Effects of Liquid-Phase Composition on Its Migration during Liquid-Phase Sintering of Cemented Carbide
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INTRODUCTION
FUNCTIONALLY graded composite materials (FGM composites) denote the composite materials with gradients of microstructural variables, such as the volume fraction of the matrix phase, the size of dispersive phase, etc. The gradients may spread from surfaces to the interior or from one part of a component to another. Compared to conventional homogeneous composite materials, FGM composites offer superior combinations of wear resistance, fracture toughness, high-temperature strength, thermal properties, and, hence, engineering performance.[1] Although the potential advantages of FGM composites are easily understood, the manufacturing of FGM composites is often difficult and challenging. For some composite material systems, liquid-phase sintering is one approach that can be used to create graded microstructures, by manipulating the distribution and migration of the liquid phase during sintering. During such processes, a volume gradient of the matrix phase can either be maintained, if there is an initial gradient in the green compact, or be created during the sintering process. During liquid-phase sintering, liquid-phase migration (LPM), also termed liquid-phase redistribution, is a physical phenomenon driven by the spontaneous tendency of the system to reduce its total interfacial energy. This phenomenon is similar to but different from the well-known capillary-driven flow in porous media, because LPM can occur in a solid-liquid two-phase system in the absence of any pore space, while the classic capillary-driven flow, resulting from the interaction among three phases (solid, liquid, and gas), relies on PENG FAN, Research Associate, JUN GUO, Ph.D. Candidate, and ZHIGANG ZAK FANG, Associate Professor, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] PAUL PRICHARD, Staff Engineer, is with Kennametal, Inc., Latrobe, PA 15650-0231. Manuscript submitted January 14, 2009. Article published online June 23, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
the existence of capillary pores.[2] In the initial stage of liquid-phase sintering, when a large volume fraction of pores exists, the liquid distribution that contributes to the densification is dependent on the capillary-driven flow, as explained by the pore-filling theory.[3] After pores are closed and eliminated with the progression of the sintering, further liquid redistribution will be controlled by the mechanism of LPM, which is, in turn, controlled by the minimization of interfacial energies; this is the subject of this study. The driving force of LPM can be measured by liquid migration pressure, Pm. If Pm is initially inhomogeneous within a system, the liquid phase will flow from a region of low Pm to a region of high Pm. It is worth noting that the liquid migration pressure acts as an imbibition pressure or negative pressure, according to its physical effect. The liquid migration or redistribution does not stop until Pm reaches homogeneity everywhere in the system.[2,4] In other words
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