EP0725166B1 - Process for plating a face of an aluminium or aluminium alloy workpiece - Google Patents

Abstract

A wear-resistant coating of Ni containing solid particles, e.g. of SiC, is applied to an Al or Al alloy surface subject to high friction by three stages. The Al is activated in a bath containing a Ni halide while acting as the anode in an electric circuit, then suractivated in a second chemical bath and finally immersed in a plating bath containing solid particles while acting as a cathode.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to the field of aluminum or aluminum alloy parts having at least at least one face or surface subjected to high friction stresses, in particular molded or forged parts of motor vehicles. These are, for example, shirts that equip internal combustion engines of motor vehicles or cylinders machined directly into the engine block. The invention relates more precisely to the surface internal, or bore, of a jacket or engine block which is subjected, cold as hot, with strong frictions and is sensitive to wear.

STATE OF THE ART

To make motor vehicle parts from aluminum alloy, one chooses in the majority of the cases of alloys which are easy to use, for example by casting or by forging, but which have insufficient characteristics of use and resistance to strong friction stresses. Such stresses can be encountered in motors, for example example on the inside surface of an engine block jacket or barrel, also called cylinder-crankcase, where the piston is guided in its alternative course and where its segments are in constant contact with said surface. In order to improve the wear resistance, we know by FR-A-1 579 266 and FR-A-2 159 179 deposit on said interior surface a coating consisting of a composite of nickel and solid particles, generally made of silicon carbide.

Patent application FR-A-1,579,266 proposes a process for the galvanic deposition of a metallic coating containing solid particles. The deposit takes place in two stages: preparatory step where a first layer of zinc is deposited chemically on the surface to be treated, and a second step which is the actual electrolytic deposition, the part to be treated being the cathode, this deposition taking place itself in two stages: first deposition of a fine almost pure nickel layer then deposit of the nickel loaded with solid particles.

This process, or its variants, is currently used widely on a large scale as for aluminum alloy engine blocks than for cast iron engine blocks or liners, because the coating thus obtained not only increases the resistance to wear but also improves lubrication because it facilitates the retention of the lubricant thanks to the carbide particles silicon emerging from the nickel surface.

Patent application FR-A-2 159 179 brings an improvement to the initial process which consists in a mechanical preparation of the surface (shot blasting) followed by a sodium attack and finally double zincing with intermediate nitric attack. Improving the adhesion of the layer filed, it is used for mass production but has the disadvantage of make a layer of irregular thickness.

Patent application EP-A-0 288 364 discloses a process for coating drums of engine blocks in cast iron where the initial deposit of zinc is replaced by a sulfuric attack electrolytic. This process allows better control of the thickness of the deposit but is not suitable for aluminum alloys.

The bore of a cylinder block is the seat of the piston stroke, it must therefore be carried out in very tight dimensional tolerances. The irregularity of the thickness of the deposited layer requires in practice a final machining, generally by abrasion-grinding, long, delicate and expensive. Good geometrical precision of the deposit would make it possible to no longer plan for recovery immediately target the thickness corresponding to the maximum wear that can be expect from this coating. In addition, to increase the life of the engine, we would still like improve the wear resistance of the coating and reduce the friction of the piston rings which move in contact with it, which would have another beneficial effect of reducing noise mechanical and engine vibrations.

OBJECT OF THE INVENTION

• une étape d'activation électrochimique où la pièce est portée à une polarité anodique et qui rend la surface à revêtir très réactive
• un traitement de suractivation qui complète l'effet de la première étape
• une étape de dépôt électrolytique où la pièce est portée à une polarité cathodique.

The object of the invention is a method of coating the face of an aluminum or aluminum alloy part intended to undergo high friction stresses. This is more particularly the bore of a liner or of an internal combustion engine block. This process comprises at least the following three successive stages:

• an electrochemical activation step where the part is brought to an anodic polarity and which makes the surface to be coated very reactive
• an over-activation treatment which completes the effect of the first stage
• an electrolytic deposition step where the part is brought to a cathodic polarity.

Profitably, these operations can be separated by rinsing with pure water and are succeed in a very short time, so that the surface to be coated does not dry between each step and without said surface being exposed to air or any other environment causing his reactivity to drop.

At each galvanic step according to the invention, an electrode with a shape close to that of the surface to be treated is placed near said surface. Advantageously, it is possible to keep the same electrode for all operations, said electrode to be simply brought to a cathode polarity at the first stage, zero at the second and anodic to the third.

The first step according to the invention is an electrochemical activation phase where the surface to treat and the electrode are in a bath containing a halogenated acid salt of nickel. Of preferably, this bath is an aqueous solution containing nickel chloride, a compound fluorinated and boric or fluoboric acid. Preferably, an aqueous solution will be used containing, per liter of electrolyte, from 100 to 250 grams of nickel chloride, from 2 to 10 grams of ammonium bifluoride, and 10 to 20 grams of fluoboric acid.

A direct current is applied between the part which acts as an anode and the electrode which acts as cathode. Preferably, the current density is between 10 and 50 A / dm ² for 30 to 120 seconds, the bath being maintained at a temperature between 40 ° C and 60 ° C.

Advantageously, one will first seek to prepare the surface to be treated by a succession of alkaline degreasing and alkaline then fluoboric-nitric pickling baths.

The second step according to the invention is an over-activation treatment having as its object complete the depassivation of the surface to be coated and dissolve the few residues from the electrochemical treatment of the first stage and likely to disturb the regularity and the homogeneity of the future deposit. Preferably this over-activation treatment is carried out with a nitric fluoboric bath and more particularly an aqueous solution comprising between 20% and 50% by volume of nitric acid concentrated to 68% and between 20% and 75% by volume of acid fluoboric concentrated at 50%. Preferably the surface in contact with this bath is maintained for a period of 30 to 120 seconds at a temperature between 20 ° C and 40 ° C

The third step according to the invention is the phase of electrolytic deposition of the composite nickel. The bath is a nickel-plating bath containing a filler composed of solid particles which can be either carbides, in particular silicon carbide, or any other hardening component the coating and improving the wear resistance of the deposit (diamond for example), i.e. component reducing the coefficient of friction (graphite for example), i.e. a mixture of components of these two categories intended to provide the best compromise between resistance to wear and coefficient of friction corresponding to the intended use.

Advantageously, said nickel-plating bath may comprise nickel sulfamate, nickel chloride, boric acid, saccharin and said charge of solid particles.

Preferably, a nickel-plating bath will be used comprising approximately, per liter electrolyte, 250 to 400 grams of nickel sulfamate, 20 to 40 grams of chloride nickel, 10 to 100 grams of boric acid and 50 to 150 grams of filler. During of the treatment the bath is maintained at a temperature between 40 ° C and 60 ° C, while its pH is maintained between 2 and 5, preferably between 2.5 and 3.5. The bath also has saccharin, which has the advantageous effect of reducing the residual stresses prevailing in the depot. Its concentration is however limited because saccharin has the other effect of decrease the deposition rate. Preferably, one liter of nickel-plating bath contains between 0.5 and 4 grams of saccharin.

A direct or pulsed current is applied between the part which acts as a cathode and the electrode which acts as an anode. Preferably, the current density is between 20 and 50 A / dm ² for the time necessary to reach the desired thickness. For example, with a current density of 30 A / dm ² , the treatment lasts 15 minutes to obtain a layer of 45 µm at a temperature close to 50 ° C.

Other characteristics and advantages of the invention stem from the synergistic effect of the combination of the first two stages and relate to the constitution of the charge in particles solid, which is enriched and better suited to the tribological properties sought in this type deposit. Thus said filler which contains particles hardening the coating, such as silicon carbide particles, can be enriched with particles improving conditions tribological of the contact such as graphite particles. In an advantageous way of the invention, this charge included, per liter of nickel-plating bath, between 5 and 50 grams of graphite powder.

On the other hand, all of the particles of said filler according to the invention can reach a size predominant between 0.5 µm and 5 µm. In a preferred embodiment of the invention, particles of silicon carbide with a particle size of between 3 μm and 5 μm are introduced, that is to say large enough to reduce the risk of seizure but not too large to avoid too much wear of the other element in contact. This same charge is enriched with particles of finer particle size graphite: 1 µm to 3 µm.

Analyzes of the surface observed just after the second step according to the invention have shown that, surprisingly due to the polarity of the part at the first stage, nickel metal settled in the cavities created by the acid attack and was not completely dissolved by the over-activation bath. These cavities constitute very reactive sites which favor the attachment of the composite nickel layer. The combination of electrochemical activation of the first step according to the invention and of the over-activation of the second step according to the invention constitutes a synergistic effect which makes it possible to immediately deposit the nickel layer composite; it is therefore not essential to deposit at the start of the third stage the fine layer of pure nickel recommended in the prior art.

The combination of the electrochemical activation of the first step according to the invention and the over-activation of the second step according to the invention improves the yield of the deposition of the third step so there is no need to reach the art bath concentrations to obtain the same charge concentration in the deposited layer. This allows identical bath viscosity, enrich the filler with the same element to improve given property, either with other elements to give it other properties; so by example, we can add to the silicon carbide powder, which improves the wear resistance, graphite powder which reduces friction at start-up and thus reduces the risk of seizure.

Still due to this synergistic effect, it is possible according to the invention to use significantly larger solid particles than in the prior art, which further improves the tribological quality of the coating by reducing the risks of seizing.

FIGURES

FIG. 1 represents the diagram of a preferred embodiment given solely by way of non-limiting example. According to this embodiment, operations are limited, waiting between steps is minimal, activation of the surface is not hampered by any oxidation or passivation. The system is dynamic, that is to say that the processing cell the part 1 is not dismantled during the process and which is introduced inside said cell 1 successively all the necessary baths. This is possible thanks to circuit 2 which includes polypropylene pipes and a pump 3 allowing the circulation of fluids between their retention tank and the treatment cell. Depending on the open or closed position of different valves 4 of the circuit, the pump first drives the activation bath of the tank 5, the rinsing bath of the tank 6, the over-activation bath of the tank 7, a new rinsing bath, finally the nickel plating bath of the tank 8.

FIG. 2 presents a block diagram of the cell for treating the part to be coated. A engine block being particularly bulky and heavy to handle, we have simplified the part by replacing it with a cylindrical jacket 12 made of the alloy AS5U3G commonly used for engine blocks. This aluminum alloy has approximately 5% silicon, 3% copper and 0.3% magnesium. The electrode 10 is held by a support 11 covering the shirt 12. The support of the shirt 13 has a centering means which makes the electrode and the jacket concentric.

The electrode support 11 and the jacket support 13 hermetically surround the jacket and pass through the cavities 14 of the shirt support 13 and 15 of the support electrode 11 the different fluids coming from the circuit of figure 1.

EXAMPLES EXAMPLE 1 Coating of fifty jacket bores with a nickel-silicon carbide composite. Preliminary step: Surface preparation

• Dégraissage alcalin sous ultrasons appliqué pendant 2 minutes dans un bain de la société Diversey, référencé D708, concentré à 30 g/l, maintenu à la température de 60°C.
• Rinçage
• Décapage alcalin appliqué pendant 2 minutes avec un bain de la société Diversey, référencé Aluminux 136, concentré à 50 g/l et maintenu à la température de 50°C.
• Rinçage
• Décapage fluoborique nitrique dans un bain composé de 50% d'acide nitrique concentré à 68% et de 20% d'acide fluoborique concentré à 50% maintenu pendant 30 secondes à la température ambiante.
• Rinçage

Different degreasing and pickling baths were first applied by dipping. One can quite imagine in a more advanced industrial phase to include them in a circuit of the type presented in FIG. 1. The following treatments have been applied:

• Alkaline degreasing under ultrasound applied for 2 minutes in a bath from the company Diversey, referenced D708, concentrated at 30 g / l, maintained at the temperature of 60 ° C.
• Rinsing
• Alkaline pickling applied for 2 minutes with a bath from the company Diversey, referenced Aluminux 136, concentrated to 50 g / l and maintained at the temperature of 50 ° C.
• Rinsing
• Nitric fluoboric pickling in a bath composed of 50% concentrated nitric acid at 68% and 20% concentrated fluoboric acid at 50% maintained for 30 seconds at room temperature.
• Rinsing

First step: Electrochemical activation

The electrochemical activation bath stored in the polypropylene tank 5 and maintained at the temperature of 50 ° C. has the following composition: NiCl ₂ 125 g / l NH ₄ HF ₂ 5 g / l H ₃ BO ₃ 12.5 g / l

It is brought to the treatment cell 1 via the pump 3, the maximum flow rate of which is 100 liters / minute. For 30 seconds, using a 40V 300A generator, a current is passed in such a way that a current density of 28 A / dm ^{2 is} established.

Second step: Over-activation

• 50% d'acide nitrique concentré à 68%
• 20% d'acide fluoborique concentré à 50%

After rinsing, and without waiting for the surface of the part to dry, the overactivation bath is passed through the cell. This bath has the following composition:

• 50% concentrated nitric acid 68%
• 20% 50% concentrated fluoboric acid

It is kept in contact with the surface for 30 seconds at 20 ° C.

Third step: Electrolytic deposition of composite nickel

The nickel-plating bath used has the following composition: Ni (NH ₂ SO ₃ ) ₂ 300 g / l H ₃ BO ₃ 30 g / l NiCl ₂ 30 g / l saccharin 2 g / l filler: silicon carbide 75 g / l, average particle size 2 micrometers

It differs from the bath of the prior art by a significantly higher chlorine content (# 9 g / l) and by a much lower pH, close to 3.

It is maintained at a temperature of 50 ° C, it circulates towards the cell with a maximum flow of 100 liters / minute, for 15 minutes for an average deposit of 50 µm The deposit obtained is characterized by its adhesion, the regularity of the thickness deposited, homogeneity of the particle distribution and by friction and wear tests The adhesion tests used follow the ASTM recommendations: B571-84 §9 (thermal shock), the intended use temperature being fixed at 200 ° C and B571-84 84 §7 (file test).

The wear and friction tests were carried out on a "PLINT" tribometer, sold by the company CAMERON and commonly used in the automotive industry. These tests, which we will call "PLINT tribology tests", allow to measure the wear of the two materials in contact (the coating and the material of the piston ring) and the coefficient of friction (Coulomb coefficient).

It is a cylinder-plane contact, the cylinder representing the segment and the plane representing engine bore. This plan is coated with the deposit to be tested. The cylinder segment is subjected to a load given normal to the plan-bore on which it rubs and moves, at a temperature given, in a direction parallel to the cylinder axis, in an alternating linear motion given amplitude and frequency.

RESULTS:

• Adhesion of the deposit: it is perfect whatever the test used.
• Thickness regularity:

After a careful adjustment of the positioning of the electrode relative to the jacket, there is good regularity in the thickness deposited: 45 to 55 micrometers for 50 micrometers targeted.

• Homogénéité de la répartition des particules de carbure de silicium: elle est bonne, de plus, aucun agglomérat de carbure de silicium n'a été observé.
• Essais de tribologie PLINT sur dépôts Ni-SiC

There was no wear of the electrode observed after these 50 deposits, which suggests that good reproducibility of the results on an industrial scale.

• Homogeneity of the distribution of the particles of silicon carbide: it is good, moreover, no agglomerate of silicon carbide was observed.
• PLINT tribology tests on Ni-SiC deposits

Three constituent materials of the segments were tested: cast iron, chromium, molybdenum. For each material, the applicant has carried out tests at two temperatures: 30 and 100 ° C. Each test was carried out under a normal load of 100 N and with a displacement 15 mm amplitude alternative.

At 30 ° C, the lubricant used is decane, the frequency of the reciprocating movement is 12 Hz, the test lasts 30 minutes.

At 100 ° C, the lubricant used is a neutral engine oil, i.e. uncharged, the frequency of the alternative displacement is 16 Hz, the test lasts 120 minutes.

These tests led to the average results in Table 1. In the latter, the wear of the coating is characterized by a loss in weight expressed in milligrams. The wear of the segments is given qualitatively according to the appearance of the contact surface of the segment at the end of the test and it is represented in the table by a number of crosses all the greater the greater . Segment material Temperature Coefficient of friction Coating wear Segment wear Start Middle End Melting 30 0.225 0.115 0.115 1.1 xxx 100 0.125 0.115 0.115 0.4 xxx Chromium 30 0.140 0.125 0.115 2.1 xxx 100 0.100 0.100 0.100 0 xx Molybdenum 30 0.130 0.115 0.115 0.9 x 100 0.115 0.105 0.105 0 x

EXAMPLE 2 Coating of a bore with a nickel-silicon carbide-graphite composite

A dozen shirts were coated with a SiC + graphite mixture. The device used, the physical parameters and the baths are identical to those of the example previous, with the only difference that we added 10, 20 or 30 g / l of carbon powder of which the grains have an average size of 2 microns.

RESULTS

• the deposit is duller and darker than in the previous example.
• the adhesion tests are excellent.
• as in the previous example, there is good regularity in the thickness deposited with the same tolerance range.
• PLINT tribology tests on Ni-SiC + graphite deposits

The same tribology tests as those presented in Example 1 were carried out on the sole temperature of 30 ° C, on two segment materials: cast iron and chromium and on three types coating corresponding to the three concentrations of graphite.

These tests led to the results averages appearing in table 2, where moreover appear for memory and for comparison the results obtained at 30 ° C with a coating without graphite. The graphite concentration is expressed in grams per liter. Segment material conc. graphite Coefficient of friction Coating wear Segment wear peak start Middle End Melting 0 0.18 0.225 0.115 0.115 1.1 xxx 10 0.110 0.120 0.125 0 x 20 0.12 0.110 0.130 0.150 0 x 30 0.110 0.140 0.150 0.1 x Chromium 0 0.49 0.140 0.120 0.115 1.7 xxx 10 0.130 0.130 0.125 0.2 x 20 0.15 0.130 0.115 0.125 0 x 30 0.130 0.115 0.145 0.2 x

Generally, there is a lower wear of these segments when the coating contains graphite. We also note that the contribution of graphite has an effect on the friction mainly at start-up, where the peak observed on the friction coefficient drops significantly with the cast iron segments and dramatically with the segments in chrome.

Finally, we note that a concentration of 20 g / l of graphite combined with 75g / l of powdered SiC corresponds to the coating which is the least used at the end of this type of test.

ADVANTAGES OF THE INVENTION

• excellent adhesion of the deposit due to the activation steps.
• uniformity of the thickness of the deposit which can vary by less than 5 μm, by adapting the conformation of the electrodes.
• homogeneity of the particle distribution (silicon carbides and graphite for example) in the depot (up to around 15% by volume).
• high deposition rate.
• consistency of the products used in all stages of this process.
• low roughness of the deposit, which allows a reduction in the running-in time of the parts as well coated.

Claims (16)

1. Process for coating the face of a part made of aluminium or of aluminium alloy to be subjected to high frictional forces, characterized in that it comprises at least three successive stages, the first being an electrochemical activation stage where the part is brought to anodic polarity in a bath containing a halogenated acid salt of nickel, the second being a stage of superactivation of the surface and the third being a stage of electrolytic deposition of a nickel layer containing particles of solid substances where the part is brought to cathodic polarity in a nickel-plating bath containing a charge of solid particles.
2. Process according to Claim 1, characterized in that said electrochemical activating bath is an aqueous solution containing nickel chloride, a fluorinated compound and boric or fluoboric acid.
3. Process according to Claim 2, characterized in that said electrochemical activating bath contains between 100 and 250 grammes of nickel chloride, 2 and 10 grammes of ammonium bifluoride and 10 and 20 grammes of fluoboric acid per litre.
4. Process according to any one of Claims 1 to 3, characterized in that, during said electrochemical activation stage, a current density of 10 to 50 A/dm² is applied for 30 to 120 seconds, the bath being kept at a temperature of between 40 and 60°C.
5. Process according to any one of Claims 1 to 4, characterized in that said electrochemical activation stage is preceded by surface preparation involving a succession of degreasing, alkaline pickling then fluoboric-nitric pickling baths.
6. Process according to Claim 1, characterized in that the bath used for the surface superactivation stage is an aqueous solution containing between 20% and 50% by volume of nitric acid concentrated to 68% and between 20% and 75% by volume of fluoboric acid concentrated to 50%.
7. Process according to Claim 6, characterized in that the bath used for the surface superactivation stage is kept in contact with the surface to be coated for a period of between 30 and 120 seconds at a temperature of between 20 and 40°C.
8. Process according to Claim 1, characterized in that the bath used in the electrolytic deposition stage contains nickel sulphamate, nickel chloride, boric acid, saccharin and a charge composed of solid particles, in particular of silicon carbide or of any other component which hardens the coating.
9. Process according to Claim 8, characterized in that the bath used in said electrolytic deposition stage contains between 250 and 400 grammes of nickel sulphamate, between 20 and 40 grammes of nickel chloride, between 10 and 100 grammes of boric acid, between 0.5 and 4 grammes of saccharin and from 50 to 150 grammes of said charge per litre.
10. Process according to any one of Claims 8 or 9, characterized in that a current density of 20 to 50 A/dm² is applied during said electrolytic deposition stage, the bath being kept at a temperature between 40°C and 60°C and at a pH between 2 and 5, preferably between 2.5 and 3.5.
11. Process according to Claim 8, in which the bath used in said electrolytic deposition phase contains a charge composed of solid particles, in particular of silicon carbide or any other component which hardens the coating, characterized in that said charge also contains graphite.
12. Process according to Claim 11, characterized in that the bath used in said electrolytic deposition stage contains between 250 and 400 grammes of nickel sulphamate, between 20 and 40 grammes of nickel chloride, between 10 and 100 grammes of boric acid, between 0.5 and 4 grammes of saccharin and from 50 to 150 grammes of said charge per litre, the charge containing between 5 and 50 grammes of graphite.
13. Process according to any one of Claims 11 or 12, characterized in that a current density of 20 to 50 A/dm² is applied during said electrolytic deposition stage, the bath being kept at a temperature between 40 and 60°C and at a pH between 2 and 5, preferably between 2.5 and 3.5.
14. Process according to any one of Claims 8 to 13, characterized in that the solid particles of said charge have a size defined by a mean diameter between 0.5 and 5 µm.
15. Process according to any one of the preceding claims, characterized in that said electrochemical activation, superactivation and electrolytic deposition stages, possibly preceded by a degreasing-pickling preparation treatment, follow one another, possibly interrupted by rinsing with pure water, in such a way that said surface to be treated does not have time to dry or to be exposed to air or to any other environment likely to reduce its activity.
16. Application of the process according to any one of Claims 1 to 15 for the electrolytic deposition of composite nickel on the bore of a casing or engine block of an internal combustion engine of a motor vehicle, made of aluminium or aluminium alloy.

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