Wettability of Cu-Based Alloys on Alumina and Joining of Alumina with Microdesigned Nickel-Chromium Alloy Interlayer

Abstract

INTRODUCTIONMonolithic ceramic materials are being widely developed for a variety of applications ranging from use as strucrural components to high-performance electronic substrates. Recently, however, major efforts have been made to use ceramics in structural applications under severe operating conditions. Joining of ceramics to ceramics and to metals is considered one of the key technologies that will either expand or restrict the potential use of ceramic materials in a wide range of applications, such as heat engines, heat exchangers and recuperato rs.Recent work by some of the authors 1 ·3 and Y. Iino4 has explored the PTLP (Partial Transienc Liquid-Phase) method of ceramic-ceramic joining. PTLP bonding is a joining method that seeks to combine the most attractive features of diffusion bonding and brazing. The technique employs a multilayer, initially inhomogeneous incerlayer. These microdesigned multilayer interlayer structures form a thin layer of a transient liquid phase (TLP) at relatively low temperatures to facilitate joining with refractory metal-based interlayers. Ar the temperature of PTLP bonding, diffusion of the less refractory cladding metal into rhe refractory core metal, reaction of the less refractory cladding metal with the refractory core metal, or a combination of these processes can lead to the disappearance of the liquid. This approach allows the formation of bonds at relatively low temperature, and interdiffusion and resulting homogenization during bonding forms a more refractory interlayer that offers the potential for high temperature use.In the case of such multilayer interlayers in PTLP bonding, the transient liquid phase is sandwiched between a ceramic and a refractory core metal. The contact angles of a liquid metal on a ceramic are high, and those on a core metal are typically quite low. A necessary condition for favorable redistribution of the liquid along the interface is that the sum of these two contact angles be less than 180°. Modifications to the transient liquid phase chemistry that reduce the contact angle on a ceramic are an important factor when modifying the process to improve strength characteristics. Such modifications can arise due to dissolution of the refractory core metal into the transient liquid film during PTLP bonding, in an effort to 407 establish local equilibrium between the liquid phase and the core layer. An understanding of the effect of the associated changes in liquid film chemistry on its wetting behavior is crucial to successful ceramic-metal joining.The present paper describes the effect of additions (Cr, Ni and 80Ni·20Cr) on the wetting characteristics of liquid Cu on Al203, and the study of PTLP (Partial Transient Liquid-Phase) bonding of Al203 using microdesigned multilayer Cu/80Ni·20Cr/Cu interlayers. EXPERIMENTAL PROCEDURES MaterialsTwo kinds of polycrystalline alumina, 99.5% and 99.9% pure (SSA-S and SSA-999W, "'98% dense; Nikkato Co. Ltd.), were used in this study. The 99.5% pure Al203 contains a small amount of glassy phase. The material wa...

Transient-Liquid-Phase and Liquid-Film-Assisted Joining of Ceramics

“…In prior work by Nakashima et al [32,34] alumina joints prepared with Cu/Ni/Cu interlayers failed exclusively along the alumina-interlayer interface, and the joint strengths varied considerably. Examination of fracture surfaces indicated that large unbonded regions persisted along the alumina-interlayer interface.…”

“…The sample preparation procedures in our current work are generally similar to those in our prior work, and detailed descriptions are available in the literature [26][27][28][29][30][31][32][34][35][36][37][38][39]. The summary provided here emphasizes those procedures or materials that differ from those used previously.…”

“…Fractography indicates that these regions are more prevalent in samples with lower fracture strength [26][27][28][29]. The wetting characteristics of the liquid film can be improved by precoating the ceramic surface with a metal, or by altering the liquid film chemistry [30][31][32][33][34]. The liquid film chemistry can be adjusted by adding directly to the cladding layer a second component that improves wetting.…”

“…Alternatively, since some dissolution of the core layer is inevitable, the addition of elements that enhance the wetting can be achieved by their incorporation in the core layer. In some of the systems examined, the implementation of such approaches has yielded assemblies in which fracture occurs primarily within the ceramic, indicating that the ceramic-metal interface has higher strength than the ceramic [34].…”

Transient-Liquid-Phase and Liquid-Film-Assisted Joining of Ceramics

“…It had been anticipated that the wetting of the liquid film on alumina would need improvement. In prior work by Nakashima et al [32,34] alumina joints prepared with Cu/Ni/Cu interlayers failed exclusively along the alumina-interlayer interface, and the joint strengths varied considerably. Examination of fracture surfaces indicated that large unbonded regions persisted along the alumina-interlayer interface.…”