Nanoindentation of diamond, graphite and fullerene films

The recently developed method of nanoindentation is applied to various form s of carbon materials with different: mechanical properties, namely diamond , graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nano indentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force-depth curve the Young's modulus as well as the hardness were obta ined reflecting a very large hardness of 95 GPa and 117 GPa for the {100} a nd {111} crystal surfaces, respectively. Investigation of a layered materia l such as highly oriented pyrolytic graphite again showed elastic deformati on for small indentation depths but as the load increased, the induced stre ss became sufficient to break the layers after which again an elastic defor mation occurred. The Young's modulus was calculated to be 10.5 GPa for inde ntation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the sof ter material of a molecular crystalline solid formed by C-60 molecules. The hardness values of 0.24 GPa and 0.21 GPa for these Alms grown by layer epi taxy and island growth on mica and glass, respectively, vary with the morph ology of the C-60 films. In addition to the experimental work, molecular dy namics simulations of the indentation process have been performed to see ho w the tig-crystal interaction turns into an elastic deformation of atomic l ayers, the creation of defects and nanocracks. The simulations are performe d for both graphite and diamond but, because of computing power limitations , for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulat ions capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However , if the speed of indentation is increased, the simulations indicate that p ermanent displacements of atoms are possible and permanent deformation of t he material takes place. (C) 2000 Elsevier Science S.A. All rights reserved .