Silicon Nitride Bonded Silicon (NBSC)
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The nitrogen bonded silicon carbide is made by firing mixtures of high purity silicon carbide and silicon or a mineral additive in a nitrogen atmosphere at high temperature (typically 1350:C to 1450:C) . The silicon carbide is bonded by the silicon nitride phase (Si3N4) developed during nitriding. The maximum operating temperature of the resulting material is around 1750:C.
The properties of nitrogen bonded silicon carbide suit many severe service conditions:
Good wear resistance in abrasive environments
High mechanical strength that is maintained up to 16500C, beyond the service temperature of common refractories and metals. NBSC is particularly strong under compressive loading.
Thermal shock resistance due to high temperature strength, low thermal expansion coefficient and high thermal conductivity. As a result NBSC can be cycled through large temperature differences without losing strength or developing cracks.
Good chemical resistance particularly to most acids, molten salts and halogens. NBSC may be attacked by oxidising agents and strong bases such as iron oxide and sodium hydroxide.
NBSC resists wetting by non-ferrous metals.
At high temperature, NBSC is a semiconductor.
Specific values of properties depend on the composition and processing conditions of the material.
A typical composition will contain between 70 and 80% silicon carbide and 20 and 30% silicon nitride. Table 1 provides mechanical properties for such a material.
Silicon Nitride Bonded Silicon Carbide Bricks
Exploiting the material's excellent stability and mechanical strength, nitrogen bonded silicon carbide bricks are used in a range of process furnaces and kilns such as the side walls of aluminium melting pots, the lower stack of blast furnaces, and as kiln furniture.
Nitrogen bonded silicon carbide is commonly found as a cast refractory for use under severe service conditions in place of alloys or other refractory materials. Near net shapes can be designed to reproduce the metal parts. The high hardness and abrasion resistance of the material means that components should be designed to avoid grinding or other finishing operations.
Applications of cast refractories exploit the physical and chemical properties of the material and the ability to form the material into complex shapes. Uses include:
Cyclone liners used in processing minerals and coal and in the chemical and paper industries.
Slurry pump parts for mineral processing and speciality chemical industries. Component lives are increased by between two and twenty times compared with metal and elastomeric materials.
Coal burner, exhaust and pulveriser parts for use in the power generation industry. Component life is reported to be increased by between two and eight times compared with other materials.
Corrosion resistant components for chemical plant.
Pump components, piping liners, valve liners, spigots and nozzles.
Non-ferrous metals processing equipment, copper, zinc and aluminium are routinely handled
with cast components.
Kiln furniture with increased life and stability.