Nanoindentation measurements of amorphous carbon coatings
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Nanoindentation measurements of amorphous carbon coatings Ashok V. Kulkarni and Bharat Bhushan Computer Microtribology and Contamination Laboratory, Department of Mechanical Engineering, The Ohio State University, Columbus, Ohio 43210-1107 (Received 7 October 1996; accepted 3 June 1997)
In this study, amorphous carbon coatings were deposited with thicknesses ranging from 20 nm to 100 nm on single-crystal silicon substrates by sputtering, ion beam, and cathodic arc deposition techniques. An indentation system with a three-plate transducer with electrostatic actuation and capacitive sensor has been used to make load displacement measurements and subsequently carry out in situ imaging of the indents. Indentation experiments were carried out using a three-sided pyramidal (Berkovich) diamond tip. Measurements include load-displacement curves and calculation of hardness and Young’s modulus of elasticity at various indentation depths, studies of hysteresis behavior, creep behavior, and strain rate effect of various carbon coatings. The cathodic arc coating exhibited the highest hardness and elastic modulus followed by the sputtered and ion beam coatings.
I. INTRODUCTION
Amorphous carbon coatings ranging in thickness from 10 to 20 nm are most commonly used as a protective overcoat for thin-film rigid disks against wear and corrosion.1 Ultrathin overcoats as thin as 5 nm on the air-bearing surfaces (ABS) of the MR-type rigid-disk head sliders are used for protection of magnetic head elements and to provide long interface durability.2,3 To maintain small gaps between a recording head and a disk, disk coatings with thickness as low as 5 nm are being developed. The tribological properties of amorphous carbon vary considerably on the process parameters. The sp 3 -bonded carbon exhibits outstanding properties of diamond such as high hardness, elastic modulus, low friction, optical transparency, and chemical inertness. Previous investigations suggest that in order to achieve higher fraction of sp 3 bonding content, the kinetic energies of the depositing species should be on the order of 100 eV or higher.4,5 As such, various deposition techniques have evolved to achieve carbon coatings with exceptional mechanical properties. Since the mechanical properties affect the friction and wear performance, study of the mechanical properties of the thin coatings is of primary concern. Among the mechanical properties, two mechanical properties measured most commonly are the hardness and the elastic modulus. Although indentation hardness testing has been in widespread use for several decades, high resolution depth sensing instruments have only recently become available. Because measurements of thin coatings require use of very small indentations, depthsensing instruments provide the necessary resolution and repeatability of hardness measurement. In order to get accurate measurements of the hardness of surface films, J. Mater. Res., Vol. 12, No. 10, Oct 1997
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