WO1988005591A1

WO1988005591A1 - Improved recording medium and process for manufacturing the same

Info

Publication number: WO1988005591A1
Application number: PCT/US1988/000215
Authority: WO
Inventors: Daniel X. Wray
Original assignee: Dynamic Disk, Inc.
Priority date: 1987-01-27
Filing date: 1988-01-26
Publication date: 1988-07-28

Abstract

Improved recording medium and method for producing the same in which a thin lubricant film is applied directly to the surface of the recording medium without the necessity of intermediate layers or additional decomposition steps. The lubricant coating, which is a single monolayer of between about 30 and about 300 angstroms in thickness, is chemically bonded to the surface of the recording medium and the molecules of the film are oriented substantially normal to the surface of the recording medium. The backbone of the molecule of the film is selected from a number of different polymeric materials to provide a desired characteristic to the film surface, such as lubricity, and the distal end of the molecule is preferably provided with a strong electronegative group so that the surface of the magnetic medium is strongly electronegative which further protects against head crash and abrasion due to contact with the magnetic slider head. The film is applied directly to a reactive magnetizable metal surface of the media. The molecules of the film to be applied to the surface are brought to the surface in solubilized form since in this form the molecules are more soluble in the solution than they are in themselves thus promoting orientation of the molecule when the terminal bonding group of the molecule reacts with the surface of the recording medium.

Description

IMPROVED RECORDING MEDIUM AND PROCESS FOR MANUFACTURING THE SAME

Field of the Invention

The present invention relates to magnetic recording media and to a process for manufacturing the same. More particularly the invention relates to an improved protective lubricant coating over the surface of the magnetic recording medium in which the coating is chemically bonded to the surface of the recording medium substrate so as to provide a uniform, durable coating thereon.

Background of the Invention

In magnetic recording media a magnetic layer, normally a powder composition comprising magnetic material and a binding agent, is applied over substrate film or metallic surface. The magnetic layer .is plated or otherwise formed over a substrate surface. The magnetic surface is subject to considerable abrasion and other stress forces due to the relative movement of the magnetic media and the pickup or recording head. In the case of magnetic disc drive systems, the magnetic disc is spun at high speed thereby forming an air gap between the disc and the head slider. However, the magnetic disc is subject to abrasion due to contact between the head slider and the disc surface when the disc is in the standby condition or should the head slider accidentally contact the surface of the rotating disc during recording or play back. This accidental contact, referred to as "head crash", can result in severe damage and/or destruction of the magnetic disc.

To protect the head slider and the magnetic media from abrasion wear or damage due to head crash, it has been proposed in the prior art to apply a lubricant layer over the magnetic layer. The prior art is replete with references to various compositions which are applied to a magnetizable metal surface to provide a protective lubricant coating thereon. Highly preferred in the prior art as a lubricant for recording media are fluorinated compounds such as the fluorinated linear polyethers described in U. S. Patents 3,715,378 and 3,847,978. A number of problems are encountered with lubricant coatings over metal surfaces, however, not the least of which being that too much lubricant can actually lead to dragging between the head and the magnetic surface which is referred to as striction. In addition the lubricant material can be easily removed from the magnetic surface so that the lubricant effect is quickly lost. Also, due to the poor interaction between the lubricant composition and the metallic surface, the lubricant may migrate, particularly in the case of spinning discs where there are high centrifugal forces on the disc surface.

Attempts in the prior art have been made to improve the bond between the lubricant layer and the surface of the recording medium. For example, U. S. Patent 4,446,193 describes a method for bonding a lubricant layer by contacting the surface with a diazoketone terminated polymeric lubricant and decomposing the diazoketone to a carbene which reacts with oxide layers on the surface of the magnetic substrate. U. S. Patent 4,529,659 discloses a method in which an intermediate layer is applied to the surface of the magnetic medium which comprises a functional silane compound. The lubricant layer, comprising a perfluoropolyether having a terminal carboxyl group or sulfonic acid group, is applied over the intermediate silane compound layer and a reaction occurs between the intermediate compound and the lubricant layer.

Methods such as those described above, although perhaps effective to create a bond between the lubricant and the substrate, present certain manufacturing and operational problems. For example, these methods require an additional manufacturing step such as a decomposition step to convert a precursor group into one which will react with the surface of the recording medium or the application on an additional intermediate coating which is capable of reacting both with the surface of the recording medium and with the lubricant layer. Such additional steps can lead to increased production time and production costs as well as problems which may occur due to the more complex manufacturing operations. In addition, the application of intermediate coatings over the surface of the recording media can increase the thickness of the deposited film and thereby adversely affect the recording capabilities of the medium. In U. S. Patent 4,536,444 there is disclosed magnetic recording medium provided with a protective layer of a phosphate terminated perfluroalkylpolyether formed on the magnetic layer. From the disclosure it appears that film thickness is critical because excess film material deposited on the magnetic layer results in a loss of output.

Summary of the Invention

The present invention provides an improved recording medium and a method for producing the same in which a thin lubricant film is applied directly to the surface of the recording medium without the necessity of intermediate layers or additional decomposition steps. The lubricant film, which is one monolayer in thickness is chemically bonded to the surface of the recording medium and the molecules of the film are oriented substantially normal to the surface of the recording medium.

The film is applied directly to a reactive magnetizable metal surface of the media. The metal surface is in a reactive condition as a result of being maintained in a non-oxidized or substantially reduced form, that is free of passivating materials such as metal oxides which interfer or prevent bonding of the film forming material to the metal surface. The film is bonded to the metal surface by a chemical bond between a terminal bonding group carried by the molecules of the film forming material and the reactive metal surface of the recording medium. The molecules of the film to be applied to the surface are brought to the surface in solubilized form since in this form the molecules are more soluble in the solution than they are in themselves thus promoting orientation of the molecule when the terminal bonding group of the molecule reacts with the surface of the recording medium. Although not completely understood, it is believed that orientation of the film molecules is further promoted by the bonding action between the terminal bonding groups and the ractive metal surface which results in a sufficiently high molecular spatial density or packing density so that the film molecules are maintained in a plane essentially normal to the reactive metal surface. An essentially non-reactive metal surface, such as a metal surface coated by an oxide layer, will inhibit the bonding of the terminal bonding groups of the film forming molecules so that an insufficient packing density of film molecules is obtained. If the packing density is too low the film molecules cannot support themselves in the desired orientation and film properties are adversely effected. By calculation it has been -found that a packing density of at least about 25% of the reactive sites on the metal surface should be reacted with terminal bonding groups in order to achieve sufficient packing density.

The molecule of the film can be thought of as comprising three major regions. The first region comprising the terminal bonding group which reacts with the reactive metal surface to bond the film thereto. The second region comprising the backbone of the molecule which provides the lubricity and other characteristics desired for the film*. The third region of the molecule comprises a functional terminal electronegative group which, due to orientation of the film molecules, presents a strongly electronegative surface which repels the electronegative surface of the slider head and reduces head crash. This group is referred to as the distal functional group.

Brief Description of the Drawinσ

FIG. 1 is a diagram showing orientation of a hydroxyl terminated perfluoroalkylpolyether bonded to the surface of a reactive metal substrate.

Description of the Invention

FIG. 1 shows diagrammatically the orientation of a perfluoroalkylpolyether comprising a terminal bonding group 12, a backbone 18 and a distal electronegative group 22. As illustrated, the terminal bonding group 12 has reacted with nickel of a magnetic surface 14 of a substrate 16 to bond the molecule to the substrate. The perfluoropolyether backbone 18 is oriented essentially normal to the surface of the substrate 16 and a distal functional group 22, in this case fluorine which is strongly electronegative, is provided on the opposite end of the molecule.

The terminal bonding group 12 may include any group reactive with the metal surface of the substrate 16 although the group can be most conveniently thought of as an oxidizing group or a Lewis base group. Good results are achieved when the terminal bonding group is oxygen or an oxygen containing compound such as a hydroxy, carbonyl, carboxy or carbonate group. Sulfur containing groups are also used with good results such as, for example, sulfide or sulfonate groups. Nitrogen containing groups such as the amines and oxime groups and phosphorus containing groups such as phosphate and thiophosphate also serve as the terminal bonding group for the molecule. Organic groups such as the epoxy and vinyl groups and ring systems capable of donating pi electrons, such as for example, cyclopentadiene, pyridine, thiophene, benzene and styrene will also serve as the terminal bonding group. The terminal bonding group may be monofunctional, that is a single bonding group or polyfunctional, such as, for example, a dicarboxylic acid or polyhydroxy alcohol.

The backbone of the molecule is organic or organo- metallic because such compositions are readily modified to provide the desired characteristics for the finished surface. Accordingly, the molecule further comprises at least one organic backbone carried by the terminal bonding group (polyfunctional terminal bonding groups may carry several backbones) which may include one or more reactive sites for addition or crosslinking reactions. Polyfunctional ethylenically unsaturated molecules, such as vinyl and halogenated vinyl, styrene and halogenated styrene, polyepoxy, polyether and aromatic and heterocyclic polyfunctional molecules, such as polyesters of aromatic dicarboxylic acids, are used with good results in that they-have sites which can be readily crosslinked or reacted with other molecules to provide a particular surface characteristic. Among the materials used to form the films of the present invention good results have been achieved with hydroxyl and carboxyl terminated perfluoroalkyl- polyethers having the formula

CF₃ - (CF₂0) _m - (C₂F₄0) _n - CF₂ - COOH (a)

where m and n are each integers of between about 0 and about 10. Perfluoroalkanoic acids of the general formula

CF₃ - (CF₂)_n - COOH (b)

where n is an integer of between about 4 and about 12 are also used with good results. The perfluoroalkylpolyether and perfluoroalkanoic backbones of (a) and (b) above may also be terminated with phosphate and sulfonate groups as represented by the following formulas

F - (A) - S0₃H (c) and

F - (A) - 0 - PO3H (d)

where A is a perfluro backbone as set forth in (a) or (b) above. The molecules as represented by formulas (a) - (d) above are further modified by substituting chlorine, iodine, hydrogen or other functionalities, such as methyl for the fluorine atom in the molecule. Also, the alkane backbone of the molecules represented in formula (b) may be ethylenically unsaturated such as with polyvinyl or other alkenes. For example, the backbone of the film forming molecule may include one or more reactive groups for crosslinking to provide a moisture resistant, protective film over the substrate surface. Compositions such as perfluorooctanoyl sulfonic acid are applied to the metallic surfaces with good results.

The distal terminal group is selected from any of fluorine, chlorine, oxygen or nitrogen. These atoms are preferred as they are electronegative. In the contact/start stop system the recording-reproducing slider head is normally composed of a refractory oxide material which includes elements of electronegative character so that the surface of the film and the slider head will repel^' each other. Although the repulsion force between the slider head and the surface of the film is not strong, it is sufficient to reduce contact therebetween when the distance between the head and the slider is reduced to about five to ten angstroms.

The deposited film is on the order of one monolayer and normally ranges from between about 30 angstroms to 300 angstroms in thickness depending upon the length of the molecule being applied. The advantage of thin monolayer - films is that the magnetic and recording properties of the substrate are not reduced by the film while film lubricity and electronegativity are achieved.

The film is deposited on any magnetizable substrate such as for example iron, nickel, chromium, cobalt and alloys thereof. The substrate surface on which the film is to be deposited must be reactive. That is to say, the substrate surface must be free of a substantial amount of a passivating material such as the metal oxide which would interfere with the formation of chemical bonds between the terminal bonding group and the metal substrate and reduce the packing density of the bonded molecules below that level required to maintain molecular orientation.. The reactive metal surface condition is achieved by utilizing freshly prepared metal surfaces and the film deposited thereon before any substantial amount of oxide coat can form. Thus a freshly plated metal surface is reactive within the meaning of that term as used herein. Alternatively, a substantial portion of the existing oxide coating can be removed from the metal surface by polishing or chemical etching under conditions which insure that the formation of oxides on the metal surface is maintained at a minimum until the film is applied in accordance with the present invention. For example, the surface may be polished in a fluid that is inert to the metal being worked and which is a solvent for the film forming molecules which is being applied to the substrate. In this manner the molecules is chemically bonded to freshly prepared reactive metal in the absence of oxygen or other substances which may react with the substrate metal to form undesirable reaction by-products which interfere with creation of a uniform, chemically bonded film on the substrate and which can result in pitting of the metal surface.

The solution of film forming material may be applied to the reactive metal surface by any of the commonly used techniques such as spraying or flooding the surface of the substrate to effect the reaction between the terminal bonding group of the molecules solubilized in solution and the metal surface.

A preferred method of coating involves immersion of the substrate into a container of the film forming solution which is maintained at sufficient heat to cause vaporization of a portion of the solvent. After immersion, the substrate is removed and maintained in the vapor phase for sufficient time to permit the removal of any excess unreacted material. The substrate may then dried for a short period of time to drive off any remaining solvent.

The invention will be further described in connection with the following examples showing certain preferred embodiments thereof. The examples are by way of illustration only and not of limitation. It should be understood that many variations of the invention are possible without departing from the spirit or scope thereof.

Example 1 Nickel plated aluminum discs were polished with one micron alumina to a brilliant mirror finish to remove as much as possible the oxide coating therefrom. Immediately after polishing the discs were washed with deionized water and rinsed with a chlorinated hydrocarbon solvent (Freon 113) . The polished discs were immediately immersed in a boiling bath comprising a solution of 250 ppm of hydroxy terminated perfluoropolyether (fluoro-terminated at the distal end) in Freon 113 solvent. The fluorine terminated perfluoropolyether carboxylic acid is sold under the name Fomblin-Dol by the Montefluos Corporation. The discs were immersed in the solution for approximately one minute and then withdrawn to the vapor phase immediately above the surface of the bath and held there approximately 5 minutes to permit excess solvent and perfluoropolyether to be removed from the surface of the disc. Following this, the discs were dried at 300^λ F for about 5 minutes.

A second set of the nickel plated discs similarly treated as above were immersed in a boiling perfluoropolyether solution comprising 25 ppm of the fluorine terminated perfluoropolyether carboxylic acid in Freon 113. After immersion for approximately one minute the discs were withdrawn to the vapor phase and held for about five minutes to remove excess solvent and film forming material and then dried at 300 F for 5 minutes.

Example 2 The coated discs prepared in accordance with Example 1 above were subjected to electron spectroscopy for chemical analysis (ESCA) . This technique involves exciting the surface of the article being examined with a probe beam of monochromatic x-rays which liberate photoelectrons from core levels of the sample atoms. By varying the angle of incidence of the probe beam to the orientation of the detector it is possible to view the surface configuration from a variety of angles and to determine the orientation of the surface molecules. This methodology is referred to as "angle resolved technique". A overview of the ECSA technique is described in an article by J. A. Buono, A. . Wisniewski, W. S. Andrus, "Surface Science Analysis Techniques",^' Solid State Technology (February 1982). Using the angle resolved technique by the ECSA analysis it was determined that the film thickness prepared in the 250 ppm solution was on the order of 57 angstroms while the discs coated in the 25 ppm solution had a coating on the order of 37 angstroms.

Using the angle resolved technique and comparing the ratio of C-C bonds to F-C bonds it was determined that the molecules of the film were oriented substantially perpendicular to the surface of the disc.

Example 3

Discs prepared from both the 25 ppm and the 250 ppm solutions of Example 1 were immersed in concentrated nitric acid. Examination of the discs indicated no observable effect from immersion in the acid.

Example 4

An aluminum substrate is electroless plated with nickel followed by an electroplated nickel and an electroplated cobalt layer. The freshly plated disc is inserted immediately into a solution comprising 250ppm of perfluoro 2,4 - octa-di-one in Freon TF. The article is then dried at about 150^λF for 10 minutes and then immersed into a solution of the sodium alkoxide of perfluorooctanol which has been prepared by reacting the alcohol with metallic sodium. Following immersion the article is washed with distilled water and dried at 150*F.

Example 5

Magnetic recording discs comprising a substrate of aluminum over which are deposited respectively layers of zinc metal, electroless nickel, electroplated nickel and a cobalt layer are coated in the 250 ppm bath in accordance with Example 1. The magnetic signal strengths of the coated discs are within specification and after 10 to 20,000 start/stop cycles the magnetic film loses less than 10 percent of the magnetic signal strength.

Having described the invention I claim:

Claims

1. A magnetic recording medium comprising a substrate having a reactive magnetizable metal surface and a thin film overlying said metal surface, said thin film comprising molecules defining a backbone and a terminal bonding group, said terminal bonding group being chemically bonded to the reactive magnetizable metal surface, the backbone of said organic polymer being oriented substantially normal to said reactive magnetizable metal surface.

2. The recording medium of claim 1 wherein said polymer further defines a terminal functional group located distally on the backbone thereof.

3. The recording medium of claim 1 wherein said said terminal bonding group is selected from the monofunctional and polyfunctional groups consisting of hydroxy, carbonyl, carboxy, carbonate, sulfide, sulfonate, amine, oxime, nitrate, phosphate, thiophosphate, epoxy, vinyl, benzyl, and ring systems capable of donating pi electrons.

4. The recording medium of claim 1 wherein said terminal functional group is electronegative. -

5. The recording medium of claim 1 wherein the backbone of said film forming molecules is selected from the group of molecules having the formula

[CX₃ - (cx₂o)_m - (c₂x₄o)_n - CX₂]

[CX₃ - (CX₂)_n] - wherein X comprises fluorine, chlorine, iodine hydrogen or methyl, m and n are integers of between 0 and about 12, and where X is hydrogen the hydrocarbon groups may be ethylenically unsaturated.

6. The recording medium of claim 1 wherein said polymer backbone is a carboxyl terminated perfluoropolyether comprising

[CX₃ - (CX₂0)_m - C₂X₄0)_n - CX₂] - m and n are integers of between 0 and about 12, and X comprises fluorine.

7. The recording medium of claim 6 wherein said terminal functional group is fluorine.

8. The recording medium of claim 1 wherein said film comprises between 1 monolayer having a thickness of between about 30 and about 300 angstroms.

9. The recording medium of claim 1 wherein said reactive magnetizable metal surface is essentially free of passivating material which interferes with the chemical bond between said terminal bonding group of said polymer and said surface.

10. The recording medium of claim 7 wherein said passivating material is the oxide of the reactive magnetizable metal surface.

11. The recording medium of claim 3 wherein said terminal bonding group is the carboxyl group.

12. A method for manufacturing a magnetic recording medium having a lubricating film bonded to a magnetizable reactive metal surface comprising the steps of a. maintaining said reactive metal surface under inert conditions to retard the formation of a passivating oxide film thereover, c. contacting said metal surface with a fluid consisting essentially of film forming molecules and a solvent therefor, said solvent being inert with respect to the reactive metal surface, said film forming molecules having a terminal bonding group for reaction with said reactive metal surface, thereby to effect chemical bonding of said molecules to said surface and to orient the molecules in a plane substantially perpendicular to the reactive metal surface.

13. The method of claim 12 wherein said reactive metal surface is maintained under an essentially oxide- free, inert fluid thereby to retard the formation of a passivating oxide film thereover.

14. The method of claim 13 wherein said medium is a fluorinated hydrocarbon.

15. The method of claim 12 wherein said fluid comprises at least about 25ppm of said film forming molecules in a solvent for said molecules said solvent being inert with respect to said reactive magnetizable metal surface.

16. The method of claim 15 wherein said solvent is trichloroethane.

17. The method of claim 15 wherein said solvent is a liquid fluorinated hydrocarbon.

18. The method of claim 12 wherein said reactive metal surface is imersed in a bath of said fluid maintained at about the boiling point of the fluid thereby to effect chemical bonding of said molecules to said surface and to promote orientation of the molecules in a plane substantially perpendicular to the reactive magnetizable metal surface.