Notes
Silicon Carbide is a nonoxide 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 SiC granular material cannot be crushed in a small ball mill. 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 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. Fast firing is required as the SiC is rapidly depleted. The Potters Dictionary has a good description of this.
There is some question about how to include SiC in glaze chemistry calculations. Perhaps the 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
Silicon carbide glazes
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Silicon carbide powder produces gases at the top end of a stoneware firing and will create glaze bubbles. You can find lots of examples by googling the term and clicking the "images" link. Many of these will have accompanying information (especially about how to fire the kiln). Be alert to the possibility that it might be more practical to put the SiC powder into a glaze you already use (using the amount of firing method explained on the resource you find).
Can SiC be added to a consumer brushing glaze?
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Yes. This was made by weighing 15g of liquid Amaco C-01 Obsidian and adding 3g of silicon carbide powder. It was fired using the C6DHSC firing schedule. The body is Plainsman M340 (although it should work on any one). Of course, the glass is filled with bubbles just under the surface, so any pressure will break many leaving razor sharp edges on the crater.
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