Densification mechanism

The various densification mechanisms at different temperatures can be modelled and displayed in HIP diagrams, in which relative temperature is plotted against temperature normalised with respect to the melting-point (Arzt el al. 1983). This procedure relates closely to the deformation-mechanism maps discussed in Section 5.1.2.2. 

Ting, C.-J. and Lu, H.-Y., Hot-pressing of magnesium aluminate spinel-I. Kinetics and densification mechanism , Acta. Mater., 1999, 47, 817-830. 

Vendange, V, and Colomban, P, Densification mechanisms of alumina, aluminosilicates and aluminoborosilicates gels, glasses and ceramics, J. Sol-Gel Set Tech-nol, 2, 407, 1994. 

Figure 4. When the densification mechanism has a higher activation energy than the coarsening mechanism, a fast heating rate to high firing temperatures (rapid sintering) can lead to the production of high densities.

In this case, T is calculated by assuming specific models for densification and grain growth, where only one densification mechanism and one coarsening mechanism are dominant. 

Several densification mechanisms can be operative in HP or HIP, as in pressureless sintering. In addition to lattice and grain boundary diffusion, plastic deformation and creep, which are unimportant in pressureless sintering, can be major mechanisms. The major densification mechanism for a given system can vary with experimental and compact conditions, such as temperature, pressure, particle size and neck size. However, the overall densification rate of a compact is, of course, the sum of the densification rates of all the mechanisms operative. 

Figure 5.9 shows an example of HIP diagrams which identify the dominant densification mechanism under various experimental conditions and shows the rate of densification that results from all the mechanisms acting together. As can be seen in the diagrams, diffusion is usually the dominant mechanism in ceramics even under a high external pressure while power-law creep can be an important densification mechanism in metals. 

At the early stage of densification where P is high, plastic yielding can be the major densification mechanism. The plastic deformation between particles may be regarded as identical to that occurring in a hardness test using an indenter. Then, if the indentation stress a,- satisfies the yielding condition. 

Power-law creep can also be a major densification mechanism in pressure-assisted sintering. At the early stage of densification achieved by power-law... 

Figure 9.5 (a) Densification mechanisms for porosity attached to a grain boundary. The arrows indicate paths for atom diffusion, (b) Densification mechanisms for porosity separated from a grain boundary. The solid arrows indicate paths for atom diffusion, and the dashed arrows indicate the direction of boundary migration. (From Ref. 1.)... 

During tableting, powder is uniaxially compressed in the die. During the compression process the powder is densified under the applied pressure. Densification of the powder occurs through particle rearrangement, particle fracture and particle deformation. The relative extent of each of the three identified densification mechanisms that occurs in a specific instance is determined by the applied pressure, the rate of the increase of pressure, the duration of the applied pressure, and the mechanical properties of the materials that are being compressed. 

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