Silicon nitride and silicon nitride ceramics have outstanding high-temperature, mechanical, wear, and thermal properties. They also have moderate thermal conductivity, a low coefficient of thermal expansion, a moderately high elastic modulus, and unusually high fracture toughness for a ceramic.
Silicon nitride ceramics are considered to be hard ceramics. Unlike soft ceramics, which have large dielectric constants and are used mainly in sensing applications, nitride ceramics have good thermal conductivity and high electrical insulation capability. This makes compounds such as silicon nitride (Si3N4) suitable for power applications.
How Silicon Nitride Ceramics are Made
Silicon nitride is synthesized by reacting silicon with nitrogen, or silicon chloride with ammonia (diimide synthesis). Direct sintering into dense parts is difficult, and most products are reaction sintered, hot pressed, or spark plasma sintered to achieve a high degree of quality.
The bonding of silicon nitride powders can be achieved at lower temperatures by adding additional materials, such as sintering aids, which commonly induce a degree of liquid-phase sintering. An alternative technique is to use spark plasma sintering in silicon nitride and silicon nitride ceramics, where heating is conducted very rapidly by passing pulses of electric current through the compacted powder.
Silicon nitride ceramics maintains their strength even at high temperatures. They possess a moderately high elastic modulus, and high fracture toughness for a ceramic. Silicon nitride also provides excellent thermal shock resistance because of its combination of moderate thermal conductivity, low coefficient of thermal expansion, and high elastic modulus. Additional advantages include corrosion resistance to many acids and alkalis, and superior wear resistance. Because of its wear properties, silicon nitride is often used in ceramic bearings.
Types and Applications
Common types of silicon nitride and silicon nitride ceramics include reaction–bonded silicon nitride (RBSN) and hot pressed silicon nitride (HPSN). Additional products include sintered silicon nitride (SSN), which include the sialons, and sintered reaction-bonded silicon nitride (SRBSN).
Silicon carbide powders are used in manufacturing silicon nitride ceramics for wear components, cutting tools, car engine parts, high temperature bearings, mechanical seal faces, thermocouple protection tubes, gas turbine components, and crucibles. Silicon nitride is also used in xerographic processes and as an ignition source for domestic gas appliances. Because of its good elastic properties, silicon nitride, along with silicon and silicon oxide, is the most popular material for cantilevers - the sensing elements of atomic force microscopes (AFM).