Abstract:
The invention is an electroformed coating of nickel formed on yttria-stabilized tetragonal polycrystalline ceramic where a hydrofluoric acid etch is utilized with the component during processing to result in an adherent, dense nickel-rich coating. Control of the sensitizing, catalyzing, and reaction enhancement processes to about 90° C. provides improved nickel deposition properties.

Description:
[0001]     This application claims the benefit under Title 35 USC 119(e) of U.S. Provisional application 60/522,941 filed Nov. 23, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a material and method of plating nickel on zirconia ceramic.  
       DESCRIPTION OF RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR 1.97 AND 1.98  
       [0003]     Presented in  FIG. 1  is the known method of forming a component assembly  2  by placing an interlayer material  8 , preferably nickel metal foil, between a ceramic part  6  and a metal part  4  where a force  10  is applied to compress the metal foil while the assembly is heated to bond by brazing. See Fey, et al., U.S. Pat. 6,521,350, which is incorporated herein by reference in its entirety.  
         [0004]     It is well known to electrolytically plate nickel on various electrical conductors. Electroless plating of nickel is also well known, however, applying a thin and adherent coating of nickel on zirconia ceramic is not known. Obtaining a thin, adherent nickel coating on zirconia has been difficult to achieve using other known approaches. Such a coating would offer advantages in the assembly of intricate parts assemblies that will be bonded by brazing, where the presence of a nickel foil has been used to advantage.  
         [0005]     Once one recognizes that an adherent nickel coating of about 10 to 16 microns (i.e., 0.0004 to 0.0006 inches) thickness is desired to be placed on a zirconia ceramic part, then plating becomes a preferred approach. Electroless nickel is a nickel-phosphorus alloy, making direct comparisons with electrolytic nickel difficult. Differences between an electroless nickel deposit and electrolytic nickel deposit are due to the difference in composition of the deposit and to inherent differences between chemical and electrolytic reduction.  
         [0006]     A principal advantage of electroless nickel solutions over electrolytic nickel solutions is the lower loss due to the low concentration of nickel in electroless solutions. Absence of anodes in electroless nickel plating eliminates a source of electrolyte contamination which is present in electrolytic plating.  
         [0007]     Solution volume is a concern in electrolytic plating. A high ratio of surface area of work to volume of bath is required to maximize speed and efficiency of deposition. It is sometimes advantageous to design the tank with a shape suitable to the work.  
         [0008]     One useful feature of electroless nickel plating is the high degree of uniformity in thickness of deposit, as contrasted with electrolytic nickel plating. On a properly catalyzed surface, the driving potential for chemical reduction is essentially constant at all points on the surface. Alternately, in electrolytic plating the amount of plating is determined by the local current density, which often varies considerably from point to point on the surface of the work. It is necessary to provide good agitation to insure uniform nickel ion distribution in the area adjacent to the work in order to obtain uniform coating thickness when depositing thick electroless nickel coatings. Electroless nickel offers advantages when plating irregularly shaped objects, such as holes, recesses, or the inside of tubes. Not only are uniform coating thicknesses obtained, but the need for conforming anodes, thieves, shields, and bipolar electrodes is eliminated. Sharp corners and edges will not build up as happens in electrolytic nickel plating.  
         [0009]     Since electroless nickel plating is catalytically controlled, the deposition is localized as desired. Compared to electrolytic nickel plating, the need for a catalyst is both advantageous and disadvantageous. If the part to be plated is a catalytic metal, it is readily plated with electroless nickel; if noncatalytic, the reaction may be initiated by first seeding the surface with a catalytic metal, such as nickel. This seeding may be accomplished electrolytically or by chemical displacement. If the noncatalytic metal is more noble than nickel, a simple method for initiating plating is to immerse the part in the bath in contact with a more active metal, such as aluminum. If the surface is nonmetallic, the reaction is catalyzed with a salt, such as palladium chloride. While certain nonconductors such as some ceramics, glasses, plastics, or wood are catalyzed and plated with electroless nickel, the successful plating of nickel on zirconia is unreported. Nickel electroplating requires that the surface first be rendered conductive, something which is difficult to accomplish on intricately shaped parts or on zirconia ceramic.  
         [0010]     The differences in physical properties of electroless versus electrolytic nickel deposits reflect the fact that electroless nickel deposits contain 3% to 15% phosphorus. Electrodeposited nickel-phosphorus alloys possess many physical properties closely resembling those of electroless nickel deposits. Compared to electrolytic nickel deposits, electroless nickel deposits are amorphous in structure, harder (500 Vickers in the as-plated condition compared to 900 Vickers after heat treatment), and more brittle. Electroless nickel deposits are usually smooth, semi-bright to bright in appearance, and possess a laminar structure similar to bright nickel. Electroless nickel deposits do not possess the full luster of bright nickel electrodeposits.  
         [0011]     The corrosion resistance of a given thickness of electroless nickel is usually superior to an equal electrolytic deposit for the following reasons: the greater uniformity in thickness of electroless deposits, eliminating the need of over plating to provide adequate corrosion protection for recessed areas; the virtual absence of porosity in electroless deposits; the homogeneous structure (that is, no crystal boundaries) of electroless deposits; and the greater corrosion resistance of nickel phosphite.  
         [0012]     A need exists for an adherent, thin electroless nickel coating on zirconia, which will facilitate bonding by, for example, brazing to other metals.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     It has now been discovered that a method for coating a ceramic component, consists of the steps of selecting the component made of yttria-stabilized tetragonal zirconia, etching the yttria-stabilized tetragonal zirconia polycrystal ceramic component in hydrofluoric acid, rinsing the ceramic component in methanol, sensitizing said ceramic component, catalyzing the ceramic component, treating the ceramic component to reaction enhance the ceramic component, placing the ceramic component in a plating bath that is configured for forming an electroformed nickel coating thereby forming a nickel coating on said ceramic component; and measuring the thickness of the nickel coating.  
         [0014]     The method may also include etching the yttria-stabilized tetragonal zirconia polycrystal ceramic component in 40% concentrated hydrofluoric acid for 30 seconds.  
         [0015]     Control of the sensitizing, catalyzing, and reaction enhancement baths to about 90° C. has been demonstrated to provide improved nickel deposition.  
         [0016]     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.  
       OBJECTS OF THE INVENTION  
       [0017]     It is an object of the invention to apply a thin, adherent coating of nickel on zirconia ceramic by electroforming.  
         [0018]     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0019]      FIG. 1  is a component assembly formed by the known nickel foil brazing approach.  
         [0020]      FIG. 2  is a component assembly formed by an electroformed nickel foil coating.  
         [0021]      FIG. 3  illustrates a flow chart of the electroless plating process.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The invention is presented in  FIG. 2  in one preferred application where a component assembly  28  is suitable for bonding by brazing. A ceramic part  6  has an intimately bonded electroformed nickel coating  22  on one end that is placed in intimate contact with metal part  4 , which is preferably titanium or a titanium metal alloy. The ceramic part  6  is preferably yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). The assembly  28  is brazed while the electroformed nickel coating  22  is held in intimate contact with the metal part  4  by force  20 . Such an assembly is suitable for implantation in living tissue where the component assembly  28  must be biocompatible. Such implantable devices are well known and in a preferred embodiment they are very small and implantable by injection, preferably having an outer diameter of 6 mm or less and an overall length of 60 mm or less. See U.S. Pat. Nos. 4,991,582, 5,193,539, 5,193,540, 5,324,316, 6,185,452, 6,208,894, and 6,315,721, for example. The instant invention has broad applicability and is not limited to such applications, however.  
         [0023]     A first step in the process of the invention, as presented in  FIG. 3  is to select a Y-TZP ceramic component  102  to which an electroformed nickel layer will be bonded by direct deposition on a selected surface of the ceramic component. The deposition process is not limited to line-of-sight deposition and the coating thickness on complex shaped parts can be controlled during the electroforming process by the use of baffles, for example. Y-TZP materials are widely known and are a phase stabilized form of yttria ceramic that has a low thermal expansion coefficient and excellent stability in certain harsh environments, such as when implanted in living tissue.  
         [0024]     A second step is to clean the Y-TZP component using a detergent cleaner  104 , such as Alconox®, available from Alconox, Inc. of White Plains, N.Y. The component is rinsed with deionized water and second rinsed with isopropyl alcohol using ultrasonic cleaning.  
         [0025]     The component is then masked in step  106  leaving the area of the component that is to be coated with electroformed nickel unmasked. For a hollow shaped component, such as a hollow tube or open cylinder, the opening may be plugged with silicone. The component surface to be protected is preferably covered with Kynar® PVDF shrink tubing, available from 3M.  
         [0026]     The inventors have found that to achieve success, the next step in a preferred embodiment is to etch the Y-TZP ceramic in hydrofluoric acid, step  108 , at a concentration of 40% hydrofluoric acid for 30 seconds. A known source of hydrofluoric acid is available from Alfa Aesar, Ward Hill, Mass. The component is then rinsed in deionized water.  
         [0027]     The component is then placed in a sensitizer treatment, step  110 , of stannous chloride (SnCl 2 ) at a concentration of 7% (i.e., 70 g of anhydrous SnCl 2  combined with 40 ml of 36.5% concentration hydrochloric acid plus deionized water to make one liter of solution) for about 7 minutes. In a preferred embodiment, the solution is held at 90° C.±2° C. This solution is unstable and cannot be stored more than one day. The component is then rinsed in deionized water.  
         [0028]     The component is next placed in a catalyzer treatment, step  112 , of PdCl 2  for about 4 minutes. The solution is about 1 gram/liter of PdCl 2  plus 20 ml of 36.5% concentrated hydrochloric acid and 40 ml of 40% concentration hydrofluoric acid combined with deionized water to make 1 liter of solution. In a preferred embodiment, the solution is held at 90° C.±2° C. This solution has been found to be stable. The component is then rinsed in deionized water.  
         [0029]     Next the component is placed in a reaction enhancement treatment, step  114 , (also known as a reducing agent) at 90° C.±20 for 10 seconds. The solution is 20% sodium hypophosphite, NaH 2 PO 2 . This is 200 grams per liter of deionized water where the water is added to make one liter of solution.  
         [0030]     The prepared component is next placed in a plating bath, step  118 , at 88° C.±2° C. or preferably at 88° C.±1° C. The inventors prefer the Advanced High Phosphorus, Semi-Bright Electroless Nickel System, Stock # 44305 from Alfa Aesar. The electroforming process proceeds according to parameters and procedures that are known to those skilled in the art. The deposition rate is preferably about 0.0004 to 0.0006 inches per hour.  
         [0031]     The inventors have found, step  120 , that, in their particular configuration, it is preferable to place the component in a flow of bubbling air that is about 0.4 to 0.5 cm between the nearest portion of the component and the source of bubbles. The flow rate is characterized as slow and is controlled to remove the hydrogen generated from the reaction to facilitate deposition of nickel.  
         [0032]     In step  122  the part is removed from the bath, optionally rinsed in deionized water before being rinsed with methanol. The part is rinsed in deionized water before a sensitizer treatment is conducted in step  124  for 15 to 30 seconds, rinsed in deionized water, followed by a catalyzer treatment for 15 to 30 seconds in step  126  followed by a deionized water rinse. The component is subject to the reaction enhancement treatment in step  128  for 10 seconds at 90° C.±2° C.  
         [0033]     The component is returned to the plating bath in step  130  for 10 minutes in bubbling air, step  132 , after which it is optionally subjected to steps  122  to  132  again, the first time through the process, if it is desired to increase the deposition thickness of the nickel. The component is deionized water rinsed in step  142 , methanol rinsed in step  144 , rinsed in deionized water and again rinsed for about 15 seconds in 40% hydrofluoric acid in step  146  before being rinsed in deionized water in step  148  and returned to the plating bath in step  150 , after which it is optionally returned the first time through the process, if it is desired to increase the deposition thickness of the nickel, to repeat steps  142  to  150  prior to being removed from the bath, rinsed in deionized water, and having its thickness measured by known means, step  152 , and the adhesion measured by known means, step  154 , to assure that the nickel to ceramic bond is acceptable.  
         [0034]     Thus, in accordance with this invention, it is now possible to apply a coating of nickel by electroforming directly on a Y-TZP ceramic component. This is surprising since prior investigators have not been able to successfully accomplish this coating process.  
         [0035]     The following example is submitted to illustrate but not to limit this invention. Unless otherwise indicated, all parts and percentages in the specification and claims are based upon weight.  
         [0036]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.