The hardness of CBN at room temperature is approximately 4,500 kg/mm2. This is about half as hard as diamond and twice as hard as conventional abrasives.
5.7.7 Wear Resistance of CBN
The differences in abrasion resistance are much more extreme. A hardness factor of 2 can translate into a factor of 100 > 1,000 in abrasion resistance depending on the abrading material. The author
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TABLE 5.1 Mechanical Properties of Typical Alumina and SiC Abrasives
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remembers, as a research student under Wilks, when the first CBN samples were supplied for abrasion-resistance measurements using the same technique used for measuring the wear resistance of diamond. The CBN was so soft in comparison to diamond that it was impossible to obtain a value on the same wear scale. As with diamond, the key is the total wear resistance to all attritious — wear processes.
Like diamond, CBN is brittle, but it differs in having six (110) rather than four (111) cleavage planes. This gives a more controlled breakdown of the grit especially for the truncated tetrahedral shape of typical CBN grains. The grain toughness is generally much less than that of blocky cubo — octahedral diamonds. This, combined with its lower hardness, provides the very useful advantage that CBN wheels can be dressed successfully by diamond (rotary) tools.