US2913385A - Method of coating - Google Patents

Description

2,913,385 METHOD OF COATING Daniel Satriana, Verona, and George Rah, Pine Brook,

N.J., assignors to Vitro Corporation of America, Verona, NJ.

No Drawing. Application May 28, 1953 Serial No. 738,291 8 Claims. (Cl. 204181) This invention relates to the preparation of coatings on the surface of a substrate.

The present invention is an improvement over the process disclosed in the copending application Serial No. 656,315, filed May 1, 1957, of Harold N. Barr, now United States Patent No. 2,878,140, issued March 17, 1959. The said application discloses and claims a process for placing pore free coatings on a substrate. The process involves the necessity of placing the substrate coated with sinterable particles in a soft rubber or plastic bag or sleeve which is then evacuated and drawn into contact with the coated substrate. The encased substrate is then isostatically pressed in a chamber filled with a liquid to densify the coating of sinterable particles prior to sintering.

Such densification procedure has been found to be desirable in preventing any tendency of the primary coating to slough off or to be damaged through mishandling. The need for densification of the coating is particularly apparent in the formation and treatment of coatings prepared by electrophoresis in accordance with a particular embodiment of the invention as described hereinafter.

One of the difficulties experienced with the above process is that in coating substrates having apertures therethrough, or having a complex or highly irregular surface the bag or sleeve cannot be satisfactorily drawn into contact with the coated substrate so that they conform to the irregular surface of the coated substrate. When pressed, the coating therefore is not uniformly and evenly densified over the surface area of the substrate. A further difiiculty encountered is that the bagging and subsequent steps necessary for isostatic pressing are somewhat intricate, tedious and time consuming.

Accordingly, it is an object of the present invention to eliminate the difliculties encountered in the prior process by permitting the densification of coatings on substrates which cannot be handled by the bagging technique.

It is also an object of the present invention to eliminate the costly and intricate steps of bagging and applying vacuum in the process of densifying coatings.

It is a further object to provide a new process for densifying coatings on regular or irregularly shaped objects.

A further object is to reduce the cost of densifying coatings to make the process more attractive economically.

It has been found that by applying a thin wax covering on the coated substrate the substrate can then be isostatically pressed using 20 to 60 tons per square inch pressure to densify the coating without the necessity of employing the bagging technique required in the prior process. The amount of pressure exerted is governed by the composition of the sinterable particle coating. While the densification can be accomplished solely by means of a wax covering, it is preferred first to cover the coated substate with a liquefied film-forming polymer which will form a film on the covered substrate and then to apply the wax to the polymer film. This new process eliminates the bagging technique and its inherant disadvantages.

The wax covering should be thin, in the range of 2 to 19 mils (preferably 5 to 15 mils), free of air bubbles and pit holes and should completely cover the coated substrate. It has been found that thicker wax coatings, 20 to 30 mils in thickness, may crack during isostatic pressing. The substrate, thus coated, is then placed in an isostatic press to densify the coating and thereafter heated to sinter the sintering particles on the substrate. If desired the substrate can be placed in a protective preformed pliable container, such as a rubber or plastic bag, filled with glycerine and then isostatically pressed in order to prevent marriug of the substrate by the pressure device.

Coatings may be formed on a variety of substrates such as metals, graphite and ceramic. Moreover, a variety of coating materials may be used, including ceramics, metals and metal oxides. However, the choice of coating material for the particular substrate should be such that the sinterable ingredient of the coating may be sintered at a temperature which is substantially below the melting point of the substrate.

The preferred procedure is first to cover the sinterable coating on the substrate with a liquefied film-forming polymer which will form a film on the coated substrate. The thin wax covering is then applied, and the substrate is then pressed to densify the coating and heated to sinter the sinterable particles on the substrate. The heat applied to sinter is sufiicient to remove the Wax and polymer film coatings before the sintering temperature is reached. These coatings can also be removed by suitable solvents or by stripping them off the substrate.

As a first step in preparing the metal substrate for treatment, the sinterable particles whether they be metal, metal oxide or ceramic or mixtures of any two or more of these types of materials, may be applied to the surface of the metal substrate by a variety of procedures such as electrophoretic deposition, spraying, dipping or painting. However, it is preferred to use electrophoretic deposition. In the electrophoretic coating process, a suspension of the sinterable particles to be deposited in a suitable liquid vehicle (preferably an organic solvent) is prepared. The suspension may be prepared by ball milling the sinterable material in the solvent, for example alcohol, to obtain a finely dispersed material. Particles ranging in diameter from 0.5 to 10 microns are preferred, although dispersions containing particles from 0.1 to 74 microns have been prepared and deposited. The electrical charge on the particle is acquired during the dispersing operation because of absorption of ionizable substances or a reaction between the solid and liquid. The organic liquids that are preferred for use as suspending vehicles prevent electrolytic reaction and the gassing of the electrodes. The article to be coated is made one of the electrodes in a bath of the suspension. The suspension is preferably agitated slowly during deposition to prevent settling. Cell voltages of 200 to 1000 volts DC. are permissible because the vehicle is an anhydrous solvent. The surface of the metal substrate is preferably thoroughly cleaned before it is immersed in the plating bath. Coating thicknesses can be varied with the deposition time, electrode spacing, voltage and suspension concentration.

Any film-forming polymer may be used which will decompose, vaporize or even oxidize (provided the oxidizing conditions will not irreparably harm the coating) before the sintering temperatures are reached leaving no deposit to poison the sinterable coating. Examples of suitable film-forming polymers are acrylic resins, polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polystyrene, cellulose acetate, etc. The covering of filmforming polymer may be applied in a liquefied form by spraying, dipping or brushing solutions, plastisols, or other dispersions, or even by applying powdered polymer to the heated substrate thereby to fuse the powder. A preferred polymer is Krylon, a commercial acrylic spray which is composed of acrylic resin, 15% chlorinated solvents, 30% aromatic hydrocarbon and 50% propellant (fluorinated hydrocarbons).

Suitable waxes are mineral, semi-mineral, animal, insect, vegetable and synthetic waxes, such as ceresine, beeswax, carnauba Wax, japan wax, montan wax, spermaceti and Carbowax 4,000. Here again any wax may be used which will decompose, vaporize or oxidize before the sintering temperatures are reached leaving no deposit to poison the sintering particle coating and which will deform without breaking at pressures ranging from 15 up to 60 tons per square inch. The Wax is preferably. applied by dipping the coated substrate into molten wax. Other methods such as spraying hot wax over the cool substrate can also be employed.

The following examples are presented as illustrations of the invention.

Example 1 A wedge of molybdenum alloy having deep holes piercing the wedge was cleaned by the standard procedure of degreasing, and was acid etched, and lightly sandblasted to roughen the surface. A dispersion was then prepared containing 104 g. nickelous oxide, 20 g. chro mium powder, and 1 g. ethyl cellulose in 500 ml. of Z-nitropropane. This dispersion was then used to coat the molybdenum wedge electrophoretically. At 500 volts DC. and 12 milliamperes current, a coating equivalent to approximately 2 mils of Nichrome was deposited in 15 seconds. The coating was reduced at 400 C. for onehalf hour in purified hydrogen. The coated molybdenum substrate was then dipped into molten ceresine, kept at a temperature from 90 to 100 C., to form a thin coating having a thickness of about mils on the substrate. The substrate was slightly agitated when submerged in the hot ceresine to insure that all the air bubbles were expelled and that complete coverage of the substrate was obtained. The thus coated substrate was removed from the hot ceresine and allowed to cool in order to harden the wax coating.

The wax covered molybdenum wedge was then placed in the isostatic press and pressed hydraulically at 25 tons per square inch. After removal from the press, the molybdenum wedge was heated at about 1100 C. in an atmosphere of hydrogen for about 20 minutes to sinter the coating. Tests showed high density and good bonding of the coating to the molybdenum base.

Example 2 The same process as employed in Example 1 was carried out with the exception that the molybdenum Wedge coated with a ceresine layer was placed in a rubber bag filled with glycerin and isostatically pressed. Here again the coating was dense, well sintered and firmly bonded to the molybdenum substrate, and had a low porosity.

Example 3 The same process as employed in Example 1 was carried out with the exception that the substrate coated with the sinterable layer was first sprayed with Krylon, a commercial acrylic resin polymer spray, to form a thin film of polymer on the surface of the coated substrate and then dried. The coated and polymer film covered molybdenum substrate was then dipped into molten ceresine, kept at a temperature of from 90 to 100 C., to form a thin wax covering on the substrate. The substrate was then further processed as set forth in Example 1.

In place of the metal powders used in Examples 1 to 3, nickel oxide, nickel, columbium carbide, chromium, iron oxide, molybdenum, and mixtures of these powders deposited on the surface of the substrate by electro- '4 phoresis, electroplating, spraying, dipping or brushing may be employed as the sinterable particle coating. In all instances, high density coatings with good bonding to the metal base are obtained.

The coatings formed by the process of this invention may have densities which are about -99% of the theoretical or intrinsic density of the coating material, which indicates a very low porosity.

The novel process of the present invention greatly simplifies the process of densification of coatings by eliminating the time consuming, tedious and costly bagging step. Further, the present process permits coatings of good quality on irregularly shaped and cast objects which cannot be satisfactorily coated by employing the bagging technique for densification.

It is to be understood, as noted above, that changes and variations may be made without departing from the spirit and scope of the invention defined in the appended claims.

We claim:

1. In the preparation of a coating on the surface of a substrate the steps of coating said surface with sinterable particles, covering said coated surface with liquefied wax, solidifying said wax covering, applying isostatic hydraulic pressure to the wax and to the coating which the wax covers thereby to densify the coating, and thereafter sintering said coating on said substrate.

2. The process as set forth in claim 1, wherein the surface of the substrate is coated by electrophoresis.

3. The process as set forth in claim 1, wherein the wax covering is applied by dipping the coated substrate into molten wax.

4. The process as set forth in claim 2, wherein the thickness of the wax covering is in the range of 5 to 15 mils.

5. The process of claim 1 wherein the wax covered and coated substrate is placed in a preformed pliable container filled with glycerin and then the isostatic pressure is applied thereto.

6. In the preparation of a coating on the surface of a substrate, the steps of coating said surface with sinterable particles, applying a liquefied film-forming polymer on said coated surface, solidifying said polymer to form a film on said coated surface, covering said coated and filmed surface with liquefied wax, solidifying said wax covering, applying isostatic hydraulic pressure to the wax and to the coating which the wax covers thereby to density the coating, and thereafter sintering said coating on said substrate.

7. The process as set forth in claim 6, wherein the liquefied film-forming polymer is sprayed onto the coated surface and the Wax covering is applied by dipping the coated substrate into molten Wax.

8. In the preparation of a coating on the surface of a metal, the steps of coating said surface with sinterable particles, spraying liquefied acrylic resin on said coated surface, drying said resin to form a film on said coated surface, covering said coated and filmed surface with about a 10 mil thickness of molten ceresine, cooling the substrate to harden the ceresine covering, applying isostatic hydraulic pressure to the ceresine and to the coating which the ceresine covers thereby to densify the coating, and thereafter sintering said coating on said substrate.

References Cited in the file of this patent UNITED STATES PATENTS 1,819,336 Penning Aug. 18, 1931 2,386,634 Robinson Oct. 9, 1945 2,389,060 Kurtz Nov. 13, 1945 2,421,652 Robinson et al. June 3, 1947 2,671,955 Grubel et al Mar. 16, 1954 OTHER REFERENCES Shyne et al. in Plating, October 1955, pages 1255- 1258.

Claims (1)

1. IN THE PREPARATION OF A COATING ON THE SURFACE OF A SUBSTRATE THE STEPS OF COATING SAID SURFACE WITH SINTERABLE PARTICLES, CONVERING SAID COATED SURFACE WTH LIQUEFIEXD WAX, SOLIDIFYING SAID WAX COVERING, APPLYING ISOSTATIC HYDRAULIC PRESSURE TO THE WAX AND TO THE COASTING WHICH THE WAX COVERS THEREBY TO DENSIFY THE COATING, AND THEREAFTER SINTERING SAID COATING ON SAID SUBSTRATE.