CA1203723A - Process for obtaining a composite material and composite material obtained by said process - Google Patents

Abstract

PROCESS FOR OBTAINING A COMPOSITE MATERIAL AND
COMPOSITE MATERIAL OBTAINED BY SAID PROCESS

ABSTRACT
A process for producing a composite material comprising a base having a nickel surface and a gold layer metallurgically bonded to the nickel of the base and having sufficient nickel to increase the hardness of the gold layer but insufficient to significantly destroy the distinctive colour of the gold layer comprises depositing the gold layer on the nickel and annealing the composite structure to provide the metallurgical bond between the nickel and gold and to interdiffuse said metals, and the composite structure obtained by said process.
The composite material is particularly useful as a coinage structure.

Description

37;~3 PR~CEgS ~OR O~'rAIN~G A COMPOSITE MATERL~L
AND COMPC)SITE MAT~RIAI. OBTAIN~D BY SAID PR(~CESS

The present invention is concerned with a composite material parti-cularly useful for tokens and coins, and more particularly with tokens or coins having a nickel base and a gold surface.
Essentially pure or low alloy content gold coins have been used since the time of the I,ydians but in today's worldS such coins must have high face value because of the intrinsic value of the metal. As of this writing gold has an intrinsic value of the order of $15.60 (Canadian) per gram. As of now, gold coins exist only essentially as collector or investment items and do not circulate in the sense of that term as applied to 25~ pieces. If widespread circulation of gold coins existed, it would be found that gold, like silver, is too soft ~or evèryday coinage use.
~or these reasons the Canadian 25~ piece was changed from a silver-copper alloy to pure nickel in 1968. In 1965 the United States 25,~ piece was changed from a ~Q silver-10 copper alloy to a ~5% Cu-25% Ni face layer sandwiching a pure copper core. Other countries ha~Fe switched to copper based coinage containing Cu plus A} or Zn or Ni or a comb;nation of the latter elements.
Actual studies of the wear of coinage alloys as a function o~ the number OI years in service are relatively few, but the wear rates Ior the Ag-Cu3- Cu-Ni and Ni are well established and are shown in Table 1.

,~

- 2- PC-2155 ~LISTORïCAL WEAR ~ATA
Wt. Average Wear Rate Year Coin Material (g) mg/yr Current U.S~ 5~ 75% Cu-25% Ni5.0 4 Pre-1965 U.S. 25~ Ag-Cu* 6.25 7 Pre-1968 Can. 25~ Ag-Cu* 5.85 6 Current (:~an. 5~ Ni* 4 55 0 g *Canada's New Nickel Coins, reprinted from Canadian Mining Journal, December, 1968.
Examination of Table 1 shows that the best alloy from a wear viewpoint is pure Ni whose wear rate is 1/4 that of cupro nic}cel (75% Cu, 25% Ni~
and 1/6 that of the 9096 Ag-10% Cu formerly used. Nickel is also superior to Cu-Ni alloys and Ag Cu alloys in terms of corrosion properties. Nickel maintains it's bright appearance for longer than it's normal service life of 20 to 40 years. ~he next best alloy as far as corrosion properties of existing coinage is cupro nickel (75% Cu - 25% Ni).
Thus, for a silver coloured coinage material pure nickel is by far the best. It also possesses unigue magnetic properties between those of iron and non-magnetic alloys which allow it to be easily discriminated in either mechanical or electrical coin operated vending m~chines.
Automated vending machines9 telephones, transit services, etc. are rapidly becoming more prevalent. For instance the number of vending machines in Canada is now estimated to exceed one million units. In many cases the objectvended exceeds the value of one dollar and requires an excessive number of 25~
coins. There is a large need for coins of face value exceeding 25~. In addition to requirements for high denomination coinage for vending machines there is an advantage to saving costs associated with circulating new paper money, eg. in excess of 5~ per year to replace a paper dollar.
Howevar~ if one, two and perhaps five dollar coins were added to the existing 5~, lOs~, 25~ coins a problem would arise. If the coins were all pure nickel and the size gradually increasing, the si~e of the dollar coins would be too large to carry around in a pncket. If the sizes were intermediate between 5~, 10~ and 25~ and all the coins were pure nickel the public would lilcely find coin discrimination difficult. If the coins were made of a brass, bronze, or cupro nickel bronze fllloy they would likely have inadequate wear properties and they would also tarnish quickly in service.

7~3

- 3- PC-2155 Cupro-nickel alloys (75% Cu - 25% Ni) have the best corrosion or tarnish resistance of all of the copper base alloys, but they have a silver colour and the level of nickel must be reduced to less than l0% in order to obtain a golden colour. At this level of nickel the alloy tarnishes. Attempts to introduce other elements such as Al, Zn into the cupro-nickel alloys have yielded golden-colored alloys of only marginally improved tarnish resistance, none being as good as cupro~nickel, and all being very inferior to pure nickel.
lf a brass, bron~e or other cupro nickel bronze alloys are employed for high denomination coins and they perform in an inferior fashion both physically and in appearance to the pure existing low denomination nickel coins, the high denomination new s~oins will likely lack public acceptance.

STATEMENT OF PROBLEM
A new material is required which has wear properties as good as or close to pure n;ckel, has the same tarnish resistance as pure nickel, and has a gold colour.

BRIEF DESCRIPTION O~ DRAWING
The accornpanying figure is a graph in which accelerated coinage wear (weight loss of coins in mg) is plotted against time in test (days).

DESCRIPTION OF TEIE INVENTION
The present invention contemplates a composite material, particularly a coin or token ~tructure, comprising a base having at least a surface of nickeland bearing on the surface thereof a layer of gold at least about 0.l llm thic!c up to e.g. about l~5 llm or even 2.5 ~Im thick. The gold layer is metallurgieally bonded to the nickel of the base and contains an amount of nickel sufficient to increase the hardness of the gold layer but insufficient to significantly destroy the distinctive colour of the gold layer.
The token or coin structure of the present invention can be made by`
depositing, e.g. by electroplnting, a layer of gold onto a nickel surfaced base and thereafter annealing the composite structure to provide a metallurgical bond therebetween and to interdiffuse nickel o~ the base and gold of the surface layer.

~1 ~r~

DETAILS OF THE INVENTION
In order to provide those skilled in the art with greater detail with respect to the present invention, the following paragraphs set forth particularsand alternatives contemplated by the inventors to be within the scope of the invention as hereinbefore described.
The base OI the composite structure of the invention is advantageously made of essentia31y pure nickel but can be made of a high nickel alloy, i.e., containing greater thRn about 50% nickel, which has corrosion resistance and workability substantially equivalent to that of pure nickel. However, nickel alloys containing less than 50~, nickel, e.g.9 7596 Cu, 25% Ni, may be used but such alloys are less preferable. ~Vhen used in this specification and claims the termt'nickel" includes such nickel alloys. Generally speaking, the ~ase oî a coin ortoken structure is monolithic, i.e~ made wholly of a single material. HowevPr, i~
desired, the base can be a composite or a sandwich structure provided the surface is nickel.
In physical form, the base of the token or coin structure is of coin thickness e.g. up to about 0.6 cm but can be of any desired width and length. For example, the base can be sheet or strip out of which coin or token blan~s can bestamped. Alternatively, the base can be a coin or token blank stamped from a sheet of nickel or any intermediate form of the coin or toXen. Specifically, thebase can be a bl~nk which has béen sized or sized and coined, or sized, coined and rimmed or sized, coined, rimmed and milled. Those skilled in the art will appreciate that gold plating on each of the alternative types of coin or token bases will result in the front and rear of the coin or token having a gold colour.
However, plating done prior to rimming and miUing will result in coins having anedge with the appearance of nickel. Alternatively, the substrate can be placed on a moving band or can be masked so that the gold deposit can occur on one side orselected areas.
The gold on the nickel surface of the base is advantageously electr~
deposited as pure ~24 KT3 gold from an electroplatin~r bath using conditions applicable to obtaining a pure electrodeposit. For purposes of this speci~ication ancl claims, however, gold need not be 100æ pure. For purposes of this inYention, the term "gold" includes not only pure gold but also yellow alloys and reddish ycllow nlloys which may contain s;lver, copper, nickel, a plntinum-group metal alld combinations thereo~. Gold contnining up to about ~ to l 0~, by ~veight .,.6~

nickel will retain its gold colour equivalent to the colour of 14 KT to 18 KT gold-silver-copper alloy.
The gold layer on the nickel surface of the base is adYantageously electrodeposited from a cyanide type bath. Such baths are usually of pro~rietarynature. However~ the general types of cyanide baths and conditions of operatio ~re set forth in standard reference sources such R5 Electroplating Engineering Hanclbook (3 ed.) A. Kenneth Graham, Van Nostrand Reinhold Company (C) 19~1 page 242 and in Re'ferences l~ 2, 29, 30, 31, 32 and 33 listed on page 255 of that work. It is also within the contemplation of the present invention to provide gold layers of the requisite thickness by means other than electroplating provided, of course, that the quality of the adhesion of the gold layer to the nickel is ~t least equivalent to that provided by electropla~ing~ As those skiUed in the art are aware, the quality of adhesion of an electroplate depends to a large extent on the care taken in surface preparation and cleaning of the base to be plated. In thisregard Chapter 3 of the aforecited Electroplating Engineering Handbook entitled Metal Surface Preparation and Cleaning is recommended to those desiring to practice the present invention.
In producing the composite structure of the present invention, the annealing step to achieve the metallurgical bond between gold and nickel and to harden the gold is an important feature. Annealing can be carried out any time after deposition of the gold layer, either before or after working or mechanicaloperation on the composite material. The conditions of temperRture and time of annealing are selected so that an interdiffused layer of gold and nickel is achieved without loss of the desired surface color. For exampl~, with a pure nickel base and a pure gold deposit averaging 0.3 ~Im thick, effectiYe annealing in 1~y~
at D~50~C rnay be achieved in a short time e.g., in the r~nge of about 2 to 5 minutes. This anne~lin~ time has been determined as residence time in a 5 cm tube furnace under laboratory conditions. Those skilted in the art will ~ppreciate that commercial scale furnacing may vary in temperature and time. Nickel has a significant whitening effect on gold. After approximately 6 minutes of annealin under those specified laboratory conditions, the vhiten;ng effect o nickel predominates and the golden colour is lost. Annealing under the same conditions for Iess than two minutes is insuf~icient to produce the required metaUurgical bond. While this example of annealing teaches an effective annealing procedure operahle in the context of the present invention, Yariations are obviously possible.

For example, annealing can be conducted at lower tempe~atures e.g., down to about 350 C for longer times. Hydrogen can be replaced as an annealing atmosphere with any atmosphere, e.g., cracked ammonia, argon, etc. which will prevent oxidation of the nickel. In addition, if alloy gold is used as the layer, less nickel diffusion can be tolerated and there~ore annealing should be carried out at temperatures lower than 450 ~nd for times less than those employed for pure goldO Still further, if gold is employed in thicker layers e.g., from 0.5 to 1.0 ym, somewhat greater $irnes or higher temperatures should be employed in ~nn~lin3o in order to permit dif~usion oe some nickel to the outer-most volume of the goldlayer in order to induce increased hardnes~ throughout the gold lay~r. Conversely~
extremely thin gold layers e.g., about 0.1 ~m must be annealed for very short times in order to retain the visual yellow effect of the gold. Annealing tlme isalso dependent upon the thickness o~ the nickel-containing base, generally longer times being necessary with thicker bases. The type of heating equiprnent also isdeterminative of annealing times. For example, with high frequency induction skin heating, shorter time~ can be used because there is no need to allow time for the entire base to come to annealing temperature. In general, annealing can be carried out at temperatures of about 350 to about 650C for about thirty minutesto about a few seconds.
The composite stru~ture of the present invsntion is particularly adapted to be useà as coins or tokens, other structures in which a golden appearance, wear resistance and corrosion resistance are desirable are within the contemplation of the invention. Such other structures include flatware, plumbingfixtures and the like.

E2~AMPLE I
The follou1ing coins were dipped into a 5 wt.~ MaCl solutiorl which had been adjusted to pH 4 with nitric acid once per dEly for four days and allowed to air dry at room temperature the balance of the day.
1. Pure Ni Canadian 25~.
2. ~ ~J.S. 25~ piece c ontaining outer faces o~ 75%Cu-25%Ni and a copper core.
3. ~ gold coated nickel coin, with the gold coating prepared according to the procedure detailed below.
. A 1983 British Pourld containing 7596Cu-1~%Zn-7%Ni.

3~23 5. A gold coated 75%Cu-25~n coin, with the gold coating prepared following the same procedure as 4.
Results The surfaces OI the pure Ni coin (1) and the new gold on Ni coin (3~
were not visibly tarnishecl. The U.S. cupro-nickel 25~ piece (2) lost its metallic luster but had no visible evidence of a tarnish. All the other coins including the British Pound (4) and the Au on brass composite (5) lost their metallic luster and had objectionabl~ tarnish formed on their sur~aces.
These results show that the latter two coins (4) and ~5) are unsuitable as high denomination coins.
Method of Preparation of the New Au on Ni Composite Coinage Material A mint Canadian 25~ pure nickel coin 1 was cle~ned in a mild acidic solution and plated at 60 C at 5 milliamps per square centimetre for 1-2 minutes from a standard cyanide-type Au plating bath obtained from Johrlson Mattlley andMallory Ltd. under the tr~demark "Orosene 999 24 KT bright gold". The weight of the Au plated was 6 mg and the apparent calculated thickness of the Au layer was0.3 ~Im. This coin was subsequently annealed at 450~C îor 3 to 4 minutes in purehydrogen. This procedure on this substrate gives a coin which is gold in colour lighter than the colour o~ 24 KT gold.

Ei~ht Canadian 25~ pure nickel coins were plated with 6 mg of Au each according to the procedure in Example 1. Four of the coins were subsequently annealed at 450C for 4 min in p~e hydrogen, the other four coin~
were unannealed~
The thus prepared coins were subjected to wear testing The appa-ratus for wear testin~, in this Example and in other ~xamples herein comprises aceramic jar mill having internal dimensions of 4 inches high (10.16 cm) and 5 inches wide (12 7 cm). The mill is closely lined with a zippered bag made of metallo~raphic polishing cloth. The coins are introduced into the bag along withan egual weight of metal shot 0.3 to 1.9 cm in diameter made of an alloy known as HASTELLOYTM alloy C containillg nominally (in weight percent~ 54% nickel, 17~
molybdenum, 1596 chromium, 5% iron and 49~ tungsten, 13 grRms of leather strip about 3.8 crn x 0.3 cm x 0.l5 cm and 7 grams OI cork No. 00 about 0.12 cm in dialneter. The leather strip is soaked with synthetic sweat solution (~0 g NaCI, 5 ~3~3 - 8- P~-2155 g Na2HP04, 4 ml lactic acid, balance, to 4 Q, distilled water), the bag is zippered closed, fitted into the ceramic jar mill and then rotated at less than critical speed for numbers of days with the leather being periodically rewet with synthetic sweat solution. The mill was rotated for 5 days at 45 rpm.
S Results The mean weight loss of the annealed coins was 6.8 mg while that for the unannealed coins was 9.0 mg. The annealed samples had a more unifor colour than the unannealed sample.

Five Canadian pure Ni 25~ pieces were plated with 24 KT Au, with the Au plated according to the procedure set forth in Example 1. The co;ns were placed into a 5 CM tube furnace having a H2 atmosphere at 450C for times ranging from about 1 to about 10 min. The surface of the Au plated coins retained their 24 KT appearance up to 3 min. After about 4 minutes the colour matched the colour of 18 KT (75% Au, 15% Ag, 10% Cu) gold. After about 6 minutes the gold colour had largely disappeared.

Two samples of the Au coated pure Ni C~n~ n 25~ pieces containing about 6 mg of Au in a layer 0.3 1I m thick annealed at 450DC for 4 minutes alongwith two U.S. cupro-nickel 25~ pieces and two Canadian pure nickel 25~ pieces were tumbled in the ceramic rnill at 45 rpm in a cloth bag containing cork and leather wetted with a synthetic sweat solution and an equal weight of HASTELLOY C shot as described in Example 2. The weight loss OI each coin was measured each day and the results plotted in ~igure 1 The data in FiPOure 1 showthat the wear rate of the 17.S. cupro-nickel is approximately twice that of purenickel and that the weight loss of the Au plated and annealed nickel coins was indistinguishable from pure nickel Canadian 25~pieces. It was not anticipated that a coin having roughly 6 mg of gold on nickel would stiLI have a good surface appearance after 20 mg had worn away. Yet, even after losing 40 mg of weight the 1~l coated coin still retained its golden colour and only on the edges of the c~oin wns Au layer abraded away.

_ g_ P~:-2155 Eight pure Ni Canadian 25~ pieces were Au plated using the procedure set forth in Example 1 and annealed at 450~C for 4 min. The measured weigl~t of gold electrodeposited was 6 to 8 mg per coin. These coins along with 8 Canadian nickel 25~ pieces and 8 U.S. cupro-nickel 25~ pieces were repeatedly placed by s hand through a mechanical vending machine coin sorting mechanism m~n-lfactured by Coineo, 868 Progress Ave., Scarborough, Ontario, CanQda.
After 8,000 cycles,- the U.S. cupr~niclcel coins had lost much of their knurled edge. The edge ~ras quite smooth. After 10,000 cycles the Canadian nickel 25~ piece showed some slight rounding of the eàge and the Au plated and annealed Canadian 25~ piece still retained their original golden colour even on the edges.
After 18,000 cycles, the knurled edges on the Canadian 25~ piece started to show a slight wear. The Au plated and annealed coins still retained their original golden colour on tile coin faces after 18,000 cycles and showed the same slight knurled edge wear as the pure nickel 25~ pieces. Some loss of golden~olour occurred on the edges after 18,U00 eycles.
After 18,000 cycles~ the total weight of Au on the coins w~s analyzed as 5 25 mg taverage) indicating only about a 1.7 m~, Au loss and 7096 Au retained during the test. Eighteen thousand cycles eorresponds to approximately 50 years of wear on a coin at an average use of once per day in a vending machine.

The same tumbling test as used in Examples 2 and 4 was rep~ated except the coinage charge consisted of 2 pure nickel S~anadian 25~ piecesJ two U.S. cupro-nickel 25~ pieces3 1 Canadian nickel 25~ piece plated with 6 mG of A~u and annealed as in Example 1 for 4 min at 450C în pure H2, plus 1 Canadian nickel 25~ piece plated with 12 mg of Au and annealed to have the same surface colou~ as the coin having 6 mg o~ Au after annealing. 'I`he tumbling mill was rotated for 1 day at 60 rpm, 3 days at 27 rpm and 1 to 5 days at 45 rpm. The mean weight loss of the coins after this treatment for 5 day and 9 day periods is shown below:

S day 9 day U.S. 25~ cupro-nickel18,9 32.7 Canadian 25~ pure ~i6.2 14.6 6 mg Au on Canadian 25~ 6.9 12 mg Au on Canadian 25~ 7.4 15.8 The 6 mg and 12 mg Au coins were indistin~uishable in visual appearance after the test.
Both retained their original golden colour even on lthe edges, Two sarnples of pure nickel-base, gold electroplated (0.3 u m~ coin stock were subjected to Auger analysis to determine the nature of the gold layerin the unannealed and annealed conditions, The annealing was carried out as set forth in Example 1. Analysis of the unannealed sample indic~ted essentially puregold in the electrodeposited layer with a very sharp boundary to pure nickel of the base. The annealed sample showed essentially R 95%Au-5~6Ni composition throu~hout the gold layer with a somewhat diff'use but relatively sharp boundaryto the essentially pure nickel of the base.

The ~ollowing specimens were dipp~d into a 5 wt.~6 NaCl solution, which had been adjusted to a pH of 4 with nitrie acid~ once per day, for 3-1/2 days and allowed to Rir dry at room temperature for the balance o~ the day.
6. A gold coated 18/8 stainless stee} base, 7. A gold coated 75%Cu-25%Ni baseO
The gold WAS plated and the composite structures anne~lled essentially as described in Example 1. The gold plated stainless steel sample (G) shawed evidence of tarnish. l`he gold plated cupro-nickel sample (?~ showed a slight green oxidation product which was easily wiped off but might remain in crevice3 in use.

While in accordance with the provisions of the statute, there is 3() illustrated and described herein specific embodiments of the invention~ Those shilled in the art will understand thRt changes may be made in the form of the invention covereà by the clrlims and that certain features oE the invention may sorrletirnes be used to advantacre without a corresponding use of the other features.

Claims (27)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A composite coinage material comprising a base having at least two opposing surfaces of nickel and bearing on each of said surfaces a layer at least about 0.1 µm thick of gold metallurgically bonded to said nickel base, said gold layer having been electrodeposited on each of said nickel surfaces and having present therein an amount of diffused nickel originating from said nickel of said base sufficient to increase the hardness of said gold layer but insufficient to significantly destroy the distinctive colour of said gold layer.

2. A composite coinage material as in claim 1 comprising a coin or token structure wherein the layer of gold is about 0.1 µm to about 2.5 µm thick.

3. A composite coinage material as in claim 1 comprising a coin or token structure wherein the layer of gold is about 0.1 µm to about 1.5 µm thick.

4. A composite coinage material as in claim 2 wherein said base is a monolithic base of essentially pure nickel and said gold layer apart from nickel is a layer of essentially pure gold.

5. A composite coinage material as in claim 2 wherein said base is a token or coin blank.

6. A composite coinage material as in claim 2 wherein said base is a nickel sheet adapted for coin blanking.

7. A composite coinage material as in claim 2 wherein said base is a token or coin.

8. A composite coinage material as in claim 2 wherein said base is a sized and rimmed coin or token blank.

9. A composite coinage material as in claim 2 wherein said base is a corrosion resistant nickel alloy.

10. A composite coinage material as in claim 2 wherein said base is a composite material, the outer surfaces being nickel surfaces.

11. A composite coinage material as in claim 1 wherein said base is ferromagnetic.

12. A process for providing a composite coinage material comprising providing a base having at least two opposing surfaces of nickel, depositing on at least two said surfaces of said base a layer of gold at least about 0.1 µm thick and thereafter interdiffusing said gold and said nickel to form a metallurgical bond therebetween and introduce into said gold layer an amount of nickel from said base sufficient to increase the hardness of said gold layer but insufficient to significantly destroy the distinctive colour of said gold layer.

13. A process for providing a composite coinage material comprising providing a base having at least two opposing surfaces of nickel, electrodepositing on each of said surfaces of said base a layer of gold at least about 0.1 µm thick and thereafter interdiffusing said gold and said nickel to form a metallurgical bond therebetween and introduce into said gold layer an amount of nickel from said base sufficient to increase the hardness of said gold layer but insufficient to significantly destroy the distinctive colour of said gold layer.

14. A process as in claim 13 wherein each layer of gold is deposited to a thickness up to about 2.5 µm.

15. A process as in claim 13 wherein the interdiffusing operation is carried out subsequent to carrying out a mechanical operation on said coin or token structure.

16. A process as in claim 15 wherein said mechanical operation is at least one of blanking, sizing, rimming and coining.

17. A process as in claim 13 wherein the interdiffusing operation is carried out prior to carrying out any mechanical operation on said coin or token structure.

18. A process as in claim 12 wherein each layer of pure gold is electro-deposited on said nickel base at a thickness of about 0.3 µm and said interdiffus-ing is accomplished by annealing at a time/temperature profile equivalent to about 3 to 6 minutes at about 450°C in a 5 cm tube furnace.

19. A process as in claim 13 wherein interdiffusing is carried out to an extent that the gold layer has a colour closely matching the appearance of 18 KTgold-silver-copper alloy.

20. A process as in claim 13 wherein interdiffusing is carried out to an extent that the gold layer has a colour closely matching the appearance of 14 KTgold-silver-copper alloy.

21. A process as in claim 13 wherein the annealing is carried out in a reducing atmosphere.

22. A composite coinage material prepared according to the process of claim 12 wherein in the annealed condition the gold layer contains up to about 10% Ni.

23. A composite coinage material as in claim 22 wherein the gold layer contains up to about 5% Ni.

24. A process as in claim 12 wherein the interdiffusing is carried out at a temperature of 350° C to about 650° C for about 30 minutes to about a few seconds, the time at temperature being inversely related to the temperature, andwherein said interdiffusing is carried out in a non-oxidizing atmosphere relative to nickel.

25. A process as in claim 24 wherein said atmosphere comprises hydrogen.

26. A process as in claim 18 wherein said interdiffusing is carried out in an atmosphere comprising hydrogen.

27. A process as in claim 24 wherein said atmosphere is selected from the group hydrogen and cracked ammonia.