WO1989010425A1

WO1989010425A1 - Method for the surface protection against corrosion and abrasion of metal objects or composite objects with metal core by deposition of silicon carbide

Info

Publication number: WO1989010425A1
Application number: PCT/FR1989/000177
Authority: WO
Inventors: Claude Sella; Martine Jamet; Jean-Claude Martin
Original assignee: Pechiney Electrometallurgie
Priority date: 1988-04-20
Filing date: 1989-04-18
Publication date: 1989-11-02
Also published as: EP0376998A1; JPH02504169A; FR2630458A1

Abstract

The invention relates to a method for the surface protection against abrasion and corrosion of metal objects or composite objects with metal matrix reinforced by fibers, of magnesium and magnesium-based alloys, aluminium and aluminium-based alloys, titanium and titanium-based alloys, zirconium and zirconium-based alloys, hafnium and hafnium-based alloys, characterized in that a deposition of silicon carbide having a thickness at least equal to 0.1 mum and preferably at least equal to 1 mum, and comprised between 1 and 5 mum is applied by a vapor phase physical deposition method.

Description

METHOD FOR SURFACE PROTECTION AGAINST CORROSION AND ABRASION

OF METAL OR COMPOSITE OBJECTS WITH A METAL MATRIX

BY DEPOSIT OF SILICON CARBIDE

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of surface protection of metallic or composite objects with a metal matrix reinforced with fibers, by depositing a thin layer of silicon carbide. Its main purpose is to protect these parts against corrosion and abrasion, including abrasion in a corrosive environment. It applies more particularly to magnesium and to magnesium-based alloys, but also to aluminum and aluminum-based alloys, to titanium and to titanium-based alloys, and sometimes to zirconium and to alloys. based on zirconia, hafnium and alloys based on hafnium.

STATE OF THE ART

Magnesium and its alloys, including the low density, the natural abundance of the base metal, the ease of processing and the moderate cost price, make the use very attractive, are limited in their applications by insufficient resistance to corrosion, especially in the presence of solutions containing chloride ions.

Attempts have been made to overcome this drawback by treating or coating the parts exposed to corrosion. Among the main known processes, we can cite:

- acid etching in a solution based on alkaline dichromate and nitric acid;

- neutral etching in a solution based on alkaline dichromate, ammonium dichromate and ammonium sulphate;

- fluorinated anodization, in an ammonium bifluoride bath;

- anodic oxidation, under conditions similar to that which is commonly carried out on aluminum;

- clogging ("surface sealing") of pores and small surface defects with a polymerizable epoxy resin around 200 ° C. This layer also constitutes a good bonding base for subsequent painting;

- paints and warts of all known types, on a first layer of etching; - plastic coatings, in particular based on "NYLON" or "RILSAN" (registered trademarks) generally carried out on a part preheated to 200 ° C which is introduced into a fluidized medium of polymerizable resin powder.

These various methods make it possible to satisfactorily resolve many cases of particular applications, but still prove to be insufficient in fields where a great longevity of the treated parts is required (automobile), or very high safety (aeronautics, marine, nuclear), which could be compromised by the appearance of corrosion points.

There is therefore a need for a method of protection against corrosion of magnesium and its alloys, the longevity and efficiency of which are significantly improved compared to known methods, in particular by a coating having simultaneously a very high abrasion resistance. .

With regard to aluminum and its alloys, protection by anodic oxidation is essentially used, but the layers thus obtained do not always have sufficient abrasion resistance. Although the resistance of aluminum to salt spray is better than that of magnesium, it is always desirable to improve it further, especially in the case of alloys for which certain heat treatments can increase the sensitivity to corrosion under stress, for example.

As for titanium, zirconium and hafnium, there are many cases where their resistance to abrasion or to corrosion could be advantageously increased: for example for titanium in the presence of fluoride or chloride ions, or for zirconium and Zircaloy type alloys, in the presence of pressurized steam.

OBJECT OF THE INVENTION

The object of the present invention is a method of protection against corrosion and abrasion of metallic or composite objects with a metal matrix reinforced by fibers, for example carbon fibers or silicon carbide, magnesium and alloys. based on magnesium, aluminum and aluminum alloys, titanium and alloys based on titanium, zirconium and zirconium-based alloys, hafnium and hafnium-based alloys, this process consisting in carrying out on said objects a surface deposit of silicon carbide, of a thickness at least equal to 0.1 tira, and preferably at least equal to 1 μm, and preferably between 1 and 5 μm on the parts to be protected.

As has been said, the invention is particularly suitable for the protection of magnesium and its alloys, but it is applicable to aluminum, titanium and their respective alloys; it can also be used for the protection of zirconium and its alloys with significantly different results.

In what follows, we will agree to designate by "magnesium" as well unalloyed magnesium as the magnesium-based alloys concerned by the implementation of the invention, this same convention being applied to aluminum, titanium, zirconium and hafnium.

A deposition of silicon carbide providing improved protection can be carried out by various physical methods, for example by physical vapor deposition (PVD), at a temperature compatible with the melting point of the metal substrate considered. But a particularly effective method is sputtering at radio frequency; it makes it possible to obtain a deposit of SiC formula, or of a perfectly defined formula, and moreover gives unexpected results as to the quality of the deposit and to the effectiveness of the protection obtained.

Deposition by CVD (chemical vapor deposition) is also possible, but it is difficult to control in order to obtain coatings of identical quality: on the one hand the stoichiometry of SiC is difficult to obtain, on the other hand it is occluded in the deposition of gases which are not neutral and can induce corrosion in the metal to be protected, which is the opposite of the desired objective. These drawbacks can be aggravated when depositing by plasma assisted CVD at low temperature, a type of deposit which would be necessary in the case of metals which are easily fusible or whose properties are very sensitive to temperature, such as Mg.

It is recalled that cathodic pul erization results from the bombardment of a target, comprising the compound to be deposited, by positive ions accelerated from a plasma. The incident ions penetrate more or less deeply into the target according to their energy and their mass, and cause a cascade of collisions which involves the ejection of a certain number of atoms being in the vicinity of the surface of the target. Part of the kinetic energy of the ions is transferred to the ejected atoms.

The spraying efficiency increases with the mass of the incident ions (in practice, Argon ions are used) and, within certain limits with the energy of the ions.

There are different spraying techniques DC, ^¬ all times, only the sputtering radio frequency allows for deposition of insulating materials. Indeed, in direct current, the target is fixed on the negatively polarized electrode relative to the substrate holder, itself connected to ground. If the target is insulating (case of silicon carbide), there is very quickly accumulation of positi¬ ves charges thereon and the Ar-i- ions end up being repelled, which causes the spraying to stop.

To avoid this drawback, reactive sputtering could be used using an Si target and a plasma containing carbon dioxide, but we would then end up with the drawbacks cited for plasma-assisted CVD. In addition, the nitrogen reacts with the target's silicon which becomes progressively insulating, which modifies the deposition parameters.

It is therefore preferable to apply an alternating radio frequency voltage between the two electrodes of the system; during an alternation, the electrode which carries the target is negatively polarized and attracts the Ar ions from the plasma; during the following alternation, the target which has become negatively polarized repels the Ar + ions and captures the electrons, thus neutralizing the excess of positive charges. This allows the pulverization of the silicon carbide (practically insulating) to continue during the next alternation.

The signal frequency should be quite high, preferably in the megahertz range and beyond 10 MHz. Indeed, the electrons have a mobility 2000 times greater than that of the ions and carry out a negative autopolarization of the target; this negative continuous polarization is superimposed at the radio-frequency voltage so that the electrode takes, with respect to the plasma, a positive voltage for a short fraction of the cycle; the plasma electrons are then attracted to the target and neutralize the positive charges which have accumulated on its surface during the rest of the cycle, that is to say when the voltage is negative with respect to the plasma, attracting thus the positive ions which pulverize the target.

The whole of the radio-frequency cathode sputtering device on which we carried out the tests comprises: - a pumping group, associated with a liquid nitrogen trap and with a subli titor, which allows to obtain a pressure from 10 ~ ^ to 10- 'hPa; - a vacuum bell made of PYREX glass, in which are arranged a cathode supporting the target, that is to say the material to be sprayed (silicon carbide crystals), and a substrate-carrying anode (metal object to be coated), connected to ground.

The distance between the target and the substrate is of the order of 30 mm, which allows a good yield, while avoiding sprays on the walls of the enclosure. The two electrodes are preferably provided with cooling by circulation of water to avoid any harmful heating of the samples. The working frequency of the generator is fixed at 13.56 MHz, taking into account the regulations for the use, in France, of radio frequencies for industrial use. The choice of this precise value therefore does not constitute a critical parameter of the operation.

The argon used is preferably of high purity, for example of the so-called N55 quality (99.9995% Ar).

The description of this apparatus and of its operating mode is given by way of example and does not constitute a limitation of the invention.

EXAMPLE OF IMPLEMENTATION OF THE INVENTION

On the principle which has just been described, a 13.56 MHz cathode sputtering installation was used, with a maximum power of 1 KVA. The enclosure comprises a target 130 mm in diameter and an anode of the same dimension, made of copper, both of which are cooled by circulation of water, the anode supporting the metallic object which it is desired to coat with 'a thin layer of silicon carbide. We carry out a first vacuum between 10 ^~ 6 and ÎO- ^ hPa to completely degas the enclosure, then it is filled with argon N55 under a pressure of 5.10 ^~ 3 hPa (a pressure located between approximately 10 ^-2 and ÎO - ^ hPa may be suitable). In the tests carried out, the high frequency power was adjusted to 300 VA.

Silicon carbide, used in the form of a powder sintered under pressure, has a purity greater than 99.5%, with, for main impurities (expressed in% by weight): A1 0.04, B 0.25, Ca 0.01, Co 0.001, Cu <0.001, Fe 0.001, Mg <0.001, Mn <0.001, Mo 0.001, Ti 0.01. It is d _* silicon carbide. , crystallized, hexagonal.

Under the conditions indicated, the growth rate of the deposition of silicon carbide on the magnesium, aluminum, titanium, zirconium or hafniun substrate is of the order of 0.6 μm per hour, or 1.2 μm in two hours.

The method offers the advantage of being able to deposit on the anode

(the metal object to be protected) at low temperature, the latter being usually always below the melting or transformation point of the metal, or even preferably at room temperature, said anode being able to be cooled; thus the mechanical and physical properties of the coated metal can be preserved, and flash phenomena which can occur at high temperature with a metal having a high vapor pressure, such as Mg, are avoided.

Preparation of the substrates: the samples of metal or alloy to be covered used in the tests are in the form of discs of 12 and 25 mm in diameter cut in turn from molded bars of magnesium; mechanical polishing is carried out dry with abrasive papers of increasing fineness up to grade 1200. This is completed by polishing with diamond paste (grains of 0.5 μm) on felt disc. The final cleaning is carried out in distilled water under ultrasound. The drying is carried out by spraying with a jet of pure nitrogen. It is preferable to avoid any degreasing with chlorinated solvents.

Other samples have been polished in a more summary manner so as to approximate the industrial conditions of implementation. Electrolytic or tribofinitioπ polishes have also given good results. Ionic pickling of the samples or metal parts to be coated can be done in radio frequency mode to eliminate the natural oxide layer which always forms on the materials, this allowing better adhesion of the deposit.

On these various samples, silicon carbide deposits having a thickness of between 1 and 2 μm were carried out in the radio-frequency cathodic sputtering device according to the description above.

Regarding the growth rate of the silicon carbide deposit, it can be adjusted, depending on the power of the radio frequency, within limits of between 1 μm per hour and 1 μm per minute, without these values constitute limits of the invention. Note, however, that it is preferable to start the deposition at a low growth rate. For optimum efficiency and adhesion, the deposition of silicon carbide must have a thickness at least equal to 0.1 μm and preferably between 1 and 5 μm.

CHARACTERIZATION OF SAMPLES

1 ° - Structure and properties of the SiC layers

X-ray diffraction analysis shows that the structure of the SiC coatings is generally amorphous but can also be micro-crystalline. The surface resistivity on a 1 cm square is greater than 20 M-TL.

2 ° - Tests of adhesion and resistance to abrasion and wear

Adhesion: all coatings resist peeling off with adhesive tape. Abrasion: the abrasion resistance is determined by the number of passages of a bundle of glass fibers of 4 mm in diameter, under a pressure of 15 grams / mm ² , leading to the complete elimination of the coating. This beam is arranged at the end of an articulated lever arm actuated by a motor fitted with a tachometer. The load is adjustable.

Under these conditions, it can be seen that a 2 μm thick SiC coating is still intact after 50,000 passages, and exhibits tears after 80,000 passages; complete elimination is achieved after 100,000 passages approximately, regardless of the nature of the metal substrate (Mg, Al, Ti, Zr, Hf or alloys).

By way of comparison, the effect of the wiper on non-alloy magnesium and aluminum leads to very rapid wear: after 500 passages, the groove dug has a depth of the order of 3 tim: we can therefore estimate that the abrasion is approximately 6 nm per pass. On dichromate-etched magné¬ sium, the etching layer disappears after only 100 passes. This confirms that these SiC coatings have a quite remarkable abrasion resistance, and, in particular, much higher than that of the alumina layers obtained by anodizing on aluminum.

Wear tests on samples of an aluminum alloy, reference 2024 according to the classification of the Aluminum Association, having the following nominal composition by weight: Si 0.5%, Fe 0.5%, Cu

3.8% to 4.9%, Mn 0.3 to 0.9%, Mg 1.2 to 1.8%, Cr 0.1%, Zn 0.25%, coated with a deposit of 2, 4 μm of SiC according to the invention were carried out by means of a Pliπt test used in the automotive industry (tribometer or high frequency friction testing machine of the Cameron-Plint type from the company Plint and Partners LTD Wokingham -England). This test consists in subjecting the surface to be tested to the friction of a cast iron segment, under precise conditions (use of a neutral lubricating oil, frequency 16 Hz, load 100 N, temperature 100 ° C, duration 2h). This test shows a total absence of weight loss and an improving coefficient of friction of 0.13 to 0.08, while, for the same uncoated samples, the weight loss is 2.2 mg and the coefficient of friction remains of the order of 0.14.

Corrosion resistance tests A number of magnesium samples were subjected to the action, on the one hand a 0.01 M sodium tetraborate solution (pH 9.3) supplemented with 100 ppm of NaCl and on the other hand, to the action of a salt-based fog, according to AFNOR standard NF41002, produced from a NaCl solution at 5% by weight, at 35 ° C, at a humidity of 85 to 90%. The duration varied from 48 to 150 h; the cited standard provides 150 hours, but we wanted to follow the evolution of the process over time. These conditions are very severe and lead to a major attack on unprotected magnesium. For these tests, the operation was carried out on a foundry alloy, of the AZ91 # type, the nominal composition of which is (in% by weight): Al 7.5-9%, Zn 0.2-1, Mn 0.15-0.6, Si <0.3, Cu < 0.2, Fe <0.05, Ni <0.01, Mg balance. The samples thus treated are examined by optical microscopy and scanning electron microscopy (SEM).

For coatings of 1 to 2 thicknesses, it can be seen that:

- after 100 hours, the surface is unaltered. No trace of corrosion is visible;

- after 150 hours, light microscopy indicates that the surface is still unaltered. The SEM examination confirms this conclusion.

These tests show that the coating of SiC constitutes a corrosion protection far superior to that of all the surface treatments known to date for magnesium.

The same tests were also carried out on another Mg alloy, ZE 41, containing by weight Zn 4%, Rare Earths 1%, Zr 0.5% and led to the same results.

An aluminum alloy, reference 2024 already cited, was covered with a layer of 1.2 μm of SiC and then was subjected to the salt spray test described above. After 2300 h there are only a few localized bites with an average depth of 134 μm and a maximum of 190 JUITS (measured on five bites). In comparison, a sample of the same uncoated alloy, subjected to the same salt spray conditions, exhibits, after only 2000 h of exposure, generalized pitting of average depth 460 u and maximum 760 u (measured on five bites).

Samples of an Al matrix carbon fiber composite were coated with a 1 µm layer of SiC and subjected to a damp oven test (40 ° C, 90% relative humidity), according to the ^' NFC 20703 standard, together with controls of the same uncoated composite. After 300 h of exposure, the controls show a general attack of the matrix with partial separation of the fibers whereas for the coated samples the attack does not appear until around 1200 h and remains localized.

Similar observations have been made on samples of titanium, zirconium and hafnium subjected to aqueous solutions containing chloride and / or fluoride ions. 4 ° - Comparative tests with other surface coatings by radio frequency sputtering.

Under the same conditions as in the case of silicon carbide, surface treatments consisting of metals (aluminum, chromium, titanium), oxides (alumina, silica) and d have been carried out, by the sputtering process described above. another carbide, titanium carbide TiC.

None of these coatings showed satisfactory resistance to the abrasion test by the glass fiber brush and to corrosion by the tetraborate chloride solution and by the salt spray.

CONCLUSIONS

The coating of silicon carbide on magnesium, aluminum, titanium, zirconium, hafnium and alloys based on each of these metals according to the invention proves to be particularly advantageous:

- it is very hard and very adherent, very homogeneous and very compact; it leads to a spectacular increase in the abrasion resistance of the metal substrate thus protected

- simultaneously, it is generally amorphous and in any case perfectly waterproof; it constitutes an excellent protection against salt corrosion which is particularly harmful in the case of magnesium and against corrosions generally difficult to control, such as chlorinated and / or fluorinated solutions for titanium and steam under pressure for zirconium and hafnium. - it is electrically insulating, so it avoids the effects of batteries observed with conductive coatings (metals, titanium carbide) and composites reinforced with carbon fibers.

- In the case of metal matrix composites, in particular with aluminum and magnesium (or their alloys) reinforced with carbon fibers, there is also a spectacular improvement in the resistance to corrosion by saline solutions.

Claims

1. Method for surface protection against abrasion and corrosion of metallic or composite objects with a metal matrix reinforced with fibers, in magnesium and alloys based on magnesium, in aluminum and alloys based on aluminum, in titanium and titanium-based alloys, zirconium and zirconium-based alloys, hafnium and hafnium-based alloys, characterized in that a deposit of silicon carbide with a thickness of at least 0 is made on said objects , 1 μm and preferably at least equal to 1 μm.

2. Method according to claim 1, characterized in that the deposition of silicon carbide is carried out by a physical vapor deposition (PVD) process.

3. Method according to claim 2, characterized in that the deposition of silicon carbide is carried out by sputtering at radio frequency of a target in silicon carbide under reduced pressure of inert gas.

4. Method according to claim 3, characterized in that the cathode sputtering is carried out under a pressure between about 10 ~ ² and 10 ^"3 hPa.

5. Method according to claim 3, characterized in that the deposition on the metal object is carried out at low temperature.

6. Method according to any one of claims 1 to 5, characterized in that the thickness of the coating of silicon carbide is between 1 and 5 μm.