WO1988008887A1

WO1988008887A1 - Stabilized electroless baths for wear-resistant metal coatings

Info

Publication number: WO1988008887A1
Application number: PCT/US1988/001517
Authority: WO
Inventors: Charles Edward Mccomas
Original assignee: Charles Edward Mccomas
Priority date: 1987-05-12
Filing date: 1988-05-10
Publication date: 1988-11-17
Also published as: JPH02503696A; EP0359784A1; AU2253588A; EP0359784A4; AU619486B2

Abstract

Electroless coating baths useful for the reductive deposition of nickel containing alloy coatings are stabilized by forming said baths to contain bath stabilizing concentrations of lead ions and tungstate ions in combination. Preferred coatings deposited from the present stabilized baths containing nickel and cobalt ions and a borohydride reducing agent exhibit high hardness and a wear-resistance unprecedented in electroless metal coatings.

Description

STABILIZED ELECTROLESS BATHS FOR WEAR-RESISTANT METAL COATINGS

Background of the Invention

This invention relates to an improved method for applying metal coatings which exhibit high hardness 'and exceptional resistance to corrosion and wear. More particularly this invention relates to electroless metal coatings comprising nickel and/or cobalt, and to the reductive deposition of said coatings on the surfaces of substrate articles from novel stabilized coating baths. The preferred coatings deposited in accordance with this invention are nickel-boron coatings characterized by hardness and wear-resistance unmatched by nickel boron or nickel/cobalt/boron coatings known in the art.

The plating or deposition of metal alloys by chemical or electrochemical reduction of metal ions on the surface of an article to modify its surface characteristics for both decorative and functional purposes is well known in the art. Of particular commercial significance is the deposition of metal alloy coatings on both metal and activated non-metal substrates to enhance surface hardness and resistance to corrosion and wear. Electroless nickel and cobalt alloy coatings deposited using hypophosphite or boron reducing agents are recognized in the art for their functional properties, including hardness and associated wear-resistance. The patent literature reflects an ongoing research and development effort in the area of electroless nickel/cobalt coatings with the goal of producing still harder, -more wear resistance coatings. See, for example, U.S. Patents 3,738,849; 3,045,334; 3,674,447; and 2,726,710.

One problem with so-called electroless coating baths is bath instability characterized by variable plating rates and premature random reduction of the metal ions in solution to form a black metallic precipitate. Practitioners have reported that both inorganic and organic bath adjuvants can help to alleviate problems of electroless coating bath instability. See, for example, bath stabilizers reported in U.S. Patents 2,762,723, 3,062,666,

3,234,031, and 3,753,667. The use of lead ions at low concentrations to promote stability of electroless metal coating baths is known in the art.

It has now been discovered that use of lead ions and tungsten-containing ions, in combination, in electroless coating baths provides exceptional bath stability and results in deposition of improved metal alloy coatings. More particularly, it has been discovered that metal alloy coatings when deposited from electroless plating baths stabilized with a combination of lead and tungsten-containing ions are consistently harder and more wear resistant than coatings of comparable composition described in the prior art. Electroless coating baths for deposition nickel and cobalt are described in the art. See, for example, U.S. Patents 3,378,400, 3,562,000, 3,715,793, 3,234,031, 3,674,447 and 3,342,338. U.S. Patents 3,562,000 and 3,715,793 exemplify deposition of metal coatings from baths containing cobalt chloride and nickel chloride, alone or in combination, using sodium hypophosphite. U.S. Patent 3,062,666 discloses that electroless metal coatings containing nickel, cobalt and boron are deposited with boron reducing agents from baths containing both nickel and cobalt salts. Electroless coating baths containing nickel chloride and cobalt chloride in combination with complexing agents and sodium borohydride has been described by Lang in Metalloberflache Vol. 19, pp. 257-262 (1965) at p. 259, footnote 4 and in Electroplating and Metal Finishing. March, 1966, pp. 86-96. The prior art provides no description of the improved coatings obtained from the novel baths stabilized in accordance with this invention. It is therefore a general object of this invention is to provide stabilized aqueous baths for electroless deposition of improved metal coatings. A further object of this invention is to provide an article of manufacture coated on at least a portion of its surface with a hard, wear and corrosion resistant electroless nickel and/or cobalt coatings deposited from electroless baths stabilized with salts containing lead and tungsten. Still a further object of this invention is to provide an electroless metal alloy coating presenting a wear-resistant surface comprising characteristic nodular deposits of nickel and/or .cobalt and boron. Yet another object of this invention is to provide improved, stabilized electroless coating baths from which a hard, wear and corrosion resistant coating can be deposited on at least a portion of the surface of a metal or activated non-metal substrate. Those and other objects of this invention will be apparent to those skilled in the art from the following summary and detailed description of the invention.

- Summary of the Invention

According to the present invention there is provided an improved method for stabilizing electroless metal coating baths, improved coating baths stabilized in accordance with that method, and an improved metal alloy composition comprising nickel or cobalt.

Electroless metal coating baths can be stabilized to minimize random reduction of solute metal ions and to enhance metal deposition efficiency and consistency by forming said baths to contain lead ions and tungsten-containing ions, preferably tungstate, in concentrations sufficient to promote bath stability. The alloy compositions deposited from said stabilized baths are particularly useful on a surface of an article of manufacture which is subject to sliding or rubbing contact with another surface under unusual wearing and bearing pressures.

Improved metal alloy coatings of the present invention comprise about 90 to about 99.5% nickel and/or cobalt and about 0.5 to about 10% boron or phosphorous. In a preferred embodiment the metal alloy coating composition of the present invention comprises about 92 to about 98 weight percent nickel, and about 2 to about 8 weight percent boron. Depending on plating bath composition the preferred coatings can also contain from trace amounts up to about 2% of cobalt, lead and/or tungsten. Such coatings are, as deposited from plating baths in accordance with this invention, remarkably hard, and they exhibit excellent corrosion and wear resistant.

In a preferred method embodiment of this invention metal coatings are applied to a substrate electrolessly by contacting the substrate with a coating bath containing nickel ions and/or cobalt ions, lead ions and a water-soluble form of tungsten, preferably tungstate anions, a metal ion complexing agent, and a borohydride reducing agent at pH about 12 to about 14 and at an elevated temperature of about 180 to about 210°F. Detailed Description of the Invention

One principal aspect of the present invention is the discovery that low concentrations of lead ions in combination with low concentrations of tungsten-containing ions in electroless plating baths provides improved bath stability and promotes deposition of alloy coatings having superior functional qualities at consistent, commercially acceptable deposition rates. Deposition of metallic coatings on suitable substrates is accomplished by contacting said substrates with a plating bath comprising an aqueous solution of nickel and/or cobalt salts, a metal ion complexing agent, bath stabilizers comprising lead and tungsten-containing salts, and a reducing agent.

In accordance with this invention an article of manufacture is coated on at least a portion of its surface with a hard, ductile, wear and corrosion resistant metallic coating comprising about 90 to about 99.5 weight percent nickel and/or cobalt, and about 0.5 to about 10 weight percent boron or phosphorous. Boron containing electroless coatings are produced where borohydride or amine-borane reducing agents are employed; phosphorous-containing coatings are produced from electroless baths employing a hypophosphite salt as the reducing agent. While preferred bath conditions, e.g. pH, temperature, metal ion complexing agents, etc., vary somewhat with the particular reducing agent selected for the electroless coating process (see above referenced prior art patents), electroless baths are generally known to exhibit some degree of instability evidenced by variable plating rates and precipitation of reduced metal. The- use of lead ions at a concentration of about 5 x 10 -6 to about 5 x 10-5 moles/gallon in combination with tungsten-containing ions at a concentration of at least 6 x 10~ moles/gallon, preferably in a concentration of about 5 x 10 to

5 x 10^" moles/gallon, has not only been found to enhance coating bath stability and efficiency, but also to impart improved coating characteristics. Particular advantage has been realized for electroless nickel baths utilizing borohydride reducing agents.

Lead tungstate is a preferred bath stabilizer in accordance with this invention and has been shown to be most effective when used in electroless coating baths at a concentration ranging from about 5 to about 15 milligrams/gallon of bath. It is expected though that the operable range of lead tungstate stabilizer concentration is from about 3 to about 25 milligrams/gallon of bath. Other sources of lead and tungsten-containing ions can be employed as described hereinbelow.

Suitable substrates for deposition of electroless coatings from the present stabilized baths are those with so-called catalytically active surfaces including those composed of nickel, cobalt, iron, steel, aluminum, palladium, platinum, copper, brass, chromium, tungsten, titanium, tin, silver, carbon, graphite and alloys thereof. Those materials function catalytically- to cause a reduction of the metal ions in the plating bath by the reducing agent and thereby result in deposition of the metal alloy on the surface of the substrate in contact with the plating bath.

Non-metallic substrates such as glass, ceramics and plastics are in general, non-catalytic materials; however, such substances can be sensitized to be catalytically active by producing a film of one of the catalytic materials on its surface. This can be accomplished by a variety of techniques known to those skilled in the art. One preferred procedure involves dipping articles of glass, ceramic, or plastic in a solution of stannous chloride and then contacting the treated surface with a solution of palladium chloride. A thin layer of palladium is thereby reduced on the treated surface. The article can then be plated or coated with the metallic composition in accordance with this invention by contact with a coating bath as detailed below. It is to be noted that magnesium, tungsten carbide and some plastics have exhibited some resistance to deposition of the present coatings. Further, where the electroless bath pH is high, aluminum must be pretreated according to art-recognized techniques.

A preferred coating bath for deposition of the present coatings comprises

(1) nickel ions, lead ions and tungsten-containing ions in the amounts indicated, expressed as moles per gallon of coating bath: nickel ions, about 0.4 to about 0.9; and lead and tungstate ions, about 5 x 10 to about 5 x 10^~5 each; (2) chemical means for adjusting the pH of the bath to between about 12 and about 14;

(3) a metal ion complexing (chelating) agent in an amount sufficient to inhibit precipitation of said ions; and (4) about 0.01 to about 0.05 moles per gallon of coating bath of a borohydride reducing agent. Cobalt ions can be substituted for the nickel ions or nickel and cobalt ions can be used in combination. When used alone or in combination the nickel/cobalt ions can be used at a level totaling about 0.4 to about 1.5 moles per gallon of bath. Deposition of cobalt is typically slower than nickel in baths stabilized in accordance with this invention. This is evidenced by the fact that a coating deposited from a bath containing a 4:1 ratio of nickel to cobalt ions was found to contain less than 1% cobalt.

The borohydride reducing agent can be selected from among the known borohydrides having a good degree of water solubility and stability in aqueous solutions. Sodium and potassium borohydrides are preferred. In addition, substituted borohydrides in which not more than three of the hydrogen atoms of the borohydride ion have been replaced can be utilized. Sodium trimethoxyborohydride [NaB(0CH₃),H] is illustrative of that type of compound. Sodium cyanoborohydride has been found to stabilize electroless coating baths utilizing other borohydride reducing agents (U.S. Patent 3,738,849). Art recognized amine borane reducing agents can also be used to effect reductive deposition of metal coatings in the improved baths of this invention.

The preferred coating bath utilizing sodium borohydride is prepared to have a pH of about 12 to about 14. Best results have been observed when the pH of the bath is maintained during the coating process within that range and more preferably at about pH 13.5. Adjustment of bath pH can be accomplished by addition of any of a wide variety of alkaline salts or solutions thereof. Preferred chemical means for establishing and maintaining bath pH are the alkali metal hydroxides, particularly sodium and potassium hydroxide, and ammonium hydroxide. Ammonium hydroxide offers an additional advantage .in that the ammonium ion can function to assist metal ion complexation in the coating bath.

Due to the high alkalinity of stabilized borohydride coating baths in accordance with this invention, a metal ion complexing or sequestering agent i_s required in the bath to prevent precipitation of the nickel and/or cobalt hydroxides or other basic salts. Importantly, too, the metal ion complexing agent functions to lower metal ion reactivity; the complexed or sequestered metal ions have minimal reactity with the borohydride ions in the stabilized bulk solution but do react at the catalytic surfaces of substrates in contact with the solution. The term catalytic surface refers to * the surface any article composed of the aforementioned catalytic materials or to the surface of a non-catalytic material which has been sensitized by application of a film of said catalytic materials on its surface.

The complexing or sequestering agents (chelating agents) suitable for use in this invention include ammonia and organic complex-forming agents containing one or more of the following functional groups: primary amino, secondary amino, tertiary amino, immino, carboxy and hydroxy. Many metal ion complexing agents are known in the art. Preferred complexing agents are ethylene diamine, diethylene triamine, triethylene tetramine, the organic acids, oxalic acid, lactic acid, citric acid, tartaric acid and ethylenediamine tetraacetic acid and nitrilotriacetic acid, and the water soluble salts thereof. More preferred for use in the present preferred coating bath are ethylene diamine, the water soluble salts of tartaric acid, ethylenediamine tetraacetic acid, nitrilotriacetic acid, and ammonia and combinations thereof. Ethylene diamine tetraacetic acid salts have been particularly useful alone and in combination with ethylenediamine for solubilizing marginally soluble stabilizer salts such as lead tungstate and maintaining in solution lead and tungstate ions derived from soluble salts containing those ions. About 2 to about 8 moles of complexing agent are used per gallon of coating bath. Best results have been obtained when about 3 to about 5 moles of complexing or sequestering agent is used for each gallon of coating bath.

The nickel, cobalt, lead and tungsten or tungstate ions in the coating bath are provided by the addition to the bath of the respective water soluble nickel, cobalt, lead and tungsten-containing salts. Any salts of those metals having an ion component which is not antagonistic to the reductive coating process is suitable. For example salts of oxidizing acid such as chlorate salts are not desirable since they will react with the borohydride reducing agent in the bath. Cobalt, nickel, and lead chlorides, sulfates, formates, acetates, and other salts whose anions are substantially inert with respect to the other ingredients in the alkaline coating bath are satisfactory. Sources of soluble tungsten include tungsten-containing compounds, preferably alkali metal or ammonium tungstates, other tungstate salts such as lead tungstate and tungstic acid. Lead tungstate is preferred as a source for both the lead and tungsten stabilizing components of the baths in accordance with this invention. Preferred concentration for the lead and tungsten-containing ions for bath stability is between about 1 x 10 —5 to about 3.5 x 10-5 moles/gallon of each ion. For the preferred stabilized electroless nickel bath using a borohydride reducing agent lead and tungstate ions are used at a concentration of about 2.3 x 10 -5 moles/gallon. Both stabilizer and reducing agent are added to the bath periodically, typically at

1/2-hour intervals, to maintain a constant «coating deposition rate. Adds are made assuming that concentration of those components has diminished to near zero levels at the time of each add.

If desired, concentration of reducing agent and stabilizer ions, as well as the nickel and/or cobalt ions in solution can be monitored using art-recognized quantitative analytical techniques. It has been found, however, that satisfactory bath performance can be achieved simply by making periodic additions of stabilizer and reducing agent. Deposition rates can be controlled somewhat by concentration of reducing agent. Thus in the above-described preferred bath embodiment of this invention a uniform deposition rate of 1 mil per hour can be maintained with sodium borohydride adds equivalent to about 1 gram/gallon every 30 minutes with

_5 addition, too, of about 2.3 x 10 moles of lead tungstate/gallon. A plating rate of 0.5 mil/hour can be maintained with the addition of 0.6 grams/gallon of

_5 sodium borohydride and 2.3 x 10 moles/gallon lead tungstate every 30 minutes. The ratio of bath volume

(cm 3) to plated surface area (cm2) is typically greater than about 2.5.

Only small amounts (less than about 1-2% total) of lead and tungsten has been detected in the coatings deposited from the preferred stabilized baths in accordance with this invention. Nonetheless, it has been found necessary in the experiments conducted to make adds of stabilizer salt(s) with the reducing agent to assure.bath stability.

The preferred coating bath is typically prepared by forming an aqueous solution of the appropriate amounts of nickel and/or cobalt salts and the stabilizer salts, adding the complexing agent(s), adjusting the pH, heating to about 195°F, filtering and finally, immediately before introducing the substrate into the bath, adding the required amounts of sodium borohydride (typically in aqueous alkaline solution) .

The article to be coated or plated using a bath in accordance with this invention is prepared by mechanical cleaning, degreasing, anode-alkaline cleaning, and finally pickling in an acid bath in accordance with the standard practice in the metal-plating art. The substrate can be masked if necessary to allow deposition of the metal alloy coating only on selected surfaces. Although the present coatings in general exhibit excellent adhesion to properly prepared substrate surfaces, in instances where coating adhesion is critical or where some adhesion problems are experienced, coating-adhesion can often be enhanced by depositing a nickel strike electrochemically on the substrate surface prior to applying the present coating. The cleaned or otherwise surface-prepared article is immersed in the hot (about 180 to about 210°F) coating bath to initiate the coating process. The process is continued until deposition of the coating has progressed to the desired thickness or until the metal ions are depleted from solution. Of course, deposition rates vary with the conditions of the process and range from about .1 mil (1 mil = one one-thousandth of an inch) to about 1 mil per hour. Some stabilizer salts, such as thallium salts, when used in electroless coatings baths are incorporated as a notable component of the electroless coating. Such salts can be used in the present baths in addition to the specified stabilizer salts of lead and tungsten to add to bath stability and to further modify coating functional performance characteristics. Where "platable" stabilizer salts are utilized, coating consistency will require systematic adds of such stabilizer salts along with the present stabilizer salt(s) and the reducing agent.

The timing of the need to replenish the present coating baths with reducing agent as well as any stabilizer salts depends on the ratio of coating bath volume to the surface area being coated. Thus, for example, replenishment of borohydride to the present preferred coating baths may not be required where but small surface areas are being treated. One gallon of bath prepared in accordance with the preferred embodiment of the present invention will coat approximately 700 square inches to a thickness of 1 mil where the bath is replenished in accordance with the above description with borohydride as that component is depleted from solution. Borohydride concentration in the coating bath can be monitored as a function of bath plating rate or by using art-recognized bath titration techniques.

Of course, the concentration of cobalt and nickel ions in the plating bath can also affect plating rate and coating composition. Coating baths depleted of nickel and cobalt ions can be replenished with added nickel and cobalt salts respectively on an intermittent or gauged-continuous basis. The frequency rate of replenishment of bath components is a function of the ratio of surface area being plated to bath volume.

"Adds" must be made more frequently or at higher rates on a continuous basis as the ratio of bath volume to plated surface is decreased.

The practical aspects carrying out electroless coating processes are well known in the art. Such processes are disclosed generally in U.S. Patents 3,338,726 issued-to Berzins on August 19, 1967; 3,096,182 issued to Berzins on July 2, 1963; 3,045,334 issued to Berzins on October 1, 1958; 3,378,400 issued to Sickles on April 16, 1968; and 2,658,841 issued to

Gutzeit and Krieg on November 10, 1953; the disclosures of which are hereby incorporated by reference.

The preferred electroless metal alloy coatings of the present invention exhibit unprecedented hardness and concomitant wear resistance. At the same time they exhibit surprising ductility allowing the coating to flex with the substrate while maintaining a strong bond to the coated material. Also, the present coatings are substantially nonporous and exhibit good corrosion resistance.

The electroless metal alloy coatings of this invention present a wear and corrosion resistant surface comprising hard, nodular deposits of metal alloy. Hardness of the present coatings can be increased by heat treatment of the coated articles. Heat treatment is accomplished at a temperature of about 375 to about 750°F for a period of about one to about 24 hours. Shorter times, about one to two hours, is preferred for the higher temperatures of between about 550-750°F while longer heat treatment times have been shown to be advantageous at the lower temperature ranges of between about 375 to about 450°F.

The metal alloy coatings prepared in accordance with the preferred embodiments (containing nickel and boron) are in the form of hard, nodular deposits. The nodular deposits are believed to be amorphous as deposited from the electroless coating bath. With heat treatment in accordance with the above description, crystalline domains of metal borides selected from nickel boride and cobalt boride are dispersed in the amorphous metal alloy matrix. The reason for the exceptional hardness of the present coating compositions is still unknown. However, as with other nickel/cobalt-boron coatings the formation of hard crystalline domains of metal borides within the nodular structures is believed to be generally responsible for the high hardness levels in the present heat-treated coati-ngs. As plated, the preferred coatings of this invention have a Knoop hardness (100 gm load) between about 850 and about 950. Heat-treated coatings in accordance with the present invention have been found to have a Knoop hardness value of between about 1330 and about 1375. These values are 15-20 percent higher than the best hardness values reported in the art for nickel boron electroless coatings.

The present coatings have a wide range of applications which will be recognized by those skilled in the art. They have particular utility for coating surfaces of articles which under normal use are subjected to highly abrasive, rubbing, or sliding conditions under high temperatures/pressures. Such high wear conditions are found at many points in construction of tools, internal combustion engines including gas turbine engines, transmissions and in a wide variety of heavy equipment construction applications.

The following examples provide details of bath compositions, process conditions, and coating compositions and properties representative of the present invention. The examples are illustrative of the invention and are not in any way to be taken as limiting the scope thereof. EXAMPLE 1

A plating bath was prepared as follows: 160 ml of a solution containing a 4:1 mixture of nickel chloride and cobalt chloride at a concentration of 6 lbs of salt/gallon was combined with a mixture of 180 ml of ethylenediamine (EDA) and 60 gm of ethylenediamine tetraacetic acid (EDTA) . Sodium hydroxide (160 gm) was added and the resulting solution was diluted with deionized water to a volume of 1 gallon.

Lead tungstate stabilizer solution: 10 gm of lead tungstate was dissolved in one gallon of deionized water containing about 10 gm of ethylenediamine tetraacetic acid disodium salt. Sodium borohydride solution: 0.8 lbs (363 gm) of sodium borohydride was dissolved in one gallon of a solution prepared by dissolving 2-1/2 lbs (1135 gm) of sodium hydroxide in one gallon of deionized water.

The plating bath was heated to 192°F and with stirring; 6 ml of sodium borohydride solution and 4 ml of lead tungstate solution were added. Twelve metal plating coupons and two 1 inch x 3 inch rate plates were suspended in the bath, and the plating rate was monitored over a four-hour period with 6 ml of borohydride solution and 4 ml of lead tungstate solution being added to the bath every 30 minutes. The bath plated at a consistent rate of 0.5 ml/hour throughout the four hour experiment. The rate plates were heat treated at 725°F for 90 minutes and tested for hardness with a 100 gm load tester: Average Knoop Reading (8 readings) 1365.75.

EXAMPLE 2

The bath from Example 1 was filtered and replenished with 25% additions of nickel/cobalt chloride solution, EDA and EDTA. The same procedure was followed: adds of 6 ml NaBH 4. solution and 4 ml of

PbWO. solution every 30 minutes. Plating rate was constant over 4-1-/2 hours at 0.5 ml/hour.

As-plated hardness - Average Knoop Reading: 897.25 Heat treated hardness - Average Knoop Reading:

1373.9

EXAMPLE 3

(A) The same procedure was followed as in

Example 1 except 10 ml of NaBH. solution and 4 ml of PbWO. solution was added to the bath every 30 minuets. Plating rate was 1 ml/hour and consistent over a 4 hour plating period. (B) The bath from (A) above was filtered and replenished with 50% additions of cobalt/nickel chloride solution, EDA and EDTA. Following the same add schedule as in (A) above, the plating rate held at a constant 1.0 mil/hour throughout a 5-hour plating run. EXA PLE 4

Using the same proportions described in Example 3 a 75-gallon plating tank was prepared with plating solution, heated with constant filtration/agitation. Plating was initiated by adding 220 ml of PbWO. solution and 550 ml of NaBH. solution initially and after each 30 minutes during the plating operation. Plating rate was 1.0 ml/hour throughout a 5-hour plating operation. A Falex Ring and Block Alpha Wear Test (ASTM G77) was performed by an independent test facility on the coating after heat treatment at 725°F for 90 minutes: Change of mass of ring (10,800 cycles) was -0.0004 gm; change of mass of block (10,800 cycles): -0.0037 gm.

EXAMPLE 5

(A) The procedure of Example 1 is followed using nickel chloride at .7 moles/gal in place of the combination of nickel and cobalt chlorides to provide a stabilized bath which deposited a nickel-boron coating at a uniform plating rate. The coating exhibits excellent hardness and wear-resistance. (B) The procedure of Example 1 is followed using cobalt chloride at .9 moles/gal in place of the combination of nickel and cobalt chlorides to provide a cobalt-boron coating at reduced yet stable plating rates with excellent hardness and wear-resistance. EXAMPLE 6

One gallon of plating bath was prepared to have the following composition Nickel sulfate: 0.50 moles/gal

Sodium hypophosphite: 1.0 mole/gal

Lactic acid: 2.0 moles/gal

Propionic acid: 0.1 moles/gal

Lead chloride: 2.3 x 10~ moles/gal Sodium tungstate: 4.0 x 10^~ moles/gal

The bath is used to plate a clean steel workpiece at a bath temperature of 200°F. After 30 minutes in the bath the workpiece is provided with a uniform nickel-phosphorous alloy coating.

EXAMPLE 7

The procedure of Example 1 is repeated with the exception that the stabilizer solution consists of two separate solutions, one containing lead acetate at 6 x

_3 10 moles/liter and the other containing a solution

_2 of tungsten iodide at 1 x 10 moles/liter in concentrated sodium hydroxide solution. Equivalent volumes of each of the stabilizer solutions are added to the bath in volumes corresponding to the volumes of lead tungstate solution used in Example 1. EXAMPLE 8

Example 7 is repeated using stabilizer solutions of lead citrate and tungsten oxychloride (in 10% NaOH solution) at concentrations of

5 x 10~ moles/liter and 6 x 10~ moles/liter, respectively.

While there has been described what are at present considered to be certain preferred embodiments of this invention, it will be understood that various modifications can be made therein, and it is intended to cover in the appended claims all such modifications as fall within the spirit of the invention.

Claims

What is claimed is:

1. In a method for depositing an electroless metal alloy coating on a substrate in a heated aqueous batjh comprising metal ions selected from nickel and cobalt, a metal ion chelating agent, a reducing agent capable of reducing said metal ions at the surface of the substrate, the improvement which comprises preparing said aqueous bath to contain about 5 x 10~ to about

_5 5 x 10 moles each of lead ions and tungsten-containing ions per gallon of bath to enhance bath stability and to improve metal alloy coating hardness and wear-resistance.

2. The improved method of claim 1 wherein the reducing agent is a boron reducing agent selected from borohydrides and amine-boranes.

3. The improved method of claim 2 wherein the bath is prepared to contain about 1 x 10 —5 to about 3.5 x 10 -5 moles each of lead ions and tungstate ions per gallon of bath.

4. The improved method of claim 3 wherein the bath is prepared to contain about 0.4 to about 0.9 moles of nickel ions per gallon.

5. The improved method of claim 1 wherein the reducing agent is a hypophosphite salt.

6. The improved method of claim 3 wherein lead tungstate is added to the bath to provide the lead and tungstate ions.

7. The improved method of claim 6 wherein the lead tungstate is added to the bath in aqueous solution with a lead chelating agent.

8. The improved method of claim 7 wherein the metal chelating agent comprises a salt of ethylenediamine tetraacetic acid.

9. An electroless metal coating deposited in accordance with the improved method of claim 1.

10. An electroless metal coating deposited in accordance with the improved method of claim 2.

11. An electroless metal coating deposited in accordance with the improved method of claim 6.

12. A stabilized electroless metal bath comprising an aqueous solution of metal ions selected from the group consisting of nickel ions and cobalt ions, a reducing agent, a metal ion complexing agent, lead ions at a concentration of about 5 x 10~ to

_5 about 5 x 10 moles/gal and tungsten-containing ions at a concentration of at least about 5 x 10~ moles/gal.

13. The electroless metal bath of claim 12 wherein the reducing agent is a borohydride or an amine-borane.

14. The coating bath of claim 13 wherein the borohydride reducing agent is selected from the group consisting of sodium borohydride, potassium borohydride, sodium trimethoxyborohydride, and potassium trimethoxyborohydride.

15. The coating bath of claim 12 wherein the metal ion complexing agent comprises a compound selected from water soluble salts of tartaric acid, citric acid, and oxalic acid, ethylenediamine, diethylenetriamine, triethylenetriamine, ethylenediamine- tetraacetic acid, nitrilotriacetic acid and ammonia.

16. The method of claim 15 wherein the metal ion complexing agent comprises ethylenediamine «nd ethylenediamine tetraacetic acid.

17. The electroless metal bath of claim 14 wherein the metal ions ae nickel ions and the tungsten-containing ions are tungstate ions.

18. The electroless nickel bath of claim 17 wherein the source of lead and tungstate ions is lead tungstate.

19. The electroless nickel bath of claim 17 wherein the concentration of lead ions and tungstate ions is each about 1 x 10 -5 to about 3.5 x 10-5 moles/gal each.

20. The electroless nickel bath of claim 19 wherein the metal chelating agent comprises ethylenediamine and ethylene-diaminetetraacetic acid.

21. The electroless nickel bath of claim 20 containing lead tungstate at a concentration of about 2.3 x 10 -5 moles/gal.

22. The electroless nickel bath of claim 21 containing, in addition, about 0.05 to about 0.4 moles of cobalt ions per gallon of bath.

23. An article of manufacture comprising a substrate and a wear resistant metal alloy coating on at least a portion of the surface of said substrate, said coating being deposited on the surface of said substrate from a coating bath in accordance with claim 12.

24. An article of manufacture comprising a substrate and a wear resistant nickel-boron coating on at least a portion of the surface of said substrate, said coating being deposited on the surface of said substrate from a coating bath in accordance with claim 17.

25. An article of manufacture comprising a substrate and a wear resistant nickel-boron coating on at least a portion of the surface of said substrate, said coating being deposited on the surface of said substrate from a coating bath in accordance with claim 22.

26. A method for depositing a hard, wear and corrosion resistant, electroless metal coating on a substrate which method comprises

(1) preparing an aqueous coating bath comprising nickel ions, lead ions and tungsten-containing ions at concentrations of about 0.4 to about 0.9, about 5 x 10^" to about 5 x 10^" , and at least 5 x 10^~ moles per gallon of bath, respectively; about 2 to about 8 moles of metal ion complexing agent per gallon of bath for complexing said ions to inhibit their precipitation from the bath during the coating process, chemical means for adjusting the pH of the bath to between about 12 and about 14, and about 0.01 to about 0.05 moles per gallon of bath of a borohydride reducing agent; (2) heating said bath to a temperature of about 180 to about 210°F; and

(3) contacting said substrate with the heated coating bath for a period of time sufficient for deposition of the coating on the surface of the substrate to a thickness of from about .2 mil to about 10 mil.

27. The method of claim 26 wherein the pH of the bath is adjusted to about 13.5 with a compound selected from alkali metal hydroxides and ammonium hydroxide.

28. The method of claim 26 wherein the temperature of the coating bath is about 185 to about 195°F during deposition of the coating on the substrate.

29. The method of claim 26 wherein the lead and tungsten-containing ions in the bath are provided by

-5 -5 the addition of about 1 x 10 to about 3.5 x 10 moles of lead tungstate.

30. The method of claim 29 wherein the metal ion complexing agent comprises ethylene diamine and ethylenediaminetetraacetic acid.

31. The method of claim 30 wherein the reducing agent is sodium borohydride and about 0.5 to about 1.5 grams of sodium borohydride in an aqueous alkali solution and about 4 to about 15 mg of lead tungstate are added on a per gallon basis to the coating bath periodically to maintain a suitable deposition rate during the deposition of the coating on the substrate.

32. In an electroless metal coating bath, the improvement which comprises forming said coating bath to contain lead ions and tungstate ions each at a concentration of about 5 x 10^" to about 5 x 10 moles/gallon.

33. The improved electroless metal coating bath of claim 32 containing lead tungstate at a concentration of about 3 milligr,ams/gallon to about 25 milligrams/gallon.

34. The improved electroless metal coating bath of claim 32 containing lead tungstate at a concentration of about 5 to about 15 milligrams/gallon.