Disk PlattersHead Crash EffectsThe size of the disks platters varies, depending onintended use, capacity, and speed. Sizes of the diskplatters commonly used are 5.25-inch, 3.5-inch and2-inch.Fixed disk systems may contain from 1 to 11platters, depending on size and capacity. The numberof platters in a drive is limited by the size of the drive.Half-height 5.25- and 3.5-inch drives contain amaximum of eight platters. Full-height drives arecurrently limited to 11 platters. Since the platters aresealed in the HDA, all of the surfaces are used for datastorage.Platters are made of aluminum alloy metal coatedwith a magnetic material (medium). The two mostcommon media for fixed disk platters are iron oxide andthin film.IRON OXIDE COATED PLATTERS. —Ironoxide platters are found in many older low-densitydrives. The oxide is applied to the platter, then curedand polished. The iron oxide is generally applied to athickness of 30 millionths of an inch. After the platteris polished, a protective lubricant is applied to helpprevent damage caused by head crashes.THIN FILM COATED PLATTERS. —Thin filmcoated platters can hold much greater data densitiesbecause the magnetic coating is much thinner and moreperfectly formed than the iron oxide coating. Twoprocesses, plating and sputtering, are used tomanufacture thin film disks.Platting —Platting is a process in which themedium is applied to the disk using an electroplatingmechanism. The final layer is a cobalt alloy ofapproximately 3 millionths of an inch.Sputtering—Sputtering is a process in which thecobalt alloy is applied in a near vacuum. The magneticmaterial, as thin as 2 millionths of an inch, is depositedon the disk in much the same way metallic films areapplied to silicon chips in the creation ofsemiconductors. A hard carbon coating is then appliedto protect the disk.The result, on both plated and sputtered disks, is anextremely thin and hard medium on the disk. The hardsurface increases the probability that the disk willsurvive a high-speed head crash with little or nodamage.10-24A head crash occurs whenever the heads come incontact with the disk’s surface. Severe damage canoccur if the heads crash with the disk spinning at fullspeed. The heads can scratch the oxide material or theheads themselves can be damaged. Whenever the diskis powered down, there is a minor head crash as the diskslows down.Many fixed disks have a designatedlanding zone for the heads, but you have to position theheads in this landing zone. To do this you should run aprogram designed to park the heads in this landing zonebefore removing power.The thinner medium requires a smaller space on thedisk to store data. Also the heads can fly closer to thedisk, further reducing the space and magnetic fieldstrength required to accurately store data and increasedensities.Read/Write HeadsThe read/write heads used infixed disk systems arevery similar to the read/write heads on the disk memoryset. There is one head for each disk surface. Theseheads are joined to the head actuator and move in unisonacross the disk. There are currently two types of headsin use: the composite ferrite head and the thin filmhead.COMPOSITE FERRITE HEAD. —Thecomposite ferrite head is the traditional type of headused in magnetic recording. It consists of an iron oxidecore wrapped with electromagnetic coils. To write dataon the disk, an electric current is passed through thecoils and a magnetic field is induced on the ferrousmaterial of the disk surface. Changing the direction ofcurrent flow through the head’s coil will result in areversal of the magnetic field on the disk.THIN FILM HEAD. —The thin film head isactually a specialized integrated circuit chip. The headhas a precise U-shaped groove in its bottom to allow theright amount of air pressure for the head to fly at theproper height. This lightweight head flies closer to thedisk than the composite ferrite heads.A thin film head’s flying height can be as little as 5millionths of an inch above the disk. The closeness ofthe head to the platter increases the signal-to-noise ratio,which increases the accuracy of the disk system.
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