[0001] The present invention relates to a ceramic coating process for valve metals, to articles
coated thereby, and to an apparatus for carrying out said process.
[0002] Valve metals exhibit electrolytic rectification, and the present invention is therefore
concerned with providing a coating process and apparatus for coating aluminium, zirconium,
titanium, hafnium, and alloys thereof.
[0003] More particularly, the present invention is concerned with an electrolytical process
using a shaped-wave, high-voltage alternating current to achieve melting during coating
of even a thick layer, such a thick layer being achieved in a short time by changing
electrolyte composition during the course of the process.
[0004] Aluminium, titanium and their alloys have favourable strength/weight ratios which
suit these metals to many applications, for example, for use in aircraft and for fast-
moving parts in internal combustion engines. As these metals do not, however, exhibit
particularly good wear properties, coatings are often used to improve wear and erosion-resistance.
The applied coatings are likely to achieve further design requirements such as resistance
to chemicals, particularly acids and alkalies; allowance of exposure to higher temperatures;
reduction of friction, and the provision of dielectric properties. While the low-cost,
widely-used anodizing process achieves some of these aims for moderate service, ceramic
coatings are required for severe service requirements.
[0005] A number of electrolytic coating processes for these metals are known, which use
direct current and/or voltages below 600 V. Such processes are described, for example,
in U.S. Patents 3,956,080; 4,082,626 and 4,659,440. These and the majority of recent
disclosures describe processes which form ceramic films using an anode spark discharge
technique, and which achieve good results regarding coating corrosion resistance and
adhesion. Such methods do, however, have two important drawbacks: low film hardness
and slow film formation.
[0006] In U.S. Patent 5,147,515, Haganata
et al. disclose the use in an electrolytic bath of a dispersion comprising an aqueous solution
of a water-soluble or colloidal silicate and/or an oxyacid salt to which ceramic particles
are dispersed. Voltage is increased during film formation from 50-200 V, and may finally
exceed 1000 V. With regard to wave form, said patent states that the output from a
power supply may be a direct current having any wave form, but preferably those having
a pulse shape (rectangular wave form), saw-tooth wave form, or DC half-wave form.
Such language does not imply recognition that a sharply-peaked wave form makes a major
contribution to providing a dense, hard film.
[0007] The speed of film formation reported in the eight examples provided in said patent
can be calculated as follows:
Example No. |
Film Thickness Microns |
Coating Time Minutes |
Formation Velocity Microns/Minute |
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 |
[0008] Such slow rates of film formation do not compare well with those of the present invention.
[0009] Also, no indication was given in U.S. Patent 5,147,515 whether it is possible, through
the method of said patent, to produce very thick coatings, e.g., in the range of 300-700
microns.
[0010] A recently-developed coating method, known as the Kepla-Coat Process, is based on
plasmachemical anodic oxidation. The cathode is the surface film of an organic electrolyte,
above which the part to be coated is suspended, forming the anode. A plasma is formed
which causes the production of a ceramic coating on the anode and heating of the workpiece.
Due to the formation of an oxide film on the anode, the process produces a film no
thicker than about 10 microns and terminates in 8-10 minutes. Workpiece heating occurs,
as the workpiece is not surrounded by liquid; non-symmetrical or slender workpieces
are likely to suffer distortion. A further disadvantage of the Kepla-Coat Process
is that the high rate of electrolyte evaporation poses an environmental problem.
[0011] It is therefore one of the objects of the present invention to obviate the disadvantages
of the prior art ceramic coating processes and to provide a process which produces
a hard film with strong adherence and minimum porosity.
[0012] It is a further object of the present invention to provide a method for producing
coatings up to 300 and more microns thick, within a moderate time span.
[0013] The present invention achieves the above objectives and others by providing 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, said process
comprising immersing said 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
said metal to be coated and said opposite electrode, wherein said modified shaped-wave
electric current rises from zero to 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 said metal to
form and weld a ceramic coating to said metal, and changing the composition of said
electrolyte while said ceramic coating is being formed, said change being effected
by adding an oxyacid salt of an alkali metal.
[0014] A still further object of the present invention is to provide an apparatus for carrying
out the above process in a cost-effective manner. The invention thus provides an apparatus
for the batch ceramic coating of articles made of a valve metal selected from the
group consisting of aluminium, zirconium, titanium, hafnium and alloys of these metals,
said apparatus comprising an electrolytic bath comprising an aqueous solution of an
alkali metal hydroxide, an electrode immersed in or containing the electrolyte liquid,
another electrode comprising at least one of said articles to be coated and means
to suspend said article in said electrolyte, a source of alternate electric current
from a high voltage source of at least 700 V, means for shaping the AC wave form whereby
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,
connector elements to complete an electrochemical circuit, and means for adding to
said bath, while the apparatus is in operation, a controlled supply of an oxyacid
salt of an alkali metal.
[0015] A distinguishing feature of the process of the present invention is its suitability
to the production of hard coatings as thick as 300 microns within a reasonable time
frame of about 90 minutes. This fast coating rate is achieved by changing the composition
of the electrolyte while the coating process is in operation. Coating quality is not
compromised by the fast formation of a thick coating, as the modified shaped current
achieves momentary melting of the layer near the metal workpiece even after the film
has built up to the stated thickness.
[0016] The invention will now be described in connection with certain preferred embodiments
with reference to the following illustrative figures so that it may be more fully
understood.
[0017] With specific reference now to the figures in detail, it is stressed that the particulars
shown are by way of example and for purposes of illustrative discussion of the preferred
embodiments of the present invention only, and are presented in the cause of providing
what is believed to be the most useful and readily understood description of the principles
and conceptual aspects of the invention. In this regard, no attempt is made to show
structural details of the invention in more detail than is necessary for a fundamental
understanding of the invention, the description taken with the drawings making apparent
to those skilled in the art how the several forms of the invention may be embodied
in practice.
[0018] In the drawings:
Fig. 1 shows a preferred type of shaped-wave pulse;
Fig. 2 depicts the relationship between coating thickness and electrolysis time;
Fig. 3 is a schematic view of an apparatus for batch coating, and
Fig. 4 is a schematic view of an apparatus for series coating.
[0019] The process of the invention will now be described. The process is used to form a
ceramic coating on aluminium, zirconium, titanium, and hafnium. The process is also
suited to alloys of these metals, provided the total of all alloying elements does
not constitute more than approximately 20% of the whole. Process parameters may be
optimized to suit the paticular metal being coated and the particular properties of
the coating considered important to a specific application.
[0020] The metal workpiece to be coated is connected as the electrode of an electrolytic
bath and is immersed therein.
[0021] For coating aluminium, the bath comprises water and a solution of an alkali metal
hydroxide. In an embodiment of the bath where it is required to optimize the coating
to provide maximum adhesion between the metal and its coating, the electrolyte consists
essentially of an aqueous solution containing between 0.5 to 2 g/liter of sodium hydroxide
or potassium hydroxide. Fine particles of various substances are added if it is required
to improve the special, for example, low friction, properties of the coating. Where
such particles are added, the electrolyte is agitated to keep the particles in suspension.
Similarly, coloured coatings are produced by adding fine particles of pigmenting substances.
[0022] The preferred opposite electrode for the process is a stainless steel bath containing
the electrolyte liquid. Where it is preferred to hold the electrolyte in a non-conducting
container, for example, for safety considerations, the electrode from ferrum, nickel
or stainless steel is inserted into the bath in the conventional manner.
[0023] A modified shaped-wave alternate electric current from a high voltage source of at
least 700 V, typically 800 V for aluminium workpieces, is then passed between the
metal workpiece and the other electrode. This results in 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 thereto. The arc microwelding is visible during
coating. A convenient and moderate-cost method of obtaining the required shaped-wave
electric pulse current is by use of a capacitor bank connected in series between the
high voltage source from 800 to 1,000 V and said metal workpiece which is being coated.
[0024] Referring now to Fig. 1, there is seen a wave form of preferred shape of current.
The effect of using alternating current in combination with a high voltage is to prolong
the life of the microarc, which causes intense, local, temporary heating, and as a
result, the welding and melting of the coating being formed on the submerged metal
workpiece. Anodizing is effected during the first positive half-cycle, the metal workpiece
being the positive electrode. Thereafter, the dielectric coating already formed fails
dielectrically, thereby starting the generation of microarcs. Arc lifetime extends
almost to the end of the first half-cycle. Burning of arc is repeated during the second
half-cycle, when the workpiece becomes the negative electrode.
[0025] Referring now to Fig. 2, there are seen time/coating thickness relationships for
processes wherein electrolyte composition is held constant, designated traces 1 to
5. Trace 1 refers to a process wherein the electrolyte is pure potassium hydroxide.
Traces 2 to 5 refer to processes wherein increasing concentrations of sodium tetrasilicate
were used.
[0026] Trace 6 refers to the process of the present invention. It has been found that much
faster coating is made possible by changing the composition of the electrolyte while
the ceramic coating is being formed. The change effected comprises adding to the electrolyte
a salt containing a cation of an alkali metal and an oxyacidic anion of an amorphous
element. Said amorphous element is selected from the group comprising 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 in a concentration of between 2 and 200 g/liter of solution. A preferred
amorphous element is silicone, and a preferred added salt is sodium tetrasilicate.
[0027] As is seen in the graph, changing of the electrolyte composition during operation
allows production of a 200-micron thick coating in approximately 50 minutes, indicating
a film formation velocity of 4 microns/minute. Tests have shown that this fast film
formation is achieved without sacrificing the quality of film adhesion to the metal
workpiece.
[0028] Obviously, once the added salt has been mixed into the electrolyte, the only practical
way of again reducing salt concentration for coating the mext batch of metal articles
is to add considerable quantities of new electrolyte liquid. This problem is solved,
however, in the apparatus to be described further below with reference to Fig. 3.
[0029] It has also been found that it is possible to produce a pore-free coating by gradual
reduction of the current flow when the film has almost reached its desired thickness.
In practice, this is effected by progressively reducing the capacitance used to shape
the wave form, thus weakening the current until the process stops.
[0030] As will be realized from the above description, the term 'modified' as used herein
refers to the fact that the wave form is other than the standard sinosidal form normally
associated with a wave of alternating current and is instead modified, e.g., as illustrated
in Fig. 1, to optimize the coating effect.
[0031] Reference is now made to Table 1, which lists various types of coatings for different
requirements. Examples are listed of aluminium alloys which have been ceramically
coated to achieve various design requirements. Examples 3 and 4 were produced by the
technique described above.
[0032] The aluminium alloy known as 'Duralumin' has an alloy designation of 2014 and, because
of its strength/weight ratio, has found extensive use in aircraft construction. This
alloy was therefore chosen for test coating. Table 2 lists characteristics of an achieved
coating and the results obtained.
TABLE 2
Item |
Units |
Value |
Metal workpiece material |
|
Duralumin |
Wave form production method |
|
Capacitors |
Transformer output voltage |
V |
800 |
Current density : |
A/dm2 |
|
Anodic |
|
6.0 |
Cathodic |
|
6.3 |
Electrolyte composition |
gram/liter water |
|
First bath: |
Potassium hydroxide |
|
0.5 |
Second bath: |
Potassium hydroxide |
|
0.5 |
Sodium tetrasilicate |
|
4.0 |
Third bath: |
Potassium hydroxide |
|
0.5 |
Sodium tetrasilicate |
|
11.0 |
Coating time : |
minutes |
|
in first bath |
|
10 |
in second bath |
|
10 |
in third bath |
|
20 |
Total coating thickness |
microns |
100 |
Average deposition rate |
microns/minute |
2.5 |
Thickness of inner layer fully melted during coating |
microns |
65 |
Substrate adhesion |
MPa |
380 |
Micro Hardness |
Vickers kgf/mm2 |
2790 |
Average Composition of layer: |
% |
|
Corrundum |
|
62 |
Alumina |
|
8 |
Alumosilicate |
|
30 |
Coating porosity |
No. of pores/cm2 |
4 - 6 |
Pore diameter |
microns |
8 - 11 |
[0033] The invention also provides a ceramically-coated metal article produced by the described
process. One example of such an article is an aluminium alloy piston for an internal
combustion engine. A second example is an aluminium engine block for an internal combustion
engine, intended to operate with minimal lubrication. A third example is a protective
tile for spacecraft, designed to survive re-entry into the atmosphere. A fourth example
is electric insulation serving also as a heat sink of an electronic board.
[0034] Fig. 3 illustrates an apparatus 10 for the batch ceramic coating of articles 12 (first
electrode) made of a valve metal selected from the group consisting of aluminium,
zirconium, titanium, hafnium and alloys thereof. The apparatus 10 has an electrolytic
40-liter bath 14, comprising an electrolyte liquid 16 of water and a solution of an
alkali metal hydroxide. Bath 14 is made of stainless steel and forms the second electrode.
Agitation means 15 are provided to stir the electrolyte.
[0035] The first electrode comprises at least one of the articles 12 to be coated, and conducting
means 18 to suspend said article in the electrolyte liquid 16.
[0036] A source of alternate electric current of at least 700 V is a 40,000 V-amp step-up
transformer 20, designed to supply up to 800, 900, or 1000 V.
[0037] The capacitor bank 22 has a total capacitance of 375 µF and it consists of capacitors
with nominal capacitance of 25, 50, 100 and 200 µF. Alternatively, such means could
be a rectifier and converter circuit (not shown), or other means of the type shown
in Fink and Beaty,
The Standard Handbook for Electrical Engineers, 12th Ed., pp. 22-96, 22-97.
[0038] Connector elements 24 are also provided to complete an electrochemical circuit. An
operator control panel 26 is seen at the left of bath 14, the latter being enclosed
behind safety doors 28. The opening of safety doors 28 cuts off the electric power.
[0039] A salt-containing feed hopper 30, having a solenoid- operated feed valve 32, provides
means for adding salt 34 to bath 14 while the apparatus 10 is in operation. Hopper
30 holds a supply of a salt 34, containing a cation of an alkali metal and an oxyacidic
anion of an amorphous element. A suitable salt 34 is sodium tetrasilicate.
[0040] Shown in Fig. 4 is apparatus 36 for serial ceramic coating of articles 12. A first
electrolytic bath 38 contains electrolyte liquid 16, comprising water and a solution
of an alkali metal hydroxide. A second electrolytic bath 40 contains an electrolyte
liquid 42, comprising water, a solution of an alkali metal hydroxide, and a low concentration
of salt 34. A third electrolytic bath 44 contains an electrolyte liquid 46, comprising
water, a solution of an alkali metal hydroxide, and a higher salt concentration than
in electrolyte 42.
[0041] For convenience, baths 38, 40, 44 can comprise a single stainless steel container
48, provided with two vertical dividers 50, forming the electrode. The other electrode
comprises at least one of articles 12 to be coated and conducting means 18, which
sequentially suspend article 12 in electrolyte liquids 16, 42, 46. Manual or automatic
manipulation means 52 allow the transfer of article 12 from the first bath 38 to the
second bath 40, and thence to third bath 44.
[0042] In apparatus 36, the electrolyte in each bath remains substantially unchanged during
operation, and may therefore be used repeatedly. The use of several electrolytes,
each having a different composition, enables coating at speeds of about 2.5-4 microns
per minute.
[0043] The electrical components used are the same as those described hereinabove with reference
to Fig. 3.
[0044] It will be evident to those skilled in the art that the invention is not limited
to the details of the foregoing illustrated embodiments and that the present invention
may be embodied in other specific forms without departing from the spirit or essential
attributes thereof. The present embodiments are therefore to be considered in all
respects as illustrative and not restrictive, the scope of the invention being indicated
by the appended claims rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims are therefore intended
to be embraced therein.
1. 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, said process
comprising:
immersing said 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 said metal to be coated and said opposite electrode,
wherein said 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 said metal to
form and weld a ceramic coating to said metal, and
changing the composition of said electrolyte while said ceramic coating is being formed,
said change being effected by adding an oxyacid salt of an alkali metal.
2. The process as claimed in claim 1, wherein said modified shaped-wave electric current
is obtained by use of a capacitor bank connected in series between said high voltage
source and said metal.
3. The process as claimed in claim 1 or claim 2, wherein said added salt is sodium tetrasilicate.
4. The process as claimed in any previous claim, optimized to provide maximum adhesion
between said metal and said coating, wherein said electrolyte consists essentially
of an aqueous solution containing between 0.5-2 g/liter of sodium hydroxide.
5. The process as claimed in any previous claim, optimized to provide maximum adhesion
between said metal and said coating, wherein said electrolyte consists essentially
of an aqueous solution containing between 0.5-2 grams per liter of potassium hydroxide.
6. The process as claimed in any previous claim, wherein said amorphous element is selected
from the group comprising 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 in a concentration of between
2 and 200 grams per liter of solution.
7. The process as claimed in any previous claim wherein fine particles of pigmenting
substances are added to produce a coating of a required colour.
8. The process as claimed in any previous claim, arranged to produce a pore-free coating,
wherein the current flow is progressively reduced to stop coating.
9. A ceramically-coated metal article produced by the process as claimed in any previous
claim.
10. An apparatus for the batch ceramic coating of articles made of a valve metal selected
from the group consisting of aluminium, zirconium, titanium, hafnium and alloys of
these metals, said apparatus comprising:
an electrolytic bath comprising an aqueous solution of an alkali metal hydroxide;
an electrode immersed in or containing the electrolyte liquid;
another electrode comprising at least one of said articles to be coated and means
to suspend said article in said electrolyte;
a source of alternate electric current from a high voltage source of at least 700
V;
means for shaping the AC wave form, whereby 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;
connector elements to complete an electrochemical circuit, and
means for adding to said bath, while the apparatus is in operation, a controlled supply
of an oxyacid salt of an alkali metal.
11. An apparatus as claimed in claim 10, wherein the high voltage source is a step-up
transformer of up to 1,000 V and said means for shaping said AC waveform is a capacitor
bank connected in series between said source and said valve metal.
12. An apparatus for the serial ceramic coating of articles made of a valve metal selected
from the group consisting of aluminium, zirconium, titanium, hafnium and alloys of
these metals, said apparatus comprising:
a first electrolytic bath comprising an aqueous solution of an alkali metal hydroxide;
a second electrolytic bath comprising an aqueous solution of an alkali metal hydroxide,
and a low concentration of an oxyacid salt of an alkali metal;
a third electrolytic bath comprising an aqueous solution of an alkali metal hydroxide,
and a higher concentration than is present m said second bath of an oxyacid salt of
an alkali metal;
electrodes immersed in or containing the electrolyte liquids;
electrodes comprising at least one of said articles to be coated and means to suspend
said article in said electrolytes;
means for transferring said articles from said first bath to said second bath and
then to said third bath;
a source of AC electric potential of at least 700 V;
means for shaping the AC wave form, whereby shaped-wave electric current rises from
zero to its maxium height and falls to below 40% of its maximum height within less
than a quarter of a full alternating cycle; and
connector elements to complete an electrochemical circuit in each bath.
13. An apparatus as claimed in claim 12, wherein the high voltage source is a step-up
transformer of up to 1,000 V and said means for shaping said AC waveform is a capacitor
bank connected in series between said source and said valve metal.