While some servers use an SSD for the installation of the operating, they almost always use traditional rotating hard disk drives. Hard disk drives were given their name due to the rigid nature of the platters, upon which a complex layer of materials is deposited, which includes the magnetic storage medium.
The vast majority of hard drives, especially those used within a server use a single spindle upon which multiple disk platters are mounted. Data is usually stored on both surfaces of each platter, across which a pair of read/write heads fly at a specified distance above the media surface. It is important that this flying distance does not fluctuate, otherwise the read/write could come into contact with the disk surface, after which data recovery will be required.
Material of Disk Platters
Hard disk platters must be manufactured using a stable material which must not interact with the magnetic recording medium or cause any magnetic disturbance. For many years it was traditional to use aluminium as the substrate material, which was suitable for many years until recording density increased requiring the read/write heads to fly at a lower height. The problem with the aluminium substrate was that the surface, even after intensive polishing and layer deposition was not smooth enough to allow the decrease in flying height without risking the heads impacting with the surface of the platter. Aluminium is also affected by changes of temperature causing them to expand with heat, so a substitute was required to enable higher capacity drives.
Glass became the alternative solution, providing increased rigidity, which also allowed for thinner platters. It is now much more common to use a platter made using a mixture of glass and ceramic, providing a further increase in rigidity and an even smoother surface after polishing. The glass and ceramic substrates do not suffer the same level of expansion as aluminium substrates, thereby allowing the read/write heads to fly at a significantly decreased height, while also reducing the likelihood of a failure.
Base Layers & Polishing Process
Although the platters undergo an extensive polishing process, the surface is still too rough for use without additional processes. A hard Nickel Phosphate layer is therefore deposited on the surface before it is polished to an almost perfect smoothness, approximately a tenth of a nanometre, the size of an atom.
Several complex layers are then deposited, actually increasing the roughness to about four tenths of a nanometre. The minimum possible flying height, while still providing reliable operation is approximately two nanometres.
Magnetic Layer, Protective Layers and Testing
Traditionally iron and nickel alloys were used as the magnetic data storage layer, which was only suitable up to a certain data density. Cobalt has for many years been used for powerful magnets, making it a perfect choice for increasing the magnetic alignment control for the magnetic storage medium. Cobalt, which also improves the signal to noise ratio, is added to the nickel and iron alloy, which is deposited onto the base substrate layers.
The magnetic recording layer is however soft, therefore requiring a protective coating of carbon approximately two nanometres in thickness is applied using ion-beam or plasma-enhanced chemical vapour deposition methods. A one nanometre thick lubricant layer is then applied to the surface as the final coating. Tape and head burnish processes are then used to remove the microscopic unevenness, known as asperites, which also helps to remove the loose particles.
Once the deposition and polishing processes are complete the platters are then attached to a glide test right, which is used to certify that all asperites have been removed from the surface of the platters. This is essential as it ensures that the read/write heads will not impact with any particles on the platter surface, which would result in a head crash, which would destroy the magnetic recording layer.