Density

Fig. 5 Grain size as a function of density for pressureless sintering of selected nanocrystalline materials

Fig. 6 Abnormal grain growth in 15 nm iron close-die sintered at 810 K (10 MPa, 1 h) followed by pressureless sintering at 1080 K for 1 h. Source: Ref 10

Fine pore size throughout the sintering process is also critical in controlling the final grain size. For these purposes, a small and uniform pore population is desired in the green compact. Most often, this pore distribution is associated with a high green density in nonagglomerated or weakly agglomerated powders. For example, it has been shown that deagglomeration of a commercial ZrO2 may reduce sintering from 4 h at 1775 k to 1 h at 1375 K (Ref 29). Efforts to produce nonagglomerated powders have intensified. A good summary of the progress for ceramics can be found in Ref 5. The effects of fine pore size and narrow pore size distribution on obtaining high densities and fine structure are well correlated (Ref 18, 26, 30, 31, 32, 33, 34).

Considerable coarsening can occur in nanocrystalline materials during heating to and cooling down from the pressureless sintering temperature. A relationship has been proposed to describe the extent of coarsening based on a modification of Eq 3 (Ref 35, 36):

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