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No More Hard Drive Crashes?

Larry Bailey
Chemical Engineering
Stanford University
May 2001

My work focuses on understanding the role of lubricants in increasing the durability of computer hard disk drives by making the data storage system more robust.  During the operation of a drive, the head, which reads data from and writes data to the disk, is supported above the disk by a thin cushion of air. T he distance, however, between the head and the disk surface is so small that the two periodically come into contact with one another.  By understanding the chemical and mechanical characteristics of this head-disk interface and the role of lubricants in decreasing wear at the interface, we intend to improve the durability of the disk drive.

The storage capacity of hard disk drives is increasing at a tremendous rate, doubling every 18 months and, in recent years, outpacing the growth in the integrated circuit industry. In order to maintain this tremendous increase in capacity, the components that make up the drives are quickly evolving and new technologies are being developed.  As storage densities increase, the distance between the read/write head and the disk surface decreases, since the signal that can be obtained from the data stored within the disk increases as the head moves closer to the disk.  This evolution has progressed to the point that, in state-of-the-art drives, the head flies only a few billionths of a meter above the disk.  If these dimensions are scaled to more physically tangible values, the system is comparable to a 747 jet flying only a few millimeters above the ground.  In a system with tolerances this tight, occasional contact between the head and the disk is inevitable.

In commercial drives, such as those found in a common home computer, the disk is a relatively complicated system made up of a substrate material, typically glass or aluminum, a number of adhesion-promotion layers, a magnetic layer, where data is stored, and finally a thin layer of hard, amorphous carbon.  This thin carbon layer protects the soft magnetic layer from damage whenever the head impacts the surface of the disk. The surface of the carbon layer is coated with an extremely thin perfluoropolyether lubricant film.  The purpose of this film is to minimize wear of the carbon layer when the head and disk come into contact. 

The lubricant film is typically composed of approximately one layer of molecules and is very important to the durability of the drive.  With the lubricant in place, disks last years before wearing out, whereas without it they wear out in a few days.  Despite their importance, the fundamental physics governing the behavior of these thin films is not yet well understood. Their extremely thin nature precludes the use of standard lubrication theories, such as those that may be used to describe lubrication in a car engine, since material properties often change dramatically when a substance is confined to molecular dimensions. 

We have hypothesized that chemical interactions between the lubricant molecules and the surfaces that they lubricate will have a significant impact on the system.  To test this hypothesis, we began monitoring the adsorption of a lubricant material onto carbon surfaces from solution, which should provide information about the interaction forces between the lubricants and the disk surface. We then used Atomic Force Microscopy (AFM) to gather information about the friction and adhesion between an idealized head and the disk surface.  Finally, we plan to chemically modify the lubricant molecules, changing the forces between the lubricants and the disk, and measure the effect of these changes using AFM.

Adsorption theories describe the amount of material adsorbed onto a surface as a function of the concentration of that material in solution. In systems that can be completely described by these theories, measuring the amount of material adsorbed on a surface as a function of the material's concentration in solution can be used to obtain the strength of interaction between the material and the surface. The stronger the interaction, the more material will be adsorbed on the surface. Thus, adsorption experiments were a natural choice in our search for a method to quantify lubricant-disk interactions. 

After making these measurements, we found that the behavior of the system is quite complicated. It cannot be described by simple theories and we are thus unable to obtain quantitative information about lubricant-surface interactions.  The adsorption kinetics, which describe the amount of material adsorbed as a function of time after the addition of lubricant to the system, have a very interesting behavior and suggest that after the lubricant comes into contact with the surface it reorganizes to form ordered structures.  Using this interpretation, we have been able to propose a model that does a reasonable job of qualitatively describing the adsorption process. 

Using AFM, we have measured the friction and adhesion between a sharp probe and a disk surface.  The friction is obtained by measuring the force needed to move the tip in a direction parallel to the disk surface.  The adhesion is obtained by measuring the force needed to separate the tip from the disk, moving the tip perpendicular to the disk surface.  This measurement provides us with quantitative information that should be directly applicable to the head-disk interface. We find that the friction is governed by the surface energy of the lubricant-disk system. The surface energy is related to molecular-level interactions between the lubricant and the disk, which strongly supports our original hypothesis that chemical interactions are important to molecular-level lubrication. However, we find that the adhesion between the head and the disk surface does not correlate with the surface energy and that the adhesive force is very dependent on the rate at which the measurement is made. 

We have concluded that these results are due to the very thin nature of the lubricant film, causing it to experience confinement that is direction-dependent.  The friction force measurements show that, in the plane of the disk, the lubricant is relatively mobile and the energetics of head-disk interactions governs the friction behavior. However, the adhesive force measurements show that, in the direction perpendicular to the surface, the lubricant is highly confined and its response times govern the behavior of the interface.

By taking these measurements and extending them to other disk-lubricant systems, we plan to develop relationships between the chemistry of the head-disk interface and its frictional and adhesive characteristics. A greater understanding of these relationships should lead to improvements in the components of the head-disk interface, allowing us to design a better interface and thereby improve the durability of hard disk drives.  A more durable head-disk interface means fewer drive crashes and, ultimately, less frustration for consumers.