WEAR CHARACTERISATION OF MAGNETIC HARD DISK COATINGS USING ATOMIC FORCE MICROSCOPE

Abstract

This project aims to characterize the wear behaviour of magnetic hard disk coatings through a series of nano/micro-wear tests carried out on commercially available glass substrate hard disk media. A TopoMetrix universal atomic force microscope (AFM), using both a specially prepared diamond tip and a normal Si3N4 tip, is employed for this investigation. The diamond tips with different radii are mechanically polished and assembled onto stainless steel cantilever for the AFM. These tips are then used to make wear marks with different loads under a fixed scan cycle or with different scan cycles under a fixed load. These give measurements for wear depth vs. load and wear depth vs. scan cycle. The images of the wear marks are then obtained by the same AFM using the normal tip. Nano/micro-wear test, the main experimental technique for the project, is used to investigate the wear behaviour of the 1.8-inch lubricated glass substrate hard disk media. The wear depth vs. wear load or scan cycle measurements are obtained and some features such as the critical point where wear behaviour undergoes a transition and the two different linear wear rates have been observed. Nano/micro-wear tests are also used to investigate the effect of lubricant and radius of diamond tip on lubricated and unlubricated 2.5-inch glass substrate hard disk media. A sharp and a blunt diamond tip are used in these tests. A critical point where wear behaviour undergoes a transition is also observed. This occurs at a wear depth which is about half of the overcoat carbon film thickness and this is found to be independent of the lubricant used. As the substrate under the carbon overcoat is the relatively softer magnetic layer, xii there is an obvious substrate effect on the wear rate of the hard coating closer to this softer layer. Nano/micro-wear tests also indicate that under the normal scan velocity of the AFM, the wear rate of lubricated hard disk is higher than that of the unlubricated one - a reversal of the expected behaviour. Only after the scan velocity is high enough, in this case, nearing the upper limit of the AFM used, the wear rate of unlubricated hard disk is found to be higher than that of the lubricated one - which is the expected result. The same phenomenon is also observed when the blunt diamond tip is used, indicating that this is independent of the tip radius. Results obtained also show that for the lubricated hard disk, wear rate by the sharp diamond tip is higher than that by the blunt diamond tip, but the results are reversed when an unlubricated disk is used. A "zoom" wear method for nano/micro-wear test was used for the first time in this project to complement the standard method, or the "separate steps" method. This new method has been found to provide reasonably satisfactory results but its ability to save time for the experimenter warrants further work to remedy some of the shortcomings discovered in the course of this project.