Ceramics

Loose powder increases the risk of receiving defects like cracks or delamination during the uniaxial pressing. We can minimize those defect appearance by using granules that have wider porosity gauge than loose powder so they provide better way out for gas phase during the material densification.

• Improves cohesion of product • Eliminates majority of internal pores • Improves mechanical properties (fatigue durability) • Helps to provide isotropic structure

Frictional forces between the compact and the die wall lead to characteristic stress gradients and density gradients in the compact. It is observed for single action compaction, using a particular die set and powder, that there is a critical compact depth below which no significant densificatnion occurs

Reason: friction between powder granules and the mould’s walls.

Uniaxial pressing allows us to receive only simply shapes of green bodies such as cuboids, cylinders, or prisms with various simple shaped base. That shape of moulded pieces is determined by a way of pressure application used in that method of pressing.

• Isostatic pressing allows us to produce more sophisticated shapes of press moulds • Isostatic pressing gives us homogenous stress distribution that minimalizes friction between granules and mould walls and reduces the “density fade” effect. • In Isostatic pressuring we don’t use mould lubricants that could contaminate the products of pressing

Due to the elastic relaxation upon pressure relief the metal part shaping the inner contour of the hollow body can be pulled out.

• Dynamic consolidation of powders using a pulsed energy source (mechanical) • Shock compaction by use of high velocity gas stream

Elimination of porosity by mass transport mechanisms

Dimensional changes - shrinkage, Property change eg. strength properties , Mass change.

Changing the shape of grain boundaries. Changing the shape and size of the grains. Elimination of porosity

Sintering = consolidation + compaction + recrystallization Consolidation - Increasing grain boundary surface area Compaction - Elimination of porosity by mass transport mechanisms Recrystallization - Grain growth

Reduction of enthalpy of free space taking into account volumes (v), limits of paragraph (b) and surface (s). The driving force of sintering is a reduction of Gibbs free energy (chemical potential or free enthalpy) by replacing high-energy free surfaces (solid-vapor) by low-energy grain-boundaries (solid-solid) and finally by minimizing also the grain boundary area (via grain growth)

The cause of elimination of pores during sintering is mass transfer

Properties such as compact strength or electrical conductivity depend on the coordination number