Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND APPARATUS FOR COATING METALS
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.
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.
More particularly, the present invention is concerned with an electroiytical
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.
Aluminium, titanium and their alloys have favourable strengthlweight
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.
A number of electrolytic coating processes for these metals are known,
which use direct current and/or voltages below 600 V. Such processes are
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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 stow film
formation.
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.
The speed of film formation reported in the eight examples provided in
said patent can be calculated as follows:
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. Example Film Thickness Coating Time Formation
No. Microns Minutes Velocity
Microns/Minute
I 35 20 1.75
2 31 20 1.55
3 28 30 0.93
4 27 20 1.35
36 30 1.20
6 14 30 0.47
7 15 30 0.50
8 28 30 0.93
Such slow rates of f lm formation do not compare well with those of the
present invention.
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.
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-IO minutes. Workpiece heating occurs, as the workpiece is not
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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.
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.
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.
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.
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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.
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.
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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.
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.
In the drawings:
Fig. 1 snows 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.
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.
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The metal workpiece to be coated is connected as the electrode of an
electrolytic bath and is immersed therein.
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.
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.
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
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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.
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.
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.
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,
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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.
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.
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.
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.
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. l, to optimize the coating
effect.
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Reference is now made to Table l, 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.
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The aluminium alloy known as 'Duralumin' has an alloy designation of
2014 and, because of its strength/weight ratio, has found extensive use in
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characteristics of an achieved coating and the results obtained.
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WO 98140541 PCT/GB97/00664
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CA 02283467 1999-09-09
WO 98/40541 PCT/GB97I00664
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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.
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.
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.
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.
The capacitor bank 22 has a total capacitance of 375 ~F and it consists of
capacitors with nominal capacitance of 25, 50, 100 and 200 ~F. Alternatively,
such means could be a rectifier and converter circuit (not shown), or other
means
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of the type shown in Fink_ and Beaty, The Standard Handbook for Electrical
En ig neers, 12th Ed., pp. 22-96, 22-97.
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.
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.
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.
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.
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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.
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.
The electrical components used are the same as those described
hereinabove with reference to Fig. 3.
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.
