Method of fabricating a bath of electrolyte for plating a platinum-based metallic underlayer on a metallic substrate

Abstract

The invention relates to a method of fabricating a bath of electrolyte for plating a platinum-based metal underlayer on a metallic substrate, comprising: a) providing a first system having ligands and amine functional groups, the first system being constituted by an aqueous solution of an amino ligand comprising at least one compound X(NH.sub.2).sub.n, where X belongs to the group constituted by (CH.sub.3, CH.sub.3CH.sub.2, CH.sub.3(CH.sub.2).sub.m), or NH.sub.3 or an xP.sup.(NH.sub.4).sup.+.sub.p salt where x is an acid radical belonging to the group constituted by (PO.sub.4.sup.3, HPO.sub.4.sup.2, H.sub.2PO.sub.4.sup., HPO.sub.4.sup.2and H.sub.2PO.sub.4.sup., SO.sub.4.sup.2, HSO.sub.4.sup., H.sub.2SO.sub.4, HSO.sub.4.sup., and H.sub.2SO.sub.4, CH.sub.3COO.sup., CH.sub.3COOH, and CH.sub.3COO.sup.), or H.sub.2SO.sub.4, or CH.sub.3COOH, and where n, m, and p are non-zero integers; b) providing a second system forming a buffer system; c) providing a third system providing a metallic salt, and constituted by an aqueous solution of platinum; d) providing a fourth system suitable for imparting the conduction property to the medium; and e) mixing together the four systems so as to obtain the electrolyte bath.

Claims

1. A fabrication method for a bath of electrolyte, the method comprising: a) providing a first system having ligands and amine functional groups, said first system comprising an aqueous solution of an amino ligand comprising: (i) at least one compound X(NH.sub.2).sub.n, where X is selected from the group consisting of (CH.sub.3, CH.sub.3CH.sub.2, CH.sub.3(CH.sub.2).sub.m; (ii) NH.sub.3; or (iii) an x.sup.P(NH.sub.4).sup.+.sub.p salt where x is an acid radical selected from the group consisting of PO.sub.4.sup.3, HPO.sub.4.sup.2, H.sub.2PO.sub.4.sup., SO.sub.4.sup.2, HSO.sub.4.sup., and CH.sub.3COO, where n, m, and p are non-zero integers; b) providing a second system forming a buffer system; c) providing a third system providing a metallic salt, and comprising an aqueous solution of platinum; the metallic salt being at least one salt of platinum of degree of oxidation IV; d) providing a fourth system suitable for imparting the property of conduction to the bath of electrolyte; forming a first solution B by grouping the first system, the second system, and the fourth system together in a single solution; forming a second solution A comprising sodium hydroxide and the least one salt of platinum of degree of oxidation IV of the third system; and e) mixing together the four systems so as to obtain the electrolyte bath of electrolyte; wherein the mixing comprises: e1) covering the first solution B and raising the first solution B to a temperature of at least 50 C. for at least 1h30 and e2) adding the second solution A to the first solution B and mixing the second solution A with the first solution B thereby forming a platinum amino complex and thus obtaining said bath of electrolyte.

2. The method according to claim 1, the first solution B comprises the x.sup.p(NH.sub.4).sup.+.sub.p salt, where x=HPO.sub.4.sup.2 and p=2, and/or x=H.sub.2PO.sub.4.sup. and p=1.

3. The method according to claim 2, further comprising f) heating the bath of electrolyte to a temperature of 80 C. to 97 C. for at least two hours; and g) electroplating a deposit of platinum on a metallic substrate using said heated bath of electrolyte.

4. The method according to claim 3, wherein prior to e2), the first solution B is raised to a temperature of 60 C.

5. The method according to claim 3, wherein the molar ratio of sodium hydroxide (NaOH) to the salt of platinum of degree of oxidation IV is 2.

6. The method according to claim 1, wherein said salt of platinum of degree of oxidation IV is Y.sub.2PtM.sub.6 with YNH.sub.4.sup.+, H.sup.+, or K.sup.+, and M=Cl.sup. or OH.sup..

7. The method according to claim 6, wherein in the second solution A, said salt of platinum of degree of oxidation IV is diammonium hexachloroplatinate of formula (NH.sub.4).sub.2PtCl.sub.6.

8. The method according to claim 1, wherein x of the x.sup.p(NH.sub.4).sup.+.sub.p salt is an acid radical selected from the group consisting of HPO.sub.4.sup.2 and H.sub.2PO.sub.4.sup.; and wherein the first system comprises diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4, ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4, or a mixture thereof.

9. The method according to claim 8, wherein a molar ratio between the ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4 and the diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4 is 2.

10. The method according to claim 1, wherein said second solution A is heated before step e2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the invention and of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

(2) FIGS. 1A-1C depict performance of test 2;

(3) FIGS. 1D-1F depict performance of test 7;

(4) FIGS. 1G-1I depict performance of test 4;and

(5) FIGS. 2A and 2B depict the stability of test 2 over time.

(6) In this way, it can be understood that preference is given to using a complex that results from bonding between an amino ligand and a platinum-based metallic salt. In particular, a ligand is chosen without a carbon chain and with only one amine function: (NH.sub.3 (ammonia) or an xNH.sub.4.sup.+ salt or an ammonium XNH.sub.2) where X is selected either as an inert molecule that is not involved in the main reaction, or else as a molecule that interacts in the formulation reaction.

(7) Preferably, the metallic salt of the third system is selected from salts of platinum of degree of oxidation IV.

(8) This solution also presents the additional advantage of making it possible to use salts of platinum of degree of oxidation IV, which are much more stable than salts of platinum of degree of oxidation II.

(9) Overall, by means of the solution of the present invention, it is possible to provide an electrolyte bath that presents improved lifetime, with plating properties that remain satisfactory and stable over time.

(10) Also according to the invention, the first system, the second system, and the fourth system are grouped together in a single solution forming a first solution B.

(11) Advantageously, the first solution B includes an x.sup.p(NH.sub.4).sup.+.sub.p salt, where x=HPO.sub.4.sup.2 and p=2, and/or x=H.sub.2PO.sub.4.sup. and p=1.

(12) Preferably, the first system forms a solution A constituted by an aqueous solution of platinum, including sodium hydroxide (NaOH) and at least one salt of platinum of degree of oxidation IV.

(13) Under such circumstances, and preferably, the molar ratio of the quantity of sodium hydroxide (NaOH) to the quantity of salt of platinum of degree of oxidation IV is 2.

(14) Also according to the invention, during step c), the third system forms a second solution A constituted by an aqueous solution of platinum comprising sodium hydroxide (NaOH) and at least one salt of platinum of degree of oxidation IV, and during step e), the following substeps are performed:

(15) e1) covering the first solution B and raising its temperature to at least 50 C. for at least 1 h30; and

(16) e2) mixing the second solution A with the first solution B to form an electrolyte bath including a platinum amino complex.

(17) In a preferred implementation, after step e), a step f) is performed during which said bath of electrolyte is heated to a temperature lying in the range 80 C. to 97 C. for at least two hours; and

(18) then a step g) is performed during which a deposit of platinum is electroplated on a metallic substrate using said bath of electrolyte.

(19) Furthermore, in the invention, during substep e2), the second solution A is added into the first solution B.

(20) Under such circumstances, and advantageously, prior to substep e2), the first solution B is raised to a temperature of 60 C.

(21) Preferably, said salt of platinum of degree of oxidation IV is defined by Y.sub.2PtM.sub.6 with YNH.sub.4.sup.+, H.sup.+, or K.sup.+, and M=Cl.sup. or OH.sup..

(22) Advantageously, in the second solution A, said salt of platinum of degree of oxidation IV is diammonium hexachloroplatinate of formula (NH.sub.4).sub.2PtCl.sub.6.

(23) Advantageously, in the first system, said x.sup.p(NH.sub.4).sup.+.sub.p amine compound comprises diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4 and/or ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4.

(24) In a preferred formulation, the first system includes diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4 and ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4 with a molar ratio of 2 between the quantity of ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4 and the quantity of diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4.

(25) One or another or several of the following provisions is/are also preferably adopted: the first solution B provided in step a) is obtained with water presenting a temperature of about 30 C.; the second solution A provided in step c) is obtained with water presenting a temperature of about 45 C.; during step b), the temperature of the first solution B is raised to at least 50 C. for at least 3 h30; and during step d), said bath of electrolyte is raised to a temperature of at least 80 C. for at least three hours (e.g. 85 C. for 3 h).

(26) The present invention also provides a method of fabricating a platinum-based metal underlayer from the bath of electrolyte obtained by the above-described fabrication method, characterized in that it comprises the following steps:

(27) f) providing a metallic substrate, in particular a substrate made of superalloy;

(28) g) heating said bath of electrolyte; and

(29) h) electroplating a deposit of platinum on said metallic substrate using said bath of electrolyte.

(30) The present invention also provides a set of solutions for fabricating a bath of electrolyte for making a platinum-based metallic underlayer on a metallic substrate, the set being characterized in that it comprises:

(31) a first solution B constituted by an aqueous solution of an amino ligand comprising at least one compound X(NH.sub.2).sub.n, where X belongs to the group constituted by (CH.sub.3, CH.sub.3CH.sub.2, CH.sub.3(CH.sub.2).sub.m), or NH.sub.3 or an x.sup.p(NH.sub.4).sup.+.sub.p salt where x is an acid radical belonging to the group constituted by (PO.sub.4.sup.3, HPO.sub.4.sup.2, H.sub.2PO.sub.4.sup., HPO.sub.4.sup.2and H.sub.2PO.sub.4.sup., SO.sub.4.sup.2, HSO.sub.4.sup., HSO.sub.4.sup., and H.sub.2SO.sub.4, CH.sub.3COO.sup., CH.sub.3COOH, and CH.sub.3COO.sup.), or H.sub.2SO.sub.4, or CH.sub.3COOH, and where n, m, and p are non-zero integers; and a second solution A constituted by an aqueous solution of platinum, including sodium hydroxide (NaOH) and at least one salt of platinum of degree of oxidation IV.

(32) Preferably, in the second solution A, said salt of platinum of degree of oxidation IV is defined by Y.sub.2PtM.sub.6 with YNH.sub.4.sup.+, H.sup.+, or K.sup.+, and M=Cl.sup.or OH.sup..

(33) Advantageously, said salt of platinum of degree of oxidation IV is diammonium hexachloroplatinate of formula (NH.sub.4).sub.2PtCl.sub.6.

(34) Preferably, the molar ratio of the quantity of sodium hydroxide (NaOH) to the quantity of salt of platinum of degree of oxidation IV is 2.

(35) In a preferred implementation, in the first solution B, said x.sup.p(NH.sub.4).sup.+.sub.p amine compound comprises diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4 and/or ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4.

(36) In a preferred variant, the first solution B includes diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4 and ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4 with a molar ratio of 2 between the quantity of ammonium dihydrogen phosphate NH.sub.4H.sub.2PO.sub.4 and the quantity of diammonium hydrogen phosphate (NH.sub.4).sub.2HPO.sub.4.

(37) Finally, the invention also provides the bath of electrolyte that results from the fabrication method of the invention. Such a bath of electrolyte for making a platinum-based metallic underlayer on a superalloy substrate is characterized in that it includes an amino complex of platinum with the wavelength of a PtNH.sub.3 or PtNH.sub.2 bond and a buffer solution.

(38) Other advantages and characteristics of the invention appear on reading the following description made by way of example and with reference to the accompanying drawings, in which: FIGS. 1A to 1I, 2A and 2B are various plots showing the characteristics and the behavior of various electrolytic baths fabricated using the fabrication method of the invention.

(39) An electrolytic bath makes it possible to deposit (i.e. electroplate) platinum using a technique that is particularly ecological and economic (performed in a short length of time, and performed under atmospheric pressure, thereby avoiding evacuation equipment) compared with techniques of chemical vapor deposition (CVD) or thermal sputtering.

(40) In addition, this plating method is compatible for use with parts having holes: the shape of the lines of current prevent any significant deposition taking place inside the holes, and in particular inside cooling holes of small size, which holes are thus not obstructed.

(41) It should also be observed that using such a method avoids having recourse to dangerous chemicals and to producing toxic waste.

EXAMPLE 1

(42) In this example, the bath is formulated from four ingredients organized as two distinct solutions A and B that are heated and stirred separately in order to cause the ingredients to react within each of the solutions, prior to mixing together the two solutions A and B.

(43) Thereafter, the mixture of the two solutions A and B is heated and stirred. Once the time for heating the A+B mixture has elapsed, the platinum electroplating bath is ready for use in performing electroplating.

(44) In particular, solution A includes, amongst other ingredients, the platinum salt(s) and solution B is the solution that contains, amongst other ingredients, the ligands (it should be recalled that a ligand is an ionic or molecular chemical entity carrying chemical functions that enable it to bond with one or more metallic entities, generally a cation, with the association of a metallic entity and one or more ligands forming a structure that is soluble in solution and known as a complex).

(45) To fabricate one liter of electrolytic bath with 8 grams (g) of platinum per liter, the procedure is as follows: Preparing solution B: in 300 milliliters (mL) of distilled water (<500 ohms ()) at 30 C., place 44.0 g of diammonium hydrogen phosphate having the chemical formula (NH.sub.4).sub.2HPO.sub.4 (i.e. 0.33 moles) and 75.0 g of ammonium dihydrogen phosphate of chemical formula NH.sub.4H.sub.2PO.sub.4 (i.e. 0.65 moles). The molar ratio between the quantity d of ammonium dihydrogen phosphate and the quantity of diammonium hydrogen phosphate is 2. Once the salts have dissolved, cover the solution and raise to 50 C. during 4 hours (h) 30 minutes (min). Preparing solution A: in 300 mL of distilled water at 45 C., place 5 g of sodium hydroxide of chemical NaOH (i.e. 0.080 moles) and 18.3 g of the platinum salts diammonium hexachloroplatinate of formula (NH.sub.4).sub.2PtCl.sub.6 (i.e. 0.040 moles). The molar ratio between the quantity of sodium hydroxide and the quantity of diammonium hexachloroplatinate salt is 2. Allow the platinum salts to dissolve within solution A. Once solution B is ready and hot, prepare solution A and add it to solution B, after raising it to 60 C. To finish, raise the A+B mixture (of pH previously adjusted to 6.3 by adding a basic solution such as, for example, sodium hydroxide, potassium hydroxide, sodium triphosphate) to 85 C. for 3 h. All of the solutions should be covered during the heating steps.

(46) More generally, with this solution B including diammonium hydrogen phosphate of chemical formula (NH.sub.4).sub.2HPO.sub.4 and ammonium dihydrogen phosphate of chemical formula NH.sub.4H.sub.2PO.sub.4, the pH of the mixture of the solutions A+B should be set to lie in the range 6 to 10, and preferably in the range 6 to 7.

(47) In the context of this formulation, and in order to identify the best operating conditions for performing platinum electroplating, an experimental design with nine baths was performed using different temperatures and times for heating solution B and then the A+B mixture, as specified in Table 1 below, with test 2 corresponding to the above-specified procedure:

(48) TABLE-US-00001 TABLE 1 Test B heat B heat AB heat AB heat No. temp ( C.) time (h) temp ( C.) time (h) 1 50 C. 1 h 30 50 C. 1 h 2 50 C. 4 h 30 85 C. 3 h 3 50 C. 8 h 95 C. 8 h 4 85 C. 1 h 30 85 C. 8 h 5 85 C. 4 h 30 95 C. 1 h 6 85 C. 8 h 50 C. 3 h 7 95 C. 1 h 30 95 C. 3 h 8 95 C. 4 h 30 50 C. 8 h 9 95 C. 8 h 85 C. 1 h

(49) For each formulated bath, test pieces were plated with platinum at different currents. Each test piece was weighed before and after plating.

(50) On the basis of the increase in weight, it was thus possible to determine: deposition rate (in grams per hour per square decimeter (g/h/dm.sup.2)) at each current; the plateau of the bath; the current at the beginning of the plateau; the mean deposition rate of the plateau; the standard deviation of the plateau; and the ratio between the minimum and maximum rates obtained over the plateau.

(51) The three tables 2-1 to 2-3 below give the results obtained with the three baths that were found to give the best results at the end of the experiment.

(52) TABLE-US-00002 TABLE 2-1 Experimental parameters Color of B heat AB heat bath temp B heat temp AB heat after ( C.) time (h) ( C.) time (h) heating Test 2 50 C. 4 h 30 85 C. 3 h Clear Test 4 85 C. 1 h 30 85 C. 8 h Clear Test 7 95 C. 1 h 30 95 C. 3 h Clear

(53) TABLE-US-00003 TABLE 2-2 Deposition rate (g/h/dm.sup.2) 8A 1A 4A 16A 24A 8A Test 2 2.4455 0.8164 1.9618 2.2618 2.2564 1.6127 Test 4 2.0782 0.1727 1.4982 2.0236 2.2891 1.4945 Test 7 2.0509 0.6782 1.5600 2.1164 1.9073 1.5109

(54) TABLE-US-00004 TABLE 2-3 Plateau characteristics Color of Mean rate Plateau Vmin/Vmax bath after Start of of plateau standard ratio of electrolysis plateau (g/h/dm.sup.2) deviation plateau Test 2 Clear 4A 2.0232 0.27 0.65 Test 4 Cloudy 4A 1.8264 0.34 0.79 Test 7 Cloudy 4A 1.7736 0.25 0.61

(55) Furthermore, the bath of test 2 provides the following advantages:

(56) This is the bath for which the greatest degree of repeatability was observed, and which, compared with a reference bath, the mean deposition rate was large for a new bath (FIG. 1A), and remained sufficiently high during operation (FIG. 1A). The bath of test 2 is repeatable since the curve for mean rate and for dispersion of fabrications 1 and 2 are superposable, which clearly demonstrates the extreme degree of reproducibility of the fabrication. In contrast, it can be seen that the fabrication curves 1 and 2 for the bath of test 7 and even more for the bath of test 4 can be distinguished, so the bath of test 4 is the least repeatable, which is why bath 4 is not preferred.

(57) Furthermore, the bath of test 2 presents good dispersion of the plateau (FIG. 1B), it being recalled that the presence of a plateau corresponds to obtaining a deposition rate that is identical regardless of the applied current and regardless of the shape of the part being treated. On each fabrication, two plateaus were implemented. One plateau was studying the gain in weight as a function of the applied current density. In in-house fabrications, the dispersion decreases with increasing amount of electrolysis performed in the bath. This is not true of the reference bath where the bath becomes increasingly dispersed with increasing number of electrolyses performed.

(58) Likewise, it can be seen that the bath of test 2presents little loss of platinum over time (FIG. 1C) and that the mean effectiveness (FIG. 2A) and the deposition rate (FIG. 2B) of the bath are practically identical after three successive regenerations. Concerning losses of platinum, we found numerous loses of platinum with the reference bath, mainly in the form of a solid precipitate of platinum on the bottom of the vessel. Furthermore, with the reference bath, the greater the number of electrolyses performed using the bath, the greater its tendency to form precipitates on the bottom of the vessel. In contrast, for baths of the invention, it is observed that platinum losses are smaller, and above all constant over time (constant with increasing number of electrolyses). Furthermore, the bath of test 2 is the bath that presented the smallest loses of platinum and thus the bath of test 2 is the most profitable from an economic point of view.

(59) Overall, and as can be seen from the curves of FIGS. 1D to 1F and 1G to 1I, the baths of tests 4 and 7give results that are very similar to those of test 2.

(60) Furthermore, as can be seen from FIGS. 2A and 2B, the electrolyte bath of test 2 gives results that are stable over time in terms of deposition rate, with this continuing after the bath has been regenerated several times: the deposition rate is practically unchanged between the first and third regenerations.

(61) In order to regenerate a bath, platinum salts are added to the bath so as to raise its platinum content. Once the platinum salts have been added, the bath is left while being stirred at 65 C. for 12 h to 24 h so that the salts become fully dissolved in the bath.

EXAMPLE 2

(62) The fabrication of the bath of electrolyte is analogous to that of the procedure of Example 1, apart from the following points.

(63) Solution B comprises 43.5 g of ammonium hydrogen sulfate of chemical formula NH.sub.4HSO.sub.4 and 76 g of diammonium sulfate of chemical formula (NH.sub.4).sub.2SO.sub.4, and water. It was raised to 50 C. for 4h30.

(64) The pH of the mixture of solutions A+B was set in the range 1 to 5.

EXAMPLE 3

(65) The fabrication of the bath of electrolyte is analogous to that of the recipe of Example 1, apart from the following points.

(66) Solution B comprises 102.4 g of ammonium acetate of chemical formula CH.sub.3COONH.sub.4 and 39.6 g of acetic acid of chemical formula CH.sub.3COOH.

(67) The solution was raised to 50 C. for 4h30.

(68) The pH of the mixture of the solutions A+B was set to lie in the range 1 to 5.

(69) In the invention, the ligand is preferably selected from aliphatic polyamines having 3 to 20 carbon atoms in a straight or branched carbon chain.

(70) Advantageously, the ligand is selected from primary polyamines such as diaminopropanes such as 1,3-diaminopropane and 1,2-diaminopropane, diethylenetriamine, 1,4-diaminobutane, 1,6-diaminohexane; secondary polyamines such as N,N dimethyl-1,3-propane-diamine; and tertiary polyamines such as N, N, N, N tetramethylethylenediamine. It is preferred to select diaminopropanes for the ligands.