Figure 2.
A rigid particle sliding on a surface rubs or abrades depending on the attack angle β. (a) Abrasion removes material by chipping, while rubbing produces plastic displacement with a prow in front of the particle displacing enamel into lateral ridges. Assuming a triangular pyramid moving facet-first on enamel (angles as shown), a sliding force F, normalized to enamel shear yield stress τy and the square of indentation depth t (= 0.5 µm to conform with experiment), abrades when β > 40°, but rubs when β < 40°. (b–e) Features of a quartz dust particle that removed an enamel chip at a fixed vertical force of 1800 µN. (b) Stereo light microscopic views show quartz attached to the titanium tip (cyanoacrylate glue produces the halo effect) prior to the scratching test. (c) The same quartz dust particle post-test, showing a clump of enamel chips (arrowed) retained on the particle after having been fractured away from the enamel surface. Inset for (c) shows a clump of these enamel chips (SEM). (d,e) Top and side views of same particle post-test, but the enamel chips having been removed (SEM). Particle has peaks, arrowed in (e), with β > 40° that removed the enamel in (c).