Microstructural aspects of impact erosion in single crystals of LiF, NaCl, KCl and CaF2 by blunt projectiles

Abstract

In an attempt to clarify the fundamental mechanism of material removal in erosion by blunt solid particles entrained in a fluid stream impinging on a solid surface, single crystals of CaF2 (relatively brittle), LiF (intermediate between brittle and ductile), and KCl and NaCl (relatively ductile) were eroded with 0.25 mm glass beads and 0.50 mm quartz sand grains. The velocity of the particles was varied between 2 and 120 m sec−1. Erosion damage was studied with optical and scanning electron microscopy. In the single-impact mode, the damage is highly dependent on the mechanical properties of the target; material spalled off or micromachined out in individual impacts, or as chips produced upon the intersection of fractures resulting from several neighbouring impacts. At normal impact, the predominant mechanism is intersecting fractures, but at impact angles away from the normal, micromachining occurs in NaCl and KCl and, in fact, becomes the major mechanism of material removal. In LiF, only a little micromachining occurs and in CaF2, none at all; hence in these materials spalling is the controlling mechanism for material loss in individual impacts.