Abstract: The friction and wear characteristics of potassium dihydrogen phosphate (KDP) single crystals are evaluated by single-tip scratch testing. The force profiles during the scratch process are captured by using high frequency force transducers. The mechanism of materials removal is characteristized by optical microscopy and scanning electron microscopy. It is concluded that the friction coefficients µ of different scratching directions increase with the increase of scratching displacements. When the scratching displacement s <3 mm, the friction coefficients µ increase smoothly and slowly, and the materials are mainly removed plastically. However, when the scratching displacement s>3 mm, the friction coefficients µ oscillate strongly, and the materials are mainly removed brittlely. The lateral cracks of the scratch groove along different scratch directions become denser with the increase of scratch displacement, and even the phenomenon of brittle fractures and break-up appears in the sides and bottom of the groove. At the same time, it is observed that the angle between the growth direction of the lateral cracks and the scratching direction is different, due to the relatively complex slide system of KDP crystals. To observe and analyze the subsurface damage induced by the scratch, section-polishing method and preferential-etching method are used and the depth of the damage is measured as well. The results show that the damage depth along [100] is the deepest and basically without fluctuation, while the damage depths along [110] and [120] fluctuate greatly. However, the damage depth along [120] relatively small in the initial stage of scratching.

Key words: Crack, Friction coefficient, KDP single crystal, Scratch, Subsurface damage