Consolidation of milled powders is an important step for reliable and reproducible determination of the physical/mechanical properties of the fabricated materials and is required for most industrial applica-tions.[22][23] Consolidation of the ball milled powders into bulk, full density compacts while retaining nanoscale grain size is a major challenge.[22] Many sintering techniques, e.g., hot pressing,[24][25] hot extrusion,[26] sintering forging,[27] and HIPing[28] have been employed to consolidate the mechanically alloyed powders. An attractive technique that has been used for consolidation of several metallic and ceramics powders is the plasma activated sintering, PAS. Recently, El-Eskandarany et al.[29]–[31] used this sintering technique for preparing bulk, fully dense nanocrystalline materials, using the mechanical alloying method as a source of the nanocrystalline powders.
In the PAS method, the powders are consolidated in vacuum at 1963 K
under pressures ranging from 19.6 to 38.2 MPa for 0.3 ks. In order to
avoid undesired grain growth, the sintering process is usually applied
for only 0.18 ks. In fact, there are three important factors governing
successful consolidation step via the PAS (Fig. 3.6). The first factor
is the application of uniaxial load, the second factor is the
application of pulsed voltage for plasma activation, and the last
factor is the resistance heating of graphite crucible and powders.
In this consolidation technique, when the direct current pulse voltage
is applied to the powders, micro discharge takes place among the powder
particles, which generates a plasma. Hence, the atoms on the surface of
each particle are activated, and the sintering procedure takes place in
a short time (consolidation is achieved within 0.3 ks). Accordingly,
grain growth during sintering can be minimized and the sintered powders
(Fig. 3.7) maintain their unique nanocrystalline properties with
improved physical and mechanical properties. In addition, the PAS
technique does not result in any compositional and/or structural
changes of the sintered powders. Thus, the as-milled and
as-consolidated samples are identical in their properties.[33]