Hardware Reference
In-Depth Information
any 3 1/2-inch drive is 12; however, most drives have six or fewer.
Platters were originally made from an aluminum/magnesium alloy, which provides both strength and
light weight. However, manufacturers' desire for higher and higher densities and smaller drives has
led to the use of platters made of glass (or, more technically, a glass-ceramic composite). One such
material, produced by the Dow Corning Corporation, is called MemCor. MemCor is composed of
glass with ceramic implants, enabling it to resist cracking better than pure glass. Glass platters offer
greater rigidity than metal (because metal can be bent and glass can't) and can therefore be machined
to one-half the thickness of conventional aluminum disks—sometimes less. Glass platters are also
much more thermally stable than aluminum platters, which means they do not expand or contract much
with changes in temperature. Virtually all modern drives use glass or glass-ceramic platters.
Recording Media
No matter which substrate is used, the platters are covered with a thin layer of a magnetically
retentive substance, called the
medium
, on which magnetic information is stored. Three popular types
of magnetic media have been used on hard disk platters:
• Oxide media (obsolete)
• Thin-film media
• AFC (antiferromagnetically coupled) media
Oxide Media
The oxide medium is made of various compounds, containing iron oxide as the active ingredient. The
magnetic layer is created on the disk by coating the aluminum platter with a syrup containing iron-
oxide particles. This syrup is spread across the disk by spinning the platters at high speed; centrifugal
force causes the material to flow from the center of the platter to the outside, creating an even coating
of the material on the platter. The surface is then cured and polished. Finally, a layer of material that
protects and lubricates the surface is added and burnished smooth. The oxide coating is usually about
30 millionths of an inch thick. If you could peer into a drive with oxide-coated platters, you would
see that the platters are brownish or amber.
As drive density increases, the magnetic medium needs to be thinner and more perfectly formed. The
capabilities of oxide coatings have been exceeded by most higher-capacity drives. Because the oxide
medium is soft, disks that use it are subject to head-crash damage if the drive is jolted during
operation. Most older drives, especially those sold as low-end models, use oxide media on the drive
platters. Oxide media, which have been used since 1955, remained popular because of their
relatively low cost and ease of application. Today, however, few if any drives use oxide media.
Thin-Film Media
Thin-film medium is aptly named. The coating is much thinner than can be achieved by the oxide-
coating method, but is much stronger. In fact, modern thin-film media are virtually uncrashable. If you
could open a drive to peek at the platters, you would see that platters coated with the thin-film
medium look like mirrors. Thin-film media are also known as
plated
or
sputtered media
because of
the various processes that deposit the thin film on the platters.
Thin-film plated media are manufactured by depositing the magnetic medium on the disk with an
electroplating mechanism, in much the same way that chrome plating is deposited on the bumper of a
car. The aluminum/magnesium or glass platter is immersed in a series of chemical baths that coat the
