CZ318599A3 - Process and apparatus for coating metals - Google Patents

Abstract

A method of forming a ceramic coating on metals for valves, selected from the group consisting of aluminum, zirconium, titanium, hafiiia and the alloys of these metals are immersed in the metal electrodes into an electrolytic tub containing an aqueous solution alkali metal hydroxide and feed alternating electric current with modified waveform from the source high voltage of at least 700 V, alternating the electric current with the modified wave increases from zero to its maximum height and falls below 40% of its maximum a height of at least a quarter of the entire AC cycle, thereby causes dielectric breakdown, heating, melting and thermal compacting a thin hydroxide layer on the metal surface wherein the composition changes during the formation of the ceramic coating electrolyte addition of an alkali metal salt of an oxo acid. Apparatus consisting of an appropriate electrolyzer tub the electrode is provided with an electrical voltage source a means for controlling current flow.

Description

METHOD AND APPARATUS FOR COATING METALS

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 performing the method.

BACKGROUND OF THE INVENTION

Metals for the manufacture of valves exhibit the phenomenon of electrolytic rectification, and the present invention therefore relates to the filament of aluminum, zirconium, titanium, hafnium and alloys of these metals, and apparatus for carrying out the process.

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

Aluminum, titanium and their alloys have advantageous mechanical strength / weight ratios that predetermine these metals for many applications, for example in aerospace or 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 properties and corrosion resistance. Applied coatings 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 the inexpensive, widely used anodic process achieves 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 direct current and / or a voltage of less than 600 V. These processes are described, for example, in U.S. Pat. U.S. Patents 3,956,080; U.S. Pat. Nos. 4,082,626 and 4,659,440. These documents and most recent discoveries describe processes in which a ceramic film is produced using an anode spark discharge technique, which achieve good results in corrosion resistance and coating adhesion, but have two important shortcomings: low hardness of the coating layer and slow formation of the coating.

• ·

U.S. Pat. U.S. Patent 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 oxo acid salt in which ceramic particles are dispersed. During layer build-up, the voltage rises from 50-200 V and may ultimately exceed 1000 V. With respect to the waveform, the patent claims that the output from the source may be direct current with any waveform but pulse (rectangular) shapes are preferred. waveform), sawtooth waveform, or DC halfwave. Such a claim does not include the recognition that the shape of sharply pointed waves largely contributes to the formation of a dense, hard layer.

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

Example number Layer thickness (in micrometers) in Coverage time (in minutes) Speed of creation layers (micrometer / minute) 1 35 20 May 1.75 2 31 20 May 1.55 3 28 30 0.93 4 27 Mar: 20 May 1.35 5 36 30 1.20 6 14 30 0.47 7 15 Dec 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 was given as to how thick coating layers could be produced according to the method of this patent, i.e. in the range of 300-700 microns.

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 ceramic anode coating and heats the workpiece. Due to the formation of a thin oxidation layer on the anode, a layer thicker than 10 microns is not formed in this way, 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 fumes causing environmental problems.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome the disadvantages of the prior art ceramic coating methods and to provide such a method that provides a hard coating with good adhesion and minimal porosity.

It is another object of the present invention to provide a method that provides a coating of 300 microns or more within a reasonable period of time.

The present invention achieves the aforementioned 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 thereof, said method comprising immersing the aforementioned a metal electrode in an electrolytic bath comprising an aqueous solution of an alkali metal hydroxide which forms an opposing electrode immersed in or containing an electrolytic fluid, the surface of said metal to be coated with a ceramic coating and said opposed electrode passing a waveform of alternating electric current a high voltage source having a value of at least 700 V, characterized in that said waveform of electric current rises from zero to its maximum height during a time which is less than a quarter of the entire alternating cycle, The dielectric breakdown, heating, melting and thermal compacting of the thin hydroxide layer formed on the surface of said metal and thereby form and weld a ceramic coating on said metal, changing the composition of said electrolyte while forming said ceramic coating, said changes being influenced by the addition of an oxyacid salt with alkali metal.

• • · · · · · · · · • »• • · · · · · · · · ¹ • ·« ^ ·· ·

Yet another object of the present invention is to provide an apparatus for carrying out the above described process at an affordable price. Accordingly, the invention provides a single step ceramic article coating device made of metal for valves selected from the group consisting of aluminum, zirconium, titanium, hafnium, and alloys thereof. The apparatus comprises an electrolytic bath containing an aqueous solution of an alkali metal hydroxide, a submerged electrode or containing an electrolyte fluid, another electrode formed by at least one of the articles to be coated, and means for securing said article in the electrolyte, an AC power source from a high voltage source at least 700 V, the AC waveform influencing means by which the shaped waveform rises from zero to its maximum height and falls back below 40% of its maximum in less than a quarter of the total AC cycle, comprising connecting elements to close the electrochemical circuit means for adding substances to said bath while the apparatus is in operation and controlled supply of the alkali metal salt of oxoacid.

A distinguishing feature of the process of the present invention is its applicability for the production of hard coatings up to 300 microns thick within an acceptable time period of about 90 minutes. This high coating rate is achieved by varying the electrolyte composition during the coating process. The quality of the coating is not a compromise for the rapid formation of a thick layer, such as when the treated waveform stream achieves flash melting of the layer near the metal workpiece, even after the specified coating layer thickness has been established.

The invention will now be described with reference to certain preferred embodiments, with reference to the following figures, for clarity.

Looking at the drawings in detail, it is to be emphasized 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 a useful and easy to understand description of the principles and concepts. aspects of the invention. Accordingly, 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 the various parts of the invention may be in practice.

BRIEF DESCRIPTION OF THE DRAWINGS

Giant. 1 shows a preferred type of shaped wave pulse

Giant. 2 shows the relationship between coating thickness and electrolysis time

Giant. 3 is a schematic view of a device for coating articles 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 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 the specific application.

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

For the coating of aluminum, the bath comprises water and an alkali metal hydroxide solution. In a bath version 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 different substances are added. In embodiments with the addition of particles, the electrolyte is stirred to keep the particles in suspension. Similarly, color coatings are formed by adding fine pigment particles.

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

A shaped AC wave 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 welding is visible during coating. A suitable and reasonably expensive way to achieve the desired waveform pulsed electrical pulse is to use a capacitor bank connected in series between a high voltage source of 800 V to 1000 V and said metal workpiece to be coated.

Giant. 1 shows a view of a preferred current 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. Anodization is effective during the first positive half cycle, during which the workpiece is a positive electrode. Then, the already formed dielectric coating departs dielectrically, whereby micro-arcs are generated. The service life of the micro-arc extends almost to the end of the first half cycle, when the workpiece becomes a negative electrode.

Giant. 2 shows a view of 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 where the electrolyte is pure potassium hydroxide. Curves 2-5 refer to the procedure where increasing concentrations of sodium tetrasilicate were used.

Curve 6 refers to the process of the invention. It has been discovered that much faster after towing is made possible by changing the electrolyte composition during the time that the ceramic coating is formed. An effective composition change involves adding salts containing an alkali metal cation and an amorphous element oxoacid 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 allows a coating of 200 microns thickness 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, once the salt is added to the electrolyte while stirring, it is expedient to reduce the salt concentration again by coating one another with a single step by adding a salt. A considerable amount of new electrolyte is present. However, this problem is solved by the apparatus described below with reference to FIG. 3.

It has also been found that it is possible to form pores without pores by gradually reducing the flow of current 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 attenuating the current until the end of the process.

As is evident 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 with different requirements. Examples are given for aluminum alloys to be ceramic coated in order to meet various structural requirements. The techniques described above were used in Examples 3 and 4.

o · o · o ··· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·

Example Functional Requirements Thickness v Vickers Hardness Porosity Notes micrometer_k_ and / mic_pore / cm ^ _

Primer for paint enamel 5-30 1800-2800 50-300 Intentionally high porosity ω

o>

ro _c> zz

Q.

-ro _c ω

ID T- -V V V • <D c

Q)

ÍN

ZJ o

CL ro c

E ín □

O

CL

Ό

CL

D> <D

XJ

-ro

XJ

O <D

W o

UΌ

O

0.

ΙΩ

O O O O O O O O O O in what what what what CN CN C \ l CN CN O O O O O O O O O O O O O O 00

O

LO

O O O O in m O O T— CM what O O O O what T— in Ν ’

c • ro>

ro

N> UCL

- <D

M— ω

O

E ro o

Ό

of O O > tí C E X w • ro N L. ω <υ 24 O £ CL > i — l 4- » > □ • <u > O ro O OT ro D -4— » ω O Q < O O ro > > O > O cz CL Έ X O ι- Ο. O N O *C E • ro io O O > • ro • ro C M— ro L- * ro ί- σ3 24 O L_ xz O ro XJ O -4— » </) O O XJ Ο 24 1 24 ro >> (Λ • Z3 C O ro ro O CL Ό C: Q O UJ «Λ N O CN what Ν ’ m CD

• £ 1 ··

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

• «

..Μα .. · re

Ο c

X o

X>

st o

C

X ω

O

-ro re n i— c ZJ <D Q X

O

Sd

O o

what o what CD CD m in o o o o o 'v o' ΣΞ

LD what about

what what <sl> I 5

>. ω X t-l t-r O > 05 / 4- » <D C 4- » <υ 4- » D 3 E 3 4— » C O O E 3 £ t-l 4- » Φ £ re 3 G 3 Ι- Ο) *-and O J— (

re

Q_

CN <

* _l

Z3 m

<

X o

X o

<3 c3 >>>

o>

O

-ř-, C '>> C her '> * 33 13 O at C *> β T > φ CL 05 / C CZ) « at, X CZ) 03 / ti T3 Ό O ω * - * '05 w oj t-l Ό Ό O CZ) «♦ - * '05 3 44 O CL *** X with > 4— » Q_ ZJ 4- * ω '> p > O X X O at. X X O O > p ω 05 / L_ Φ Ό X O to. Ό ώ 05 / at- 4— » re > O CL l-l O > P p CZ) Έ O i-l £ C 05 / > Q 05 / * -> O 4— · O «♦ -» 2ϊί > “» X ni C _C 05 / C 05 / 4— » > » .C 05 / C Φ 4- » w re > o > <D C IT > φ C 05 / > > <D C re re sz 4— » > <Z) ZJ Yeah X O E ω O re re -Q Φ φ re > > '05 05 / > 3 Έ 'Φ SW > _O 4— < '03 C X O CZ) O M— </) _C '05 X O C O re Έ (p Έ C _C O at_ 0) > 1— re 44 HIM C CL O "O X φ > 1— '05 > O C χυ G ° 3 C C > N < > N CL Q H > > > _x CL 05 / > <D O O Φ tx N H m ω CL O Ph

HIM

T o

CL g

<U

X>! U

X

MU g

O} -l o3 <S

O

N> *>

ω

1—>

Έ

Έ>

4— »> <Λ

O □

+ '05

L_

4— »(Z)

CZ)

4- »ω

o>

C • · ·

re

4->

O c

T3

O

X

4->

O c

O

Φ o

σ>

cn

CN what what

<<

E o

•O

CL o

> o o

Q_ £

o o

£ «w p

TABLE 2 (continued) re> N _O

O

0.

>,>

ω

L_>

03 '03> N ω <n

£ is about

cn

O

C

E i

T o

CL (/) o

>

o ιΌ

CL · □

O

CL> <D

E o

and_

Ol • ·

The invention also provides ceramic coated metal articles made according to the described process. One example of such a product is an aluminum alloy piston for an internal combustion engine. A second example is an aluminum engine block for an internal combustion engine designed to operate with minimal lubrication. A third such example is the spacecraft shield plates aimed at survival as they enter the atmosphere. A fourth example is electrical insulation that also serves as a heat sink for an electronic panel.

Giant. 3 discloses 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 thereof. The apparatus 10 has a 40 liter electrolyte bath 14 comprising an electrolyte fluid 16 consisting of water and an alkali metal hydroxide solution. The Γ4 tub is made of stainless steel and creates 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.

Here, an AC power source of at least 700 volts is a transformer 20 which increases the voltage to 40,000 volts and is designed to consume up to 800, 900 or 1000 volts.

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

The connecting elements 24 also ensure the completeness of the electrochemical circuit. The control panel 26 is to the left of the bath 14. More recently, it is surrounded by a security door 28. Opening the security door will interrupt the electrical energy.

The salt-containing dispensing hopper 30 has a dispensing valve 32 operating by a solenoid and is part of the means for adding salt 34 to the bath 14 while the apparatus 10 is in operation. The hopper 30 forms a supply of salt 34 containing an alkali metal cation and an oxo acid anion of the amorphous element. A suitable salt 34 is sodium tetrasilicate.

• · • · · 13..· ' • · · · · • · · · · ·· ··· · · ·

Giant. 4 shows an apparatus 36 for ceramic coating articles 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 that is water, an alkali metal hydroxide solution, and a salt of 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 article 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, achieves a coating at a rate of about 2.5-4 microns per minute.

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

3.

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

ts nií-dl

Claims (13)

A method for forming a ceramic metal coating for valves selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys thereof, comprising: immersing said metal as an electrode in an electrolytic bath comprising an aqueous alkali hydroxide solution, providing an opposite 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 to the surface of said metal; is to be coated and said counter electrode, wherein said shape-altered wave 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 breakdown, heating, melting and heat compacting a thin 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 y are influenced by the addition of an alkali metal salt of the oxoacid. 2. The method of claim 1 wherein said altered shaped waveform is obtained using a capacitor bank connected in series between said high voltage source and said metal. The process according to claim 1 or 2, wherein said addition salt is sodium tetrasilicate. Method according to any one of the preceding claims, optimized to provide 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. A method according to any one of the preceding claims, optimized to provide maximum adhesion between said metal and said coating, characterized in that: The 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 of 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 flow 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 comprising an aqueous solution of an alkali metal hydroxide, an electrode immersed in or containing an electrolytic fluid, another electrode comprising at least one of said articles to be coated, and means for suspending said article in said electrolyte, an AC power source from a high voltage source; at least 700 V, AC waveform influencing means 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 entire alternating cycle, by means of closing the electrochemical circuit, 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. • · 11. The apparatus of claim 10 wherein the high voltage source is a transformer increasing voltage up to 1000 volts and said means for influencing the shape of said AC wave is a capacitor bank connected in series between said voltage source and said valve metal. . 12. Apparatus for ceramic coating articles in multiple 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 alkali metal salt concentration of a third electrolytic bath containing an aqueous alkali metal hydroxide solution and a higher alkali metal salt concentration than that of said second bath 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 with a source of AC electrical potential of at least 700 V, means for influencing the AC waveform by means of which the electric current with the altered waveform is raised from uly up to its maximum and falls back below 40% of its maximum height in less than a quarter of the entire alternating cycle and fasteners to close the electrochemical circuit in each bath. 13. The apparatus of 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 the AC voltage. valves.