EP0297982B1 - Process for electolytically codepositing a nickel-cobalt matrix with ceramic particles, and coating thus obtained - Google Patents

Description

The invention relates to a method of protection against oxidation and erosion by friction of parts used up to 600 ° C made of alloy steel or nickel-based superalloy.

The assemblies of parts of turbomachines operate frequently without conventional lubrication by oil or lubrication circuits.

These various assemblies such as the fitting of compressor blades in the disc pockets, the main shaft splines, the passages of the tie rods, the suspension shafts, the articulations, or even the HP compressor disc dampers are the site of local contacts between parts often associated with very small displacements and wear by micro-deflections (seizure). There are also a number of wear cases caused by significant relative displacements.

These phenomena can generate fatigue cracks and lead to premature failure of parts.

In addition, in the veins of the compressor, a fairly particular and equally penalizing type of degradation is observed: degradation by erosion.

All the types of wear mentioned cause deterioration of the surfaces and / or changes in dimensions in the blade assemblies at the vertices, leading edges and trailing edges, with the practical consequences of reducing the service life. blades and cause significant performance losses of the HP compressor. This sensitivity is linked to the materials constituting the blades, mainly the moving parts.

Thus studies carried out on HP compressors in operation have shown that after 3000 flight cycles of 3 hours and 30 minutes, significant losses in efficiency, up to 14% loss of flow, were due to the following factors: - above mentioned in particular by wear of the tips of blades and erosion of the blades.

Certain remedies can be brought to the problems of dry wear by micro-deflections by using massive materials like steels with high carbon content such as Z100 CD 17 (AFNOR standard - steel comprising 1% C, 17% Cr and 0,5 % Mo) of high hardness when cold or cobalt-based alloys forged, cast or melted. Coatings sprayed with plasma such as tungsten carbides with 17% cobalt, chromium carbides, cobalt-based alloys can be used but are only applicable in homogeneous torque, in order to obtain sufficiently low coefficients of friction.

The use of electrolytic coatings of cobalt and around 30% of chromium carbide indicated in the document FR-A-2-412 626 makes it possible to improve the behavior in dry friction in a temperature range between 300 ° and 750 ° C.

However, the methods listed above do not make it possible to simultaneously solve the problems of wear by micro-deflections or by erosion.

• lorsque la géométrie des pièces à traiter est complexe, la projection de revêtements plasma peut être difficile sinon impossible (cas par exemple d'alésages de faible diamètre),
• chaque fois que l'on désirera des dépôts d'épaisseurs faibles à moyennes homogènes.
• lors du remplacement de certains revêtements projetés, qui appliqués sur des pièces en frottement, pourraient conduire à des déformations par échauffement dûes à la projection,
• suivant le type de pièces, les revêtements électrolytiques peuvent être meilleurs marchés que les revêtements plasma, donc présenter un meilleur compromis efficacité/coût,

Le document US 3 152 971 mentionne la possibilité d'un codépôt de nickel électrolytique avec des poudres du type céramique afin de réaliser un revêtement de finition satiné anti-reflet ce qui constitue un but tout différent de celui de l'invention. En outre ce document ne révèle la réalisation effective d'aucune combinaison nickel-cobalt-céramique et n'enseigne pas les moyens d'optimiser une concentration en particules céramiques homogène en tout point dans le revêtement.
L'invention porte donc sur un procédé électrolytique permettant le co-dépôt nickel-cobalt et particules céramiques donnant un composite dur ayant une bonne résistance à l'usure et à l'érosion.
La difficulté de tels co-dépôts nickel-cobalt provient du fait suivant : pour que deux ou plusieurs espèces puissent se décharger simultanément sur la cathode lors d'une électrolyse , ces espèces doivent se trouver sous des formes ioniques telles qu'elles aient des potentiels de décharge voisins au cours de la déposition, ce qui n'est pas le cas du nickel et du cobalt.
L'obtention d'un tel alliage électrolytique Ni-Co-céramique nécessite :

• le rapprochement des tensions d'équilibre des métaux présents en solution,
• l'augmentation de la surtension du métal le plus positif,
• la diminution de la surtension du métal le plus négatif.

The production of coatings by electrolytic means is desirable in many cases, for example:

• when the geometry of the parts to be treated is complex, the projection of plasma coatings can be difficult if not impossible (case for example of small diameter bores),
• whenever we want deposits of small to medium thicknesses homogeneous.
• during the replacement of certain projected coatings, which applied to friction parts, could lead to deformation by heating due to projection,
• depending on the type of parts, electrolytic coatings can be cheaper than plasma coatings, therefore presenting a better compromise between efficiency and cost,

Document US 3,152,971 mentions the possibility of co-depositing electrolytic nickel with powders of the ceramic type in order to produce a satin finish anti-reflective coating which constitutes an entirely different object from that of the invention. In addition, this document does not reveal the effective production of any nickel-cobalt-ceramic combination and does not teach how to optimize a homogeneous concentration of ceramic particles at all points in the coating.
The invention therefore relates to an electrolytic process allowing the co-deposition of nickel-cobalt and ceramic particles giving a hard composite having good resistance to wear and erosion.
The difficulty of such nickel-cobalt co-deposits stems from the following fact: in order for two or more species to be able to discharge simultaneously onto the cathode during electrolysis, these species must be in ionic forms such that they have potentials neighboring discharge points during deposition, which is not the case for nickel and cobalt.
Obtaining such a Ni-Co-ceramic electrolytic alloy requires:

• approximation of the equilibrium tensions of the metals present in solution,
• the increase in the most positive metal overvoltage,
• the reduction of the most negative metal overvoltage.

Document DE-A-1 533 441 describes general points relating to the production by electrolysis of massive pieces of nickel-cobalt, with ceramic particles. One of the aims of the invention is to find the best compromise between all the operating conditions to achieve effective electrolytic deposition of cobalt nickel and to optimize the Ni / Co ratio during the electrolysis process in order to allow good dispersion of the ceramic particles in the final coating and sufficient homogeneity of said coating.

Thus, according to the subject of the invention, a method of protection against oxidation and erosion by friction at a temperature below 600 ° C., of a substrate made of alloy steel or of nickel-based superalloy is carried out by codeposition electrolytic coating of a binary nickel-cobalt matrix comprising a homogeneous dispersion of ceramic particles M chosen from the group of oxides and carbides, for example SiC, Al2 O3 and Cr2 O3, said coating comprising a mass content of particles ceramics M between 3.5% and 10%.

• densité de courant (ddc) comprise entre 2,5 et 15 A/dm².
• température comprise entre 50° et 54°C
• PH compris entre 3,5 et 4,5.

According to a feature of the invention, the electrolytic deposition is carried out in a bath in a sulfamate medium comprising a total content of metal salts (Ni + Co) of between 70g / l and 100g / l, the total concentration of particles M in suspension in the bath is between 50g / l and 300g / l while the Ni / Co mass ratio of the electrolysis bath will be equal to 15, under the conditions:

• current density (ddc) between 2.5 and 15 A / dm².
• temperature between 50 ° and 54 ° C
• PH between 3.5 and 4.5.

• la figure 1 montre en coupe une cuve à électrolyse du type utilisé dans le processus selon l'invention.
• la figure 2 montre l'influence du rapport Ni/Co et de la densité de courant sur la composition du revêtement,
• la figure 3 montre l'influence de la concentration en sels métalliques (Ni + Co) dans le bain sur la composition du revêtement.
• la figure 4 est une photomicrographie (grossissement 1000 fois) de la coupe d'un revêtement Nickel-Cobalt - SiC obtenu par le procédé décrit dans l'invention,
• la figure 5 est une vue similaire d'un revêtement Nickel-Cobalt + Cr2 O3 selon l'invention,
• la figure 5a est également une coupe d'un revêtement Nickel-Cobalt Al2 O3, au même grossissement,
• les figures 6a à 6h montrent des micrographies (grossissement 1000 fois) du revêtement Ni-Co-SiC pour présenter l'influence de la température et du temps de maintien à température sur la morphologie et l'oxydation du revêtement,
• les figures 7a et 7b montrent des micrographies (grossissement 5000 fois) du même revêtement à l'état brut 100h à 600°C sous air,
• les photos 8a, 8b, 9a, 9b, 10a, 10b, montrent respectivement des images X du silicium (8), du nickel (9) et du cobalt (10) respectivement à l'état brut (indice a) et après 100h à 600°C (indice b), d'un revêtement Ni-Co-SiC (grossi 5000 fois),
• les figures 11 à 13 donnent les courbes d'usure d'un revêtement Ni-Co-SiC en couples homogènes (matériaux identiques opposés l'un à l'autre) et en couples hétérogènes (matériaux différents) respectivement à 20°, 250° et 400°C pour les couples suivants :

Ni-Co-SiC sur Ni-Co-SiC (à 400°C) Ni-Co-SiC sur Z12 C13 (à 20, 250° et 400°C) Ni-Co-SiC sur NC 20 K 14 Other characteristics of the method according to the invention, and the results obtained will be explained below with reference to the appended figures among which:

• Figure 1 shows in section an electrolytic tank of the type used in the process according to the invention.
• FIG. 2 shows the influence of the Ni / Co ratio and the current density on the composition of the coating,
• Figure 3 shows the influence of the concentration of metal salts (Ni + Co) in the bath on the composition of the coating.
• FIG. 4 is a photomicrograph (1000 times magnification) of the section of a Nickel-Cobalt - SiC coating obtained by the process described in the invention,
• FIG. 5 is a similar view of a Nickel-Cobalt + Cr2 O3 coating according to the invention,
• FIG. 5a is also a section of a Nickel-Cobalt Al2 O3 coating, at the same magnification,
• FIGS. 6a to 6h show micrographs (magnification 1000 times) of the Ni-Co-SiC coating in order to present the influence of the temperature and the temperature holding time on the morphology and the oxidation of the coating,
• FIGS. 7a and 7b show micrographs (magnification 5000 times) of the same coating in the raw state 100 h at 600 ° C. in air,
• photos 8a, 8b, 9a, 9b, 10a, 10b, respectively show X images of silicon (8), nickel (9) and cobalt (10) respectively in the raw state (index a) and after 100h at 600 ° C (index b), of a Ni-Co-SiC coating (magnified 5000 times),
• Figures 11 to 13 show the wear curves of a Ni-Co-SiC coating in homogeneous couples (identical materials opposite to each other) and in heterogeneous couples (different materials) respectively at 20 °, 250 ° and 400 ° C for the following couples:

Ni-Co-SiC on Ni-Co-SiC (at 400 ° C) Ni-Co-SiC on Z12 C13 (at 20, 250 ° and 400 ° C) Ni-Co-SiC on NC 20 K 14

The various experiments carried out were carried out in a rectangular or cylindrical electrolysis tank such as that shown in FIG. 1. If we refer to it, in a tank 1 filled with water heated by a heating resistor 2, we has arranged an electrolytic cell 3 comprising Ni and Co salts and ceramic particles in suspension and the composition of which will be specified below. Anodes 4 supplied by a current generator (not shown) being made up of nickel balls S arranged in titanium baskets.

A combined bath agitation system was implemented. Indeed, the simple stirring modes have proved to be ineffective in maintaining the dispersion of the particles within the electrolyte and in obtaining in the final coating an acceptable and reproducible occlusion rate.

The combination of several appropriate stirrings has made it possible to control the suspension of the fine ceramic particles in the electrolyte and thereby ensure better incorporation into the metallic matrix. Thus, a particular installation was developed comprising on the one hand a double stirring of the electrolyte by compressed air 5 and by the use of a turbine disperser 7 rotating at high speed, the tests carried out between 1750 and 2250 revolutions per minute having shown that a speed of rotation of 2000 rpm of the disperser 7 combined with air mixing was preferred. On the other hand the parts 6 arranged on the cathode were installed on a translation mechanism "up and down" at controlled speed having a variable stroke according to the nature and size of the parts to be coated.

Before incorporating ceramic particles in the bath, the latter was optimized and to do this, various electrolyte compositions were tested and nickel-cobalt baths in sulfamate medium were chosen because of the high-performance coatings which they produce both mechanical properties and stability of the alloy composition. In addition, unlike other baths commonly used (chloride, acetate or pyrophosphate), they provide coatings with low internal stresses while accepting high deposition rates.

• des constituants primaires
  • . le sulfamate de nickel, source d'ions Ni⁺⁺ indispensable au revêtement de nickel ;
  • . le sulfamate de cobalt, source d'ions Co⁺⁺ permettant le dépôt simultané avec le nickel,
  • . l'acide borique, agent tampon utilisé pour maintenir constant le pH au sein du bain et à l'interface cathode-solution,
  • . le chlorure de nickel (en concentration maximale 10 g/l), car bien qu'étant à l'origine de contraintes internes, il favorise la corrosion des anodes, donc l'augmentation du rendement cathodique du bain et en variante,
  • . le bromure de nickel qui peut être employé à la place du chlorure de nickel, si le bain contient peu de cobalt (1,5 g/l) ; mais crée un risque de passivation anodique si la teneur en cobalt dans le bain est élevée.
• des constituants secondaires (agents d'addition) :
  • . des produits mouillants (tensio-actifs et/ou surfactants) qui accélèrent le départ des bulles d'hydrogène à la surface des pièces et permettent d'éviter la formation de piqûres dans le revêtement,
  • . des produits nivellants qui diminuent la présence de fortes excroissances aux densités de courant cathodiques élevées,
• des produits réducteur de tensions internes.

The baths used included:

• primary constituents
  • . nickel sulfamate, a source of Ni⁺⁺ ions essential for coating nickel;
  • . cobalt sulfamate, a source of Co⁺⁺ ions allowing simultaneous deposition with nickel,
  • . boric acid, a buffering agent used to keep the pH constant in the bath and at the cathode-solution interface,
  • . nickel chloride (at a maximum concentration of 10 g / l), because although being the source of internal stresses, it promotes corrosion of the anodes, therefore increasing the cathodic yield of the bath and, as a variant,
  • . nickel bromide which can be used in place of nickel chloride, if the bath contains little cobalt (1.5 g / l); but creates a risk of anodic passivation if the cobalt content in the bath is high.
• secondary constituents (addition agents):
  • . wetting products (surfactants and / or surfactants) which accelerate the departure of hydrogen bubbles on the surface of the parts and make it possible to avoid the formation of pits in the coating,
  • . leveling products which reduce the presence of strong growths at high cathode current densities,
• internal stress reducing products.

9 combinations were tested, corresponding to Ni / Co ratios equal to 5.15 and 20 respectively, ie a cobalt content in solution of the order of 16.6 - 6.2 and 4.3% and to mass contents of metal salts Ni + Co equal respectively to 75 g / l (normal concentration 2.54 N), 87.5 g / l (concentration 2.96 N) and 100 g / l (concentration 3.40 N). These 9 sulfamate bath compositions are detailed in Table 1.

• mettre dans la cuve la moitié du volume d'eau déionisée nécessaire,
• ajouter le volume de nickel sulfamate calculé à partir des solutions concentrées réceptionnées,
• chauffer la solution à 40 - 45°C et agiter constamment.
• ajouter la quantité de chlorure de nickel par petites quantités et agiter la solution jusqu'à dissolution complète,
• ajouter l'acide borique peu à peu, (il est recommandé de dissoudre l'acide borique dans une solution d'eau déionisée chauffée à 65-70°C pour accélérer cette opération),
• ajouter le volume de sulfamate de cobalt requis et homogénéiser la solution ,
• traiter la solution au charbon actif pour éliminer les impuretés organiques, la durée du traitement est de 3 heures.
• filtrer la solution,
• ajuster le pH avec une solution d'acide sulfamique diluée à 10% en masse,
• amener au volume requis avec de l'eau déionisée,
• ajouter l'agent mouillant (lauryl-sulfate de sodium).

The mounting conditions for the baths used were carried out according to the following sequence:

• put half the volume of deionized water required in the tank,
• add the volume of nickel sulfamate calculated from the concentrated solutions received,
• heat the solution to 40 - 45 ° C and stir constantly.
• add the quantity of nickel chloride in small quantities and stir the solution until complete dissolution,
• gradually add boric acid (it is recommended to dissolve boric acid in a solution of deionized water heated to 65-70 ° C to speed up this operation),
• add the required volume of cobalt sulfamate and homogenize the solution,
• treat the solution with activated carbon to remove organic impurities, the duration of the treatment is 3 hours.
• filter the solution,
• adjust the pH with a solution of sulfamic acid diluted to 10% by mass,
• bring to the required volume with deionized water,
• add the wetting agent (sodium lauryl sulfate).

The evolution of the cobalt element in the nickel-cobalt alloy formed could be followed over a whole range of current densities taking into account variable parameters such as the Ni / Co ratio and the total concentration of metal salts (Ni + Co).

The analyzes carried out by plasma emission spectrometry (ICP) on the samples having received an electrolytic coating in the 9 baths C to K (see Table 1), made it possible to select four alloy compositions (23 ± 3, 42 ± 4 , 64 ± 2 and 72 ± 2% of cobalt) with a view to characterizing them in structure, hardness and level of internal stress and these last tests led to retaining only the coatings with 29 and 42% of cobalt corresponding to baths F and G. The 29% Co coating was chosen for its low internal stresses (around 100 MPa), the 42% Co coating for its high hardness (550HV).

Characterization tests have shown that the 29% Co alloy is slightly better than pure nickel and the 42% alloy in the areas of resistance to wear at 20 ° C in heterogeneous pairs, resistance abrasion and resistance to erosion at low impact angles.

For these reasons, the desired final coating, incorporating ceramic particles, was chosen as the binary nickel-cobalt matrix containing 29% cobalt.

However, to obtain this final cobalt value taking into account the presence of the ceramic particles M in the bath, the Ni / Co ratio was chosen equal to 15 and the total content (Ni + Co) equal to 87.5 g / l (bath G in Table 1), to arrive at the desired cobalt concentration.

These Ni-Co codeposition parameters having been optimized, the incorporation of M ceramic powders of three kinds was tested in the electrolyte baths. Thus, silicon carbide SiC, Cr2 O3 and alumina Al2 O3 were used.

In the three cases, the mass content of the ceramic particles M in suspension in the electrolyte was retained in the range between 70 and 150 g / l.

The numerous tests carried out have shown that the homogeneity of the dispersion of the ceramic particles in the coating obtained was satisfactory with a particle size of the powder M of between 1 and 5 micrometers, as confirmed by the photomicrographs of FIGS. 5 to 7.

It has also been found that, before addition of the ceramic particles in the electrolysis baths and in particular with SiC and Al2 O3, it is important to remove all traces of annoying metallic impurities (for example iron) and, to do this , provision is made in the invention to subject the ceramic powders to a prior decontamination operation by acid washing in hydrochloric medium.

• densité de courant (ddc) comprise entre 2,5 et 15 A/dm2
• température comprise entre 50 et 54°C,
• pH compris entre 3,5 et 4,5.

The tests carried out with the three types of ceramic mentioned above were carried out with the following electrolysis parameters:

• current density (ddc) between 2.5 and 15 A / dm2
• temperature between 50 and 54 ° C,
• pH between 3.5 and 4.5.

The determination of these parameters was dictated by the following reasons:
The choice of a particular temperature lies in the fact that the fundamental advantage of a temperature rise is to increase the maximum admissible current density in order to obtain high rates of electrochemical reactions and diffusion. However, there are problems with the rate of transport of the particles, which can decrease the rate of occlusion.

In a temperature range between 20 and 60 ° C., the rate of incorporation of the particles Al2 O3 or of SiC remains constant, but that of Cr3 C2 increases with temperature.

The current density is an important parameter which depends on the molar concentration of the electrolyte and the temperature of the bath. During the formation of a coating, the current density conditions its deposition speed, its structure, its distribution ...
With regard to composite coatings, the amount of particles incorporated is directly related to the current density applied. So the overall percentage of occluded particles decreases with increasing current density and that is why it is preferable to work at low current density (5A / dm2) to increase the rate of particles in the metal matrix since the speed of metal ions is greater than the adsorption rate of the particles.

This observation is verified in particular in the case of incorporation of Al2 O3 particles and those of SiC. More precisely in the case where the ceramic particles are respectively SiC, Al2 O3, Cr2 O3, the previous parameters were chosen with the following values: SiC Al2 O3 Cr2 O3 ddc = 5 A / dm2 5 A / dm2 3 A / dm2 Temperature 52 ° ± 2 ° C 52 ° ± 2 ° C 52 ° C ± 2 ° C pH = 4 ± 0.2 4 ± 0.2 4 ± 0.2
and the coatings obtained had for weight compositions:
66 ± 2% Ni, 30 ± 2% Co, 4 ± 2% SiC.
63 ± 3% Ni, 32 ± 2% Co, 4.5 ± 0.5 Al2 O3
58 ± 3% Ni, 33 ± 3% Co, 9 ± 1% Cr2 O3

The aforementioned coatings were deposited on two types of substrates, a high-strength alloy steel Z 85 W CD V6 (AFNOR standard - steel comprising 0.85% C + 6% W + 5% Cr + 4% Mo + 2% Va) and two NC 19 FeNb superalloys (Standard AFNOR - Ni base alloy comprising 19% Cr + 18% Fe + 5% Nb) and NC 22 FeD (AFNOR Standard - nickel base alloy comprising 22% Cr + 19% Fe + 9% Mo).

These substrates are significant for iron base and nickel base alloys because the coatings according to the invention exhibit good adhesion to these alloys and, therefore, all iron base or nickel base substrates can also be covered by the protective coatings according to the invention. 'invention.

These substrates must undergo a surface preparation before undergoing the electrolytic coating. The surface preparation ranges including degreasing and surface activation operations are conventional. A pre-nickel plating bath containing a higher or lower hydrochloric acid content must be used to ensure maximum adhesion before applying the nickel-cobalt-ceramic coating on the aforementioned substrates.

The part is then able to receive the nickel-cobalt ceramic deposit indicated above.

After this coating has been formed under the aforementioned conditions, a thin electrolytic chromium plating can be carried out, for example between 2 and 10 micrometers in order to increase the resistance to hot oxidation of the nickel-cobalt-ceramic coating.

• Après dépot du revêtement protecteur, des examens de structure ont été effectués, les exemples donnés ici ayant été choisis dans le cas d'un revêtement Ni-Co-SiC. Les examens ont été réalisés sur état brut après revêtement et après des maintiens de 2 à 100 H à des températures variant de 400 à 600°C. Les méthodes suivantes ont été utilisées :
• microscopie optique sur état poli non attaqué
• microscopie électronique à balayage (MEB) après polissage électrolytique dans une solution sulfurique/méthane (1/7 - 0°C - 10 sec.)
• analyses en dispersion d'énergie (ADE).

As regards the particular case of the Ni-Co-SiC coating, the consolidation of the particles in the matrix can be improved by a subsequent heat treatment of diffusion of the silicon under inert gas for a period of between 1 hour and 3 hours at a temperature from 550 ° C to 620 ° C.

• After depositing the protective coating, structural examinations were carried out, the examples given here having been chosen in the case of a Ni-Co-SiC coating. The examinations were carried out in the raw state after coating and after maintenance of 2 to 100 hours at temperatures varying from 400 to 600 ° C. The following methods were used:
• light microscopy on unattacked polished state
• scanning electron microscopy (SEM) after electrolytic polishing in a sulfuric / methane solution (1/7 - 0 ° C - 10 sec.)
• energy dispersion analysis (ADE).

• A l'état brut (figures 6a à 6h) les particules SiC (en gris foncé) sont réparties uniformément dans le revêtement. Les examens MEB mettent en évidence le contact intime entre ces particules et la matrice métallique nickel-cobalt.
• Parallèlement l'état vieilli 100 H à 600°C a fait l'objet d'une caractéristique particulière.

At the end of these examinations, the following important points with regard to the annexed tables and plates were retained:

• In the raw state (Figures 6a to 6h) the SiC particles (in dark gray) are distributed uniformly in the coating. SEM examinations reveal the intimate contact between these particles and the nickel-cobalt metallic matrix.
• In parallel, the 100 H aged state at 600 ° C was the subject of a particular characteristic.

The micrographs (photos 7a and 7b) show a change in coloration of the SiC particles (from dark gray to light gray) corresponding to a depletion in silicon. Indeed, microanalysis X reveals a phenomenon of diffusion of the silicon of the particles in the nickel-cobalt metallic matrix (photos 8 to 10).

During the preparation of the samples, we note the good bonding of the particles in the matrix.

• La comparaison de différents états vieillis 100 h à 450, 500, 550 et 600°C a permis de montrer l'évolution de comportement des revêtements (photos 6b à 6h) :
• après 100 h à 450°C, on constate que le changement de coloration (gris foncé à gris clair) ne s'est pas réalisé totalement. Cependant de nombreuses particules présentent deux zones de coloration différentes : une zone gris foncé avec une teneur en silicium comparable à celle obtenue à l'état brut et une zone gris clair appauvrie en silicium.

To verify the possible presence of substrate-coating diffusions, filiations of compositions of the main chemical elements by means of X microanalysis were carried out. At the end of these examinations, only diffusions of the chromium and iron elements in the nickel-cobalt matrix could be observed over respective depths of 10 and 20 micrometers relative to the substrate / coating interface.

• The comparison of different states aged 100 h at 450, 500, 550 and 600 ° C made it possible to show the evolution of the behavior of the coatings (photos 6b to 6 h):
• after 100 h at 450 ° C., it can be seen that the change in color (dark gray to light gray) has not been completely achieved. However, many particles have two different coloring zones: a dark gray zone with a silicon content comparable to that obtained in the raw state and a light gray zone depleted in silicon.

So after 100 h at 450 ° C, the diffusion of silicon from the particles to the nickel-cobalt matrix is still incomplete.

• Des essais d'usure par abrasion sur abrasimètre TABER comportant deux meules circulaires chargées d'un lest entrainées par frottement par la pièce à tester animée d'un mouvement circulaire, dont les résultats sont résumés au tableau 2, montrent que les revêtements Ni-Co-SiC présentent une excellente tenue à l'usure par abrasion, comparativement à des revêtements traditionnels Ni-Co à 29% ou à 42% de Co sans adjonction de céramique. Jusqu'à 700°C pendant 2 h le Ni-Co-SiC présente un meilleur comportement que les revêtements Ni et Ni-Co sans adjonction de céramique ;
• des essais de résistance à l'usure en frottement alterné à sec des revêtements composites selon l'invention ont été réalisés en couple homogène et en couple hétérogène. Les couples testés ont été les suivants :

Ni-Co-SiC / Ni-Co-SiC (à 400°C) Ni-Co-SiC / Z12 C13 (à 20, 250 et 400°C) Ni-Co-SiC / NC 20 K14 Photos 6a to 6h also show that the oxidized depth at the surface is linked to the temperature maintenance time but that this layer is thick substantially constant for 100 h at 500,550 or 600 ° C.

• Abrasion wear tests on a TABER abrasimeter comprising two circular grinding wheels loaded with ballast driven by friction by the part to be tested with a circular movement, the results of which are summarized in Table 2, show that the Ni-Co coatings -SiC have excellent resistance to abrasion, compared to traditional Ni-Co coatings with 29% or 42% Co without adding ceramic. Up to 700 ° C for 2 hours, Ni-Co-SiC exhibits better behavior than Ni and Ni-Co coatings without the addition of ceramic;
• tests of resistance to wear in alternating dry friction of the composite coatings according to the invention were carried out in homogeneous couple and in heterogeneous couple. The couples tested were as follows:

Ni-Co-SiC / Ni-Co-SiC (at 400 ° C) Ni-Co-SiC / Z12 C13 (at 20, 250 and 400 ° C) Ni-Co-SiC / NC 20 K14

The results are given in FIGS. 11 to 13 and show that, if the Ni-Co-SiC coating has an average behavior in homogeneous couples, the behavior in heterogeneous couples is good at 400 ° C. and that this behavior can still be significantly improved. by prior correction of the coating.

Similar tests carried out with Ni-Co-Al2 O3 and Ni-Co-Cr2 O3 coatings (although not illustrated here), show equivalent results with, however, better erosion resistance for Ni-Co-Al2 O3 and better friction resistance for Ni-Co-Cr2 O3.

The type of composite coatings according to the invention is interesting because it allows, depending on the opposing material, to obtain a lower friction torque than those encountered with other types of protective coatings.

Another advantage is that it allows the use of heterogeneous couples of materials in friction.

These composite protective coatings therefore have notable advantages and may also prove to be advantageous for the sealing function at the top of compressor blades, where the guarantee of a minimum clearance is imperative in terms of performance. Sealing is, in this case, ensured by the abradable material located on the casing which can be partially machined by the vane opposite. The composite coatings according to the invention make it possible to ensure dawn / abradable interference without significant wear. They indeed allow to play on the ductility of the "nickel-cobalt" matrix and on the brittleness / hardness of the abrasive grain "ceramic particles" so as to ensure the wear of the abradable by the blade in the ratio 90 / 10 and thus limit the reduction in engine performance.

In the case where these coatings have undergone the additional electrolytic chromium plating step, they will find a preferred use in friction parts subjected to temperatures greater than or equal to 400 ° C. in an oxidizing atmosphere.

TABLEAU 2 Résultats d'essais d'abrasion TABER du NiCoSiC. Traitements Thermiques/air Variation de masse mg/4000 cycles Indice d'usure TABER Volume abrasé (mm3) sans 8 2 0,9 2 h 400°C 8 2 0,9 2 h 500°C 8 2 0,9 2 h 600°C 9 2,25 1 100 h 600°C 14 3,5 1,6 2 h 700°C 12 3 1,4 On the other hand, the coatings according to the invention, having undergone the additional diffusion heat treatment, will see their lifetime in friction significantly increased.

TABLE 2 TABER abrasion test results for NiCoSiC. Heat / air treatments Mass change mg / 4000 cycles TABER wear index Abrasive volume (mm3) without 8 2 0.9 2 h 400 ° C 8 2 0.9 2 h 500 ° C 8 2 0.9 2 h 600 ° C 9 2.25 1 100 h 600 ° C 14 3.5 1.6 2 h 700 ° C 12 3 1.4

Claims (14)

1. Process for protection against the oxidation and erosion by friction at a temperature below 600°C of a substrate made of alloy steel or of nickel-based super-alloy, characterised in that it is carried out by electrolytic codeposition of a coating made up of a nickel-cobalt binary matrix containing a homogeneous dispersion of ceramic particles M which are chosen from the group of oxides and carbides and especially made up of SiC, Al₂O₃ and Cr₂O₃, the said coating comprising a mass content of ceramic particles M of between 3.5 % and 10 %, the said electrolytic codeposition being performed in a sulphamate bath comprising a metal (Ni + Co) salt content of between 70 g/l and 100 g/l with a Ni/Co mass ratio of the bath equal to 15, in that the mass content of the particles M in suspension in the bath is between 50 g/l and 300 g/l, and in that the deposition is conducted with the following parameters:

   - current density (cd) of between 2.5 and 15 A/dm²;

   - temperature of between 50 and 54°C

   - pH of between 3.5 and 4.5.
2. Process for protection according to Claim 1, characterised in that the particle size of the ceramic particles M in suspension is between 1 and 5 micrometers.
3. Process for protection according to either of Claims 1 and 2, characterised in that before the introduction of the ceramic particles M into the electrolyte bath they are subjected to a decontamination operation by acidic washing in a hydrochloric medium.
4. Process for protection according to any one of Claims 1 to 3, characterised in that the anodes consist of nickel S beads arranged in titanium-baskets.
5. Process for protection according to one of Claims 1 to 4, characterised in that the electrolytic Ni-CO-M codeposition is carried out with stirring of the electrolyte bath by combined means of a turbine disperser and of compressed-air stirring and with a "rise and fall" translational movement of the cathode.
6. Process for protection according to Claim 5, characterised in that the electrolytic codeposition is carried out in a vessel containing at least one turbine disperser with a vertical axis at a speed of rotation of between 1750 and 2250 rev/min and a pressurised air delivery at the bottom of the vessel, the component to be coated, subjected to a cathodic voltage, being arranged between the disperser and the compressed air delivery on a "rise and fall" arm.
7. Process for protection according to any one of Claims 1 to 6, characterised in that an additional stage of electrolytic chromium plating with a thickness of between 2 and 10 micrometers is performed.
8. Process for protection according to any one of Claims 1 to 7, in which the particles M are SiC particles, characterised in that the operating conditions of electrolytic codeposition are:

   - cd = 5 A/dm²

   - temperature = 52°C

   - pH = 4
9. Process for protection according to Claim 8, characterised in that an additional operation of thermal treatment of diffusion of silicon into the matrix is performed under a neutral gas for a period of between 1 hour and 3 hours at a temperature of between 550°C and 620°C.
10. Protective coating obtained by the process according to either of Claims 8 and 9, characterised in that it comprises a weight composition equal to
    Ni : 63 ± 2 %, Co = 30 ± 2 %, SiC = 4 ± 2 %.
11. Process for protection according to any one of Claims 1 to 6, in which the particles M are Al₂O₃ particles, characterised in that the operating conditions of electrolytic codeposition are:

    - cd = 5 A/dm²

    - temperature = 52°C

    - pH = 4.
12. Protective coating obtained according to the process of Claim 11, characterised in that it comprises a weight composition equal to:
    Ni = 63 ± 3 %, Co = 32 ± 2 %, Al₂O₃ = 4.5 ± 0.5 %.
13. Process for protection according to any one of Claims 1 to 6, in which the particles M are Cr₂O₃ particles, characterised in that the operating conditions of electrolytic codeposition are:

    - cd = 3 A/dm²

    - temperature = 52°C

    - pH = 4.
14. Protective coating obtained according to the process of Claim 13, characterised in that it comprises a weight composition equal to:
    Ni = 58 ± 3 %, Co = 33 ± 3 %, Cr₂O₃ = 9 ± 1 %.

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