EP1276920A1

EP1276920A1 - Oxidising electrolytic method for obtaining a ceramic coating at the surface of a metal

Info

Publication number: EP1276920A1
Application number: EP01929704A
Authority: EP
Inventors: Jacques Beauvir
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-04-26
Filing date: 2001-04-25
Publication date: 2003-01-22
Anticipated expiration: 2021-04-25
Also published as: US6808613B2; IL152307A; CN1426496A; BR0110339A; US20020112962A1; KR100868547B1; WO2001081658A1; CA2405485A1; FR2808291B1; KR20030011316A; AU775598B2; IL152307A0; CN100482867C; JP2003531302A; EP1276920B1; FR2808291A1; RU2268325C2; AU5640701A; ATE517200T1

Abstract

Process for obtaining a ceramic coating on the surface of a metal having semiconducting properties, such as aluminium, titanium, magnesium, hafnium, zirconium and their alloys, by a physico-chemical transformation reaction of the treated metal. This process consists in immersing the metal workpiece (5) to be coated in an electrolytic bath (3) composed of an aqueous solution of an alkali metal hydroxide, such as potassium hydroxide or sodium hydroxide, and of an oxyacid salt of an alkali metal, the metal workpiece forming one of the electrodes, and in applying a signal voltage of overall triangular waveform to the electrodes, that is to say a signal having at least a rising slope and a falling slope, with a form factor that can vary during the process, generating a current which is controlled in its intensity, its waveform and its ratio of positive current to negative current.

Description

ELECTROLYTIC OXIDATION PROCESS FOR OBTAINING A CERAMIC COATING ON THE SURFACE OF A METAL

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

(in particular for moving parts, with high mechanical deformation loads and constraints), etc.

However, since these metals do not naturally exhibit suitable tribological and mechanical properties (hardness, coefficient of friction, resistance to abrasion, etc.), coatings are often used to improve the limiting characteristics of these materials.

They often make it possible to satisfy additional requirements, such as resistance to corrosion in an acid and / or alkaline medium, the possibility of temporarily withstanding high temperatures, or obtaining dielectric properties.

Several electrolytic coating methods are currently used. The most used, in protection against wear and / or corrosion, is hard anodizing. However, it presents limits of use which are reached fairly quickly. This anodizing process makes it possible to form protective oxide layers on the aluminum parts. However, the coatings produced by this method are limited in thickness and only have an average hardness (maximum of around 500 Hv).

A certain number of other techniques have been developed to produce more efficient coatings, in particular ceramic, in order to meet more severe requirements of use: plasma projection by arc discharge, flame projection or techniques of vacuum deposits.

However, the disadvantage is that in order to obtain good adhesion of the coating, these techniques require a high substrate temperature, as well as preliminary methods for preparing the surface. Furthermore, these methods cannot compete with traditional anodization in terms of coating uniformity and / or in production costs.

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. By increasing the anode potential to more than 200 V, 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,

Gunterschulze and Betz were the first to evoke in 1932, the process of anodic deposit by sparkling called 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.

The earliest practical applications of ASD was their use as an anti-corrosion coating on magnesium alloys, which dates from 1 936. and were included in a military specification in 1 963. Since then, major research efforts were led by Gruss, McNeill and colleagues at Frankford Arsenal in Philadelphia, and Brown, Wirtz, Kriven and collaborators at the University of I ^* Illinois at Urbana-Champaign. At the same time, research was carried out in East Germany, mainly by Krysmann, Kurze, Dittrich and collaborators. The process in German is called "Anodic oxidation by spark discharges", the acronym of which is ANOF. Reports of this work in international literature refer to patents in the German language.

It is clear that this research brings significant progress, however it remains summary and the compounds of the coating formed, have not been clearly identified (only the constituents α-AI ₂ 0 _3. And γ-AI ₂ O ₃ ( OH) (Bohemian) were identified by X-ray).

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.

Several modifications have been made to this process called "silicodizing", 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 drawback for such methods is the use, in terms of signal form, of a direct current (DC) of a few mA, at voltages less than 500 V. This results in stopping of the sparking after a few minutes (the largest amount of deposit is formed in the first few minutes). Such operating conditions do not allow produce only very small coating thicknesses and thus limit its physical properties.

Other methods describe the use, in electrolytic baths of variable compositions, of alternating voltages which can exceed 1000 V, associated with a direct or alternating current.

It should also be noted that the use in certain cases of high voltages with high current densities makes these processes difficult to use industrially.

On the other hand, the excellent adhesion to the substrate of this type of coating, the physical and tribological characteristics (high hardness, resistance: thermal, electrical, abrasion, corrosion, etc.), the wide variety of alumino-silicate mixtures for coating purposes, and the fact that the coating can be carried out inside narrow surfaces and of complex geometry, are among the many advantages of this process. We describe below a different type of micro-arc process capable of following, imposing and controlling the evolution of a ceramic coating process in its different phases. 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.).

Three main process phases can be identified, according to the descriptions found in the numerous scientific works and other publications on the subject generally called MICRO ARC OXYDATION and described previously. The parts to be treated and the electrodes immersed in the electrolyte constitute a dipole, to which the electrical energy supplied by a generator is applied.

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. A wide variety of solutions are described in the numerous publications on the subject.

In 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. In the second phase, a breakdown of 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.

In the third phase, 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 γ-AI ₂ O ₃ type elements on X-rays. (bohemian) and -AI ₂ 0 ₃ corendum.

When using a generator delivering continuous or alternative electrical energy with fixed parameters, there is a drop in intensity during the process with differentiation of the voltage-current curves recorded on an oscilloscope. This is the result of the process itself regardless of any intervention. In this case, one of the determining factors is the dielectric property and the thickness of the ceramic layer formed.

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. There are also described 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. In addition, 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.

To this end, 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,

- And applying to 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.

It 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. Thus, 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.

According to an embodiment of this method, it 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.

According to another of these characteristics, 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,

- a module for modifying the slope and the signal form factor,

- a module for varying the frequency in different types of cycles, and

- an electrical energy management module depending on the energy set and the energy used.

Advantageously, 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.

According to another characteristic of the invention, this generator is controlled by a processor of the PC type making it possible to manage the various parameters as the process proceeds.

The combined variations in frequency, voltage, signal factor and current factor play an essential role in the method according to the invention.

The frequency sweep combined with variations in the forward slope of our triangular signal makes it possible to excite inactive interior zones in turn and exterior zones with large vectors of natural excitation.

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;

- significant reduction in the roughness of the surface of the layer;

- improvement of the adhesion of the coating to the substrate;

- progressive growth of the oxide layer; - rapid growth of the α-AI ₂ 0 ₃ type ceramic layer

(corendum);

- makes it possible to effectively control and reduce the residual porosity rate inherent in the fundamental micro-arc process itself, especially with certain alloys; - improvement of treatment on categories of highly alloyed aluminum;

- obtaining a thicker and denser layer in a time reduced by more than half (50%);

- allows relaunching the micro-arcs in the advanced phase of the treatment to obtain greater thicknesses (depending on the alloys) from 40 μm to 300 μm without destroying the existing layer;

- reduction of energy consumption by more than 50%;

- reduction by a factor of 35% of the calorific power emitted; - obtaining better homogeneity outside the current leakage lines due to the angles and contours of the parts to be treated; - possibility of vacuum impregnation, by dipping or spraying or other, of elastomeric polymer resin or other organic compound.

For an identical compared capacity materialized in dm ² of treated surface, it is possible to use with this new process a section of power cable reduced by 50%.

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.

It follows a strong economy and a substantial reduction in the cost of treatment with increased quality. When you know that one of the main industrial pitfalls lies in this high consumption of electrical energy, this process already offers an important advantage in this area.

Seen in another aspect, 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.

The invention is explained below with reference to the attached schematic drawing showing an embodiment of the device for implementing the method as well as some curves illustrating the method:

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. Inside the electrolyte sink 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. In FIG. 1, there is also shown 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.

The curves illustrated in 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. As is apparent from the above, 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.

Claims

1. Electrolytic process of oxidation by plasma micro-arc in order to obtain a ceramic coating on the surface of a metal, having properties of semiconductor, such as aluminum, titanium, magnesium, hafnium, zirconium and their alloys, by physicochemical reaction of transformation of the treated metal, characterized in that it consists in:

- immerse the metal part (5) to be coated in an electrolytic bath (3) composed of an aqueous solution of alkali metal hydroxide, such as potassium or sodium, and an oxyacid salt of an alkali metal, the part metallic forming one of the electrodes,

2. Method according to claim 1, characterized in that the front and rear slopes of the voltage signal are substantially symmetrical.

3. Method according to claim 1, characterized in that the front and rear slopes of the voltage signal are asymmetrical and of variable angles during electrolysis.

4. Method according to one of claims 1 to 3, characterized in that it consists in changing the value of the triangular voltage between 300 and 600 V rms, during the process.

5. Method according to one of claims 1 to 4, characterized in that it consists in varying the frequency of the triangular signal between 100 and 400 Hz, during the process.

6. Method according to one of claims 1 to 5, characterized in that it consists in changing or fixing the value of the current independently of the value of the voltage.

7. Method according to all of claims 1 to 6, characterized in that it consists in varying independently during the process the different parameters: form factor, potential value, frequency, current value.

8. Method according to all of claims 1 to 6, characterized in that it consists in varying simultaneously during the process at least some of the different parameters: form factor, potential value, frequency, current value, UA / IC ratio.

9. Method according to one of claims 1 to 8, characterized in that it consists in separately controlling in its forms and values the electrical energy Ul in positive phase and / or in negative phase.

10. Electronic generator of current source type for implementing the method according to one of claims 1 to 9, comprising a connection block (9) to a single-phase or three-phase power supply from the sector and a connection block to the electrolysis tank, characterized in that it comprises:

- a module (10) for transforming the alternating sinusoidal signal supplied by the network into a trapezoidal or sawtooth signal,

- a module (1 2) for modifying the slope and the signal form factor,

- a module (1 3) for varying the frequency in different types of cycles, and

- A module (14) for managing the electrical energy as a function of the configured energy and of the energy used.

1 1. Generator according to claim 10, characterized in that it comprises, at 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 in use for electrical protection, with one of the poles connected to earth.

1 2. Generator according to one of claims 1 0 and 1 1, characterized in that it is controlled by a processor (8) of the PC type making it possible to manage the different parameters as the process proceeds.