Pressure sintering and hot isostatic pressing

Pressure sintering and hot isostatic pressing 3.8.1. Applying a pressure during sintering 

Furthermore, it can be possible to obtain the sintered part with its exact dimensions net shape), without the need for a machine finishing in applications that require high dimensional accuracy. The other side of the coin is the technical complexity of the process and the high costs incurred, as well as the limitations on the geometry of the parts, which can only have simple forms and a rather reduced size. We must have pressurization devices manufactured in materials that resist the temperatures required by sintering - and even if these temperatures are lower 

The creep eqrration, modified for pressure sintering, can be written as  

Mechanism Grain size exponent, m Stress exponent, n Coefficient of diffusion, D 

In most cases, the use of fine grained ceramics on the one hand, and the high level of plastic flow required by iono-covalent crystals on the other, are such that the diffusion terms (Nabarro-Herring or Coble) override the plastic flow. Grain boundary diffusion dominates over volume diffusion all the more when the grains are finer and the temperature lower, because the volume exponent of the former is 3 whereas that of the latter is only 2, and the activation enthalpy of Dj is in general lower than that of Dy. 

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