Abstract: Nano-indentation test with a Berkovich indenter, Vickres indentation test and nano-scratch test with a spherical indenter have been conducted along 0°, 45° and 90° orientations on the third harmonic generation(THG) crystal face of potassium dihydrogen phosphate(KDP) crystal. There is obvious indentation size effect on the sample. The different mechanical properties, critical loads and depths of the brittle-ductile transition of three orientation samples are obtained by the tests. Results show that the THG crystal face of KDP crystal has strong anisotropy. The critical loads of the brittle-ductile transition are 20.4 mN, 33.6 mN and 63.2 mN, while the critical depths are 1.26 μm, 1.67 μm and 3.45 μm. 90° is the optimal processing orientation because the sample has the least hardness, the largest fracture toughness, and the deepest critical depths of the brittle-ductile transition of the three. The sample which is scratched in 90° had the most ductile removal and the latest transition point from ductile to brittle. The prediction model of nano-scratch test on brittle materials with a single point spherical diamond indenter is established according to the results.

Key words: Anisotropy, Brittle-ductile removal transition, Indentation test, Potassium dihydrogen phosphate(KDP) crystal, Scratch test