Color silicon carbides

Pure silicon carbide is colorless, but iron impurities normally impart an almost black color to the crystals. Carborundum is an excellent abrasive because it is very hard, with a diamondlike structure that fractures into pieces with sharp edges (Fig. 14.43). 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

Properties For silicon carbide, crystalline form ranges from small to massive crystals in the hexagonal system, the crystals varying from transparent to opaque, with colors from pale green to deep blue or... 

Reduction of CeOs in a stream of Hg is the best method. It is carried out in a silicon carbide boat. The Hg must be carefully purified (free of oxygen) and dried. Very pure CeOg (3 g.), prepared as described on p. 1133, requires about 80 hours at 1000°C (or 45 hours at 1100 °C) for complete reduction. Traces of La and Nd moderately increase the rate of reduction, while Pr and Tb do so markedly. The reduction is complete when the blue-black color of the partially reduced intermediates changes to the pure golden yellow of CegOa. 

Powder is blended with the paste at an addition level of 1 wt%. The paste is spread coated at a thickness of 2 mm by knife over roller over bed on to a non-woven support. Particles of silicon carbide and colored quartz are each scattered on the surface at the rate of 100 g/m. The paste is then gelled for 3 minutes at 170°C. Powder at the rate of 50 g/m is applied and further silicon carbide is applied at the rate of 100 g/m. The product is then fused at 190°C for 3 minutes and embossed. 

The conditions for color etching and subsequent grain boundary etching of silicon carbide and grain boundary etching of boron carbide are given below as examples. 

Figure 84. Pressureless-sintered silicon carbide doped with boron carbide. Electrolytically etched, BF. Grain face etching. Color etching reveals different grain orientations.

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