Contact Fatigue of a Silicon Carbide with a Heterogeneous Grain Structure

A comparative study of cyclic fatigue damage from Hertzian contacts in silicon carbide ceramics with homogeneous microstructure (fine, equiaxed grains, strong grain boundaries) and heterogeneous microstructure (coarse, contiguous elongate grains, weak interphase boundaries) is presented. Observations of the surface and subsurface damage patterns using optical microscopy reveal fundamentally different cyclic fatigue mechanisns: in the homogeneous material, by slow growth of a well-developed cone crack outside the contact area; in the heterogeneous material, by progressive mechanical degradation within a distributed damage zone below the contact area. Scanning electron micrographs of the latter material show copious fine debris in the damage zone, consistent with a degradation mechanism by frictional attrition by forward-reverse sliding at the weak interphase boundaries. Acoustic emission is recorded during both load and unload half-cycles, confirming hysteresis in the sliding process. Flexure tests indicate initially slight strength losses from the cyclic contact damage in both microstructures, followed by accelerated losses at higher numbers of cycles. The underlying basis for establishing an analytical model of damage accumulation in the heterogeneous microstructure in terms of shear-fault sliding, and for designing micro-structures for optimal properties in fatigue and wear applications, is foreshadowed.     

Evaluation of stamping lubricants at various temperature levels using the ironing test

Lubricants are employed in stamping operations in order to (a) improve the material flow into the die cavity, (b) reduce wear and galling in the die and (c) obtain good surface finish of the part. Process conditions such as high temperatures and pressures could cause the lubricant to fail, thus resulting in galling or tearing of the part, damage to the tooling, and lost production. Therefore, selection of an appropriate lubricant based on the process conditions is important in the stamping industry. Several benchmark tests emulating stamping operations have been developed and are used to evaluate the performance of candidate lubricants. The major drawback of most of these tests is their inability to emulate high contact pressures and sliding velocities, which are crucial parameters for lubricity, especially in the case of high-speed progressive or transfer die operations involving ironing. Moreover, most of these tests are conducted at room temperature, while in reality; the process temperature can reach as high as 200 °C. The ironing tribotest developed at the Engineering Research Center for Net Shape Manufacturing (ERC/NSM) induces high contact pressures and temperatures, thus emulating the conditions in a production environment. Application of the test to screen candidate lubricants for stamping operations involving the ironing process is discussed in this paper.     

A testbed for performance evaluation of load‐balancing strategies for Web server systems

To improve response time of a Web site, one replicates the site on multiple servers. The effectiveness of a replicated server system will depend on how the incoming requests are distributed among replicas. A large number of load‐balancing strategies for Web server systems have been proposed. In this paper we describe a testbed that can be used to evaluate the performance of different load‐balancing strategies. The testbed uses a general architecture which allows different load‐balancing approaches to be supported easily. It emulates a typical World Wide Web scenario and allows variable load generation and performance measurement. We have performed some preliminary experiments to measure the performance of a few policies for load balancing using this testbed. Copyright © 2003 John Wiley & Sons, Ltd.     

Estimation of critical stability parameters by asymptotic analysis in multilayer extrusion

In industrial coextrusion operations, multiple layers are frequently brought in contact with each other at different temperatures to “match” viscosities and to reduce interfacial instability. Here, a linear stability analysis of multilayer plane Poiseuille flow is carried out for real, specific polymer melts in terms of process parameters, such as temperatures and flow rates. However, heat transfer across the layers is neglected as a first approximation. Previously, it was observed that below a certain Reynolds number and above a certain relaxation time, the flow-rate ratio (or the depth ratio), above which the flow is stable to all wavelengths, can be estimated by the long-wavelength asymptotic analysis (1,3). The Reynolds numbers for the industrial flows are smaller, and the relaxation times are larger than the corresponding critical values by several orders of magnitude. Therefore, the critical stability parameters can be estimated in the present study for industrial cases using the asymptotic stability analysis at long wavelengths. Although a large number of earlier parametric studies of model fluids indicate the influence of various theological parameters, only the viscosity stratification plays an important role in the interfacial instability within the operating range of industrial coextrusion processes.     

Alumina Supported Co–K–Mo Based Catalytic Material for Diesel Soot Oxidation

Alumina supported Co–K–Mo based mixed metal oxide type catalytic materials have been prepared by co-impregnation. These catalysts show excellent activity for carbon as well as diesel soot oxidation, which could be due to the redox properties of Mo and Co as well as to a synergistic effect of molybdenum, cobalt, and K contents. The catalyst containing 5 wt% molybdenum shows a lowering of carbon oxidation by about 190 °C under loose contact conditions as compared to the non-catalyzed reaction, as well as to bare alumina. Characterization studies suggest a composite nature of these materials, while thermal stability investigations confirm the stable nature. The selected catalyst has been studied by XPS, however, it is difficult to conclude which are the important factors contributing to the catalytic activity. It appears to be a synergistic effect of Co, K, and Mo components as these catalysts show much improved activity as compared to the individual components in supported and unsupported forms.     

Tailoring of nanozeolite NaX for enhanced removal of a phytoestrogen from its aqueous solutions

This study represents the application of nanozeolite NaX (NZX) synthesized by hydrothermal treatment without addition of any expensive structure directing agent for the highly effectual adsorption of biochanin A from aqueous solution. The characteristics and surface morphology of NZX were determined using FT-IR, SEM, TEM, BET, and XRD. The adsorption performance of NZX was analyzed at different conditions such as temperature, pH, contact time, adsorbent dose, and initial adsorbate concentration. The adsorption mechanism was well verified by pseudo-second-order kinetic and Redlich–Peterson isotherm models. The thermodynamic parameters revealed that the removal process was exothermic and spontaneous.     

Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part I,YAlO3 and γ-Y2Si2O7

The high-temperature (1500?°C) interactions of two promising dense, polycrystalline EBC ceramics, YAlO₃ (YAP) and γ-Y₂Si₂O₇, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Despite the fact that the optical basicities of both the EBC ceramics and the CMAS are similar, they both react with the CMAS. In the case of the Si-free YAlO₃, the reaction zone is small and it comprises three regions of reaction-crystallization products, including Y-Ca-Si apatite solid-solution (ss) and Y₃Al₅O₁₂ (YAG)(ss). In contrast, only Y-Ca-Si apatite(ss) forms in the case of Si-containing γ-Y₂Si₂O₇, and the reaction zone is an order-of-magnitude thicker. These CMAS interactions are analyzed in detail, and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb₂Si₂O₇ and β-Sc₂Si₂O₇) in the accompanying Part II paper. This is attributed to the presence of the Y in the YAlO₃ and γ-Y₂Si₂O₇ EBC ceramics.     

A Mechanism for Carbon Depletion at Bondline of High-Frequency Electric-Resistance-Welded X70 Pipeline Steel

Metallurgical and Materials Transactions A - The bondline of electric-resistance-welded (ERW) linepipe steel, often etched white (i.e., ferrite) in optical microscopy, is generally believed to be...