Article having a decorative and protective multilayer coating simulating brass
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/329,383, filed Oct. 26, 1994, now U.S. Pat. No. 5,476,724 issued Dec. 19, 1995 which is a continuation of application Ser. No. 08/252,793, filed Jun. 2, 1994, now U.S. Pat. No. 5,413,874 issued May 9, 1995. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to substrates, in particular brass substrates, with a multi-layer decorative and protective coating. 
     BACKGROUND OF THE INVENTION 
     It is currently the practice with various brass articles such as lamps, trivets, candlesticks, door knobs, door handles, door escutcheons and the like to first buff and polish the surface of the article to a high gloss and to then apply a protective organic coating, such as one comprised of acrylics, urethanes, epoxies, and the like, onto this polished surface. While this system is generally quite satisfactory it has the drawback that the buffing and polishing operation, particularly if the article is of a complex shape, is labor intensive. Also, the known organic coatings are not always as durable as desired, particularly in outdoor applications where the articles they are exposed to the elements and ultraviolet radiation. It would, therefore, be quite advantageous if brass articles, or indeed other metallic articles, could be provided with a coating which gave the article the appearance of highly polished brass and also provided wear resistance and corrosion protection. The present invention provides such a coating. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a metallic substrate having a multi-layer coating disposed or deposited on its surface. More particularly, it is directed to a metallic substrate, particularly brass, having deposited on its surface multiple superposed metallic layers of certain specific types of metals or metal compounds. The coating is decorative and also provides corrosion and wear resistance. The coating provides the appearance of highly polished brass, i.e. has a brass color tone. Thus, an article surface having the coating thereon simulates a highly polished brass surface. 
     A first layer deposited directly on the surface of the substrate is comprised of nickel. The first layer may be monolithic or preferably it may consist of two different nickel layers such as a semi-bright nickel layer deposited directly on the surface of the substrate and a bright nickel layer superimposed over the semi-bright nickel layer. Disposed over the nickel layer is a layer comprised of nickel-tungsten-boron alloy. Over the nickel-tungsten-boron alloy layer is a layer comprised of chrome. Over the chrome layer is a layer comprised of a non-precious refractory metal such as zirconium, titanium, hafnium or tantalum, preferably zirconium or titanium. A top layer comprised of a zirconium compound, titanium compound, hafnium compound or tantalum compound, preferably a titanium compound or a zirconium compound such as zirconium nitride, is disposed over the refractory metal layer, preferably zirconium layer. 
     The nickel, nickel-tungsten-boron alloy and chrome layers are preferably applied by electroplating. The refractory metal such as zirconium and refractory metal compound such as zirconium compound layers are applied by vapor deposition such as sputter ion deposition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of a portion of the substrate having the multi-layer coating deposited on its surface. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The substrate 12 can be any platable metal or metallic alloy substrate such as copper, steel, brass, tungsten, nickel alloys, and the like. In a preferred embodiment the substrate is brass. 
     The nickel layer 13 is deposited on the surface of the substrate 12 by conventional and well known electroplating processes. These processes include using a conventional electroplating bath such as, for example, a Watts bath as the plating solution. Typically such baths contain nickel sulfate, nickel chloride, and boric acid dissolved in water. All chloride, sulfamate and fluoroborate plating solutions can also be used. These baths can optionally include a number of well known and conventionally used compounds such as leveling agents, brighteners, and the like. To produce specularly bright nickel layer at least one brightener from class I and at least one brightener from class II is added to the plating solution. Class I brighteners are organic compounds which contain sulfur. Class II brighteners are organic compounds which do not contain sulfur. Class II brighteners can also cause leveling and, when added to the plating bath without the sulfur-containing class I brighteners, result in semi-bright nickel deposits. These class I brighteners include alkyl naphthalene and benzene sulfonic acids, the benzene and naphthalene di- and trisulfonic acids, benzene and naphthalene sulfonamides, and sulfonamides such as saccharin, vinyl and allyl sulfonamides and sulfonic acids. The class II brighteners generally are unsaturated organic materials such as, for example, acetylenic or ethylenic alcohols, ethoxylated and propoxylated acetylenic alcohols, coumarins, and aldehydes. These Class I and Class II brighteners are well known to those skilled in the art and are readily commercially available. They are described, inter alia, in U.S. Pat. No. 4,421,611 incorporated herein by reference. 
     The nickel layer can be comprised of semi-bright nickel, bright nickel, or be a duplex layer containing a layer comprised of semi-bright nickel and a layer comprised of bright nickel. The thickness of the nickel layer is generally in the range of from about 100 millionths (0.000100) of an inch, preferably about 150 millionths (0.000150) of an inch to about 3,500 millionths (0.0035) of an inch. 
     As is well known in the art before the nickel layer is deposited on the substrate the substrate is subjected to said activation by being placed in a conventional and well known acid bath. 
     In a preferred embodiment as illustrated in the Figure, the nickel layer 13 is actually comprised of two different nickel layers 14 and 16. Layer 14 is comprised of semi-bright nickel while layer 16 is comprised of bright nickel. This duplex nickel deposit provides improved corrosion protection to the underlying substrate. The semi-bright, sulfur-free plate 14 is deposited, by conventional electroplating processes, directly on the surface of substrate 12. The substrate 12 containing the semi-bright nickel layer 14 is then placed in a bright nickel plating bath and the bright nickel layer 16 is deposited on the semi-bright nickel layer 14. 
     The thickness of the semi-bright nickel layer and the bright nickel layer is a thickness effective to provide improved corrosion protection. Generally, the thickness of the semi-bright nickel layer is at least about 50 millionths (0.00005) of an inch, preferably at least about 100 millionths (0.0001) of an inch, and more preferably at least about 150 millionths (0.00015) of an inch. The upper thickness limit is generally not critical and is governed by secondary considerations such as cost. Generally, however, a thickness of about 1,500 millionths (0.0015) of an inch, preferably about 1,000 millionths (0.001) of an inch, and more preferably about 750 millionths (0.00075) of an inch should not be exceeded. The bright nickel layer 16 generally has a thickness of at least about 50 millionths (0.00005) of an inch, preferably at least about 125 millionths (0.000125) of an inch, and more preferably at least about 250 millionths (0.00025) of an inch. The upper thickness range of the bright nickel layer is not critical and is generally controlled by considerations such as cost. Generally, however, a thickness of about 2,500 millionths (0.0025) of an inch, preferably about 2,000 millionths (0.002) of an inch, and more preferably about 1,500 millionths (0.0015) of an inch should not be exceeded. The bright nickel layer 16 also functions as a leveling layer which tends to cover or fill in imperfections in the substrate. 
     Disposed on the bright nickel layer 16 is a layer 20 comprised of nickel-tungsten-boron alloy which as seen below has at least about 0.05 weight percent boron. More specifically, layer 20 is comprised of an amorphous composite alloy of nickel, tungsten and boron. Layer 20 is deposited on layer 16 by conventional electroplating processes. The plating bath is normally operated at a temperature of about 115° to 125° F. and a preferred pH range of about 8.2 to about 8.6. The well known soluble, preferably water soluble, salts of nickel, tungsten and boron are utilized in the plating bath or solution to provide concentrations of nickel, tungsten and boron. 
     The nickel-tungsten-boron alloy layer generally is comprised of from about 50 to about 70 weight percent nickel, about 30 to 50 weight percent tungsten, and from at least about 0.05 to about 2.5 weight percent boron, preferably from about 55 to about 65 weight percent nickel, about 35 to about 45 weight percent tungsten, and from about 0.5 to about 2.0 weight percent boron, and more preferably from about 57.5 to about 62.5 weight percent nickel, about 37.5 to about 42.5 weight percent tungsten, and from about 0.75 to about 1.25 weight percent boron. The plating bath contains sufficient amounts of the soluble salts of nickel, tungsten and boron to provide a nickel-tungsten-boron alloy of the afore-described composition. 
     A nickel-tungsten-boron plating bath effective to provide a nickel-tungsten-boron alloy of which a composition is commercially available, such as the Amplate™ system from Amorphous Technologies International of Laguna Niguel, Calif. A typical nickel-tungsten-boron alloy contains about 59.5 weight percent nickel, about 39.5 weight percent tungsten, and about 1% boron. The nickel-tungsten-boron alloy is an amorphous/nano-crystalline composite alloy. Such an alloy layer is deposited by the AMPLATE plating process marketed by Amorphous Technologies International. 
     The thickness of the nickel-tungsten-boron alloy layer 20 is generally at least about 20 millionths (0.00002) of an inch, preferably at least about 50 millionths (0.00005) of an inch, and more preferably at least about 100 millionths (0.0001) of an inch. The upper thickness range is not critical and is generally dependent on economic considerations. Generally, a thickness of about 2,500 millionths (0.0025) of an inch, preferably about 2,000 millionths (0.002), and more preferably about 1,000 millionths (0.001) of an inch should not be exceeded. 
     Disposed over the nickel-tungsten-boron alloy layer 20 is a layer 21 comprised of chrome. The chrome layer 21 may be deposited on layer 20 by conventional and well known chromium electroplating techniques. These techniques along with various chrome plating baths are disclosed in Brassard, &#34;Decorative Electroplating--A Process in Transition&#34;, Metal Finishing, pp. 105-108, June 1988; Zaki, &#34;Chromium Plating&#34;, PF Directory, pp. 146-160; and in U.S. Pat. Nos. 4,460,438, 4,234,396, and 4,093,522, all of which are incorporated herein by reference. 
     Chrome plating baths are well known and commercially available. A typical chrome plating bath contains chromic acid or salts thereof, and catalyst ion such as sulfate or fluoride. The catalyst ions can be provided by sulfuric acid or its salts and fluosilicic acid. The baths may be operated at a temperature of about 112°-116° F. Typically in chrome plating a current density of about 150 amps per square foot, at about 5 to 9 volts is utilized. 
     The chrome layer generally has a thickness of at least about 2 millionths (0.000002) of an inch, preferably at least about 5 millionths (0.000005) of an inch, and more preferably at least about 8 millionths (0.000008) of an inch. Generally, the upper range of thickness is not critical and is determined by secondary considerations such as cost. However, the thickness of the chrome layer should generally not exceed about 60 millionths (0.00006) of an inch, preferably about 50 millionths (0.00005) of an inch, and more preferably about 40 millionths (0.00004) of an inch. 
     Disposed over chrome layer 21 is a layer 22 comprised of a non-precious refractory metal such as hafnium, tantalum, zirconium or titanium, preferably zirconium or titanium, and more preferably zirconium. 
     Layer 22 serves, inter alia, to improve or enhance the adhesion of layer 24 to layer 21. Layer 22 is deposited on layer 21 by conventional and well known techniques such as vacuum coating, physical vapor deposition such as ion sputtering, and the like. Ion sputtering techniques and equipment are disclosed, inter alia, in T. Van Vorous, &#34;Planar Magnetron Sputtering; A New Industrial Coating Technique&#34;, Solid State Technology, Dec. 1976, pp 62-66; U. Kapacz and S. Schulz, &#34;Industrial Application of Decorative Coatings--Principle and Advantages of the Sputter Ion Plating Process&#34;, Soc. Vac. Coat., Proc. 34th Arn. Techn. Conf., Philadelphia, U.S.A., 1991, 48-61; and U.S. Pat. Nos. 4,162,954, and 4,591,418, all of which are incorporated herein by reference. 
     Briefly, in the sputter ion deposition process the refractory metal such as titanium or zirconium target, which is the cathode, and the substrate are placed in a vacuum chamber. The air in the chamber is evacuated to produce vacuum conditions in the chamber. An inert gas, such as Argon, is introduced into the chamber. The gas particles are ionized and are accelerated to the target to dislodge titanium or zirconium atoms. The dislodged target material is then typically deposited as a coating film on the substrate. 
     Layer 22 has a thickness which is at least effective to improve the adhesion of layer 24 to layer 21. Generally, this thickness is at least about 0.25 millionths (0.00000025) of an inch, preferably at least about 0.5 millionths (0.0000005) of an inch, and more preferably at least about one millionth (0.000001) of an inch. The upper thickness range is not critical and is generally dependent upon considerations such as cost. Generally, however, layer 22 should not be thicker than about 50 millionths (0.00005) of an inch, preferably about 15 millionths (0.000015) of an inch, and more preferably about 10 millionths (0.000010) of an inch. 
     In a preferred embodiment of the present invention layer 22 is comprised of titanium or zirconium, preferably zirconium, and is deposited by sputter ion plating. 
     Reactive ion sputter is generally similar to ion sputter deposition except that a reactive gas which reacts with the dislodged target material is introduced into the chamber. Thus, in the case where zirconium nitride is the top layer 24, the target is comprised of zirconium and nitrogen gas is the reactive gas introduced into the chamber. By controlling the amount of nitrogen available to react with the zirconium, the color of the zirconium nitride can be made to be similar to that of brass of various hues. 
     Layer 24 is comprised of a hafnium compound, a tantalum compound, a titanium compound or a zirconium compound, preferably a titanium compound or a zirconium compound, and more preferably a zirconium compound. The titanium compound is selected from titanium nitride, titanium carbide, and titanium carbonitride, with titanium nitride being preferred. The zirconium compound is selected from zirconium nitride, zirconium carbonitride, and zirconium carbide, with zirconium nitride being preferred. 
     Layer 24 provides wear and abrasion resistance and the desired color or appearance, such as for example, polished brass. Layer 24 is deposited on layer 22 by any of the well known and conventional plating or deposition processes such as vacuum coating, reactive sputter ion plating, and the like. The preferred method is reactive ion sputter plating. 
     Layer 24 has a thickness at least effective to provide abrasion resistance. Generally, this thickness is at least 2 millionths (0.000002) of an inch, preferably at least 4 millionths (0.000004) of an inch, and more preferably at least 6 millionths (0.000006) of an inch. The upper thickness range is generally not critical and is dependent upon considerations such as cost. Generally a thickness of about 30 millionths (0.00003) of an inch, preferably about 25 millionths (0.000025) of an inch, and more preferably about 20 millionths (0.000020) of an inch should not be exceeded. 
     Zirconium nitride is the preferred coating material as it most closely provides the appearance of polished brass. 
     In order that the invention may be more readily understood the following example is provided. The example is illustrative and does not limit the invention thereto. 
     EXAMPLE 1 
     Brass door escutcheons are placed in a conventional soak cleaner bath containing the standard and well known soaps, detergents, defloculants and the like which is maintained at a pH of 8.9-9.2 and a temperature of 180°-200° F. for 30 minutes. The brass escutcheons are then placed for six minutes in a conventional ultrasonic alkaline cleaner bath. The ultrasonic cleaner bath has a pH of 8.9-9.2, is maintained at a temperature of about 160°-180° F., and contains the conventional and well known soaps, detergents, defloculants and the like. After the ultrasonic cleaning the escutcheons are rinsed and placed in a conventional alkaline electro cleaner bath for about two minutes. The electro cleaner bath contains an insoluble submerged steel anode, is maintained at a temperature of about 140°-180° F., a pH of about 10.5-11.5, and contains standard and conventional detergents. The escutcheons are then rinsed twice and placed in a conventional acid activator bath for about one minute. The acid activator bath has a pH of about 2.0-3.0, is at an ambient temperature, and contains a sodium fluoride based acid salt. The escutcheons are then rinsed twice and placed in a semi-bright nickel plating bath for about 10 minutes. The semi-bright nickel bath is a conventional and well known bath which has a pH of about 4.2-4.6, is maintained at a temperature of about 130°-150° F., contains NiSO 4 , NiCL 2 , boric acid, and brighteners. A semi-bright nickel layer of an average thickness of about 250 millionths of an inch (0.00025) is deposited on the surface of the escutcheon. 
     The escutcheons containing the layer of semi-bright nickel are then rinsed twice and placed in a bright nickel plating bath for about 24 minutes. The bright nickel bath is generally a conventional bath which is maintained at a temperature of about 130°-150° F., a pH of about 4.0-4.8, contains NiSO 4 , NiCL 2 , boric acid, and brighteners. A bright nickel layer of an average thickness of about 750 millionths (0.00075) of an inch is deposited on the semi-bright nickel layer. The semi-bright and bright nickel plated escutcheons are rinsed three times and placed for about forty minutes in a nickel-tungsten-boron plating bath available from Amorphous Technologies International of California as the AMPLATE bath. The bath utilizes insoluble platinized titanium anode, is maintained at a temperature of about 115°-125° F. and a pH of about 8.2-8.6. A nickel-tungsten-boron layer of an average thickness of about 400 millionths (0.0004) of an inch is deposited on the bright nickel layer. The nickel-tungsten-boron plated escutcheons are then rinsed twice. 
     The nickel-tungsten-boron plated escutcheons are then placed in a conventional, commercially available hexavalent chromium plating bath using conventional chromium plating equipment for about seven minutes. The hexavalent chromium bath is a conventional and well known bath which contains about 32 ounces/gallon of chromic acid. The bath also contains the conventional and well known chromium plating additives. The bath is maintained at a temperature of about 112°-116° F., an utilizes a mixed sulfate/fluoride catalyst. The chromic acid to sulfate ratio is about 200:1. A chromium layer of about 10 millionths (0.00001) of an inch is deposited on the surface of the nickel-tungsten-boron layer. The escutcheons are thoroughly rinsed in deionized water and then dried. The chromium plated escutcheons are placed in a sputter ion plating vessel. This vessel is a stainless steel vacuum vessel marketed by Leybold A. G. of Germany. The vessel is generally a cylindrical enclosure containing a vacuum chamber which is adapted to be evacuated by means of pumps. A source of argon gas is connected to the chamber by an adjustable valve for varying the rate of flow of argon into the chamber. In addition, two sources of nitrogen gas are connected to the chamber by an adjustable valve for varying the rate of flow of nitrogen into the chamber. 
     Two pairs of magnetron-type target assemblies are mounted in a spaced apart relationship in the chamber and connected to negative outputs of variable D.C. power supplies. The targets constitute cathodes and the chamber wall is an anode common to the target cathodes. The target material comprises zirconium. 
     A substrate carrier which carries the substrates, i.e., escutcheons, is provided, e.g., it may be suspended from the top of the chamber, and is rotated by a variable speed motor to carry the substrates between each pair of magnetron target assemblies. The carrier is conductive and is electrically connected to the negative output of a variable D.C. power supply. 
     The plated escutcheons are mounted onto the substrate carrier in the sputter ion plating vessel. The vacuum chamber is evacuated to a pressure of about 5×10 -3  millibar and is heated to about 400° C. via a radiative electric resistance heater. The target material is sputter cleaned to remove contaminants from its surface. Sputter cleaning is carried out for about one half minute by applying power to the cathodes sufficient to achieve a current flow of about 18 amps and introducing argon gas at the rate of about 200 standard cubic centimeters per minute. A pressure of about 3×10 -3  millibars is maintained during sputter cleaning. 
     The escutcheons are then cleaned by a low pressure etch process. The low pressure etch process is carried on for about five minutes and involves applying a negative D.C. potential which increases over a one minute period from about 1200 to about 1400 volts to the escutcheons and applying D.C. power to the cathodes to achieve a current flow of about 3.6 amps. Argon gas is introduced at a rate which increases over a one minute period from about 800 to about 1000 standard cubic centimeters per minute, and the pressure is maintained at about 1.1×10 -2  millibars. The escutcheons are rotated between the magnetron target assemblies at a rate of one revolution per minute. The escutcheons are then subjected to a high pressure etch cleaning process for about 15 minutes. In the high pressure etch process argon gas is introduced into the vacuum chamber at a rate which increases over a 10 minute period from about 500 to 650 standard cubic centimeters per minute (i.e., at the beginning the flow rate is 500 sccm and after ten minutes the flow rate is 650 sccm and remains 650 sccm during the remainder of the high pressure etch process), the pressure is maintained at about 2×10 -1  millibars, and a negative potential which increases over a ten minute period from about 1400 to 2000 volts is applied to the escutcheons. The escutcheons are rotated between the magnetron target assemblies at about one revolution per minute. The pressure in the vessel is maintained at about 2×10 -1  millibar. 
     The escutcheons are then subjected to another low pressure etch cleaning process for about five minutes. During this low pressure etch cleaning process a negative potential of about 1400 volts is applied to the escutcheons, D.C. power is applied to the cathodes to achieve a current flow of about 2.6 amps, and argon gas is introduced into the vacuum chamber at a rate which increases over a five minute period from about 800 sccm (standard cubic centimeters per minute) to about 1000 sccm. The pressure is maintained at about 1.1×10 -2  millibar and the escutcheons are rotated at about one rpm. 
     The target material is again sputter cleaned for about one minute by applying power to the cathodes sufficient to achieve a current flow of about 18 amps, introducing argon gas at a rate of about 150 sccm, and maintaining a pressure of about 3×10 -3  millibars. 
     During the cleaning process shields are interposed between the escutcheons and the magnetron target assemblies to prevent deposition of the target material onto the escutcheons. 
     The shields are removed and a layer of zirconium having an average thickness of about 3 millionths (0.000003) of an inch is deposited on the chromium layer of the escutcheons during a four minute period. This sputter deposition process comprises applying D.C. power to the cathodes to achieve a current flow of about 18 amps, introducing argon gas into the vessel at about 450 sccm, maintaining the pressure in the vessel at about 6×10 -3  millibar, and rotating the escutcheons at about 0.7 revolutions per minute. 
     After the zirconium layer is deposited a zirconium nitride layer having an average thickness of about 14 millionths (0.000014) of an inch is deposited on the zirconium layer by reactive ion sputtering over a 14 minute period. A negative potential of about 200 volts D.C. is applied to the escutcheons while D.C. power is applied to the cathodes to achieve a current flow of about 18 amps. Argon gas is introduced at a flow rate of about 500 sccm. Nitrogen gas is introduced into the vessel from two sources. One source introduces nitrogen at a generally steady flow rate of about 40 sccm. The other source is variable. The variable source is regulated so as to maintain a partial ion current of 6.3×10 -11  amps, with the variable flow of nitrogen being increased or decreased as necessary to maintain the partial ion current at this predetermined value. 
     The pressure in the vessel is maintained at about 7.5×10 -3  millibar. 
     The zirconium-nitride coated escutcheons are then subjected to low pressure cool down, where the heating is discontinued, pressure is increased from about 1.1×10 -2  millibar to about 2×10 -1  millibar, and argon gas is introduced at a rate of 950 sccm. 
     While certain embodiments of the invention have been described for purposes of illustration, it is to be understood that there may be various embodiments and modifications within the general scope of the invention.