Nanoindentation of pure and gas-saturated fullerite C60 crystals: Elastic-to-plastic transition, hardness, elastic modulus

Abstract

Elastic-plastic transition at nanoindentation of (111) plane of pure C60 fullerite single crystals was studied. The onset of plastic deformation in the contact was noted due to the plateau formation in the initial part of loading curve. The estimated stress of plasticity beginning was found to be on the order of the theoretical shear stress required for homogeneous dislocation nucleation in the ideal crystal lattice of C60. The empirical values of elastic modulus E ∼ 13.5 GPa, hardness of the ideal crystal lattice H ∼ 1.4 GPa, and hardness at different indentation loads were obtained. The hardness vs load dependence was found consistent with the model of geometrically necessary dislocations. The loading diagrams shape and the dependencies of contact pressure vs indentation depth were strongly affected by gaseous interstitial impurities (hydrogen, oxygen, nitrogen) in C60 crystal; transition stress was essentially less and plateaus formation was observed at elevated indentation loads and depths as compared with pure fullerite crystal. For crystals, saturated with hydrogen, the enhanced value of elastic modulus (∼ 20.4 GPa) and hardness (∼ 1.1 GPa) were obtained. The results acquired at room temperature for C60 with face-centered cubic lattice are important for the description of the physical-mechanical properties of simple cubic lattice phase of C60 below 260 K (S. V. Lubenets, L. S. Fomenko, V. D. Natsik, and A. V. Rusakova, Fiz. Nizk. Temp. 45, 3 (2019) [Low Temp. Phys. 45, 1 (2019)]).