Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/026—Anodisation with spark discharge
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/005—Apparatus specially adapted for electrolytic conversion coating

Definitions

• the present invention relates to an electrical oxidation process by plasma micro-arc in order to obtain a ceramic coating on the surface of a metal having semiconductor properties.
• Aluminum, titanium, their alloys and all metals which have valve (diode) properties have an attractive mechanical strength / weight ratio and are suitable for a wide range of applications such as aeronautics, automotive, mechanical
• a relatively old process (1,932) of anodic oxidation by micro-arc discharges or sparking discharges at the anode was developed in order to obtain ceramic coatings for aluminum, titanium and magnesium parts and their alloys, as a means of protection against severe abrasion and corrosion.
• Micro-arc oxidation forms an insulating barrier film on valve effect metals such as aluminum and titanium.
• the barrier film is broken and micro-arcs appear. If a high voltage is maintained, many micro-arcs strike and move quickly over the entire submerged surface of the sample.
• These dielectric breaks cause chimneys which cross the oxide layer (barrier) formed.
• Complex compounds are synthesized inside these chimneys. They consist of substrate material, surface oxides and electrolyte addition elements. Chemical interactions in the plasma phase occur in multiple surface discharges and result in the formation of an increasing coating in both directions from the surface of the substrate. This causes a gradual change in the composition of the coating properties from the metal alloy inside the substrate to a complex ceramic compound in the coating. According to the description of the history of this process,
• Anodic Spark Deposition (ASD). They observed that the material underwent deposition of the electrolyte during the dielectric rupture of a growing insulating film on the anode. This dielectric rupture causes sparks which appear and disappear while being distributed on all the surface of the anode, giving an effect of movement.
• a patented process in 1 974 was put in place to compete with the coating on aluminum for architectural purposes.
• the method allows the aluminum substrate to act as an anode in a potassium-silicate solution, so that a silicate-alumina coating of gray-olive color is applied by the use of a rectified half-wave 400 V direct current.
• the process takes place through a dielectric breakdown of the barrier layer, causing visible sparks or flickers on the anode substrate, while Bakovets, Dolgoveseva and Nikiforova postulate three parallel mechanisms during film formation: electrochemical, plasma oxidation and chemical oxidation.
• sicodizing including the addition of carboxylic acids and vanadium components in the bath. Ceramic or tetrafluoroethylene resins were also added to the bath to provide hardness or lubrication qualities to the coating.
• the excellent adhesion to the substrate of this type of coating is among the many advantages of this process.
• the physical and tribological characteristics high hardness, resistance: thermal, electrical, abrasion, corrosion, etc.
• the wide variety of alumino-silicate mixtures for coating purposes are among the many advantages of this process.
• a suitable device makes it possible to establish the optimum programming, as a function of different parameters (nature of the alloy, or of the metal of the parts to be treated, the characteristics of the ceramic that one wishes to obtain, etc.).
• the electrolyte is an aqueous-based solution, preferably demineralized and comprises at least one oxyacid salt of an alkali metal and one hydroxide of an alkali metal.
• the first phase which lasts according to the alloys, from a few seconds to a few minutes, an insulating layer consisting of hydroxide is formed, this thin layer is a dielectric.
• this dielectric layer is observed with a micro-arc activity which increases, depending on the electrical energy applied.
• This second phase lasts, depending on the above parameters, between 1 5 and 30 minutes.
• the formation of a thick ceramic layer is gradually obtained.
• the composition and physical properties of the coating during this training are subject to change. We were able to identify the majority of ⁇ -AI 2 O 3 type elements on X-rays. (bohemian) and -AI 2 0 3 coumblem.
• the generators used and described in the various publications deliver: either a rectified and / or direct current, or a single-phase or three-phase sinusoidal alternating current.
• Capacitors in series are interposed in particular to limit the current in the secondary use circuit and a particular form of current follows.
• alternating generators powered by three-phase current and using the three phases sequentially using thyristors or equivalent electronic devices.
• the shape of the current is only the result of the process itself and cannot be changed in its shape.
• Document US 5 61 6 229 relates to a method of producing a ceramic coating by this technique, in which the voltage applied to the electrodes is at least 700 V. Below this voltage value, it is not possible to obtain a coherent ceramic, but powder. This therefore results in a very high energy consumption, especially when the parts to be coated with ceramic have a large surface area.
• the object of the invention is to provide an electrolytic oxidation process by plasma micro-arc in order to obtain a ceramic coating on the surface of a metal having semiconductor properties, such as aluminum, titanium, magnesium, hafnium, zirconium and their alloys by physico-chemical reaction of transformation of the treated metal.
• the aim is to reduce the porosity of the ceramic layer by obtaining a very dense layer of uniform thickness over the entire surface of the part.
• an object of the invention is to reduce the growth time of the ceramic on the surface of the metal part while reducing the electrical energy consumed.
• the process which it relates to is characterized in that it consists in: - immersing the metal part to be coated in an electrolytic bath composed of an aqueous solution of alkali metal hydroxide, such as potassium or sodium , and an oxyacid salt of an alkali metal, the metal part forming one of the electrodes,
• a signal voltage of generally triangular shape that is to say having at least a front slope and a rear slope, with variable form factor during the process, generating a current controlled in its intensity , its shape and its relationship between positive and negative intensity.
• the shape of the voltage wave is thus possible to adapt the shape of the voltage wave to the different stages of the process as well as to the type of alloy and to the different electrolytic bath solutions.
• This waveform also has a variable frequency parameter, which greatly improves the qualities of the ceramic coating compared to those obtained by known methods. Different modes of implementation of this method are possible.
• the front and rear slopes of the voltage signal can be substantially symmetrical, or asymmetrical and of varying angles during the process. It is also possible, during the process, to change the frequency of the triangular signal between approximately 100 and 400 Hz.
• this method consists in changing the value of the triangular voltage during the electrolysis between about 300 and 600 V rms.
• the value of the current can also be modified or fixed independently of the voltage.
• the different parameters (form factor, potential value, frequency, current value, UA / IC ratio) can be changed simultaneously or independently of each other during the process.
• this method consists in separately controlling in its forms and values the electrical energy Ul in the positive phase and / or in the negative phase.
• An electronic generator of the current source type for implementing this method comprising a block for connection to a single-phase or three-phase electrical supply from the sector and a block for connection to the electrolysis cell, is characterized in that it comprises : - a module for transforming the sinusoidal alternating signal supplied by the network into a trapezoidal or sawtooth signal,
• this generator comprises, at the output, an isolation transformer with capacitors in series in the primary or secondary, for filtering the DC component in order to avoid saturation of the magnetic circuit while inserting optimum safety of use for the electrical protection, with connection of one of the poles to earth.
• this generator is controlled by a processor of the PC type making it possible to manage the various parameters as the process proceeds.
• the steep front slope makes it possible to induce the initiation of micro-arcs very actively without raising the average voltage.
• the slow slope maintains a constant current for the time necessary for the physico-chemical reaction within the plasma.
• Rear slope control has also repercussions on the negative current.
• the negative current peak helps to diffuse the al ions necessary for the continuity of the formation of the ceramic layer in certain phases of the process. It is also used to obtain a reduction in residual porosity at the end of the process.
• the symmetrical slopes of the signal favor a rapid and regular growth of the ceramic layer, and allow the inclusion of additive elements which can be added to the bath and according to the characteristics of the ceramic coating which it is desired to obtain for the optimal use of parts. This situation is much more effective than that obtained from a sinusoid or a direct current described in the documents of the prior art.
• the implementation of the process according to the invention has the following main advantages: - optimal formation of the hydroxide layer;
• the energy power of the network which supplies the electrical power is reduced in the same proportions as is the subscription of the metering bracket for the electrical energy consumed.
• this same installation is capable, from an electrical energy of a certain value, of doubling the processing capacity compared to a conventional generator using the sinusoidal signal of the distribution network.
• the voltage / current curves obtained show the fundamental differences of the positive and negative energy peaks obtained by the process. Full control of these parameters highlights the possibility of obtaining the desired values and current forms at any stage of growth of the layer during the treatment.
• Figure 1 is a very general view of the installation
• Figure 2 is a view of a block diagram of the current generator
• Figures 3, 4 and 5 are three illustrative diagrams respectively of the drive voltage signal when it is balanced, of the corresponding intensity / voltage signal taken across the load and related positive and negative power curves;
• Figures 6, 7 and 8 are three views corresponding respectively to Figures 3, 4 and 5 in the case where the front slope of the voltage signal is steeper than the rear slope;
• Figures 9, 10 and 1 1 are three views respectively corresponding to Figures 3, 4 and 5 in the case where the rear slope of the voltage signal is greater than the front slope.
• FIG. 1 illustrates the general arrangement of an installation, in which the tank is designated by the general reference 2 and contains an electrolytic bath 3 constituted by an aqueous solution of alkali metal hydroxide, such as potassium or sodium, and d 'an oxyacid salt of an alkali metal.
• an electrolytic bath 3 constituted by an aqueous solution of alkali metal hydroxide, such as potassium or sodium, and d 'an oxyacid salt of an alkali metal.
• a counter electrode (cathode) 4 and an "anode” 5 which is constituted by the part to be coated by transformation of the metal itself, this part being made of a metal or metal alloy having semiconductor properties.
• a current supply unit 6 a voltage generator 7 and a microcomputer 8 controlling and controlling the variable parameters according to the sequences of the process.
• FIG. 2 represents, in more detail, the generator 7.
• the power supply is carried out on the left-hand side of FIG. 2, at the location designated by the reference 9.
• This generator comprises a module 10 for transforming the 50 sinusoidal alternating signal into triangular or trapezoidal signal.
• the module 1 2 is intended to make modifications to the slope and the form factor of the voltage signal.
• the module 1 3 controls the variation of the frequency in different types of cycles, for example from 70 to 400 Hz.
• the module 14 connected to the microcomputer 8 manages the electrical energy as a function of the configured energy and of the energy actually used.
• the output signal is designated by the reference 1 5. It is possible to have at the output an isolation transformer, not shown with capacitor in series in the primary or secondary to filter the DC component, in order to avoid saturation of the magnetic circuit, while inserting optimal safety of use for electrical protection, with connection of one of the poles to earth.
• FIGS. 3 to 1 1 clearly show the consequences of the variation of the front and rear slope of the voltage signal, in particular on the electrical power, and on the distribution of the positive and negative phases thereof. It is remarkable to note that the adjustment of the power is easily achieved by varying the front and rear slopes of the voltage signal.
• the invention brings a great improvement to the existing technique by providing a very economical implementation method making it possible to produce a ceramic deposit of uniform thickness, and of excellent quality, on metal parts , even of large area.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Coating By Spraying Or Casting (AREA)
• Chemical Treatment Of Metals (AREA)
• Application Of Or Painting With Fluid Materials (AREA)

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Publications (2)

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• 2000
  • 2000-04-26 FR FR0005321A patent/FR2808291B1/fr not_active Expired - Fee Related
• 2001
  • 2001-04-25 RU RU2002128612/02A patent/RU2268325C2/ru not_active IP Right Cessation
  • 2001-04-25 BR BR0110339-3A patent/BR0110339A/pt not_active IP Right Cessation
  • 2001-04-25 KR KR1020027014269A patent/KR100868547B1/ko not_active IP Right Cessation
  • 2001-04-25 IL IL15230701A patent/IL152307A0/xx active IP Right Grant
  • 2001-04-25 US US10/018,709 patent/US6808613B2/en not_active Expired - Fee Related
  • 2001-04-25 WO PCT/FR2001/001269 patent/WO2001081658A1/fr active IP Right Grant
  • 2001-04-25 EP EP01929704A patent/EP1276920B1/de not_active Expired - Lifetime
  • 2001-04-25 JP JP2001578724A patent/JP2003531302A/ja active Pending
  • 2001-04-25 AT AT01929704T patent/ATE517200T1/de not_active IP Right Cessation
  • 2001-04-25 AU AU56407/01A patent/AU775598B2/en not_active Ceased
  • 2001-04-25 CN CNB01808656XA patent/CN100482867C/zh not_active Expired - Fee Related
  • 2001-04-25 CA CA002405485A patent/CA2405485A1/fr not_active Abandoned
• 2002
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