摘要:

High-resolution electron microscopy (HREM) in conjunction with high-spatial resolution analytical electron microscopy (AEM) was used to characterize the internal interface structure and grain-boundary chemistry in SiC whisker-reinforced Si3N4-based ceramic composites. Investigations on the model system of Al2O3bicrystals are compared with studies on the complex microstructures of real composites. When relating structural features to the mechanical response of the composite, a distinction between macroscopic features, such as overall size and morphology of both matrix grains and reinforcements, and microscopic structures, such as structural and chemical width of interfaces, is made. The necessity of AEM in combination with HREM studies is shown in order to understand the performance of the composite under severe service conditions.

DOI:10.1163/156855493X00239

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Because of the inherent coefficient of thermal expansion (CTE) mismatch between the fiber and matrix within metal and intermetallic matrix composite systems, high residual stresses can develop under various thermal loading conditions. These conditions include cooling from the processing temperature to room temperature as well as subsequent thermal cycling. As a result of these stresses, within certain composite systems, radial, circumferential, and/or longitudinal cracks have been observed to form in the fiber-matrix interface region. A number of potential solutions for reducing this thermally induced residual stress field have been proposed recently. Examples of some potential solutions are high CTE fibers, fiber preheating, thermal anneal treatments, and engineered interfaces (e.g. a compensating/compliant layer concept or the graded layer concept). In the strict sense, an engineered interface is one that provides a compromise between the various pertinent chemistry and mechanics issues for the application and the system under consideration. Here, the focus is on designing an interface (using a compensating/compliant layer concept) to reduce or eliminate the thermal residual stress field and, therefore, the initiation and propagation of cracks developed during thermal loading. Furthermore, the impact of the engineered interface on the composite's mechanical response when subjected to isothermal mechanical load histories is examined.

DOI:10.1163/156855493X00248

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Many previous researchers have used high surface area silica as a model substrate for glass. In this study, the electrokinetic behavior of natural silica and glasses of various compositions have been measured. In all cases, the unmodified samples were found to have points-of-zero charge values between pH 2 and 3.3. Also, the electrokinetic behavior of the samples after the adsorption of an amino-organofunctional silane coupling agent was investigated. For these samples a reversal of the zeta potential occurred, causing the silica and glass samples to possess a net basic surface. Finally, the electrokinetic responses of silica and a glass sample were compared after the adsorption of aluminum metal ion species. The adsorbed metal ion species caused a reversal in the zeta potential at approximately pH 3 for both the silica and the glass samples. In all cases, despite wide variations in the bulk compositions of the silica and glass samples, the electrokinetic behavior was found to be almost identical. Thus, the electrokinetic results found herein support the practice of using silica as a model substrate for glass.

DOI:10.1163/156855493X00257

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A stress analysis was carried out in order to determine the local stress distribution around spherical particles in particulate filled composites. The results were introduced into the criteria for micromechanical deformations and the conditions for the initiation of each mechanism were calculated. Results of the calculations have shown that shear yielding, and possibly crazing, do not depend on thermal stresses and adhesion, but debonding does. The results of the analysis were in accordance with literature and experimental data. The calculated debonding stresses correspond to the observed values, but both debonding and yield stress depend strongly on composition, a phenomenon the analysis does not account for. The discrepancy can be explained with matrix/filler interaction and the role of the interphase. The results indicate that although stress analysis helps to predict deformation and failure in particulate filled composites, it must be further refined to find more exact solutions and the effect of interfacial interactions must also be considered.

DOI:10.1163/156855493X00266

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Hydroxyl groups on silica surfaces can be removed partially or completely under mild conditions, including washing of silica with dry hexane, prolonged storage over phosphorous pentoxide, chemical reaction with phenyl isocyanate, ethyl isocyanate or trimethylsilyl trifluoromethanesulfonate. The removal of these hydroxyl groups was performed at much lower temperatures than usually required for this process.

DOI:10.1163/156855493X00275

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor