DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

2.5.3 Solid State Sintering
In this dissertation, solid state sintering is the first densification method
investigated. For reference, a classic solid state sintering process is described where
idealized conditions of mono-sized spherical particles in point contact that consolidate
under isothermal conditions are assumed. In reality, most sintering is carried out using
powder compacts containing non-spherical particles that have a wide particle size
distribution and packing density. Sintering occurs over a range in temperature rather than
isothermally. Also, gradients introduced by thermal stress and atmosphere interactions
have a significant influence to the process. Nevertheless, the classic model gives a basic
view of sintering process upon which more sophisticated models and computer
simulation may be built.
2.5.3.1 Mass Transport
Driven by reduction of free energy, various mass transport phenomena occur
during the sintering process, including: surface diffusion, bulk diffusion, grain boundary
diffusion, evaporation-condensation, viscous flow, and plastic flow. 38 The actual mass
transport process is a mixture of these individual mechanisms and incorporates three
major stages as shown in Figure 2.17 and Table 2.4.
Figure 2.17 Illustration of the densification of three adjacent particles. 37
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