Even then, though hard disk engineers wanted to put lots of platters in a particular model, the standard “slimline” hard disk form factor is limited to 1 inch in height, which limits the number of platters that can be put in a single unit. Of course, engineers are constantly working to reduce the amount of clearance required between platters, so they can increase the number of platters in drives of a given height.
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 therefore alternatives to aluminum, such as glass, glass composites, and magnesium alloys have been proposed. It now is looking increasingly likely that glass and composites made with glass will be the next standard for the platter substrate. Compared to aluminum platters, glass platters have several advantages:
- Better Quality:
- Improved Rigidity:
- Thinner Platters:
- Thermal Stability:
One disadvantage of glass compared to aluminum is fragility, particularly when made very thin.
The substrate material of which the platters are made forms the base upon which the actual recording media is deposited. The media layer is a very thin coating of magnetic material which is where the actual data is stored. It is typically only a few millionths of an inch in thickness.
Older hard disks used oxide media. Oxide media is inexpensive to use, but also has several important shortcomings. The first is that it is a soft material, and easily damaged from contact by a read/write head. The second is that it is only useful for relatively low-density storage. It worked fine for older hard disks with relatively low data density, but as manufacturers sought to pack more and more data into the same space, oxide was not up to the task: the oxide particles became too large for the small magnetic fields of newer designs.
Today's hard disks use thin film media. Thin film media consists of a very thin layer of magnetic material applied to the surface of the platters. Special manufacturing techniques are employed to deposit the media material on the platters.
Compared to oxide media, thin film media is much more uniform and smooth. It also has greatly superior magnetic properties, allowing it to hold much more data in the same amount of space. After applying the magnetic media, the surface of each platter is usually covered with a thin, protective, layer made of carbon. On top of this is added a super-thin lubricating layer. These materials are used to protect the disk from damage caused by accidental contact from the heads or other foreign matter that might get into the drive.