GB2560969A

GB2560969A - Electroless plating

Info

Publication number: GB2560969A
Application number: GB1705145.9A
Authority: GB
Inventors: Little Kenny William; Charles Gauld Kenneth
Original assignee: Ajt Eng Ltd
Current assignee: Ajt Eng Ltd
Priority date: 2017-03-30
Filing date: 2017-03-30
Publication date: 2018-10-03
Also published as: GB201705145D0; WO2018178709A1

Abstract

A process for electroless plating comprises contacting a substrate with a first plating fluid comprising one or more first plating metals in the presence of one or more first reducing agents and contacting the substrate with a second plating fluid comprising one or more second plating metals in the presence of one or more second reducing agents, wherein the molar concentration of the first reducing agents is higher than the molar concentration of the second reducing agents and/or the molar ratio of first reducing agents to first plating metals is higher than the molar ratio of second reducing agents to second plating metals. The first and second reducing agents may be selected from sulphites, nitrites, hydrides, phosphites, hypophosphates, borohydrides, hydrazines, boranes and borane adducts. The first and second plating metals are preferably selected from one or more of nickel, cobalt, aluminium, copper and tin. Also disclosed is plated substrate produced by the method of the invention and a plated substrate comprising first and second metal platings each comprising phosphorus or boron or both, wherein the total amount of phosphorus and boron is greater in the first plating than in the second plating.

Description

(54) Title of the Invention: Electroless plating

Abstract Title: Processes for electroless plating substrates with first and second plating metals and electrolessly plated substrates (57) A process for electroless plating comprises contacting a substrate with a first plating fluid comprising one or more first plating metals in the presence of one or more first reducing agents and contacting the substrate with a second plating fluid comprising one or more second plating metals in the presence of one or more second reducing agents, wherein the molar concentration of the first reducing agents is higher than the molar concentration of the second reducing agents and/or the molar ratio of first reducing agents to first plating metals is higher than the molar ratio of second reducing agents to second plating metals. The first and second reducing agents may be selected from sulphites, nitrites, hydrides, phosphites, hypophosphates, borohydrides, hydrazines, boranes and borane adducts. The first and second plating metals are preferably selected from one or more of nickel, cobalt, aluminium, copper and tin. Also disclosed is plated substrate produced by the method of the invention and a plated substrate comprising first and second metal platings each comprising phosphorus or boron or both, wherein the total amount of phosphorus and boron is greater in the first plating than in the second plating.

Electroless Plating

Technical Field [0001] The invention relates to a process for electroless plating of a substrate, and to a plated substrate that can result from such a method.

Background [0002] Electroless plating is a widely used technique to deposit a metallic plating onto a substrate, and is often also referred to as chemical or autocatalytic plating. The process involves converting dissolved metal ions to metal in the presence of a chemical reducing agent, the resulting metal becoming deposited onto the surface of a substrate. Examples of electroless plating processes are described in WO 90/00261, WO 03/020443, WO 2004/101848 and WO 2007/134182.

[0003] Electroless plating can be used to improve certain properties of a substrate, for example to improve protection against corrosion, improve hardness or wear resistance, impart conducting or magnetic properties to a non-conducting or non-magnetic substrate, or to refurbish worn or mis-machined parts. An advantage of the electroless plating technique over other techniques, such as electroplating methods, is the ability to form a more even deposit on a surface.

[0004] The present invention relates to a new processing method which results in unexpectedly improved plating performance.

Summary of Invention [0005] The invention is directed to a process for electroless plating of a substrate, comprising;

(i) contacting a substrate with a first plating fluid comprising one or more first plating metals in the presence of one or more first reducing agents to form a first plating on the substrate; and (ii) contacting the substrate with a second plating fluid comprising one or more second plating metals in the presence of one or more second reducing agents to form a second plating on the substrate;

wherein the molar concentration of the one or more first reducing agents in the first plating fluid is higher than the molar concentration of the one or more second reducing agents in the second plating fluid; and/or the molar ratio of first reducing agent(s) to first plating metal(s) in the first plating fluid is higher than the molar ratio of second reducing agent(s) to second plating metal(s) in the second plating fluid.

[0006] The invention is also directed to a plated substrate that can be obtained by such a process.

Description of Embodiments [0007] Electroless plating is often used to improve various characteristics of a substrate, typically a metal substrate. Such features include hardness/wear resistance, corrosion resistance, and also visual appearance such as brightness and lustre.

[0008] Conventional treatments typically use only one type of plating fluid in order to achieve a specific result. However, it has now been found that using a two-step plating process using two separate plating fluids can not only combine the advantages of the different plating fluids, but can also result in improved performance for each characteristic.

[0009] Thus, in the present invention, two plating fluids are employed with different concentrations of reducing agent. Fluids with relatively low concentrations of reducing agent are often used to apply a hard, wear-resistant plating to a substrate, whereas fluids with relatively high concentrations of reducing agent are used to apply a corrosion resistant plating. It has now been found that properties of a plated substrate close to or equivalent to the properties expected from applying two separate layers in two separate procedures can actually be achieved using thinner individual coatings than would otherwise be expected.

[0010] In the present invention, the substrate can be any substrate on which a metallic plating can be applied. Typically, they are metals or metal alloys, examples being iron, aluminium, copper, titanium, nickel and magnesium, and alloys of any one or more thereof, and also other alloys such as stainless steel, alloy steels, cast iron, and beryllium alloys, such as beryllium-iron, beryllium-aluminium, beryllium-nickel and beryllium-copper alloys. In other embodiments, they may be non-metallic substrates that already have a metallic coating or plating.

[0011] The first and second plating fluids are typically liquid-phase solutions, usually aqueous solutions, containing dissolved metal or metals which are to be deposited as the plating metal. The plating metals are typically selected from one or more of nickel, cobalt, aluminium, copper and tin. Based on the metal content, the total concentration of metals in each of the first and second plating fluids is typically in the range of from 1 to 600 mM, for example from 10 to 300 mM or from 50 to 150 mM. Other dopant metals can also be present, typically at lower concentrations such as less than 1 mM in each plating fluid, for example at concentrations of 0.01 to 0.5 mM. Examples of dopants include lead, which can act as a stabiliser.

[0012] In embodiments, the concentration of the dopant in the solution is sufficient to ensure the resulting plating comprises up to 0.2 wt% of the dopant metal such as in a range of from 0.0001 to 0.2 wt%. In embodiments, the dopant metal is present in the plating at less than 0.1 wt%, i.e. in a range of from 0.0001 to less than 0.1wt%. In embodiments, lead is present as a dopant, the resulting plating containing less than 0.1% lead by weight. The total concentration of the one or more first plating metals in the first plating fluid can be the same as or different from the total concentration of the second plating metals in the second plating fluid. For example, typical total concentrations of the one or more first plating metals in the first plating fluid can be in the range of from 60 to 150 mM, for example from 90 to 110 mM. Typical total concentrations of the one or more second plating metals in the second plating fluid can be in the range of from 40 to 130 mM, for example in from 75 to 105 mM.

[0013] The one or more metals that make up the plating metals in the first plating fluid (i.e. the first plating metals) and second plating fluid (i.e. second plating metals) can comprise a common metal, and in further embodiments all of the first plating metals are the same as the second plating metals. In embodiments there is only a single first plating metal and a single second plating metal.

[0014] The process of the invention is particularly suited for electroless nickel plating, in which nickel is the major constituent of the first and second plating metals. In further embodiments, nickel is the only member of the first and second plating metals.

[0015] The one or more first plating metals and second plating metals are usually provided in the form of salts that are water-soluble. For example each salt can be independently selected from a halide (such as chloride, bromide or iodide), sulfate, sulfamate or acetate. Other salts that can be used include nitrate, phosphate, tetrafluoroborate, hexafluorophosphate or oxalate. In embodiments, nitrate salts are avoided [0016] The first and second plating fluid can each comprise one or more additional components.

[0017] One or more chelating agents can be present, to stabilise and maintain solubility of the first and second plating metals to enable more controlled deposition. Examples of chelating agents are organic acids or their salts, typically those which comprise more than one carboxyl group, and also those comprising a single carboxyl group and at least one additional -OH group. The number of carbons atoms in the organic acid can be in the range of from 36. Specific examples include citric acid, lactic acid, tartaric acid, succinic acid, malic acid, maleic acid, itaconic acid and gluconic acid, or salts thereof. In certain embodiments, the organic acid is selected from citric acid, malic acid, itaconic acid, and salts thereof.

[0018] Other chelating agents include polyamines, typically those comprising 2 to 4 amine groups, such as ethylenediamine and diethylenetriamine; polyethers such as polyethylene glycol, polypropylene glycol and crown ethers, typically those containing from 3 to 8 oxygen atoms, for example from 4 to 6 oxygen atoms; and porphyrins.

[0019] Salts of the chelating agents typically include those having monovalent or divalent cations, such as alkali metal or alkaline earth metal cations.

[0020] The chelating agents can be added separately from the salts of the plating metals. In an alternative embodiment, any of the first and/or second plating metals can be combined with a chelating agent before addition to the first or second plating fluid.

[0021] The pH of the first and/or second plating fluids can be controlled. Typically, the pH is maintained at a neutral or slightly acidic value to avoid premature precipitation/deposition of the one or more first and second plating metals, for example by maintaining the pH at a value in the range of from 4 to 7.

[0022] The first and second plating fluids may be maintained at different pH values. For example, the first plating fluid may be maintained at a value in the range of from 4 to 6, such as from 4.5 to 5.5, or 4.8 to 5.2. The second plating fluid can be maintained at a value in the range of from 5.5 to 7, for example from 6 to 6.6.

[0023] The pH can be adjusted by one or more pH adjustment agents. The acidity of the fluids can increase during the plating process, so the pH can be maintained or adjusted throughout the process by adding one or more pH adjustment agents to either or both of the first and second plating stages to maintain the pH within the desired range. Agents which raise the pH are typically selected from hydroxides, carbonates and bicarbonates, for example ammonium, alkali metal or alkaline earth metal hydroxides, carbonates and bicarbonates. In one embodiment, ammonium hydroxide is used as the pH adjustment agent. In another embodiment, potassium carbonate is used. Agents which lower the pH can be selected from mineral acids, such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, fluoroboric acid, or organic acids, such as carboxylic acids, for example carboxylic acids (including fatty acids) having from 2 to 24 carbon atoms, for example carboxylic acids having from 2 to 8 carbon atoms.

[0024] A composite plating can be achieved by including in the first and/or second plating fluid a composite material. These can be incorporated into the metallic plating, to modify and improve properties such as wear, abrasion resistance or lubricity of the deposit. Composite materials are generally included in the plating fluid as a fine suspension, optionally in the presence of surface active agents which help to stabilise the suspension and control the deposition during the plating process. Examples of composite materials are fluoropolymers such as polytetrafluoroethylene (PTFE), natural or synthetic diamonds, ceramics, chromium carbide, silicon carbide, magnetite and aluminium oxide.

[0025] The first and/or second plating fluids can comprise one or more additional components, such as one or more stabilisers and/or surfactants. Lead, for example in the form of lead acetate, can act as a stabiliser, which can help to avoid indiscriminate and uncontrolled plating. Surfactants, such as silicone glycol surfactants, for example polyalkoxylated silicone glycol surfactants, can help to assist the fineness of the grain of the plating metals. They can also help stabilise dispersed composite material that may also be present in the first and/or second plating fluid.

[0026] Each of the first and second plating media comprises one or more reducing agents. The total concentration of the one or more first reducing agents in the first plating medium is higher than the total concentration of the one or more second reducing agents in the second plating medium.

[0027] The reducing agent causes reduction of the soluble metal plating ions to metal, which then deposit on the substrate. More than one reducing agent can be used in each of the first and second plating fluids, in which case the concentration of reducing agent refers to the total concentration of reducing agent in the respective fluid.

[0028] The total concentration of reducing agent in each of the first and second plating fluids is typically in the range of from 50 to 750 mM.

[0029] The total concentration of the one or more first reducing agents in the first plating fluid is typically, in the range of from 150 to 500 mM, for example from 250 to 350 mM.

[0030] The total concentration of the one or more second reducing agents in the second plating fluid is typically in the range of from 50 to 300 mM, for example from 150 to 220 mM.

[0031] In embodiments, the ratio of molar concentrations of the one or more first reducing agents to the one or more first plating metals in the first plating fluid is in the range of from 2 to 4, for example in the range of from to 2.3 to 3.4.

[0032] In embodiments, the ratio of molar concentrations of the one or more second reducing agents to the one or more second plating metals in the second plating fluid is in the range of from 1.2 to 3.2, for example from 1.7 to 2.7.

[0033] Examples of reducing agents suitable for use in the present invention include sulfites, nitrites, hydrides, phosphites, hypophosphites and borohydrides, typically provided as salts selected, for example, from alkali metal salts, alkaline earth metal salts, organic “R” salts, where “R” is defined below (such as CH₃ ⁺ or C2H5⁺ salts), or quaternary ammonium salts of general formula NH4._XR_X where x is from 0 to 4, and wherein the “R” groups are defined below, for example tetramethyl ammonium salts. Other examples of reducing agents include hydrazines, and also boranes and borane adducts such as amine boranes or organoamine boranes of general formula NH₃_yR_y: borane in which y is from 0 to 3, the “R” groups being defined below, an example being dimethylamine borane. The borane is typically BH₃.

[0034] Each “R” group referred to above can be selected from Ci-s alkyl groups, C₃_8 cycloalkyl groups, C₃_8 heterocycloalkyl groups, C₃_8 aryl groups and C₃_8 heteroaryl groups the heterocycloalkyl and heteroaryl groups comprising at least one carbon atom, and having from 1 or 3 heteroatoms, each selected from O, S and N. Ci-s alkoxy groups can optionally be substituted with one or more groups selected from halide, Cw alkoxy and Cm haloalkoxy groups. C₃_8 cycloalkyl, C₃_8 heterocycloalkyl, C₃_8 aryl and C₃_8 heteroaryl groups can optionally be substituted with one or more groups selected from halide, Cm alkyl, C1.4 haloalkyl, Cm alkoxy groups and Cm haloalkoxy groups.

[0035] Specific examples of reducing agents that can be used include sodium hypophosphate, dimethylamine borane, diethylamine borane and sodium borohydride.

[0036] In embodiments, the first and/or second reducing agents contain phosphorus or boron. Use of phosphorus or boron-containing reducing agents can result in incorporation of phosphorus or boron in the plating on the substrate, which can impart certain advantages to the deposited metal plating. For example, incorporation of increasing amounts of phosphorus into a metal plating, for example a nickel plating, can improve corrosion resistance of the substrate, while at the same time reducing hardness of the plating. Incorporation of boron can also improve hardness.

[0037] In embodiments there is only a single first reducing agent, and a single second reducing agent. In further embodiments, alkali metal hypophosphites such as sodium hypophosphite, are used as the first and second reducing agents.

[0038] In embodiments, the first plating on the substrate is a corrosion resistant layer, and the second plating is for improving the hardness and/or wear characteristics.

[0039] In embodiments, for corrosion resistant layers, a phosphorus or- boron-containing reducing agent is used, for example a hypophosphite or a borohydride. The amount of phosphorus or boron in the resulting plating is typically in the range of from 8-17% by weight, for example 10-13% by weight. Nickel can be the plating metal in such plating layers.

[0040] In embodiments, for hard and/or wear resistant layers, phosphorus- or boroncontaining reducing agents can be used, for example a hypophosphite or a borohydride. The amount of phosphorus or boron in the resulting plating is typically in the range of from 1-6% by weight, for example from 1-3% by weight. Again, nickel can be the plating metal in such plating layers.

[0041] The temperature of the two plating treatments can be the same or different. They are typically operated at above ambient temperature, for example in the range of from 60 to 95°C, such as in the range of from 80-93°C.

[0042] In one embodiment, the first plating fluid is maintained at a temperature higher than that of the second plating fluid.

[0043] In embodiments, the first plating fluid is maintained at a temperature in the range of from 85-91°C. In embodiments, the second plating fluid is maintained at a temperature in the range of from 82-91 °C.

[0044] The first and/or second plating fluid can be agitated during use, and the composition adjusted to ensure sufficient plating metals, reducing agent, other components and properties such as pH are maintained within desired limits. In embodiments, plating takes place in a plating bath, in which plating fluid is removed and analysed. The composition of the plating fluid is then adjusted to ensure the desired characteristics are maintained. The fluid can be filtered before being returned to the main plating bath. In embodiments, this is a continuous process, with plating fluids being recirculated through a recirculation loop that comprises one or more adaptions so that components can be added to adjust the composition, and/or to enable sample to be removed for analysis. In embodiments, analysis is carried out on-line using suitable sensors built into the recirculation loop and/or in the plating bath itself.

[0045] Steps (i) and (ii) of the process can take place in the same plating bath, optionally with suitable washing and preparation of the plating bath between treatments. In other embodiments, the plating is carried out in separate baths, the substrate being removed from one bath after the first treatment and immersed in a second bath optionally after washing and/or rinsing the substrate.

[0046] Plating thickness for each step can be controlled by known means, for example by the length of time of immersion in the plating fluids, the temperature, the concentration of plating metals and reducing agents, the choice and concentration of chelating agents, and use of other components such as stabilisers.

[0047] The plated substrate can optionally be heat treated between and/or after the two plating steps. When heating is carried out, it is typically to a temperature in the range of from 150 to 400°C. For improved hardness and wear characteristics, temperatures above 260°C are often used, typically in the range of from 320 to 370°C, because this can cause crystallisation of the plating metal(s) with additives or dopants in the plating, such as phosphorus or boron from the reducing agent.

[0048] In embodiments where one or more of the layers is for improving corrosion resistance of the substrate, heat treatment is preferably avoided, particularly in embodiments where the layer incorporates elements such as phosphorus or boron from the reducing agent, since crystallisation of such species can disrupt the uniformity and coverage of the protective plating.

[0049] The thickness of plating for each stage is typically in the range of from 5 to 125 pm, for example from 6 to 60 pm, or from 10 to 30 pm.

[0050] The process of the present invention results in a plated substrate with enhanced properties.

[0051] In embodiments, the substrate comprises a first plating and a second plating, in which the first plating is a metal plating comprising phosphorus and/or boron, and the second plating is a metal plating comprising boron and/or phosphorus, in which the total amount of phosphorus and boron in the first plating is greater than that in the second plating.

[0052] In embodiments, the total amount of phosphorus and/or boron in the first plating is in the range of from 8 to 17% by weight. In embodiments, the total amount of phosphorus and/or boron in the second plating is in the range of from 1 to 6% by weight.

[0053] In embodiments, the first and/or second plating metals are selected from one or more of nickel, cobalt, copper, aluminium and tin. In embodiments, all of the first plating metals and second plating metals are the same, and in further embodiments are nickel or predominantly nickel.

[0054] In embodiments, the first plating is for improving corrosion resistance, and the second plating is for improving wear resistance and/or hardness.

[0055] The substrate can be as defined above.

[0056] The second plating is applied on top of or over the first plating, and typically covers the first plating in its entirety.

Experimental [0057] The following outlines procedures for providing a nickel plating on a steel substrate that was pre-passivated by dipping in an acidic solution comprising citric acid and sulfamic acid, and rinsed thoroughly with cold deionized water before use.

Comparative Example 1 [0058] An anodically protected steel plating bath was cleaned and passivated using a 3025 50% v/v nitric acid solution for 4 - 8 hours. The bath was drained, and rinsed to ensure there was no residual nitrate (confirmed by using nitrate test strips).

[0059] 500 litres deionized water were added to the bath, followed by 45 litres of a commercially available aqueous high phosphorus electroless first nickel-containing solution comprising nickel sulfate and sulfuric acid. 113 litres of a first hypophosphite solution containing sodium hypophosphite were then added and the bath was then made up to 750 litres by adding more deionized water.

[0060] The resulting plating fluid contained 6.0 g/litre (102 mM) nickel and 30 g/litre (283 mM) sodium hyphophosphite monohydrate. The pH was measured, and adjusted where necessary with dilute ammonium hydroxide (50% v/v) or dilute sulfuric acid (10% v/v) to achieve a pH of 4.8.

[0061] The plating medium was then heated to 88°C, and the steel substrate was then immersed in the plating fluid. The temperature throughout the plating procedure was maintained within a range of 80-90°C. The loading of substrate in the bath was maintained such that the surface area of substrate per litre of plating fluid was in the range of 0.25 - 2.5 dm²/litre.

[0062] The medium was continuously recirculated and filtered using a 1 micron filter at a rate of between 3-7 bath volumes per hour. Recirculation also acted to agitate and mix the solution. Further agitation was achieved by mild bubbling of air.

[0063] At regular intervals the plating fluid was analysed for nickel concentration (using EDTA titration), hypophosphite concentration (using thiosulfate titration), and pH.

[0064] Nickel concentration (as nickel, as opposed to nickel sulfate) was maintained in the range of from 5.4-6.3 g/litre (92-107 mM) by addition of an appropriate amount of a 50/50 vol/vol mixture of (i) the abovementioned first nickel-containing solution and (ii) an alternative hypophosphite solution containing about three times the concentration of sodium hypophosphite present in the abovementioned first hypophosphite solution, and also containing ammonia.

[0065] Hypophosphite concentration (as sodium hyphophospite monohydrate) was maintained within a range of 27-33 g/litre (255-31 lmM) by addition of a suitable amount of a proprietary solution containing approximately 600 g/litre (5.7 M) sodium hypophosphite monohydrate.

[0066] The pH was maintained within a range of 4.8-5.2, by adding dilute ammonium hydroxide (50% v/v) or dilute sulfuric acid (10% v/v).

[0067] Under these conditions, a plating rate of 8-15 pm per hour is achievable, usually about 10-13 pm if the initial values of pH, nickel concentration and hypophosphite concentration can be maintained. The process was carried out to achieve a plating thickness of 25 pm.

Comparative Example 2 [0068] A similar procedure to that outlined in Comparative Example 1 was used, except that different sources of plating metal and reducing agent were used.

[0069] In place of 45 litres of the first nickel-containing solution (the nickel sulfatecontaining solution) used in Comparative Example 1, 22.5 litres of a high purity second nickel-containing solution comprising nickel disulfamate solution was used. In place of 113 litres of the first hypophosphite solution used in Comparative Example 1, 150 litres of a proprietary second hypophosphite solution containing sodium hypophosphite (monohydrate) and ammonia was used.

[0070] The resulting plating fluid contained 5.4 g/litre (92 mM) nickel and 20 g/litre (189 mM) sodium hyphophosphite monohydrate. The pH was measured, and adjusted where necessary with dilute ammonium hydroxide (50% v/v) or dilute sulfuric acid (10% v/v) to achieve a pH of 6.4.

[0071] The plating medium was then heated to 85°C, and the steel substrate was then immersed in the medium. The temperature throughout the plating procedure was maintained within a range of 82-91°C. The loading of substrate in the bath was maintained such that the surface area of substrate per litre of plating fluid was in the range of 0.5 - 2.5 dm /litre.

[0072] Nickel concentration (as nickel, as opposed to nickel sulfamate) was maintained in the range of from 4.6-5.7 g/L (78-97 mM) by addition of an appropriate amount of a 50/50 vol/vol mixture of (i) an alternative nickel disulfamate solution containing a lower concentration of nickel disulfamate than is present in the abovementioned second nickelcontaining solution but also containing ammonia, and (ii) an alternative proprietary solution containing sodium hypophosphite (monohydrate) but no ammonia, wherein the concentration of sodium hypophosphite in the alternative proprietary solution is about 3 times the concentration in the abovementioned second hypophosphite solution.

[0073] Hypophosphite concentration (as sodium hyphophospite monohydrate) was maintained within a range of 18-22 g/litre (170-208 mM) by addition of a suitable amount of proprietary solution containing approximately 600 g/litre (5.7 M) sodium hypophosphite monohydrate.

[0074] The pH was maintained within a range of 6.0 - 6.6, by adding dilute ammonium hydroxide (50% v/v) or dilute sulfuric acid (10% v/v).

[0075] Under these conditions, a plating rate of 7-18 pm per hour is achievable, usually

12.5 pm per hour if the initial values of pH, nickel concentration and hypophosphite concentration are maintained. The process was carried out to achieve a plating thickness of 25 pm.

Example 1 [0076] The process of Comparative Example 1 was carried out, but for a shorter period of time so as to achieve a plating thickness of 12.5 pm. After rinsing the plated substrate, the process of Comparative Example 2 was carried out on the same substrate, to achieve a further coating of 12.5 pm, resulting in a total nickel coating of 25 pm.

[0077] Results comparing the different coating procedures are shown in Table 1.

Table 1 - Characteristics of Plated Substrates

	Thickness (pm)	Hardness¹	Hardness¹After Heat Treatment²	Taber Wear³	Taber Wear³After Heat Treatment²	Coefficient of Friction⁴	Salt Spray⁵ (hrs)	Salt Spray⁵(hrs) After Heat Treatment²
Comp.Ex. 1	25	460 HV 480 HK	875 HV 900 HK	18-24	10-15	0.3	>1500	too
Comp.Ex.2	25	715 HV 750 HK	1090 HV 1000 HK	7-12	6-10	0.3	36	25
Example 1	25	715 HV 750 HK	1090 HV 1000 HK	7-12	6-10	0.3	>1000	75

¹ HV = Vickers Pyramid Number. HK = Knoop hardness test. Tests conducted according to method ASTME384.

² Heat treatment at 400°C for 1 hour.

³ Taber wear test - ASTM D4060.

⁴ Measured according to ASTM DI894.

⁵ Appearance of corrosion on exposure to salt spray according to ASTM BI 17.

[0078] The results demonstrate that plated substrate resulting from the two-step plating method according to the invention has properties that are superior than would otherwise be expected from a mere sum of the individual coatings.

[0079] In particular, even though the outer coating of Example 1 is only half that of Comparative Example 2, the hardness and wear characteristics of the plated substrate are identical to a substrate having received an individual coating of twice the thickness. Similarly, even though the inner coating of Example 1 is only half that of Comparative

Example 1, the corrosion resistance of the plated substrate is still high and only marginally lower than the result of a single, individual coating having twice the thickness.

[0080] Heat treating the plated substrate of Example 1 reduced its corrosion resistance. Therefore, if corrosion resistance is desirable, heating of the plated substrate is preferably avoided.

[0081] In summary, the process of the present invention, comprising two plating steps, provides unexpectedly enhanced properties which can enable a substrate to benefit, for example, from improved corrosion resistance and improved hardness/wear characteristics using thinner individual platings without detriment to either property.

Claims

1. A process for electroless plating of a substrate comprising;

wherein the molar concentration of the first reducing agent in the first plating fluid is higher than the molar concentration of the second reducing agent in the second plating fluid, and/or the molar ratio of first reducing agent(s) to first plating metal(s) in the first plating fluid is higher than the molar ratio of second reducing agent(s) to second plating metal(s) in the second plating fluid.

2. A process as claimed in claim 1, in which at least one of the first and/or second reducing agents are selected from sulfites, nitrites, hydrides, phosphites, hypophosphates, borohydrides, hydrazines, boranes and borane adducts.

3. A process as claimed in claim 2, in which at least one of the first and/or second reducing agents are provided as a salt selected from alkali metal salts, alkaline earth metal salts, organic R salts, quaternary ammonium salts of formula NHXR4_X where x is from 0 to 4, hydrazines, boranes, and amine-borane adducts of general formula NHyR3.y: borane where y is from 0 to 3, and where each R in the above formulae is independently selected from Ci-8 alkyl groups, C3-8 cycloalkyl groups, C3-8 heterocycloalkyl groups, C3-8 aryl groups and C3-8 heteroaryl groups comprising at least one carbon atom, the C3-8 heterocycloalkyl and C3-8 heteroaryl groups having from 1 or 3 heteroatoms, each selected from O, S and N, and where Ci_8 alkoxy groups are optionally substituted with one or more groups selected from halide, C1.4 alkoxy and C1.4 haloalkoxy groups, and where C3.8 cycloalkyl, C3.8 cycloalkyl, C3.8 aryl and C3-8 heteroaryl groups are optionally substituted with one or more groups selected form halide, C1.4 alkyl, C1.4 haloalkyl, C1.4 alkoxy and C 1.4 haloalkoxy groups.

4. A process as claimed in claim 3, in which at least one of the first reducing agents and at least one of the second reducing agents are selected from hypophosphites and borohydrides.

5. A process as claimed in any one of claims 1 to 4, in which the substrate is a metal or metal alloy, or comprises a plating of a metal or metal alloy.

6. A process as claimed in claim 5, in which the substrate or substrate plating is selected from iron, aluminium, copper, titanium, nickel, magnesium, alloys of any two or more thereof, stainless steel, alloy steels, cast iron and beryllium alloys selected from berylliumiron, beryllium-aluminium, beryllium-nickel and beryllium-copper alloys.

7. A process as claimed in any one of claims 1 to 6, in which at least one of the first and/or second plating metals are selected from one or more or nickel, cobalt, aluminium, copper and tin.

8. A process as claimed in any one of claims 1 to 7, in which all of the first plating metals are the same as the second plating metals.

9. A process as claimed in any one of claims 1 to 8, in which the molar ratio of first reducing agent(s) to first plating metal(s) in the first plating fluid is in the range of from 2 to 4, and the molar ratio of second reducing agent(s) to second plating metal(s) in the second plating fluid is in the range of from 1.2 to 3.2.

10. A process as claimed in any one of claims 1 to 9, in which the total concentration of the one or more first reducing agents in the first plating fluid is in the range from 150 to 500 mM, and the total concentration of the one or more second reducing agents in the second plating medium is in the range of from 50 to 300 mM.

11. A process as claimed in any one of claims 1 to 10, in which the total concentration of one or more first plating metals in the first plating fluid is in the range of from 60 to 150 mM, and the total concentration of the one or more second plating metals in the second plating fluid is in the range of from 40 to 130 mM.

12. A process as claimed in any one of claims 1 to 11, in which the pH of the first plating fluid is maintained at a value in the range of from 4 to 6, and the pH of the second plating fluid is maintained at a value in the range of from 5.5 to 7.

13. A process as claimed in any one of claims 1 to 12, in which the first plating is for improving corrosion resistance, and the second plating is for improving wear resistance and/or hardness.

14. A process as claimed in any one of claims 1 to 13, in which the thickness of each of the first and second platings on the substrate is in the range of from 5 to 125 pm.

15. A plated substrate produced by a process according to any one of claims 1 to 14.

16. A plated substrate comprising a first plating and a second plating, in which the first plating is a metal plating comprising phosphorus and/or boron, and the second plating is a metal plating comprising boron and/or phosphorus, in which the total amount of phosphorus and boron in the first plating is greater than that in the second plating.

17. A plated substrate according to claim 16, in which the total amount of phosphorus and/or boron in the first plating is in the range of from 8 to 17% by weight, and the total amount of phosphorus and/or boron in the second plating is in the range of from 1 to 6% by weight.

18. A plated substrate according to claim 16 or claim 17, in which the metal in the first and/or second plating is selected from one or more of nickel, cobalt, copper, aluminium and tin.

19. A plated substrate according to any one of claims 16 to 18, in which all of the first plating metals and second plating metals are the same.

20. A plated substrate according to claim 19, in which the first and second plating metals are nickel or predominantly nickel.

21. A plated substrate according to any one of claims 16 to 20, in which the first plating is for improving corrosion resistance, and the second plating is for improving wear resistance and/or hardness.

22. A plated substrate according to any one of claims 16 to 21, in which the thickness of the first plating and second plating is in the range of from 5 to 125 pm.

23. A plated substrate according to claim 22, in which the thickness of the first and second plating is in the range of from 10 to 30 pm.

Intellectual

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Office

Application No: GB1705145.9