... of ZrO increases.3. Only no harder than Boron carbide and Diamond, friable, its girt is sharp, and ... holdout, durableSuitable for form grinding, knives, Measuring instrumentGreen silicon carbideGCgreenOnly no harder than Boron carbideSuitable for grinding Hard ... Optical glass, vitrify and other hard friable materials.Black silicon carbideCblack Harder than corundum, crisper, less flexibility.Suitable ...