Solubility liquid-phase sintering

FIGURE 1 49 Schematic of the effects cl solubility on densification or swelling during liquid phase sintering. Taken from German [116, p. 69]. 

The element-rich solid solubility alloys are of importance in the so-called heavy metal alloys. The tungsten phase formed during liquid-phase sintering is saturated with iron (and nickel) and, vice versa, the nickel-iron binder phase with tungsten. 

During liquid-phase sintering, the compositions of the starting solids are such as to result in the formation of a liquid phase upon heating. The liquid formed has to have an appreciable solubility of the solid phase and wet the solid. In the next sections, the reasons these two requirements should be met and the origin of the forces at play during liquid-phase sintering are elucidated. 

In a solid-liquid two phase system, the solid can be dissolved in the liquid and the liquid can also be dissolved in the solid. A high solubility of the solid in the liquid will facilitate potentially a high mass transport rate through the liquid layer, which is the basic requirement of liquid-phase sintering. However, the dissolving of the liquid in the solid is totally prevented, otherwise the compact would be severely swelling due to the formation of transient liquid phases. The effect of solubUily on densification and swelling of the compact is demonstrated schematically in Fig. 5.33 [73]. 

Fig. 5.46 Model binary phase diagram showing the composition and sintering temperature associated with liquid-phase sintering in the L + S2 phase field. The favorable characteristics for liquid-phase sintering include a suppression of the melting temperature, high solid solubility in the liquid, and low liquid solubility in the solid. Reproduced with permission from [73]. Copyright 2009, Springer...

Homogeneous systems As the ceramic is heated to sintering temperatures, a liquid phase is formed which gradually disappears as it is soluble in the matrix. 

Another key parameter is the solubility of the solid in the liquid. The common requirement is for the solid to be soluble in the liquid but for the liquid to have a low solubility in the solid. This requirement leads to a liquid that persists during sintering. A high liquid solubility in the solid leads to a transient liquid phase and swelling of the compact during heating. 

The Si-B-C system was mainly investigated with view to the understanding of the sintering mechanisms of SiC with boron in combination with carbon and the sintering of boron carbide with silicon [4]. The silicon solubility of about 2.5 at.% in B4C at 2323 K and the comparatively low temperatures of liquid phase formation in the ternary system enhance the sintering of boron carbide. 

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