US3891542A - Method for insuring high silicon carbide content in elnisil coatings - Google Patents

Abstract

A highly wear-resistant nickel-silicon carbide composite is applied to a cast metal surface by a process in which the cast metal surface is cathodically subjected to high current density in an aqueous acid treatment bath, preferably containing nickel sulfamate in a concentration of at least 600 grams per liter. A dispersion of silicon carbide particles is maintained in the bath in a concentration of 90 to 150 grams per liter. The bath temperature is regulated to be at least 150*F and cathodic current density is maintained at a level of at least 200 a.s.f. The anode is shaped to conform with the internal configuration of the cathode surface to be coated; significant bath agitation by air is employed to maintain the silicon carbide particles in continuous suspension to insure such particles settle out along with the electrodeposited nickel in a predetermined uniform and highly dense distribution. The process is continued until a coating thickness of at least 25 mils is deposited within a time period of no greater than 180 minutes.

Description

United States Patent [1 Cordone et al.

[ METHOD FOR INSURING HIGH SILICON CARBIDE CONTENT iN ELNISIL COATINGS [75] Inventors: Leonard G. Cordone, Allen Park;

William A. Donakowski; John R. Morgan, both of Dearbom Heights; Karl Roemming, Detroit, all of Mich.

[73] Assignee: Ford Motor Company, Dearborn, Mich.

[51] Int. Cl. C25D /02; CD 3/12 [58] Field of Search 204/3, 4, 16, 49

[56] References Cited UNITED STATES PATENTS 3,06l,525 lO/l962 Grazen ..204/3 3,l32,927 5/l964 Borner 204/9 3,326,782 6/1967 Kendrick et al. 204/3 3,640,799 2/1972 v Stephan 204/38 B [451 June 24, 1975 Primary Examiner-T. M. Tufariello Attorney, Ager'lt, 0r Firm.loseph W. Malleck; Keith L. Zerschling 5 7 1 ABSTRACT A highly wear-resistant nickel-silicon carbide composite is applied to a cast metal surface by a process in which the cast metal surface is cathodically subjected to high current density in an aqueous acid treatment bath, preferably containing nickel sulfamate in a concentration of at least 600 grams per liter. A dispersion of silicon carbide particles is maintained in the bath in a concentration of to grams per liter. The bath temperature is regulated to be at least l50F and cathodic current density is maintained at a level of at least 200 a.s.f. The anode is shaped to conform with the internal configuration of the cathode surface to be coated; significant bath agitation by air is employed to maintain the silicon carbide particles in continuous suspension to insure such particles settle out along with the electrodeposited nickel in a predetermined uniform and highly dense distribution. The process is continued until a coating thickness of at least 25 mils is deposited within a time period of no greater than minutes.

5 Claims, 2 Drawing Figures [412 Ava/72727107? METHOD FOR INSURING HIGH SILICON CARBIDE CONTENT IN ELNISIL COATINGS BACKGROUND OF THE INVENTION The process of the present invention is particularly applicable to the manufacture of composite articles requiring a functional coating, such as in a rotor housing for rotary internal combustion engine, with the exposed coating ofthe composite housing capable of serving ad mirably under severe wear conditions.

Most commonly employed wear-resistant materials are heterogeneous in structure, that is, they contain two or more distinct, chemically different phases since, in most cases, pure homogeneous metals have notoriously poor wear characteristics. Since wear is entirely a surface phenomenon, engineers for many years have employed techniques to form a dispersion at the surface of parts subject to wear. Such techniques include operations such as nitriding, case hardening, hard facing flame, metalizing and alike. In these surface treatments, metallurgical laws continue to place severe restrictions on the physics and the chemistry of the coatings produced.

The process of the present invention is concerned with a new approach to the preparation of dispersions. A dispersion coating is deposited herein by electrolytic means in such thicknesses to constitute a functional part, but the coating must be deposited within a reasonably short period of time so that the cost of fabrication is not exorbitant. A simultaneous electrodeposition of a metal and settling out of electrically inert additive particles is promoted under controlled conditions to produce a composite dispersion structure in which the additive particles retain their discrete identity and are homogeneously dispersed within the electroplated metal deposit. The selection of the chemistry of the particular additive particles will, of course, be governed by the characteristics desired in the final product. Where the primary concern is the improvement of wear-resistance, material such as silicon carbide, aluminum oxide, tungsten carbide, titanium carbides, Zirconium oxide, boron carbide, chromium carbide, or diamonds may be employed. I

It has been found most difficult to deposit a functionally thick coating onto a rotor housing of an internal combustion engine which is shaped as an epitrochoid and wherein the additive particles are in sufficient quantity and distribution to promote the characteristics of wear that is expected of the rotor housing. Since the wear characteristic is highly dependent upon the density and distribution of the silicon carbide inert particles, it has been extremely difficult to obtain satisfactory results while attempting to increase the speed of plating to make such a functional coating economically feasible and commercially practical.

SUMMARY OF THE INVENTION A primary object of this invention is to provide a process capable of electrolytically depositing a heavy thick functional dispersion coating more quickly and more efficiently than the prior art.

Still another object of this invention is to provide a method for electrodepositing a matrix of nickel and a dispersion of silicon carbide particles, the carbide particles being distributed with a high degree of homogeniety and uniform density to promote a high wear resistance for a coating useful as a wear surface in a rotary internal combustion engine.

Yet another object is to provide a method for electrodepositing a composite of nickel and silicon carbide onto a cathodic article to be coated, the process utilizing a semiconforming anode shaped predominantly to the configuration of the surface to be coated, the spacing between the anode and cathode article normally inhibiting a uniform deposited dispersion but compensated for by the method.

SUMMARY OF THE DRAWINGS FIG. 1 is a photomicrograph of a portion of a rotor housing for a rotary internal combustion engine having a chemistry similar to the deposited coating of the invention but having unsatisfactory wear characteristics;

FIG. 2 is a photomicrograph of a portion of a rotor housing having a functionally thick coating of nickel and silicon carbide according to the teaching of this invention and having a highly satisfactory wear-resistant characteristic.

DETAILED SPECIFICATION 1 electrolytically deposited from a bath containing nickel sulfamate in a uniquely high concentration and silicon carbide particles in a unique agitated concentration, can be electrolyzed by a high current density to deposit out a relatively thick coating having an unprecedented uniform, dense'and homogeneous distribution of silicon carbide particles rendering unusually high wear resistance. It is now known that the bath may contain any nickel salt selected from the group consisting of nickel sulfate, nickel chloride, nickel fluoroborate, nickel sulfamate and any mixtures thereof. In addition, the particles may be any inert particle havingthe required hardness shape and temperature stability.

APPARATUS The problem of uniformly distributing carbide particles in a nickel matrix is particularly evident when attempting to electrolytically deposit a coating on the epitrochoid surface forming the internal wall of the rotor housing of a rotary internal combustion engine.

A preferred method mode for carrying out the present invention essentially comprises:

a. providing an aqueous acidic solution containing nickel sulfamate in a concentration of at least 600 grams/liter, nickel chloride in a concentration of about 17 grams/liter, boric acid (H in a concentration of about 45 grams/liter, saccharin (acting as a stress reliever) in a concentration of about 2.2 grams/liter, and silicon carbide particles in a concentration of about grams/liter, the latter having a fine particle size of about 3 microns and substantially spherically shaped.

b. continuously introducing air into said solution at a plurality of stations disposed uniformly throughout a bottom horizontal zone of said solution, the flow rate of said air being controlled to be in the range of 75-l25 c.c./minute/cm to agitate the ingredients of said solution and prevent laminar conditions or phase variation therein.

c. applying an electrical potential between a cathodically disposed article and a nickel containing anode assembly in said solution to provide a current density at said article of at least 200 amps/ft and preferably at 400 amps/ft during a predominant later stage of said deposition.

d. continuing said deposition until a composite coating of at least 25 mils is obtained in a time period no greater than l80 minutes.

A typical apparatus mode for carrying out the present invention may comprise a solution tank in which is disposed a series'of stacked rotor housings which have been previously cast from aluminum metal. The rotor housings have an internal epitrochoid surface and flat side walls so that it is convenient to separate them by electrically inert material such as plastic sheets and to protect the exposed top and bottom walls of the stack by means of aluminum plates coated with silicone rubber to eliminate deposition thereon. The stacked housings may be carried on a harness for movement into i and out of the solution. The housings are each connected by suitable means to an electrical potential so housings in a predetermined spaced relation no greater than 0.75-l.0 inches. Masking bands may be mounted about the sleeve'to block current throw at intervals between housings or around edges to promote a uniform coating thickness on the cathode epitrochoid surface. The anode assembly may be constructed as a titanium mesh basket wherein the sleeve is constituted of expanded sheet metal or-woven wire; nickel pieces are ar- 1 ranged in intimate contact with each other within the basket. The titanium mesh basket is placed in contact with a suitable voltage source tooperate as a current transfer agent, being anodically inert due to the development of an oxide coating. The nickel piecesin contact with the mesh basket thus become the anode while the basket serves as a constant anode area.

BATH MAKEUP The bath makeup should principally and preferably be comprised of an aqueous solution of nickel sulfamate Ni (NH- S0 having a concentration of at least 600 grams per liter, a concentration of silicon carbide particles of 100-150 grams/liter.

Satisfactory nickel sulfamate compositions are available commercially, for example, as M&T sulfamate from Metals and Thermite of Matawan, N.J., or as Bar- I rett sulfamate from Allied Research Company of Baltimore, Md. Any commercially available nickel sulfamate which is controlled in the amounts of contaminate present by prior substantial removal of iron and similar metals can be satisfactorily used in the sulfamate plating bath of this invention.

The sulfamate plating bath should also contain angrams/liter, preferably 17 grams/liter. An ionizable metal salt, such as one or more of sodium chloride. sodium fluoride, sodium bromide, potassium chloride, potassium bromide. and the like, (Ni (31 -611 0) can be used to supply the necessary ingredient. Nickel chloride is a particularly preferred ionizable metal salt for use in the sulfamate bath of the present process. The function of the ionizable salt is to promote smooth dissolution of the consumable, nickel anode pieces. If the anode pieces are not reacted, the current density of the anode cannot be increased beyond 200 amps/ft without causing the detrimental evolution of gas.'ln order to plate an article at the cathode current densities required of this invention, the surface area of the anode would normally have to be increased two or vthree times beyond'the'surfaee area of conventional anode assemblies. It will be readily appreciated that this is difficult to arrange where substantially conforming anodes are needed. Nickel chloride further increases the stress in the deposits formed. Nonetheless, the concentration of the nickel chloride should be kept as low as practicable because coatings formed at the highest currentdensi ties are under considerable stress already and the increase in the stress is not of such great importance.

Boric acid (H 80 should also be present in the sulfamate plating bath of this invention in a concentration of between 37-45 grams/liter, and preferably about 45 grams/liter. Boric acid acts as a buffer in the plating solution and ifpresent in the required amount, is effective in providing a smooth, uniform and non-graining plated surface. l

A sufficient amount of a wetting agent'is added'tothe sulfamate plating solution to regulate the surface tension of thebath between about-2O and dymes/centimeter Baths having a surface tension in this range have been found to be more amenable to proper agita- .tion duringelectroplating and therefor allow; the production of a 'more uniform composite. A particularly helpful wetting agent is saccharin or sodium lauryl sulfate in the range of 1.7-2.7

V grams/liter or preferably about 2.5 grams/liter.

The pH of the sulfamate plating bath must be mainacid; boric acid, or nickelcarbonate may be added to i the plating bath, as needed to maintain the proper operating range. V

Nickel sulfamate plating solutions provided in accordance with this invention have certain additional advantages over known plating solutions. n particular, the throwing power of the electrolyte is superior to that s of other solutions. A method for measuring throwing 1 other ionizable metal salt in a concentration of l5l9f powerutilizes a hull cell described in the paper entitled if a dominant nickel sulfamate bath isnot used. the r V nickel salt should be selected from the group consisting of nickel sulfate, nickel chloride, nickel fluoroborate, nickel sulfamate, and any mixtures thereof.

It is important to avoid metallic powders in these baths, for example, poorly cast niekel'anodes which might powder during the use and dispersed nickel particles in the bath definitely can cause roughness and also high concentrations of activated carbon bath cause very undesirable roughness. Carbon from rolled or cast nickel anodes, however, does not usually cause roughness when floating in the baths. High concentrations of iron dissolved as ferrous or ferric iron in the baths do cause settling roughness or gross pitting effects in the baths at pH values of 3.8 to 5.5. Zinc or cadmium ions can be present in the baths in concentration as high as about 0.02 grams per liter without detrimentally effecting the plating. The presence of sodium and magnesium salts are not harmful.

The selection of the particular additive particles will, of course, be governed by the characteristics desired in the final product. Where the primary concern is the improvement of wear-resistance, and abrasion resistance,

and the like, materials such as silicon carbide, aluminum oxide, tungsten carbide, titanium carbide, zirconium oxide, boron carbide, chromium carbide, thorium oxide or uranium oxide, may be employed. The shape should preferably be acicular or spherical and of a particle size no greater than 400 mesh or 3 microns. The concentration of the additive particles must be in the range of lOO-l50 grams/liter.

The deposition of the additive particles under the surface of the cathodic particle to be coated does not require any cataphoretic action, but it is due solely to the settling of the particles through the body of the liquid and on to the article surface. Since the voltage employed in this and conventional electroplating operations is usually on theorder of 12 volts or less, the voltage gradients present in the electrolyte are insufficient to cause any cataphoretic deposition of the additive particles.

CURRENT DENSITY It has been found that as the concentration of the nickel salt, nickel sulfamate, is varied, the maximum current density and the internal stress in the deposit both vary. As the concentration of nickel sulfamate is varied from 550 up to 900 grams per liter, the maximum current density that may be applied rises substantially to as much as 400 amps/ft in the range of 600-700 grams per liter and then'falls again.

The higher the cathode current density the greater is the speed of deposition. But under given plating conditions, there is a practicai maximum current density above which so-called burning occurs. The value of this practical maximum current density depends not only on the temperature of operation but also on the rate of agitation and the deposition of the anode and the cathode in the plating bath. A typical maximum current density for the prior art is about 200 amps/ft for a plating solution containing about 300 grams/liter of nickel sulfamate at a temperature of about 60C. in accordance with this invention a maximum current density is about 400 amps/ft for a plating solution containing about 600-700 grams/liter of nickel sulfamate at a temperature of about 160F. Initially the plating sequence may use only about 200 amps/ft but the predominant portion of the plating process can be carried out ator close to the maximum current density.

The temperature of operation has a material affect on the maximum current density as well as upon the internal stress. If the temperature is reduced from 65C to 50C, the internal stress moves rapidly in the tensile direction. Below 50C, tensile stress in the deposit is so high and the maximum practical current density is so low that the use of the solution offers no significant advantage over conventional solutions. However, as the solution is heated to higher temperatures, the maximum current density is accordingly raised, with a compressive stress which is higher but the tensile stress being lower. This phenomenon can be employed in electrodeposition or electroforming since the electroform product should have a low internal tensile stress if it is to have a dimension stability required for many purposes such as a uniform coating on the inside of an epitrochoid surface. The required bath temperature herein is between 7l-77C or preferably about 160F.

PRODUCT The resulting product of practicing the above method will produce a dispersion coating having certain unique features. A high silicon carbide content in the range of 35% by weight will be assured. The distribution, density and character of the dispersed silicon particles will be of a highly desirable type as exhibited in the photomicrograph of FIG. 2. This is in contrast to the randomness and heterogeneity of the silicon carbide particles when processed according to the prior art, as shown in FIG. 2. For this latter exhibit, the bath concentration was about 320 grams/liter of nickel sulfamate, silicon carbide in a concentration of about grams/liter, no independent bath agitation, and a cathode current density below 200 amps/ft in some embodiments, a substantially pure nickel strike may be deposited on the cast aluminum substrate in advance of depositing the dispersion coating thereover, such as shown in FIG. 2.

Tests were performed to compare engine wearresistance of composite coating herein as a function of the percentage weight or volume of silicon carbide in the end product. When the bath contained between 86-97 grams/liter of silicon carbide, the end product had an average carbide weight of 3.3% and an average volume content of 9.5%. After hours of engine test work, considerable chatter (grooves in the coating) and gouling occurred with the electroplated coating. When the bath contained between 100-112 grams/- liter, the average carbide weight was 4.3% and the average volume 11%. Engine wear tests showed such a coating to have good wear resistance. When the bath contained about -150 grams/liter, the average carbide weight was in excess of 4.7% and the average volume was in excess of 12.1%. At the end of 100 hours of engine testing, there was perceivable wear and no measurable wear was observed at 400 hours of testing. Thus, the latter concentration must be considered superior.

We claim as our invention:

1. A method for electrolytically depositing a highly uniform wear-resistant dispersion coating onto a cast substrate, comprising:

a. providing an aqueous acidic solution containing at least one nickel salt selected from the group consisting of nickel sulfate, nickel chloride, nickel fluoroborate, and nickel sulfamate and any mixtures thereof, said salt being present in a concentration of at least 600 grams/liter, said solution containing also a suspension of inert wear-resistant particles in a concentration of about l50 grams/liter and a prec. applying an electrical potential between a nickel 7 containing anode assembly and said cast substrate to provide a current density at said cast substrate of about 400 amps/ft? 2. The method of claim 1, in which said aqueous solution being particularly comprised of at least 600 grams/liter of nickel sulfamate, about l7 grams/liter of nickel chloride, about 45 grams/liter of boric acid, and about 150 grams/liter silicon carbide, said substrate is defined as an epitrochoid' surface and said anode assembly is substantially conforming: in configuration to said surface with spacing therebetween being extending up to 1.0 inch as a result of the increased throwing powerof said solution at said high current densities.

3. The method of claim I, in which said step (c) is carried out for a period of time no greater than 180' minutes with a deposited dispersion no less than 25 mils thick. g

4. The resulting product of practicing the method of claim 1 in which the content of inert particles in said dispersion coating is homogeneous and yet in the range of 35%' by weight and the volume in the 'range'of 5. The resulting product as in claim 4, in which said dispersion coating is effective toJexperience no greater than 0.0005 inches of wear over a period of 100 hours i of engine performance when said substrate is the rotor housing of a rotary internal combustion engine.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 5 Patent No. 3 891 542 Dated June 24 197 Leonard G. Cordone et al. Page 1 of 3 Inventor s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Add Figures 1 and 2, as part of Letters Patent 3,891,542,

as shown on the attached sheets.

Signed and Scaled this Fifth Day of Apn'l1977 [SEAL] Attesi:

RUTH C. MASON C. MARSHALL DANN Anesn'ng Officer Commissioner oflarents and Trademarks Patent No. 3,891,542 Page 2 Page 3 Patent No. 3,891,542

A/Zm/J/Ww

Claims (5)

1. A METHOD FOR ELECTROLYTICALLY DEPOSITING A HIGHLY UNIFORM WEAR-RESISTANT DISPERSION COATING ONTO A CAST SUBSTRATE, COMPRISING: A. PROVIDING AN AQUEOUS ACIDIC SOLUTION CONTAINING AT LEAST ONE NICKEL SALT SELECTED FROM THE GROUP CONSISTING OF NICKEL SULFATE, NICKEL CHLORIDE, NICKEL FLUOROBORATE, AND NICKEL SULFAMATE AND ANY MIXTURES THEREOF, SAID SALT BEING PRESENT IN A CONCENTRATION OF AT LEAST 600 GRAMS/LITER, SAID SOLUTION CONTAINING ALSO A SUSPERSION OF INERT WEAR-RESISTANT PARTICLES IN A CONCENTRATION OF ABOUT 150 GRAMS/LITER AND A PREDOMINANT PARTICLE SIZE ON GREATER THAN 3 MICRONS AND HAVING A SHAPE WHICH IS SPHERICAL OR ACICULAR SO AS TO DESPERSE QUICKLELY THROUGH SAID SOLUTION, SAID SOLUTION BEING MAINTAINED AT A TEMPERATURE NO LESS THAN 160*F, B. INTRODUCING AN INERT GASEOUS MEDIUM INTO SAID SOLUTION AT A PLURALITY OF STATIONS DISPOSED UNIFORMLY THROUGHOUT A BOTTOM HORIZONTAL ZONE OF SAID SOLUTION AND AT A FLOW RATE OF BETWEEN 75-125 C.C./MINUTE/CM2 TO TURBULATE SAID SOOLUTION IN A MANNER FOR PREVENTING LAMINAR CONDITONS IN SAID SOLUTION, AND C. APPLYING AN ELECTRICAL POTENTIAL BETWEEN A NICKEL CONTAINING ANODE ASSEMBLY AND SAID CAST SUBSTRATE TO PROVIDE A CURRENT DENSITY AT SAID CAST SUBSTRATE OF ABOUT 400 AMPS/FT2.

2. The method of claim 1, in which said aqueous solution being particularly comprised of at least 600 grams/liter of nickel sulfamate, about 17 grams/liter of nickel chloride, about 45 grams/liter of boric acid, and about 150 grams/liter silicon carbide, said substrate is defined as an epitrochoid surface and said anode assembly is substantially conforming in configuration to said surface with spacing therebetween being extending up to 1.0 inch as a result of the increased throwing power of said solution at said high current densities.

3. The method of claim 1, in which said step (c) is carried out for a period of time no greater than 180 minutes with a deposited dispersion no less than 25 mils thick.

4. The resulting product of practicing the method of claim 1 in which the cOntent of inert particles in said dispersion coating is homogeneous and yet in the range of 3-5% by weight and the volume in the range of 10-15%.

5. The resulting product as in claim 4, in which said dispersion coating is effective to experience no greater than 0.0005 inches of wear over a period of 100 hours of engine performance when said substrate is the rotor housing of a rotary internal combustion engine.

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