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The oxidation process of non-oxide ceramics (NOCs) is accompanied with the formation of the compressive stress generated within the oxide film, which in turn affects the oxidation behavior. To shed light on the influence of the compressive stress, a modified model combining the stress equilibrium equation and RPP model has been herein proposed. This modified model can not only quantify the effect of stress evolution on the oxidation process but also accurately describe the relationship between oxidation fraction and other factors including time and temperature, etc. The comparison between experimental data and theoretical calculations for the oxidation of different NOCs verifies the validity of the modified model. It is predictable that this modified model can be applied to treat the oxidation behavior of different NOCs with the consideration of compressive stress.

With the progress in science and technology, non-oxide ceramics (NOCs) are playing a vital role in various fields at high temperature. However, under harsh service conditions, NOCs tend to be confr...

Thermodynamic and diffusion models are given to describe morphological evolution of the reaction zone during diffusion-limited interaction between non-oxide Si-containing ceramics (SiC and Si3N4) and transition metals (Cr, Mo, Ti, Ni, Co, Pt). In the case of diffusion-controlled process in the ternary metal-ceramic systems, reaction phenomena can be rationalized using chemical potential diagrams. However, in some cases, a periodic layered morphology is found in the transition zone, which is not fully understood, and it is difficult to predict a priori. Silicide formation in systems based on dense Silicon Nitride and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Silicon at the interface, and hence, the product phases in the diffusion zone. Traces of Oxygen in the ambient atmosphere might affect the interaction in non-oxide ceramic/transition metal systems. The thermodynamic stability of the condensed phases in the systems where volatile species may form can be interpreted using predominant area-type diagrams.

In this chapter, the non-oxide powder surface characteristics before and after treatment, and the effect of surface modification on the solids loading of its aqueous suspension were studied. For Si3N4 powders, a universal method to prepare highly concentrated suspensions has been established. The results show that Si3N4 with high reliability was obtained via processing strategy, for example, using a combination of coating, oxidizing, gelcasting, cold isostatic pressing, and gas-pressure sintering (GPS).

Fracture toughness is one of the most important parameters for ceramics description. In some cases, material failure occurs at lower stresses than described by KIc parameter. In these terms, determination of fracture toughness only, proves to be insufficient. This may be due to environmental factors, such as humidity, which might cause subcritical crack propagation in a material. Therefore, it is very important to estimate crack growth velocities to predict lifetime of ceramics used under specific conditions. Constant Stress Rate Test is an indirect method of subcritical crack growth parameters estimation. Calculations are made by using strength data, thus avoiding crack measurement. The expansion of flaws causes reduction of material strength. If subcritical crack growth phenomenon occurs, critical value of crack lengths increases with decreasing stress rate due to longer time for flaw to grow before the critical crack propagation at KIc takes place. Subcritical crack growth phenomenon is particularly dangerous for oxide ceramics due to chemical interactions occurring as a result of exposure to humidity. This paper presents results of Constant Stress Rate Test performed for alumina, zirconia, silicon carbide and silicon nitride in order to demonstrate the differences in subcritical crack propagation phenomenon course.

[Liu, Z. Y.; Wang, Q. Z.; Xiao, B. L.; Ma, Z. Y.] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China.;Xiao, BL (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;blxiao@imr.ac.cn

Several models have been proposed in the literature to describe the thermal and electrical conductivities of particulate composites. Among the proposed models, the Lewis–Nielsen model appears quite attractive as it is simple to use and it predicts the correct behavior when filler concentration (ϕ) approaches the maximum packing concentration (ϕm). In this paper, the Lewis–Nielsen model is evaluated in light of a vast amount of experimental data available on thermal and electrical conductivities of particulate composites. The Lewis–Nielsen model is found to describe the experimental data for both thermal and electrical conductivities reasonably well.

Sensing indentation tests at different maximum loads were carried out on two crystalline ceramic materials using Berkovich and spherical indenters. Hardness and Young´s modulus were estimated from load – displacement curves through Oliver and Pharr method. Values from mechanical properties change with the penetration depth depending on indentation size, material stiffness and geometry of the indenter. To estimate the indenter deformation effect, finite elements simulations were developed to simulate the indentation process on an elastic material with the properties of the stiffer material (Boron carbide).En este trabajo se han realizado ensayos de indentación instrumentada sobre dos materiales cerámicos empleando indentadores Berkovich y esférico a diferentes cargas máximas. Se ha determinado su dureza y el módulo de elasticidad a partir del análisis de las curvas carga – desplazamiento con el método de Oliver y Pharr. Se ha observado que los valores de ambas propiedades cambian con el tamaño de la indentación. Sin embargo, estas variaciones dependen de diversos factores como el efecto del tamaño de la indentación, la rigidez del material indentado y la geometría del indentador. Con el fin de acotar el efecto que la deformación del indentador tiene sobre las tendencias observadas, se han realizado simulaciones por elementos finitos del ensayo de indentación sobre un material elástico lineal con las propiedades del material cerámico más rígido estudiado (carburo de Boro) y analizando los casos de indentador rígido y elástico.

The ceramic yield of a new polycarbosilane is discussed based on its structure, including vinyl functionality, branched structure, as well as polymer molecular weight. The cross-linking behavior of this polymer is studied up to 400 °C. According to the cross-linking mechanism, the optimized heat-treatment conditions can be obtained for a high ceramic yield. The product pyrolysized at 1400 °C has a composition close to 1.1:1 Si:C with a ceramic yield of about 70%.

Ambient temperature mechanical properties of Al–8.5Fe–1.3V–1.7Si/15 vol.%SiC P composite sheet prepared by a spray deposition (SD)–rolling process were investigated. The experimental results show that the densification method of the as-deposited preforms has an obvious effect on the mechanical properties of the as-rolled sheets. The σ b and δ were 535 MPa and 7%, respectively, for the sheet with the preforms extruded at 490 °C and then rolled at 490 °C. When the preforms were pressed at 450 °C and then rolled at 490 °C, the obtained σ b and δ were up to 620 MPa and 9.5%. The microstructures, density and rigidity are also examined.

The technically important quasi-ternary section Si3N4-AlN-Al2O3-SiO2 of the Si-Al-O-N system has been then-nodynamically reassessed. Improved descriptions for the Gibbs energies of the beta and O'- ...

The carbothermal reduction processing of partially mineralized fir (Abies alba) samples was used to obtain highly-porous SiC ceramics with cellular structure. The infiltration of TEOS (tetraetilortosilikat, Si(OC2H5)(4)) as a silica source, was conducted in order to carry out the mineralization process. Synthesis of the SiC was achieved with a C/SiO2 replica annealing at 1723 K in Ar atmosphere. The obtained samples were characterised using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The experimental results revealed that the hierarchical bimorphous wood structure was preserved even after high-temperature treatment. Microstructural characterization of the ceramics revealed the presence of the beta-SiC phase and traces of the alpha-SiC phase. 5th Young Researchers Seminar, Dec 25-26, 2006, Belgrade, Serbia

International audience; À " basse température " (entre 400 et 600 ◦C), l'oxydation de MoSi2 entraîne sa désintégration en poudre (phénomène de " peste "). De récents travaux ont montré que l'utilisation de MoSi2 dense et nano-organisé permettrait de ralentir ce phénomène de " peste ". Le défi de produire des matériaux denses et nano-organisés peut être relevé par le frittage " flash " réactif sous champ électrique à partir des poudres mécaniquement activées (Mechanically-Activated Spark Plasma Sintering, MASPS). Le contrôle de la composition et de la microstructure du composé intermétallique MoSi2 nécessite de déterminer les paramètres du frittage SPS (température, rampe de montée en température, pression uniaxiale appliquée et temps de maintien) lorsque des poudres activées mécaniquement sont utilisées. À l'heure actuelle, par ce procédé, les échantillons présentent une phase majoritaire, MoSi2 à plus de 97 %, et une phase secondaire, Mo5Si3. Il est donc indispensable de comprendre les transitions de phase qui peuvent avoir lieu au cours d'un frittage impliquant simultanément une consolidation et une réaction SHS. Il a donc été essentiel de développer un nouveau type d'équipement pour reproduire partiellement ce procédé. Ceci a été réalisé avec succès en mettant en oeuvre une analyse par diffraction des rayons X " in-situ " et en temps réel couplée à une analyse par thermographie IR.

Effects of frequency on fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite (C/SiC) were investigated at an elevated temperature of 550 °C. Tension–tension fatigue tests were conducted at three frequencies: 0.1, 10 and 375 Hz to establish stress versus cycles to failure ( S–N ) relationships. There was an increase in cycles to failure at a given stress level as frequency increased from 0.1 to 375 Hz at elevated temperature. This trend was different at room temperature where cycles to failure decreased when frequency changed from 40 to 375 Hz but remained almost same below 40 Hz. There was a reduction in cycles to failure at frequencies less than 40 Hz but cycles to failure remained same at a higher frequency of 375 Hz when test environment changed from room temperature to 550 °C. Analysis of damage mechanisms showed that the oxidation of carbon fibers was the major difference between the room and elevated temperatures, which caused a reduction in cycles to failure with lower frequencies at elevated temperature in comparison to that at room temperature. However, oxidation of carbon fibers was almost absent or negligible at higher frequency at elevated temperature, which caused practically no reduction in cycles to failure at elevated temperature in comparison to their counterparts at room temperature.

Tensile creep tests of two-dimensional-C/SiC specimens with double-edge arc notches have been carried out at 1100, 1300 and 1500 °C in vacuum. The matrix cracks on the surface and resonance frequency were examined at different creeping times. At 1100 °C, the creep strains of both smooth and notched specimens were concentrated at the transient stage and the steady creep rates were nearly zero, whereas steady creep rates of notched specimens and smooth specimens were similar at 1500 °C. It has been observed that the creep damage mainly concentrated at the area near the notches. Micro-cracks appeared in the area near the notches and on the cross-points of the woven fiber bundles, and the longitudinal fibers near the notches fractured easily. Both types of curves, namely quantity of micro-cracks vs. time and micro-crack width vs. time, were extremely similar as for the creep curves. In general, micro-cracks developed fast during the first 10 h. It has been noticed that within the first 2 h, the micro-cracks near the notches grow faster than those far from the notches, whereas the growth rate of micro-cracks far from notches was faster than those near the notches after 2 h. This phenomenon indicates the stress redistribution during creep. Damage curves at 1300 and 1500 °C have similar trend, though the damage and the quantity of micro-cracks at 1500 °C are higher than those at 1300 °C.

The effect of ZrO2-3% Y2O3 addition on densification, sintering behavior and mechanical properties of a boron carbide matrix was investigated. When adding 0–30 wt% of ZrO2-3% Y2O3 to B4C sintered densities were increased from 75% to 98.5%. Sinterability was significantly improved by addition of a small amount of ZrO2-3% Y2O3. As a result of density improvement, mechanical properties such as hardness, strength and fracture toughness were increased remarkably. However, when the amount of ZrO2-3% Y2O3 exceed from 20 wt%. Hardness started to reduce.

La necesidad de desarrollar nuevos implantes basados en materiales bioactivos que sean capaces de soportar grandes cargas mecánicas ha llevado a la producción de sustratos metálicos recubiertos con cerámicas bioactivas. Recientemente se ha propuesto un dispositivo alternativo que consiste en un sustrato de carburo de silicio (SiC) biomórfico recubierto con vidrio bioactivo, mediante la técnica de Depósito por Láser Pulsado (PLD), y que dispone de la resistencia mecánica adecuada, además de gran ligereza y una porosidad intrínseca muy favorable de cara a la implantación. En este trabajo se presenta un estudio interdisciplinar de este nuevo material centrado en la morfología y porosidad de sustratos de SiC provenientes de diferentes maderas, la bioactividad de los recubrimientos producidos por PLD y en la evaluación in vitro con células de osteosarcoma MG-63 con la que se ha determinado la citotoxicidad de estos materiales y se ha estudiado la influencia de los mismos en la adhesión y la proliferación celular. In the past years there was a need to develop new tough bioactive materials capable to resist high loads when implanted in the body, that led to the production of bioactive coatings on metallic substrates. A new approach, which consists of biomorphic silicon carbide (SiC) coated with bioactive glass by Pulsed Laser Deposition (PLD), was recently presented. This new material joins the high mechanical strength, lightness and porosity of biomorphic SiC and the bioactive properties of PLD glass films. In this work, a multiple evaluation of this new material is presented starting from the biomorphic SiC morphology and porosity, following with the bioactivity in simulated body fluid of the coatings, and ending with a deep in vitro study with MG-63 cells. The citotoxicity of the SiC coated and uncoated and the cell proliferation and attachment were studied.

Incorporation of carbon nanotubes (CNTs) into the precursor-derived Si-(B-)C-N ceramics has been investigated for the reinforcement of the materials. Different types of CNTs consisting of multi-wall (MW) and single-wall (SW) were examined as the reinforcement of the Si-(B-)C-N ceramics to make a comparison of the effect. Mechanical properties demonstrated in the Si-(B-)C-N/CNT nanocomposites have been discussed in connection with their microstructural features characterized by scanning electron (SEM) and transmission electron microscope (TEM). Other material properties of the nanocomposites as revealed on the thermal stability and the crystallization behavior have been also considered in relation to the microstructural characteristics of the nanocomposites. Dense Si-C-N/CNT nanocomposites containing different types of MWCNTs were successfully prepared by casting of a mixture of MWCNTs and a liquid precursor polymer followed by cross-linking and thermolysis. In these processes, the sonication for deagglomeration and dispersion of CNTs in the precursor polymer as well as the thermolysis condition for ceramization of the cross-linked precursor/CNT nanocomposites was examined to obtain homogeneously CNT distributed Si-C-N ceramics. Fracture toughness behavior of the Si-C-N/CNT nanocomposites has been evaluated by a thermal loading technique on the disc shaped materials. The results reveal a dependence of the fracture toughness on the type of the MWCNTs. The MWCNTs showing high integrity in the tube structure exhibit a remarkable increase in the fracture toughness at the CNT content of 1 – 2 mass %, whereas the other ones possessing amorphous nature exhibits no effect. The microstructural analyses at the fracture surfaces have demonstrated different features of CNTs between both nanocomposites, where pulling out and breaking of CNTs are considered to be reasons for the observed fracture toughness increase. No significant influences observed on the material properties of the Si-C-N/CNT nanocomposites besides the toughening indicates that CNTs can simply work as the reinforcement for the Si-C-N ceramics. SWCNTs incorporation into the Si-C-N materials has revealed toughening effect with similar microstructural features to the MWCNT reinforced Si-C-N nanocomposites. In this system, it was found that the deagglomeration and debundle of the SWCNTs are major issues to make the best use of SWCNTs as the reinforcements. Concerning the Si-B-C-N/CNT nanocomposites, preparation processes via a casting and a warm pressing from different types of boron-containing precursors have been investigated to produce rigid MWCNT nanocomposites. The observed pulling out and breaking CNTs structure at the fracture surfaces of the Si-B-C-N/CNT nanocomposites indicate the toughening effect of CNTs similar to Si-C-N/CNT ceramics. Moreover, the interaction between CNTs and the matrix has appeared to be changed with increasing thermolysis temperature. However, the crystallization of the Si-B-C-N matrix and the deterioration of thermal stability have been disclosed in the Si-B-C-N/CNT nanocomposites. It is revealed that embedded CNTs have an effect to accelerate or to generate nucleation sites for the crystallization of Si-B-C-N matrix. Si-(B-)C-N-Keramiken, die durch die Thermolyse von prakeramischen Polymeren erzeugt werden, weisen eine bemerkenswerte Hochtemperatur-, Korrosions- und Kriechbestandigkeit auf. Die Bestandigkeit dieser Precursorkeramiken basiert grundsatzlich auf deren kovalentem Charakter und der hohen Reinheit dieser Materialien, welche keine Sinteradditive enthalten. Der amorphe und dadurch bedingte sprode Charakter dieser Werkstoffe fuhrt jedoch auch zu einer Verschlechterung der mechanischen Eigenschaften, insbesondere der Bruchzahigkeit. Ein Ansatz zur Verstarkung von sproden Keramiken ist die Einlagerung von zusatzlichen Phasen bzw. die Erzeugung von Verbundwerkstoffen. In diesem Zusammenhang bietet sich der Einbau von Kohlenstoff-Nanorohrchen an, da diese eine enorme mechanische Festigkeit sowie ein hohes Lange/Durchmesser-Verhaltnis besitzen. In dieser Arbeit wurden Kohlenstoff-Nanorohrchen (carbon nanotubes, CNTs) im Hinblick auf deren Eignung zur Verstarkung von Si-(B-)C-N-Precursorkeramiken untersucht. Eingesetzt wurden sowohl Nanorohrchen vom Typ „multi-wall“ (MW) als auch „single-wall“ (SW). Dabei wurden insbesondere die mechanischen Eigenschaften in Verbindung mit den vorliegenden Gefugen der Si-(B-)C-N/CNT-Verbundmaterialien behandelt. Auserdem war die thermische Stabilitat und das Kristallisationsverhalten Gegenstand der Arbeiten. Zur strukturellen Charakterisierung der Materialien wurden vor allem die Rasterelektronenmikroskopie (REM) und die Transmissionselektronenmikropie (TEM) herangezogen. Dichte Si-C-N/CNT-Nanocomposite mit einem Gehalt von 0 – 2 Masse% an CNTs wurden erfolgreich durch Giesen einer Mischung aus Nanorohrchen und dem Precursor Ceraset TM sowie anschliesender Vernetzung (360 °C, 6h) und Thermolyse (1000 °C, 1h) erzeugt. Es zeigte sich, dass eine Ultraschallbehandlung der Nanorohrchen in einem Losungsmittel sowie eine niedrige Heizrate bei der anschliesenden Thermolyse im Hinblick auf eine Erzeugung von dichten und rissfreien Verbunden mit einer homogenen Verteilung der Nanorohrchen wesentliche Grosen darstellen. Diese Prozesse ermoglichen prinzipiell eine endkonturnahe Herstellung von Si-C-N/CNT-Formteilen mit kleinen und komplexen Geometrien. Die Untersuchung der mechanischen Eigenschaften dieser Nanocomposite mittels einer Thermoschockmethode zeigte, dass die Bruchzahigkeit von der Art der zur Verstarkung eingesetzten MWCNTs abhangt. So konnte die Bruchzahigkeit bereits bei einer Zugabe von lediglich 2 Masse% an Nanorohrchen in einem Fall im Vergleich zur reinen Precursorkeramik um 70 % gesteigert werden. Dagegen zeigte ein zweiter eingesetzter MWCNT-Typ keinen Effekt dieser Art. Im Falle der Materialien mit verbesserten Eigenschaften ergibt sich aus rasterelektronenmikroskopische Aufnahmen, dass hierzu verschiedene Mechanismen („pull-out-Effekt“, Uberbruckung sowie das Brechen der Rohrchen) beitragen. Weitere Untersuchungen mittels der Transmissionselektronenmikroskopie zeigen auserdem den Erhalt der typischen Struktur der Nanorohrchen und das Vorliegen einer definierten CNT/Matrix-Grenzflache. Im Falle der Nanorohrchen, durch welche die mechanischen Eigenschaften nicht beeinflusst werden, liegt dagegen eine sehr starke CNT/Matrix-Wechselwirkung vor. Zudem weisen diese Rohrchen eine amorphe Struktur auf, so dass weder die oben beschriebenen Effekte noch verbesserte mechanische Eigenschaften beobachtet werden. Aus diesen Untersuchungen geht somit hervor, dass der Einbau der Nanorohrchen in die Matrix und die CNT/Matrix-Wechselwirkung wesentliche Parameter in Bezug auf die mechanische Verstarkung der Materialien darstellen. Im Falle der Herstellung von Si-B-C-N/CNT-Verbundmaterialien zeigte sich, dass deren Herstellung uber den oben beschrieben Giesprozess nicht moglich ist. Es konnten aufgrund der Sprodigkeit der vernetzten Si-B-C-N-Precursoren sowie wegen deren starken Haftung an der Wand der Giesformen keine stabilen Formteile erhalten werden. Eine alternative Erzeugung uber Warmpressen liefert jedoch Materialien, die Bruchflachen aufweisen, welche denen der oben beschriebenen Si-C-N/CNT-Verbundmaterialien mit verbesserten mechanischen Eigenschaften ahneln, was auf einen Verstarkungseffekt der Nanorohrchen auch im Falle der Si-B-C-N-Precursorkeramiken hinweist. Die Verstarkung der Wechselwirkung zwischen den Nanorohrchen und der Matrix mit steigender Auslagerungstemperatur die mit einem Ubergang von pull-out-Strukturen zum Brechen der Nanorohrchen verbunden ist, bietet prinzipiell die Moglichkeit, die fur die Verstarkung erforderliche optimale Thermolysetemperatur zu bestimmen. Die Erhohung der Thermolysetemperatur geht jedoch mit einer zunehmenden thermischen Zersetzung sowie der Kristallisation der Materialien einher, was diese Moglichkeit zur Erhohung der Zahigkeit wiederum eingeschrankt. Dabei ergibt sich aus Untersuchungen mittels der Rontgendiffraktometrie und der Transmissionselektronenmikroskopie, dass die Nanorohrchen als Template fur die Kristallisation unter Bildung von Siliciumcarbid wirken.

A volatility diagram was calculated for temperatures of 1000, 1800, and 2500 K to understand the oxidation of ZrB2. Applying the diagram, it can be seen that exposure of ZrB2 to air produces ZrO2 (cr) and B2O3 (l) over the temperature range considered. The pressure of the predominant vapor species was predicted to increase from ∼10−6 Pa at 1000 K, to 344 Pa at 1800 K, and to ∼105 Pa at 2500 K. Predictions were consistent with experimental observations that ZrB2 exhibits passive oxidation below 1200 K, but undergoes active oxidation at higher temperatures due to B2O3 (l) evaporation.

Accepted: 2004-09-28, 資料番号: SA1003436000

Ultimate tensile strength of five different continuous fiber-reinforced ceramic matrix composites (CMCs), including SiCf/BSAS (two dimensional (2D), 2 types), SiCf/MAS (2D), SiCf/SiC (2D), and Cf/SiC (2D, 2 types), was determined as a function of test rate at 1100–1200 °C in air. All five CMCs exhibited a significant dependency of ultimate tensile strength on test rate such that the ultimate tensile strength decreased with decreasing test rate. The dependency of ultimate tensile strength on test rate, the applicability of preload technique, and the predictability of life from one loading configuration (constant stress-rate loading) to another (constant stress loading) all suggested that the overall, phenomenological delayed failure of the CMCs would be governed by a power-law type of slow crack growth.

The R-curve for Si{sub 3}N{sub 4}-40 wt% TiN composites was estimated by the indentation-strength method and compared to that of monolithic Si{sub 3}N{sub 4} with duplex microstructure. Both materials exhibited rising R-curve behavior. The Si{sub 3}N{sub 4}-TiN composites, however, displayed better damage tolerance and higher resistance to crack growth. From TEM observation, it was inferred that this superior performance of Si{sub 3}N{sub 4}-TiN composites can be attributed mainly to stress-induced microcracking at heterophase (Si{sub 3}N{sub 4}/TiN) boundaries.

Five laboratories tested NIST-supplied, pin-loaded, 76-mm-long tensile creep specimens at 1400°C under a 150 MPa load using flag-based, laser extensometry. The laboratories reported failure time and strain and supplied the individual creep curves. Only one of the laboratories produced failure times that were significantly less than the others. It is likely that their reduced failure times resulted from small load calibration and test temperature errors. After steps were taken to ameliorate these problems, three additional tests yielded failure times that agreed with those of the other four laboratories. Although the times to failure from the four laboratories that initially agreed were statistically indistinguishable, their creep curves exhibited subtle differences. These differences probably arose because the laboratories used different gage length definitions. When we recalculated the creep curves to the same gage length definition, the differences between the four laboratories whose times to failure agreed, vanished. Although a number of the specimens exhibited edge chips, creep cracks, and obvious chemical interactions with the flags, the presence of these defects did not reduce the time or strain to failure. Two additional creep tests in our laboratory, using specimens that were grossly misaligned, yielded failure times and strains that were commensurate with those from well-aligned specimens.

An evaluation of measurements of crack opening displacements (CODs) on a commercial sintered reaction-bonded silicon nitride (SRBSN) was performed. To determine the intrinsic fracture toughness of this material, a new evaluation method is presented, which takes into account not only the near tip CODs, but the CODs of the complete crack profile. The method is applied on through-thickness cracks in bend bars and contrasted to CODs of Vickers radial cracks. A crack-tip toughness of ∼1.7 MPa·m1/2 is obtained.

This paper examines the classification and quantification of initial or manufacturing porosity of a 10 layer laminate ceramic matrix composite DLR-XT. Both optical and scanning electron microscopy (SEM) techniques are used to identify different classes of porosity and to measure and quantify their extent. Four classes of porosity have been identified, namely inter-fibre micro-porosity; trans-tow cracks; matrix cracks; and denuded matrix regions. The presence of each of these types of porosity has been confirmed by both optical and SEM examination. In order to quantify the degree of porosity, two approaches have been used. For the case of void-like porosity, area fraction has been used; and, for crack-like porosity, the periodicity of the cracks and their lengths has been measured. The paper concludes by tabulating porosity types and their levels observed. No interpretation is made of the significance of these magnitudes since this will be subsequently carried out in the context of their influence on thermal and mechanical properties. This will form the subject of a series of subsequent papers.

Environmentally conscious, biomorphic ceramics (Ecoceramics) are a new class of materials that are manufactured from renewable resources and wastes. In this study, silicon carbide and oxide-based biomorphic ceramics have been fabricated from pine and jelutong wood precursors. A carbonaceous preform is produced through wood pyrolysis and subsequent infiltration with oxides (ZrO2 sols) and liquid silicon to form ceramics. These biomorphic ceramics show a wide variety of microstructures, densities, and hardness behavior that are determined by the type of wood and infiltrants selected.

The relationships between mechanical properties of Si 3 N 4 /BN multilayer materials and the geometrical factors, number of layers ( N ) and layer thickness ratio ( λ h ), have been studied. At a given number of layers, with the increase of layer thickness ratio, the toughness and bending strength both increase, and when it continues to increase, the two curves both level off. Keeping the layer thickness ratio of about 10 and increasing the number of layers, the bending strength decreases slightly, and a maximum toughness was obtained at N ≈30. Furthermore, a mechanical model was established and used to analyze the above experimental results. According to the experimental and calculated results, suitable geometrical parameters were advised in the design of Si 3 N 4 /BN multilayer materials as λ h ≈10 and N ≈30.

Joining of dissimilar materials allows the properties of both materials to be exploited in a device or structure. The main reasons for the incorporation of dissimilar materials are to achieve function, improve efficiency and to reduce cost.

Microwave preparation of MoSi2–SiC composites has been studied using several strategies. Direct sintering of MoSi2 and SiC mixtures using 2.45 GHz microwaves has been found to be the most successful approach. Sintered composites exhibit higher than 96.4% of theoretical density. Sintering times required are very short and of the order of only 10 min. The observed accelerated sintering has been discussed in the context of unique microwave effects. Vickers hardnesses and fracture toughnesses of the composites are in the range of 9.25–13.2 GPa and 3.2–4.5 MPa m1/2, respectively. Microwave sintered composites do not exhibit any pesting tendency at 550 °C even after 72 h of exposure to air. Reactive sintering of elemental powders and partially reactive sintering based on the use of one of the compounds (MoSi2 or SiC) in the premix, do not result in good sintering and lead to the formation of unintended products such as Mo5Si3, Si3N4 and Si2ON2.

The microstructure of silicon nitride with aligned silicon nitride whisker seeds was examined by electron microscopy. Silicon nitride sintered with yttria and alumina showed a “core-rim” structure. A few grains of the sample sintered with yttria and alumina had more than one core within one grain. This was explained in terms of coalescence of the grains growing from separate cores. A boundary with small misfit was observed by TEM, also supporting the possibility of grain coalescence.

B4C based ceramic composites with 0−25 mol% CrB2 were fabricated by hot-pressing at 1900 °C, and their mechanical properties were examined. The B4C−CrB2 composites with both high strength of 630 MPa and modest fracture toughness of 3.5 MPa m1/2 could be obtained by the addition of 20 mol% CrB2. It seems that improvement in fracture toughness is due to the formation of microcracks and deflection of propagating cracks caused by the thermal expansion mismatch between CrB2 and B4C.

Titanium carbide (TiC) particles have been added to molybdenum disilicide (MoSi2) to form TiC–MoSi2 composites. Benefits associated with the addition of the TiC to MoSi2 include finer microstructure, higher strength, higher fracture toughness and higher hardness. Under the wear condition of this test, the wear resistance of MoSi2 increases significantly by the addition of TiC particles. The wearing rate of TiC–MoSi2 composites is as low as tenth of monolithic MoSi2. The effects of fracture toughness and hardness on wear resistance are discussed.

A 3D C/SiC composite and a bulk CVD SiC material were prepared. The effects of the CVD SiC coating and the heat treatment on the longitudinal and transverse thermal diffusivity of the C/SiC composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400 °C which includes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200 °C with activation energy of 77 kcal/mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200 °C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature.

The vaporization behaviour of pure Al 2 O 3 , Y 2 O 3 and SiC as well as SiC–Al 2 O 3 and SiC–Al 2 O 3 /Y 2 O 3 mixtures has been analysed by thermodynamic calculations in an open system. Pure Al 2 O 3 and Y 2 O 3 evaporate congruently in the 1200–2300 K temperature range. Pure SiC vaporizes in a non-congruent manner leading to graphite formation as by-product. A SiC–Al 2 O 3 mixture evaporates congruently according to the main vaporization reaction, 2 SiC(s) + Al 2 O 3 (s) +Al 2 O(g) ⇆ 2 SiO(g) + 2 CO(g) +4 Al(g), but the overall composition changes: for SiC rich samples, the mixture tends towards pure SiC in time, and for Al 2 O 3 rich samples towards pure Al 2 O 3 . A SiC–Al 2 O 3 /Y 2 O 3 mixture shows similar behaviour.

The thermal shock behavior of a three-dimensional carbon fiber reinforced SiC matrix fabricated by chemical vapor infiltration (CVI) technique was studied using the air quenched method. Damage to composites was assessed by a destructive technique of measuring mechanical properties using three-point flexure and SEM characterization. C/SiC composites displayed good resistance to thermal shock, and retained 83% of the original strength after quenching from 1300 to 300°C 100 times. The critical ΔT of C/SiC in combustion environment was 700°C. The critical number of thermal shocks for the C/SiC composite was about 50 times. When the number of thermal shocks was less than 50 times, the residual flexural strength of C/SiC composites decreased with the increase of thermal shock times. When the number of thermal shocks of C/SiC was greater than 50, the strength of C/SiC did not further decrease because the crack density was saturated.

MoSi2 and its composite with TiB2 (10 and 20 wt.%) particulates were synthesized by the hot pressing technique. Dry sliding wear experiments were done on these samples by pin-on-disc method on an EN-24 steel disc. It was observed that the densification and the reinforcement of the matrix are beneficial in reducing the friction and wear Of MoSi2. The tribofilm that was formed during sliding contained both the disc and the pin material. (C) 2002 Elsevier Science B.V. All rights reserved.