The majority of RAID arrays use hard disk drives with the data stored on rotating platters, upon which the magnetic storage media is carefully deposited using a complex layer of materials. The rigid nature of the platters is how the name hard disk drive originates.
The majority of hard disk drives used in RAID arrays contain multiple platters which are mounted on a single spindle, with the data stored using both surfaces. Data stored on the platter is accessed using a pair of read/write heads, flying at a specified distance above the surface of the recording medium. In the event of damage to any platter, the complexity of performing a data recovery will be severely increased, making it important to avoid any contaminants being present.
The most essential properties of the material used to manufacture hard disk platters are that they be stable and do not interact with the recording medium at the magnetic level. The traditional material used to manufacture these platters for many years was from aluminium. Technology has progressed swiftly, with significantly higher recording density which requires the read/write heads to fly at much lower distances, which in turn requires the surface of the substrate to be much smoother. Initially glass was used as an alternative, but it is now more common to use platters made from a composite of glass and ceramic.
The enhanced rigidity of glass and ceramic materials allow the manufacture of thinner platters, which in turn allows more platters to be contained within the hard drive. Another important property of glass and ceramic substrates is that they expand less than aluminium at higher temperatures, allowing the engineering tolerances to be improved, leading to failures being less likely.
Magnetic Layer Deposition & Polishing
The platter substrates are highly polished, but not smooth enough without further processing to improve the roughness. The first action is to deposit a hard Nickel Phosphate layer, which is polished after which the surface is almost perfectly smooth. The resulting roughness is less than a tenth of a nanometre; approximately the size of an atom.
Over the base layer several complex layers are deposited, which increases the roughness to approximately four tenths of a nanometre. Most read/write heads current fly above the platter at a distance of two nanometres, the minimum flying height for the reliable operation of the drive using substrate roughness of four tenths of a nanometre.
Magnetic Data Layers & Protective Coating
A composite of iron and nickel was the traditional material used in magnetic data storage layers. Advances in materials technology has improvements in storage capacities through using a layer made from a composite of cobalt, nickel and iron alloy. Cobalt has long been associated with high quality magnets, making it a perfect choice for improving the control of the magnetic orientation, also increasing the signal to noise ratio.
Due to the magnetic data storage layer being soft, it is essential to provide a hard protective coating. This deposited protective layer is a two nanometre thick layer of carbon which is applied using either ion-beam or plasma-enhanced chemical vapour deposition techniques. The final layer deposited is a one nanometre layer of lubricant. Microscopic unevenness (called asperities) must be removed which is done by the platters undergoing tape burnish and head burnish processes, which has the additional benefit of removing any loose particles.
The platters are placed in a glide test rig for the final testing process, used to certify that there are no asperities present on the platter surface. This ensures that the read/write heads will not come into contact with the disk surface, which would cause a head crash and destroy the magnetic recording layer.