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COURTNEY

During liquid phase sintering, microstructural coarsening takes place. One mechanism by which this occurs is Ostwald ripening. Alternatively, particle coalescence also leads to a concomitant reduction in the solid particle surface area per unit volume. In isolated structures in which particle-particlecontacts are made, the rate of coarsening by coalescence is limited by the time between particle contacts, for this is long compared to the time to fuse two particles together. In skeletal structures the "coalescence time" limits coarsening by coalescence since this is long in comparison to the time between contacts. Expressions for the rate of particle coarsening are developed for the different mechanisms and different particle morphologies. The results of these calculations are combined with the microstructure maps developed in Part I of this paper to refine these maps so that they predict both the morphology developed and the dominant mechanism of coarsening in liquid phase sintered systems. I. INTRODUCTION

IN the first part of this paper,I the type of microstructure developed during liquid phase sintering was related to the time required to make particleparticle contacts via Brownian motion relative to that time necessary for particles to fuse once such contact was established. If the latter is short in comparison to the time between contacts, an isolated microstructure develops; if the opposite is the case, a highly contiguous or skeletal structure appears. Isolated structures were also predicted to occur under conditions of complete liquid-solid wetting and for systems with a large distribution in particle sizes. Subsequent to densification, microstructural coarsening also occurs during liquid phase sintering. The resultant decrease in solid-liquid interfacial area per unit volume is caused by the reduction in surface energy accompanying this process. The extent of coarsening can be quantitatively ascertained by measuring the number of particles per unit volume (Nv), the size of these particles, or the liquid-solid surface area to solid volume ratio (Sv). For isolated microstructures, Sv is unambiguously related to particle density (Nv) and size; for skeletal structures, however, this relationship is more obscure because of the ambiguity associated with defining a "particle." In the present paper, therefore, we shall discuss microstructural coarsening in terms of Sv although, under the appropriate circumstances, this parameter can be directly converted to particle size and/or density. The extent of coarsening, and the dominant mechanism thereof, will depend on the microstructure developed during sintering. For isolated structures characterized by a value of zero for the parameter Ps, the probability of particles "sticking" subsequent to particle-particlecontact, the only mechanism available for microstructural coarsening is that of T. H. COURTNEY is Professor, Department of Metallurgical Engineering, MichiganTechnological University, Houghton, MI 4993 I. Manuscript submitted June 14, 1976. METALLURGICA