Investigations have shown that the densification is controlled by grain boundary diffusion over most of the densification range, unless at very high densities it becomes controlled by lattice diffusion.
Since the motion of grain boundaries, necessary for grain growth, is impeded by the presence of pores, grain coarsening proceeds at a higher-rate above 97% density. Grain sizes of the as-sintered ingots are commonly in the range of 1 0 to 30µm.
Besides temperature and time, several other parameters influence densification, such as powder particle size, green density, sintering atmosphere, powder purity, compact size/weight, heating rate, thermal gradients, and the presence of insoluble phases such as oxides (Th02, La203, Ce02, 2r02) or metallic potassium (NS-tungsten).
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