It depends on the pelletization and sintering process. During the sintering process, some gases/organic mater (if used) could evolve from the surface making it a bit less dense. This could also be due to inefficient pellet making process or diffusion of gases/moisture immediately after the pelletization (before sintering). So its not uncommon.

This phenomenon is associated with the manufacturing process. In case of isostatically pressed samples it does not occur. In case of axially pressed disks a density gradient in the green body can occur if the pressing force is not high enough.

These density gradients in the green body either persist in the final sintered part - resulting in inhomogenous density from rim to center - or - if the powder is sinterable enough the density is homogenous but the part either cracks due to the created stress or is warped. The regions in vicinity of the walls of the mold are less compacted due to friction phenomena. Axial pressing has therefore been considered an inferior shaping technology for a long time. Still the technology is the most performing for mass production.

With modern axial pressing technology these effects have become much less pronounced. Today it is possible to press large and thin plates with homogeneous thickness and density without warping.

(see: Kaiser, A: “Hydraulic pressing of advanced ceramics”, CFI, Ber. DKG, 84[6], E27-E32, 2007.)

Thank you very much Prof. Sher and Prof. Kern for your answers.

Dear Son Nguyen

I´ve only worked with glass sintering. We can talk of ceramic density or of course of porosity (higher porosity= lower density).

When you begin sintering a powder compact, porosity is interconected. Necks among particles develop. Densification begins and gases (air, atmosphere gas, evolving gases from particles) can escape through the interconnected porosity.

From relative density about 0.8, close porosity appear and the entrapped gases can only go out by diffusion. The dynamics of this diffusion process will be an important variable in determining the final density distribution of your sample.

I have experienced a same problem. As said earlier, on the edges because of the friction between die and powder sample particles and loose contact during uni axial cold pressing, porosity is higher on the edges (less densification).  isostatic pressing has resolved these issues and dies can also be modified to give better results. Plus if you are using any lubricant in the die during cold pressing to reduce the friction or ease in taking the sample out of the die, e.g. bromide solution, then it is important to heat treat the green compact first to evaporate the entrapped solution or anything which can be converted into gasses during high temperature sintering processes.