WO1988005473A1

WO1988005473A1 - Metal surface coating

Info

Publication number: WO1988005473A1
Application number: PCT/US1988/000214
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

The combination of a metal substrate and a thin surface film of a reaction product of the metal substrate and the film forming material. The molecules of the film are oriented essentially normal to the reactive metal substrate surface. The film molecules comprise a monofunctional or a polyfunctional terminal bonding group for chemically bonding with the metal substrate surface. It is preferred that a terminal bonding group be located at a terminus of the molecular backbone to insure that the desired molecular orientation is achieved. Additional sites may be provided for reaction with other molecules or for crosslinking to provide a film on the substrate surface having the desired characteristics. The terminal bonding group is selected from the group consisting of carbonyl, carboxy, carbonate, sulfide, sulfonate, amine, oxime, nitrate, phosphate, thiophosphate, epoxy, vinyl, benzyl, and ring systems capable of donating pi electrons. The backbone of 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 or hydrogen, m and n are integers of between 1 and about 32, and where X is hydrogen the hydrocarbon groups may be ethylenically unsaturated. A method for producing the article is provided.

Description

METAL SURFACE COATING

Field of the Invention

The present invention relates to a coating for a reactive metal surface and to a method for producing the same.

Background of the Invention

Metals are highly reactive with oxygen and form on their surfaces oxide metal coatings. For example, aluminum is very highly reactive and in fact does not exist as the pure metal but requires a passive surface coating to prevent the complete oxidation of the metal to its oxide. In metal processing, the oxide coatings begin to form almost immediately upon completion of the processing operation. The surface chemistry is further complicated by the presence of organic cutting fluids and the like which also may react on the surface of the freshly processed metal to form unstable reaction products which subsequently become entrapped under the oxide coating. These entrapped materials can result in pitting on the surface of the metal which interferes with surface uniformity. In addition, the formation of the metal oxide on the metal surface can produce a coating which is relatively . inert with respect to subsequent coating materials such as decorative finishes or the like which may be desired to be laid over the metal surface.

There are many types of coating materials disclosed in the prior art for use on metal surfaces. For example, in the case of aluminum the anodizing process has long been practiced to apply a passive aluminum oxide, sulfide or phosphate coating over aluminum substrates. In addition, a substantial amount of work has been done in the area of providing lubricating or protective coatings on metal surfaces utilizing other types of materials such as organic polymeric materials which are readily modified for the purpose of creating various desired surface characteristics. Certain serious problems are encountered with present coating and film forming techniques. Of major concern is the provision of a uniform coating on the metal substrate surface. A non-uniform film or coating can result in poor surface characteristics for the finished article. For example, a pitted or rough surface condition can result when a poor protective film is applied over the metal substrate surface.

Yet another problem with the application of coatings and films over metal surfaces is that of poor bonding to the metal surface. This problem is very apparent with lubricant films for metal surfaces although the problem is not limited to such films. Thus, lubricant films are subject to "weeping" , that is, migration of the lubrication material particularly on magnetic discs and the like which are subjected to high centrifugal forces due to the spinning action of the disc. In addition, the lubricant films are easily eroded away or require the use of intermediate layers of material to improve the adherence of the lubricant film onto the substrate.

To overcome the forgoing problems various techniques have been put forward. For example, U. S. Patent 4,536,444 (Sumiya et al.) discloses magnetic recording media having a protected layer comprising a phosphate terminated perfluoroalkylpolyether. However, as Sumiya teaches, coating thickness and/or density is critical and a loss of output may occur if the coating is greater than 30 mg/in² or there may be no effect from the coating if it is less than 0.01 mg/in². Other prior art methods have been employed to improve the bond which involve the formation of an intermediate product to react with the metal oxide coating which is naturally formed on the metal substrate surface and which will also react with organic polymers and the like to provide the lubricant property to the finished film. An example of this type of methodology is set forth in U.S. Patent 4,446,193, Afzali-Ardakani, et al, in which a lubricant is chemically bonded to an information carrying solid surface by contacting the surface with a diazoketone-terminated polymer lubricant and decomposing the diazoketone-terminated polymeric lubricant to form a carbene which reacts with the solid surface. In yet another method of bonding a coating U.S. Patent 4,529,659, Hoshino et al, discloses a technique which teaches the formation of a silicon oxide layer on a metal oxide substrate such as by applying a layer of tetra-alkoxysilane in a suitable solvent such as ethanol followed by heat treatment. This results in the silicon forming an inorganic polymer which is then subsequently reacted with an aminosilane or epoxysilane compound to form a condensation reaction product which then can be further reacted with a fluorinated hydrocarbon polymer to form a lubricant layer.

The foregoing prior art methods require several reaction steps which are time consuming and which add to the expense of the manufacturing process. In addition, the prior art methods do not control the normally formed oxide coating on the metal surface so that the surface is subject to the imperfections described above in connection with normally formed oxide coatings.

Accordingly, it would be highly desirable to provide on a reactive metal surface a uniform coating which bonds with the metal surface, can be readily modified to provide various surface characteristics to the metal, does not adversely affect the performance or functions of the substrate and which provides a uniform surface for the metal substrate.

Summary of the Invention

In accordance with the present invention there is provided the combination of a metal substrate ^«and a surface film of a reaction product of the metal substrate and the film forming material. The molecules of the film are oriented essentially normal to the reactive metal substrate surface. The film molecules comprise a terminal bonding group for chemically bonding with the metal substrate surface. The terminal bonding groups are located at a terminus of the molecular backbone to insure that the desired molecular orientation is achieved. Additional sites may be provided for reaction with other molecules or for crosslinking to provide a film on the substrate surface having the desired characteristics.

The terminal bonding group of the film forming molecule film is monofunctional or polyfunctional and characterized by its ability to chemically bond with the reactive metal substrate. The chemical bond is most generally formed by an oxidation - reduction reaction and the application of the film to the reactive metal surface can be considered as a controlled oxidation reaction. However, this does not account for bonding in all cases, such as the case where the substrate is aluminum which is a powerful Lewis acid and the bonding of the molecules of the film to the substrate surface occurs as a Lewis acid-base reaction. In yet other cases, the bond appears to be a coordination complex. Accordingly the precise mechanism by which the bond between the terminal bonding group or groups of the film molecules and the metal substrate is not completely understood and most likely depends upon the nature of the terminal bonding group and the metal substrate. It is recognized that the chemical bond, regardless of its nature, is essential to the retention of the film on the substrate and to the orientation of the film molecules. The terminal bonding group may be selected from any group which is capable of chemical bonding with the reactive metal surface. Good results are achieved when the 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 oxi e groups and phosphorus containing groups such as phosphate and thiophosphate groups can be used as the terminal bonding group of the film forming material. 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 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 styrene and halogenated styrene, polyepoxy molecules, polyether molecules 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. An important feature of the film of the present invention is the orientation of the film molecules. As previously mentioned, the molecule backbone is oriented essentially normal to the metal substrate with the 5 bonding group chemically bonded to the metal substrate and the molecular backbone lying in a plane essentially perpendicular to the substrate. The distal end of the molecules of the film may carry functional groups such as electronegative radicals which modify the surface rσ> properties of the film.

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

15 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)

20 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

25 the following formulas

F - (A) - S0₃H (c) and

F - (A) - O - PO3H (ά) where A is a perfluro backbone as set forth in (a) or

20 (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

35 in formula (b) may be ethylenically unsaturated such as with polyvinyl or other alkenes. The film is deposited on any metal substrate such as for example aluminum, iron, nickel, chromium, cobalt and alloys thereof. The substrate surface on which the film is to be deposited must be reactive. That is to 5 say, the substrate surface must be free of a continuous layer 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.This is particularly critical for substrates

IJD such as the Group III metals and aluminum in particular. Aluminum is a powerful Lewis acid and rapidly forms an oxide layer over those portions exposed to the atmosphere. Other metals also react in substantially the same fashion and readily form oxide

15 layers on their surface, although at a slower rate than is the case for aluminum. The metal oxide films thus formed are substantially inert and resist application of other types of decorative coatings, protective films, lubricants and the like.

20 The reactive metal surface condition is achieved by removing the existing oxide coating from the metal surface by machining or chemical etching under conditions which insure that the formation of an oxide on the metal surface is maintained at a minimum until

2.5, the film is applied in accordance with the present invention. Accordingly it is preferred to prepare the surface in a fluid that is inert to the metal being worked and which is a solvent for the film forming molecules which are being applied to the substrate. In

30 this manner the molecules are 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, 5 chemically bonded film on the substrate and which can result in pitting of the metal surface. The film is applied on the reactive metal surface by coating the metal surface with a solution of the film forming molecule in a solvent for the film material which is inert with respect to the reactive metal surface. In solution the desired orientation is promoted because the molecules, being more soluble in the solvent than they are in themselves, are in a relaxed, uncoiled condition. 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 substrate may be sprayed with the solubilized film material or dipped in the solution. Depending on the reactivity of the metal, the solution may be maintained at room temperature or the solution temperature may be elevated to increase the rate of chemical bonding. The dipped or sprayed metal substrate is then subjected to a heating operation to complete the bonding reaction. It is preferred that the solution be maintained at about its boiling point if the substrate is coated by immersion because the heating step can be carried out by suspending the substrate in the vapor phase above the solution. In this manner a separate heating oven is unnecessary and the vapor 5 phase assists in removing any excess, unreacted solution. Concentration of the molecule in the solution is not critical although high concentrations provide a thicker film layer than less concentrated solutions. Concentrations of 25 ppm to about 250 ppm ϊ.α? of the film forming molecules are preferred.

Film thickness (apart from spatial density considerations as described above) is not a critical factor in the performance of the film although for best results the film should not exceed a single monolayer

15 between about 25 angstroms and about 250 angstroms. As the film thickness increases the orientation of the molecules becomes less uniform, particularly at the extending ends which define the film surface, and the desired surface characteristics may become less 0 defined. By way of illustration, if it is desired to react an additional coating material on the substrate, well defined orientation is highly desired in order to effect the strongest bonding between the outer coating and the film. Less defined orientation reduces the 5 number of bonding sites between the film and the outer coating material thus weakening the adherence of the coating.

Description of the Preferred Embodiment

The invention will be further described in 0 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 5 thereof. Example 1 Nickel plated aluminum discs were polished with one micron alumina until a brilliant mirror finish was obtained. After polishing the discs were washed with deionized water and rinsed with a halongonated hydrocarbon (Freon 113) . The polished discs were immediately immersed in a bath consisting of a solution comprising 250 ppm of a hydroxy terminated perfluoroalkylpolyether in Freon 113. The perfluorpolyether is distributed by Montefluos under the trade name "Fomblin-Dol". The bath was maintained at its boiling point (47*C) . The discs were maintained in the bath for approximately one minute then withdrawn and dried at approximately 150*C for 5 minutes. A second set of the nickel plated discs similarly treated as above were dipped in the perfluoroalkylpolyether solution comprising 25 ppm of the perfluoroalkylpolyether in Freon 113. After an immersion of one minute the discs were withdrawn and dried at 150 C for 5 minutes.

The discs were • then subjected to electron spectroscopy for chemical analysis (ESCA) to determine the nature of the coating thereon. This technique is described in an article by J. A. Buono, A. . Wisniewski, W. S. Andrus, "Surface Science Analysis Techniques", Solid State Technology (February 1982). The 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 beam to the angle of orientation of the detector it is possible to determine the orientation of the film molecules. Also this technique is utilized to determine the thickness of the film. By the ECSA analysis of the discs it was determined that the thickness of the film on the discs that were immersed in the 250 ppm solution was on the order of 57 angstroms while the thickness of the film on the discs which had been immersed in the 25 ppm solution was on the order of 37 angstroms. Discs prepared from both solutions indicate orientation of the molecules substantially normal to the plane of the substrate surface when examined by the ESCA technique.

Examples 2 - 6

Aluminum discs are ground to remove the oxide coating from the surface of the discs and to prepare a reactive aluminum surface. During grinding the disc surface is continuously flooded with a cutting fluid that is inert with respect to the aluminum and in which is dissolved 25ppm of film forming molecules. Immediately following the grinding operation, the discs are dried at about 150^XC. The cutting fluid and the film forming molecules dissolved in the cutting fluid for each the Examples 2 - 6 are set forth below. Example 2

Film forming molecules of Example 1 in trichloroethane.

Example 3 Chromium complex of perfluoroacrylic acid solubilized in Freon TF Example 4

Phosphate terminated perfluoroalkylpolyether (Phosmer FO distributed by Uni Chemical Company) solubilized in Freon TF.

Example 5 Alkenylcarboxylic acid/chromium complex (Volane manufactured by DuPont) solubilized in Freon TF.

Example 6 Perflurooctanoyl sulfonic acid solubilized in Freon TF.

The ground surface of the discs of Examples 2 - 6 are not etched by a zincating bath consisting of zinc oxide in 10% sodium hydroxide or by concentrated nitric acid.

Example 7 An aluminum substrate is electroless plated with nickel and 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 65*C for 10 minutes. The foregoing examples illustrate various films which are bonded onto reactive metal surfaces and a method for carrying out such bonding operations. While I have described the bonding of the chromium complexes of various polymeric materials it will be clear that any of the transition metal complexes, such as for example titanium and vanadium, may be employed with equally good results.

The choice of the molecular backbone to be bonded through the terminal bonding group to the reactive metal surface is largely dependent on further processing operations such as the reaction of the film with other materials and the characteristics desired for the finished coating on the metal substrate.

An important feature in obtaining the bonded film in accordance with the present invention resides in the condition of the reactive metal surface. That is to say it should be substantially in the non-oxidized or reduced form in order that the chemical bonding between the metal of the substrate and the terminal bonding group of the film material take place. The coating operation can occur after surface preparation, as by machining to remove any metal oxide coating, or may be formed concurrently by having the film forming material solubilized in the cutting fluid. Having described the invention I claim:

Claims

1. An article of manufacture comprising a substrate having a reactive metal surface and a film comprising molecules defining a backbone and a terminal bonding group, said terminal bonding group being chemically bonded to the reactive metal surface, the backbone of said film molecules being oriented substantially normal to said reactive metal surface.

2. The article of manufacture of claim 1 wherein 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.

3. The article of manufacture of claim 1 wherein the backbone of said film forming molecules is selected from the group of molecules having the formula

[CX₃ - (CX₂0)_m - (C₂X₄0)_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.

4. The article of manufacture of claim 1 wherein said film comprises 1 monolayer having athickness of between about 25 angstroms and about 250 angstroms.

5. The article of manufacture of claim 1 wherein said reactive metal surface is essentially free of passivating material which interferes with the chemical bond between said terminal bonding group of said polymer and said metal surface.

6. The article of manufacture of claim 1 wherein said terminal bonding group is the carboxyl group.

7. The article of manufacture of claim 1 wherein said reactive metal surface is selected from the group consisting of aluminum, iron, nickel, chromium, cobalt and combinations and alloys thereof.

8. A method for applying a film on a metal substrate comprising the steps of a. removing passivating material from the surface of said substrate thereby to provide a reactive metal surface, b. maintaining said reactive metal surface under inert conditions thereby to prevent the formation of a passivating film thereover, c. contacting said metal surface with a solution which is inert with respect to the reactive metal surface, said solution comprising a 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.

9. The method of claim 8 wherein said solution comprises a solvent for said molecules which is inert with respect to said metal whereby said reactive metal surface is maintained essentially free of passivating material which interferes with the chemical bond between said terminal bonding group of said polymer and said metal surface.

10. The method of claim 8 wherein said terminal bonding group of said film forming molecule is selected from the group consisting of carbonyl, carboxy, carbonate, sulfide, sulfonate, amine, oxime, nitrate, phosphate, thiophosphate, epoxy, vinyl, benzyl, and ring systems capable of donating pi electrons.

11. The method of claim 8 wherein said terminal bonding group is the carboxyl group.

12. The method of claim 8 wherein said terminal bonding group is the hydroxyl group.