Silicon nitride is an all-rounder in the ceramics industry
Silicon nitride (Si3N4) has the most versatile combination of mechanical, thermal and electrical properties of any engineering ceramic material. It is a high-performance engineering ceramic that is extremely hard and has exceptional thermal shock resistance and impact strength. It exceeds the high temperature properties of most metals and has an excellent combination of creep and oxidation resistance. Its low thermal conductivity and high wear resistance also make it an excellent material to withstand the harshest conditions in the most demanding industrial applications. Silicon nitride is an excellent choice when high-temperature and high-stress capabilities are required.
Applications of silicon nitride
Rotating ball bearings and roller bearings
Cutting tools
Engine components: Valves, rocker arm supports, sealing surfaces
Supports for induction heating coils
Turbine blades, vanes, blades
Welding and brazing fixtures
Heating element components
Crucibles
Rollers and dies for forming metal tubes
TIG/plasma welding nozzles
Welding positioners
Precision shafts and axles in high wear environments
Thermocouple sheaths and tubes
Process equipment for semiconductors
Material properties of silicon nitride
High strength over a wide temperature range
High fracture toughness
Good flexural strength
Resistant to mechanical fatigue and creep
Low weight - low density
High hardness and wear resistance, both from impact and friction
Excellent thermal shock resistance
Low thermal expansion
Electrical insulator
Good oxidation resistance
Good chemical corrosion resistance
Wear resistant
High stiffness
Gas overpressure sintered silicon nitride
This is the most popular process for producing high-strength and complex silicon nitride components. The GPSN process uses a silicon nitride powder mixed with sintering aids to promote liquid phase sintering (usually yttria, magnesia and/or alumina) and binders to improve the mechanical strength of the ceramic green body. The powder is pressed into the desired shape, and green machining can be performed. The compacts are then placed in a nitrogen pressure atmosphere furnace to aid in compaction and prevent evaporation/decomposition of silicon, nitrogen and additives.
Precision Ceramics stocks a range of standard silicon nitride ceramic rods, all of which are precision turned and have excellent surface finishes. These rods can be used as bearings, pistons, engine components or in a variety of other assemblies. In addition, Precision Ceramics offers comprehensive machining/grinding services for the manufacture of custom silicon nitride components.
Hot pressed silicon nitride
HPSN is produced by uniaxial pressing of silicon nitride powder with sintering additives while applying heat. This process requires a special type of press and mold. It yields a silicon nitride with excellent mechanical properties. However, only simple molds can be made. Since it is impossible to green machine a hot pressed part, diamond grinding is the only way to produce complex geometries. Due to the high cost and difficulties associated with diamond grinding and hot pressing, the use of this process is usually limited to the production of simple components in small quantities.
Hot isostatically pressed silicon nitride
In this process, the silicon nitride powder is solidified under high pressure and high temperatures. A silicon nitride body with closed porosity is isostatically pressed by an inert gas at up to 2000 bar (uniform pressure on all sides), while the chamber is simultaneously heated. This process forces the pores/defects out of the material during sintering and brings the density closer to the theoretical density. The HIP process improves mechanical properties and reliability, but it is an expensive process that is usually used only in very selected cases.
Silicon nitride machining
Silicon nitride can be processed in the green state, as a cookie, or in the fully dense state. In the green or cookie state, it is relatively easy to process into complex geometries. However, the sintering process required to fully densify the material causes the body to shrink by about 20%. This shrinkage means that it is impossible to maintain very tight tolerances when machining before sintering. To achieve very tight tolerances, the fully sintered material must be machined/ground with diamond tools. In this process, the material is removed with a very precise diamond-coated tool/wheel until the desired shape is achieved. This can be a time consuming and costly process due to the toughness and hardness of the material.
Frequently asked questions
What is silicon nitride used for?
Rotating ball bearings and roller bearings
Cutting tools
Engine components: Valves, rocker arm supports, sealing surfaces
Supports for induction heating coils
Turbine blades, vanes, blades
Welding and brazing fixtures
Heating element components
Crucibles
Rollers and dies for forming metal tubes
TIG/plasma welding nozzles
Welding positioners
Precision shafts and axles in high wear environments
Thermocouple sheaths and tubes
Process equipment for semiconductors
What are the advantages of silicon nitride?
Compared to other engineering ceramics, its low coefficient of thermal expansion provides good thermal shock resistance. It is extremely hard, surpassing the high-temperature capabilities of most metals, and also has excellent oxidation resistance. Consequently, silicon nitride can withstand the harshest conditions in the most demanding high-temperature and high-load applications.
Even NASA scientists recognized its unique properties when silicon nitride bearings were used in the main engines of the Space Shuttle. It was identified as one of the few monolithic ceramic materials that could withstand the severe thermal shock and thermal gradients generated in hydrogen/oxygen rocket engines and proved to be completely reliable throughout the space shuttle program.
What is the formula for silicon nitride?
The formula for silicon nitride is Si3N4.