Abstract: Mechanical properties of potassium dihydrogen phosphate (KH2PO4, KDP) single crystal at the nano scale are one of important factors for probing its ultraprecision machining technology. Nanoindenting tests on (001) crystallographic plane and the tripler plane of KDP single crystal at room temperature are investigated by using nanoindentation tester with a standard Berkovich indenter with its tip radius around 50 nm.The results show that the nanohardness H and elastic modulus E exhibit significant peak load dependence. And the nonlinear proportional specimen resistance of normal load versus contact depth is responsible for the observed size dependence of H and E by Meyer’s law and the modified proportional specimen resistance (MPSR) model. Furthermore, according to the of Raman spectral analysis, the nonlinear amplitude of them is influenced greatly by the surface residual stress of the processed sample. Moreover, pop-in events are also observed in load-displacement curves, which are closely correlated to the transition from elasticity to plasticity of the examined single crystals.

Key words: Elastic modulus, Indentation size effect, KDP single crystal, Nanohardness, Pop-in event