Nanoindentation Of Pure And Gas-Saturated Fullerite C-60 Crystals: Elastic-To-Plastic Transition, Hardness, Elastic Modulus

Elastic-plastic transition at nanoindentation of (111) plane of pure C-60 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 C-60. The empirical values of elastic modulus E similar to 13.5 GPa, hardness of the ideal crystal lattice H similar to 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 C-60 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 (similar to 20.4 GPa) and hardness (similar to 1.1 GPa) were obtained. The results acquired at room temperature for C-60 with face-centered cubic lattice are important for the description of the physical-mechanical properties of simple cubic lattice phase of C-60 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)]).