Activated and Liquid Phase Sintering

In activated sintering, the rate of sintering is enhanced compared to that observed in compacts of a single metal powder or a homogeneous alloy powder. The process to which the term activated sintering is applied are the solid-state sintering processes. In contrast, liquid-phase sintering refers to processes in which a liquid phase is formed. Of the two types of liquid-phase sintering discussed in this article, the solution and reprecipitation process frequently is used to provide enhanced liquid-phase sintering. Transient liquid-phase sintering, however, may be used for applications where rapid densification is undesirable, or where alloy formation is desired.

Enhanced sintering, whether achieved by activated or liquid-phase sintering, is commonly used with refractory metals. This is due to the difficulties associated with the extremely high sintering temperatures of these metals. The problems in understanding conventional sintering are compounded in enhanced sintering by the presence of a second phase or supplemental treatment. Most enhanced sintering studies have been conducted with little or no theory to predict beneficial treatments. The eventual success of this approach cannot be denied. A theory to explain such behavior has evolved (Ref 15).

Sintering enhancement generally results from an increased driving force, through physical or chemical treatments. Many such processes are well known. While most attention has focused on tungsten, several other materials, including molybdenum, rhenium, iron, tantalum, uranium, tin, copper, aluminum, titanium, and several ceramic materials, have been investigated.

Enhancement of the sintering process generally is attributed to one or more changes in the fundamental material properties resulting from a special treatment. The strongest effects are those associated with changes in the interfacial properties (higher surface energy or lower grain-boundary energy). Alternatively, a less common means is to induce the operation of a normally dormant mass transport mechanism.

To the ceramist, many of these phenomena are commonplace. Impurities and stoichiometry departures can provide enhanced sintering of many ceramic materials. Likewise, the sintering atmosphere can have a profound influence on sintering rate, as well as the sintering mechanism. Any change in a material that induces an enhanced defect concentration or higher atomic mobility or that promotes the operation of new mass transport processes is considered enhanced sintering.

Typically, enhanced mass flow during the sintering cycle is beneficial. However, in the fabrication of filters, porous bearings, and flow restrictors, enhanced sintering can be detrimental. For most P/M materials fabricated for structural, magnetic, radiation, thermal, or electrical applications, improved service properties are associated with greater mass flow during sintering. Hence, any technique that delivers a greater degree of sintering is beneficial to these applications.

Usually, superior sintered properties are ensured by a higher sintered density. Pressing to high densities (above 90% of theoretical) before sintering is difficult. Consequently, specific techniques have evolved to enhance densification of powder compacts during sintering. Although the mathematics describing densification are somewhat formidable, they provide a concise description of the effects of the various process parameters. In a qualitative sense, there are some useful concepts worthy of review. For example, a smaller grain size (or smaller particle size) aids sintering densification and final properties. Higher sintering temperatures have a significant effect (because of an exponential term), thus increasing the rate of densification.

In a similar manner, a lower process activation energy has the same effect as an increase in sintering temperature. By contrast, a large pore size inhibits densification. Sintering time has a nominal effect on densification; generally, prolonged sintering is not advantageous, because grain size is increased. The grain size increase reduces the amount of grain-boundary area, thus reducing the beneficial effects of the enhanced diffusion rates at grain boundaries. Furthermore, time at temperature is expensive and usually is avoided for economic reasons.

Phase Changes. As previously discussed, compacts made from powders of body-centered cubic metals exhibit more rapid sintering than compacts made from face-centered cubic metals. This is directly related to the higher diffusivities in metals with a body-centered cubic lattice structure compared to metals with a face-centered cubic lattice structure.

In sintering compacts of iron-base compositions, it may be desirable to achieve maximum shrinkage rather than dimensional control. The body-centered cubic phase of iron (ferrite) is unstable at the usual sintering temperatures of 1000 to 1300 °C (1830 to 2370 °F). However, this phase may be stabilized in iron alloys with silicon or molybdenum. This is achieved by sintering compacts of mixtures of iron powder and ferrosilicon or molybdenum powders. Densification increases with the amount of ferrite stabilized at the sintering temperature. Increased densification is most likely due to the higher diffusivity and the fact that the phase boundary is a good vacancy sink. Additionally, the mixed phase microstructure resists grain growth during sintering.

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