1. Field of the Invention
The present invention relates to measuring lubricant mobility on a surface of a disk used in hard disk drives.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
The disks are rotated by a spindle motor of the drive. Rotation of the disks creates an air flow within the disk drive. Each head has an air bearing surface that cooperates with the air flow to create an air bearing between the head and the adjacent disk surface. The air bearing eliminates or minimizes the mechanical wear between the head and the disk. The height of the air bearing is commonly referred to as the flying height of the head.
The magnetic field detected by the head is inversely proportional to the flying height of the head. Likewise, the strength of the magnetic field written onto the disk is also inversely proportional to the fly height. A larger fly height will produce a weaker magnetic field on the disk.
It is desirable to minimize the fly height of the head to maximize the magnetic field strength. Consequently, disk drives have been designed to have contact or near contact between the heads and the disks. To minimize friction and damage to the disk most disk surfaces are covered with a very thin layer of lubricant.
The disks spin during operation of the disk drive. The centrifugal forces associated with such spinning motion can cause some of the lubricant to move across the disk. This movement of lubricant can vary the thickness of the disk at certain disk locations. When flying, the slider carries the air bearing force which also results in disk lubricant movement. Some head designs move out the pole-tip protrusion to reduce head disk spacing at the air gap. The higher pole-tip protrusion will increase the air bearing pressure locally near the pole-tip. The localized high pressure will generate high lube depletion forces and high lube redistribution. It is desirable to measure lubricant mobility so that the disk drives can be designed to account for movement of the lubricant.
One way to test lubricant mobility is to remove the lubricant from a portion of the disk surface, spin the disk and then measure the height of lubricant that moved into the clean area of the disk. There are various disadvantages to this approach. First, the boundary between the lubricant and the area where lubricant is removed is not crisp. It is desirable to obtain a sharp edge at this boundary so that a more accurate reading of lubricant mobility can be achieved. Second, the lubricant moves in both a radial and circumferential manner during disk spin. It is desirable to measure just the circumferential movement of the lubricant. It is difficult to measure circumferential lubricant movement with the present technique. The circumferential lube movement caused by the slider air bearing force becomes more critical at lower flying height when using a higher pole-tip protrusion. Third, only one data point can be acquired for each disk. Multiple data points require the washing, spinning and measuring of multiple disks. It would be desirable to provide a method for determining lubricant mobility that overcomes the disadvantages noted above.