The magnetic patterns that comprise your data are recorded in a very thin media layer on the surfaces of the hard disk's
platters; the bulk of the material of the platter is called the substrate and
does nothing but support the media layer. To be suitable, a substrate material must be
rigid, easy to work with, lightweight, stable, magnetically inert, inexpensive and readily
available. The most commonly used material for making platters has traditionally been an
aluminum alloy, which meets all of these criteria.
Due to the way the platters spin with the read/write heads floating just above them,
the platters must be extremely smooth and flat. With older, slower spindle drives and
relatively high fly heights, the uniformity of the platter surface was less of an issue.
Now, as technology advances, the gap between the heads and
the platter is decreasing, and the speed that the platters spin at is increasing,
creating more demands on the platter material itself. Uneven platter surfaces on hard
disks running at faster speeds with heads closer to the surface are more apt to lead to head crashes. For this reason many drive makers began
several years ago to look at alternatives to aluminum, such as glass, glass composites,
and magnesium alloys.
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Hard disk platters are very smooth, right? Well, not to
a scanning electron microscope!
The image on the left is of the surface of an aluminum alloy platter; the one on the right
is a glass platter. The images speak for themselves. The scale is in microns.. |
Composed from two original images © IBM Corporation
Images used with permission. |
It now is looking increasingly likely that glass and composites made with
glass will be the next standard for the platter substrate. IBM has been shipping drives
with glass platters for several years and in 2000 is introducing them into the IDE/ATA
consumer drive market. Compared to aluminum platters, glass platters have several
advantages:
- Better Quality: The first and most important reason for going to glass
is probably that glass platters can be made much smoother and flatter than aluminum,
improving the reliability of the hard disk and making low flying heights and faster
spindle speeds more feasible.
- Improved Rigidity: Another important consideration is that glass is
more rigid than aluminum for the same weight of material. Improved rigidity, one of the
reasons why platter sizes are also shrinking in size,
is important for reducing noise and vibration with drives that spin at high speed.
- Thinner Platters: The enhanced rigidity of glass also allows platters
to be made thinner than with aluminum, allowing more platters to be packed into the same
drive dimensions. Thinner platters also weigh less, reducing spindle motor requirements
and reducing start time when the drive is at rest.
- Thermal Stability: When heated, glass expands much less than does
aluminum. With some hard disk platters now containing 35,000 tracks per inch or more, even
a small amount of expansion can causes these tracks to "move around". The
drive's servo mechanism compensates for expansion and
contraction, but it is still preferable to use materials that move as little as possible
because this reduces the amount of adjusting the hard drive has to do, improving
performance.
One obvious disadvantage of glass compared to aluminum is fragility,
particularly when made very thin. For this reason some companies are experimenting with
glass/ceramic composites. One of these is a Dow Corning product called MemCor,
which is a glass made with ceramic inserts to reduce the likelihood of cracking. Sometimes
these composites are just called "glass", much the way aluminum alloy platters,
which usually contain other metals, are just called "aluminum".
Next: Magnetic Media
