Micro-Wear Scan Test on the Carbon Overcoats as Thin as 6 nm or Less
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ABSTRACT Nitrogen-doped carbon (CNx) overcoats, ranging from I to 6nm in thickness, were deposited on magnetic recording disks by a DC-sputtering process. A critical load, based on the first occurrence of coating damage, was used as a semi-quantitative measure of the mechanical strength of these overcoats. It was found that the critical load decreased in a nearly linear manner with the CNx thickness from 6nm down to -2nm regime. However, the lnm thick CNx coating deviated from this trend with a significant decrease in critical load. High-resolution SEM was employed to find the critical loads as well as to reveal the details of the coating wear morphology and the CNx failure mechanism. INTRODUCTION There is much interest in understanding the mechanical properties of few nano-meter thick coatings, particularly, the diamond-like carbon coatings. The latter coatings are used in today's magnetic recording industry to protect the disk surfaces where the coating thickness has been pushed down to few nano-meters in order to meet the magnetic spacing requirement demanded by storage capacity. Limits to how far the carbon thickness can be scaled are starting to be investigated [1-3]. In fact, when the coating thickness is decreased, there are more limits other than the mechanical one that prevail, for example, the disk surface coverage [4,5] or corrosion protection, etc. In this report, we focus on understanding the role of thickness in the sputtered nitrogen-doped carbon coatings on mechanical properties. By combining a micro-wear scan technique [6] and high resolution scanning electron microscopy, we were able to establish an empirical relationship between the coating mechanical strength and its thickness. Furthermore, we were able to study the thickness limit effect based on the behavior of the critical load, failure mechanism, and the coating wear morphology.
EXPERIMENT Micro-wear Scan Technique and High Resolution SEM Fig. 1 shows the schematic of the micro-wear tester. In the course of a micro-wear scan, while the indenter is oscillating at a frequency of -2Hz along the x-direction, the sample scans at a speed of 0.18 iim/sec along the y-direction. At the same time, the indenter is being pushed toward the sample surface to increase the normal loading. A diamond conical indenter with a nominal tip radius of 1 micron was employed for the test. Because the existing micro-indenter currently is not equipped with sufficient force sensitivity in either the normal load or tangential force measurements, we do not anticipate resolving the critical load issue through any direct force sensing signals. However, because of the tester's unique wear-scan motion and ramped409
Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
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scheme [6], the micro-wear scan capable of creating a 20[tm wide by -90gtm long wear track and preserving progressing wear morphology inside. ,the Z the i.e. two features, The morphology preservation and relatively large track area, are important, because they allow us to utilize
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