75

000 800 1000 1200 1400

Temperature, ^C

Fig. 7 Pressureless sintering curves for Ti02 (2 h), showing accelerated densification of 40 nm particles (closed circles) compared to commercial, fine-particulate material (0.2 /Jm, open circles). Source: Ref 40

Pressure-Assisted Consolidation. Classical temperature-pressure interactions offer distinct advantages to nanocrystalline powder consolidation due to the ability to restrict grain growth by lowering consolidation temperature. Illustrations of the pressure effect on nanopowder densification are shown in Fig. 8, 9, and 10. Applied pressure adds an additional component to the curvature related driving pressure for densification described earlier. At elevated temperature, it activates new plasticity-driven densification mechanisms, such as local yielding, creep, and stress-assisted diffusion. Applied pressure can induce particle rearrangement and large pore collapse, increasing the number of particle contacts and thereby accelerating densification.

o cn

41

%

35

j

t2'

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.3 Applied pressure, GPa

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.3 Applied pressure, GPa

Fig. 8 Effect of compaction pressure on the density of a 17 nm silicon nitride preform. Density after precompaction in WC-Co piston-cylinder die. Source: Ref 40

Fig. 8 Effect of compaction pressure on the density of a 17 nm silicon nitride preform. Density after precompaction in WC-Co piston-cylinder die. Source: Ref 40

Fig. 9 Effect of compaction pressure on density of (a) iron and (b) copper powder. In each figure, "A" refers to micrometer-sized powder (2 /Jm Fe, 50 /Jm Cu) and "B" to nanocrystalline powder (50 nm Fe, 58 nm Cu). Two regions of consolidation are evident from the plots. Low-pressure Region I is associated with particle sliding and elastic deformation between adjacent particles. High-pressure Region II is associated with plasticity at interparticle necks. Source: Ref 41

Pressure, GPa

Fig. 10 Cold compaction of 13 nm 7-alumina. Source: Ref 42

The role of room temperature compaction pressure, P, on relative density, A, has been studied extensively (Ref 43). The generally accepted form is:

where k and B are constants, and ¿"is the fractional porosity (Ref 44). Changes in densification mechanism, such as particle rearrangement, plastic deformation, and agglomerate fracture, effect changes in the constants as demonstrated in Fig. 9.

Pore collapse scales with the shear stress level. The shear component of stress is minimal in hot isostatic pressing (HIP) and is present increasingly in quasi-isostatic pressing, hot pressing, sinter forging, and extrusion (Ref 45). Additionally, shear stress is beneficial for mechanical disruption of surface oxide layers, which improves interparticle bonding.

Most pressure-assisted consolidation methods have been applied to nanocrystalline powder: HIP, forging and hot pressing, extrusion consolidation, cold sintering (high-pressure consolidation), dynamic consolidation, and field activated sintering. Details of these techniques are discussed in other articles.

The classical pressure-assisted sintering diagrams developed by Ashby and coworkers (Ref 46, 47, 48) have been used to describe nanocrystalline powder consolidation (Ref 49, 50, 51). Densification maps illustrate both the change in mechanism and accelerated densification relative to conventional powders. This is shown for HIP of nanocrystalline zirconia in Fig. 11(a) (Ref 50). The primary densification mechanism is boundary diffusion driven primarily by curvature effects, even though 25 MPa external pressure was applied. Densification was complete after 1 h at 1500 K. Figure 11(b) shows the computed densification behavior under identical conditions for 2 Cm zirconia. Densification is controlled by creep and volume diffusion driven by the applied pressure, and full density is not obtained below about 2200 K.

Fig. 11 Densification maps computed for zirconia HIPed with 25 MPa pressure, (a) 40 nm particulate, (b) 2 /'m particulate. Symbols represent experimental data for 1 h HIP of 40 nm zirconia calcined at 823 K and 1273 K. Source: Ref 50
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