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Silicon Nitride - Reaction Bonded Silicon Nitride (RBSN)

Chemical Formula

Si3N4

Background

Reaction bonded silicon nitride is made by heating a compact of silicon powder in a nitrogen gas atmosphere.  Reaction starts at 1200 °C. The nitriding cycle is usually carried out at 1450 °C and takes between 150 and 200 hours.  Nitrogen - hydrogen or nitrogen- hydrogen-helium gas mixtures are often used as they give faster and more easily controlled reaction rates and result in higher strength materials.

Reaction bonding enables the production of complex near net shapes.  Although silicon expands by approximately 20% on conversion to the nitride, the expansion is accommodated with the void space of the silicon compact.  There is little change in the overall volume of the components, accurately dimensioned shapes can be made without expensive and time consuming finishing operations.

Reaction bonded silicon nitride components have lower density than components made by hot pressing routes due to the difficulty in ensuring complete reaction throughout the volume of the silicon pack.  As a consequence, RBSN components have inferior mechanical properties such as strength and Young’s modulus.

Key Properties

        Good thermal shock resistance

        Resistant to wetting by molten metals

        High electrical resistivity

        Low density

        High hot strength

        Mechanical fatigue and creep resistance

        Oxidation resistance

        Ability to be formed into complex shapes relatively easily

Table 1. Typical Physical and Mechanical Properties of Reaction Bonded Silicon Nitride

Property Value
Density (g.cm-3) 2.5
Young’s Modulus (GPa) 200
Bend Strength (MPa) 300
Fracture Toughness K 1c (MPa.m0.5) 2
Hardness (GPa) 10
Thermal Expansion Co-eff., (x10-6)(°C) 3.0
Thermal Conductivity (W/m.K) 10
Decomposition Temperature (°C) 1900

Applications

Various parts manufactured from RBSN (photo courtesy of CERAM Research Ltd).

Figure 1. Various parts manufactured from RBSN (photo courtesy of CERAM Research Ltd).

Thermocouple Sheaths

Due to silicon nitrides’ good resistance to thermal shock and it resistance to wetting by molten metals, it is often employed as a material of choice for thermocouple protection sheaths, in particular for non-ferrous metal applications.

Nozzles

Internally threaded nozzles for inert gas welding and cutting torches are often made from reaction bonded silicon nitride due to its ability to resist thermal shock and molten metal attack.

Specialised Kiln furniture

Some specialised kiln furniture is made from silicon nitride due to its low thermal mass combined with its ability to resist thermal shock.

Other Applications

Reaction bonded silicon nitride is also used for fixtures to position and transfer metal parts for processes such as induction heating and resistance welding, brazing and soldering.

Source: Ceram Research

For more information on this source please visit Ceram Research Ltd.

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