Lubricating heated hard drives
Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge February 2010
Recent experiments show how lubricants are lost from computer hard disk drives due to laser heating.
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Since the first computer hard disk drive was introduced in the mid-1950s, there has been an inexorable increase in storage capacity from the first systems that required 50, two-foot diameter disks to store only five megabytes of memory to today’s storage densities of 300 GBit per square inch.
Because the flying height of the slider is equivalent to a jet airliner cruising a few millimeters above the earth, much of this growth in storage density has been due to the development of novel perfluoropolyether lubricants.
However, further increases in memory density will require making smaller and smaller magnetic domains for the magnetic switches that form the bits on the hard disk. Since magnetism is a collective phenomenon, it requires a certain size to form the magnetic domain, which means as the domain size gets smaller the resulting magnet becomes weaker.
One solution to this problem has been to use magnetic materials with a high coercivity that result in higher magnetic fields at smaller domain sizes. However, switching materials with higher coercivities also requires larger magnetic fields and thus is limited by the properties of magnetic materials in the write head.
Since the coercivity decreases with increasing temperature, eventually becoming zero at the Curie temperature, one approach has been to rapidly heat the region of the hard drive where the bit will be written in a technique called thermally assisted magnetic recording. It is anticipated that this approach could lead to magnetic recording densities in excess of 1 TBit per square inch.
This process, of course, subjects the lubricant to thermal stresses that can lead to lubricant loss. In order to address this issue, professor Norio Tagawa’s group at the High Technology Research Center at Kansai University in Japan measured the laser-induced depletion of various thicknesses of two commonly used hydroxyl-terminated hard disk lubricants, Zdol2000 and Ztetraol2000. These were deposited onto flat glass slides coated with a thin tantalum film and a carbon overcoat to mimic the disk surface, and were rotated at 8 m/s and illuminated with a 5 mW laser. The resulting film thickness profile was measured.
They found that lubricant was lost and the depletion depth increased with radiation time and formed raised ridges on the outer edge of the depleted region. The extent of depletion increased with film thickness but only for films thicker than one monolayer; lubricants bonded directly to the surface were not removed.
Two possible mechanisms for lubricant loss were identified. The first was lubricant evaporation due to heating and the second was the effect of a decrease in surface tension with increasing temperature that would tend to cause the lubricant to move away from the heated region. They were able to discount the second effect since excess lubricant should accumulate uniformly around the depleted region, while it did not.
In order to explore whether evaporation was responsible for lubricant loss, the heating effect of the laser beam was simulated and found to result in a maximum temperature increase of ~90 C. The change in lubricant film thickness when heating to about this temperature was then measured for both lubricants. Lubricant loss was found, and since this temperature is much lower than the lubricant decomposition temperature (~350 C), this was ascribed to lubricant evaporation.
When this evaporation rate was compared with the rate of lubricant removal rate in the laser beam, almost exactly identical results were found. It seems that solving tribological problems will continue to be at the heart of designing even higher capacity hard drives.
FOR FURTHER READING:
Tagawa, N., Andoh, H. and Tani, A. (2009) “Study on Lubricant Depletion Induced by Laser Heating in Thermally Assisted Magnetic Recording Systems: Effect of Lubricant Thickness and Bonding Ratio,”
Tribology Letters, Online First, DOI: 10.1007/s11249-009-9533-4.
Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.
Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.