Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Organic nitrogen compound
Reexamination Certificate
2000-03-23
2002-10-22
Johnson, Jerry D. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Organic nitrogen compound
C508S562000, C508S582000, C508S428000, C508S383000
Reexamination Certificate
active
06468947
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lubricant compositions for lubricating magnetic data, particularly rotatable magnetic recording media, such as thin film magnetic disks having textured surfaces and a lubricant topcoat for contact with cooperating magnetic transducer heads.
BACKGROUND OF THE INVENTION
Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. In operation, a typical contact start/stop (CSS) method commences when a data transducing head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk, where it is maintained during reading and recording operations. Upon terminating operation of the disk drive, the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a “head crash.” Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by technique generally referred to as “texturing.” Conventional texturing techniques involve mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the texture on the surface of the substrate is intended to be substantially replicated in the subsequently deposited layers.
A typical longitudinal recording medium is depicted in FIG.
1
and comprises a substrate
10
, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al—Mg)-alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, glass-ceramic materials and graphite. Substrate
10
typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer
11
,
11
′, a cobalt (Co)-base alloy magnetic layer
12
,
12
′, a protective overcoat
13
,
13
′, an a lubricant topcoat
14
,
14
′. Cr underlayer
11
,
11
′ can be applied as a composite comprising a plurality of sub-underlayers
11
A,
11
A′.
The protective overcoat desirably possesses high durability, density and hardness to protect the underlying magnetic layer providing wear resistance and encouraging durability of the magnetic recording medium arrangement. Typically, a thin film of zirconium oxide, silicon oxide or carbon is used as a protective overcoat.
Chromium underlayer
11
,
11
′, Co-base alloy magnetic layer
12
,
12
′ and protective overcoat
13
,
13
′ are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture which is substantially reproduced on the disk surface.
In accordance with conventional practices, a lubricant topcoat is uniformly applied over the protective overcoat to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat increases friction between the head and disk, thereby causing catastrophic drive failure. Conversely, excess lubricant at the head-disk interface causes high stiction between the head and disk. If stiction is excessive, the drive cannot start and, likewise, catastrophic failure occurs.
The drive towards ever increasing recording density, and faster data transfer rates and the resulting smoother disk surfaces and lower flying heights, has served as an impetus for the development of new lubricants to serve as a lubricating topcoat overlying the protective overcoat. Such lubricants must perform a variety of different purposes requiring diverse characteristics and attributes. For example, the lubricant forming the topcoat is preferably chemically inert, possesses a low vapor pressure, low surface tension, high thermal stability, stability under high shear stress and good boundary lubrication properties. Moreover, it is critical that the lubricant tightly adheres to the underlying surface over the lifetime of the magnetic recording media.
The entire disc surface of a magnetic recording disc, however, is not ideal for reading and writing data. In particular, disc surfaces have asperities, i.e. protrusions on surfaces of the disks, which interfere with the flying characteristics of the data head, as well as the read and write operations of the data head. In operation, the head can come into contact with asperities while the head flies above the surface of the disc. Potentially, this undesirable contact can cause data written to a particular location on a disc to be lost. In an effort to alleviate such occurrences, manufactures commonly burnish the surfaces of disks to reduce asperities located thereon. Typical burnishing processes, however, cause contamination of ceramic oxides, such as aluminum oxide, on the disk's surface which can catalyze the decomposition of the lubricant topcoat layer resulting in reduced tribological properties.
Several classes of lubricants may satisfy some of the desired properties. Among the many lubricants available, liquid perfluoropolyethers (PFPE) are the most utilized for forming topcoat lubricants on magnetic recording media. PFPE's have been reported for use as lubricating magnetic media in, for example, U.S. Pat. No. 3,778,308. PFPE having a variety of polar end-groups are known (see, e.g. U.S. Pat. Nos. 3,810,874; 4,085,137 and 4,647,413) and have been used in an attempt to improve adhesion of the lubricant to the magnetic medium (see, e.g. U.S. Pat. Nos. 4,268,556; 4,696,845; 4,889,939; 5,128,216). Their preparation has also been widely reported (see, e.g., U.S. Pat. Nos. 3,810,874 and 5,506,309)
Typical conventional lubricants, such as perfluoroalkylpolyether (PFPE) fluids such as Fomblin Z-DOL, Fomblin TX, and Fomblin Z-Tetraol, etc., generally have 2-4 polar groups at either end of a linear perfluorinated polyalkylether backbone. The functionalized end groups are considered necessary to provide direct bonding, and thus, improved adhesion of the lubricant topcoat to the recording media. Polar functional groups, however, are not necessarily chemically inert and consequently, such conventional lubricants may disadvantageously undergo chemical reactions prior to their application or while on the magnetic medium tending to decrease their adhesion to the disk surface. Undesirable chemical reactions further include degradation of the lubricant itself. Contamination by Lewis acids, such as aluminum oxide, on magnetic recording media further promote degradation of the lubrications.
Thus, a significant factor in the performance of a lubricant topcoat is the ability of the lubricant to resist decomposition over time, particularly decomposition by acid catalysis.
Falcone Samuel J.
Lee Seog W.
Stirniman Michael J.
Johnson Jerry D.
Seagate Technology LLC
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