Alternate Names: SiC
|HMOH - Hardness (Moh)||9+|
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)
Free silicon(Sif): <0.2%
Free carbon(Cf): 0.6-1.0%
Total oxygen (?o): 0.7-1.5%
Average diameter range: 0.05-0.5mm
Particle shape: Ball or cubic body
Dispersion: good hard agglomerate
Specific surface area: 20-40m2/g
A high refractory of the form SiC, very popular as the principal material in compositions for making kiln shelves and shelf bracings, particularly for high temperature firings (over 1000ºC), owing to its high resistance to thermal shock, warping, high refractoriness under load, resistance to contaminations from acids, metals, salts, and so forth, and especially an excellently uniform dispersal of heat.
Glaze Color - Red
Small silicon carbide additions to copper glazes fired in an oxidation kiln will produce reduction effects.
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Hot pressing is a process used to fabricate ceramic parts (often from non-oxide powders like BIN, SiC or B4C). The process involves the simultaneous application of pressure and heat to a 'green' component or powder under a controlled atmosphere (depending on the material being processed). Since pres...