SK121099A3 - Process and apparatus for coating metals - Google Patents

Abstract

The invention provides a process for forming a ceramic coating on a valve metal selected from the group consisting of aluminium, zirconium, titanium, hafnium and alloys of these metals. The process includes immersing the metal as an electrode in an electrolytic bath comprising an aqueous solution of an alkali metal hydroxide, providing an opposite electrode immersed in or containing the electrolyte liquid, passing a modified shaped-wave alternate electric current from a high voltage source of at least 700 V through a surface of the metal to be coated and the opposite electrode, wherein the modified shaped-wave electric current rises from zero to its maximum height and falls to below 40 % of its maximum height within less than a quarter of a full alternating cycle, thereby causing dielectric breakdown, heating, melting, and thermal compacting of a hydroxide film formed on the surface of the metal to form and weld a ceramic coating to the metal, and changing the composition of the electrolyte while the ceramic coating is being formed, the change being effected by adding an oxyacid salt of an alkali metal. The invention also provides apparatus for carrying out this method.

Description

Technical field

The present invention relates to a method of forming a ceramic coating on valve metals, to articles coated therewith, and to an apparatus for carrying out the method.

BACKGROUND OF THE INVENTION

The metals for producing the valves provide an electrolytic rectification phenomenon, and the present invention therefore relates to the coating of aluminum, zirconium, titanium, hafnium and alloys of these metals and apparatus for performing the method.

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In particular, the present invention relates to an electrolytic process that utilizes a high-voltage AC waveform to achieve coating even with a thick layer during the melting process, said layer being achieved in a short time by varying the electrolytic composition during an ongoing process.

Aluminum, titanium and their alloys have advantageous mechanical strength / weight ratios that predetermine these metals for many applications, for example in the aerospace industry or in fast moving components in internal combustion engines. Unfortunately, these metals do not have particularly good wear resistance, and so they are often coated with a coating to improve wear and corrosion resistance. The coatings applied are promising to meet other planned requirements, such as resistance to chemicals (especially acids and bases), the possibility of exposure to higher temperatures, reduced friction and ensuring dielectric properties. While inexpensive, widely used anodic processes achieve some of these objectives for routine work, ceramic coatings are required for high stress operations.

A number of electrolytic coating processes for these metals are known using a direct current or voltage of less than 600 V. These processes are described, for example, in U.S. Pat. U.S. Patents 3,956,080; No. 4,082,626 and 4,659,440. These documents and most recent discoveries describe processes in which a ceramic film is formed using an anode spark discharge technique, which have good results in corrosion resistance and coating adhesion, but have two important drawbacks: low hardness of the coating layer and slow formation of the coating.

U. U.S. Pat. No. 5,147,515 to Haganata et al. Discloses the use of a dispersion in an electrolytic bath containing an aqueous solution of a water-soluble or colloidal silicate or oxoacid salts in which the ceramic particles are dispersed. During the formation of the layer, the voltage rises from 50-200 V and may ultimately exceed 1000 V. With respect to the waveform, said patent claims that the output from the source may be a direct current with any waveform, but pulse ( rectangular waveform), sawtooth waveform, or DC waveform. Such a claim does not include the recognition that the shape of sharply pointed wines largely contributes to the formation of a dense, hard layer.

The layer formation rate given in the eight examples of this patent can be calculated as follows:

Example number Layer thickness (in micrometers) in Coating time (in minutes) Layer formation rate (Micron / min) 1 35 20 1.75 2 31 20 1.55 3 28 30 0.93 4 27 20 1.35 5 36 30 1.20 6 14 30 0.47 7 15 30 0.50 8 28 30 0.93

This slow layer formation rate cannot be compared well with the layer formation rate of the present invention.

Also in U.S. Pat. No. 5,147,515, no indication has been given as to how thick coating layers can be produced according to the method of this patent, i. in the range of 300-700 micrometers.

Recently, a coating process known as the Kepla-Coat Process has been developed which is based on plasmachemic anodic oxidation. The cathode is a surface layer of organic electrolyte above which is suspended the part to be coated forming the anode. A plasma is formed which causes a ceramic coating of the anode and heats the workpiece. Due to the formation of a thin oxidation layer on the anode, this does not produce a layer thicker than 10 micrometers with an increase in growth in 8-10 minutes. Since the workpiece is not immersed in the liquid when heated, irregular or thin bodies are usually deformed. Another disadvantage of the Kepla-Coat Process in the future will be the high level of electrolyte vapors causing environmental problems.

SUMMARY OF THE INVENTION

Thus, it is one object of the present invention to overcome the disadvantages of the prior art ceramic coating methods and to provide such a method which provides a hard coating with good adhesion and minimal porosity.

Another object of the present invention is to provide a method that provides a coating of 300 or more microns in thickness over a reasonable period of time.

The present invention achieves the above and other objects, providing a method for forming a ceramic coating on metal for valves, wherein the metals are selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, said method comprising immersing the above mentioned a metal electrode in an electrolytic bath comprising an aqueous solution of an alkali metal hydroxide forming an opposing electrode immersed in or containing an electrolytic fluid, the surface of said metal to be coated with a ceramic coating and said opposing electrode passing a waveform of alternating current from the a high-voltage source of at least 700 V, characterized in that said shape-altered wave of electric current rises from zero to its maximum height over a period of time which is less than a quarter of the total alternating cycle, thereby causing dielectric breakdown, heating, melting and thermal compacting of a thin hydroxide layer formed on the surface of said metal to form and weld a ceramic coating on said metal, changing the composition of said electrolyte while forming said ceramic coating, said changes being affected by the addition of an oxyacid salt with alkali metal.

Yet another object of the present invention is to provide an apparatus for carrying out the above-described process at an affordable price. The invention thus provides a device for coating articles with a ceramic coating in one step made of metal for valves selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals. The apparatus comprises an electrolytic bath containing an aqueous solution of an alkali metal hydroxide, a submerged electrode, or an electrolyte fluid, another electrode formed by at least one of the objects to be coated, and a means for securing said article in the electrolyte; a voltage of at least 700 V, the AC waveform influencing means by which the shaped electric wave rises from zero to its maximum height and falls back below 40% of its maximum within less than a quarter of the entire AC cycle, comprising connecting elements for closing the electrochemical means for adding substances to said bath while the apparatus is in operation and controlling the supply of the alkali metal salt of the oxo acid.

A distinguishing feature of the process of the present invention is its applicability for producing hard coatings up to 300 microns thick within an acceptable time period of about 90 minutes. This high coating rate is achieved by changing the electrolyte composition during the coating process. The quality of the coating is not compromised by the rapid formation of a thick layer as well as by the modified waveform which achieves momentary melting of the layer near the metal workpiece, even after the specified coating layer thickness has been formed.

The invention will now be described with reference to certain preferred embodiments with reference to the following figures, in order to be more readily understood.

Looking at the drawings in detail, it should be pointed out that the details given are merely exemplary and are merely illustrative of the discussion of preferred embodiments of the present invention and are provided in order to provide the most useful and easy to understand description of the principles and conceptual aspects of the invention. . In view of this, there is no attempt to show the constructional details of the invention in more specific views than is necessary for a basic understanding of the invention, and the description of the figures will be apparent to those skilled in the art what a particular form of various parts of the invention may be in practice.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a preferred type of shaped wave pulse

Fig. 2 illustrates the relationship between coating thickness and electrolysis time

Fig. 3 is a schematic view of a device for coating objects in one step

Fig. 4 is a schematic view of a device for coating objects in a plurality of successive steps.

The process according to the invention will now be explained. The process is used to form a ceramic coating on aluminum, zirconium, titanium and hafnium. The use of the process is also suitable for alloys of these metals, provided that the sum of all these alloy elements does not constitute more than about 20% of the total. The process parameters can be optimized with respect to the particular metals to be coated and to the specific properties of the coating under consideration which are important for specific wear.

The metal workpiece to be coated is connected and immersed as an electrode in the electrolytic bath.

For the coating of aluminum, the bath comprises water and an alkali metal hydroxide solution. In a bathing embodiment where it is desired to optimize the coating to ensure maximum adhesion between the metal and its coating, the electrolyte consists essentially of an aqueous compound containing between 0.5 to 2 g / liter of sodium hydroxide or potassium hydroxide. When it is necessary to achieve improved special coating properties, for example low friction, fine particles of various substances are added. In embodiments with particle addition, the electrolyte is agitated to keep the particles in suspension. Similarly, color coatings are formed by adding fine pigment particles.

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A preferred counter electrode for this process is a stainless steel bath containing liquid electrolyte. Where a non-conductive electrolyte container is preferred, e.g. For safety reasons, an electrode of iron, nickel or stainless steel is inserted into the bath in the usual manner.

The AC waveform from a high voltage source of at least 700 V, typically 800 V for aluminum workpieces, then passes between the metal workpiece and the second electrode. The result is an electrical breakdown, heating, melting and thermal compaction of a thin layer of hydroxide formed on the metal surface that forms and welds the ceramic coating. Arc microwelding is visible during coating. A suitable and reasonably expensive way to achieve the desired shaped wave pulse is to use a capacitor bank connected in series between a high voltage source having a voltage of 800 V to 1000 V and said metal workpiece to be coated.

Fig. 1 shows a view of a preferred waveform. The effect of using alternating current is combined with high voltage to extend the life of the microcircuit, which causes intense, local, temporary heating, and as a result of welding and melting the coating formed on the submerged metal workpiece. The anodization is effective during the first positive half cycle, during which the workpiece is a positive electrode. Then, the already formed dielectric coating weakens the dielectric, whereby the microspheres begin to be generated. The lifetime of the microwaves extends almost to the end of the first half cycle, when the workpiece becomes a negative electrode.

Fig. 2 shows the relationship of time and thickness of the coating, where the electrolyte composition is kept constant as shown by curves 1 to 5. Curve 1 refers to a process wherein the electrolyte is pure potassium hydroxide. Curves 2 to 5 refer to the procedure where an increasing concentration of sodium tetrasilicate was used.

Curve 6 refers to the process of the invention. It has been found that much faster coating is made possible by changing the electrolyte composition during t

the time at which the ceramic coating is formed. An effective composition change involves adding salts containing an alkali metal cation and an amorphous oxo acid anion to the electrolyte. Said amorphous element is selected from the group consisting of B, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, P, Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn and Fe, said salt being added at a concentration of between 2 and 200g / liter of solution. The preferred amorphous element is silicon and the preferred addition salt is sodium tetrasilicate.

As shown in the graph, the change in electrolyte composition during the operation makes it possible to form a coating of 200 microns in about 50 minutes, with a thin film formation rate of 4 microns per minute. Tests have shown that this rapid layer formation is achieved without compromising the quality of adhesion of the layer to the metal workpiece.

Obviously, as soon as salt is added to the electrolyte as it is stirred, it is expedient to reduce the salt concentration again by adding a considerable amount of new electrolyte to coat other metal objects in one step. However, this problem is solved by the apparatus described below with reference to FIG. Third

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It has also been found that it is possible to form pores without pore by gradually reducing the flow rate when the layer reaches almost the desired thickness. In practice, this is affected by the gradual reduction of the capacitance used to form the waveform, thus reducing the current until the end of the process.

As can be seen from the description above, the term "altered" as used herein refers to the fact that the wave form is other than the standard sinusoid form normally associated with an AC wave and is instead altered, for example, as illustrated in FIG. 1 to optimize the coating effect.

Reference is now made to Table 1, which lists the different types of coating for different requirements. Examples are given for aluminum alloys to be ceramic coated to meet different structural requirements. The techniques described above were used in Examples 3 and 4.

notes Intentionally high porosity Strong adhesion It is subjected to machining before use Porosity porosity / cm ² 50-300 <5 in in Not applicable 5- 10 Hardness (according to Vickers) kgf / mm ² ABOUT about 00 1 ABOUT ABOUT oo 1,000 - 2,400 1,000 - 2,300 1,000- 2,300 1,000 - 2,300 1 800 - 2 800 Thickness in microns 5-30 10-50 30-150 10-250 50 -300 40-100 Functional requirements Substrate for enamel coating or a Teflon layer Decorative coating Corrosion protection Electrical insulation Shields protecting the spacecraft during heating caused by entering the atmosphere resistance to wear Example number rs m about- • n \ABOUT

Aluminum alloy known as alloy with alloy designation 2014 for its strength to weight ratio finds widespread use in aircraft construction. This alloy was therefore used for the test coating. Table 2 shows the characteristics of the coating obtained and the results obtained.

Table 2

entry units Value Metal workpiece material dural Waveform production method capacitor Transformed voltage output IN 800 Normal density: A / dm ² anode 6.0 cathode 6.3 Electrolyte composition gram / liter of water First bath: potassium hydroxide 0.5 Second bath: potassium hydroxide 0.5 sodium tetrasilicate 4.0 Third bath: potassium hydroxide T 1 0.5 sodium tetrasilicate 11.0 Coating time minute in the first tub 10 in the second tub 10 in the third tub 20 Total coating thickness micron 100 Average documented ratio Micron / minute 2.5 The thickness of the inner layer completely melted during coating micron 65 Adhesion of substrate MPa 380 microhardness Vickers kgfi'mm ² 2790 Average layer composition: % Corundum (corrundum) 62 alumina 8 Alumosilicate (alumosilicate) 30 Porosity of the coating Number of pores / cm ² 4-6 Pore diameter micron 8-11

The invention also provides ceramic coated metal articles made according to the described process. One example of such a product is an aluminum piston alloy for an internal combustion engine. A second example is an aluminum engine block for an internal combustion engine designed for minimal lubrication operation. A third such example is the spacecraft shield plates aimed at survival at the entrance to the spacecraft

into the atmosphere. A fourth example is an electrical insulation also serving as a thermal tank for an electronic panel.

Fig. 3 describes an apparatus 10 for coating articles 12 in one step (first electrode) made of valve metals selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals. Apparatus 10 has a 40 liter electrolyte bath 14 comprising an electrolyte fluid 16 consisting of water and an alkali metal hydroxide solution. The bath 14 is made of stainless steel and forms a second electrode. Agitation means 15 is provided for agitating the electrolyte.

The first electrode comprises at least one of the articles 12 to be coated and a conduction means 18 for fastening the article in the electrolytic fluid 16.

A source of alternating current of at least 700 V is a transformer 20, increasing the voltage to 40,000 V-amp, designed to draw up to 800, 900 or 1000 V.

The capacitor bank 22 has a total capacitance of 375 gF and is composed of capacitors with nominal capacitances of 25, 50, 100 and 200 gF. Alternatively, such means may be a current rectifier and reverse circuit (not shown) or other means of the type shown in The Standard Handbook for Electrical Engineers (Fink and Beaty), 12th. Ed., P. 22-96 22-97

The connecting elements 24 also ensure the completeness of the electrochemical circuit.

The operating control panel 26 is to the left of the bath 14. More recently, it is surrounded by a security door 28. The opening of the security door interrupts the electricity.

The salt dispensing hopper 30 has a dispensing valve 32 operating by a solenoid and is part of the means for adding salt 34 to the tub 14 while the apparatus 10 is operating. The hopper 30 comprises a supply of salt 34 which comprises an alkali metal cation and an oxo acid anion of the amorphous element.

A suitable salt 34 is sodium tetrasilicate.

Fig. 4 shows an apparatus 36 for ceramic coating objects 12 in a number of subsequent steps. The first electrolyte bath 38 comprises an electrolyte fluid 16 which is water and an alkali metal hydroxide solution. The second electrolyte bath 40 comprises an electrolyte fluid 42 which is water, an alkali metal hydroxide solution and a low concentration salt 34. The third electrolyte bath 44 comprises an electrolyte fluid 46 which is water, an alkali metal hydroxide solution, and a salt with a higher concentration than the electrolyte 42.

For simplicity, one stainless steel container 48 provided with two vertical electrode dividers 50 forming the tubs 38. 40. 44. The second electrode consists of at least one object 12 to be coated with the conduction means 18 by means of which the object 12 is gradually suspended in the electrolytic fluids 16. 42, 46.

Manual or automatic handling means 52 allow the object 12 to be moved from the first tub 38 to the second 40 and from there to the third tub 44.

In the device 36, the electrolyte remains substantially unchanged in each tub during operation and can thus be reused. Using several electrolytes, each with a different composition, it achieves a coating speed of about 2.5-4 microns per minute.

The electrical components used are the same as described above with reference to FIG. Third

It is understood that the invention is not limited to the details of the above-mentioned embodiments to those skilled in the art, and that it may be illustrated by other specific forms in the spirit of its essential properties. Thus, the present embodiments are intended in all respects as illustrative and not limiting. The scope of the invention is set forth in the appended claims rather than the foregoing description, and all changes that are contemplated and equally within the scope of the claims are intended to be included in the invention.

Claims (12)

PATENT CLAIMS A method of forming a ceramic metal coating for valves selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, characterized in that it comprises: immersing said metal as an electrode in an electrolytic bath containing an aqueous solution of an alkali metal hydroxide, providing a counter electrode immersed in the electrolytic fluid, or containing the electrolytic fluid, passing an alternating current waveform from a high voltage source of at least 700 V surface of said metal to be coated and said counter electrode, wherein said waveform of electric current rises from zero to its maximum height and falls below 40% of its maximum height in less than a quarter of the entire alternating cycle, causing dielectric breakthrough, heating, melting and thermal compacting of the thin a hydroxide layer formed on the surface of said metal to form and weld a ceramic coating on said metal, and altering the composition of said electrolyte while forming said ceramic coating, said changes being affected by by extracting the alkali metal salt of the oxo acid. Method according to claim 1, characterized in that said altered shaped electric current wave is obtained by using a capacitor bank connected in series between said high voltage source and said metal. Process according to claim 1 or 2, characterized in that said added salt is sodium tetrasilicate. Method according to any one of the preceding claims, optimized for ensuring maximum adhesion between said metal and said coating, characterized in that said electrolyte consists essentially of an aqueous solution containing between 0.5-2 g / liter of sodium hydroxide. The method according to any one of the preceding claims, optimized to ensure maximum adhesion between said metal and said coating, characterized in that said electrolyte consists essentially of an aqueous solution containing between 0.5-2 g / liter of potassium hydroxide. Method according to any one of the preceding claims, characterized in that said amorphous element is selected from the group consisting of B, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, P, Ti, Zr, V. , Nb, Ta, Cr, Mo, W, Mn and Fe, said salt being added at a concentration between 2 and 200 grams per liter of solution. Method according to any one of the preceding claims, characterized in that fine pigment particles are added to form a coating with the desired color. Method according to any one of the preceding claims, adapted to form a pore-free coating, characterized in that the incoming current is progressively reduced until the coating is stopped. A ceramic coated metal object formed by the method of any preceding claim. 10. Apparatus for ceramic coating articles in one step, made of metal for valves selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, characterized in that it consists of: an electrolytic bath containing an aqueous solution of an alkali metal hydroxide, an electrode immersed in a fluid or comprising an electrolytic fluid, another electrode comprising at least one of said objects to be coated, and means for suspending said object in said electrolyte, an alternating current source of high voltage a value of at least 700 V, by means of an AC waveform, by which the affected waveform current rises from zero to its maximum and drops back below 40% of its maximum height in less than a quarter of the total AC cycle, by means of electrochemical closure and means for adding a controlled dose of the alkali metal salt of the oxo acid to said bath while the device is in operation. The apparatus of claim 10, wherein the high voltage source is a transformer increasing voltage up to 1000 volts and by said means for influencing the shape of said alternating current waveform a capacitor bank is connected in series between said voltage source and said valve metal. . 12. Apparatus for ceramic coating articles in a plurality of the following steps, made of metal for valves selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, characterized in that it consists of: a first electrolytic bath containing an aqueous alkali metal hydroxide solution, a second electrolytic bath containing an aqueous alkali metal hydroxide solution and a low concentration of the alkali metal oxoacid salt a third electrolytic bath containing an aqueous alkali metal hydroxide solution and a higher concentration of the alkali metal oxoacid salt in said second alkali metal salt electrodes immersed in or containing electrolytic fluids, electrodes comprising at least one of said articles to be coated, and means for suspending said article in said electrolytes, means for transferring said articles from said first tub to said second tub and then to said third tub by means of an AC electric potential source with a voltage of at least 700 V, by means of an AC waveform, by means of which the modified current waveform rises from nu They are reduced to their maximum and fall back below 40% of their maximum height in less than a quarter of the entire alternating cycle and the fasteners to close the electrochemical circuit in each bath. Apparatus according to claim 12, wherein the high voltage source is a transformer increasing voltage up to 1000 volts and said means for influencing said AC waveform is a capacitor bank connected in series between said high voltage source and said metal for a voltage source. valves.