Platters were originally made from an aluminum/magnesium alloy, which provides both strength and light weight.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.
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Glass platters offer greater rigidity than metal (because metal can be bent and glass cant) and can therefore be machined to onehalf 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: ox10e meala (oosolete) Thin-film media AFC (antiferromagnetically cupled) 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 ith a syrup containing ironoxide 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,
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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 oxidecoating 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 uch 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 platter with several layers of metallic film. The magnetic medium layer itself is a cobalt alloy about 1 0-inch thick. Thin-film sputtered media are created by first coating the aluminum platters with a layer of nickel phosphorus and then applying the cobalt-alloy magnetic material in a continuous vacuum-deposition process called sputtering. This process deposits magnetic layers as thin as 1 0-inch or less on the disk, in a fashion similar to the way hat silicon wafers are coated with metallic films in the semiconductor industry.
The same sputtering technique is again used to lay down an extremely hard, 1 [I-inch protective carbon coating. The need for a near-perfect vacuum makes sputtering the most expensive of the processes described here. The surface of a sputtered platter contains magnetic layers as thin as 1 [I-inch. Because this surface also is smooth, the head can float more closely to the disk surface than was previously possible. Floating heights as small as 10nm (nanometers, or about 0. 4 0-inch) above the surface are possible.
When the head is closer to the platter, the density of the magnetic flux transitions can be increased to provide greater storage capaclty A001tlonally, tne Increased Intenslty 0T tne magnetic TlelO during a closer-proximity read provides the higher signal amplitudes necessary for good signal-to-noise performance. AFC Media The latest advancement in drive media is called AFC media and is designed to allow densities to be pushed beyond previous limits. Any time density is increased, the magnetic layer on the platters must be made thinner.See More on Digital