Ultra-Thin Carbon Coatings for Head-Disk Interface Tribology
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J.N. GLOSLI*, J. BELAK*, and M.R. PHILPOTFT** University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550 **IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099
ABSTRACT Molecular dynamics computer simulations of the growth processes and microstructural properties of amorphous carbon (a:C) and amorphous hydrogenated carbon (a:CH) ultra-thin films have been performed. Films 1 to 10 nm thick were grown on a diamond (100) surface using Brenner's[ 1-2] bond-order potential for hydrocarbons. The stoichiometry, radial distribution function, chemical bonding (amount of sp 2 and sp 3 hybridization) and residual stress are presented.
INTRODUCTION Amorphous carbon films approximately 20 nm thick are used as protective coatings on magnetic recording disks. As storage densities increase, the role of the overcoat becomes increasingly important because of smaller spacings between the recording head and the spinning disk. Furthermore, future generation disks call for an overcoat thickness of 5 nm or less. These small length scales and the high speed of the spinning disk (10-30 m/s) suggest that a molecular dynamics (MD) model[3] might provide useful insight into friction and wear mechanisms when the head and disk make contact. One of the necessary inputs required to carry out such an MD model is a specification of the position of all of the atoms in the simulation, i.e. a detailed specification of the material microstructure. Such a detailed understanding of the microstructure of amorphous carbon overcoats does not presently exist. Neutron[41 and electron[5] diffraction studies demonstrate that the material is amorphous. Previous classical MD simulations[6-8] yield pair distribution function in qualitative agreement with the diffraction studies, but they all differ in detail. More recent, quantum mechanical tight-binding MD (TBMD) studies[9-1 I] give a better description of the interatomic interactions and the chemical hybridization (sp 2 -graphite-like versus sp 3-diamond-like). However, these studies are presently limited to rather small system sizes and rapid quench rates. In this paper we present both quench and deposition simulations of the formation of amorphous carbon using Brenner's[1-2] bond-order potential model. This classical potential mimics the quantum mechanics allowing carbon to form strong chemical bonds with a variety of hybridizations. METHODS and RESULTS We have performed two types of simulations: quench from a high temperature liquid and deposition onto a room temperature surface. During a quench simulation, we use periodic boundary conditions and constrain the volume to be 85% of diamond density (PD= 3 .54 gms/cc). The initial state is a well equilibrated liquid at 6000K. We have used system sizes ranging from 64 to 8192 carbon atoms and quench rates ranging from 5 to 40 K/ps. It was found that the system formed unphysical bonding configurations without an added torsional energy between sp 2 hybrid749 Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Ma
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