Indentation size effect in Mg2C60 governed by depth-dependent activation of shear bands and localized amorphization

Indentation size effects in crystalline solids are commonly associated with dislocation-mediated strain gradients. Here, we show that the layered molecular crystal Mg2C60 exhibits a distinct small-scale response. Continuous stiffness nanoindentation reveals that both hardness and modulus increase as the indentation depth decreases from 1 μm to 100 nm, notably deviating from classical dislocation-based predictions. Cross-sectional focused ion beam–transmission electron microscopy further reveals an evolution from relatively sparse, preferentially oriented shear bands at 0.5 μm to a spatially extended, multi-oriented shear-band network at 1 μm, with localized amorphization confined to selected shear-localized regions. The mechanical measurements and structural observations support a depth-dependent evolution from a more confined deformation state toward distributed shear localization at larger indentation depths. The results demonstrate that bonding hierarchy and structural discreteness, rather than conventional strain-gradient hardening alone, govern the depth-dependent indentation response of Mg2C60.