Toughness, Breaking Behavior Life Cycle and Sustainability of Abrasive Tools

Today, grits are available in a large range of toughness suitable for many appli­cations [MARI04, p. 347]. Hard and friable abrasives are generally applied in precision grinding, whereas tough, large grits are more suitable for heavy-duty grinding [MALK08]. Grit types for resin bonds are usually more friable than for bronze bonding [BUTT68, p. 73].

Toughness is the resistance of a material against breakage and crack propagation and is often measured under dynamic conditions [LINK15]. Higher toughness implies that the grit is less likely to fracture when engaging the workpiece. The breaking or fracture behavior describes how the grits break, i. e. breakout of large or small particles, leaving a rough surface with many cutting edges or a smooth surface with only one cutting edge (see Sect. 2.1.5 “Performance of Corundum”). Jackson and Davim introduce the term friability as inverse term of fracture toughness [JACK11, p. 11]. The sum of toughness and friability, both in [N], accounts to 100 [SCHT81]. The “toughness index” is one industrial measure [VOLL12]. Temperatures during grinding can induce grit defects that change the grit breaking behavior, so “temperature toughness index” is another useful measure. The “friability index” is a measure for the loss of abrasive material by splintering [JACK11, p. 11].

On the one hand, grits that are too tough for a special application will become dull and increase friction. This leads to unnecessary thermal damage of the workpiece material and the danger of process vibrations. On the other hand, too friable grits wear away quickly resulting in short tool life and possibly form errors. As optimum, the grits should have a controlled breakdown behavior, so that they regenerate sharp cutting edges and the tool works in the so called self-sharpening mode.

Conventional grits are usually produced by crushing of coarser material resulting in fewer surface defects and in less friable grits [BREC73, MALK08]. In the same way, smaller diamond grits are commonly tougher because of fewer structural defects [FIEL81]. The toughness of conventional abrasives can be changed through the sintering temperature of the grinding tool [SCHT81].

Natural and synthetic diamond grits show different breakage behavior. Natural diamond collapse with several breakage events, synthetic grits, however, fail with one breakage event [HIMM90].

Toughness, Breaking Behavior
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