Mechanical Behavior of Lightweight Ceramics Based on Sintered Hollow Spheres

A simple micromechanical model, describing the Young's modulus, fracture toughness, and density of a lightweight ceramic structure produced by the sintering of hollow spheres and based on shell theory, was developed. The approach combined an analysis of the elastic deformation using shell theory and a simple analysis of the densification in terms of the micro-structural parameters. The results were compared with experimental data for materials produced by the sintering of hollow glass spheres and gave reasonable to good agreement, especially in the prediction of the nonlinear variation of Young's modulus and fracture toughness with density. The model should provide a useful basis for rationalizing the relation between mechanical properties and fabrication procedure for such materials.     

Processing, Mechanical Properties, and Oxidation Behavior of Silicon Oxynitride Ceramics

Silicon oxynitride ceramics were prepared by hot-pressing an equimolar Si₃N₄+ SiO₂ mixture with 3 mol% CeO₂. The Ce₂O₃/SiO₂ ratio of intergranular phase (liquid phase) increased as the formation of Si₂N₂O proceeded. The intergranular liquid remained as a glass on cooling until the Ce₂O₃/SiO₂ ratio exceeded a certain value, at which point the liquid crystallized. There were great differences in thermal and mechanical properties and oxideation behavior between the specimen containing intergranular glassy phase and the one containing intergranular crystalline phase (Ce₅(SiO₄)₃N–Ce_4.67(SiO₄)₃O). The specimen containing the intergranular glassy phase showed excellent hightemperature strength and oxidation resistance.     

Processing and Mechanical Properties of Zirconium Diboride-Based Ceramics Prepared by Spark Plasma Sintering

Zirconium diboride (ZrB₂) reinforced by nano-SiC whiskers has been prepared by spark plasma sintering (SPS). Of most interest is the densification of ZrB₂–SiCw composites accomplished by SPS at a temperature as low as 1550°C. The relative density of ZrB₂–SiCw composites could reach to 97% with an average grain size of 2–3 μm. Both flexural strength and fracture toughness of the composites were improved with increasing amount of SiCw. Flexural strengths ranged from 416 MPa for monolithic ZrB₂ to over 545 MPa for ZrB₂–15 vol% SiCw composites. Similarly, fracture toughness also increased from 5.46 MPa·m^1/2 to more than 6.81 MPa·m^1/2 in the same composition range. The relative density of ZrB₂–SiCw composites could be further improved to near 100% by adding some sintering aids such as AlN and Si₃N₄; however, the effects of different sintering additives on the mechanical properties of the composites were different.     

Influence of Stoichiometry on the Electrical and Mechanical Behavior of Yttrium Oxide Ceramics

Because of their method of preparation, ceramics contain point defects, the concentration of which may vary from the center to the boundary of the grains, which may change the bulk properties of the material. The electronic structure of the oxygen-related defects has been studied by electron spectroscopy in yttria ceramics submitted to various heating conditions under controlled atmospheres. The differences in the mechanical and electrical behaviors are observed to correlate with the defect content.     

纳米陶瓷及其发展趋势

纳米陶瓷改变了传统陶瓷的脆性，大幅度提高了材料的强度、韧性和超塑性，同时对传统陶瓷材料的电学、热学、磁学、光学等性能产生了重要影响。本文综述了纳米陶瓷的性能以及发展趋势。     

Microstructure and Properties of Plasma Ceramics

Plasma spraying, originally a surfacing technology used for ceramic coating of metal and nonmetal substrates, is suitable for producing large, full ceramic bodies, such as plates and tubes, in situ to almost the desired dimensions; i.e., conventional methods of designing by molding and casting green bodies followed by sintering are not required. These "plasma ceramics" exhibit characteristic mechanical and thermomechanical properties. Their porous laminar grain structure results in low E -modulus and strength, and in extreme thermal shock resistance, which is enhanced by the comparably high fracture resistance of the material. Because of these properties and their low hardness, plasma ceramics can be easily machined with almost conventional methods of metal processing.     

Radio-Frequency Plasma Spraying of Ceramics

This study was aimed at developing a novel spraying process using a radio-frequency (rf) plasma. Experiments of Al₂O₃ and ZrO₂–8 wt% Y₂O₃ spraying showed that the initial powder size was the most important parameter for depositing dense coatings. The optimum powder sizes of Al₂O₃ and ZrO₂–8 wt% Y₂O₃ were considered to be around 100 and 80 μm, respectively. The use of such large-size powders compared with those used by conventional dc plasma spraying made it possible to deposit adherent ceramics coatings of 150 to 300 μm on as-rolled SS304 substrates. It was also shown that low particle velocity of about 10 m/s, which is peculiar to rf plasma spraying, was sufficient for particle deformation, though it imposed a severe limitation on the substrate position. These experimental results have proved that rf plasma spraying is an effective process and must be a strong candidate to open new fields of spraying applications.     

石英陶瓷的应用

介绍了石英陶瓷在玻璃行业以外，冶金、化工、航空航天等领域的应用，包括应用的石英陶瓷制品、应用特点及条件。     

车用陶瓷及其应用解读

特种陶瓷在化学组成、内部结构、性能和使用效能各方面均不同于传统陶瓷,它具有各种优异、独特的性能。正是由于特种陶瓷这些优异的性能,对减轻车辆自身重量、提高发动机热效率、降低油耗、减轻排气污染、提高易损件寿命,完善汽车智能性功能都有重要意义。根据车用陶瓷材料的性能特点与应用前景,分别介绍了氧化物陶瓷、氮化物陶瓷、碳化物陶瓷...     

陶瓷超塑性及其应用

本文阐述了材料超塑性特征和陶瓷材料超塑性的最近发展，讨论了超塑性在陶瓷成型加工过程中的应用。     

Fracture Behavior of Open-Cell Ceramics

Cellular structures are commonly used by nature as a means of optimizing the strength-to-weight ratio of a body. Cellular ceramics may have potential in structural applications requiring a high modulus-to-weight ratio. The models that are used to predict the mechanical behavior of these materials describe them as an interconnected network of columns and plates and predict a dependence on density and cell size. The fracture toughness and strength of several open-cell alumina materials were measured and compared to a theoretical model. Image analysis was used to fully characterize the macrostructure of the samples and provided a basis for evaluating the toughness and strength behavior. It was observed that the properties of the bulk material depend on the strength of the individual struts in the structure. The fit of the data to an open-cell model was found to be dependent on the agreement between the actual macrostructure and that assumed in the model. Both microscopic and macroscopic flaws contribute to the variability of the strength in these materials and it was found that there was a wide distribution of bulk and strut strengths. Both of these flaw populations must be controlled to maximize the mechanical properties of these open-cell ceramics.     

Fibrous Monolithic Ceramics: IV, Mechanical Properties and Oxidation Behavior of the Alumina/Nickel Systein

Fibrous monolithic ceramics were fabricated in the alumina/nickel system. The microstructure consists of high-aspect-ratio polycrystalline cells of alumina separated by thin cell boundaries of nickel. The nickel content in the material is 3 to 8 vol%. The fibrous monolith with uniaxially aligned cells fails noncatastrophically in flexure. Bridging ligaments of nickel, crack deflection along cell boundaries, and crack branching in the axial direction are observed in flexure bars and notched beams. Strength values range from 246 to 375 MPa. Indentations cause controlled damage on the surface but do not introduce strength-degrading flaws. The alumina/nickel fibrous monoliths also show potential for use at high temperatures in oxidizing environments. Noncatastrophic fracture behavior is observed at room temperature after 10 h at 1200°C in air. The Ni cell boundary network is oxidized to a depth of 50 to 100 μm by this heat treatment. The NiO oxidation product in the cell boundaries reacts partly with alumina from the cells to form NiAI₂O₄, which would provide better protection.     

Thermal Shock Behavior of Duplex Ceramics

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, Δ T _c , is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before Δ T _c is reached. The effects are accompanied by a gradual reduction in strength. At Δ T _c , critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K ^R -curve behavior.     

BaPbO_3导电陶瓷的研究与应用

本文综述了BaPbO3 导电陶瓷研究的最新成果 ,并对其结构性能、导电机理、制备及应用作了详细的论述     

AlN陶瓷制备过程中氧化行为研究

本论文以氮化铝陶瓷的制备为研究对象,围绕其在烧结过程中的氧化进行了研究,分析了氮化铝陶瓷在烧结过程中氧化的机理,提出了防止AlN陶瓷制备过程氧化的措施,确定了合适的埋粉碳粉和AlN粉体,并确定了合适的配比点。     

Fabrication and Mechanical Properties of Titanium Boride Ceramics

Dense TiB ceramics (99.6% of theoretical) with a grain size of ∼5 µm have been fabricated by reaction hot pressing of TiB₂ and titanium for 2 h at 1900°C and 28.5 MPa. The TiB ceramics exhibit a fracture toughness ( K_IC ) of 4.5 MPa·m^1/2 and a bending strength (sigma_b) of 360 MPa. Electrical resistivity (rho) is 3.4 × 10^-7 Omega·m at room temperature.     

Crack Propagation Behavior of Y-TZP Ceramics

Y-TZP ceramics present the twofold advantage of a very fine grain size and a phase transformation leading to some reinforcement, making them useful in hip joint head prostheses. The subcritical crack growth of such a material that mainly drives the lifetime of the components has been evaluated for wide crack velocity ranges down to 10^-11 m/s by using both double torsion and static fatigue tests. It appeared that subcritical crack growth was activated by H₂O. It is shown that two types of flaws should be considered: surface flaws caused by machining located in the compressive layer present at the surface, and so-called volume flaws. The subcritical crack growth and the toughness of both types of defects are different. The relevance of subcritical crack growth analysis for such surface cracks is discussed. The compressive layer at the surface leads to a threshold value below which no crack growth occurs.     

Elastic Behavior of Polymer-Impregnated Porous Ceramics

The effect of polymer impregnation on the elastic behavior of porous ceramics was investigated. Large increases in the Young's, shear, and bulk moduli and Poisson's ratio were observed. On the basis of a two-dimensional model containing elliptical inclusions, it was shown that the primary role of the polymer on impregnation is to significantly offset the original reduction in elastic moduli caused by the pore phase. The relative effect of polymer impregnation on elastic behavior increases with increasing pore volume fraction and pore ellipticity. These conclusions are also expected to explain the elastic behavior of polymer-modified cements and concretes.     

Fracture Behavior of Low-Density Fibrous Ceramics

It was established that subcritical crack growth occurs in the silica fiber-based tiles that are currently in use as part of the thermal protection system (TPS) of the Space Shuttle. By measuring the tensile strength distribution and the subcritical crack-growth parameters, it was possible to predict the material's behavior during proof-testing. A similar time dependence of dynamic strength was also found for structural systems using this ceramic tile, suggesting that the ceramic is controlling their strength behavior. It was possible, therefore, to predict the influence of the proof testing currently in use for the Shuttle. These predictions were found to be in reasonable agreement with the experimentally determined strength distributions after proof-testing. This study confirmed that proof-testing and tile densification should lead to improved reliability of the TPS. The research further suggests that the subcritical crack-growth behavior in these fibrous tiles is simiilar to that observed in bulk glass of similar composition and that failure occurs by the tensile fracture and fragmentation of individual fibers.