Disk Platters Head Crash Effects The size of the disks platters varies, depending on intended  use,  capacity,  and  speed.  Sizes  of  the  disk platters  commonly  used  are  5.25-inch,  3.5-inch  and 2-inch. Fixed disk systems may contain from 1 to 11 platters, depending on size and capacity. The number of platters in a drive is limited by the size of the drive. Half-height  5.25-  and  3.5-inch  drives  contain  a maximum of eight platters. Full-height drives are currently limited to 11 platters. Since the platters are sealed in the HDA, all of the surfaces are used for data storage. Platters are made of aluminum alloy metal coated with a magnetic material (medium). The two most common media for fixed disk platters are iron oxide and thin  film. IRON OXIDE COATED PLATTERS.—  Iron oxide platters are found in many older low-density drives. The oxide is applied to the platter, then cured and polished. The iron oxide is generally applied to a thickness of 30 millionths of an inch. After the platter is  polished,  a  protective  lubricant  is  applied  to  help prevent damage caused by head crashes. THIN FILM COATED PLATTERS.—  Thin  film coated  platters  can  hold  much  greater  data  densities because  the  magnetic  coating  is  much  thinner  and  more perfectly formed than the iron oxide coating. Two processes,   plating   and   sputtering,   are   used   to manufacture thin film disks. Platting —Platting is a process in which the medium is applied to the disk using an electroplating mechanism. The  final  layer  is  a  cobalt  alloy  of approximately 3 millionths of an inch. Sputtering —Sputtering is a process in which the cobalt alloy is applied in a near vacuum. The magnetic material, as thin as 2 millionths of an inch, is deposited on the disk in much the same way metallic films are applied   to   silicon   chips   in   the   creation   of semiconductors. A hard carbon coating is then applied to  protect  the  disk. The result, on both plated and sputtered disks, is an extremely thin and hard medium on the disk. The hard surface increases the probability that the disk will survive  a  high-speed  head  crash  with  little  or  no damage. A head crash occurs whenever the heads come in contact  with  the  disk’s  surface.  Severe  damage  can occur if the heads crash with the disk spinning at full speed. The heads can scratch the oxide material or the heads themselves can be damaged. Whenever the disk is powered down, there is a minor head crash as the disk slows  down. Many fixed disks have a designated landing zone for the heads, but you have to position the heads in this landing zone. To do this you should run a program designed to park the heads in this landing zone before removing power. The thinner medium requires a smaller space on the disk to store data. Also the heads can fly closer to the disk, further reducing the space and magnetic field strength required to accurately store data and increase densities. Read/Write Heads The read/write heads used in fixed disk systems are very similar to the read/write heads on the disk memory set. There is one head for each disk surface. These heads are joined to the head actuator and move in unison across the disk. There are currently two types of heads in use: the composite ferrite head and the thin film head. COMPOSITE   FERRITE   HEAD.—   The composite ferrite head is the traditional type of head used in magnetic recording. It consists of an iron oxide core wrapped with electromagnetic coils. To write data on the disk, an electric current is passed through the coils and a magnetic field is induced on the ferrous material of the disk surface. Changing the direction of current  flow  through  the  head’s  coil  will  result  in  a reversal of the magnetic field on the disk. THIN FILM HEAD.—  The thin film head is actually a specialized integrated circuit chip. The head has a precise U-shaped groove in its bottom to allow the right amount of air pressure for the head to fly at the proper height. This lightweight head flies closer to the disk than the composite ferrite heads. A thin film head’s flying height can be as little as 5 millionths of an inch above the disk. The closeness of the head to the platter increases the signal-to-noise ratio, which increases the accuracy of the disk system. 10-24