Silicon Nitride (Si₃N₄)

Silicon nitride is a light gray to black ceramic, which is used in high-temperature technology in particular because of its good mechanical properties up to 1800 °C. Silicon nitride ceramics are characterized by a low density, high fracture toughness, low coefficient of friction and very good thermal shock resistance. However, their properties are highly dependent on the manufacturing process used and the additives added.

The requirement for the production of dense silicon nitride ceramics is the presence of a liquid phase under sintering conditions, since the strongly covalent Si-N bonds limit the mass transfer by solid-state diffusion even at high temperatures [LAN91]. Therefore, the addition of additives is essential. In addition, Si₃N₄ decomposes at temperatures above 1900 °C.

The most common method for producing dense workpieces from Si₃N₄ is the sintering of shaped silicon nitride powders in a nitrogen atmosphere with the addition of additives such as aluminum or yttrium oxide or a mixture thereof. These additives form a glass (Y-Si-Al-O-N) or crystalline phase (Yttrium Aluminum Garnet (YAG)) enveloping the Si₃N₄ grains. In order to lower the necessary additive content and to improve the mechanical properties can be sintered under elevated pressure [HEI10].

For reaction bonded Si₃N₄ (RBSN), the correspondingly shaped silicon powder is nitrided in nitrogen atmosphere below its melting temperature. In this case, almost no shrinkage occurs, whereby a costly reworking usually omitted. In addition, the raw materials used are relatively cheap. Due to the poor diffusion of nitrogen in the Si₃N₄ formed so only small workpieces can be made. In addition, RBSN materials have a high porosity which, compared to dense Si₃N₄ ceramics, leads to inferior mechanical properties. A subsequent sintering process allows higher densities, but also leads to shrinkage of the component [KOL18].