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Platter Substrate Materials
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.
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:
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