Pressure sintering hot pressing

Densification method Low gas pressure sintering Gas pressure sintering Hot pressing Hot isostatic pressing (HIP) Reaction bonding Reaction bonding and postsintering... 

Pressure sintering. Whereas solid-state sintering under normal conditions, dealt with so far, is based on the diffusion mechanism, during pressure sintering (hot pressing) the yield stress may be exceeded as a result of external pressure which results in plastic deformation of the particles. 

The less pure aluminas are blended with silicates so that they can be sintered at 1350 °C or less. The highest-purity materials require a temperature of 1750 °C at atmospheric pressure or hot-pressing. As cost increases with sintering temperature, the grade used in practice is usually the least pure that has adequate properties. 

Application of pressure during sintering results in a lower final porosity as compared with pressureless processes. External pressure increases the pressure difference created by curved surfaces, Eq. (3.6), and facilitates pore elimination. For the usual pressures in hot press sintering (or pressure-assisted sintering), this increase in pressure difference is 5-10 times (McColm Clark, 1988). As a result, sintering temperatures can be reduced by 300°C and higher final densities are obtained. 

Additives which promote liquid phase formation at sintering temperatures are particularly beneficial. Another approach is to increase the effective Sintering stress by applying external pressure using hot pressing or hot isostatic pressing... 

Hot pressing (or pressure sintering) is the simultaneous application of pressure and heat to a powder mass enclosed in a die. This technique allows the use of lower temperatures and pressures and shorter processing times than those for conventional sintering and thus permits the production of bodies with finer grain size, lower porosity and higher purity. 

The most delicate part of this system is obviously the die, whose material must resist the simultaneous action of heat and pressure (adequate strength and creep resistance) must not react chemically with the material being hot pressed and with the environment must have a low thermal expansion coefficient, i.e. lower than the material being sintered (otherwise hot ejection is necessary to avoid the sample cracking or the die splitting) and must have good thermal-shock resistance. 

---