Silicon Nitride

• High strength over a large temperature range
• High fracture toughness
• Good flexural strength
• Mechanical fatigue & creep resistant
• Lightweight – Low density
• High hardness and wear resistance, both impingement and frictional modes
• Superior thermal shock resistance
• Low thermal expansion
• Electrical insulator
• Good oxidation resistance
• Good chemical corrosion resistance
• Wear-resistant
• High stiffness

There are several different methods used to produce Silicon Nitride, all producing materials with slightly differing properties.

• Reaction-Bonded Silicon Nitride (RBSN)
• Sintered Reaction-Bonded Silicon Nitride (SRBSN)
• Gas Pressure Sintered Silicon Nitride (GPSN)
• Hot Pressed Silicon Nitride (HPSN)
• Hot Isostatically Pressed Silicon Nitride (HIP-SN)

In the early silicon nitride days it was all made via reaction bonding due to the difficulty of producing a silicon nitride powder. With this method, a silicon compact is heated it in a nitrogen rich atmosphere – the nitrogen binds to the silicon powder to produce a porous and not fully dense silicon nitride body. There is little shrinkage in this process making green-machining (machining the compact prior to sintering) very effective. The mechanical properties of RBSN are not good due to its limited density (approximately 70 – 85%). In addition, the pore structure cause by its low density leads to high oxidation rates and to poor erosion resistance. Typically it is useed in high-temperature/thermal shock applications. Reaction-Bonded Sintered Silicon Nitride (RBSSN)

The Reaction Bonded Sintered method uses the same process as above, however, sintering aids are included in the original powder (to promote liquid phase sintering) and additional sintering process follows. After the silica compact has been nitrided, the porous silicon nitride body is then put into a high-pressure nitriding furnace to remove the pores and produces a much denser product. This method is fairly dated and was used predominantly because of the difficulty associated with manufacturing silicon nitride powder in the early days.

This is the most popular method for producing high-strength and complex geometry silicon nitride components. The GPSN method uses a silicon nitride powder that has been mixed with sintering aids to promote liquid phase sintering (typically yttria, magnesium oxide, and/or alumina) as-well-as binders to improve the mechanical strength of green ceramic body.

The powder is pressed into the desired form and green-machining can take place. The compacts are then placed into a furnace that has a pressurized nitrogen atmosphere to aid with densification and prevent the evaporation/decomposition of the silicon, nitrogen and additives.

HPSN is produced by uniaxially pressing silicon nitride powder (with sintering additives) while applying heat at the same time. A special type of press and die is required for this process. It produces a silicon nitride with excellent mechanical properties, however, only simple shapes can be produced.

Because it is impossible to green-machine a component that is hot-pressed, diamond grinding is the only way to create complex geometries. Because of the high costs and difficulties associated with diamond grinding and hot-pressing, its use is typically limited to the production of simple components in small quantities.

This method consolidates the silicon nitride powder by using high pressures and high temperatures. A silicon nitride body with closed porosity is isostatically pressed (uniform pressure on all sides) via an inert gas at up to 2000 bar while the chamber is simultaneously heated.

This process effectively squeezes any pores/defects from the material while it is sintering and brings the density closer to theoretical. HIPing improves the mechanical properties and reliability, however, it is an expensive process that is typically only used in very select circumstances.

Precision Ceramics stocks a range of standard silicon nitride ceramic rods, all precision turned to an excellent surface finish. These rods can be used as bearings, pistons, engine components, or in a variety of other assemblies. In addition, Precision Ceramics offers full machining/grinding services for custom silicon nitride component manufacturing.

Silicon Nitride can be machined in green, biscuit, or fully dense states. While in the green or biscuit form it can be machined relatively easily into complex geometries. However, the sintering process that is required to fully densify the material causes the Silicon Nitride body to shrink approximately 20%. This shrinkage means that it is impossible to hold very tight tolerances when machining Silicon Nitride pre-sintering. In order to achieve very tight tolerances, fully sintered material must be machined/ground with diamond tools. This processes uses a very precise diamond coated tool/wheel to abrade away the material until the desired form is created. Due to the inherent toughness and hardness of the material, this can be a time-consuming and costly process.