Silicon carbide: harder than aluminium oxide, but not automatically the better choice
In 1891, the American inventor E. G. Acheson was looking for a way to produce synthetic diamonds. He found something else instead: dark, irregularly shaped crystals made of silicon and carbon, namely silicon carbide (SiC for short).
Harder and harder, but still not always right
SiC has a Knoop hardness of approximately 2,600 to 3,000, making it considerably harder than aluminium oxide at 1,850. CBN at 4,500 to 5,000 and diamond at 5,000 to 8,000 are harder still.
This might appear to settle the matter: always use the harder grain. It is not quite that simple. When selecting an abrasive grain, hardness is only one consideration. Toughness, thermal behaviour and chemical compatibility with the workpiece also matter.
Why we recommend aluminium oxide for steel
When grinding steel, we generally recommend aluminium oxide rather than silicon carbide. At sufficiently high temperatures in the grinding zone, SiC can react with iron. This chemical reaction attacks the surface of the abrasive grain. It can weaken the grain’s cutting edge and increase grain wear. The harder grain is therefore not automatically the better grain.
Where silicon carbide is the right choice
The rule of thumb for conventional abrasives is therefore:
- Aluminium oxide for steel materials, particularly unalloyed, low-alloy and quenched and tempered steels.
- Silicon carbide for hard, brittle and short-chipping materials, such as glass, ceramics, stone, non-ferrous metals and cemented carbide.
This rule of thumb is no substitute for evaluating the individual grinding process. The specification that suits your application depends not only on the material, but also on the machine, process parameters and required surface finish.
Which material is currently causing the most difficulty in your grinding process? Tell us the material and grinding task, and we will recommend a suitable specification.