CA1190512A - Protective coating means for articles such as gold- plated jewelry and wristwatch components, and method of forming such coating means - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The surface of a metal article which is exposed to the atmosphere and abrasion during normal use of the article is protected from scratches and/or corrosion during such use by a thin transparent abrasion-resistant film of an inert non-metallic material such as SiO2, SiC, Si3N4, TiO2, MgO, Al2O3, Ta2O5, Nb2O5, GeO2 , spinel and selected colorless glass compositions. The protective film is preferably deposited by RF-sputtering techniques and undesirable coloration of the article by optical in-terference effects from incident light rays is avoided by properly correlating the film thickness with the refrac-tive index of the particular material used to form the film. The invention permits the use of thinner gold plating on such items as articles of expensive jewelry and bracelets and cases for fine wristwatches without detract-ing from the quality, durability or appearance of the merchandise. Alternative embodiments in which several films of various selected non-metallic inert materials are combined to form composite protective coatings that pro-vide additional cost and manufacturing advantages are also disclosed along with methods for sputter-depositing the protective films in the proper thicknesses to avoid op-tical discoloration effects, either on articles that have been previously plated with gold or which have been pro-vided with a sputter-deposited layer of gold by sequen-tially operating the sputtering apparatus.

Description

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PROTECTIVE COATING MEANS FOR ARTICLES SUCH AS
GOLD-PLATED JEWELRY AND WRISTWATCH COMPONENTS, AND METHOD OF FORMING SUCH COATING MEANS
BACKGROUND OF THE INVENTION
This invention generally relates to the art of protecting articles that have metallic surfaces which are susceptible to abrasion damage or corrosion and has par-ticular reference to proiecting gold-plated articles of jewelry and wristwatch components (such as bracelets and cases) with a transparent coating of one or more selected inert materials. The invention also provides methods for coating such articles with one or more overlying protec-ti-ve films that are transparent and composed of abrasion-resistant material.
As is well known, many articles of merchandise have metallic surfaces which inherently become dull or tarnished in the environment in which the article is used.
For example, hardware items such as building-identifica-tion plaques, handrails, doorknobs, decorative door-knockers~ etc. that are composed of brass or a similar metal oxidize quite rapidly and require constant polishing and waxing to maintain a brilliant pleasing appearance.
This is also an age old problem with articles such as flatware, trays, trophies, etc. that are made of silver or are silver plated.

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While articles such as ~ine jewelry and the like that are m2de from solid gold or have gold-plated surfaces do not tarnish and thus do not present such a maintenance problem, they scratch easily and soon become unsightly 5 when subjected to the constant abrasion and "rubbing action" encountered during normal everyday use. Since gold is a relatively soft material, it also wears away quite rapidly when subjected to such conditions. If the article is gold-plated, this frequently exposes the base lO metal and creates an unsightly corrocled appearance in the case of articles (such as chains, rings, lockets, watch-bands, etc.) that are in direct contact with the person's body. These characteristics thus present serious problems in the production and marketing of such items as gold-15 plated jewelry and gold-plated bracelets and cases for ` wristwatches. In order to compensate for the loss of gold , that occurs during use by the customer, relatively thick gold plating is customarily used on high ~uality merchan-dise of this type to ensure that the article will retain r 20 its original pleasing appearance. However, in view of the ~iv extremely high cost of gold and the likelihood that it will become even more expensive in the future, the use of such heavy gold plating presents a serious economic prob-lem in the watch and jewelry industries.
A practical and reliable means for protecting gold and gold plated articles such as wristwatch compon-ents and the like from rapid wear and unsightly scratching (as well as corrosion if the gold plating has worn through~ without materially changing its "natural" finish ~$ 30 or appearance would, accordingly, not only be very desir-able from a quality and marketing standpoint but would be very advantageous from a production and cost reduction standpoint. Such protective means would also be very useful in preventing skin reactions and similar problems that are sometimes encountered by certain individuals when they wear a ring, chain or similar article that is made from a particular metal or alloy.

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SUMMARY OF T~E INVENTION
All of the foregoing objectives are achieved in accordance with the present invention by coating the metallic surface of the article with a thin substantially transparent and colorless film of a selected inert and non-metallic material which tenaciously adheres to the surface and has sufficient "hardness" to provide a very durable and abrasion-resistant protective finish and covering. In accordance with a preferred embodiment, the protective film is composed of a dielectric type material such as silicon dioxide, magnesium oxide, aluminum oxide, titanium dioxide, spinel, silicon nitride, silicon carbide and various types of glasses that have the proper combina-tion of hardness, transparency and thermal expansion characteristics. Other dielectric type materials which are substantially transparent in film thicknesses and are also suitable are tantalum oxide, niobium oxide, and germanium oxide. Certain types of glasses can also be used as the protective covering or as "buffer" layers be-tween the protective films and the substrates to compen-sate for differences in the thermal expansion characteris-tics of the substrate material and protective material.
The protective film of selected inert material is preferably deposited by RF-sputtering techniques and its thickness is controlled to prevent undesirable dis-coloration of the article by optical interference effects produced by incident light rays which enter the film.
Such interference effects are exhibited by transparent films when the optical thickness (that is, the true thick-ness of the film multiplied by the refractive index of thefilm material) is comparable to the wavelength of light and thus falls within the range of from about 3,000 to 8,000 Angstroms (the visible portion of the spectrum).
Since the refractive index for the various film materials is different, the optimum thickness range will also vary depending upon the particular material used to form the protective film.

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Composite type films which include one or more additional layers of another material are also employed in accordance with another embodiment of the invention to enhance the adhesion of the protective film as well as the adhesion of a layer of a different metal that is sputtered onto the substrate (as in the case of an article of base metal such as a watchband that is first coated with a sputtered layer of gold or another precious metal). A
composite coating consisting of very thin films of a precious metal (such as gold) and interposed alternately-arranged transparent films of a protective material are employed in accordance with another embodimen-t to further reduce the amount of precious metal required per article.
Various methods of forming the protective films and also sequentially metal-coating and then protectively-coating various articles composed of a base metal employing sput-tering-deposition apparatus and techniques are also dis-closed.
BRIEF DESCRIPTION OF THE DRAWING
A better understanding of the invention will be obtained from the exemplary embodiments shown in the accompanying drawing, wherein:
Figure 1 is a plan view of a gold-plated band or bracelet for a wristwatch which has been protectively coated in accordance with the invention;
Fig. 2 is a fragmentary cross-sectional view, on a greatly enlarged scale, through a portion of the watch bracelet shown in Fig. 1 and depicts the manner in which the thin plating of gold is protected by an overlying transparent film of inert abrasion-resistant material;
Fig. 3 is a similar cross-sectional view of a conventional watch bracelet, on the same scale, and illus-trates the much thicker gold plating commonly employed in the prior art for such watch components in the absence of the protective coating means of the present invention;
Eig. 4 is a similar cross-sectional view of another embodiment wherein the substrate is provided with s~
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an adhesion-promoting primer layer before being coated with a sputtered layer of gold or the like and then pro tectively coated;
Fig. 5 is a fragmentary cross-sectional view on an enlarged scale of still another embodiment wherein a buffer or transition layer of a selected glass is employed between the transparent protective fili~ and the plated - surface o~ the substrate to compensate for the difference in the thermal expansion coefficients of the plated sub-strate and protective film;
Fig. 6 is a similar view of an alternative embodiment in which two buffer or transition layers of different glasses are employed;
Fig. 7 is a cross-sectional view of yet another embodiment wherein a transparent protective film of a selected inert material is deposited directly onto the ` unplated surface of a substrate or article that is com-posed of tarnishable metal;
Fig. 8 is a cross-sectional view of another embodiment of the invention wherein alternating very thin films of a precious metal (such as gold) and a transparent protective material are employed to further reduce the amount of precious metal reguired to plate an article; and Fig. 9 is a plan view of a case for a wristwatch which is representative of other types of metallic arti-cles that can be protectively coated in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
;, While the present invention can be used with advantage to protectively coat various kinds of articles having metallic surfaces that are subject to attack or corrosion by the environment in which they are used as well as pieces of jewelry and the like tha-t are plated with a layer of a precious metal of a type which is easily scratched or rapidly worn away during normal use of the jewelry, it is especially adapted for use in conjunction with gold-plated articles such as bracelets and cases for 6 48,4~8I
wristwatches and the like and it has accordingly been so illustrated and will be so described.
A representative watchband or bracelet 10 is shown in Fig. 1 and consists of the usual intercoupled links L and a suitable latching member or clasp C. As illustrated in Fig. 2, such components are typically fabricated from a suitable base metal 12 (such as brass or stainless steel) which serves as a substrate for a plating 14 of gold, a gold alloy, or other precious metal that provides the desired attractive lustrous finish. As is customary in the gold~plating art, a thin coating 13 of nic~el or other suitable metal is deposited on the sub-strate before the plating operation is performed by elec-trode position or other well known means. Such an initial coating is referred to as a "strike" in the art and is re-quired to insure that the gold plating bonds properly to the substrate and that a smooth lustrous gold finish is produced if the substrate has a rough surface. Nickel "strikes" on brass substrates typically. have a thickness 20 in the order of 0.10 micron (1,000 Angstroms) or so. How ; ever, the thickness of such bonding layers is not critical and can be varied according to the plating requirements and the composition and condition of the base metal.
In accordance with one of the important advan-tages afforded by the present invention, the thickness of the yold plating 14 is drastically reduced and the plated surface of the watch bracelet 10 (or other article) is protected from scratching and abrasion by a film 16 of a selected inert and non-metallic material that tenaciously adheres to the gold-plated surface of the substrate 12.
In order to provide adequate long-term protection for the thin "soft" plating 14 of gold without altering its ap-pearance, the protective film 16 must be formed from a material which is much "harder" than gold and is substan-tially transparent and substantially colorless in thinfilm form.

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Materials which meet all of these requirements and are thus suitable for use as protective films in accordance with the invention are silicon dioxide (SiO2~, aluminum oxide (Al2O3), titanium dioxide (TiO2), silicon nitride (Si3N4), magnesium oxide (MgO), spinel (MgO-3.5Al2O3), Corning Glass No. 0080 (soda-lime glass), ~';l Corning Glass No. 7070, Corning Glass No. 7740 (PYREX
glass), and Corning Glass No. 7059.
The various properties of these materials ~hich make them suitable for use as protective coatings pursuant to the invention are listed in Table I below. For compar-ison, electroplated gold has a Knoop hardness of about 130 and the thermal expansion coefficients for stainless steel and brass are 173 and 169 x 10 7/oC, respectively.
TABLE I
~ Protective Thermal Expansion i Coating Hardness RefractiveCoefficient Material ~ Index (n)(x10 /C) _ .

f SiO2 741 1.46 8 A1203 1370 1.78 73 ~'.
~ TiO2 879 2.71 96 . .
Si3N4 2500 1.87 45 MgO 692 1.69 120 Spinel 1140 1.73 59 25 Corning Glass 400 1.512 92 ~j No. 0080(approx.) Corning Glass 418 1.469 32 No. 7070(approx.) Cornlng Glass 418 1.474 33 No. 7740 Corning Glass 424 1. 53 46 No. 7059 S~
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The hardness data given in Table I is the Knoop microhardness for bulk materials and thus provides an indication of the abrasion resistance of the various materials and their ability to protect the underlying plating of gold (or other precious metal).
Corning Glass No. 0080 is a soda-lime silicate type glass that i5 used in the electric lamp industry for lamp bulbs and the like. Such glasses typically contain 60 to 75% (by wt.) SiO2, 5 to 18% Na2O, 3 to 13% CaO or MgO (or a mixture thereof~ and minor amounts of additional materials such as A12O3, and K2O. A specific example of a glass composition of this type is as follows: 73O6% SiO2, 16% Na2O, 3.6% ~gO, 5.2% CaO, 1% A12O3 and 0.6% K2O.
Corning Glass Nos. 7070 and 77~0 are borosili-cate type glasses that contain major amounts of SiO2 and B2O3 and various minor constituents. Glass No. 7740 is marketed by Corning under the trade name "Pyrex" glass. A
specific example of a No. 7740 type glass is as follows:
80-5% (by wt-) Si2' 12-9% B2O3, 2-2~ A12O3, and 0.4% K2O.
A specific example of a No. 7070 type glass composition is as follows: 70% (by wt.) SiO~, 28% B~03, 1-2% Li2O, 1.1% A12O3, 0.2% MgO, 0.5% K2O and o.l% CaO.
Corning Glass No. 7050 is an aluminoborosilicate type glass which typically has the following composition;
50.2% (by wt.) SiO2, 25.1% BaO, 13.0% B2O3, 10.7% A12O3 and 0.4% As2O3.
Other dielectric type materials that are suit-able for use as protective films in accordance with the invention are silicon carbide (SiC) which has a Knoop hardness of 2500, tantalum oxide (Ta2O5), niobium oxide (Nb2O5) and germanium oxide (GeO2). Another glass which ; has also been found suitable is a high-lead-content solder glass which has a thermal expansion coefficient of 117 x 10 7/oC and typically contains 85% (by wt.) PbO, 7.5% B2O3 and 7.5% SiO2.
As indicated by the data given in Table I, the material which is used to form -the protective film 1~ should have a refrac~ive index that is in the range of from about 1.~ to about 2.8.

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In general, any material that has a Knoop hard ness of 400 or more, is inert, and has the ability to be o~

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deposited in thin adherent films that are of controlled thickness and are also substantially transparent and colorless in such thicknesses can be used as the protec-tive coating. If the thermal expansion characteristic of the protective material relative to the substrate is such that flakiny, cracking or peeling o the film occurs, then an intervening layer (or layers) of other materials must be used as hereinafter disclosed to correct the mismatch.
Various types of clear glass compositions (such as the aforesaid solder glass) that have thermal expansion co-efficients which approximate that of stainless steel, for example, can also be used.
While the protective film 16 can be formed on the plated substrate 12 by various means including elec-tron beam evaporation and chemical vapor-deposition tech-niques, deposition by RF-sputtering is preferred because sputtered films, in general, exhibit excellent adhesion, t are dense and free from pinholes, provide satisfactory substrate coverage, and have the proper stoichiometry.
20As is well-known, transparent films will produce coloration due to optical interference or so-called "New-ton ring" effects when the optical thickness (the product of the true or mechanical thickness and the refractive index) of the film is roughly of the same order of magni-2Stude as the wavelength of light (from about 3,000 to 8,000 Angstroms). The film thickness range which enables inci-dent light rays to produce such interference color effects thus depends upon the refractive index of the coating ` material and generally lies between 0.05 micron (500 30Angstroms) and 1.5 microns (15,000 Angstroms) for the materials which are listed in Table I or referred to as being suitable. Hence, in order to avoid such undesirable discoloration of the gold-plated surface of the watchband 10 (or other article which is being protectively coated~, the thickness of the protective film 16 must either be less than about 500 Angstroms or greater than about 15,000 Angstroms for these materials.

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Since films with thicknesses less than 500 Angstroms would be too thin to provide adequate "long term" abrasion protection, protective films formed from the aforementioned materials must have thicknesses that are greater than about 15,000 Angstroms. Protective films that are too thick, however, must also be avoided since they will tend to crack or peel away from the substrate.
Since the film thickness above which optical interference coloration effects are not discernible varies inversely with the refractive index of the coating material, a thinner film of a high refractive index material can be employed. This is desirable from a manufacturing stand point since shorter ~ilm-deposition times will be required and peeling will be inhibited. The following materials are preferred in this respect since they have high indices of refraction (shown in parenthesis): SiC (2.73), Nb2O5 (2-24), Ta2O5 (2-21), TiO2 (2.71), GeO (2.1) and Si3N4 (1.87).
The sputtering yield provides a relative indi-cation of how easily a given material can be sputtered .~ from a target and, hence, how rapidly a protective film of that material can be formed by sputter-deposition. This is an important consideration when coating in mass-produc-tion quantities is involved. In general, the ideal mate-rial for the protective coating from both a quality and manufacturing standpoint is thus a material which has high values for hardness, refractive index, and sputtering yield.
TEST DATA AND SPECIFIC EXAMPLES
' 30 Preliminary tests performed on small stainless-steel plates which were coated with a 2.5 microns (25,000 Angstroms) thick electroplated layer of 24 K gold have indicated that protective coatings of SiO2, Al2O3, TiO2, Si3N4 and SiC which were formed by RF-sputtering exhibited excellent adhesion and protection of the substrates against abrasion. A portion of each of the gold-plated test plates was masked during deposition of the protective 11 48,448I
film to provide an uncoated "gold reference" surface for evaluation. Film adhesion was checked by a so-called "tape test" which consisted of firmly pressing a piece of "Scotch'r-brand adhesive tape onto a film-coated portion of the test plate and then stripping the tape away. This is a very demanding test since any material which is not firmly bonded or securely anchored to the substrate will be lifted from its surface when the adhesive tape is stripped away. Abrasion resistance was determined by vigorously rubbing the coated and uncoated portions of the test plates with a pencil eraser and then with steel wool.
The appearance of the coated portions of the plates was evaluated visually by noting any discoloration effects or undesirable altering of the natural color of the gold plating.
These preliminary tests verified that very thin films of the tested materials in the thickness range below that were required to avoid optical interference effects (that is, less than about 0.05 micron or 500 Angstroms) would not provide adequate "long-term" abrasion protection of the gold plated substrates. The sputtered films of SiC
had a yellowish-brown tint and altered the natural gold color of the samples to a certain degree. Such films would, accordingly, be satisfactory only where a slight discoloration of the plated surface could be tolerated (or might even be desirable). Although the sputtered films of TiO2 were colorless and (due to their high refractive index) were devoid of any optical interference coloration even though they were only 1.5 microns or 15,000 Angstroms thick, the high refractive index of this material caused the film surface to have a high reflectivity which tended to mute the gold color of the substrate and slightly modify its appearance. The Al2O3 sputtered very slowly and would thus probably not lend itself to mass production operations.
Experiments have indicated that the sputtering yield of Si3N4 is comparable to that of SiO2 (0.13 mole-~3~

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cule per ion at 1 kilovolt target voltage) and these two materials are thus good selections for protectively coat-ing gold plated substrates where coating times and costs are critical factors.
Test data obtained with SiO2 films of varying thickness on gold-plated stainless steel sample plates have shown that protective films of this material that were apprvximately 1.4 microns or 14,000 Angstroms thick exhibited pale pink and green interference colors which indicated that the films were too thin. When films of SiO2 5 microns or 50,000 Angstroms thick were deposited on such gold-plated sample substrates, the coatings cracked and flaked from the substrates due to high stresses pre-sent within the thick films. SiO2 films 3.4 microns 15 (34,000 Angstroms) thick adhered well to the gold plated substrates and also had excellent abrasion resistance, on the basis of the "eraser and steel wool" test. The color of the coated plates was indistinguishable from the origi~
nal gold-plated substrate. Pursuant to these experimental data, the optimum thic~ness range for SiO2 protective films on gold-plated articles is accordingly within the range of about 1.5 to about 4 microns (that is, from about 15,000 to about 40,000 Angstroms).
Similar tests conducted on gold-plated watch bracelets 10 of the type shown in ~ig. 1 confirmed the foregoing early test data obtained on plate samples.
These additional tests revealed that cleanliness of the gold-plated substrates prior to deposition of the pro-tective film is quite important. Some of the sample : 30 watchbands apparently had an organic film or coating on their surfaces which may have been intended to serve as a protective coating by the manufacturer. This contamina tion produced undesirable brown stains when SiO2 protec-tive films were applied and also caused flaking of the films. These coating problems were solved by subjecting the watchbands to a cleaning procedure which consisted of boiling the components in a suitable detergent, rinsing 5~

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them in deionized water and methyl alcohol, and then drying them in air at around 120C.
The additional series of tests also indicated that the optimum thickness of SiO2 protective films was somewhat less for ~he gold-plated watchbands than for the gold-plated test blanks of metal. Films between about 2 and 3~5 microns thick tended to flake from the watchbands and no undesirable interference color effects were pro-duced if the film thickness was greater than about 1.4 microns. The optimum thickness for protective films of SiO2 in the case of gold-plated watchbands of the kind shown in Fig. ] is accordingly within the range of from about 1.4 to about 2 microns (that is, from about 14,000 to about 20,000 Angstroms~.
The differences in the observed interference effects for SiO2 films deposited on the test blanks of gold-plated metal and on the gold-plated watchbands may have been due to the smooth surface finish of the test blanks and the fact that the watchbands had a textured surface finish.
A most important advantage afforded by the present invention from a cost standpoin-t is the fact that wristwatch bracelets and other components for fine watches (as well as various articles of fine jewelry) can be provided with a much thinner plating of gold without detracting in any way from the quality or appearance of the components or articles since the natural appearance of the gold plating is preserved by the transparent film of protective material. As shown in Figs. 1 and 2, a watch-band 10 of high quality having a stainless steel substrate12 which is provided with a nickel "strike" 13 about 1,000 Angstroms thick and then plated with a layer 14 of gold approximately 0.5 micron or 5,000 Angstroms thick (dimen-sion "t1") can accordingly be made by protectively coating the thin layer of gold with a film 16 of 5iO2 (or similar transparent material) that is approximately four -times as thick as the gold plating--that is, a protective film of sj ~

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SiO2 approximately 2 microns or 20,000 Angstroms thick.
The relative thicknesses of the gold plating, the bonding layer or "s-trike" and the protective film are substan~
tially as shown in Fig. 2. Hence, the gold plating 14 is only one-fourth as thick as the protective film 16, and the nickel "strike" 13, in turn, is only one-fifth as thlck as the gold plating.
If the article or substrate is such that it can be coated with a thinner layer o~ gold which still retains the natural appearance and color of solid go:Ld, then gold coatings in the order of about 0.2 or 0.3 micron (about

2,000 or 3,000 Angstroms) can be used in combination with the protective film of the invention.
In contrast, conventional watch bracelets 11 (shown in Fig. 3) of good quality having similar sub-strates 18 of stainless steel which is primed by a nickel "strike" 19 about 1,000 Angstroms thick generally have a gold plating 20 that is approximately 10 microns or 100,000 Angstroms thick (dimension "t2") -that is, twenty times thicker than the gold plating 14 employed in accord-ance with the invention. Figs. 2 and 3 are drawn to the same scale so that the relative thicknesses of the two gold platings 14 and 20 is accurately shown in and apparent from the drawing.
Hence, the present invention permits the thick-ness of the gold platings employed on such fine watch components to be reduced by at least 95% (5,000 Angstroms versus lO0,000 Angstroms) and up to 9~% or so (2,000 Angstroms versus 100,000 Angstroms) with a corresponding reductions in the manufacturing cost of the watches. In view of the high cost of gold and other precious metals, the cost saving is very significant and constitutes an important competitive advantage, not only in the watch industry but in the manufacture of fine jewelry and simi-lar articles that are composed of a base metal and pres-ently require heavy coatings or platings of a precious metal to preserve their appearance.

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ALTERNATIVE "PRIMER LAYER" EMBODIMENT (FIG. 4) Further tests on protective films for gold-plated articles such as watch bracelets have shown that an additional reduction in their manufacturing cost can be reali~ed by depositing the gold layer on the watch brace-lets by sputtering techniques (rather than electroplating) so that both the gold-coating and protective~coating operations can be performed sequentially within the vacuum chamber of the RF-sputtering apparatus. Experiments con-firmed that this could be achieved quite readily by pro-~viding one target of gold and another target of SiO2 (or other material from which the protective film is to be formed) within the vacuum chamber and then simply operat-ing the sputtering apparatus in two different modes which selectively bombarded the targets to first deposit the required layer of sputtered gold onto the watchbands of ase metal and then coat the resulting gold-coated surface with a film of sputtered SiO2, the required thicknesses being obtained by properly controlling the target voltage, power input and the length of the sputtering operation in ~, each mode.
A further advantage afforded by this method of sequentially-coating was the ability to sputter-coat the watchbands with a layer of 24K gold instead of the 14K
gold conventionally used in the electroplating process.
I'he sputtered-gold layers thus had a deeper and richer . gold color (due to the higher gold content) compared to watchbands with gold-electroplated coatings--even though the sputtered-gold layers were much thinner and used less gold.
During the course of these experiments, it was also discovered that both the adherence and durability of the sputtered-gold coatings could be improved by deposit-ing a very thin layer of titanium (Ti) on the stainless steel substrate before sputter-depositing the gold layer.
The use of a sputtered film of Ti about 200 Angstroms thick permitted a layer of sputtered gold less than 0.5 16 48,448I
micron thick (5,000 Angstroms) to pass the "tape test" for adhesion. Such a preliminary or "primer" layer of Ti accordingly enables sputtered gold layers approximately 0.25 or 0.3 micron thick (2,500 or 3,000 Angstroms) to be employed on substrates--thus providing a corresponding further reduction in coating and material cost without detracting from the appearance of the finished article as regards its natural gold "finish" and appearance.
A watch bracelet lOa (or other article) having a 10 substrate 12a of a base metal (such as stainless steel or the like) that is provided with the composite sputtered coating according to this embodiment as shown in Fig. 4.
As will be noted, the substrate 12a has a thin primer layer 21 of titanium deposited on its surface to promote 15 the adhesion of a layer 22 of sputtered gold which, in turn, is protected by a film 16a of SiO2 or other suitable material that is substantially transparent and does not noticeably alter the natural appearance or finish of the gold layer.
The film thickness of the adhesion-promoting primer-layer 21 of titanium is not especially critical and can be in the range of from about 50 to 400 Angstroms.
Suitable thin films of other metals such as chromium, nickel and NichromeC~ype alloys that have a similar adhe-25 sion-promoting effect can also be used. Nichrome alloys are well known in the art and are composed of about 80%
(by wt.) nickel and about 20% chromium.
The manufacture of the watch bracelet lOa or other article will also be facilitated if the primer layer 30 21 of titanium (or other metal) is sputter-deposited on the substrate 12a in sequential fashion with the overlying layers of gold and protective material in a common vacuum chamber of a properly modified and controlled RF-sputter-ing apparatus.
ALTERNATIVE "BUFFER LAYER7' EMBODIMENT (FIG. 5) Anot:her form of composite protective coating for gold-plated watch components and similar articles is shown 17 48,448I
in Fig. 5 and was developed to overcome an adherence problem encountered during trial runs using a production type sputtering system which operated at high deposition rates. When sputtering protective films of SiO2 onto gold-plated stainless steel watchbands using such a sys~
tem, it was discovered that the protective films sometimes flaked from the watchbands upon removal from the sputter-ing apparatus. While the exact cause of this problem is unknown, it is believed that it may be due to excessive heating of the watchbands produced by the high rate at which the sputtered material was deposited--in combination with the subsequent cooling and a thermal expansion co-efficient mismatch between the SiO2 film and stainless steel substrate which induced stresses in the fi.lms with resultant flaking.
It was found that this problem could be solved by using an additional transparent layer of a suitable material between the SiO2 film and the gold-plated stain-less steel substrate, which additional layer is composed of a material that has a thermal expansion coefficient between that of SiO2 (8 x lO 7/oC) and that of stainless steel (173 x lO 7/oC). The additional layer thus serves as a buffer or "transition" layer that compensates for the expansion mismatch between the substrate and protective SiO2 film without interfering with the ability of the film to protect and preserve the natural appearance of the gold plating.
The soda-lime silicate type glass marketed by the Corning Glass Company under the trade designation Corning Glass No. 0080 (listed in Table I) is a material particularly suitable for use as such a buffer layer since it has a thermal expansion coefficient of 92 x lO 7/oC
(about midway between that of stainless steel and ~iO2) and is transparent and colorless in the thicknesses re-~5 quired. This glass composition also has an index of re-fraction of l.512 and a Knoop hardness of about 400 as indicated in the Table. However, any of the glasses list-~, ~
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ed in Table I (as well as the aforementioned solder glass)can be used as a buffer material since they all have thermal expansion coefficients that are much higher than that of SiO2. Since materials such as TiO2 and A1203 have high coefficients of thermal expansion, they are also especially suited for use as a buffer material.
Tests have shown that stainless steel watchbands provided with a gold layer (either plated or sputtered) approximately 5,000 Angstroms thick can be provided with a sputtered composite transparent protective coating that e~hibits excellent adhesion and durability and consists of a layer of Corning No. 7059 Glass about 1.5 microns thick (15,000 Angstroms) that was covered by a film of SiO2 approximately 0.5 micron (5,000 Angstrom) thick. The relative thicknesses of the transition or buffer layer of glass and the SiO2 film are not critical and can vary considerably from the stated values. For example, the film thickness of the SiO2 can be within the range of from ~; about 0.2 micron to about 2 microns (2,000 to 20,000 Angstroms) and the buffer layer of glass can be from about 1 to 4 microns thick (10,000 to 40,000 Angstroms). How-ever, the combined thicknesses of the protective film and buffer layer must be sufficient to prevent the occurrence of optical interference effects and undesired coloration of the gold-coated surface of the article.
As illustrated in Fig. 5, a watchband lOb or other article according to this embodiment consists of a substrate 12b of stainless steel (or other base metal), a "strike" or bonding layer 13b of nickel or the like, a thin plating 14b of gold (or other precious metal) that is protectively shielded from scratching and other damage by a substantially transparent and colorless composite coat-ing which consists of the buffer layer 24 of a selected glass and a much thinner layer 25 of SiO2 or other suit-able abrasion-resistant material.

19 48,448I
ALTERNATIVE MULTIPLE "BUFFER-LAYER" EMBODIMENT (FI~
The invention is not limited to the use of a single layer of a buffer material to correct the mismatch of the thermal expansion and contraction characteristics of the gold-plated substrate and the protective film but includes within its scope the use of two or more buffer layers for this purpose. A multiple buffer-layer embodi-ment 10c is shown in Fig. 6.
As illustrated, this embodiment comprises a sub strate 12c of a base metal, the usual very thin "strike"
or bonding layer 13c of nickel or the like, a gold plating 14c of reduced thickness, two buffer layers 26, 27 of two different substantially transparent colorless materials that have thermal expansion coefficients which provide a "two-step" transition from the high expansion coefficient of the plated substrate 12c to the much lower expansion of the transparent protective film 28.
In the case of a stainless steel gold-plated substrate and a protective film of SiO2, a first buffer layer of soda-lime silicate glass (No. 0080 glass) and a second buffer layer of Corning No. 7059 Glass would be a good combination of buffer materials since they would provide a '7balanced" transition from 173 to 92 to 46 to 8 (in terms of the expansion coefficients of the respective materials, starting with the substate).
The thickness of the buffer layers is not criti-cal but they obviously should be thin enough to maintain the composite coating below about 40,000 Angstroms or so.
They may be of equal thickness (as shown in Fig. 6) or their relative thickness can be varied and correlated with the expansion coefficients of the particular materials to provide an optimum "gradation" of stress forces at the interface of the substrate and protective film. However, in Fig. 6 the total thickness of the illustrated composite coating (that is, buffer layers 26, 27 and the protective film 28) is the same as the thickness of the composite 20 48,448I
coating used in the single~buffer layer embodiment shown in Fig. 5. This is preferred since it would reduce the sputter~coating times to a minimum.
ALTERNATIVE SIN~LE-COATING EMBODIMENT (FIG. 7) The invention is also suitable for use in pro-tectively coating articles that are entirely composed of a material (such as brass or silver) that is rapidly de-graded or becomes tarnished by chemical attack from oxygen or pollutants in the atmosphere in which they are used.
As shown in Fig. 7, in accordance with this embodiment the unplated article 10d itself (a silver bow]
or tray, or a brass name~plate, for example) constitutes the substrate 12d that is provided with the substantially transparent and colorless protective film 30 of SiO2 (or other suitable abrasion-resistant material such as those listed in Table I and in the text immediately following the Table). The thickness of the film 30, as in the pre-vious embodiments, must be properly correlated with the refractive index of the parti.cular protective material to avoid ~optical interference effects and resultant undesir-able coloration that would otherwise be produced by inci-dent light rays. Since the aforementioned solder glass has a thermal expansion coefficient of 117 x 10 7/oC, it can also be used to form the protective film 30 in those instances where the article is composed of a metal that also has a high expansion coefficient. E'or example, a protective film of this type glass (or any of the other glasses listed in Table I) would be suitable for use on silver trays, brass commemorative pla~ues or brass name-plates used on bui.ldings and the like, especially sincesuch articles are not subjected to severe abrasion during normal use.
If the protective film 30 is composed of a much harder material such as TiO2, SiO2, MgO or the like, then it can be used in conjunction with watchbands or watch cases that are composed of solid brass and are thus not coated or plated with gold or another precious metal.

21 48,448I
Experimental tests have demonstrated that sputtered SiO2 films adhere very well to brass articles and produce a body color that is very similar to gold. SiO2 films 2.58 microns thick (25,800 Angstroms) were free of blemishes and abrasion-resistant when sputter-deposited on a clean brass watchcase.
ALTERNATIVE "MULTI-THIN-FILM"
EMBODIMENT (FIG. 8) Another form of composite layer that permits even thinner coatings and smaller amounts of gold or other precious metals to be employed on substrates of a base metal is shown in Fig. 8.
According to this embodiment, an article lOe (such as a bracelet for a wristwatch or a piece of jewel-ry) that is composed of a base metal (such as stainless steel or the like that serves as a substrate 12e) is coat-ed with a primer layer 13e of titanium or a similar mater-ial and then with a plurality of very thin films 31,3~-35,37 of sputtered gold (or other precious metal) and a plurality of interposed thin films 32,~4--36,38 of sput-tered SiO2 (or other transparent and inert protective material). The series of alternately-disposed sputter-deposited films of gold and SiO2 form a very hard and durable composite coating whose outer surface consists of an SiO2 film 38 and whose inner surface is a layer 31 of gold that is bonded to the primer-coated substrate 12e.
While a total o~ eight interposed and overlapped films 31-38 are shown in Fig. 8, any suitable or required number can be employed (as indicated by the "break-away" in the composite coating). Tests conducted with substrates having a total of ten such films gave satisfactory re sults.
The alternating films can be very thin (for ex-ample, less than about 100 Angstroms thick) and provide several advantages in that they greatly reduce the sput-tering times (and thus the overall coating cost) but still produce an article 10e with a finish that has the natural 22 48,~48I
look of gold but has excellent abrasion~resistant proper-ties and actually contains a very small total amount of gold metal. As the top films "wear away" due to their extreme thinness, the next layer (of gold or SiO2) provide the desired gold "finish" appearance.
The total amount of gold in such multi-film composite coatings can be further reduced by making the gold films thinner than the films of protective material--for example, gold films that are around 50 Angstroms thick in combination with SiO2 films about 100 Angstroms thick.
Hence, a composite coating having a total of forty such films would have an overall thickness of only 3,000 Ang-stroms (with the aggregate or "total" film thickness of the gold films being only 1,000 Angstroms and thus re-quiring a very small quantity of gold).
Since the interposed protective films are sothin, they do not produce any optical interference effects or discoloration of the underlying gold films and can be sputter-deposited very rapidly. Flaking or peeling of the films is also not a problem since the sputtered materials are intimately bonded to one another and are not thick or brittle enough to create stresses due to mismatches of thermal expansion coefficients, either with respect to the interposed films themselves or with respect to the base-metal substrate.
While the combination of interposed films ofSiO2 and gold have been referred to in the embodiment just described, it will be apparent to those skilled in the art that various other combinations of materials can be used--depending upon the type of article involved (for example, alternating films of silver and spinel or a suitable glass, alternating films of platinum and TiO2 or MgO, etc.).
WATCHCASE EMBODIMENT (FIG. 9~
The invention is not limited to protectively coating bracelets or bands for wristwatches but includes within its scope the provision of transparent abrasion-23 43,4~8I
resistant coatings (consisting of one or several films) on other components for wristwatches such as watchcases 40 of the type shown in Fig. 9. Such cases can either comprise a gold-plated base metal (such as stainless steel or brass) or they can be composed of a metal such as brass that is not plated with gol.d but has a body color which is very similar to gold.
The various protective films and composite pro-tective coatings of the present invention can accordingly be used on any article of manufacture that has a metallic surface which is degraded or tarnished by chemical attack from the atmosphere or from pollutants in the atmosphere in which the article is used. The films and coating can also be used on articles such as jewelry and decorative components which have a substrate that is composed of a base metal (such as stainless steel or the like) that is coated with a relatively soft metal (such as gold) which , is easily scratched and has poor 'iwearing" characteris-, tics.
~X 20 Another more limited but important benefit afforded by the protective films and coatings of the present invention is in the prevention of "allergic" type reactions which some persons experience when wearing rings or chains, etc., that are composed of a certain metal or alloy. Since all of the materials listed are chemically inert and stable, a thin film of such material physically - isolates the wearer's skin from the reaction-causing metal and thus permits the ring or other article to be worn without any undesirable biological effects or reactions.

3~ This is especially true in the case of the glass or glass-like film~forming materials listed or mentioned previous-ly .
SPECIFIC EXAMPLE OF SPUTTER-COATING PROCESS
Following is a specific example of the manner in which the substantially transparent abrasion resistant films or coatings of the present invention are applied to articles or substrates by sputter-deposition using experi-mental apparatus.

24 48,448I
The articles or substrates are loaded into an RF-sputtering apparatus along with a suitable target of the selected coating material, a 6~inch diameter target of SiO~ for example. The sputtering chamber is then evac-uated to a pressure of about 5 x 10 7 Torr and filled withapproximately 1.4 x 10 2 Torr of a mixture of 90% argon and 10% oxygen which serves as the sputtering gas. An RF
target voltage of 750 volts is then applied to the target so that the power input is around 200 watts. The sputter ing apparatus was operated for approximately 10 hours under these conditions and films of SiO2 having a thick~
ness of from 1.5 to 2.0 microns (15,000 to 20,000 Ang-stroms) were deposited on the substrate. If a production type magnetron RF-sputtering apparatus were used, the time required to deposit SiO2 films of such thicknesses could be reduced to approximately 1 or 2 hours.
As will be appreciated to those skilled in the art, the operating parameters of the sputtering apparatus can be adjusted in accordance with the sputtering yields, etc., of the various coating materials so that films of the thicknesses required for each of the described embodi-ments can be readily and efficiently deposited.
; ADDITIONAL SPECIFIC E~AMPLES OF VARIOUS EMBODIMENTS
In addition to the experimental and test samples and data described previously, following are specific examples of additional embodiments that have been made and evaluated:
Fig. 2 Embodiment ~ a stainless steel watchband was first coated with a primer layer of Ti 260 Angstroms thick which was sputter-deposited in a chamber evacuated to a pressure of 4 x 10 7 Torr and then filled with argon (the sputtering gas) to a pressure of 1.4 x 10 Torr.
The Ti film was deposited in approximately eleven minutes using an RF target voltage of 950 volts and a power input of around 200 watts. A gold film 4,900 Angstroms thick was then sputter-deposited (without removing the watchband from the chamber) by operating the sputtering apparatus 25 48,448I
for twenty minutes (with a 24K gold target) at a target voltage of 750 volts and lO0 watts power input. A protec-tive film of SiO2 approximately 20,000 Angstroms thick was then sputter-deposited by operating the apparatus at a target voltage of 750 volts and 200 watt power input for ten hours (using an SiO2 target). The coatings adhered very well to the substrate and the SiO2 film was colorless and passed the abrasion and adhesion tests described previously.
~0 Fig. 4 Embodiment - A stainless steel watchband, which was previously electroplated with gold in the con-ventional ~anner, was provided with a buffer layer of Corning No. 7059 glass that was 1.5 microns (15,000 Ang-stroms) thick and then coated with a protective film of 15 SiO2 0.5 micron (5,000 Angstroms) thick by operating an RF-sputtering apparatus in sequential fashion with two different targets. The sputtering chamber was first evacuated to a pressure of 6 x lO 7 Torr and then filled with a sputtering gas consisting of 90% argon and 10%
oxygen at a pressure of 1.4 x lO 2 Torr. The buffer layer of glass was deposited by operating the apparatus at a target voltage of 450 volts and a power input of 300 watts for four and one-half hours, and the SiO2 film was depos-ited by using a target voltage of 750 volts and operating the apparatus for three hours at 200 watts input.
_g 7 Embodiment - A brass watchcase was coated with a transparent colorless protective film of SiO2 2.58 microns (25,800 Angstroms) thick by first evacuating the sputtering chamber to a pressure of 5 x 10 7 ~orr, then filling it with a sputtering gas consisting of 90% argon and 10% oxygen at a pressure of 1.4 x 10 2 Torr and oper-ating the apparatus for fifteen hours at 200 watts input and 750 volts applied to the SiO2 target. The SiO2 film adhered well, had no visual defects and passed the afore-mentioned "tape" and abrasion tests.
As will be apparent to those skilled in the art,when the precious metal coating or plating on the article , 48,448I
consists of gold, it need not be composed of pure (24K) gold but can comprise a suitable gold alloy (for example, lOK or 14K gold, etc.).

Claims (38)

1. I claim as my invention:
   l. In combination with an article of manufacture that is composed of metal and has a metallic surface which enhances the aesthetic appeal of the article but is susceptible to abrasion damage and/or environmental degradation such as corrosion and tarnishing, said article being of a type that is subjected to frequent human contact during normal usage of the article and is thus exposed to additional surface damage by such contact and usage, an adherent coating that protects said article from such abrasion damage and degradation without materially altering the natural aesthetic appearance of the metallic surface, said coating consisting essentially of a thin substantially trans-parent film of a sputter-deposited non-metallic inert material that has;
   a Knoop hardness in the range of from about 400 to about 2500 and is thus of sufficient hardness to be abrasion-resistant, and a refractive index in the range of from about 1.4 to about 2.8, said protective film having a thickness that is in the range of from about 14,000 to about 40,000 Angstroms and is also correlated relative to the refractive index of the non-metallic inert material comprising said film that the film, in addition to being substantially transparent, is (a) substantially devoid of discoloration due to interference effects that other-wise would be produced by incident light rays and thus alter the natural aesthetic appearance of the underlying metallic surface of the article, and (b) is of sufficient thickness to protect the metallic surface of the article from abrasion damage and degradation when in use and subjected to frequent human contact.
2. 2. The protectlvely-coated article of claim 1 wherein the metallic surface comprises a coating of precious metal that has a thickness substantially less than 100,000 Angstroms.
3. 3. The protectively-coated article of claim 2 wherein the article comprises a piece of jewelry or a wristwatch com-ponent and the coating of precious metal is composed of gold or a gold alloy.
4. 4. The protectively-coated article of claim 3 wherein the article comprises a bracelet or case component for a wristwatch and the gold or gold alloy coating has a thickness of from about 2,000 to about 5,000 Angstroms.
5. 5. The protectively-coated article of claim 2 wherein the sputter-deposited non-metallic material comprising said film is a dielectric type material from the group consisting essentially of Si.02, SiC, Si3N4, TiO2, A1203, MgO, GeO2, Ta205, Nb205, and spinel.
6. 6. The protectively-coated article of claim 2 wherein the sputter-deposited non-metallic material comprising said film is a glass from the group consisting of soda-lime silicate, borosilicate, aluminoborosilicate and a lead solder type glasses.
7. 7. The protectively-coated article of claim 5 or 6 wherein the article comprises a bracelet or case component for a wristwatch that is composed of stainless steel and the coating of precious metal comprises a coating of gold or gold alloy that (a) has a thickness that does not exceed about 5,000 Angstroms and (b) is bonded to the stainless steel substrate by a layer of nickel.
8. 8. The protectively-coated piece of jewelry or wristwatch component of claim 3 wherein the protective material comprises SiO2 having a film thickness in the range of from about 14,000 Angstroms to about 40,000 Angstroms.
9. 9. The protectively-coated article of claim l wherein, the metallic surface comprises the surface of said article and the article is composed of a metal that has a thermal expansion coefficient greater than the material which constitutes the protective film, and material which is substantially colorless and trans-parent and has a thermal expansion coefficient greater than that of the protective material and less than that of the metal article is disposed between the protective film and said article and thus serves as a buffer means which reduces stresses caused by the difference in the thermal expansion coefficients of the metal article and protective film and thus enhances the adhesion of the protective film.
10. 10. The protectively coated article of claim 9 wherein said buffer means comprises a single layer of stress-reducing material.
11. 11. The protectively-coated article of claim 9 wherein said buffer means comprises a plurality of layers of stress-reducing material with the material in each layer having a different thermal expansion coefficient.
12. 12. The protectively coated article of claim 9 wherein;
    the metallic surface of said metal article comprises a coating of precious metal, and said buffer means comprises two layers of stress-reducing glass-like materials that provide a graded transition from the thermal expansion characteristic of the coated article to that of the protective film.
13. 13. The protectively-coated article of claim 9 wherein the thickness of the buffer material is greater than that of the protective film and is sufficient, in conjunction with the film thickness, to prevent discoloration of the coated article by optical interference effects from incident light rays.
14. 14. The protectively-coated article of claim 1 wherein;
    said article is composed of brass, and said protective film is a material selected from the group consisting of SiO2, TiO2, and MgO.
15. 15. The protectively-coated article of claim 1 wherein;
    said article is composed of a base metal at least a portion of which is covered with a precious metal coating having a thickness substantially less than 100,000 Angstroms, and said protective film is disposed on the metal-coated surface of the article.
16. 16. The protectively-coated article of claim 15 wherein a layer of an adhesion-enhancing material is disposed between the base metal and the Coating of precious metal, said adhesion-enhancing layer having a thickness that is less than that of the precious-metal coating.
17. 17. The protectively-coated article of claim 16 wherein the coating of precious metal comprises an electro-plated layer of gold or a gold alloy having a thickness which does not exceed about 5,000 Angstroms, and said adhesion-enhancing layer comprising a bonding layer of nickel.
18. 18. The protectively-coated article of claim 16 wherein;
    the coating of precious metal comprises a sputtered layer of gold or a gold alloy having a thickness of from about 2,000 to 5,000 Angstroms, and the adhesion-enhancing layer comprises a primer layer of titanium, chromium, nickel or a Nichrome alloy.
19. 19. The protectively-coated article of claim 15 wherein;
    the base metal has a thermal expansion coeffi-cient which is greater than that of the abrasion-resistent material which constitutes the protective film, and at least one layer of a substantially transparent and colorless material having a thermal expansion coefficient greater than that of the material comprising the protective film and less than that of the base metal is disposed between the protective film and the base metal and thus serves as a buffer means which reduces thermally-induced stresses and thus enhances the adhesion of the protective film.
20. 20. The protectively-coated article of claim 19 wherein said buffer means comprises two layers of different type glasses.
21. 21. The protectively-coated article of claim 19 wherein said buffer means comprises a single layer of a selected glass.
22. 22. The protectively-coated article of claim 20 or 21 wherein the base metal is coated with gold, and protective film comprises a coating of a material from the group consisting essentially of SiO2, SiC, TiO2, MgO, Si3N4, A1203, Ta205, Nb205, Ge02 and Spinel.
23. 23. The protectively-coated article of claim 15, wherein;
    said coating of precious metal comprises gold or a gold alloy, and said protective film is composed of a material of the group consisting essentially of 5iO2, SiC, Si3N4, MgO, TiO2, A1203, Ta205, Nb205, GeO2, spinel and a glass that is substantially colorless and has a Knoop hardness of at least about 400.
24. 24. The protectively-coated article of claim 23 wherein;
    said article comprises a piece of jewelry or a component for a wristwatch, and the gold or gold alloy coating has a thickness that does not exceed about 5,000 Angstroms.
25. 25. A protectively-coated wristwatch component according to claim 24 wherein said component comprises a watch bracelet.
26. 26. A protectively-coated wristwatch component according to claim 24 wherein said component comprises a watch case.
27. 27. A wristwatch bracelet according to claim 25 wherein the base metal of said bracelet comprises stain-less steel or brass, and the coating of gold or gold alloy has a thickness in the range of from about 2,000 to about 5,000 Angstroms.
28. 28. The method of coating a metallic article with a different metal and providing the coated surface of the article with a protective film of a selected non-metallic abrasion-resistant material which does not sub-stantially alter the color or appearance of said coated surface, which method comprises;
    placing the metallic article in the coating chamber of a radio-frequency type sputtering apparatus along with a quantity of the metal to be deposited on the article and a quantity of the abrasion-resistant material from which the protective film is to be formed, evacuating said chamber and, after introducing a sputtering gas therein, operating the sputtering apparatus in a first mode such that a sputtered layer of the coating metal is deposited on said article, and while the metal-coated article is still in the coating chamber, operating the sputtering apparatus in a second mode such that a sputtered film of the abrasion-resistant material is deposited over the metal-coated surface of the article, the second sputtering operation being of such duration that the deposited protective film of abrasion-resistant material is substantially transparent and of sufficient thickness to prevent discoloration effects due to optical interference produced by incident light rays.
29. 29. The method of claim 28 wherein;
    said metallic article is composed of a base metal and the metal with which it is coated comprises a precious metal, and the abrasion-resistant material which comprises the protective film is a material of the group consisiting essentially of SiO2, SiC, Si3N4, TiO2, MgO, A1203, Ta205, Nb205, GeO2, spinel and selected glasses that have a Knoop hardness of at least about 400.
30. 30. The method of claim 28 wherein said metal-lic article comprises a component for a wristwatch that is composed of stainless steel or brass and is coated with a layer of gold or gold alloy which is deposited by the first sputtering operation.
31. 31. The method of claim 30 wherein the sputtered layer of gold or gold alloy has a thickness that does not exceed about 5,000 Angstroms, and the protective film of abrasion-resistant material comprises SiO2 that has a thick-ness between about 14,000 Angstroms and 40,000 Angstroms.
32. 32. The method of protecting a selected portion of an article that is exposed to the atmosphere during normal use of the article, which method comprises;
    placing the article in the deposition chamber of a sputtering apparatus along with a target composed of a selected non-metallic material that is chemically inert, has a predetermined index of refraction and, when sputter-deposited, forms a film that is substantially transparent, orienting the article so that the selected portion thereof which is to be protected will be exposed to and be coated with sputtered material from said target when the apparatus is operated, and then operating said apparatus and thereby sputter-depositing a protective film of the said non-metallic material on the selectea portion of the article, the time period which said apparatus is operated and the thickness of the resulting sputter-deposited protective film of material both being so correlated with the reflective index of said material that the sputtered film is substantially devoid of discoloration effects produced by optical interference of incident light rays which enter the transparent protective film.
33. 33. The method of claim 32 where said non-metallic film-foxming material is a material selected from the group consisting essentially of SiO2, SiC, Si3N4, TiO2, MgO, A1203, Ta205, Nb205, GeO2, spinel and selected glasses that have a refractive index in the range of from about 1.4 to about 2.8.
34. 34. The method fo claim 32 wherein;
    said article is composed of a base metal and the said selected portion of the article has a coating of precious metal thereon which is susceptible to scratching and other damage from abrasion during normal use of the article, and the substantially transparent and colorless protective film of sputtered non-metallic material is formed from a material that has a Knoop hardness of at least 400 so that said film is also abrasion-resistant and thereby permits the precious metal coating to be thinner than the coating which would otherwise be employed to avoid exposing the base metal after prolonged use of the article.
35. 35. The method fo claim 34 wherein said pre-cious metal coating is composed of gold or a gold alloy.
36. 36. The method of claim 35 wherein said article comprises a piece of jewelry.
37. 37. The method of claim 35 wherein said article comprises a case or bracelet component for a wristwatch.
38. 38. A wristwatch case according to claim 26 wherein the base metal of said wristwatch case comprises stainless metal or brass, and the coating of gold or gold alloy has a thickness in the range of from about 2,000 to about 5,000 Angstroms.