Driving Force for Sintering

The driving force, DF, for sintering is the reduction in the total free energy of the system 

TABLE 16.2 Surface Energy fw Ceramics Using the Equation y - A + BT(K) J/m  

AGj = yg (Final surface area — Initial surface area) (16.1b) 

Recently, the sintering driving force has been modified to the so-called sintering stress [4,5] (or, in other terminology, sintering pressure 

FIGURE 16A Sintering of a fine AI2O3 powder = 0.8 /im) with and without pressure. Sintering is under constant heating rate conditiCHis eS 7.3°C/min. Data from Reed [1]. 

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In die operation of die first source, the driving force for sintering is the difference in curvature between the neck and the surface of the sphere. The curvature force A l, is given by... 

The values of m given above conform to Hemng s scaling law (1950) which states that since the driving force for sintering, the transport length, the area over which uansport occurs and the volume of matter to be transported are proportional to a, and respectively, the times for equivalent change in two powder samples of initial particle size ai q and 2,0 are... 

This experiment is discussed here in some detail both beeause it casts light on the driving force for sintering and because it is a beautiful example of the ingenious... 

The driving force for sintering is the reduction in the total free energy of the particulate system, AG, which is composed of free energy changes of volume, AGy, boundaries, AGb, and surfaces, AGt. 

Because the surface energy per volume is larger for small particles and because the fundamental driving force for sintering is surface-energy reduction, compacts composed of smaller powders will typically sinter more rapidly. Smaller powders are more difficult to produce and handle therefore, predictions of sintering rate dependence on size are used to make choices of initial particle size. Herring s... 

Figure 7.30 Two adjacent particles. (a) Before sintering, the particles have two solid-vapor surfaces, each with a surface energy ysv. (b) After the onset of sintering, a necked region forms between particles, replacing two solid-vapor surfaces with a lower-energy solid-solid surface. yss < ysv, providing the thermodynamic driving force for sintering.

Commonly, the applied pressure is much greater than the driving force for sintering, and as a result, equation (e) of (17.2.3.5.1) becomes ... 

External pressure can be applied to provide the driving force for sintering in addition to surface curvature, which could be comparatively smaller in this case. The pressure should be applied during the key state of sintering. There are two ways to apply pressure hot pressing and hot isostatic pressing. By applying an external... 

Chemical reactions can provide driving force for sintering only when they have contribution to the densification process. The change in free energy of a chemical reaction is given by... 

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