Chemical thermodynamics as a predictive tool in the reactive metal brazing of ceramics
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I.
INTRODUCTION
THE science of thermodynamics and transport phenomena have long served as cornerstones for our present understanding of materials science. In the past 2 decades, there has also been considerable interest in the area of metal-ceramic joining. In particular, reactive metal brazing has emerged as the joining process of choice for fabricating metal-ceramic joints. Although reactive metal brazing is a widespread industrial process, the basic science behind the process is as yet unclear. A qualitative understanding does exist, but predictive capabilities are clearly lacking. In this article, we present efforts to understand reactive metal brazing using classical chemical thermodynamics. The science of thermodynamics may be applied to both reversible and irreversible systems. Reversible thermodynamics, nevertheless, has been exclusively used by most researchers to understand reactive metal brazing. We first summarize the work of other researchers followed by our own novel ideas involving the use of bulk reversible thermodynamics. Bulk reversible thermodynamics provides a useful framework for analyzing and understanding reactive metal brazing. It is not yet possible to be fully predictive, however, and the state-ofthe-art is best described as semiquantitative. The second part of this article utilizes the framework of i r r e v e r s i b l e thermodynamics to understand reactive metal brazing. The basic theoretical framework and the governing equations are presented to develop full predictive capabilities. II. S E L E C T I O N O F R E A C T I V E E L E M E N T S USING REVERSIBLE THERMODYNAMICS A key question in designing a reactive metal brazing alloy is the choice of the reactive element. In the search for effective reactive element additions for reactive brazing alloys, bulk chemical thermodynamics provides a convenient and useful predictive tool. Aksay et al. ttl used this approach to estimate the reactive wetting by calculating the free energy change for the reaction between a reactive metal oxide and a ceramic (alumina). McDonald and Eberhart r2] correlated the work of adhesion between G. WANG, Senior Research Engineer, is with Edison Welding Institute, Columbus, OH 43212. J.J. LANNUTTI, Assistant Professor, is with The Ohio State University, Columbus, OH 43210-1179. Manuscript submitted March 29, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
alumina and a liquid metal with the free energy of formation of the corresponding oxide. Chatain et al.[3] correlated the work of adhesion between alumina and a liquid metal to a linear combination of the enthalpy of formation of the metal oxide and the enthalpy of dissolution of aluminum into the liquid melt. Yost and Romig taj proposed a thermodynamic analysis relating the advance of the contact line between a liquidmetal drop and a ceramic solid to the Gibbs free energy change of the interfacial reaction. They treated the system as a static problem involving four phases: gas, solid ceramic, liquid braze, and solid interfacial reaction product. Although th
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