Coating of solid substrates with magnetically propelled particles

Abstract

Particulate material is coated onto a solid substrate surface by exposing the surface in a confined volume containing small magnet elements mixed with the particulate material, and establishing within an effective distance of the confined volume, a magnetic field varying in direction with time. The magnetic field is of sufficient intensity to propel the magnet elements and the particulate material at a velocity which causes the mixture to impinge upon the surface and the particulate material to adhere thereto as a layer.

Description

BACKGROUND OF THE INVENTION

This invention relates to the coating of solid substrates with various materials. More particularly, the present invention is directed to the coating of various particulate materials on the surface of solid substrates by utilizing particles propelled by magnetic forces.

DESCRIPTION OF THE PRIOR ART

Numerous techniques are known in the art for coating solid articles. An article may be coated to modify its surface properties such as corrosion resistance, electrical contact resistance, reflectivity, color, abrasion resistance, solderability, coefficient of friction, etc.

The common methods of coating are chemical reduction, electroplating, spraying, hot dipping, mechanical plating and vacuum metallizing. Chemical reduction requires stringent temperature control, generally produces noxious fumes, and is not economical. The quality of chemically reduced deposits is inferior to that of either electro-plated metal deposits or vacuum-metalized deposits with respect to dimensional control, durability and reflectivity. The processing temperatures involved with chemical reduction practices generally exceed the heat-distortion point of most plastics, thereby precluding the use of this process for coating plastics.

Electroplating is limited in the number of metals which can be plated, causes hydrogen embrittlement and has the disadvantage of requiring a conductive substrate, thereby also precluding the coating of plastic by this technique unless the plastic substrate is provided with a conductive surface.

Metal spraying, applicable primarily for heavy deposits, produces a coating which is porous, dimensionally nonuniform, and usually requires thermal treatment to improve adherence. The finish of sprayed metal coatings is rough and unattractive.

Commercial hot-dipped coatings are limited to low melting metals such as zinc, tin, lead and aluminum. Additionally, hot dipping requires an extremely clean, grease- and oxide- free surface to obtain a uniform adherent coating.

The process of mechanical plating has been known for perhaps a quarter of a century. The broad principles of the process are well known; see, e.g., British Pat. No. 534,888, U.S. Pat. Nos. 2,689,808, and Re. 23,861, and other publications. The process is typically carried out by placing in a tumbling barrel metallic parts to be plated, plating metals in the form of minute malleable particles, impact media such as glass beads and cullet, water, and, optionally, a chemical promoter. As the tumbling barrel is rotated, the plating metal particles are hammered against the surface of the metallic parts to be plated, the impact media and the parts themselves serving to flatten the metal particles into a continuous coat. Mechanical plating may produce adequate results but is generally limited to only a few metals such as tin, zinc, cadmium, and brass.

Mechanical plating may also be accomplished by projecting an air borne mixture of coatable particles and hard peening particles onto a substrate causing hammering of the coatable particles on the surface as a layer. Such a process is limited by the trajectory of the stream of the air borne mixture to relatively flat and uniformly shaped substrates.

SUMMARY OF THE PRESENT INVENTION

In accordance with the process of the invention, particulate material is plated on a substrate surface by exposing the surface in a confined volume containing small magnet elements mixed with the particulate material and establishing within an effective distance of the confined volume a magnetic field varying in direction with time.

Without being bound by any theory or scientific explanation of precisely how the present invention functions, it is believed that the magnetic field imparts motion to the magnet elements which in turn imparts a motion to the particulate material mixed therewith. These materials then impinge upon the surface of the substrate in a sufficient amount and with sufficient force to clean the surface and hammer the particulate material thereon to form a uniform coating. Microscopic examination of the coated surface, prior to completion of the coating, indeed reveals a multitude of flattened particles adhered to the substrate surface, in some cases.

The present invention provides a coating process which permits simple or very complex shaped articles of plastic, metal, or any hard material to be coated with any of a variety of materials including plastics, metals, inorganic materials and others. The process utilizes simple economical apparatus and produces no undesirable waste products which require removal or disposal. The process, which requires no toxic chemicals, does not utilize molten metal and therefore eliminating the danger of burns and fires caused thereby. The process can be used to coat fragile articles, very complex articles, and articles not capable of being coated by conventional techniques. The process provides uniform coatings of good quality from very thin to very thick, with no modification thereof merely by continuing coating for the appropriate time. No hydrogen embrittlement is produced by the process of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a coating apparatus in accordance with the invention; and

FIG. 2 is a vertical section view, taken at lines 2--2 of the apparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIGS. 1 and 2, the apparatus utilized for coating in accordance with the invention is comprised of a magnetic field generating device 10 capable of producing a magnetic field which varies in direction with time, small magnet elements 11 capable of being moved by the magnetic field, and particulate material 12 which is to be coated. A container 13 confines the mixture of magnet elements' particulate material 12, and substrate 14 being coated, in a predetermined volume. (It should be noted that magnetic field generating device 10 is shown as a solid annulus merely for purposes of illustration, and it will actually have other parts such as wires, cores, etc. as will be apparent from the description which follows.)

The magnetic field may be generated by means of osillators, oscillator/amplifier combinations, solid-state pulsating devices, motor generators, and mechanical vibrators. The magnetic field may also be provided by means of air or metal core coils, stator devices or the like. The preferred device for generating the magnetic field is capable of generating a rotating magnetic field. With such a device, the field which is generated rotates about a central axis defined by the device itself.

A preferred device for generating such a rotating magnetic field, is described in assignee's copending application to Lovness and Feldhaus, Ser. No. 334,000, filed Feb. 20, 1973, the disclosure of which is incorporated herein by reference. This device has at least four overlapping electrical coils arranged in a generally circular pattern of opposed pairs and energized by two or more out-of-phase sources of alternating current so that opposed coils are of opposite polarity and of the same phase. A rudimentary version of this type of field generator device is the stator of a two pole alternating current electric motor.

The container or surface for confining the magnet elements and particulate coating material within a predetermined area should be formed of a non-magnetic material such as glass, synthetic organic plastics, for example, polytetrafluoroethylene (e.g., "Teflon"), polyethylene, polypropylene, and the like, ceramics, non-magnetic metals such as stainless steels, bronze, lead, etc.

Any one of a variety of particulate materials of varying degrees of hardness and shape is contemplated for use as the coating material of the present invention. For the most part, the coating materials are metal powders but other materials have also been found suitable for coating. Illustrative of metal powders which may be coated are aluminum, iron, lead, zinc, cadmium, copper, indium, tantalum, chromium, magnesium, nickel, tungsten, silver, and gold. Illustrative metal alloy powders which have been found useful for coating include stainless steels, aluminum/zinc alloys, and tin/lead alloys. Non-metal powders found useful for coating in the present invention include graphite, molybdenum disulfide, and organic resins such as polytetrafluoroethylene and polyvinyl chloride.

The shape of the particulate material being coated need not be in any particular form since it has been found virtually all shapes will provide a suitable coating, e.g., round, flake, etc. The particulate material may range in size from 0.1 micron or less in maximum dimension to several hundred microns or more; preferably the particle size is within the range of 0.5 to 50 microns. The mass of each magnet element is preferably at least twice the mass of each individual fragment of particulate material being coated or else very large magnetic forces are required to provide uniform and permanent coatings.

As previously mentioned, the process of the invention utilizes small magnet elements, each of which is an individual minute permanent magnet and hence susceptible to the influence of a moving magnetic field. Such elements include gamma iron oxide (Fe.sub.2 0.sub.3), hard barium ferrite, (Ba0.6Ee.sub.2 0.sub.3), particulate aluminum- nickel- cobalt alloys, or mixtures thereof. Suitable magnet elements have been found to have a magnetization (M) in excess of 10 gauss per gram, magnetization being a measure of the magnetic field intensity of the material from which the particles are prepared. Hard barium ferrite has a magnetization of about 70 gauss/gm and gamma iron oxide has a magnetization of about 50 gauss/gm. Also, it has been found that suitable magnet elements should have a magnetic coercivity (defined as the opposite sign field necessary to reduce the magnetization to zero) greater than the magnetic field (H) applied to cause physical movement of the element. Magnetic fields of about 100 to about 600 oersteds and higher have been used to move the particles. Hard barium ferrite has a magnetic coercivity of about 3000 oersteds and the gamma iron oxide has a magnetic coercivity of about 300 oersteds. Magnet elements having a magnetic coercivity less than about 100 oersteds have been found not to be particularly suited for use in the invention because application of external magnetic fields sufficiently strong to move the elements causes demagnetization.

The size of the magnet elements will vary over a considerable range depending upon the coatable particulate material and upon the particular substrate being coated. As previously stated, the mass of the magnet elements being used should be at least twice the mass of the particulate material being coated. Typically, the size of the magnet elements will vary between 1 micron in maximum extent to about several hundred microns or more. The size of the magnet elements should be sufficiently small to enter any openings or perforations in the article being coated, if it is desired to coat the inner surface of such openings.

The amount of magnet elements used with the coatable particulate material will also vary depending upon the coatable particulate material being used and upon the substrate being coated. Functionally stated, the total mass of magnet elements is that sufficient to cause the coatable particulate material to impinge upon the surface of the substrate being coated and to provide a coating thereon. Since the magnet elements should be at least twice the mass of the coatable particulate material, the total mass of the magnet elements will likewise be at least twice the mass of the coatable particulate material. Usually an excess of the amount of coatable particulate material desired to be coated on the substrate is used or eventually added during the coating operation.

For some applications, other substances may be used with the magnet elements coatable particulate material mixture. For example, with some substrates it may be desired to have the additional abrasive action of a suitable abrasive material in the mixture to provide a smooth clean finish to the surface being coated. Additionally, the mixture may contain hard dense particles such as glass beads, metal shot, ceramic beads and the like to aid in hammering the particulate material onto the surface of the substrate.

Of particular advantage, is the fact that two or more different coatable materials can be coated simultaneously by the process of the invention. This feature permits the simultaneous coating of two metals or of a non-metal and a metal together in composite layers. In this manner, composite layers can be made from any of a wide variety of starting mixtures.

The process of the invention is generally carried out under normal atmospheric conditions; however, for some materials (either coatable materials or substrates) it may be desirable to coat in an inert atmosphere such as dry nitrogen, argon, or helium, or to carry out the entire operation in a vacuum or near vacumm. For example, when utilizing magnesium powder as the coatable particulate material, it is preferred to carry out the process in a dry inert atmosphere. Additionally, while it is generally unnecessary, various coating additives or promoters may also be utilized in the process. Such materials may provide a more uniform coating for some coatable particulate materials and for some substrates. Such additives and promoters are well known in the mechanical plating art and useful in the present process.

Another remarkable advantage of the invention is that the substrates to be coated do not generally require a clean surface. In other words, the substrate may have a surface layer of rust, scale, paint or grease, before it is subjected to the magnetic particle/coatable particulate material mixture and yet be coated with a uniform layer. Extremely thick layers of surface contamination are preferably removed prior to commencement of coating to shorten the amount of time required to achieve such coating.

In some instances the magnet elements themselves may actually coat onto the substrate along with the material nominally being coated. If such a situation occurs and is not desired, magnet elements encased in a protective shell such as a hard polymeric resin coating may be used. A typical coating is of polyurethane.

Substrates which can be coated or plated in accordance with the present invention include any hard material. Such materials include metals, alloys, wood, plastic, ceramics and the like. Such substrates may have any shape including "blind" holes, threads, sharp angles, knurls, and the like. As long as the surface of such an article is in communication with the coatable particulate material and magnet elements, it will be coated.

The following examples illustrate the invention.

EXAMPLE 1

Powdered aluminum was coated on a copper substrate surface by utilizing a rotating magnetic field generating device and barium ferrite magnetic particles. The rotating magnetic field generating device, originally the stator of a 1/2 horsepower electric motor, was a ring-like structure having a 5.5 inch outer diameter and a 2.8 inch inner diameter, with windings formed of insulated copper wire forming a two pole single phase arrangement.

A 500 ml "Pyrex" glass beaker to contain magnet elements and the aluminum particles was situated within the opening of the stator described above. A 1/2 inch .times. 1 inch .times. 0.010 inch strip of copper to be plated was held on the wall of the beaker by a strip of double faced pressure sensitive tape. The magnet elements were barium ferrite speaker magnets which had been crushed to provide a particle size which passed through a U.S. Standard Screen mesh size of 12 and were retained on a 40 mesh (approximately 0.4- 2 mm). The barium ferrite particles had been previously magnetized by brief exposure in a 11,000 gauss magnetic field.

The powdered aluminum was that sold by U.S. Bronze Powder Company as "Venus Aluminum Powder Atomized No. 610 medium mesh" having a particle size of about 20 microns and a bulk powder density of 1.0 g/cc. About 2.5 grams of powdered aluminum were used.

Coating was accomplished by energizing the rotating field generating device to an operating current of 10 amps for a period of one hour. A 3 mil thick coating having a matte finish uniformly covering the exposed surface article was produced.

EXAMPLE 2-168

Utilizing the device described above the substrate shown below was coated with the coatable particulate material shown in the table.

__________________________________________________________________________ Ex. No. Substrate Coating Material __________________________________________________________________________ 2 aluminum aluminum (Same as Ex. 1) 3 anodized aluminum " 4 stainless steel " 5 nickel " 6 copper " 7 titanium " 8 aluminized steel " 9 303 stainless steel " 10 zinc " 11 magnesium " 12 430 stainless steel " 13 glass " 14 ceramic " 15 nylon " 16 polystyrene " 17 polytetrafluoroethylene " 18 polycarbonate " 19 styrene polymer " 20 polyethylene " 21 aluminum barium ferrite (Magnets of alloy Ex. 1) 22 anodized aluminum " 23 steel " 24 nickel " 25 copper " 26 titanium " 27 aluminized steel " 28 zinc " 29 magnesium " 30 430 stainless steel " 31 glass " 32 ceramic " 33 nylon " 34 polystyrene " 35 polytetrafluoroethylene " 36 polycarbonate barium ferrite (Magnets of alloy Ex. 1) 37 styrene polymer " 38 polyethylene " 39 aluminum tin (2.5 micron) 40 anodized aluminum " 41 stainless steel " 42 nickel " 43 copper " 44 titanium " 45 aluminized steel " 46 303 stainless steel " 47 zinc " 48 polystyrene " 49 aluminum lead (6 micron) 50 steel " 51 nickel " 52 copper " 53 titanium " 54 aluminized steel " 55 303 stainless steel " 56 zinc " 57 aluminum zinc (4 micron) 58 anodized aluminum " 59 steel " 60 nickel " 61 copper " 62 titanium " 63 aluminized steel " 64 303 stainless steel " 65 zinc " 66 polystyrene " 67 aluminum cadmium (7 micron) 68 anodized aluminum " 69 steel " 70 nickel " 71 copper " 72 titanium " 73 aluminized steel " 74 303 stainless steel " 75 zinc " 76 aluminum copper (8 micron) 77 anodized aluminum " 78 steel " 79 nickel " 80 copper " 81 titanium " 82 aluminized steel " 83 303 stainless steel " 84 430 stainless steel " 85 glass " 86 aluminum graphite 87 anodized aluminum " 88 steel " 89 nickel " 90 copper " 91 titanium " 92 aluminized steel " 93 303 stainless steel " 94 zinc " 95 aluminum molybdenum disulfide (microsize powder) 96 anodized aluminum " 97 steel " 98 nickel " 99 copper " 100 titanium " 101 aluminized steel " 102 303 stainless steel " 103 430 stainless steel " 104 aluminum indium (200 mesh) 105 anodized aluminum " 106 steel " 107 nickel " 108 copper " 109 titanium " 110 aluminized steel " 111 303 stainless steel " 112 aluminum tantalum (4.8 micron) 113 anodized aluminum " 114 steel " 115 copper " 116 titanium " 117 aluminized steel " 118 303 stainless steel " 119 430 stainless steel " 120 aluminum chromium 121 anodized aluminum " 122 steel " 123 nickel " 124 copper " 125 titanium " 126 aluminized steel " 127 303 stainless steel " 128 430 stainless steel " 129 aluminum tin (50)/- lead (50) 130 anodized aluminum " 131 steel " 132 nickel " 133 copper " 134 titanium " 135 aluminized steel " 136 303 stainless steel " 137 zinc " 138 aluminum polytetra- fluoroethylene 139 anodized aluminum " 140 steel " 141 nickel " 142 copper " 143 titanium " 144 aluminized steel " 145 303 stainless steel " 146 430 stainless steel " 147 aluminum silver 148 steel " 149 nickel " 150 copper " 151 aluminized steel " 152 zinc " 153 ABS plastic " 154 polystyrene " 155 aluminum gold (2.3 micron) 156 anodized aluminum " 157 steel " 158 titanium " 159 aluminized steel " 160 303 stainless steel " 161 ceramic " 162 nickel " 163 molybdenum nickel (0.2% max. + 200 mesh 2.0% max. + 325 mesh) 164 aluminum magnesium (-400 mesh) 165 glass " 166 steel aluminum/zinc- alloy 167 steel tungsten carbide/ cobalt alloy (-325 mesh) 168 aluminum iron (-325 mesh) __________________________________________________________________________

The degree of adhesion of each coating tabulated above was qualitatively measured by a "tape test," using a 3/4 inch wide and 11/2 inch long strip of pressure-sensitive adhesive tape sold under the trade designation "Scotch Brand Magic Mending Tape" by the 3M Company. For the test, one-half inch of the strip adjacent one end was adherently bonded with finger pressure to the coated surface, and then the free end of the strip was doubled back on itself at 180.degree. and slowly pulled away to completely remove the tape from the article. An adequately adhered coating was one which did not split or fail under the test. The coatings in all of the examples remained intact when subjected to the tape test just described, except for those of graphite and molybdenum disulfide (Examples 86-103) which, of course, would not be expected to do so, since they have a weakly cohesive nature.

EXAMPLES 169-174

The following examples show the effective weight ratio of particulate material to magnet elements useful in the invention. For each example, a copper piece was attached inside the container consisting of an 8 ounce paper drinking cup which also contained 100 grams of magnet elements. The rotating magnetic field generating device was operated at 11 amperes for 30 minutes in each case. The amount of aluminum powder used for each example is shown in the table below. The rotating field generating device, the copper pieces, the magnet elements, and the aluminum powder are all described in Example 1.

After the coating was accomplished, the coating thickness and weight were measured. Results are as follows: Ex. Powder Coating Thickness Coating Weight No. Weight (g) (mil) (g) ______________________________________ 169 2 .25 .0016 170 5 .26 .0022 171 10 .27 .0025 172 20 .25 .0016 173 40 .22 .0009 174 80 .15 .0009 ______________________________________

As can be seen, the efficacy of coating is reduced somewhat if the weight of particulate material is greater than about 1/10th the weight of the magnet elements, indicating that it is preferred to maintain a relatively small amount of particulate material with respect to the magnet elements.

Claims

What is claimed is:

1. A process for coating particulate material upon the surface of a substrate comprising:

exposing said substrate surface in a confined volume to a plurality of disconnected and independently movable small permanent magnet elements and said particulate material, and

establishing within said volume, in addition to the magnetic field of said permanent magnet elements, a magnetic field varying in direction with time and of sufficient intensity to impart movement to the magnetic elements to cause the particulate material to impinge upon and to coat said exposed substrate surface.

2. The process of claim 1 wherein said established magnetic field rotates about a central axis.

3. The process of claim 1 wherein said magnetic elements have an electromagnetic field of at least about 100 gauss.

4. The process of said claim 1 wherein said magnetic elements have a magnetization of at least 10 gauss per gram.

5. The process of claim 1 wherein said magnetic elements are barium ferrite.

6. The process of claim 1 wherein said particulate material is powdered aluminum.

7. A coated substrate comprising a substrate having a surface coating of a multitude of flattened particles adhered to said surface, said coating containing magnet elements.