As with diamond, CBN grain grades are most commonly characterized by toughness and by shape. Toughness is measured both at room temperatures and at temperatures up to >1,000°C comparable to those used in wheel manufacture, the values being expressed in terms of a toughness index (TI) and thermal toughness index (TTI). The details of the measurement methods are normally proprietary but, in general, grains of a known screened-size distribution are treated to a series of impacts and then rescreened. The fraction of grain remaining on the screen is a measure of the toughness. For TTI measurements, the grains may be heated in a vacuum or a controlled atmosphere or even mixed with the wheel bond material, which is subsequently leached out. TI and TTI are both strongly influenced by doping and impurity levels. Additional degradation of the grain within the wheel bond during manufacture can also occur due to the presence of surface flaws that may be opened up by penetration of bond.
The surface roughness of CBN is a more pronounced and critical factor than for diamond in terms of factors influencing grinding wheel performance. A rough angular morphology provides a better, mechanical anchor. Of the examples illustrated in Figure 5.17, GE Type 1 abrasive is a relatively weak irregular crystal. The coated version GE Type II abrasive used in resin bonds has a simple nickel-plated cladding. However, GE 400 abrasive is a tougher grain with a similar shape
Tetrahedral
but with much smoother, flaw-free faces. The coated version GE 420 is, therefore, first coated with a thin layer of titanium to create a chemically bonded roughened surface to which the nickel cladding can be better anchored.
Only a relatively few grades of CBN are tough and blocky with crystal morphologies shifted away from tetrahedral growth. The standard example is GE 500 used primarily in electroplated wheels. De Beers also has material, ABN 600, where the morphology has been driven toward the cubo-octahedral.