FR2656632A1 - PROCESS FOR COATING SURFACES INCLUDING A PRIOR METALLURGIC DEPOSIT OF AT LEAST ONE LAYER OF ALUMINUM AND ITS HARD ANODIZATION, PARTS TREATED FOLLOWING THIS PROCESS, AND PROCESS FOR BONDING SUCH PARTS. - Google Patents

Abstract

A surface coating method involving a preliminary metallurgical deposition on a conductive solid material of aluminium having a thickness sufficient for hard anodizing. The method is characterized in that a series of metal, metalloid or ceramic layers are deposited on the sealed, adhesive layer of alumina thus formed on the solid material in order to develop a stable, sealed multilayer complex having lasting protective or functional properties. A method for bonding the treated parts to each other or on other untreated parts is also described. This method is particularly designed to obtain chemically and electrochemically inert, sealed multilayer coatings on various massive materials which are subject to high levels of stress or aggression, or which need to be bonded.

Description

METHOD FOR COATING SURFACES INCLUDING A DEPOSIT
METALLURGICAL PRIOR TO AT LEAST ONE LAYER OF ALUMINUM AND
ITS HARD ANODIZATION, TREATED PARTS FOLLOWING THIS PROCESS, AND
PROCESS FOR GLUING SUCH PARTS.

 The present invention relates to a process for coating multilayer surfaces, comprising at least one layer of alumina produced by hard anodization of a prior metallurgical deposit of aluminum, the parts treated according to this process, as well as a process for bonding these parts, between them or on other parts not treated.

 There is a wide variety of methods for treating and coating surfaces. In both cases, the objective is mainly to "protect" from an aggressive environment given a massive material, used for its intrinsic mechanical qualities, while preserving them as much as possible. The border between surface treatment and surface coating is blurred, especially with regard to the most modern techniques. Surface treatment thus aims to modify the behavior or the nature of the surface layers of a solid material; coatings, for their part, provide a wider choice, because we know how to deposit many materials, organic, metallic, metalloid, or mineral, on a given material, thereby improving their resistance to corrosion or resistance to wear. ; similarly, the roughness, the hardness, or even the surface appearance are modified.

 Furthermore, a coating can itself undergo a complementary surface treatment, or serve as a base for attachment to another coating.

This is why, in order to better specify the context in which the present invention is situated, it appears necessary to cite, of course without any exhaustiveness, some examples of the most used coating techniques, namely
- coatings galvanically (or electrochemically), and chemically (Ni, paints, ...),
- deposits by projection ("shooping", plasma torch, ...), or by reloading by welding (oxyacethylene torch),
- physical vapor deposition (vacuum evaporation, ion bombardment, sputtering, etc.),
- deposits by plating, or by compacting metal powders or metal oxides ("sherardization", isostatic pressing, ...).

Techniques similar to these are also used in certain surface treatments, and aim to complement them functionally with a heat treatment. These are mainly
- the diffusion of metallic layers (chromisation,.),
- the diffusion of metalloids (carburizing, nitriding assisted by plasma, ...),
- the diffusion of minerals (phosphating, ...).

 Whatever the process used, the main quality of the protective deposit is its adhesion, since its effectiveness and its lifetime depend mainly on it. As such, the main factors playing a role in adhesion are: corrosion, dry and wet, diffusion phenomena at the interface between the solid material and the deposit, the plasticity of the deposit, and its mode of implementation. artwork. In addition, to achieve good adhesion, the surface on which a protective or functional coating is deposited must be as clean as possible, that is to say be free of grease, and more generally of foreign bodies.

These two essential qualities which are consequently the adhesion and the cleanliness make it possible to discern, inter alia, certain disadvantages of the techniques previously mentioned, namely
- the galvanic deposits carried out in aqueous baths cause embrittlement by hydrogen of the underlying solid material (steels in particular), initiating deep adhesion-inhibiting defects.

 - many organic coatings, such as paints, are permeable and can only be used as an undercoat.

 - spray deposits have other drawbacks. First of all, the metallization technique with a spray gun (projection by "shooping") induces stresses in the coating due to the brutal cooling of the droplets of projected molten metal, not favoring its attachment to the underlying material. , reloading with an oxy-ethylene torch strongly raises the surface temperature of the solid material, causing an ionic interdiffusion between the deposit and the material Finally, ceramic deposits, hard, inert and refractory, generally produced by projection by means of a plasma torch , manage to undergo detachment in a very corrosive environment, due to their porosity (crazing phenomenon) and the underlying corrosion of the resulting solid material.

 - The vapor phase deposits subject the material, in most of the known techniques, to an excessive temperature, creating here again stresses which favor the migration of the ions of the coating, and the pollution of the massive material. Among these techniques, ion bombardment, if it raises the temperature less, nevertheless results in coatings that are insufficiently thick for certain applications.

 - Finally, compacting, in particular by isostatic pressing, remains a mechanical coating method which in no way allows metallurgical adhesion to be obtained between the coating and its substrate.

 It is therefore understood that the coating techniques known at present do not, in general, provide sufficient mechanical strength to the protective or functional layer deposited, limiting its service life, the latter then becoming the site of generating surface ruptures. , as we know, deeper ruptures.

 The present invention aims to remedy these drawbacks by proposing to interpose a bonding layer having good plasticity, and having a great seal, between the solid material to be coated and a functional surface coating of a type of those mentioned above. . Another object of the invention is to provide, in addition, a highly refractory intermediate layer, which can serve as a thermal shield for the solid material, and allowing the deposition of coatings by techniques usually inducing surface stresses in said material. Another objective of the invention is also to provide an intermediate layer adhering metallurgically to the underlying solid material, the mechanical strength of a possible complex deposit, produced on this base layer, being thus improved. invention is to allow the marriage of incompatible materials in applications where it would prove useful to stick them, in a very safe and very reliable way.

 The invention thus relates to a process for coating surfaces by successive deposition of metallic, metalloid or ceramic layers, on a solid material, characterized in that a preliminary metallurgical deposition of aluminum is carried out on said material whose thickness is sufficient for allow its hard anodization, and obtaining a layer of aluminum and a layer of alumina on this material.

 Although the advantages of an aluminum coating have been known for a long time, it has never been envisaged, in accordance with the present invention, to make it an intermediate bonding layer for a multilayer coating, this intermediate layer being , in addition, passivated by hard anodization to form a mechanical, chemical, electrochemical, and thermal screen with respect to subsequent deposits.

 A multilayer coating produced according to the present invention therefore has a structure where each layer can fully fulfill its functional role. Thus, the solid material is protected first of all by an anodizable aluminum layer, the qualities of which are well known with respect to protection against dry or wet corrosion.

On the other hand, even if said aluminum layer is anodized, its functional characteristics, although excellent, do not necessarily lend themselves to all applications; for example, alumina, an excellent refractory material used, in this respect, in the ceramic industry, exhibits a microhardness providing good wear resistance, but this microhardness decreases if its subsequent "clogging" is carried out - this latter operation consisting in hydrating the alumina to reinforce its resistance to corrosion.

 It is therefore advantageous, according to the invention, to be able to deposit on the anodized aluminum layer, produced in a thick layer by known means, at least one coating by a traditional deposition process: in this way it is possible to compensate for the lack of tightness of such a coating, lack of adhesion of another, or the insufficient hardness of the solid material vis-à-vis a very hard coating (which causes a differential stress which can lead to separation). Unexpectedly, it is then possible to use functional coatings that have been deposited with little success on certain solid materials, without modifying anything in their own deposition technique, while obtaining excellent results. which has a progressive hardness in its thickness (from the low hardness of ductile aluminum to the high hardness of pure crystalline alumina), thus makes it possible to define zones of progressive stresses inside a multilayer coating.

 Since, moreover, by means of a more or less significant "clogging", means are available for varying the corrosion resistance to wear resistance ratio of an anodized aluminum layer, it is clear that 'We can favor one or the other of these characteristics according to the "weaknesses" of a subsequent coating. So as to have a great latitude in this choice, while retaining the assurance of the existence of 'A mechanical, chemical, electrochemical, and thermal screen underlying, it is also understood that only the hard anodization of a thick aluminum layer is suitable for the implementation of the invention.

 Other characteristics and advantages of the process according to the invention will emerge more clearly from the description which follows of an implementation given by way of non-limiting example thereof.

 The aluminum coating is preferably carried out electrolytically, in an organic bath or a molten bath; the latter alternative provides, at present, the best results, in that it is known to deposit layers of a very pure aluminum, crystallographically close to the deformable aluminum, and therefore having a very high ductility. It is, moreover, a technique that does not raise the temperature of the massive material that is covered, which makes it possible to preserve the intrinsic qualities of the latter, by avoiding ionic interdiffusion generating surface constraints.

 It is also known to obtain, by this technique, layers of significant thickness, commonly going up to a value close to 150 micrometers, which we are sure will be quickly exceeded to reach some 500 micrometers; however, from a thickness of ten micrometers, an aluminum layer deposited by electrolysis in a molten medium is free of pores; therefore, such a layer is waterproof.

 Furthermore, it has been shown that the adhesion of this same layer is a metallurgical adhesion (shear resistance tests show that this adhesion is greater than the shear resistance of the coating itself). There is, therefore, no risk of delamination of the aluminum layer, which it is usual, mainly for this reason, to use the corrosion protection properties.

 In addition, in accordance with the invention, the thickness of the aluminum deposit is chosen to be greater than that which is necessary for its hard anodization.

 As such, anodic oxidation of aluminum is a passivation technique that can be operated in various ways. To achieve the previously mentioned objectives of the invention, sulfuric anodization and chromic oxidation are two inappropriate techniques, since the alumina layer obtained then has a significant porosity or a thickness insufficient to be considered as waterproof.

 Preferably, a hard anodization of the previously deposited aluminum layer is therefore carried out. The available thickness of this aluminum layer guarantees, as such, the obtaining of a perfectly waterproof alumina surface coating, providing a chemical screen. and electrochemical between the underlying aluminum and any other subsequent deposit.

 Furthermore, the alumina layer constitutes a hard mechanical screen, resting on a very ductile aluminum sublayer, the adhesion between aluminum and alumina being excellent. This property is very important with regard to the invention, because it is understood that the mechanical strength of the base coating (aluminum / alumina) thus formed allows a large number of coatings to be deposited there, without fear of detachment due to constraints internal.

The improvement in mechanical strength is thus very clear compared with known projection techniques by blown plasma, and with vapor deposition techniques; in these cases already mentioned, the purely mechanical adhesion of the ceramic layers deposited, for example of alumina, leads to their detachment under severe conditions of use (in particular very corrosive medium).

Another characteristic of the coating process according to the invention is also to allow electrically isolating a solid material treated from the outside. As such, a sufficient hard anodization of the deposited aluminum layer is carried out so that there is no porosity in the alumina layer, and that the latter behaves as a good insulator; In most of the targeted applications, the thickness of the alumina layer will be taken so that it is insulating when it is subjected to a potential difference at least equal to 1000 volts in a dry atmosphere. although this insulating surface layer makes it possible in certain cases to reduce, or even eliminate, the propagation of induced surface currents, of the type in particular of
Eddy in rotating parts, or of the type of high frequency currents in parts subjected to a strong vibration. This results in a significant reduction in high-frequency interference, present for example in the automotive field, this reduction being capable of improving the operation of electronic sensors used in such cases.

 It is, moreover, particularly interesting to note the role of thermal screen of the alumina film. Alumina is a refractory material whose melting point exceeds 2000 C.

The aluminum itself melts at around 450 "C. It is obviously out of the question to subject massive materials treated according to the invention to such temperatures.

Moreover, it is true that few spray coating techniques raise the temperature above 1500C, these techniques being today the richest in possibilities and in variety of deposits. At the very least, it is a question of attenuating, beyond the hard anodized aluminum layer, the surface constraints inherent in deposition techniques raising the surface temperature, so as to provide metallurgists with higher working temperatures. allowing them to improve the possibilities of said techniques.

Thanks to the present invention, it is known that, in any case, the solid material will be preserved.

 It is also important to note that the roughness of the solid material has little effect on the roughness of the final multilayer coating. Indeed, the roughness of a layer of aluminum deposited electrolytically in a molten medium depends only on its thickness, and beyond a certain thickness, it is well known that this roughness decreases. Since, on the other hand, all the anodic oxidation techniques, and in particular the hard anodization, preserve, and even reveal the surface condition of the underlying aluminum, it is advantageous, in accordance with another characteristic of the invention, to control, through the thicknesses of the aluminum layer and the alumina layer, the roughness, and therefore the adhesion, of any subsequent deposition.

 In particular, it has been shown that ceramic layers deposited by a usual method of projection by means of a plasma torch, from metal oxide powders of given composition, on a layer of anodized aluminum, have exhibited durations life multiplied by a factor at least equal to two under very aggressive conditions of use (valve elements for the "off-shore" oil drilling industry), the stripping of the coatings only taking place due to damage to the alumina layer by mechanical fatigue.

 The effective control of the roughness of the alumina layer is of interest in another way than that of the subsequent deposition of at least one functional overlayer. An important problem of the metallurgical industries is to be able to marry between them materials which are a priori incompatible, whether it is a question of depositing a noble coating on a solid material with little adhesion (or else soiled because of its shaping technology), or whether it's sticking two materials together. Such a possibility can considerably simplify certain industrial processes where the manufacturing of certain parts requires significant and costly pre-treatments.

 It is therefore proposed, in accordance with the present invention, to deposit, on at least one of the two materials to be bonded, an aluminum layer, in a layer thick enough to be hard anodized. Thus, by the technique already described, a first control of the roughness of the surface alumina layer of the material or the treated materials is obtained.

Then, by a preferably mechanical action of the type of microsandblasting, the surface condition of the treated material, or of the two materials, is adapted so that the surface layer of alumina, while remaining thick enough to be perfectly waterproof, can serve as a bonding layer to an adhesive, for example ceramic or acrylic, of which we know the exceptional durability.

 It is then clear that a wide variety of materials can be bonded to a support previously treated in accordance with the teachings of the invention. In particular, steel coated with aluminum and alumina is bonded with great efficiency to a very different material such as carbon. Likewise, between materials that it is customary to bond, the mechanical strength of the interface is clearly superior to that of the bondings known to date. For this purpose, any type of common adhesive can be used in such cases, that is to say the ceramic and acrylic adhesives already mentioned, but also, at higher temperature, the epoxy adhesives.

 Other characteristics of the present invention will now be described, intended to allow its implementation on massive materials as varied as possible.

 A first case is that where the attachment of the aluminum deposited electrolytically in a molten medium, on the solid material, while being good, risks not being sufficient to serve as an undercoat for a subsequent coating. In accordance with the invention, it is then planned to deposit a bonding layer on the solid material, this layer preferably being constituted by nickel, the thickness of which is small, of the order of 1 micrometer. It is therefore a cladding of the solid material which is carried out before the electrolytic deposition of aluminum.

 A second case is that where the solid material must be preserved from pollution by aluminum, this pollution taking place by diffusion under a thermal effect; such a manufacturing constraint is common in the nuclear industry. In accordance with the present invention, a layer of a material which diffuses little in the solid material is then deposited, or which is not considered to be a pollutant if it diffuses. As such, we know the qualities of a titanium coating, its thickness being small, and hardly exceeding a few micrometers.

 A third case is that where it is proposed to cover massive materials which it is unusual to cover; these are mainly composite materials, in particular fiber composites, or even carbon. According to the invention, it is thus possible to cover with aluminum, by electrolysis in a molten medium, compounds incorporating carbon, boron, glass, kevlar fibers, etc. This layer of aluminum can be anodized, and the alumina layer then serves as a basis for the subsequent deposition of certain coatings hitherto impossible to deposit on composite materials, due, in particular, to an inexistent ionic diffusion, or to a ductility of the coatings insufficient for the forces in torsion requested, when, for example, the solid material to be treated is a cable, a wire, or any other object of the same kind; the very ductile aluminum layer, and which we know moreover that it can undergo an elongation greater than 80%, is particularly suitable for this use.

 As we know, moreover, that fiber composite materials offer little mechanical resistance when they undergo a shear transverse to the main axis of the fibers, it is common to sheath them, or to arm them, with a material such than steel, brass, etc. This amounts to coating them by plating, with the drawbacks already mentioned. According to the invention, an anodisable layer of aluminum is deposited around the fiber composite materials, that is to say which may have, as it stands, a surface hardness capable of protecting them very effectively against transverse collicitations. In addition, a multilayer coating can be subsequently deposited on the alumina layer, so that these composite materials have unexpected functional properties at their surface.

 In addition, the technique of depositing aluminum by the electrolytic route in a molten medium is particularly suitable for covering parts with complex geometries, or even wires. As regards fiber composite materials, these parts with complex geometries can be obtained, for example, by a pultrusion technique.

Advantageously, the process according to the invention makes it possible, in certain cases, to simplify the production of parts subjected to isotropic stresses, which we are currently trying to achieve by orthogonal "weaving" of fibers, d 'a type already mentioned, which is then drowned in a material serving as a matrix.

 The invention therefore also relates to the parts treated according to the coating process previously developed.

 Furthermore, the coating method and the bonding method in accordance with the invention are of course not limited by the description which has just been made, within the framework of a particular implementation thereof.

 This process is in particular intended for obtaining multilayer, waterproof, chemically and electrochemically inert coatings on various solid materials subjected to stresses or to significant aggressions, or which it is desired to bond.

Claims (10)

 1 - Method for coating surfaces by successive deposition of metallic, metalloid or ceramic layers, on a solid material, characterized in that a prior metallurgical deposition of aluminum is carried out on said material, the thickness of which is sufficient to allow its hard anodization , which makes it possible to obtain a layer of aluminum and a layer of alumina on this material.  2 - A method of coating surfaces according to claim 1, characterized in that the surface alumina layer obtained by hard anodizing has a thickness sufficient to be, on the one hand waterproof, and on the other hand electrically insulating when it is subject to a potential difference of the order of 1000 volts in a dry atmosphere.  3 - A method of coating surfaces according to any one of the preceding claims, characterized in that one proceeds to a more or less partial "clogging" of the alumina layer, which provides the means of varying the resistance ratio corrosion on the wear resistance of said layer.  4 - A method of coating surfaces according to any one of the preceding claims, characterized in that the aluminum coating is preferably carried out electrolytically, in an organic bath or a molten bath.  5 - A method of coating surfaces according to any one of the preceding claims, characterized in that one controls, through the thicknesses of the aluminum layer and the alumina layer, the roughness, and therefore the adhesion of the coating subsequently deposited on said alumina layer.  6 - Process for coating surfaces according to any one of the preceding claims, characterized in that a bonding layer is deposited under the aluminum layer, this bonding layer preferably being made up of nickel, in low thickness, not exceeding about one micrometer.  7 - A method of coating surfaces according to any one of claims 1 to 5, characterized in that is deposited, under the aluminum layer, a layer of a material which diffuses only slightly in the solid material, or n 'not being considered a pollutant if it diffuses into said material.  8 - Parts of a solid material, treated in accordance with the surface coating process according to any one of the preceding claims.  9 - Parts according to claim 8, characterized in that the solid material is a composite material, and in particular a fiber composite material, which gives this composite material a surface hardness capable of protecting it from stresses applied orthogonally to the usual direction of the efforts exerted there.  10 - A method of bonding parts of a solid material according to any one of claims 8 or 9, characterized in that, to bond two parts together, an aluminum layer is deposited on the surface of at least one of said parts which is hard anodized, then, by a mechanical action, for example of the type of a micro-sandblasting, the surface condition of the surface alumina layer obtained is adapted, so that the facing surfaces to be bonded have a roughness likely to allow their lasting bonding by means of an acrylic adhesive, ceramic, epoxy, or any other type of adhesive of usual use in such a case.