Densification and structural development in liquid phase sintering
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INTRODUCTION
PROCESSESoccurring during both solid state and liquid phase sintering (LPS) are driven by surface energy. The gravity force also influences structural development in LPS. However, momentarily neglecting this factor, .we can say that the surface energy driving force and kinetic factors can be considered to control both the densification rate and the microstructure developed during both types of sintering. Even though the driving force does not, and the diffusional mechanisms may not, change during solid state sintering, it is frequently divided into "stages". In effect the "stages" correspond to different geometrical morphologies of the pore-solid particle array. For example, stage I sintering corresponds to a pore-particle array in which both of the "phases" individually demonstrate a high degree of connectivity. In contrast, during stage III sintering the pores occur as isolated pockets within a continuous solid matrix; the transition in morphology is, of course, a continuous one. Since the driving force and kinetics may not change during the transition, geometrical factors only define the various stages. The division of solid state sintering into distinct geometrical stages has proven useful in clarifying the mechanisms and kinetics of the process. This approach has, for example, led to the development of sintering diagrams 1,2 T. H. COURTNEY is Professor of Metallurgical Engineering and Dean of the Graduate School, Michigan Technological University, Houghton, MI 49931. This paper is based on a presentation delivered at the symposium "Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds" held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME. METALLURGICALTRANSACTIONS A
which define the dominant neck growth mechanisms (e.g., boundary, surface, or volume diffusion) and the rate of neck growth in terms of the sintering time and temperature. In a different form, 3 solid state sintering diagrams are also used to predict densification kinetics. Liquid phase sintering also has traditionally been discussed in terms of "stages". However, discussion of LPS is considerably less well defined than for solid state sintering. Firstly, the time scale for densification and structural development in LPS is relatively short; in fact, it is experimentally difficult in many cases to separate conveniently the several "stages" of the process. More important, the morphological changes in LPS, while occurring in response to the same thermodynamic driving force as for solid state sintering, involve different mechanisms taking place on different time scales. As expected the various processes affect densification and structural development differently, and this is one distinction between LPS and solid state sintering. Another one is that LPS involves three (solid, liquid, and pore) phases whereas only the solid and pore phases are present during solid state sintering. In LPS, the fluid liquid phase allows f
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