Spreading and interlayer formation at the copper-copper oxide/polycrystalline alumina interface
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I.
INTRODUCTION
CERAMIC-metal brazing is an essential process in combining materials that have very different but equally desirable properties. Ceramics have good wear resistance, corrosion resistance, and high-temperature properties. However, they are typically brittle, expensive relative to metals, and must be joined to a more ductile metallic alloy to complete a cost-effective assembly. Examples of this concept are carbide-tipped machining tools and silicon carbide rotor blades joined to stainless steel shafts. Joining metals to ceramics is challenging, because the materials have such intrinsically different atomic structures. When these materials are brought together, the two quite different types of bonding must be accommodated in some manner to form a successful joint. A metalceramic joint of high integrity requires intimate contact between the interfacial product, the ceramic substrate, and the metal. Intimate contact is best achieved when one component of the joint is in the liquid state. Wettability and spreadability have often been used to quantify the extent of intimate contact achieved between a liquid and a solid surface. A common definition of wettability is given by the Young equation, which balances the surface energies at the point of contact between the solid, liquid, and vapor: ,)/sv = ,yst + ,)/iv COS 0
[1]
In Eq. [1], 7% ya, and ytv are the surface energies of M.D. BALDWIN, formerly Research Associate, Center for Welding and Joining Research, Department of Metallurgical and Materials Engineering, is Member of Technical Staff, Liquid Metal Processing Laboratory, Sandia National Laboratories, Albuquerque, NM 871851134. P.R. CHIDAMBARAM, Research Assistant Professor, and G.R. EDWARDS, Professor and Director, are with the Center for Welding and Joining Research, Department of Metallurgical and Materials Engineering, Golden, CO 80401-1887. Manuscript submitted August 23, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
the solid/vapor, solid/liquid, and liquid/vapor interfaces, respectively, and 0 is the contact angle. Spreading has been discussed in terms of a spreading coefficient, S: tq S -~- ysv _ (~lsl .~_ ~llv)
[2]
Obviously, S in Eq. [2] must be positive for spreading to continue. The operative attractive forces during wetting can be described either as physical or chemical.tz] Physical wetting occurs when a liquid-substrate interface is energetically favored by reversible physical forces such as those of Van der Waals. Physical wetting systems are not frequently used commercially because of the associated low bond energy values. Physical wetting can be accurately predicted by the Young equation, which assumes an equilibrium and reversible condition. In chemical wetting systems, the irreversible reaction results in an interfacial product with higher bond energy values, and therefore, chemical wetting systems are more commonly used commercially. When an interfacial product forms, new surface energies associated with the product are introduced, which are not accounted for in Eqs. [1] and [
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