Definition according to ISO 3252: Sintering is a "thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles".
Sintering processes for ceramics are commonly divided into several categories, these include solid-state sintering (e.g. Al2O3 - with MgO additive), liquid-phase sintering (e.g. MgO with a silicate glass), pressure sintering (e.g. Si3N4 with Al2O3 and Y2O3 additives under nitrogen gas pressure or under an externally applied pressure), viscous sintering (e.g. SiO2 gel), and vitrification (e.g. Porcelain).
Depending on material and to be sintered and processes different atmospheres and atmosphere compositions are used.
Messer Processes = Hydrotherm, Neutrotherm