2.9.3.1 Friatest
Manufacturers of superabrasives evaluate the grit fracture behavior by impact strength tests, so called friability test or friatester [ODON76, MARI04]. A grit sample with a defined weight and a steel ball are encased in a capsule, which is then shaken with a defined cycle number [VOLL12]. The impact load breaks a portion of the grits. The percentage of the non-destroyed grits is defined as toughness index (TI) [VOLL12]. The friability index (FI) defines the number of cycles needed to break 50 % of the grits [VOLL12]. Diamond grit manufacturers qualify their grits by room temperature toughness (TI) and thermal toughness after heating (TTI), for example at 1000 °C [MARI07]. [LINK15]
In general, the friability characteristics do not have a sufficient correlation to the grit performance and more research is needed. Another problem of the friability test method is that several characteristics intermingle in the test results.
2.9.3.2 Single Grit Toughness Test
The single grit breakage test enables a particle related description of grit strength. This method is based on the maximum force to break a particle, measured along one axis [VOLL03a, VOLL03b]. Some systems allow combining a two-dimensional picture analysis of grit size and grit shape, and measurement of the specific breakage force. In the procedure, a single grit is positioned between two anvils, which are pressed together with increasing, pneumatic force until the grit brakes [LIST06]. The maximum uniaxial force needed to break the particle is measured. The force divided by the grit area defines the breakage strength.
Diamonds with a diameter of 250 pm can have average breakage strength of 5000 N/mm2, so that the pressing forces have to be sufficiently high [LIST06]. The single grit fracture test has similarities to the real grinding process, where the normal forces are higher than the tangential forces.
Benea and Griffin introduced a strength testing method for micron powder [BENE03b].