Hardness Testing by Grit Breakout Test or Scratch Test

Merbecks developed the grit breakout test and scratch test following ideas from Opitz and Peklenik [MERB03, PEKL60]. These tests are well applicable to superabrasive grinding tools as they do not damage the abrasive layer, leave holes, or measure the whole body elasticity [KLOC05c, MERB03]. In the single grit breakout test, a cemented carbide tip is fed in a defined angle towards a single grit on the wheel surface. In the grinding wheel scratch test, the carbide tip is fed along
the grinding wheel surface at a certain depth and with a defined scratch rate. Its depth of should be one third of the smallest grit size tested, e. g. depth of 30 qm for a B91 CBN grit. The breakage phenomena are monitored by video, force measure­ment, and/or acoustic emission measurement. Grit breakage, grit pullout, and bond breakage are the three breakage phenomena. The grinding wheel hardness is characterized by the proportion of these wear mechanisms.

As example, higher bond hardness results in an increased portion of grit breakage as consequence of the stronger bond bridges and stronger grit embedding [KLOC05c]. Larger grit sizes at a constant scratch depth results in higher pro­portion of grit pull-out due the larger lever arm and resulting momentum on the grits [KLOC05c]. Linke used the test method to understand the wear behavior of vitrified bonded grinding tools after dressing [LINK07].