A Basic Understanding of Platter Substrate Material
Your data is made up of magnetic patterns that are stored in a very thin media layer on the surface of the platters in the hard drive disks. The majority of the material that makes up the platter is known as the substrate and simply supports the media layer. The most widely used material for producing platters has traditionally been an aluminum alloy, which is rigid, lightweight, inexpensive, easy to work with, readily available, magnetically inert, and stable.
The platters sitting inside the hard drive spin with the read/write heads hovering just above them so its critical that the surface of the platters be as level and smooth as possible. The older, slower spindle drives usually had very generous fly heights therefore; the consistency of the surface of the platter was not as crucial. Today, as technology continues to advance, the size of the gap between the platter and the heads continues to decrease as the rate of spin increases. This creates more demands on the platter material. Irregular surfaces on the platter in hard drives running at increased speeds along with the heads being suspended closer to the surface more readily lead to head crashes. This is one reason many drive manufacturers began looking for alternatives to aluminum, such as glass, glass composites, and magnesium alloys. More and more it looks like glass and composites made with glass will be the next accepted material for the platter substrate. IBM has been shipping drives with glass platters for several years. Glass platters have several advantages over aluminum ones:
----Better Quality: The primary reason for moving to glass is perhaps that glass platters can be made a great deal smoother and flatter than aluminum. This alone improves the dependability of the hard drive disk. It also makes increased spindle speeds and low flying head heights possible.
----Enhanced Rigidity: Given the same weight of material, glass is more rigid than aluminum. Enhanced rigidity, is imperative for dropping noise levels and vibration in hard drives that spin at high speeds. It is also one of the reasons why platters are decreasing in size.
----Thinner Platters: With the enhanced rigidity platters can be made thinner than with aluminum. Thinner platters weigh less which means reduced spindle motor requirements, reduced start time when the drive is at rest and it also allows an increased number of platters to be placed into the same size hard drive.
----Thermal Constancy : As aluminum heats it expands, while glass does expand as it heats it does so at a much reduced rate compare to traditional aluminum. Some platters contain 35,000 tracks per inch or more and even a tiny amount of expansion can cause these tracks to move around. The hard drive's servo mechanism compensates for expansion and contraction, but it is still preferable to use materials that move as little as possible.
Because one of the biggest disadvantages of glass would be its fragile nature, especially when it is very thin, some companies are experimenting with glass/ceramic composites.