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Silicon carbide (SiC) and boron carbide (B4C) are among the world’s hardest known materials and are used in a variety of demanding industrial applications, from blasting-equipment nozzles to space-based mirrors. But there is more to these “tough guys” of the materials world than hardness alone-these two ceramic carbides have a profile of properties that are valued in a wide range of applications and are worthy of consideration for new research and product design projects.
Silicon CarbideUse of this high-density, high-strength material has evolved from mainly high-temperature applications to a host of engineering applications. Silicon carbide is characterized by:
- High thermal conductivity
- Low thermal expansion coefficient
- Outstanding thermal shock resistance
- Extreme hardness
- Semiconductor properties
- A refractive index greater than diamond
For example, within the two categories of silicon carbide production-sintering and reaction bonding-different methods of compaction can result in various advantages and limitations of the finished material. In one such case, reaction-bonded silicon-infiltrated silicon carbide (SiSiC) is produced by impregnating a porous skeleton of silicon carbide and free carbon with molten silicon. This creates a bond matrix of silicon carbide and leaves some silicon in unbonded form. The material does not contract during densification, resulting in dimensional stability that enables the production of large-scale components and complex shapes. On the down side, the free silicon limits the operation temperature to about 1380°C.
Other compaction methods produce silicon carbide sintered without pressure (SSiC), hot-pressed or hot isostatically pressed silicon carbide (HPSiC/HIPSiC), recrystallized silicon carbide (RSiC), and liquid-phase sintered silicon carbide (LPSiC). These methods variously alter porosity, strength, durability, corrosion resistance, thermal spalling resistance or fracture toughness.
Applications for silicon carbide are wide-ranging but grounded in the remarkable toughness of the material. Some of the most common applications include:
- Automotive components and seal faces, requiring superior resistance to wear and thermal shock and the ability to last the lifetime of the vehicle
- Mechanical seals, requiring superior resistance to corrosion, abrasion and shock
- Blast and atomization nozzles, requiring excellent wear resistance, light weight and long life (silicon carbide lasts 50% longer than tungsten carbide)
- Process industry valve applications, requiring outstanding corrosion resistance, particularly in acids
- Semiconductor production, requiring exceptional resistance to wear and corrosion, which in turn leads to reduced maintenance and component recycling
At the much lower operational temperatures associated with spacecraft, silicon carbide-based electronics would lend themselves to a more compact spacecraft power system and would also not require as much cooling by a spacecraft’s thermal radiators as conventional silicon-based electronics. These thermal radiators could, in turn, be made smaller and lighter, reducing overall weight and, potentially, cost. In addition, the use of high-temperature, radiation-hardened silicon carbide-based circuits would require less shielding in nuclear-powered spacecraft, further reducing weight.
Boron CarbideBoron carbide’s hardness ranks behind only diamond, cubic boron nitride and boron oxide (see Table 2). Difficult to sinter to full density, boron carbide is usually produced with the addition of sintering aids such as fine carbon or silicon carbide. Boron carbide is typically characterized by:
- Extreme hardness
- Low thermal conductivity
- High elastic modulus
- High compressive strength
- Good nuclear properties
- Low density
In addition to applications requiring excellent wear resistance and high strength, other applications can take advantage of the distinctive properties of boron carbide. The following are a few of this ceramic carbide’s varied applications:
- Abrasives, requiring extreme hardness for use in polishing and lapping applications, dressing diamond tools, and water jet cutting
- Nozzles, requiring wear and abrasion resistance for slurry pumping, grit blasting, and water jet cutters
- Nuclear applications, since 20% of natural boron is the 10B isotope, boron carbide can be used as an absorbent for neutron radiation in nuclear power plants
- Body armor, requiring a critical balance of hardness, compressive strength, high elastic modulus and low specific density
Looking AheadGiven their unique properties and proven versatility, silicon carbide and boron carbide offer researchers and design engineers opportunities to investigate other exciting but currently unexplored uses for these remarkable advanced ceramics. The “tough guys” surely have a lot more to offer.
For more information regarding ceramic carbides, contact the authors at firstname.lastname@example.org or email@example.com.
SIDEBAR: Form Follows FunctionSilicon carbide and boron carbide can be found in a range of forms to facilitate the development of new products and applications.
Note: Not all forms are available for both silicon carbide and boron carbide.