PLATING COMPOSITION AND PROCESS

Abstract: Small amounts of lead can be used in cobalt electroless plating compositions in order to refine the structure of the deposited cobalt. The lead also stabilises the composition, particularly when additive particles are present and the composition is adapted to deposit a composite cobalt deposit. The additive particles in the composite may have been pre-treated with lead, cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony, and/or lead may be added to the plating composition. Cadmium may also be present. Lactic acid has been found to have a beneficial moderating effect on the plating operation.

Description: PLATING COMPOSITION AND PROCESS This invention relates to electroless cobalt plating. In particular, it relates to the production of composite cobalt plates, produced by electroless plating, in which the cobalt matrix contains heterogeneous particles. Electroless deposition in general is a known technique. It is a process whereby metal ions in solution are reduced to metal atoms on a surface which is to be plated. Electroless plating is distinguished from electroplating, in which the reducing power is supplied by electricity, by the provision of a chemical reducing agent. Electroless cobalt deposition is itself known. Pearlstein and Weightman (J. Electrochem. Soc. (1974) 1023-1028) have described the electroless deposition of cobalt from acid baths. They reported that the coercivity of certain electroless cobalt-boron deposits was low, and electroless cobalt has been used in the preparation of magnetic memory surfaces. Although nickel is perhaps the most common metal deposited by electroless techniques, cobalt does have certain advantages: for example, electroless cobalt depositions have been able to withstand higher temperatures than corresponding nickel depositions, which is advantageous in some applications. Electroless plating has in the past been used to produce composite coatings. Composites are coatings in which additive particles (for example silicon carbide) in the plating solution become entrapped in the plated metal as it is forming a layer on the substrate. Composites are often produced where it is desired to impart to the coated substrate good wear characteristics, particularly at elevated temperatures. For example, internal combustion engine valves have been plated with composite coats produced by electroless deposition. Because, in an electroless plating bath, the reducing agent is present at the same time as the metal ion to be reduced, the bath is thermodynamically unstable. As a practical matter, therefore, the metal ions which are to be deposited are usually complexed so as to prevent their reduction to metal in the bulk of the solution; but the metal ions should not be so strongly complexed that the deposition of the metal onto the substrate to be plated will be prevented. The surface of the substrate will usually be to some degree catalytic (either inherently or by being rendered catalytic) for the reduction of the metal ion to the metal. The problem of stability is particularly acute in electroless plating baths for producing composite platings. This is because such baths will contain often irregularly shaped additive particles, and so there is a problem in how to prevent the metal in solution from being plated onto the additive particles. Thallium has previously been reported as a stabiliser for certain electroless nickel deposition systems. Unfortunately, thallium does not seem to be particularly effective for cobalt when added to the bulk of the bath, and in any case it is extremely toxic. There is therefore a need for a means of stabilising baths for producing electroless" cobalt composite plates. It has now been discovered that, in appropriate amounts, lead can act as a stabiliser for such baths. It has also been discovered that lead can refine the structure of the electroless cobalt deposit produced, whether or not additive particles are present. " According to a first aspect of the present invention, there is provided an electroless cobalt ' plating composition comprising a source of cobalt ions, a reducing agent and a source of lead in an an amount sufficient to refine the structure of the deposit. According to a second aspect of the invention, there is provided an electroless cobalt composite plating composition comprising a source of cobalt, ions, a reducing agent, additive particles and a source of lead in an amount sufficient to stabilise the composition but insufficient substantially to prevent cobalt deposition on a surface of a substrate on wtiich the cobalt is to be deposited. For the source of cobalt, any convenient and sufficiently soluble salt of cobalt Iriay be used. Cobalt sulphate or cobalt chloride is generally found to be convenient. The source of cobalt may be present in an amount of from 1 to 100 g/1, for example 10 to 45 g/1, typically about 30 g/1, calculated as cobalt sulphate hexahydrate. The cobalt deposit may contain a minor amount, for example up to 15, 20, 25 or 30%, of an alloying metal such as nickel. In such a case, the amount of cobalt in the plating composition may be (but is not necessarily) slightly less than the middle of the broad and preferred ranges given above. A source of alloying metal such as nickel may be present to provide metal ions in an amount of from 0.1 to 3 g/1, preferably 0.5 to 1.5 g/1; these amounts could be realised by providing, for example, 0.45 to 13.5 g/1, and preferably 2.25 to 6.75 g/1, nickel sulphate hexahydrate. As a practical matter, a complexor will usually be present to maintain the cobalt and/or alloying metal in solution under the appropriate operating conditions. Any suitable complexor may be used, but carboxylic acids (for example C-^-C 8 carboxylic acids) , particularly dicarboxylic acids (for example C 2 -C 10 carboxylic acids) may be advantageous. Succinic acid has been found to be the most appropriate, but other suitable acids include acetic, glycollic, oxalic, glutaric and adipic acids. The complexor may be present in an amount of from 1 to 100 g/1, for example 5 to 40 g/1, typically about 25 g/1. It will be appreciated that both the source of cobalt and the complexor may be provided as a single source, for example cobalt succinate. (References in this specification to weak (eg organic) acids are to be taken to include their salts, " -and vice versa, as the exact nature of the species present will depend on the pH.) Any appropriate reducing agent may be used. Hitherto, dimethylamino borane (DMAB) has been found to be the most appropriate. It appears that the use of such a reducing actually yields electroless deposits of cobalt-boron, much in the way as the ""use of hypophosphite in electroless nickel plating produces nickel-phosphorus. It is to be understood that in this specification the word "cobalt", when referring to the deposited metal includes such entities as cobalt-boron alloys, cobalt-phosphorus alloys and compositions of cobalt, boron and phosphorus. Boron hydrides and their derivatives in general and alkyl amino boranes (including DMAB) in particular are suitable reducing agents. Mixtures of more than one reducing agent (such as DMAB and hypophosphite-] can be used. The reducing agent may be present in an amount of from 0.5 to 20 g/1, for example from 2 to 6 g/1, typically about 4 g/1. The composite particles may be of any suitable material. Additive particles which may be used include graphite, polyfluorocarbons such as polytetra- fluoroethylene (ptfe) , carbides, oxides, borides and nitrides; even non-catalytic metallic particles such as molybdenum can be co-deposited to provide another family of composites. Silicon carbide is a particularly preferred additive particle because it is available in purified form and a wide range of particle sizes, is reasonable in cost and has been found to be excellent in performance. Chromium carbide and chromium oxide particles are the additive particles of choice in the aeronautical industry. The particles may range from 0.5 to 50 microns in size, for example 1 to 5 microns and typically about 2 microns. The additive particles may be present in the electroless deposition composition in an amount of from 0.1 to 50 g/1, typically 0.5 to 10 g/1, for example about 2 g/1. The amount of lead that is to be present is to be chosen so as to be sufficient to refine the structure of the deposit produced and/or to stabilise the composition (particularly when depositing composites) , but insufficient substantially to prevent the cobalt being deposited on the substrate. Too high a concentration of lead is believed to poison the catalytic qualities of the substrate. The amount of lead that is suitable for use in the invention may vary depending on the composition conditions, but will generally be in the order of from 0.5 to 40 ppm, for example about 1 to 20 ppm. Appropriate amounts can be determined by simple experimentation within the general teaching of this specification. The lead may be provided simply by adding a source of lead ions to the composition (for example in an amount of from 1 to 10 ppm, typically 3 to 5 ppm) and/or it may be provided, in the case of composite plating compositions, by pre-treating additive particles (when present) with lead and adding the so-treated particles to the plating composition. This pre-treatment technique is preferred when composites are to be deposited, as it tends to ensure that there is a higher concentration of lead on the surfaces of the additive particles rather than on the surface which is intended to bear the deposited plate, thereby effectively directing plating* onto the desired surface. It is not, however, essential, as the lead in the plating composition may preferentially adsorb onto the additive particles, particularly if the additive particles are angular rather than smooth. As an alternative to lead, additive particles may be pre-treated with cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony. For convenience, these elements, including lead, will be referred to as "pre-treatment metals". The additive particles may be pre-treated by contacting them with a solution of a soluble pre-treatment metal salt, for example lead or cadmium acetate. For example, additive particles may be soaked (for -example for 1 to 30 minutes, typically 5 to 20 minutes) in a solution of lead acetate or another suitable pre-treatment metal salt such as a nitrate, which is at a concentration of from 0.1 to 10 g/1 lead acetate (or equivalent), for example 0.5 to 2.5 g/1 in water or a dilute acid such as 10% v/v acetic acid. Prior to use, the treated particles may be rin≤ed to remove excess salt. The pre-treatment is believed to cause lead or another pre-treatment metal to adsorb onto the additive particles. The pre-treatment metal may be present in ionic form or covalently bound in a compound or as the free metal. The plating composition may contain cadmium, which acts as a further stabiliser and can act as a brightener and/or grain refiner and/or it can promote bath life. The use of cadmium in electroless cobalt plating baths is the subject of another application filed today by the same applicants as are named in this application and claiming the same two priorities; where the law allows, the other application is herein incorporated by reference. The amount of cadmium that is to be present may be chosen so as to be sufficient to stabilise the bath, in combination with the other bath components, and/or sufficient to have any of the other beneficial effects of cadmium, but insufficient substantially to prevent the cobalt being deposited on the substrate. Too high a concentration of cadmium is believed to poison the catalytic qualities of the substrate. The amount of cadmium that is suitable for use in the invention may vary depending on the bath conditions, but will generally be in the order of less than 15 ppm, for example from 0.1 to 8 ppm, typically from 0.5 to 7 ppm, and preferably about 6 ppm. Appropriate amounts can be determined by simple experimentation within the general teaching of this specification. The cadmium may be added in ionic form or covalently bound in a compound or as the free metal, in which case it will generally dissolve to form a salt. It is preferably supplied as a soluble salt, such as cadmium acetate. The amount of cadmium present is calculated by reference to the metal. Other stabilisers, brighteners, grain efinfers and/or bath life promoters can additionally or alternatively be present. The plating composition may contain an accelerator. Accelerators act to improve the plating' rate. Sulphur-containing compounds in general are good accelerators, although their use in excess can be marked by a deterioration in surface quality. Some sulphur-containing compounds such as thiourea are initially effective but prone to hydrolysis. Preferred accelerators are therefore sulphur-containing compounds which are generally stable to hydrolysis. An example of such a compound is mercaptobenzothiazole (MBT) which may be used in an amount of from 0.1 to 20 ppm, for example 1 to 5 ppm and typically about 2 ppm. The composition may contain a moderator. Lactic acid has been found to have an effect on the overall quality of the plating operation, for example by enhancing stability and improving the structure of the " deposit. Lactic acid is known for use in nickel-hypophosphite electroless plating compositions, where it has the effect of removing an unwanted by-product. Here it is having a different effect. According to a third aspect of the invention, there is therefore provided an electroless cobalt plating composition comprising a source of cobalt, a reducing agent, other than hypophosphite and lactic acid present in an amount to moderate the plating. The lactic acid may be present in the composition in an amount of from 0,1 to 30 ml/1, for example 1 to 10 ml/1, typically about 5 ml/1. The composition may contain a wetting agent, for example to reduce surface tension as an aid to the release of gas bubbles. A wide range of surfactants are suitable, particularly nonionic and anionic surfactants. Ethoxylated alkyl phenols, exemplified by those sold under the trade mark TRITON XT 100, are a suitable class of nonionic surfactants, and sulphosuccinates, such as those sold under the trade mark LUTENSIT ABP are a suitable class of anionic surfactants. The wetting agent may be present in an amount of from 1 to 75 ppm for anionic surfactants or from 1 to 40 ppm for nonionic surfactants. Amounts in the range of from 1 to 5 ppm are typically used. The pH of the plating composition will generally be on the acid side, but if the pH is too low, the plating rate is reduced. It is therefore preferred for the composition pH to be in the range of from 4.5 to 6.5, for example about 4.8 to 5.5. The temperature of the composition may be adjusted as desired. A higher temperature results in a faster plating rate, but there are corresponding sacrifices in the energy consumption of the system. It is generally preferred for the temperature to be in the range of from 60 to 90°C, for example from 70 to 85°C. 11 According to a fourth aspect of the invention, there is provided a process for plating an electroless cobalt deposit onto a substrate, the p-rocess comprising contacting the substrate with an electroless cobalt plating composition comprising a source of cobalt ions, a reducing agent and a source of lead in an amount sufficient to refine the structure of the deposit. According to a fifth aspect of the invention', there is provided a process for plating a composite electroless cobalt deposit onto a substrate,* the process comprising contacting the substrate with a plating composition comprising a source of cobalt ions, a reducirfg agent, additive particles and a source of lead in an amount" sufficient to stabilise the composition but insufficient substantially to prevent cobalt depfosition on the substrate. The method of contacting the surface of the srSbstrate to be plated with the composition will usually be immersion. However, it should be understood that immersion is the not necessarily the only suitable method. It may, for example, be possible to develop a suitable spray-plating method. The time of contact between the substrate and the plating composition will naturally depend on the thickness of the deposit required and the plating rate. Contact times in the order of 5 minutes to 5 hours may be appropriate, with contact times of from 30 minutes to 2 hours being typical. The plating rate itself will generally be optimised to a degree that is compatible with the quality required of the finished product. Plating rates in the range of 20 to 30 microns per hour are generally achievable by means of the invention, but higher quality deposits may be achievable by more modest plating rates. Plating rates in the range of from 10 or 15 to 20 microns per hour, for example about 17 microns per hour, have been found to be optimal. In order to obtain a sufficiently even distribution of additive particles in the composite, it is usual to agitate the composition during plating. Agitation may be achieved by any suitable means such as mechanical stirring, pump agitation, magnetic stirring and air agitation. The plating may take place on any suitable substrate. Steel and other metals are preferred substrates. It is not essential that any pre-treatment of the substrate take place, although for non-conductive substrates an activator such as palladium may be deposited. Cleaning and desmutting operations may be appropriate, as will be known to those skilled in the art. When working with steel, a thin electroless nickel deposit may be used with advantage as an undercoat. Any suitable electroless nickel plating composition may be used, such as that supplied by Lea Manufacturing Co Ltd of Buxton, England under the trade mark ALLIED KELITE 794. Plating may take place for as little as 5 minutes to produce a very thin substantially pure nickel layer. According to a sixth aspect of the invention, there is provided a process for preparing additive particles for use in a composite electroless cobalt plating composition, the process comprising treating the particles with lead, cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony in aι " amount sufficient to stabilise the composition but insufficient substantially to prevent cobalt deposition on a surface of a substrate on which the cobalt is to be deposited. According to a seventh aspect of the invention, there are provided particles of material, suitable for inclusion in plated cobalt by means of an electroless plating composition to form a cobalt composite plate, the particles comprising lead, cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony in an amount sufficient to stabilise the composition. According to an eighth aspect of the invention, there is provided an article having a surface bearing a cobalt-boron composite coating. Preferred features of all the aspects of the invention are as for the first aspect, muta is mutandis. The invention will now be illustrated by the f llowing examples . Example 1 Silicon carbide particles (3 to 5 microns diameter) were soaked for 15 minutes in a 1 g/1 lead acetate in 10% aqueous acid. This was then rinsed until the pH of the effluent became neutral. Example 2 A cobalt electroless plating composition of the following composition was prepared: Pre-treated silicon carbide, prepared in accordance with Example 1, was added to the above solution at 2 g/1. The solution remained stable for 45 minutes during heat up. A small test coupon of steel (38 cm ) was plated for 30 minutes, by which time the silicon carbide particles had begun to be incorporated into the deposit. The test coupon was rotated throughout the plating process, and the bath itself was agitated continuously. A good deposit was obtained. Example 3 The procedure of Example 2 was followed, except that the plating solution additionally contained 0.1 ml/1 LUTENSIT ABP. A good deposit was obtained. Example 4 The procedure of Example 2 was repeated^ except that lead was added as lead acetate to the composition in an amount of 5 ppm (lead) . No plating out occurred after four hours at operating temperature and air agitation. A steel coupon (38 cm 2 ) was then plated for one hour, good results were obtained. Example 5 This example was carried out in 3 litre tall form beaker at 70°C on a stirrer hotplate with digital thermocouple temperature control. An electroless cobalt solution was prepared containing 6g/l cobalt, 2ppm lead (added as lead acetate) and 0.1 ml/1 LUTENSIT ABP but otherwise in accordance with the electroless cobalt plating solution as described in Example 2. Test coupons were plated with additions made of cobalt sulphate and dimethylamine borane only to maintain the bath chemistry. When 18g of cobalt metal had been deposited and the additi©ns made, plating out occurred on the bottom of the beaker. The solution was filtered and reheated to operating temperature whereupon decomposition began to occur. Filtration was again carried out and the solution regenerated by the addition of cobalt sulphate and dimethylamine borane. The lead in the bath was replenished by the addition of 6mg lead to give a working concentration of 2ppm. The bath was reheated to 70°C and plating resumed. No sign of solution decomposition was evident until a further 18g metal had been deposited. The deposits produced from the above experiments, although smooth, had a matt appearance. When viewed under a metallurgical microscope (magnification 200X) the surface of the coating exhibited a nodular structure. Example 6 The procedure of Example 5 was repeated, but in order to refine the structure of the deposit, 2ppm cadmium was added (as cadmium acetate) to the plating bath. No deleterious affect on plating rate was noticed. The deposit produced from this solution had a high degree of reflectivity. When viewed under the microscope (magnification 200X) , the surface structure appeared to be laminar as opposed to nodular when no cadmium is present in the solution. Example 7 This is a comparison example. A 390g sample of 1-4 / xm chromium carbide powder (London and Scandinavian Metallurgical Co Ltd) was prepared by washing in 50% nitric acid solution for 1 hour. The particles were washed repeatedly with demineralised water until a neutral effluent was achieved. The 17 sample was then introduced into a 26 litre pilot composite plating tank containing an electroless cobalt plating solution as described in Example 2 at 70°C (with the additional presence of 0.1 ml/1 LUTENSIT ABP) to give a particle concentration of 15g/l. After a period of 30 minutes deposition occurred on the particles leading to complete solution decomposition. Example 8 The procedure of Example 7 was repeated, except that the additive particles sample was further rinsed in 500c of 2g/l (as metal) acidic cadmium acetate solution for 15 minutes. This was followed by rinsing until a neutral pH was achieved. The particles were introduced into the plating bath as in Example 7 to give a concentration of 15g/l. The solution chemistry remained stable for 6 hours of continuous operation followed by cooling and standing for 18 hours. After this period the bath was reheated to 70"C and operated for a further 6 hours with no signs of solution decomposition.

Claims: CLAIMS 1. An electroless cobalt plating composition comprising a source of cobalt ions, a reducing agent and a source of lead in an amount sufficient to refine the structure of the deposit. 2. An electroless cobalt composite plating composition comprising a source of cobalt ions, a reducing agent, composite particles and a source of lead in an amount sufficient to stabilise the composition but insufficient substantially to prevent cobalt deposition on a surface of a substrate, on which the cobalt is to be deposited. 3. A composition as claimed in claim 1 or 2, wherein the additive particles comprise silicon carbide and/or chromium carbide. 4. A composition as claimed in claim 1 or 2, wherein the source of cobalt ions comprises cobalt sulphate. 5. A composition as claimed in claim 1 or 2, wherein the source of cobalt ions is present in an amount of from 1 to 100 g/1, calculated as cobalt sulphate hexahydrate. 6. A composition as claimed in claim 1 or 2, wherein a complexor for cobalt ions is present. 7. A composition as claimed in claim 1 or 2, wherein the reducing agent is dimethylamino borane. 8. A process as claimed in claim 1 or 2, wherein the lead is present in an amount of from 0.5 to 40 ppm. 9. A composition as claimed in claim 1 or 2, wherein the additive particles are pre-treated with lead, cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony. ■ * 10. A composition as claimed in claim 1 or 2, wherein lead is present in the composition in an amount*^ o from 1 to 10 ppm. 11. A composition as claimed in claim 1 or 2, . hich contains an accelerator. 12. A composition as claimed in claim 1 or 2, which comprises lactic acid in an amount to moderate the plating. 13. A composition as claimed in claim 1 or 2*, * which contains cadmium ions. 14. An electroless cobalt plating composition comprising a source of cobalt, a reducing agent other than hypophosphite and lactic acid present in an amount to moderate the plating. "3 15. A process for plating an electroless cobalt deposit onto a substrate, the' process comprising contacting the substrate with an electroless cobalt plating composition comprising a source of cobalt ions, a reducing agent and a source of lead in an amount sufficient to refine the structure of the deposit. 16. A process for plating a composite electroless cobalt deposit onto a substrate, the process comprising contacting the substrate with a plating composition comprising a source of cobalt ions, a reducing agent, additive particles and a source of lead in an amount sufficient to stabilise the composition but insufficient substantially to prevent cobalt deposition on the substrate. 17. A process for preparing additive particles for use in a composite electroless cobalt plating composition, the process comprising treating the particles with lead, cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony in an amount sufficient to stabilise the composition but insufficient substantially to prevent cobalt deposition on a surface of a substrate on which the cobalt is to be deposited. 18. Particles of material, suitable for inclusion in plated cobalt by means of an electroless plating composition to form a cobalt composite plate, the particles comprising lead, cadmium, tin, zinc, mercury, thallium, bismuth and/or antimony in an amount sufficient to stabilise the composition. 19. An article having a surface bearing a cobalt-boron composite coating. 20. A process for plating a composite electroless cobalt deposit substantially as described herein with reference to the Examples. 21. An electroless cobalt composite plating composition substantially as described herein with reference to the Examples.

Inventors: ARCHER PHILIP JOHN, MCDONALD IAN GERALD, ARCHER, PHILIP, JOHN, MCDONALD, IAN, GERALD
Applicants: POLYMETALS TECH, POLYMETALS TECHNOLOGY LIMITED
Application year: 1990
Main classification: C23C18
IPC codes: C23C18