Sintering is commonly regarded as taking place in three stages：
*During the early stage, necks are formed between individual particles and grow by diffusion, increasing the interparticle contact area. The powder aggregate shrinks，involving center to center approach of the particles. In this stage, the degree of densification is still low and the pore structure is open and fully connected.
*With increasing neck formation (intermediate stage), the necks become and lose their identity. The pores are assumed to be cylindrical. Their radii vary along their lengths and, with increasing shrinkage, the pore channels break up into small, still partly interconnected segments. During this stage (channel closure stage), pronounced densification occurs and significant grain growth occurs concurrent with shrinkage.
*Finally, in the last stage (isolated pore stage), the pore segments further break up into chains of discrete, isolated pores of more or less spherical symmetry. This stage occurs when about 90% of the theoretical density is achieved. The sintering density then approaches asymptotically the practical limit of 92-98%.
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