This is part of a series of posts about abrasives. This is mostly cool SEM pictures, but I find them interesting. It’s quite difficult to image small grains at large magnifications, so there’s very limited information and pictures online about them. Head over to the archive to see the other snippets on abrasives.

Today’s abrasive is SiC. Silicon carbide is a fantastic engineering material, and also quite hard. Depending on the polytype, you get between 2000 and 3000 HV, which makes it kinda like the 3rd or 4th hardest commercially available abrasive. It is also super temperature resistant and very chemically inert. Again, depending on the polytype, it is either green (4H-SiC) or black (6H-SiC).

What’s the difference? The crystal structure. While SiC is typically hexagonal oriented, the different polytypes show different stacking orders. While 6H is stacked ABCACB, 4H is stacked ABCB

Crystal structure of 6H-SiC.

This property is called polymorphism, many minerals exhibit this, but SiC is unique in terms of there being 250 different known polymorphs.

The SiC we are looking at was bought at a German sharpening supply, sharpeningstones.de. They declare it as FEPA 300, 400 and 1000 grit, which would correspond to (roughly) 34, 17 and 4.5 µm.

FEPA 300 SiC

SEM Micrographs of FEPA 300 SiC particles. Instrument: Zeiss GeminiSEM 560.

FEPA 400 SiC

SEM Micrographs of FEPA 400 SiC particles. Instrument: Zeiss GeminiSEM 560.

FEPA 1000 SiC

SEM Micrographs of FEPA 1000 SiC particles. Instrument: Zeiss GeminiSEM 560.