A Fine Solution For Advanced Applications

October 1, 2001
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Extremely fine sizes of silicon carbide and boron carbide are expanding applications for the ceramic industry.

SiC and B4C in finer and cleaner sizes are helping ceramic manufacturers achieve smaller pore sizes and stronger, denser shapes.
Silicon carbide (SiC) and boron carbide (B4C) are two very versatile raw materials with a diverse range of properties that have made their use critical in a number of applications over the years. Both materials exhibit high hardness and wear resistance, thermal shock resistance and chemical resistance, and can be used as abrasives, engineered ceramics, refractory materials and electrical materials.

For about the last 100 years, these materials have been primarily used in large macrogrit and mesh sizes (approximately 10 to 325 mesh), as well as dust collector fines (DCFs), and such sizes still comprise the largest use of these materials. However, over the past five to 10 years, there has been a growing need for SiC and B4C in finer (microgrit) and cleaner sizes as ceramic manufacturers have tried to achieve smaller pore sizes and stronger, denser shapes. As the demand for advanced technologies in consumer and industrial markets drives the development of even smaller, higher quality components, refinements of these old, stable and well-known raw materials will play an ever more important role in the ceramic industry.

Superfine Silicon Carbide

Recently, a new line of extraordinarily clean and very fine sizes of SiC superfine powders was developed, including 800/1000 (7 micron) water classified black and green silicon carbide, F1200 (3 micron) black and green silicon carbide, 1200/finer (2.5 micron) black and green silicon carbide and 2 micron air-classified green silicon carbide.

These superfine sizes have led to new applications in metal matrix composites, ceramic matrix composites, polyurethane and resin matrix composites, high temperature cements, conductive and wear-resistant coatings, refractory bond construction, refractory castables, slip cast bodies and self-bonded silicon carbide bodies.

Metal Matrix Composites. Green and black SiC in sizes ranging from F400 to F1000 are used by aluminum smelters to reinforce aluminum ingot, making it stronger than steel. These composites are used to reduce weight in automobiles, airplanes, bicycles and many other applications.

Ceramic Matrix Composites. Green SiC preforms (in any shape, but open-ended shallow boxes are typical) made from superfine sizes are infiltrated with molten aluminum to make very light, strong bases for electrical components.

Polyurethane and Resin Matrix Composites. Superfine SiC finds a variety of uses in weather- and heat-resistant coatings and epoxy wear-resistant fillers. It is also used to strengthen polyurethane shapes. Another application is in epoxy molds, where it is used for short runs as a much cheaper alternative to tool steel.

High Temperature Cements. Refractory cements that call for fine SiC bonds can use 1200/F (2.5 micron) or F 400-W (20 micron) SiC as an ingredient. Others use the 800/1000 washed size to reduce the reactivity with acidic binders used in steel mill formulations.

Conductive Coatings. Superfine SiC in sizes from F400 to F800 can be used in paints that conduct electrical charges away from large motors.

Wear Resistant Coatings. Superfine SiC can be mixed with an epoxy or resin matrix to create chemical- and wear-resistant coatings for tanks, tubes and fillers in diamond wheels.

Refractory Bond Construction. Superfines are used in the bond constituent area of refractory body construction. They each have somewhat different surface areas and micron sizes, and should not be considered as interchangeable grains for this use. For instance, the 800/1000 and F1200 are very clean at 99% (green SiC) and 98% (black SiC) and can be used to reduce the outgassing that is experienced with normal DCF types of grains. The 1200/finer product contains some impurities (green SiC is 96% pure and black SiC is 94% pure) and is suitable for bonding where micron size matters and where some impurities add strength to the bonding mechanism of fine formulations.

Refractory Castables. Fine-grained SiC is used to give strength and flow characteristics in cements and self-leveling castables.

Slip Cast Bodies. Fine-grained SiC can be used to design slips for a variety of applications. The reactivity of very fine sizes helps make bonds that fire at a lower temperature and have special characteristics, such as the ability to control viscosity and change the bond characteristics of the slip cast body.

Self-Bonded SiC Bodies. The cleanness of fine-grained SiC is especially useful in making self-bonded silicon carbide bodies. Larger SiC particles are usually too coarse to achieve the required properties, but because the superfine SiC particles are so clean, other bond constituents can be added in a very controlled manner.

Fine-Grained Boron Carbide

New industrial applications have also been developed as a result of the availability of fine-grained B4C in sizes such as F800, F1000 and F1200 (water classified) and 2 micron (air classified). These sizes are used as refractory anti-oxidants, ceramic parts, nuclear pellets and boronizing/boriding materials.

Refractory Anti-Oxidants. Sizes of -200 to -300 mesh are used in formulas (only 1⁄2 to 1%) of castable cements to line blast furnace troughs and runners. These formulas form B2O3, a very tough, corrosion-resistant skin that prevents further oxidation. Fine-grained B4C is also used in alumina-carbon and magnesia-carbon brick for blast furnaces to achieve the same purpose.

Ceramic Parts. Sizes of -400 mesh to -10 micron are hot pressed or cold pressed and sintered to make blasting nozzles, crush rolls and wear parts.

Nuclear Pellets. -400 mesh sizes are hot pressed or cold pressed and sintered to make control rods that absorb stray neutrons in nuclear reactors.

Boronizing and Boriding Materials. -200 mesh sizes are used in formulas with aluminum oxide, cryolite and silicon carbide to impregnate the surfaces of wear parts (such as oil well pipes and tools) with a thin layer of very hard iron boride (Fe2B).

Nano-Sized Particles

Fifteen to 20 years ago, SiC and B4C in large particle sizes was the norm. Creating smaller sized particles was very expensive, making the end product cost-prohibitive for ceramic manufacturers and other end users. Today, however, the technology has evolved to the point where suppliers can efficiently produce smaller and smaller particles in large quantities and at affordable prices. These smaller sizes are allowing the industry to develop products, such as those listed above, that couldn’t otherwise be developed.

As the demand for advanced technologies continues to increase, even finer sizes of SiC and B4C will be required. In anticipation of those needs, suppliers are in the process of developing raw materials that are in the nano-sized particle range. While the properties of the formulations in this range of particle sizes are still being explored, such developments will undoubtedly generate a new wave of applications for ceramic manufacturers in the future.

For More Information

For more information about superfine SiC and B4C, contact Electro Abrasives Corp., 701 Willet Road, Buffalo, NY 14218; (716) 822-2500; fax (716) 822-2858; e-mail info@electroabrasives.com; or visit www.electroabrasives.com.


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