A micromechanistic model of the combined combustion synthesis-densification process
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A series of computer experiments has been conducted in order to study the combined combustion synthesis-densification process, in which a mechanical load is applied to a sample as it undergoes a combustion synthesis process. The current work is an extension of a theoretical model of the combustion synthesis process that was developed previously. 12 In this work, the appropriate constitutive equations for sample deformation have been incorporated, in order to account for the pore-volume change that may take place when the mechanical load is applied, thus densifying the sample. It was shown that the brief appearance of a liquid phase in the combustion wave front provides an important opportunity for densification when the self-propagating combustion synthesis process is conducted in conjunction with an applied mechanical load. That is, the concomitant decrease in the (local) total volume fraction of the solid phases—due to the elementary melting and dissolution processes that also occur (locally)—effectively lowered the (local) apparent yield strength of the sample, thus allowing for the compaction and densification of the sample (i.e., locally). Results indicated that the mechanical load should be applied at the instant at which the sample is ignited, in order to ensure that articles whose density is uniform throughout the sample can be fabricated. This work provided a more detailed and quantitative understanding of this unique process for preparing dense articles by the self-propagating combustion synthesis process, that is, when it is conducted in conjunction with an applied mechanical load.
I. INTRODUCTION Dense ceramic materials have many important uses in harsh environments (e.g., high-temperature, high-wear, and corrosive conditions), particularly when they possess a high toughness, high strength, low thermal expansion coefficient, high thermal-shock resistance, and corrosion resistance. Conventionally, such dense ceramic bodies are produced by solid-state reaction sintering.3 For the more refractory ceramic materials, sintering aids are often used, in order to promote the formation of a liquid phase that facilitates the densification of the green body. The subsequent reduction in the (high-temperature) creep resistance of such sintered articles, however, can often be attributed to (i) creep of the sintering aid that typically resides at the grain boundaries following sintering, or (ii) chemical reactions that may occur between the sintering aids and the ceramic material at high temperatures. Alternative fabrication processes, which often require that little or no sintering aid be used, include hot-pressing, hot isostatic pressing, and high-temperature forging.4'5 Unfortunately, the design of the furnaces that are used in these processes often limits the shape of the dense articles that can be prepared to relatively simple geometries. Moreover, in all of these approaches, the combination of an expensive, external supply of heat, 1828
J. Mater. Res., Vol. 10, No. 7, Jul 1995
elevated processing pressures, an
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