A model of work of adhesion for reactive metal/ceramic systems
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I. INTRODUCTION
THE wetting and adhesion behavior of ceramics with metal is important in metal-ceramic joining, composite processing, and the manufacture of electronic devices. The value of the work of adhesion, which affects the bonding strength of the metal-ceramic interface, is a basic parameter in obtaining some composite materials. In addition to its great technical importance, the study of metal-ceramic interfaces is very attractive from a theoretical point of view, considering the differences in their electronic and structural properties. The thermodynamic work of adhesion, W, is defined as the work per unit area necessary to increase the separation of a unit area of solid/liquid interface from some equilibrium distance to infinite distance in the present article. It is given by W ⫽ sv ⫹ lv ⫺ sl
[1]
where sv , sl , and lv are the interfacial free energies between two phases (s ⫽ solid, l ⫽ liquid, and v ⫽ vapor). Liquid metal-ceramic systems can be classified into two categories,[1] depending on the sign of the Gibbs free energy change (⌬Gbulk) for the reaction of metal/ceramic. Most of the literature on model of work of adhesion refers to nonreactive systems, that is the systems with a positive value of ⌬Gbulk. In reactive systems, with a negative value of ⌬Gbulk, wetting of the liquid on the ceramic occurs simultaneously with the formation of a new solid phase at the interface. Metals are known to wet ceramics essentially by chemical bond formation.[2] However, the interfacial reaction mechanism that governs the wetting and spreading of liquid reactive metals in contact with ceramic bodies are not clearly understood. There is still no existing model that can calculate the work of adhesion in reactive systems at the present time. Although the definition of reactive systems and nonreactive systems are different, there is no obvious distinction between them. In fact, some dissolution of the oxide ceramic
JIAN CHEN, Postdoctoral Fellow, and MINGYUNA GU, Professor, are with the State Key Laboratory of MMCs, Shanghai Jiaotong University, Shanghai 200030, People’s Republic of China. FUSHENG PAN, Professor, is with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China. Manuscript submitted August 22, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
into metallic phases always occurs in pure liquid metalionocovalent oxide systems, and some chemical bonds exist at the interface in these nonreactive systems. For this type of nonreactive system, the work of adhesion can also be calculated by the same formula as the reactive systems. Thus, the systems studied in this work will include not only the reactive systems with a new phase formed, but also the nonreactive systems with localized chemical bonds at the interface. A general model has been proposed to calculate the work of adhesion for both systems. II. PREVIOUS MODELING OF WORK OF ADHESION Different models were proposed to estimate the work of adhesion in metal-oxide systems. The majority of
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