Activated Liquid Phase Sintering

Activated LPS can be used to improve the sintering behavior of systems with little or no solubility of the solid in the liquid. Densification is enhanced by the addition of elements that segregate to grain boundaries and lower the activation energy for solid-state diffusion. Such elements can be identified by phase diagram characteristics. Limited additive solubility in the liquid helps to ensure segregation to grain boundaries, while the formation of low-melting temperature intermetallic phases with the base metal provides the high-diffusivity phase needed for enhanced solid-state sintering in the presence of the liquid. For liquid volume fractions of 35 vol% or more, high sintered densities can be achieved in systems that lack mutual solubility, because grain shape accommodation is not required. Thus, activators are only needed for compositions with low volume fractions of liquid. In this case, volume diffusion through a high-diffusivity interboundary phase provides the grain shape accommodation necessary to achieve high sintered densities.

Systems in which ALPS is particularly applicable are those in which high electrical or thermal conductivity is desired, such as W-Cu, Mo-Cu, and WC-Cu (Ref 52, 53). In the case of W-Cu, the base metal is practically insoluble in the liquid, but other transition elements, such as cobalt, iron, nickel, and palladium have substantial solubility for tungsten. Traditionally nickel, which has complete solubility with copper, is added to W-Cu to increase densification by solution reprecipitation through the Cu-Ni liquid phase, but the alloying of copper and nickel degrades the electrical and thermal conductivity. High conductivities can be maintained if the amount of activator (i.e., nickel) is small; however, this results in only a small increase in solubility in the liquid phase, which is not highly beneficial to improved density. On the other hand, cobalt and iron also have substantial solubility for tungsten but have only limited solubility in copper. As shown in Fig. 16, they are much more effective activators of W-Cu, Mo-Cu, and WC-Cu, which all have similar phase relationships with these four activators.

Fig. 16 Effects of transition elements on the sintered densities of W-Cu, Mo-Cu, and WC-Cu

Increasing the amount of additive in a system that densifies by ALPS above a certain amount hinders densification and lowers properties, as shown in Fig. 17. The optimal amount of additive for ALPS corresponds closely with the amount of activator needed to coat the particle surfaces plus the amount of activator that remains in solution in the liquid phase. For systems that densify via solution reprecipitation, such as W-Cu-Ni, increasing the solubility of the liquid phase by increasing the nickel concentration improves the densification behavior.

Activator concentration, wt%

Fig. 17 Effects of activator concentration on W-10Cu for samples sintered at 1300 °C for 1 h in H2

Densification in an ALPS system is further improved by a high initial homogeneity and a fine particle size. In fact, with particle sizes below 1 /'m. solid-state diffusion in the presence of the liquid phase can provide near-theoretical densities without the need for an activator (Ref 54). Low compaction pressures are beneficial because much of the densification occurs via rearrangement. When there are low volume fractions of liquid a greater portion of densification occurs through the surface layer, due to more contacts, and thus, there is more opportunity for solid-state sintering. Slow heating rates are detrimental to the densification behavior due to the formation of solid-state sinter bonds, which must be dissolved prior to rearrangement. Sintering temperatures and times can be optimized according to the desired properties and specific powder characteristics.

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