Anisotropic plasticity and fracture mechanisms of single-crystal CaF2 under indentation: Insights from crystal plasticity simulation and experiment

Single-crystal calcium fluoride (CaF 2 ) is a key optical material in lithography system for advanced semiconductor manufacturing, yet its ultra-precision machining remains challenging due to its soft-brittle nature and pronounced crystallographic anisotropy. This paper systematically elucidates nanoscale incipient plasticity, deformation mechanism and crack formation of CaF 2 under indentation, with particular emphasis on anisotropic effects, through a combined approach of experimental measurements and crystal plasticity finite element simulations. Results show that compared with the (111) sample, the (001) sample exhibited a lower pop-in load (0.44 mN vs. 1.18 mN) and smaller pop-in size (20.7 nm vs. 50.33 nm). Both slip trace patterns and crack initiation sites demonstrate strong crystallographic anisotropy. Quantitative analysis of accumulated shear strain within the {100}<110> slip systems determine the dominant slip systems responsible for the plastic flow, accounting for the observed anisotropic slip trace patterns. Furthermore, cracks on the (001) and (111) samples preferentially propagate along their respective dominant cleavage planes, as directly evidenced by the distributions of maximum principal stress and cleavage stresses on {111} planes. This work advances the mechanistic understanding of anisotropic, multi-scale indentation behaviour in brittle materials and offers valuable guidance for achieving high-quality ultraprecision manufacturing of CaF 2 .