Nitride Bonded Silicon Carbide (NBSC)

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Materials based on silicon carbide are generally employed for their good wear resistance and thermomechanical properties. Pure silicon carbide tends to densify when heated at high temperature and pressure; however, the process is not a commercially feasible one.

High-density silicon carbide materials are manufactured by adding a sintering aid that creates a second bonding phase. Most often, the thermomechanical properties are restricted by the bond phase. For instance, the highest operating temperature of reaction bonded silicon carbide is 1375 °C, which is close to the silicon’s melting point.

Production

The nitrogen-bonded silicon carbide is produced by firing mixtures of high-purity silicon carbide and silicon, or a mineral additive in a nitrogen atmosphere, at high temperature (usually 1350 ºC to 1450 ºC). The silicon carbide is bonded by the silicon nitride phase (Si₃N₄) formed during nitriding. The highest operating temperature of the ensuing material is about 1750 ºC.

Key Properties

The properties of nitrogen bonded silicon carbide (NBSC) are suitable for many adverse service conditions:

• High-mechanical strength is sustained up to 1650 °C, surpassing the service temperature of typical metals and refractories. NBSC is especially robust under compressive loading.
• Good chemical resistance specifically to most molten salts, acids, and halogens. NBSC may be attacked by oxidizing agents and strong bases like sodium hydroxide and iron oxide.
• Thermal shock resistance owing to low-thermal expansion coefficient, high-temperature strength, and high-thermal conductivity. Consequently, NBSC can be cycled through large temperature variations without the loss of strength or formation of cracks.
• NBSC resists wetting through non-ferrous metals.
• Good wear resistance in abrasive settings.
• NBSC acts as a semiconductor at high temperatures.

Precise values of properties rely on the material’s composition and processing conditions. A standard composition will have 70%–80% silicon carbide and 20%–30% silicon nitride. Table 1 shows the mechanical properties of such a material.

Table 1. Typical mechanical and thermal properties for silicon nitride (20%–30%) bonded silicon carbide (70%–80%)

Note: Properties can differ widely for nitride bonded silicon carbide materials, based on factors like composition, grain size, porosity, etc. Users are strongly recommended to seek additional information from manufacturers/suppliers.

Applications

Silicon Nitride Bonded Silicon Carbide Bricks

Leveraging the material’s superior mechanical strength and stability, NBSC bricks are used in a variety of process kilns and furnaces such as the lower stack of blast furnaces, the side walls of aluminum melting pots, and as kiln furniture.

Cast Refractories

NBSC is typically found as a cast refractory for use under extreme service conditions instead of alloys or other refractory materials. Near net shapes can be engineered to replicate the metal parts. The material’s high hardness and abrasion resistance means that parts can be designed to prevent grinding or other finishing operations.

Applications of cast refractories manipulate the chemical and physical properties of the material and have the ability to develop the material into intricate shapes. Applications include:

• Coal burner, exhaust, and pulverizer parts for use in the power generation sector. Component life is said to be maximized by two to eight times when compared to other materials.
• Cyclone liners are used in coal and mineral processing, and also in the paper and chemical sectors.
• Kiln furniture has better stability and service life.
• Slurry pump components are used in mineral processing and specialty chemical sectors. The service life of components is increased by 2 to 20 times when compared to elastomeric and metal materials.
• Pump components, spigots, valve liners, piping liners, and nozzles.
• Non-ferrous metals processing equipment, aluminum, zinc, and copper are routinely handled with cast components.
• Corrosion-resistant parts for chemical plants.