Densification, sintering with/without

Sintering can occur with or without densification. Sintering without densification, as illustrated in Figure 7.8fl, occurs by surface-based mass transport mechanisms that... 

Silicon nitride is a highly covalent bonded compound with self-diffusion coefficient of the nitrogen atoms of 6.3 x 10 2(1 cm2/s at 1400°C.22 Therefore, densification without any sintering additives is nearly impossible. In 1961, Deeley and Herbert.23 was the first to report that Si3N4 ceramics could be... 

If we consider the microstructural changes to the particles, it is clear that the spherical particles will alter their shape with time to that of a prolate ellipsoid of revolution by tins transfer of material finm the spherical surface to that of the neck. In fact, the distance between the centers of the particles is not affected by vapor phase transport and only the shape of the pores is changed. Without a decrease in the center-to-center distance there is no densification or sintering. Therefore, this is one example of coarsening during the initial stage sintering. 

Figures 98 and 99 illustrate the shrinkage behavior and densification rates of the alloys with and without Ti additions. Soon after the spontaneous start of densification, the sintering rate decreases drastically, but eventually begins to increase again. Without Ti addition shrinkage starts at 1140°C but in the alloy containing Ti, sintering begins at 1250°C. After this temporary decrease, the densification rate increases in both materials above 1320°C. Densification comes to an end after 2 h isothermal sintering at 1700°C.

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