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0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 Fraction of Theoretical Density

0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 Fraction of Theoretical Density

FIGURE 10.8 Example formulation for powder metallurgy processing of graded Ni-Al2O3 composites utilizing varying binder contents. (Reprinted from Winter [41].)

stacking under centrifugal forces [42, 43] and slip casting under pressure-induced flow [44-46].

Consolidation and sintering are also achieved in a manner similar to methods used for ceramics and metals. These include pressureless sintering, hot pressing, hot isostatic pressing, and microwave sintering. Gradients in temperature during densification may be used as a method to modify sintering rates. Spark plasma sintering (SPS) (also called pulse electric current sintering or plasma-activated sintering [47]) has been relatively recently shown to be an effective method for manufacturing graded materials, particularly when dissimilar materials must be sintered simultaneously [48]. In SPS an electric current is applied to the compact while it is pressed at elevated temperature. The electric current results in local "Joule" heating as well as heating from the creation of localized plasma between nearby powder particles, as indicated schematically in Fig. 10.9. Since the amount of Joule heating and plasma heating are highest in less dense areas of the compact, regions that would not sinter well under only radiative heat and pressure experience enhanced sintering rates.

10.4.5 Preform Methods

A second phase may be intruded by solid-, liquid-, or vapor-phase diffusion. Solid-state diffusion has been used for a long time to produce desired compositional and/or microstructural gradients, for example, in steel processing. Other examples of solid-state diffusion are discussed elsewhere [3]. In liquid- or

FIGURE 10.9 Schematic showing ways in which heat is generated in spark plasma sintering (SPS). (Courtesy of M. Tokita, Sumitomo Coal Mining Group at Izumi Technology Company.)

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