WO2001007686A2

WO2001007686A2 - Article in which a non-conductive substrate is coated with electroplated metal and process for its manufacture

Info

Publication number: WO2001007686A2
Application number: PCT/IL2000/000444
Authority: WO
Inventors: Natalie Perene; Sergey Chapligin; Nadegda Mirochnik; Lev Vinnikov
Original assignee: Nickel Rainbow Ltd; Natalie Perene; Sergey Chapligin; Nadegda Mirochnik; Lev Vinnikov
Priority date: 1999-07-26
Filing date: 2000-07-26
Publication date: 2001-02-01
Also published as: IL131113A0; WO2001007686A8; AU2000263121A1

Abstract

An article comprises a non-conductive substrate, the surface of which has been chemically modified by treatment with an acidic reactant selected from oxidants and etchants, and subsequently coated with a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB, IVA and IVB, the sulfide being formed on the surface in absence of elemental noble metal and platinum group metal compounds, and being coated with at least one electroplated metal superimposed directly on the sulfide film.

Description

ARTICLE IN WHICH A NON-CONDUCTIVE SUBSTRATE IS COATED
WITH ELECTROPLATED METAL AND PROCESS FOR ITS MANUFACTURE
FIELD OF THE INVENTION
The present invention relates to an article in which a non-conductive substrate is coated with electroplated metal in absence of an electrolessly-deposited metal layer, and to a process for making it.

BACKGROUND OF THE INVENTION
There is an increasing demand for metal-plated articles made from non-conductive materials, particularly plastics. Such articles find application in, for example, the automotive, home appliance and radio and television industries.

Among the various plastic materials used for this purpose, acrylonitrile/butadiene/styrene copolymers (ABS) account for some 70-80%, because they are easily processed and mechanically strong. Also industrially applicable in this field are polypropylene, polycarbonate, polysulfones, polyacetals, impact-resistant polystyrene and epoxy resins, while for special applications, there may also be mentioned polyethylene, phenol-formaldehyde resins, polyethers, polyamides, polyethylene terephthalate and polytetrafluoroethylene.

Non-conductive surfaces are conventionally metallized by a sequence of steps including deposition of colloidal noble metal particles, usually Pd, and electroless deposition of an intermediate metal layer, such as Cu or Ni, to provide a basis for subsequent electroplating of metal. The electroless deposition step is frequently a source of faults in the coated product leading to a relatively short service life, while the use of noble metals leads to a high-cost product.

Attempts have previously been made to avoid the need for the fault-prone electroless coating, by directly electroplating a metal onto a non-conductive surface which has been treated with colloidal noble metal particles, or with a semi-conductive film of metal sulfide.

Thus, for example, in U. S. Patent No. 4,952,286, there is described a
method for electroplating a nonconductor surface, wherein the latter is treated with
an acid solution of a reduced noble metal electroless metal plating catalyst which
is thereby deposited on at least a portion of the surface, and then with a solution of
dissolved sulfide, selenide or telluride capable of reacting with the metal plating catalyst to form a corresponding chalcogenide, and finally electroplating. In practice, this method is limited to electroplating small areas such as the holes of printed circuit boards. U. S.

Patent No. 5,276,290 similarly describes a printed circuit board having metallized through holes, with electroplated copper on the walls of the holes and an adsorbed layer of colloidal metal particles of catalytic metal sulfide dispersed between the copper and the circuit board material.

Use of a semi-conductive metal sulfide layer on which a metal may be electroplated has been described, for example, in U. S. Patent No. 3,658,661, wherein rough or porous aromatic polymer surfaces, particularly of phenolic resins, are sequentially pretreated with aqueous solutions of an alkali metal sulfide and a
Group IB, IIB, VIIB or VIII metal salt, before electroplating. In U. S. Patent No.

4,783,243, a polymeric surface impregnated with a metallic sulfide such as copper sulfide (made e. g. by treatment with aqueous copper and sulfide solutions) is electroplated. In such known processes using an intermediate sulfide layer for electroplating, deposition of sulfide is slow and lacks uniformity, with the result that a relatively large number of immersion cycles in the two solutions is required. U. S.

Patent No. 5,207,888 attempts to overcome these drawbacks by a process including treatment of the non-conducting surface with a preformed colloid of a catalytic metal chalcogenide (such as palladium sulfide). However, the product is expensive and also has a relatively short service life.

In U. S. Patent No. 5,762,777, there is described a multi-step process for electroplating onto a non-conductive substrate, in which in the first step, the substrate surface is modified by treatment with organic hydrocarbons or polymers.

It will be appreciated that treatment with organic materials generally adds to the expense of the process including the cost of waste disposal.

More recently in U. S. Patent No. 5,908,543, it has been proposed to
electroplate non-conductive materials including molded components, which have
been pretreated with an acidic hydrosol containing palladium, stannous and
copper compounds, followed by contact with aqueous alkali. Such products
nevertheless suffer from the expense consequent on the use of palladium
compounds.

The entire contents of the above-mentioned U. S. Patents are deemed to be
incorporated herein by reference.

OBJECTS OF THE INVENTION
It is an object of the present invention to provide a low-cost and reliable process for electroplating metal onto non-conductive materials, such as plastics, or onto articles made therefrom.

It is a further object of the invention to provide such a process in which a relatively large surface area of non-conductive material is adequately and uniformly coated with electrodeposited metal.

Other objects of the invention will appear from the description which follows.

SUMMARY OF THE INVENTION
It has surprisingly been found in accordance with the present invention, that electroplated articles based on non-conductive substrates can be made where the electroplated metal covers a relatively large area-rather than being confined to areas of the order of size of the holes in a circuit board-in absence of an intermediate electroless metal coating underlying the electroplated metal, in absence of elemental noble metals and platinum group metal compounds, and in absence of an initial treatment with organic materials.

One result of this discovery is that by this means, a relatively large area of the surface of electrically non-conductive substrates may be rendered electroconductive. Another result is, that selective plating of ABS polymers can be effected, without simultaneous metal deposition on the insulation material of the racks.

The present invention accordingly provides in one aspect, an article which comprises a non-conductive substrate, the surface of which has been modified by treatment with an acidic reactant selected from oxidants and etchants, and subsequently coated with a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB and IV (i. e. IVA and IVB), said sulfide being formed on said surface, and in absence of elemental noble metal and
platinum group metal compounds, with at least one electroplated metal coating
superimposed directly on said sulfide film.

The present invention further provides in another aspect, a process for
manufacturing the article defined in the preceding paragraph, which process
includes subjecting the substrate to the following sequential steps:
an initial treatment step with an aqueous oxidant or etchant solution at acid pH, resulting in chemical modification of the surface of said substrate,
an intermediate treatment step with, in either order, (i) an aqueous solution of at least one water-soluble compound of at least one metal selected from
Periodic Groups IB, IIB and IV and at least one complex-forming ligand and (ii) an aqueous solution of at least one water-soluble sulfide selected from alkali and alkaline earth metal sulfides,

and
a step in which the substrate bearing a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB and IV, is electroplated.

References herein to the sulfide being formed on the surface of the substrate are intended to convey that water-insoluble sulfide (s) of at least one metal selected from Periodic Groups IB, IIB and IV, are formed on the surface of the substrate modified by oxidant or etchant (and optionally further treated as described herein), by interaction of sequentially added reactants (in either order), being the corresponding metal ions on the one hand, and sulfide or polysulfide ions on the other hand. In other words, the sulfide on which electroplated metal is deposited in accordance with the present invention is not pre-formed, before being applied to the substrate surface.

DETAILED DESCRIPTION OF THE INVENTION
The non-conductive substrates to be electroplated in accordance with the present invention are those known to persons of the art for this purpose. They
include those referred to above and in particular those mentioned in the
above-cited U. S. Patents.

In the initial treatment step, the aqueous oxidant or etchant solution
preferably includes additionally, at least one species of metal ions, for example
selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze
surface modification of the substrate by the oxidant or etchant.

It will be apparent that the purpose of the intermediate treatment step is to
react together solutions (i) and (ii) in either order i. e. first (i) and then (ii), or first (ii)
and then (i), so that there will deposit on the substrate, at least one water-insoluble
sulfide of at least one metal selected from Periodic Groups IB, IIB and IV.

It is presently preferred to subject the substrate to an additional step, between the initial and intermediate treatment steps, of treatment with an agent adapted to promote adhesion of said at least one water-insoluble sulfide thereto.

Suitable such agents may include at least one ingredient selected from the group consisting of Zn, Ti, Ge, Sn, Pb and Fe ions, or an ingredient which preferably contains one or more of hydroxide, sulfate, carboxylate or amino groups, and in particular, may be, selected from the group consisting of azo, thiazino and triarylmethine dyes, in soluble acid form. Exemplary such dyes are methylene blue, methyl orange, methyl red, aniline blue and crystal violet.

In the intermediate treatment step, the at least one water-soluble compound in aqueous solution (i) of at least one metal selected from Periodic Groups IB, IIB and IV may be selected, for example, from the group consisting of Cu, Ag, Zn, Cd,
Hg, Sn and Pb, and may optionally include at least two such compounds, wherein the metal components have differing redox potentials. Suitable ligands are, for example, chelating agents selected from compounds forming ammine-or aqua
complex groups, or from such complex-forming compounds as acetic, citric,
maleic, succinic and tartaric acids, ammonia, or monoethanolamine or
ethylenediamine derivatives such as EDTA. Aqueous solution (ii) may contain at
least one polysulfide.

The invention will be illustrated by the following non-limitative Examples.

Examples I and IA
In these and other Examples, all aqueous solutions and washing water are
at ambient temperature, unless otherwise specified, and the numerals in
parentheses identify individual solutions.

Example 1: An ABS shower part of surface area 3.5 dm2 and 21 cm in length was
metallized. The sample was subjected to an initial treatment at 55 C for 1 minute,
with an aqueous oxidant solution (1-2) containing 30 9/1 chromium trioxide and 25
vol. % sulfuric acid and about 75 vol. % phosphoric acid, and thereafter was
washed under running water for about 2 minutes. It was then immersed in an
aqueous solution (11-5) containing 3 g/l lead nitrate.

After washing with deionized
water for about 0.1-0.5 minute, the sample was treated for 0.1-1 minute with an
aqueous solution (IV-2) prepared by dissolving sulfur powder to saturation in 300
g/l NaOH. After washing with water for about 1 minute, the sample was immersed for 0.1-1 minute in an aqueous solution (111-5) containing 40 g/I copper sulfate hydrate and 5 gui hydroquinone. A uniformly colored sulfide film was thus formed on the sample surface, by 2-3 cycles of repeated immersion in the solutions (IV-2 and 111-5). The sulfide film was uniformly distributed on the treated sample of complicated configuration.

The sample was washed under running water for 1-2 minutes, then with deionized water. This product as cathode was immersed in a
Watts-type electroplating bath containing 280 gui nickel sulfate, 45 g/I nickel chloride and 35 g/I boric acid, and a nickel anode. The bath was maintained under air agitation at 50-52 C, pH and a cathode current density of 3.5 A/dm2.

The sulfide film was covered with nickel in about 2 minutes, but electrodeposition was continued for 40 minutes. The PVC insulation of the electroplating rack was not simultaneously electroplated. The adhesion rating of the electroplated coating was rated as high (ASTM D3359).

Example 1A: A similar sample was treated under the same conditions as in
Example 1, except that use of solution 11-5 was omitted. Repeated immersion for 7-8 cycles, using solutions (IV-2 and 111-5) was necessary in order to reach the proper color, density and distribution of the sulfide film. During the electroplating step, not only the ABS sample, but also the PVC insulation of the electroplating
rack was electroplated. The metal/ABS adhesion was rated as low (ASTM D-3359).

Examples 2-7
The procedure described in Example 1 was repeated, with variation of the
non-conductive sample to be treated, the treatment solutions and in some cases the order of applying the water-insoluble sulfide forming solutions (i) and (ii).

Details are given in Tables 1 and 2.

Table 1 : Variation of aqueous solutions appiied in different cases.

Example Sample Oxidant Sulfide-adhesion Insoluble suifide-
or Etchant Agent forming sequence
1 ABS 1-2 11-5 IV-2, 111-5
1A ABS 1-2-IV-2, 111-5
2 impact-resistant I-2 IV-2III-2,
polystyrene
3 ABS 1-1 11-1 111-2, IV-2
4 polycarbonate 1-4 11-4 IV-2, 111-1
5 II-2III-1,IV-1I-2
6 II-3IV-2,III-3I-3
7 epoxy resin I-5 IV-1III-4,
Table 2: Key to code numbers of aqueous solutions (concentrations in g/l)
Solution Initial Step Additional Step Intermediate Step
No.

Solution (I) Solution (II) Solution (III) * Solution (IV) **
CrO3 (300-400) TiCl3(4-6) Pb(II)acetate(10-50) Na2S (10-50)
-1 H2SO4 (380-420) HCI (40-60) Na citrate (40-60) NaOH (to
60-70 C, 8-20 min. NH40H (50-100) pH 12-12.5)
CrO3 (10-40) FeSO4 (10-60) CuS04 (40-80) polysulfides:
H2SO4 (400-500) H2SO4 (10-30) Pb (II) acetate (10-30) saturated
-2 H3PO4 (1200-1300) Na citrate (40-60) solution of
50-70 C, 1-5 min.

NH40H (50-100) S in NaOH (200-400)
methyleneblueCdSO4(10-40)CrO3(50-80)
-3 H2SO4 (1000-1500) (0.5-2.0) K-Na tartrate (20-60)
CuSO4 (1-3) NH40H (50-100)
50-70 C, 8-15 min.

K2Cr207 (40-80) methyl orange CdS04 (10-40)
-4 H2SO4 (1000-1500) (0.5-2.0) ZnSO4 (40-80)
60-70 C, 4-10 min. NH40H (150-250)
HF (8-18%) lead CuSO4 (10-100)
-5 H2SO4 (55-75%) nitrate hydroquinone
50-70 C, 10-35 min. (0.5-5) (1-20)
for sub-steps * (i), ** (ii)
(Note: sulfates are used in the form of their hydrates.)
It is presently contemplated that for certain applications, the sulfide film on the substrates in the above Examples may in the alternative be electroplated with copper from electrolytes where the redox potential of the deposited metal is higher than the potential of chemical reduction of the metal forming the sulfide film.

Optionally, the copper plating may be over-electroplated with nickel.

ADVANTAGES OF THE INVENTION
The method of the invention, particularly when including the optional additional step as described herein, presents the following advantages over similar known processes: 1. Absence of electroless deposition implies also a reduction of production costs and improved economy of operation. Since electroless deposition frequently involves use of hypophosphite, its absence simplifies wastewater treatment.

2. Absence of noble metals implies a longer service life as well as reduced production costs because these of the expense of such materials.

3. Minimizing the number of treatment cycles and operations before the electroplating step, e. g. by reducing the number of cycles from 7-8, to 2-3.

4. An option of improved adhesion between the electroplated metal and a wide range of non-conductive substrates.

5. An option of selective plating of ABS without undesirable metal deposition
on the insulation material of the racks.

While particular embodiments of the invention have been particularly
described hereinabove, it will be appreciated that the present invention is not
limited thereto, since as will be readily apparent to skilled persons, many
modifications or variations can be made. Such modifications or variations which
have not been detailed herein are deemed to be obvious equivalents of the
present invention.

Claims

CLAIMS 1. An article which comprises a non-conductive substrate, the surface of which has been chemically modified by treatment with an acidic reactant selected from oxidants and etchants, and subsequently coated with a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB and IVB, said sulfide being formed on said surface, and in absence of elemental noble metal and platinum group metal compounds, with at least one electroplated metal coating superimposed directly on said sulfide film.

2. An article according to claim 1, wherein said at least one electroplated metal is selected from copper and nickel.

3. An article according to claim 1, wherein said at least one electroplated metal consists of electroplated copper, with an over-electroplating of nickel.

4. A process for manufacturing an article as defined in any of claims 1-3, which includes subjecting said substrate to the following sequential steps : an initial treatment step with an aqueous oxidant or etchant solution at acid pH, resulting in chemical modification of the surface of said substrate, an intermediate treatment step with, in either order, (i) an aqueous solution of at least one water-soluble compound of at least one metal selected from Periodic Groups IB, IIB and IVB and at least one complex-forming ligand and (ii) an aqueous solution of at least one water-soluble sulfide selected from alkali and alkaline earth metal sulfides,

and a step in which said substrate bearing a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB and IVB is electroplated.

5. A process according to claim 4, wherein said solutions (i) and (ii) are used in that order, and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent adapted to promote adhesion of said at least one water-insoluble sulfide thereto; (b) said oxidant or etchant solution inclues additionally at least one species of metal ions adapted to catalyze surface modification of said substrate by said oxidant or etchant; (c) said at least one water-soluble compound in aqueous solution (i) of at least one metal selected from Periodic Groups IB, IIB and IVB includes at least two such compounds wherein the metal components have differing redox potentials ;

(d) said aqueous solution (ii) contains at least one polysulfide.

6. A process according to claim 4, wherein said solution (ii) is used prior to solution (i), and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent adapted to promote adhesion of said at least one water-insoluble sulfide thereto ; (b) said oxidant or etchant solution inclues additionally at least one species of metal ions adapted to catalyze surface modification of said substrate by said oxidant or etchant; (c) said at least one water-soluble compound in aqueous solution (i) of at least one metal selected from Periodic Groups IB, IIB and IVB includes at least two such compounds wherein the metal components have differing redox potentials;

(d) said aqueous solution (ii) contains at least one polysulfide.

7. A process according to claim 4, wherein said solutions (i) and (ii) are used in that order, and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient selected from the group consisting of Ti, Ge, Sn, Pb and Fe ions, adapted to promote adhesion of said at least one sulfide thereto; (ss) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of said substrate by said oxidant or etchant;

(y) said at least one metal selected from Periodic Groups IB, IIB and IVB is selected from the group consisting of Cu, Ag, Zn, Cd, Hg, Sn and Pb.

8. A process according to claim 4, wherein said solution (ii) is used prior to solution (i), and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient selected from the group consisting of Ti, Ge, Sn, Pb and Fe ions, adapted to promote adhesion of said at least one water-insoluble sulfide thereto;

(ss) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of said substrate by said oxidant or etchant; (y) said at least one metal selected from Periodic Groups IB, IIB and IVB is selected from the group consisting of Cu, Ag, Zn, Cd, Hg, Sn and Pb.

9. A process according to claim 5, wherein at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient, in soluble acid form, selected from the group consisting of azo, thiazino and triarylmethine dyes, adapted to promote adhesion of said at least one water-insoluble sulfide thereto; (P) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of said substrate by said oxidant or etchant;

10. A process according to claim 6, wherein at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient, in soluble acid form, selected from the group consisting of azo, thiazino and triarylmethine dyes, adapted to promote adhesion of said at least one water-insoluble sulfide thereto ; (p) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of substrate by said oxidant or etchant;

11. An article which comprises a non-conductive substrate, the surface of which has been chemically modified by treatment with an acidic reactant selected from oxidants and etchants, and subsequently coated with a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB and IV, said sulfide being formed on said surface, and in absence of elemental noble metal and platinum group metal compounds, with at least one electroplated metal coating superimposed directly on said sulfide film.

12. An article according to claim 11, wherein said at least one electroplated metal is selected from copper and nickel.

13. An article according to claim 11, wherein said at least one electroplated metal consists of electroplated copper, with an over-electroplating of nickel.

14. A process for manufacturing an article as defined in any of claims 11-13, which includes subjecting said substrate to the following sequential steps: an initial treatment step with an aqueous oxidant or etchant solution at acid pH, resulting in chemical modification of the surface of said substrate, an intermediate treatment step with, in either order, (i) an aqueous solution of at least one water-soluble compound of at least one metal selected from Periodic Groups IB, IIB and IV and at least one complex-forming ligand and (ii) an aqueous solution of at least one water-soluble sulfide selected from alkali and alkaline earth metal sulfides,

and a step in which said substrate bearing a film of at least one water-insoluble sulfide of at least one metal selected from Periodic Groups IB, IIB and IV is electroplated.

15. A process according to claim 14, wherein said solutions (i) and (ii) are used in that order, and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent adapted to promote adhesion of said at least one water-insoluble sulfide thereto; (b) said oxidant or etchant solution includes additionally at least one species of metal ions adapted to catalyze surface modification of said substrate by said oxidant or etchant; (c) said at least one water-soluble compound in aqueous solution (i) of at least one metal selected from Periodic Groups IB, IIB and IV includes at least two such compounds wherein the metal components have differing redox potentials ;

(d) said aqueous solution (ii) contains at least one polysulfide.

16. A process according to claim 14, wherein said solution (ii) is used prior to solution (i), and at least one of the following conditions applies : (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent adapted to promote adhesion of said at least one water-insoluble sulfide thereto; (b) said oxidant or etchant solution inclues additionally at least one species of metal ions adapted to catalyze surface modification of said substrate by said oxidant or etchant; (c) said at least one water-soluble compound in aqueous solution (i) of at least one metal selected from Periodic Groups IB, IIB and IV includes at least two such compounds wherein the metal components have differing redox potentials;

(d) said aqueous solution (ii) contains at least one polysulfide.

17. A process according to claim 14, wherein said solutions (i) and (ii) are used in that order, and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient selected from the group consisting of Zn, Ti, Ge, Sn, Pb and Fe ions, adapted to promote adhesion of said at least one sulfide thereto; (p) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of said substrate by said oxidant or etchant;

(y) said at least one metal selected from Periodic Groups IB, IIB and IV is selected from the group consisting of Cu, Ag, Zn, Cd, Hg, Sn and Pb.

18. A process according to claim 14, wherein said solution (ii) is used prior to solution (i), and at least one of the following conditions applies: (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient selected from the group consisting of Zn, Ti, Ge, Sn, Pb and Fe ions, adapted to promote adhesion of said at least one water-insoluble sulfide thereto;

(ss) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of said substrate by said oxidant or etchant; (y) said at least one metal selected from Periodic Groups IB, IIB and IV is selected from the group consisting of Cu, Ag, Zn, Cd, Hg, Sn and Pb.

19. A process according to claim 15, wherein at least one of the following conditions applies : (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient, in soluble acid form, selected from the group consisting of azo, thiazino and triarylmethine dyes, adapted to promote adhesion of said at least one water-insoluble sulfide thereto; said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of said substrate by said oxidant or etchant; (y) said at least one metal selected from Periodic Groups IB, IIB and IV is selected from the group consisting of Cu, Ag, Zn, Cd, Hg, Sn and Pb.

20. A process according to claim 16, wherein at least one of the following conditions applies : (a) said substrate is subjected to an additional step, between said initial and intermediate treatment steps, of treatment with an agent including at least one ingredient, in soluble acid form, selected from the group consisting of azo, thiazino and triarylmethine dyes, adapted to promote adhesion of said at least one water-insoluble sulfide thereto;

(p) said oxidant or etchant solution includes additionally at least one species of metal ions, selected from the group consisting of Cu, Ag, Ti and Fe ions, adapted to catalyze surface modification of substrate by said oxidant or etchant ; (y) said at least one metal selected from Periodic Groups IB, IIB and IV is selected from the group consisting of Cu, Ag, Zn, Cd, Hg, Sn and Pb.