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 total 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 if the drive overheats, 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 several layers, starting from the substrate;