A Phenomenological Analysis of Sintering Mechanisms of W-Cu from the Effect of Copper Content on Densification Kinetics
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THE W-Cu composite materials combine a hard and refractory W phase having a low thermal expansion coefficient with a ductile Cu phase having high electrical and thermal conductivities. They are used in applications, such as electrical contacts, electroerosion segments, and heat sinks for packaging in microelectronic devices. Functionally graded W-Cu components are also considered for application as first wall assemblies in nuclear fusion experiments.[1–3] These materials are traditionally processed by Cu infiltration of W preforms. Processing high density materials through liquid-phase sintering of W-Cu powder compacts is possible only if the phase size in the powder mixture is submicrometric,[4–12] due to the low solubility of W in solid and liquid Cu. The classic liquid-phase sintering system consists of a mixture of a refractory phase with a minor metallic binder phase and densifies in three overlapping stages:[13–15] (1) a first stage of particle rearrangement after liquid formation; (2) an intermediate stage of solution precipitation, with corresponding shape accommodation and grain growth; and (3) a final stage of sintering of the solid skeleton. The corresponding mechanisms can already operate in the solid state, below the melting point of the binder phase, for powder mixtures with micrometric or submicrometric particle J.-J. RAHARIJAONA, formerly Postdoctoral Student, Laboratoire de Science et Inge´nierie des Mate´riaux et Proce´de´s, SIMAP, Grenoble INP/UJF/CNRS, Domaine Universitaire, is now Engineer, with the ITER Organization, St. Paul-lez-Durance, France. J.-M. MISSIAEN, Associate Professor, and D. BOUVARD, Professor, are with the Laboratoire de Science et Inge´nierie des Mate´riaux et Proce´de´s, SIMAP, Grenoble INP/UJF/CNRS, Domaine Universitaire, BP 75, F-38402 Saint-Martin d’He`res, France. Contact e-mail: [email protected] Manuscript submitted September 11, 2010. Article published online February 2, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
size of the refractory phase.[16] For the W-Cu system, densification is reduced to a two-stage process, due to the negligible solubility of W in solid and liquid Cu:[17] (1) a first stage of particle rearrangement and (2) a second stage of sintering of the solid W skeleton. The rearrangement stage was analyzed with the help of experiments and simulations.[18–20] It is described as a discontinuous process: the local pore filling by the liquid phase results in a capillary-assisted densification, followed by further repacking and densification of clusters of increasing size. The process amplifies the heterogeneity in pore sizes, first because migration of the liquid phase usually leaves large pores behind and also because capillary forces are lower for clusters with lower packing fraction. The efficiency of particle rearrangement on densification is then strongly dependent on the homogeneity of the initial powder mixture. Another limit of particle rearrangement results from the competition with the solid-state sintering of the W skeleton, since the form
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