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Hard Disk Platters

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The platters in a hard disk are the media on which the data is stored. Made from a composite of materials they are the most critical part of a hard disk drive.

Modern hard disk drives can have several platters, each with two sides to make up the capacity of the drive. Some SCSI hard disk drives can have 10 or more platters using 20 heads.

The substrate is usually made from an Aluminium alloy, Glass or Ceramic material. Some problems with aluminium platters are that as the drives get hot, the aluminium alloy expands at different rate to the other materials, this is also known as 'Differential Thermal Expansion' (DTE). In severe cases it can cause the coating to delaminate from the substrate resulting in a head crash.

Manufacturers take DTE into account when designing drives by having several layers of coatings which expand at different rates so that the effect on the topmost layer is negligible. The composite materials consist of sever layers, starting from the substrate;

• Substrate material Al Alloy, Glass, Ceramic
• Base layer Ni Phosphorous (Ni-P) alloy is usually applied by electroless plating techniques, Ni-P is a very hard material and is finely polished and then textured with circumferential grooves, these help the magnetic anistropic properties of the disk. Typically, this layer is 10 microns thick.
• Intermediate layer Chromium is used as the intermediate layer as it has epitaxial properties that are similar to that of the cobalt based magnetic coating. Typical thickness of this coating is 0.5 microns.
• Magnetic Layer Cobalt alloys are used as the magnetic coating. Alloys of Cobalt-Chromium-Tantalum (CoCrTa), Cobalt-Platinum-Chromium (CoPtCr) and Cobalt-Platinum-Nickel (CoPtNi) alloys are the most commonly used. 0.3 microns is a typical thickness for this type of coating.
• Protective Coating A 0.1 micron layer of a Carbon based polymer is sputtered onto the magnetic layer to help protect it from head contact wear and minor shocks. It also acts as a support for the lubrication layer
• Lubrication Layer The last layer is an organic polymer lubricant material to minimise head-to-platter contact friction. When the heads park on the platters, it is this layer that prevents excessive wear to the heads as they take-off and land