Introduction

SINTERING is a process in which particles bond together when heated to a sufficiently high temperature. The driving force is the net reduction in the surface energy. Liquid-phase sintering (LPS) involves the formation of a liquid phase to promote higher densification rates and lower the sintering temperatures. Because of cost and productivity advantages, it is estimated that over 70% of sintering products are processed using liquid-phase sintering. Common systems for liquidphase sintering include Cu-Co, W-Cu, W-Ni-Fe, W-Ag, Cu-Sn, Fe-Cu, WC-Co, TiC-Ni, and Fe-Cu-P. Key factors in determining whether a liquid phase forms during sintering of a specific material system include surface energies, solubilities, and diffusivities.

There are two main forms of LPS. When a liquid phase is obtained by inducing melting in a mixture of powders and is persistent throughout the high-temperature portion of the sintering cycle, the process is termed as persistent LPS. Classic LPS systems such as W-Ni-Fe and WC-Co are excellent examples of LPS with a persistent liquid. A persistent liquid can also be obtained by partially melting a prealloyed powder above its solidus temperature (termed as supersolidus LPS, or SLPS) and is widely used for processing tool steels, stainless steels, and superalloys. In some systems (for example W-Cu and Mo-Cu) with a low inter-solubility even in the presence of a persistent liquid, an activator can be used to enhance sintering. This is termed as activated liquid-phase sintering (ALPS). Alternatively, transient liquid-phase sintering (TLPS) involves liquid that disappears due to dissolution into the solid or formation of a new phase/compound. Although each material system has its own set of fabrication concerns, many general LPS concepts can be used to guide their processing. The following sections outline these concepts and provide specific examples of how they apply to industrially relevant materials.

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