Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/0635—Carbides

Definitions

• 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.
• 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.
• 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.
• 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 ⁇ m, and preferably between 1 and 5 ⁇ m on the parts to be protected.
• 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.
• 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.
• PVD physical vapor deposition
• 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.
• 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.
• the signal frequency should be quite high, preferably in the megahertz range and beyond 10 MHz.
• 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 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.
• 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.
• 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 ⁇ m per hour, or 1.2 ⁇ m in two hours.
• 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.
• 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 ⁇ m) 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.
• silicon carbide deposits having a thickness of between 1 and 2 ⁇ m were carried out in the radio-frequency cathodic sputtering device according to the description above.
• 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 ⁇ m per hour and 1 ⁇ m 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.
• the deposition of silicon carbide must have a thickness at least equal to 0.1 ⁇ m and preferably between 1 and 5 ⁇ m.
• 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.
• 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.
• 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.
• 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.
• Samples of an Al matrix carbon fiber composite were coated with a 1 ⁇ m 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.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physical Vapour Deposition (AREA)
• Laminated Bodies (AREA)

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• 1988
  • 1988-04-20 FR FR8805713A patent/FR2630458A1/fr not_active Withdrawn
• 1989
  • 1989-04-18 WO PCT/FR1989/000177 patent/WO1989010425A1/fr not_active Application Discontinuation
  • 1989-04-18 JP JP50491189A patent/JPH02504169A/ja active Pending
  • 1989-04-18 EP EP19890905129 patent/EP0376998A1/de not_active Withdrawn

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