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(11) | EP 1 223 232 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process for depositing a metal coating containing nickel and boron |
[0001] The present invention relates to a process for depositing a metal coating, in particular to a process for electroless nickel plating. Furthermore, it relates to a plating bath for use in said process, and a plated article as produced by means of said process.
[0002] The document US-PS-6,066,406 discloses corrosion and wear resistant metallic coatings containing nickel and boron. The coatings as described according to said prior art reference are preferably deposited on catalytically active substrates from an electroless coating bath containing nickel ions, a stabilizer, a metal ion complexing agent, and a borohydride reducing agent, at a pH of about 10 to about 14. However, the hardness of said coating layers is still limited as well as the other properties being important for technical applications, which limitations result from the insuppressible occurrence of substantive co-depositing of impurities during the plating process. Especially, the coating baths according to the state of the art tend to spontaneous decomposition caused by the presence of unwanted elements that themselves would become activated plating sites that, once plating become larger in mass until they become so large that they fall out of solution, thereby dropping to the bottom of the plating tank causing said tank to plate or worse, additionally falling onto the work item to be plated, resulting thereby in a rough coating surface. Furthermore, in most cases the coating baths according to the state of the art additionally do not provide a proper reduction of the present nickel ions, leading to a bath solution that requires discarding, thereby also contributing to a worse quality of the coating surface.
[0003] Therefore, there remains a strong demand for a process for depositing a metal coating containing nickel and boron on a substrate, which avoids completely such phenomena being caused by undesired co-depositing of impurity particles, affecting unfavourably the application properties of the resulting nickel containing coatings as produced according to the state of the art.
[0004] This problem is solved by the process according to the elements of attached claim 1 or preferably any of dependent claims 2 to 8, making use of a plating bath according to any of the subsequent claims 9 to 16 and resulting in a coated article according to any of claims 17 to 20.
[0005] Especially, the process according to the present invention is characterized by the use of most accurately deionized water with a conductivity in the range of about 0.05 to about 0.1 µS for the preparation of the plating bath, which measure in particular avoids completely the undesired co-deposition of impurity particles in the coatings, resulting surprisingly in coatings with exceptionally unexpected physical properties in regard of particularly hardness, corrosion resistance, wear resistance, abrasion resistance, etc.(see below under section "K" of the working example).
[0006] In particular, the use of said deionized water prevents spontaneous decomposition of the coating bath by removing unwanted elements that themselves would become larger in mass until they become so large in mass that they fall out of the solution dropping to the bottom of the plating tank, thereby causing the tank to plate or worse, additionally falling onto the work item to be coated, thereby resulting in a rough coating surface. Furthermore, when using deionized water, the elements found in tap water which usually tend to interfere with the stabilizing elements of the bath are absent, resulting in a proper reduction of the nickel. Therefore, the above discussed decomposition of the coating bath can be avoided.
[0007] The further details of the teaching according to the present invention are described by means of the following working example.
WORKING EXAMPLE
A. BATH COMPONENTS
[0008]
Cemkote A = Make-up ( = premixed solution of 24g/l of total tank volume of Sodium Hydroxide pellets (Electronic Grade) mixed with high-quality deionized water.
Cemkote B = Reducer ( = solution comprising an effective amount of a borohydride reducing agent and an effective amount of a metal ion complexing agent, preferably of ethylenediamine);
Cemkote C = Stabilizer ( = preferably a solution comprising an effective amount of lead tungstate).
Cemkote D = Nickel Replenisher ( = a solution comprising both Ni-based plating solution, ammonium hydroxide, and a metal ion complexing agent, preferably ethylenediamine according to effective amounts, respectively).
B. MAKE-UP PROCEDURE
[0009]
a. Rinse tank thoroughly with deionized water. Tank must be free of debris and plate-out residue.
b. Place new 5-micron filters in the filter chamber.
c. Fill the tank half way with deionized water having a conductivity in the range of 0.05 to 0.1 µS.
d. Add 20% by volume of the total tank volume of Cemkote A.
e. Add premixed solution of 24g/l of total tank volume of Sodium Hydroxide pellets (Electronic Grade) mixed with high-quality deionized water. Caution: this solution heats up when mixed.
f. Fill tank to working level with high-quality deionized water.
g. Start pump.
C. BATH PARAMETERS
[0010]
| Temperature | 90-92 degrees Celsius; |
| pH | 10.5 to 14; |
| Bath load | 0.12 dm2 / ml to 0.16 dm2 / ml; |
| Nickel concentration | 4.76 to 5.6 g/l; |
| Plating rates | 18 to 24 µm per hour. |
D. PLATING
[0011] After proper preparation, parts are to be placed into the Cemkote solution for the required time in order to obtain the desired thickness of plating.
E. AGITATION
[0012] Air agitation is not recommended for this process. Continuous filtration with 5-micron filters and the total tank turnover rate of 10 times per hour is recommended.
F. TANK CLEANING
[0013] A solution of 30% to 50% Nitric acid solution should be placed into the tank after every third day of plating to clean out all the plate-out, if any, as well as passivate the tank. Let circulate over night. After Nitric acid solution is removed, refill the tank with a 2% to 3% ammonium hydroxide solution to neutralize any residual nitric acid (pH to be 10.5 or higher). Let circulate for one to two hours, then drain and flush with high-quality DI water.
G. NICKEL REPLENISHMENT
[0014] Nickel Titration Apparatus:
a. 250 ml Erlenmeyer flask
b. 2 ml pipette
c. 10 ml graduated cylinder
d. 25 ml burette
e. Stir plate
f. Pipette bulb
g. Stir bar
PROCEDURE:
[0015]
a. Add 100 ml of deionized water to a 250 ml Erlenmeyer flask.
b. Pipette 2 ml sample of the plating solution into same flask.
c. Add 10 ml of concentrated Ammonium Hydroxide solution to flask.
d. Place flask on the stir plate and turn on stirrer.
e. Add one Murexide indicator tablet (4mg).
f. Zero burette with 0,0575 EDTA standard solutions.
g. Titrate.
h. Color change is from yellow to purple.
[0016] Monitor nickel level every half hour to maintain within recommended range. When adding Cemkote D to an operating plating bath, add slowly.
H. REDUCER REPLENISHMENT
[0017]
a. Cemkote B and Cemkote C components are first mixed together before adding them to an operating plating bath.
b. Adds should be made per addition schedule every half hour of plating.
c. These adds should be made as slow as possible.
I. ADDITION SCHEDULE
[0018] Please note that this schedule is based on a Total Tank Volume (TTV) of 100 Gallons ( = about 500 liters), and will vary based on the Total Tank Volume (TTV) of your particular tanks.
| LOAD [dm2] | CEMKOTE "B" Addition [ml] |
CEMKOTE "C" Addition [ml] |
| 135 | 1087.5 | 870 |
| 162 | 1305.0 | 1044 |
| 203 | 1631.25 | 1305 |
| 243 | 1957.5 | 1566 |
| 270 | 2175.0 | 1740 |
| 310 | 2501.25 | 2001 |
| 350 | 2827.5 | 2262 |
| 404 | 3262.5 | 2610 |
| 444 | 3588.75 | 2871 |
| 485 | 3915.0 | 3132 |
| 539 | 4350.0 | 3480 |
| 579 | 4676.25 | 3741 |
| 620 | 5002.5 | 4002 |
| 674 | 5437.5 | 4350 |
K. PHYSICAL PROPERTIES OF RESULTING COATINGS
[0019]
1. Composition: approximately 95 wt.-% nickel and 5 wt.-% boron.
2. Coating density: 8.0 to 9.4 g/cm3.
3. Deposition thickness range: 1 µm to preferably approximately 250 µm (maximum deposit thickness generally unlimited), particularly 25 - 50 µm averagely for common industrial standard applications.
4. Structure: columnar growth; nodular topographic surface.
5. Uniformity of deposit: +/- 10 % of total deposit.
6. Magnetic Properties: slightly magnetic.
7. Surface finish distortion: increase of 20 -32 RMS from starting value of 1 RMS (mild steel, standard grit-blasted preparation, 25 µm deposit).
8. Bond Strength: metallurgical bond to metallic substrates, exceeds known bond tests
resulting in secondary attachment material failure, epoxy/silver solder.
Greater than 3200 kilo/2.54cm3, 6061 t aluminium (epoxy failure).
Greater than 5000 kilo/2.54cm3, 6061 t titanium (epoxy failure).
Greater than 35,000 kilo/2.54cm3, ferrous alloys.
9. Hydrogen Embrittlement: Less than 1 % induction of hydrogen; ASTM-F1940, Hard chrome: 38-44 %; Electroless nickel: 23-27 %.
10. Internal Stresses; Extremely low tensile: 30 MPa. Hard Chrome: 1000 MPa.
11. Corrosion resistance. ASTM B-117. Although Cemkote® should not be considered for corrosion resistance alone, in applications requiring wear resistance along with some corrosion resistance, a duplex layer should be incorporated. With an underlying layer of nickel 10 - 12 µm, the coatings as produced in accordance with the teaching of the present invention can resist oxide development up to 240 hours. In addition, post-plating sealers have been employed to further enhance corrosion resistance. It should also be mentioned that due to the durability of the coatings produced in accordance with the teaching of the invention, in common, practical applications, these coatings far exceed the useful life of other coatings such as cadmium and zinc.
12. Wear Resistance, Falex ASTM-D2714, ring and block; mass loss: 0.0002g.
Electroless nickel, mass loss: 0.0022 g.
Hard chrome, mass loss: 0.0017 g.
Tungsten Carbide, mass loss: 0.0014 g.
13. Abrasion resistance, ASTM-G65, Abrasive sand, mass loss: 0.008 g;
Thermal spray tungsten carbide, mass loss: 0.009 g;
PVD titanium nitride, mass loss: 0.065 g; PVD Cr nitride, mass loss: 0.0039 g;
Hard chrome, mass loss: 1.1g; Electroless nickel, mass loss: 1.4 g.
14. Ductility: Extremely ductile because of the true columnar growth.
15. Hardness: The coatings as manufactured according to the teaching of the present invention represent an alloy coating comprising of a softer nickel and boron matrix with harder nickel boride crystalline clusters dispensed through-out the deposit. Knopp and Vickers indenters capture mostly the softer matrix and not the much harder nickel boride clusters. So reading are commonly found to be 1100 - 1400 KpH. The nickel-boride clusters 1600 - 2000 KpH.
16. Temperature limitations: -100 °C to 980 °C.
17. Melting Point: 1400 °C.
18. Electrical conductivity: about 15 - 190 microOhms.
19. Coefficient of friction: 0.08 - 0.06 ASTM-D2714; Hard chrome: 1.3 - 1.35.
preparing a plating bath having a pH in the range of about 10.5 to 14 and a temperature above ambient temperature, comprising:
(a) nickel ions according to a nickel concentration in the range of about 4.76 to about 5 g/l of plating bath;
(b) an effective amount of a stabilizer;
(c) a metal ion complexing agent in an amount effective to inhibit precipitation of said metal ions from the plating bath;
(d) an effective amount of a borohydride reducing agent; and
(e) optionally up to 0.25 moles of cobalt per liter of plating bath;
immersing said substrate to be coated into said plating bath, electrolessly depositing the coating on the substrate and optionally heat treating said metal coating for about one to about 24 hours subsequently, characterized in that the plating bath is prepared on the basis of deionized water having a conductivity in the range of about 0.05 to about 0.1 µS.
(a) nickel ions according to a nickel concentration in the range of about 4.76 to about 5 g/l of plating bath;
(b) an effective amount of a stabilizer;
(c) a metal ion complexing agent in an amount effective to inhibit precipitation of said metal ions from the plating bath;
(d) an effective amount of a borohydride reducing agent; and
(e) optionally up to 0.25 moles of cobalt per liter of plating bath; and being
characterized in that the plating bath comprises deionized water having a conductivity in the range of about 0.05 to about 0.1 µS.