Lubricants for magnetic media

Abstract: Amphiphilic compound having one of the following structures: (X-T) n -A-Y m and (X-T) n -R-(T-A) n wherein X is a fluorocarbon terminal group, T is a polar linking group, A is a nucleus other than saturated lower aliphatic, Y is a polar nonlinking group, R is a saturated or unsaturated, substituted or unsubstituted, straight or branched lower aliphatic group, n is at least one, and the sum of n and m is at least two. Such compounds are useful as lubricants for magnetic media. Also, a lubricated magnetic medium comprising a planar, polar surface covered with a monolayer comprising a compound having the structure described above, except that A can also be a saturated lower aliphatic group.

Description: The present invention relates generally to novel amphiphilic compounds. (Amphiphilic, compounds are defined as compounds having at least one polar portion and at least one nonpolar portion, which are capable of forming Langmuir-Blodgett films.) The present invention relates more particularly to amphiphilic compounds in which each nonpolar portion is a fluorocarbon and each polar portion is capable of bonding to a polar surface. These compounds have utility as lubricants to protect metal, metal oxide, carbon, or other surfaces of thin film magnetic recording media useful in data, audio, and video recording from wear and corrosion. The present invention also relates to a novel thin-film magnetic medium comprising a polar surface of the magnetic medium, coated by and bonded to a monolayer of at least one of the present amphiphilic compounds. Background of the Invention Magnetic media are used in the form of tapes, floppy disks, hard disks, and the like to store magnetic impulses received from a recording or write head scanned on the medium and to reconstruct the same impulses in a playback or read head scanned on the same part of the medium. The write and read functions of a computer disk system are commonly performed by a single read/write head. The head moves rapidly with respect to the magnetic medium and closely approaches it, momentarily coming into direct contact with it when the head starts or stops scanning. The industry has found it necessary to lubricate the magnetic medium so friction between the medium and head does not rapidly destroy the head or the medium. Computer data storage media such as oxide disks have been coated with a 30 to 120 Angstrom layer of fluorinated oil to reduce friction while the head is in contact with the disk, and to protect the disk from corrosive atmospheric contaminants. This coating has worked well for systems achieving densities of no more than about 60 to 100 megabytes per square inch, wherein the read/write heads fly relatively high over the recording medium, riding on an air cushion. Other materials which have found utility as lubricants for magnetic media used with high-flying read-write heads include fluorinated oils (particularly perfluoropolyethers), fatty acids and their esters, organosilanes, and organoaminosilanes. In higher-density magnetic storage systems currently under development, the read/write heads will fly lower. The lubricant layer for such heads will typically be much thinner -- perhaps less than 30 Angstroms thick. Such a thin layer of a conventional lubricant will not lubricate the magnetic medium sufficiently to allow it to withstand the increased friction resulting from repeated scanning by low-flying heads. Another problem in the art is degradation of the lubricant. Fatty aliphatic chains (broadly defined herein as those having from 6 to 22 carbon atoms) degrade with each read/write cycle, forming a buildup of sludge. This sludge causes higher friction between the magnetic medium and the head, head instability, and increased spacing between the head and the medium surface. Deterioration of performance is the practical result. Thus, a need has arisen for new lubricants which will durably adhere to the disk in thin layers, will allow repeated read/write cycles without an increase in friction or sludge buildup, will offer corrosion protection, and will provide better lubrication in the environment of a low flying read/write head. US-A-4,871,625 discloses certain polyphosphzenes substituted with single chain, linear fluoroalkyl substituents as a lubricant for a magnetic recording medium. US-A-4,613,548 discloses certain polyphosphazenes substituted with single chain, linear fluoroalkyl substituents as a magnetic tape lube. Summary of the invention The present invention provides a lubricated magnetic medium comprising a planar, polar surface covered with a monolayer according to claim 1. The present lubricants bond more tightly to the substrate, are less easily degradable, and provide better lubrication and corrosion protection than the lubricants of the prior art. Brief Description of Drawings Figure 1 is a diagrammatic perspective view of the surface of a magnetic medium, coated with a lubricant according to the present invention. Figure 2 is a fragmentary side elevational view of the structure shown in Figure 1. Detailed Description of the Invention While the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to the preferred embodiments. In the present disclosure, it is assumed that where more than one substituent of a particular kind is to be selected, each selection is made independently, so the several substituents of the same kind may be the same or different. Compounds as described herein may be made using any of a variety of synthetic methods. A preferred method contemplated herein, and illustrated in the examples, is to react 1) a fluorocarbon having a terminal functional group with 2) a precursor of the nucleus having one or (preferably) more functional groups. The reaction of the functional groups of the fluorocarbon and the precursor provides a polar linkage joining the two to form a compound according to the present invention. Lubricated Magnetic Medium Another aspect of the invention is a lubricated magnetic medium comprising a planar, polar surface of the medium covered with a monolayer comprising a compound having the structure described in the preceding description, except that A can also be a saturated lower aliphatic group in this embodiment of the invention. The monolayer may be a Langmuir-Blodgett (LB) monolayer less than 120 (12), preferably less than 70 (7), most preferably less than 30 Angstroms (3 nm) thick. The monolayer may also be applied to the substrate in another manner. A background on the nature and formation of LB films and the amphiphilic materials capable of forming LB films is provided in Agarwal, "Langmuir-Blodgett Films," Physics Today , June, 1988. "Amphiphilic" is also defined in this reference. The definition of "amphiphilic" and the description of LB films in the Agarwal reference are hereby incorporated herein by reference. Generally speaking, an LB film is a tightly packed single layer of amphiphilic molecules oriented with their hydrophilic ends adjacent to the substrate and their hydrophobic ends standing above the substrate. While the novelty or scope of the invention does not depend on the accuracy of this theory, it is contemplated that the compounds of the present invention provide superior, durable lubrication because the polar linking group and non-polar linking group moieties of the present compounds, as well as any polar moieties included in the structure of the nucleus and lower aliphatic group, are bonded securely to the polar surface of the magnetic medium by Van der Waals forces between the free electron pairs of the polar moieties and oxygen atoms present in the magnetic medium. (A typical thin-film magnetic medium has either a metal oxide surface or a carbon surface which oxidizes due to its exposure to air.) Dipole interactions and ionic or electrostatic attraction also contribute to bonding. For the present compounds the planar and aromatic heterocyclic ring will interact with the pi-electrons of an amorphous carbon magnetic recording disk protective coating. Because of these bonds between each lubricant molecule and the substrate, the nucleus and lower aliphatic moieties associated with the polar groups are closely packed (due to Van der Waals attractive forces between the polar moieties of adjacent molecules), and are fixed substantially in a lubricant plane parallel to the polar surface of the magnetic medium. The fluorocarbon terminal group moieties, which are very nonpolar, extend substantially perpendicularly above the lubricant plane, which is how they are oriented when an LB film of the lubricant is applied to the magnetic medium. Since each fluorocarbon moiety is linked to the nucleus by a polar linking group, the base of each fluorocarbon chain is anchored to the substrate. The several fluorocarbon moieties of a lubricant are preferably identical, are each anchored at one end in identical fashion, and extend perpendicularly upward from the substrate. The fluorocarbon chains are zig-zag chains, as dictated by their tetrahedral bond geometry. This symmetrical arrangement means that the corresponding atoms of each fluorocarbon chain are disposed the same distance above the substrate, so the fluorocarbon chains have a tendency to nest, allowing close packing of the chains. The nuclei of the monolayer are tightly packed and the fluorocarbon terminal group moieties are numerous and preferably identical, so the fluorocarbon terminal group moieties stand like blades of grass above the substrate, providing a dense, durable lubricating layer which interfaces with a scanning read/write head to reduce friction. The lubricants described herein are applied in essentially conventional fashion to thin-film magnetic media, for example, using the LB coating technique. While only one Z-type monolayer of the present lubricants can bond directly to the substrate, more than one Z-type layer (with the fluorocarbon chains extending away from the substrate, as shown in the figures) can be built up by the LB film formation method. This may be done to ensure complete coverage of the substrate by a monolayer, particularly if the substrate surface is uneven. The present lubricant layers can also be overcoated with another lubricant, for example a fluorocarbon oil which is compatible with the fluorocarbon chains of the LB monolayer. Before applying a LB monolayer of the present lubricants, it is important to thoroughly clean the magnetic medium surface, as by immersion in a solvent such as isopropanol followed by rinses of deionized water, until the surface to be coated has a high electrical resistance (for example, about 18 megohms per square). Example The following working examples provides further exemplification of the scope of the present invention, and enable one of ordinary skill in the art to carry out the present invention. Example 1 . The condensation product of phosphonitrilic chloride and a trifluoroalkylcitrate was prepared. The product contained 18 fluorocarbon chains on a phosphorus and nitrogen aromatic heterocyclic ring. 1.1g of phosphonitrilic chloride trimer (P 3 N 3 Cl 6 ) having the following nuclear formula: was dissolved in chloroform (25 ml), then was added dropwise to 11.4 g of a trifluoroalkyl citrate having this structure: (mw ~ 1600, n is 6, 8, 10, 12, or 14, mostly 8 or 10), in FREON 112 (50 ml) and triethylamine (10 ml) while stirring and heating. Refluxed overnight, 10 g of light amber material were recovered. Major IR bands were observed near 2960 cm-1 (νCH 2 ), 1750 cm-1 (νCH 2 ), 1750 cm-1 (νC=O, -CO 2 ), 1200 cm-1 (CF 2 , CF 3 broad), and 1040 cm-1 (possibly P-O-C). This spectrum is consistent with six terminal groups having the following structure: on the following nucleus: The IR absorption bands due to phosphonitrilic chloride near 1240 cm-1 (main) and 1300, 1190, 1140 and 980 cm-1 (minor) were masked by the absorption bands of the CF 2 and CF 3 fluoroalkyls.

Claims: A lubricated magnetic medium comprising a polar magnetic medium surface covered with a monolayer which has the nuclear structure : substituted at each free position with the substituent: wherein s is between 6 and 14 inclusive. The lubricated magnetic medium of claim 1, wherein said lubricant is a Langmuir-Blodgett monolayer. The lubricated magnetic medium of claim 1 or 2, wherein the monolayer is less than 7 nm thick. The lubricated magnetic medium of any one of claims 1 to 3, wherein each substituent of the nuclear structure is identical.

Inventors: NG QUOCK, NG, QUOCK
Applicants: QUANTUM CORP, QUANTUM CORPORATION
Application year: 1991
Main classification: C07F9
IPC codes: C07D239, C10N30, C10M105, G11B5, C07C335, C07F9, C10N40