Lubricant overcoat for recording media and a process for...

Details Lubricant overcoat for recording media and a process for... Lubricant overcoat for recording media and a process for...

2001-12-21

2003-09-30

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S216000, C428S336000, C428S421000, C428S690000, C428S690000

Reexamination Certificate

active

06627302

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to recording medium with a lubricant overcoat, particularly, a lubricant overcoat with at least two layers, and a method of making the same.
BACKGROUND
Most modern information storage systems depend on magnetic recording due to its reliability, low cost, and high storage capacity. The primary elements of a magnetic recording system are the recording medium and the read/write head. Magnetic discs with magnetizable media are used for data storage in almost all computer systems.
FIG. 1
shows the schematic arrangement of a magnetic disc drive
10
using a rotary actuator. A disc or medium
11
is mounted on a spindle
12
and rotated at a predetermined speed. The rotary actuator comprises an arm
15
to which is coupled a suspension
14
. A magnetic head
13
is mounted at the distal end of the suspension
14
. The magnetic head
13
is brought into contact with the recording/reproduction surface of the disc
11
. The rotary actuator could have several suspensions and multiple magnetic heads to allow for simultaneous recording and reproduction on and from both surfaces of each medium. A voice coil motor
19
, as a kind of linear motor, is provided to the other end of the arm
15
. The arm
15
is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion
17
.
A conventional longitudinal recording disc medium is depicted in FIG.
2
and typically comprises a non-magnetic substrate
20
having sequentially deposited on each side thereof an under-layer
21
,
21
′, such as chromium (Cr) or Cr-alloy, a magnetic layer
22
,
22
′, typically comprising a cobalt (Co)-base alloy, and a protective overcoat
23
,
23
′, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat
24
,
24
′ to the protective overcoat. Under-layer
21
,
21
′, magnetic layer
22
,
22
′, and protective overcoat
23
,
23
′, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy under-layer.
A conventional longitudinal recording disc medium is prepared by depositing multiple layers of metal films to make a composite film. In sequential order, the multiple layers typically comprise a non-magnetic substrate, a seedlayer, one or more under-layers, a magnetic layer, and a protective carbon layer. Generally, a polycrystalline epitaxially grown cobalt-chromium (CoCr) magnetic layer is deposited on a chromium or chromium-alloy under-layer.
The seed layer, under-layer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 Å thick.
Lubricants conventionally employed in manufacturing magnetic recording media typically comprise mixtures of long chain polymers characterized by a wide distribution of molecular weights and include perfluoropolyethers, functionalized perfluoropolyethers, perfluoropolyalkylethers (PFPE), and ftnctionalized PFPE. “Molecular weight” is the sum of the atomic weights of all the atoms in a molecule. A “functionalized” hydrocarbon, such as functionalized perfluoropolyethers, is a hydrocarbon in which an atom or a group of atoms, acting as a unit, has replaced a hydrogen atom in the hydrocarbon molecule and whose presence imparts characteristic properties to this molecule.
PFPE do not have a flashpoint and they can be vaporized and condensed without excessive thermal degradation and without forming solid breakdown products. The most widely used class of lubricants includes perfluoropolyethers such as AM 2001®, Z-DOL®, Ausimont's Zdol or Krytox lubricants from DuPont.
There is a demand in computer hard drive industry to develop an areal storage density of 100 Gbits/inch
2
and higher in the future. With this high areal density, the flying height between the read-write head and the media has to be minimized. Current magnetic hard disc drives operate with the read-write heads only ~10 nanometers above the disc surface and at rather high speeds, typically a few to a few tens of meters per second. Because the read-write head can contact the disc surface during operation, a thin layer of lubricant overcoat is coated on the disc surface to reduce wear and friction. The overcoat thickness of the hard disc on these future disc-drives is estimated to be less than 3 nm.
With the carbon overcoat as thin as ≦3 nrm, the overcoat may fail to completely cover the magnetic recording layer. Corrosion of the magnetic layer at those locations where overcoat coverage is imperfect could be a major cause of drive failure. To ensure good corrosion resistance, applicants have found that an overcoat having the ability to minimize or prevent corrosion of the magnetic layer is required and a traditional overcoat material, such as hydrogenated (a-C:H) or nitrogenated (a-C:N) carbon, could be insufficient in protecting the hard disc media or read-write head from corrosion at the thickness level of less than 5 nm.
As hard disc drive technology is pushed to higher and higher recording densities, the industry is faced with an increasingly difficult task of maintaining the tribological robustness of the head-disc interface (HDI). The carbon overcoat and the lubricant film on top of the magnetic recording layer are the main components of a hard disc that control its tribological performance. Tribological performance is characterized in several aspects, including wear durability and corrosion resistance of the media. Higher areal density recording demands reduced head-medium spacing, which in turn requires reduced carbon overcoat thickness and, to a lesser degree, reduced lubricant film thickness. However, if the thickness of the carbon overcoat is about 50 Å or less, the corrosion protection it provides is greatly compromised. Although there are ways to improve corrosion resistance provided by the carbon overcoat properties, e.g., by increasing its density and changing its electrical conductivity, there is a limit as to how much corrosion resistance a carbon overcoat just a few atomic layers thick can provide.
Wear and friction have been recognized as potential problems for the head/disc interface. One solution for improving the wear resistance of the media is proposed in U.S. Pat. No. 5,674,638 (Grill). Grill suggests using a thick fluorinated diamond-like carbon layer of thickness in the range between 3 nm and 30 nm. Grill uses a thick fluorinated carbon overcoat to improve wear resistance, which generally increases with increased thickness.
One way of improving upon the inherently poor corrosion performance of thin carbon overcoats is to consider the lubricant in combination with the carbon overcoat as a means to maximize corrosion protection. Towards this end, solutions mostly involved the use of a lubricant additive, such as phosphazene or phosfarol, which provide an increase in corrosion protection. These additives react in some fashion with high-energy sites on the carbon surface, where it is believed that corrosion is most likely to initiate. Thus, the blocking of these sites from contact with corrosion sources in the environment, such as moisture and oxygen, provides higher corrosion resistance. However the use of lubricant additives can lead to other problems, such as phase separation of the additive and lubricant, which can degrade the flying behavior of the read/write head and cause the drive to fail.
As the fly height decreases, contact between the media and read/write head will likely increase. As a result, there is a need for a high performance carbon overcoat/lubricant system with enhanced protection characteristics. U.S. Pat. No. 6,110,330 discloses a recording media with a read/write zone and a takeoff/landing zone. Bec

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