by 3D fiber network. These fibers as channel underprops were treated by surface coating with acidic macromolecule glue. The solidified coated fibers in ceramic block could form run-through channels and also etch stripes on channel walls for cell attachment. The channels formed in sintering process by fibers volatilization may be directed artificially according to beforehand design of structure for ceramic block. The results showed that the pore characteristics and the inner structure of the sample made from this technique have settled for essential requests of porous bioceramic. The channels in sintered scaffold shown in SEM (scanning electron microscopy) micrographs have directional connection, equal distribution, intact configuration and existence of thin stripes on inner walls. All of these characteristics have met requests for tissue cell developing, transplanting and attaching. The samples have appropriate interconnectivity and reasonable structure with equable pore-arranging and uniform size of pore. XRD (X-ray diffraction) patterns of sample indicated no major change of the crystalline structure.     

Interfacial reactions and wetting in Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique

The present paper considers the microstructures of Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique. The composites were produced using an Al–10 wt.% Mg alloy with two oxide ceramic foams, spinel (MgAl₂O₄) and mullite (Al₆Si₂O₁₃), at 915 °C in a flowing N₂ atmosphere. Full infiltration of the aluminium alloy into the ceramic preform has been achieved with good bonding between the metal and ceramic phases. The composites were characterised by a range of techniques and compared with those for alumina from the literature. It has been found that the metal–ceramic interface of the composite consisted of an oxide layer near the ceramic phase and a nitride layer from Mg₃N₂ to AlN near the metal phase. The improvement of Al wetting and adhesion on the oxide ceramics by the addition of Mg and in the presence of N₂ was studied by a sessile drop technique to clarify which compound that formed at the interface contributed to the spontaneous infiltration.     

Microhardness of ceramics produced from different alumina nanopowders by different techniques

Fine-grained ceramics (with a grain size on the order of a micron) have been produced by the spark plasma sintering (SPS) of various alumina nanopowders. We have compared the microhardness of ceramic samples prepared from 11 alumina nanopowders and that of composites based on such powders. The ceramics have been prepared by both SPS and a conventional technique (sequential pressing and sintering). We examine the effect of the phase composition and average particle size of the starting nanopowder on the microhardness of the ceramics.     

Processing of B4C Particulate-reinforced Magnesium-matrix Composites by Metal-assisted Melt Infiltration Technique

In fabricating magnesium-matrix composites, an easy and cost-effective route is to infiltrate the ceramic preform with molten Mg without any external pressure. However, a rather well wettability of molten Mg with ceramic reinforcement is needed for this process. In order to improve the wettability of the metal melt with ceramic preform during fabricating composites by metal melt infiltration, a simple and viable method has been proposed in this paper where a small amount of metal powder with higher melting point is added to the ceramic preform such that the surface tension of the Mg melt and the liquid-solid interfacial tension could be reduced. By using this method, boron carbide particulate-reinforced magnesium-matrix composites （B4C/Mg） have been successfully fabricated where Ti powder immiscible with magnesium melt was introduced into B4C preform as infiltration inducer. The infiltration ability of molten Mg to the ceramic preform was further studied in association with the processing conditions and the mechanism involved in this process was also analyzed.     

Sintering and technological properties of alumina/zirconia/nano-TiO2 ceramic composites

The present work focuses on studying the effect of nano TiO₂ (0.0–25 mass%) on the sintering behavior and mechanical properties of alumina/zirconia ceramic composites. Al₂O₃–ZrO₂–TiO₂ oxides mixture was sintered at 1600 °C to obtain the desired composites. The sinterability and the technological properties of these ceramic composites, i.e. the sintering parameters and microhardness as well as thermal shock resistance were investigated. Moreover, phase composition and microstructure of the sintered bodies were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results revealed that nano TiO₂ is a beneficial component for alumina/zirconia ceramic composites. The batch containing 20 mass% TiO₂ exhibited the highest sintering and mechanical properties as well as resistance to thermal shock. The obtained microstructure exhibited high compacted ceramic matrix composites.     

The displacive compensation of porosity (DCP) method for fabricating dense, shaped, high-ceramic-bearing bodies at modest temperatures

A novel process is introduced for the fabrication of dense, shaped ceramic/metal composites of high ceramic content: the Displacive Compensation of Porosity (DCP) method. In this process, a metallic liquid is allowed to infiltrate and undergo a displacement reaction with a porous oxide preform. Unlike other displacement-reaction-based processes (e.g., the C⁴, RMP, and AAA processes), a larger volume of oxide is generated than is consumed, so that composites with relatively high ceramic contents can be fabricated. Bar- and disk-shaped MgO/Mg-Al composites were produced by the infiltration and reaction of molten Mg with porous Al₂O₃ preforms at 1000 °C. By varying the relative density of the preforms (from 53.3 to 71.0% of theoretical), the magnesia content of the final composites could be adjusted from 70.4 to 85.6 vol %. Because the increase in oxide volume associated with the conversion of alumina into magnesia was accommodated by the prior pore volume of the preforms, the composites retained the shapes and dimensions (to within a few percent) of the starting preforms. The MgO/Mg-Al composites were lightweight (2.94–3.30 g/cm³), dense (97.7–99.0% of theoretical), and resistant to hydration. Bar-shaped MgO/Mg-Al composites exhibited average flexural strength and indentation toughness values of 244 MPa and 5.4 MPa · m^1/2, respectively.