EP0725166A1 - 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 one face or surface subjected to high friction stresses, in particular molded or forged parts of motor vehicles. These are, for example, the liners which equip internal combustion engines of motor vehicles or else cylinders machined directly in the engine block. The invention relates more precisely to the internal surface, or bore, of a jacket or of an engine block which is subjected, cold as hot, to high stresses of friction and is sensitive to wear.

STATE OF THE ART

In order to produce motor vehicle parts from aluminum alloy, in the majority of cases, alloys which are easy to use, for example by molding or forging, are chosen, but which have insufficient characteristics of use and resistance. against high friction stresses. Such stresses can be encountered in engines, for example on the inner surface of a jacket or barrel of an engine block, also called cylinder-crankcase, where the piston is guided in its reciprocating stroke and where its segments are in constant contact with said surface. In order to improve the wear resistance, it is known from FR-A-1,579,266 and FR-A-2,159,179 depositing on said interior surface a coating consisting of a composite of nickel and solid particles, generally in silicon carbide.

Patent application FR-A-1,579,266 proposes a process for the galvanic deposition of a metal coating containing solid particles. The deposition is carried out in two stages: a preparatory stage where a first layer of zinc is deposited by chemical means on the surface to be treated, and a second stage which is the actual electrolytic deposition, the part to be treated being the cathode , this deposition taking place in two stages: first deposition of a thin layer of almost pure nickel then deposition of the nickel loaded with solid particles.

This process, or its variants, is currently used widely on a large scale for both aluminum alloy engine blocks and cast iron engine blocks or liners, since 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 particles of silicon carbide which emerge from the nickel surface.

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

Patent application EP-A-0 288 364 discloses a process for coating drums of cast iron engine blocks in which the initial deposit of zinc is replaced by an electrolytic sulfuric attack. 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, so it must be performed within very tight dimensional tolerances. The irregularity of the thickness of the deposited layer imposes 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 provide for a resumption of machining and to aim straight away at the thickness corresponding to the maximum wear that can be expected from this coating. In addition, to increase the life of the engine, it would be desirable to further improve the wear resistance of the coating and to reduce the friction of the piston rings which move in contact with it, which would have the other beneficial effect of reducing mechanical noise 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.

Advantageously, these operations can be separated by rinsing with pure water and follow one another in a very short time, so that the surface to be coated does not dry between each step and without said surface having been exposed to air or any other environment causing its 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 of the operations, said electrode having to be simply brought to a cathode polarity at the first stage, zero at the second and anodic at the third.

The first step according to the invention is an electrochemical activation phase in which the surface to be treated and the electrode are in a bath containing a halogenated acid salt of nickel. Preferably, this bath is an aqueous solution containing nickel chloride, a fluorinated compound and boric or fluoboric acid. Preferably, an aqueous solution will be used comprising, per liter of electrolyte, from 100 to 250 grams of nickel chloride, from 2 to 10 grams of ammonium bifluoride, and from 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, it will first be sought to prepare the surface to be treated by a succession of alkaline degreasing baths and alkaline then fluoboric-nitric pickling.

The second step according to the invention is an over-activation treatment whose purpose is to complete the depassivation of the surface to be coated and to dissolve the few residues resulting from the electrochemical treatment of the first step and liable to disturb the regularity and the homogeneity of the future deposit. Preferably, this overactivation treatment is carried out with a nitric fluoboric bath and more particularly an aqueous solution comprising between 20% and 50% by volume of concentrated nitric acid at 68% and between 20% and 75% by volume of concentrated fluoboric acid. 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 may be either carbides, in particular silicon carbide, or else any other component hardening the coating and improving the wear resistance of the deposit (diamond for example), either a component reducing the coefficient of friction (graphite for example), or a mixture of components of these two categories intended to provide the best compromise between wear resistance 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 of electrolyte, from 250 to 400 grams of nickel sulfamate, from 20 to 40 grams of nickel chloride, from 10 to 100 grams of boric acid and from 50 at 150 grams of charge. During 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 contains saccharin, which has the advantageous effect of reducing the residual stresses prevailing in the deposit. Its concentration is however limited because saccharin has the other effect of reducing 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 arise from the synergistic effect of the combination of the first two stages and relate to the constitution of the charge in solid particles, which is enriched and better suited to the tribological properties sought in this type of deposit. Thus said filler which contains particles hardening the coating, such as particles of silicon carbide, can be enriched in particles improving the tribological conditions of contact such as particles of graphite. In an advantageous embodiment 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 predominant size of 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 sufficiently large to reduce the risks of seizure but not too much to avoid too strong wear of the other element in contact. This same charge is enriched with graphite particles of finer particle size: 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 in the first step, nickel metal is deposited in the cavities created by the attack acidic and was not completely dissolved by the overactivation bath. These cavities constitute very reactive sites which favor the attachment of the layer of composite nickel. 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 constitutes a synergistic effect which makes it possible to immediately deposit the layer of composite nickel; it is therefore not essential to deposit at the start of the third step the thin layer of pure nickel recommended in the prior art.

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

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

FIGURES

FIG. 1 represents the diagram of a preferred embodiment given solely by way of nonlimiting example. According to this embodiment, the operations are limited, the waiting time between the steps is minimal, the activation of the surface is not hampered by any oxidation or passivation. The system is dynamic, that is to say that the treatment cell for the part 1 is not dismantled during the process and that all of the baths are successively introduced inside the said cell 1. required. 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 the various valves 4 of the circuit, the pump first drives the activation bath of the tank 5, the rinsing bath of the tank 6, the overactivation 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. As an engine block is particularly bulky and heavy to handle, we have simplified the part by replacing it with a cylindrical jacket 12 made of the AS5U3G alloy commonly used for engine blocks. This aluminum alloy contains approximately 5% silicon, 3% copper and 0.3% magnesium. The electrode 10 is held by a support 11 covering the jacket 12. The support of the jacket 13 has a centering means which makes it possible to make the electrode and the jacket concentric.

The electrode support 11 and the jacket support 13 hermetically surround the jacket and allow the various fluids coming from the circuit of FIG. 1 to pass through the cavities 14 of the jacket support 13 and 15 of the electrode support 11.

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%

II est maintenu en contact de la surface pendant 30 secondes à 20°C.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 rate 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 recommendations of ASTM: B571-84 §9 (thermal shock), the temperature of use target being set at 200 ° C and B571-84 §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", used 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 the engine bore. This plan is coated with the deposit to be tested. The cylinder-segment is subjected to a given load normal to the plan-bore on which it rubs and moves, at a given temperature, in a direction parallel to the axis of the cylinder, according to a reciprocating linear movement of amplitude and frequency data.

RESULTS:

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

After a careful development of the positioning of the electrode relative to the jacket, there is a 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

Trois matériaux constitutifs des segments ont été testés: la fonte, le chrome, le molybdène. Pour chaque matériau, la demanderesse a effectué des tests à deux températures : 30 et 100°C. Chaque essai a été effectué sous une charge normale de 100 N et avec un déplacement alternatif d'amplitude 15 mm.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 an alternative displacement of amplitude 15 mm.

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

At 100 ° C, the lubricant used is a neutral engine oil, that is to say uncharged, the frequency of the reciprocating movement 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 . Table 1 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 previous example, with the only difference that 10, 20 or 30 g / l of carbon powder have been added, the grains of which have an average size of 2 microns.

RESULTS

• le dépôt est plus mat et plus foncé que dans l'exemple précédent.
• les tests d'adhérence sont excellents.
• comme dans l'exemple précédent, on constate une bonne régularité dans l'épaisseur déposée avec la même fourchette de tolérance.
• Essais de tribologie PLINT sur dépôts Ni-SiC + graphite

Les mêmes essais de tribologie que ceux présentés dans l'exemple 1 ont été menés à la seule température de 30°C, sur deux matériaux de segment: la fonte et le chrome et sur trois types de revêtement correspondant aux trois concentrations de graphite.

• 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 at the only temperature of 30 ° C, on two segment materials: cast iron and chromium and on three types of 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. Table 2 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 less wear of these segments when the coating contains graphite. On the other hand, it can be seen that the contribution of graphite has an effect on the friction essentially at start-up, where the peak observed on the coefficient of friction drops appreciably with the cast iron segments and dramatically with the chromium segments.

Finally, it can be seen that a concentration of 20 g / l of graphite combined with 75 g / l of SiC powder 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 distribution of the particles (silicon carbides and graphite for example) in the deposit (up to approximately 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 thus coated.

Claims (16)

Process for coating the face of an aluminum or aluminum alloy part intended to undergo high friction stresses, characterized in that it comprises at least three successive stages, the first being a stage of electrochemical activation where the part is brought to an anodic polarity in a bath containing a halogenated acid salt of nickel, the second being a step of over-activation of the surface and the third being a step of electrolytic deposition of a layer of nickel containing particles of solid matter where the part is brought to cathodic polarity in a nickel-plating bath containing a charge of solid particles. Process according to claim 1, characterized in that said electrochemical activation bath is an aqueous solution containing nickel chloride, a fluorinated compound and boric or fluoboric acid. Process according to Claim 2, characterized in that the said electrochemical activation bath contains, per liter, between 100 and 250 grams of nickel chloride, 2 and 10 grams of ammonium bifluoride, 10 and 20 grams of fluoboric acid. Method according to any one of Claims 1 to 3, characterized in that during the said electrochemical activation step a current density of 10 to 50 A / dm ² is applied for 30 to 120 seconds, the bath being maintained at a temperature between 40 and 60 ° C. Process according to any one of Claims 1 to 4, characterized in that the said electrochemical activation step is preceded by a preparation of the surface comprising a succession of degreasing baths, of alkaline pickling then of fluoboric-nitric pickling. Method according to claim 1, characterized in that the bath used for the surface over-activation step is an aqueous solution comprising between 20% and 50% by volume of concentrated nitric acid at 68% and between 20% and 75% by volume of 50% concentrated fluoboric acid. Method according to claim 6, characterized in that the bath used for the surface over-activation step is kept in contact with the surface to be coated for a time between 30 and 120 seconds at a temperature between 20 and 40 ° C . Method according to claim 1, characterized in that the bath used in the electrolytic deposition step comprises nickel sulfamate, nickel chloride, boric acid, saccharin and a filler composed of solid particles, in particular in silicon carbide or any other component hardening the coating. Process according to Claim 8, characterized in that the bath used in the said electrolytic deposition step comprises, per liter, between 250 and 400 grams of nickel sulfamate, between 20 and 40 grams of nickel chloride, between 10 and 100 grams of boric acid, between 0.5 and 4 grams of saccharin and 50 to 150 grams of said charge. Method according to either of Claims 8 and 9, characterized in that during the said electrolytic deposition step a current density of 20 to 50 A / dm ² is applied, the bath being maintained at a temperature between 40 ° C and 60 ° C and at a pH between 2 and 5, preferably between 2.5 and 3.5. A method according to claim 8, the bath used in said electrolytic deposition phase comprises a filler composed of solid particles, in particular silicon carbide or any other component hardening the coating, characterized in that said filler also comprises graphite. Process according to claim 11, characterized in that the bath used in said electrolytic deposition step comprises, per liter, between 250 and 400 grams of nickel sulfamate, between 20 and 40 grams of nickel chloride, between 10 and 100 grams of boric acid, between 0.5 and 4 grams of saccharin and 50 to 150 grams of said charge, the latter comprising between 5 and 50 grams of graphite. Process according to either of Claims 11 and 12, characterized in that during the said electrolytic deposition step a current density of 20 to 50 A / dm ² is applied, the bath being maintained at a temperature between 40 and 60 ° C and at a pH between 2 and 5, preferably between 2.5 and 3.5. Process according to any one of Claims 8 to 13, characterized in that the solid particles of said charge have a size defined by an average diameter of between 0.5 and 5 µm. Method according to any one of the preceding claims, characterized in that the said steps of electrochemical activation, of over-activation and of electrolytic deposition, optionally preceded by a preparation treatment by degreasing-pickling, follow one another, possibly interspersed with rinses with pure water, so that said surface to be treated has neither the time to dry nor to be exposed to air or any other environment capable of causing its reactivity to drop. Use of the method according to any one of claims 1 to 15 for carrying out the electrolytic deposition of composite nickel on the bore of a jacket or an engine block of an internal combustion engine of a motor vehicle made of aluminum or an alloy of aluminum.

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