Crack Formation during Scratching of Brittle Materials

Abstract

In order to investigate crack formation of brittle materials during grinding, scratching experiments with diamond pyramids have been carried out in air under varying loads and scratching speeds on several materials: single crystalline MgO, polycrystal-line Mn Zn- and Ni Zn ferrite and a variety of glasses. Loads between 0.05 and 5 N and speeds between 0.4 and 400μ/s were applied. Additional experiments on one glass at about 0.5 m/s were carried out. When polycrystalline material is scratched with a leading pyramidal plane one observes with increasing speed first chips and then separate lateral cracks. With increasing load successively pure ductile grooving, lateral cracking and finally chipping are seen. On glass with increasing speed one observes successively chipping plus irregular median cracking, lateral cracking plus irregular median and sub-surface cracking and only smooth median and subsurface cracking. With increasing load the sequence of events is: ductile grooving, smooth median and subsurface cracking, irregular median and subsurface cracking plus lateral cracking and finally irregular median cracking plus chipping. This behaviour is maintained when the speed is increased to 0.5 m/s. In single crystalline MgO lateral cracks are formed at any load and at any speed. Minor median and subsurface cracks are also formed. When the experiments are carried out with a leading edge, then median cracks are also observed in poly-crystalline materials together with chipping and lateral cracking.

For the lateral and medium cracks expressions are derived relating tangential force to stress intensity factor, crack width and groove width. For the lateral cracks plots of scratching speed versus stress intensity factor are given for Mn Zn ferrite, for one glass and for MgO. K values of the glass obtained at certain crack speeds in a double torsion experiment in air are shown to fall within the error bars of K_I values derived from lateral cracks obtained at the corresponding scratching speeds. The groove width at constant load is seen to decrease with increasing scratching speed.

The number of lateral cracks per unit scratch length increases with increasing load and with decreasing speed.