Atomistic Simulation of the Nanoindentation of Diamond and Graphite Surfaces
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ATOMISTIC SIMULATION OF THE NANOINDENTATION OF DIAMOND AND GRAPHITE SURFACES
J. A. HARRISON, R. J. COLTON, C. T. WHITE, and D. W. BRENNER Chemistry Division, Code 6170, Naval Research Laboratory, Washington, DC 20375-5000
ABSTRACT Molecular dynamics simulations which make use of a many-body analytic potential function have been used to study the nanometer-scale indentation of diamond and graphite. We find that the simulation correctly reproduces experimentally determined trends in load versus penetration data. As a result, trends in mechanical properties, e.g. Young's modulus, are also reproduced.
I. INTRODUCTION Microindention has long been used to characterize bulk properties of materials. An indentation curve plots the relationship between load and penetration depth, h, continuously measured and recorded during a hardness test.' A limitation of traditional indentation techniques is the need to image the surface after indentation to determine the penetration depth of the plastically deformed material. For microindentation, SEM and even TEM images are required."' 2 Furthermore, shallow indents are often hard to find and difficult to image. As the interest in thin films has increased, so has the need for reliable characterization methods. This and other considerations have prompted the use of instruments such as the Nano-indentor 3 and the atomic force microscope (AFM)2,4," to study the nanomechanical properties of solids and thin films. Recently, the AFM has been used to measure the nanomechanical properties (e.g. elastic modulus and hardness) of an elastomer, graphite, gold films2 , and diamond surfaces, 5 and to examine the surface forces of monolayer films' with depth and force resolution superior to other methods. To better understand the strengths, limitations and interpretation of nanoindentation for characterizing materials and thin film properties, we have been using molecular dynamics (MD) to simulate indentation at the atomic scale. In this work we report initial studies of the indentation of hydrogen terminated (111) and (100) surfaces of diamond and the basal plane of graphite with a diamond tip. We find that the force on the simulated tip varies linearly with the penetration depth and that the slopes reflect the correct trends for the moduli of the various surfaces. We interpret the differences in moduli for the two diamond surfaces in terms of the relative displacement of bond angles and bond lengths during indentation.
Mat. Res. Soc. Symp. Proc. Vol. 239. 01992 Materials Research Society
574
II. APPROACH Molecular dynamics calculations were carried out by integrating Newton's equations of motion with a third order variable time step Nordsieck predictor corrector.' The particle forces are derived from an empirical hydrocarbon potential that is capable of modeling intramolecular chemical bonding in both diamond and graphite lattices, as well as in a variety of small hydrocarbon molecules.8 The potential used here is potential II of reference 8 with additional terms that better describe torsiona
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