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Silicon Carbide

Alternate Names: SiC

Notes

Silicon Carbide is a non oxide ceramic and is used in a wide range of products that must perform in thermally (high heat and heat shock) and mechanically demanding applications. It is employed in both abrasives and wear resistant parts for its hardness; in refractories and ceramics for its resistance to heat and low thermal expansion; and in electronics for its thermal conductivity and other properties. The only materials harder than SiC are boron carbide and diamond.

SiC itself can be bonded in a number of ways and parts can be fabricated in a variety of ways. Hot pressed and reaction bonded parts are usually porous, non-homogeneous and less thermally conductive and shock resistant. By contrast single crystal SiC has optimal properties but is very expensive to make. CVD furnaces, on the other hand, can be used to make solid pure SiC parts that are uniform and dense. Surprisingly, 100% SiC powders can also be cast using the traditional slurry deflocculation and plaster casting method (provided that a very fine grade SiC powder is employed). SiC cast parts separate best from completely dry molds and special measures may be needed to get the dispersant to mix in properly. SiC casting mixes can also contain some plastic clay to affect better suspension and enable using a coarser grade of material (for refractory setters, for example). Items need to be fired to 1500C.

In ceramics the most common use of SiC is for high heat duty kiln shelves. But this material is increasingly being used to make a wide range of products having low expansion, high heat endurance and resistance to abrasion.

SiC powder has some curious uses in ceramic glazes. It is employed to make crater and foam glazes. The silicon part takes up available oxygen to make SiO2 and the carbon combines with oxygen to make the CO2 that creates the blisters and bubbles. Using this mechanism it is possible to create reduction effects in oxidation firings, but with obvious challenges (blistering and bubbling). The carbon that silicon carbide particles release acts to reduce metallic oxides like iron and copper. Additions of tin oxide will aid color development, especially for copper reds. The Potters Dictionary has a good description of this.

There is some question about how to include SiC in glaze chemistry calculations. Perhaps that best answer is not to do it. Treat it as a recipe level additive (having predictable effects as such) while looking at the rest of the ingredients as suppliers of oxides to the glaze that the SiC is affecting.

The properties of SiC ultrafine powders are tightly controlled and typical data is presented as follows:

Crystalline type: Cubic (b-type)
Purity(SiC): 97.0-98.0%
Free silicon(Sif): <0.2%
Free carbon(Cf): 0.6-1.0%
Total oxygen (?o): 0.7-1.5%
Chloride(Cl): <0.25%
Average diameter range: 0.05-0.5mm
Particle shape: Ball or cubic body
Dispersion: good hard agglomerate
Specific surface area: 20-40m2/g

Related Information

Links

Oxide Analysis Formula
Materials Alumina
Materials Barium Titanate
Materials Boron Carbide
Materials Boron Nitride
Glossary Hot Pressing
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Non-Oxide Material
Many of these materials are not used in traditional ceramics but they have such incredible properties that some have been included in this database.
Typecodes Abrasive Resistant Super Hard Material
These materials are generally available in granular form, the particles are cemented together using frits to produce abrasive products. However powdered and slurried forms of these materials can also be formed and fired by various means to produce hard materials.
URLs http://en.wikipedia.org/wiki/Silicon_carbide
Silicon Carbide at Wikipedia
URLs http://skylinecomponents.com/SiC.html
SiC inforation at Skyline Components

Data

Hardness (Moh)9+

Mechanisms

Glaze ColorSmall silicon carbide additions to copper glazes fired in an oxidation kiln will produce reduction effects.

By Tony Hansen


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