Phase equilibria effects on the enhanced liquid phase sintering of tungsten- copper
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I.
INTRODUCTION
DUE to its potential for high-thermal conductivity and low-thermal expansion coefficient, W-Cu is a promising material for thermal management applications such as microelectronic packaging. These applications require W contents of 80 to 90 wt pet to achieve the necessary thermal expansion coefficient, but processing via liquid phase sintering of these compositions is hindered by the limited solubility of W in liquid Cu. Traditionally, Ni, which has substantial solubility for W, has been added to increase densification by solution-reprecipitation, resulting in high-sintered densities and good mechanical properties. tt'21 To achieve high-sintered densities, several weight percent Ni are often added, but to achieve high-thermal conductivities with the W-Cu system, additives must be minimized. Besides Ni, other a-ansition elements including Co, Fe, Lnd Pd also have substantial solubility for W; thus, they are preci~ctg2 to enhance the sintering of W-Cu. The effects of transition g~gments on the enhanced sintering of W have been w e l l documt~.rIted'13-?3l and a comprehensive theory of activated solid-stm',~ sintering has been developed.Ill.J71 However, the effects of tra2~ iti~ elements ",'n eAtenon the sintering behavior of W-Cu have not bt," sively investigated. Contrary to the case of the enha,riced sintering of W, Co has been reported as more effective than Ni in the activated liquid phase sintering of W - C u . I24-271 This indicates a need for reconsideration of the combined W, Cu, and activator phase equilibria effects. The present work demonstrates the connection between phase diagram features and the enhanced sintering JOHN L. JOHNSON, Research Assistant, and RANDALL M. GERMAN, Brush Chair in Materials, are with Pennsylvania State University, University Park, PA 16802-6809. Manuscript submitted January 14, t993. METALLURGICAL TRANSACTIONS A
of W-Cu. The sintering behavior and properties of W-Cu are evaluated and explained by a combination of liquid phase sintering and activated solid-state sintering theories.
II.
BACKGROUND
Similar to liquid phase sintering, activated solid-state sintering results from rapid mass transport through a second phase, but a segregated solid phase, rather than a liquid phase, reduces the activation energy for diffusion. IJz-~SI Consequently, no capillary forces are produced to aid densification. Specific solubility requirements must be met to insure that the second phase remains segregated at the grain boundaries to provide a short-circuit diffusion path. Favorable systems for the activated solid-state sintering of W can be identified through phase diagram characteristics including solubilities, melting temperatures, and intermediate phase relationships. 1+4'zStThe solubility of W in the additive should be much higher than the solubility of the additive in W. This is necessary for both the solubility and the segregation requirements. Also to ensure segregation, the liquidus and solidus should decrca;g. 'E!th increasing add;,t~,'~ c~3.c_ep.ff~tio_n_,tam A low ]
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