US 2968578 A
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CHEMICAL NICKEL PLATING ON CERAMIC MATERIAL John M. Mochel, Painted Post, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York No Drawing. Filed Apr. 18, 1958, Ser. No. 729,245 Claims. (Cl. 117-54) Ag, Ni, Au, Co, Pd, Al, and Cu, by merely contacting such metal with a plating bath comprising an aqueous solution containing a salt of nickel or cobalt and an alkali metal hypophosphite in the presence of a sodium salt of a carboxylic acid and/or an ammonium salt. In such process the nickel ions in the solution are reduced to metal by the hypophosphite ions and deposited according to the reaction,
wherein the hypophosphite is oxidized to phosphite.
The chemical deposition of nickel and cobalt on nonmetallic surfaces such as synthetic plastics, hard rubber, Wood, quartz, glass, ceramics, or the like is also known from Patent No. 2,690,401. Such process requires first activating the non-metallic surface with a catalytic metal either by incorporating the finely divided metal into the composition of the article and removing the surface to lay bare the metal particles or by cleaning the surface by sanding, blasting, brushing, tumbling, chemical etching, etc. and dusting or blasting the cleaned surface with such metal in powder form or otherwise contaminating the cleaned surface with the metal. The activated surface is thereafter immersed in a hypophosphite plating bath containing a salt of the metal to be plated and preferably containing as a buffer two carboxyl groups for every metal ion.
In the latter process the activated surface may first be immersed in ,a preliminary plating bath having a pH of about 4.85 in order to produce a flash coating of metal thereon and once a metal plating on the surface of the material has been initiated or has proceeded to the point where the surface is covered with a continuous film of the metal of the plating bath, even a monomolecular layer, the conditions for continued plating are identical to those prevailing in the direct plating of the catalytic metals referred to above.
It has been proposed in Patent No. 2,690,402 to improve such process by activating the cleaned non-metallic surface by immersing it briefly in an aqueous solution of a palladium salt, rinsing it with water and then immersing it in a soluble chemical reducing agent, such as, hypophosphorous acid, hydrazine-hydrochloride, or hydroquinone, thereby reducing the palladium salt. As a result, sufiicient metallic palladium is deposited on the cleaned surface to activate it and cause the deposition thereon of nickel or cobalt when the activated surface is subsequently immersed in the plating bath.
While the prior processes are said to be capable of producing continuous films of nickel or cobalt on nonmetallic surfaces so treated, I have found it impossible by the disclosed methods to produce films of nickel or cobalt on silicate glass which are sufficiently uniform and adherent for purposes, such as the formation of soldered joints thereto. The conventional chemical etching of a silicate glass surface by aqueous hydrofluoric acid, referred to in the latter patent, results in a clean fresh surface but neither nickel nor palladium is sufficiently adherent thereto for subsequent plating with nickel and the formation of a soldered joint. The alternate treatment of such an etched surface with a solution of a salt of nickel or palladium and a solution of a reducing agent followed by immersion of the activated surface in a conventional nickel and hypophosphite plating bath results in metal films which adhere so slightly to the glass that they may be rubbed off, even by a stream of water.
Abraded surfaces also are ineifective for the production of a properly adherent metal coating. For instance, two strips of glass, when rubbed together with a mixture of 30 grit silicon carbide and powdered nickel between them, acquire a matte surface upon which nickel can be deposited locally in spots when the abraded and rinsed strips are immersed in a conventional nickel and hypophosphite plating bath. The coatings become loosened and separated from the glass when an attempt is made to solder thereto, even when the coated glass is previously heated to eliminate water at the glass-metal interface.
I have found that such non-uniformity and poor adherence of nickel and cobalt films produced on glass surfaces according to the prior methods are due to poor bonding of the metal to the glass caused by lack of an effective anchorage in the surface of the glass. To obtain a sufficiently strong anchorage I have discovered that, instead of being merely roughened. or chemically etched, the surface of the glass must be leached so as to provide it with a submicroscopically porous surface film consisting mostly of silica which is still a part of the molecular network.
Such a porous surface film is produced in accordance with the invention preferably by vapor-leaching the surface by exposure to HP vapor for 510 minutes, the temperature of the glass being high enough to prevent condensation of moisture thereon. Such condition may be obtained by preheating the glass to -250" C. before exposing it to the HF vapor and removing it from the vapor before it has cooled to the vapor temperature.
It is important that condensation of moisture on the surface of the glass during the vapor treatment be prevented, since this forms an aqueous solution of HF which dissolves the surface of the glass and prevents the formation of a porous film thereon. In the absence of such condensation it is believed that the constituents other than SiO in the surface of the glass are converted to fluorides which accumulate in a thin layer on the surface and may subsequently be washed off leaving the outer silicon atoms of the network as a submicroporous film. Hydration of the exposed silicon atoms with OH radicals is prevented by the fluoride ions which block the free silicon bonds. Hydration of the porous silica film would render it ineffective as an anchorage for subsequent metal plating.
Preferably the warm glass is supported or suspended in a covered vat or container composed of or lined with insoluble material such as a synthetic plastic or lead and containing a strong aqueous solution of hydrogen fluoride or other solution which will evolve HF vapor. Areas of the glass which are not to be plated may be protected with a coating of a resist such as wax or parafiin. An exposure of 5-10 minutes to the vapor is sufficient, although a longer exposure can also be used if desired. Leaching is continued until the porous film, after washing, is about 1000 angstroms thick, as may be determined by observing the order of its interference color. This is ordinarily first order yellow, although thicker films are also effective.
aaesms I have also found that phase-separable glasses or glasses which tend to form a soluble flux-containing phase and a. less soluble silica-rich phase when heat treated in their annealing ranges, alsomay be leached in suitable acid solutions. Such glasses are of the type known as boros'ilicates containing over 10% B and 55-85% SiO Inimany cases during the forming operations they undergo sufficient heat treatment to cause a phase separation at their surfaces sufficient to make them amenable to solution-leaching; however, if in their formation they are rapidly cooled as by being blown, a subsequent heating for 1 to 2 hours at GOO-700 C. is usually necessary to j nake their surfaces suitably leachable by this method. Such glasses may be leached by immersing them for at least one minute in an aqueous solution of a strong acid containing fluoride ions. By strong acid is meant an acid which is at least 10% ionized in a 1 normal aqueous solution, such as, the so-called mineral acids and fiuoboric acid. For this method, I prefer to use a 10% aqueous solution of fluoboric acid for at least one minute or a "15% aqueous solution of fluosilicic acid, preferably at about 45 C.','for at least /2 hour. In such solution-leachalso, the hydration of the porous leached surface film is'prevented by the presence of fluoride ions which unite withand block the silicon bonds which are broken by the'leaching.
In both methods the leaching is facilitated by first etching the glass by momentarily dipping it in aqueous hydro fluoric acid to remove and clean its surface.
When the surface of a glass, which has been leached in accordance with the invention, is alternately immersed in an aqueous solution of a salt of nickel, cobalt or palladium, and then in an aqueous solution of a suitable reducing agent, the metal ions which are adsorbed in the subrnicroporous surface are reduced in situ before they can diffuse away in the reducing solution. For best results, a rapidly acting reducing agent is necessary and I have found that a 1 -10%, and preferably 2% solution of one or more of the alkali metal borohydrides, KBH NaBH and LiBH4, is particularly suitable. *Only the borohydrides of potassium and sodium are readily available on the market. They are sufliciently soluble in water for the present purpose but an organic solvent, such as dimethyl ether of diethylene glycol may be used if desired. Lithium borohydride is not presently available and its reaction with water is objectionably strong, although it is more amenable in the organic solvent. It may be formed, however, in mixture with the borohydride of either potassium or sodium by adding 2 "to 4 weight percent of lithium chloride to a non-aqueous solution of either of the other borohydrides. Such a mixture is somewhat more effective than either the potassium or the sodium compound alone.
In carrying out the invention, the surface of the warmed glass is provided with a microporous surface by leaching and preferably by vapor-leaching with anhydrous HF as shown above. It is then rinsed with water and activated by alternately immersing it at room temperature, preferably first in an aqueous solution containing 2% by weight of a salt of a catalytic metal, such as a chloride of nickel, cobalt, or palladium and suflicient HCl or NH OH to provide a pH of 56, rinsing it with water and then immersing it in a 2% aqueous solution of a reducing agent, preferably KBH With larger amounts of the metal salt, about 4% of NI-I Cl may be added to form a complex with the nickel or cobalt salt so that the corresponding hydroxide will not be precipitated on contact with the alkaline reacting KBH since such precipitation interferes with the proper reduction and distribution of the metallic particles. One or more repetitions of such activation treatment is usually desirable to provide an adequate initial coating of the catalytic metal. In succeeding cycles, a pH less than 5 may cause resolution of some of the initially deposited metal and interfere with its uniform distribution. On the other hand, if
cool sn o 50 g. sodium hydroxyacetate and 10 g. sodium hypophosphite in water per liter, the pH being adjusted" to 44.5 with HCl.
The alkaline plating bath comprises 30 g. NiCl -6H O or CoCl -6H O, 100 g. sodium citrate, 50 g. NH Cl, and 10 g. sodium hypophosphite in water per liter, the pH being adjusted to 8-10 with NH OH. The strong ammonia fumes of the hot alkaline plating bath may be objectionable but it has good stability.
The nominal rate of deposition of the two plating baths is about 0.6 mil per hour and the adhesion of the metal coating to glass surfaces whichhave beenvapor leached and activated by alternate treatment with a salt of nickel cobalt or palladium and an'alkali metal borohydride, in accordance with the invention, is much greater than that of metal coatings on glass surfaces which have" not been so treated. l
Sudden local heating, as in soldering, of a metal film, even though made in accordance with the invention, may result in blistering and loosening of the film by the expansion of water vapor at the metal-glass interface. Such difficulty, however, may be avoided and the adhesion further improved by heating the plated glass at about 200 C. for about /2 hour to remove absorbed water from the interface. The metal coatings are readily wet by ordi nary soft solder even if the coating is so thin a s' t o be substantially transparent.
So as to demonstrate the superiority of the metal coatings deposited by the method of the inventionand their strong adherence and utility as a base for'soldered glass to metal joints, the following experiments were performed:
Several strips of borosilicate glass (composition B of Patent No. 1,304,623) /8 inch x inch x 6 inches long were broken at their longitudinal centers and the broken ends were dipped for a few seconds in 10% aqueous HF to clear them, rinsed with water and dipped for at least one minute into 10% fiuoboric acid and again rinsed with water. The acid treated ends of the glass strips were then dipped for a few seconds into an aqueous solution containing 2% of NiCl -6H O and 4% of NH Cl having a pH of 5 to 6 and then into a 2% aqueous solution of KBH rinsed with water and once again were dipped alternately in the two solutions as before and rinsed with water. As a result of such treatment the treated portions of the glass strips had become coated with thin practically transparent films of metallic nickel.
The metal-coated portions of the glass strips were then immersed in the above described acid plating bath at C. until the metal coating was 0.15 to 0.3 mil thick. Such thickness was determined by the fact that the electrical resistance of the coating attained a value of 1 to 2 ohms between probes spaced 1 inch apart.
The plated glass strips, after being rinsed with water, were dried and slowly heated up to 250 C. to eliminate moisture at the glass to metal interface. They were then tinned with half and half tin-lead solder in the usual manher while heated and butt joints were formed between the matched pairs of strips by flowing solder between them while in close juxtaposition. When the joined pairs were cool any external fillets of solder were scraped off. The strength of the joints and the. adhesion of the nickel plate to the glass were measured in pounds per s'quare inch by means of a Dillon Multilow Range Tensile Tester.
The method comprised supporting each individual jointed pair horizontally on knife edges spaced 3 /2 inches apart and loading it on 2 knife edges inch apart and centrally spaced from the lower knife edges, the loaded knife edges being astride the joint and evenly spaced therefrom. The average fiexural strength of 5 such joints was 2,617 p.s.1.
Although several attempts were made to produce similar soldered joints by the prior methods referred to above, metal coatings to which solder could successfully be applied could not be produced thereby.
What is claimed is:
1. In a method of chemically depositing nickel or cobalt on the surface of a silicate glass by chemically etching it, washing it, activating the etched surface by alternately immersing it in an aqueous solution of a catalytic metal selected from the class consisting of Ni, Co and Pd, and then in a solution of a reducing agent, washing it with Water and thereafter treating the activated surface with a conventional plating bath containing a salt of the metal to be deposited, hypophosphite ions and a salt of a carboxylic acid, the step in which a submicroporous siliceous film is formed on the chemically etched surface of the glass by exposing the surface of the glass to HP vapor for 5-10 minutes, the temperature of the glass being high enough to prevent condensation of moisture thereon.
2. The method of chemically depositing a strongly adherent film of nickel or cobalt on the surface of a silicate glass, which comprises leaching a submicroporous siliceous film thereon by exposing the surface of the glass to HP vapor for 5-10 minutes, the temperature of the glass being high enough to prevent condensation of moisture thereon, washing the leached surface with 'water, alternately immersing it in a solution of catalytic metal selected from the class consisting of Ni, Co, and Pd, and in a 1% to solution of an alkali metal boro-hydride, washing it with water and thereafter immersing it in a plating bath containing a salt of the metal to be deposited, hypophosphite ions and a salt carboxylic acid.
3. The method of claim 2 in which the alkali metal borohydride is KBH 4. The method of chemically depositing a strongly adherent film of nickel or cobalt on the surface of a phase separated borosilicate glass containing over 10% B 0 and 55-85% SiO which comprises leaching a submicroporous siliceous film thereon by immersing it in an aqueous solution of an acid which is at least 10% ionized in a 1 normal solution and which contains fluoride ions to extract non-siliceous constituents therefrom, Washing the leached surface with water, alternately immersing it in a solution of a catalytic metal selected from the class consisting of Ni, Co and Pd, and in a 1% to 10% solution of an alkali borohydride, washing it with water and thereafter immersing it in a plating bath containing a salt of the metal to be deposited, hypophosphite ions and a salt of a carboxylic acid.
5. The method of claim 4 in which the surface of the glass is leached by immersing it in a 10% aqueous solution of fluoboric acid for at least one minute.
6. The method of claim 4 in which the surface of the glass is leached by immersing it in a 15% aqueous solution of fluosilicic acid for at least /2 hour.
7. The method of claim 4 in which the alkali metal borohydride is KBH 8. In a method of chemically depositing nickel or cobalt on the surface of a silicate glass by chemically etching it, washing it, activating the etched surface by alternately immersing it in an aqueous solution of a catalytic metal selected from the class consisting of Ni, Co and Pd, and then in a solution of a reducing agent, washing it with water and thereafter treating the activated surface with a plating bath containing a salt of the metal to be deposited, hypophosphite ions and. a salt of a carboxylic acid, the step in which a submicroporous siliceous film is formed on the chemically etched surface of the glass, said glass consisting of a phase separated borosilicate glass containing over 10% B 0 and 85% SiO by immersing it in an aqueous solution of an acid which is at least 10% ionized in a 1 normal solution and which contains fluoride ions.
9. The method of claim 8 in which the surface of the glass is leached by immersing it in a 10% aqueous solution of fluoboric acid for at least one minute.
10. The method of claim 8 in which the surface of the glass is leached by immersing it in a 15% aqueous solution of fluosilicic acid for at least A hour.
References Cited in the file of this patent UNITED STATES PATENTS 2,327,978 Birdseye et al Aug. 24, 1943 2,617,742 Olson Nov. 11, 1952 2,690,402 Crehan Sept. 28, 1954 2,872,312 Eisenberg Feb. 3, 1959 OTHER REFERENCES Paul: Compt. Rend. Acad. Sci., vol. 232, February 12, 1951, 627-629, CA45:1043H.