Failure Behavior of Cellular-Core Ceramic Sandwich Composites

The mechanical behavior of a novel ceramic sandwich composite system was investigated. This system was comprised of a low-density, cellular alumina core bonded to dense alumina faceplates. The flexural strength and elastic properties of the core material alone were measured as a function of relative density. For the sandwich system, failure mechanisms and failure loads for a variety of core densities and face thicknesses were recorded. Initial failure mechanisms were observed to be exclusively core fracture but in a non-catastrophic manner. In order to successfully predict these failure loads with existing theory, Weibull statistics were incorporated into the analysis. This allowed flexural strengths measured independently on the core material to be scaled to those expected from the stress distribution in the sandwich core.     

Unique Opportunities for Microstructural Engineering with Duplex and Laminar Ceramic Composites

Duplex (two-phase) microstructures and laminar composites offer some unique opportunities for improving the room-temperature mechanical reliability (e.g., flaw tolerance) and the high-temperature microstructural stability (e.g., resistance to grain growth and creep damage) of structural ceramic materials. Examples illustrating the approach to designing novel multiphase microstructures and laminar composites with enhanced structural reliability are given. These are based on current work on various alumina-based ceramics as well as zirconia-, silicon nitride-, and silicon carbide-containing ceramics. Critical issues and areas for future work are discussed.     

Electrodeposition Method for Terminals of Multilayer Ceramic Capacitors

A method using a combination of electroless and electrolytic plating was developed to provide an alternative method for forming the end terminals of multilayer ceramic capacitors. Electrodeposited terminals were formed by using electroless copper followed by electrolytic nickel plating. The electrical characteristics of the electrodeposited terminal capacitors were compared with standard capacitors and found to be identical.     

TiC／Si3N4导电陶瓷复合材料的制备

通过气压烧结工艺制备了含１,５,２０,２５,３５ｗｔ％ＴｉＣ的Ｓｉ３Ｎ４陶瓷复合材料研究了其导电特性,该复合材料由充当绝缘体的Ｓｉ３Ｎ４和作为导电添加剂的ＴｉＣ组成,其电阻率主要取决于其中的ＴｉＣ含量。复合材料的渗流阈值ＶＣ为１６．４５－１８．５０ｖｏｌ％,当ＴｉＣ含量达到或超过该阈值时复合材料中就形成导电通路,电阻率迅速降低。     

氧化物纤维/氧化物陶瓷基复合材料研究概述

氧化物纤维，氧化物陶瓷基复合材料可以在高温氧化环境下长时间工作，是最有发展潜力的高温结构陶瓷材料之一。决定氧化物纤维，氧化物陶瓷基复合材料性能最主要的2个因素是氧化物纤维的性能和界面材料的组成与结构。笔者介绍了氧化物纤维和界面材料的发展，以及界面材料涂覆方法，并探讨了氧化物纤维，氧化物陶瓷基复合材料的发展趋势。     

陶瓷部件挤出成型工艺

本文以蜂窝陶瓷生产工艺为例，讨论了陶瓷部件挤出成型过程中，塑性泥料的制备、成型、干燥和烧成等工艺技术和装备问题。     

Fabrication of Macrochannelled-Hydroxyapatite Bioceramic by a Coextrusion Process

A coextrusion process was used to fabricate macrochannelled hydroxyapatite (HA). To improve the powder characteristics, the HA powder was calcined at 900°C in air for 1 h. A feedrod composed of HA (shell) and carbon black (core) was coextruded at 120°C through a symmetric 8:1 square reduction die. After the first coextrusion, the individual pieces were bundled, warm-pressed, and then coextruded again. After removing the binder, the billet was sintered at 1350°C for 1h in air. Uniform, 27 vol% macrochannels with a diameter of 270 μm were formed in the dense HA through removal of the carbon black. The compressive strength of the test specimens was strongly dependent on the macrochannel direction due to the stress concentrations related to the macrochannel geometry.     

Grain Growth Phenomena in the Interface Region of α-Alumina Bilayer Composites

Microstructural development in the interface region of α-Al₂O₃ bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO₂, residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO₂ doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO₂-Al₂O₃ liquid at the interface region, thereby strongly influencing α-Al₂O₃ grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO₂ dopant in the α-Al₂O₃ bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.     

Mullite—Alumina Composites by Extrusion

The rheological properties of a paste containing chopped alumina fiber and particulate silica suspended in a gelled boehmite liquid phase have been evaluated using a physically based extrusion model. When sintered, the paste formed a mullite-alumina fiber composite. Extrudates with fiber volumes up to 30% in the sintered product were prepared. During extrusion, the pressure drop was largely independent of extrudate velocity, fiber length, and the fiber concentration. All pastes showed significant yield behavior leading to good postextrusion shape retention. For any given fiber length, it was shown that there exists a critical volume fraction above which fiber-fiber interactions are so great that both yield and wall shear stresses increase. At these high concentrations of fiber, inhomogeneities also increase. Up to the critical volume fraction, dispersed wet fibers produced lower extrusion parameters than when dry fibers were used as the starting material. The observed behavior is explained in terms of low viscosity liquid formation above the yield point of the boehmite gel.     

废旧PP塑木复合材料的研制

以废旧PP、锯末木粉为原料，采用双螺杆挤出机造粒生产环保型塑木复合材料。研究了木粉、相容剂（PP-g—MAH）对复合材料性能的影响，探讨了塑木型材的挤出成型工艺。结果表明，PP—g—MAH可以增加木粉与PP的界面结合，提高复合材料的性能，合适的用量是6～9份（质量份数，下同）；增加木粉含量（低于100份）可以提高复合材料的拉伸弹性模量、弯曲性能和热变形温度，但韧性和硬度呈降低趋势，木粉的理想用量是70～100份左右。塑木型材的挤出成型温度过高容易造成质量缺陷，须严格控制。     

Mechanochemical Formation of Metal–Ceramic Composites

A mechanical activation technique has been used to form composites of alumina with titanium carbide, nitride, or carbonitride, both with and without elemental iron. The composites were formed by reacting elemental aluminum with either ilmenite (FeTiO₃) or rutile (TiO₂) concentrates in the presence of carbon and/or nitrogen in a ball-mill at ambient temperature. The reaction was complete for the ilmenite samples after milling but was completed only for rutile under hot pressing conditions. Microhardness measurements indicated that the composites had hardnesses in the range 19–30 GPa (1740–2750 VHN), with only a small variation within each sample. Elemental mapping of the pressed pellets indicated that titanium and aluminum were evenly distributed on a submicrometer level whereas iron tended to coalesce into <20 μm particles in the presence of TiC. The coalescence decreased with the carbon content of the hard material until iron was evenly distributed with TiN. A superstoichiometric amount of aluminum led to the formation of iron–aluminum phases which decreased the iron coalescence. The XRD crystallite size of the alumina was 30–50 nm and was 25–50 nm for the titanium phases, confirming the extremely fine microstructure.     

SiC-Whisker-Reinforced Cellular SiO2 Composites

Fabrication of lighweight, cellular ceramic composites by foaming sol–gels is presented. Foams of 20-vol%-SiC-whisker-reinforced SiO₂ can be tailored with relative densities as low as 10%, with either open or closed cell structures. In addition to reducing drying shrinkage and thus gel cracking, whisker reinforcement significantly increases the strength of the composite foams relative to pure silica foams.     

Ceramic Composites with Multilayer Interface Coatings

Silicon carbide matrix composites have been fabricated from either ceramic-grade Nicalon^TM or Hi-Nicalon^TM fibers coated with an interface material consisting of six alternating carbon and silicon carbide layers. Initial efforts involved the use of chemical vapor infiltration to produce minicomposites (single tows of fibers). In subsequent work, forced-flow thermal-gradient chemical vapor infiltration was used to produce a single composite plate with a multilayer interface from ceramic-grade Nicalon fabric and two plates from Hi-Nicalon fabric, one with a single carbon layer and one with a multilayer interface. Tensile testing of the minicomposites and of specimens cut from the plates revealed typical composite behavior and strengths for the as-processed samples. Exposure of tensile specimens to 950°C air for 100 h resulted in large losses in strength and strain tolerance regardless of the interface coating. The results demonstrate that forced-flow thermal-gradient chemical vapor infiltration can be used to prepare multilayer interface material. The results also verified that relatively thick (>100 nm) single or multiple carbon layers are susceptible to oxidation that causes the loss of composite properties.     

尼龙/碳纳米管复合材料的制备和性能

碳纳米管是一种一材多能和一材多用的功能材料和结构材料,尼龙/碳纳米管复合材料具有优异的导电性、超强的力学性能和良好的导热性,可望用于汽车、飞行器制造、电子机械等领域。对尼龙/碳纳米管复合材料的制备方法、主要性能和应用进行综述。     

Fabrication of Continuous-SiC-Fiber-Reinforced SiAlON-Based Ceramic Composites by Reactive Melt Infiltration

A technique for fabrication of β'-SiAlON-based ceramics in three-dimensional woven fabrics of BN-coated SiC (Hi-Nicalon™) fibers was developed by reactive melt infiltration in a controlled N₂ atmosphere. β'-SiAlON was produced in situ by the reaction of β-Si₃N₄, AlN, and Y-Al-Si-O molten glass. The wettability of the fibers with the molten glass was improved by infiltration and pyrolysis of perhydropolysilazane, resulting in fully dense matrix composites. The reaction between the fiber and molten glass could be depressed by increasing the N₂ partial pressure during the melt infiltration. The inhibition of the interfacial reaction may be related to the formation of carbon and oxynitride on the SiC fiber, in agreement with thermodynamic calculations as a function of N₂ partial pressure. The fabricated composites had a high ultimate flexure strength and a large work of fracture at room temperature. Degradation of the mechanical performance of the composites was small, even at 1773 K in an argon atmosphere.     

Extrusion of Metal–Ceramic Composite Pipes

Metal–ceramic composite pipes were prepared through simultaneous extrusion of different pastes by a multi-billet extrusion method. ZrO₂ and stainless steel powders were chosen, and an aqueous solution of water-soluble polymer, hydroxypropyl methylcellulose (HPMC), was used as binder. The maximum extrusion pressure and the minimum amount of binder required reached their lowest values when the mixing fraction of ZrO₂ powder was 0.4. The minimum amount of binder for forming the outer layers was 4%–5% higher than that for inner layers, even for the same powder. It was possible to decrease the binder content and broaden the extrudable range of the binder content by means of mixing coarse and fine powders.     

Al2O3 Nanoparticulate LZS Glass–Ceramic Matrix Composites for Production of Multilayered Materials

This work reports the processing steps of Al₂O₃ (1–5 vol%) nanoparticulate (d_V.50 = 13 nm) LZS glass–ceramic matrix (19.58Li₂O·11.10ZrO₂·69.32SiO₂, mol%, d_v.50 = 3.5 μm) composites for production of multilayered materials with thermal expansion gradients obtained by tape casting. Suspensions were prepared in water to solids contents ranging from 40 to 47 vol% using ammonium polyacrylate as a deflocculant, and an acrylic copolymer and polyvinyl alcohol as binders. Optimum performance was achieved by sonication and controlling the rheological properties for every step of the process. To prepare the composites, different concentrations (1, 2.5 and 5 vol%) of nanoalumina were added to fresh, as‐prepared LZS suspensions, by changing the solid contents as required to maintain similar viscosities. Green tapes with high uniformity, without macroscopic defects and easy to handle were sintered to relative densities between 89% and 94%. Dense and homogeneous laminates with gradual composition with increasing concentrations of alumina were obtained.     

Piezoelectric Lead Zirconate Titanate Ceramic Fiber/Polymer Composites

Piezoelectric lead zirconate titanate (PZT) ceramic fiber/polymer composites were fabricated by a novel technique referred to as "relic" processing. Basically, this involved impregnating a woven carbon-fiber template material with PZT precursor by soaking the template in a PZT stock solution. Careful heat treatment pyrolized the carbon, resulting in a PZT ceramic relic that retained the fibrous template form. After sintering, the densified relic was backfilled with polymer to form a composite. Optimized relic processing consisted of soaking activated carbon-fiber fabric twice in an intermediate concentration (405-mg PZT/(1-g solution)) alkoxide PZT solution and sintering at 1285°C for 2 h. A series of piezoelectric composites encompassing a wide range of dielectric and piezoelectric properties was prepared by varying the PZT-fiber orientation and polymer-matrix material. In PZT/Eccogel polymer composites with PZT fibers orientated parallel to the electrodes, K = 75, d ₃₃= 145 pC/N, d _h= 45 ± 5 pC/N, and d _hg_h= 3150 × 10⁻¹⁵ m²/N were measured. Furthermore, in composites with a number of PZT fibers arranged perpendicular to the electroded surfaces, K = 190, d ₃₃= 250 pC/N, d _h= 65 ± 2 pC/N, and d _h g_h= 2600 × 10⁻¹⁵ m²/N.     

碳热还原氮化法制备SiAlON陶瓷材料

碳热还原氮化工艺是近年来制备低成本高性能SiAION陶瓷材料的一种实用方法．具有产业化生产潜力。本文对碳热还原氮化法制备SiAION的进展进行了综述,归纳分析了不同条件对生成物性能的影响,对今后的研究进行了展望。