Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to composite electro-
deposited coating. Such coatings may be deposited by an electro-
lytic or an electroless method and comprise a metal matrix
incorporating particle additions, the metal and the particles being
deposited from a plating bath in which the particles are substan-
tially insoluble.
It has been proposed to employ various metals as the
matrix, for example nickel and cobalt, and it has been proposed to
employ various particles, for example silicon carbide, chromium
carbide and graphite.
The present invention is directed to providing a coating
which is highly resistant to oxidation and/or to corrosion, for
example in a sulphur-containing environment, at high temperatures.
Such conditions might occur in jet or rocket engines.
Preferably the resistant coating of the invention remains
substantially unaffected by thermal cycling.
According to one aspect of the invention, there is
provided a composite electrodeposited coating on a substrate, the
coating comprising a metal matrix electrodeposited from a plating
bath, the matrix incorporating particles deposited simultaneously
from the bath in which the particles are substantially insoluble,
and in which the particles comprise in combination, up to 40% by
weight of a chromium-containing substance, up to 25% by weight of
an aluminium-containing substance, and 0.01 to 5% by weight of at
least one oxide selected from the group consisting of rare earth
metals and oxides of metals from Group IV of the Periodic Table
of the Elements.
In another aspect, the invention comprises a method of
-- 1 --
forming a composite coating on a substrate which comprises
electrodepositing a metal matrix from a plating bath and simultan-
eously depositing particles which are substantially insoluble in
the plating bath and in which the particles include, in combination,
up to 40% by weight of a chromium containing substance, and 0.01
to 5% by weight of at least one oxide selected from the group
consisting of rare earth metals and oxides of metals from Group IV
of the Periodic Table of the Elements, the method further including
incorporating up to 25% by weight of aluminium in the coating.
The particles may comprise pure chromium powder and pure
aluminium powder, or an alloy of chromium and aluminium in powder
form, or an alloy of chromium and aluminium in powder form mixed
with either both or one of pure aluminium or chromium, or one
further possibility is to have a small quantity of a rare earth
metal or a Group IV metal alloyed with either chromium or aluminium
or both.
Preferably the metal matrix comprises nickel or cobalt
and the coating may be used to coat any suitable substrate for
example steel, high temperature creep-resistant nickel alloys,
nonferrous and light alloys and non-metallic substances with an
outer conductive layer.
The oxide present in the coating may be an oxide of
zirconium, titanium or hafnium, and preferably comprises "lime~
stabilised" zirconia. The oxide may constitute from 0.01 to 5% of
the coating, preferably about 2% by weight, and may be present in
the size range up to 10~.
The chromium present in the coating may comprise 10-20%
by weight of the coating, more preferably 15%. The aluminium
-- 2
3L~7t~o~
present in the coating may comprise up to 20% by weight of the
coating and more preferably comprises 10 to 20%.
The particles may be up to 10~ in diameter and preferably
fall largely within the range of 1 to 2~.
- 2a -
According to another aspect of the present invention
there is provided a method of forming a composite coating comprises
electrodepositing a metal matrix from a plating bath and simultane-
ously depositing particles which are substantially insoluble in
the plating bath and in which the particles include, in combination,
a chromium containing substance, and an oxide of a rare earth metal
and/or oxide of a metal from Group IV of the Periodic Table of
the Elements, and by subsequently aluminising the coating.
After deposition the coating may be further heat treated, either
b~ for~ ~se O
~10 in use or/for example by being held at 1000 C for 4 hours.
It has been found that a layer of aluminium oxide forms
over the surface of the coating by aluminium atoms diffusing to the
surface and oxidising. This process carries on until the surface
is covered. After the initial oxide layer has covered the surface,
it is believed that further oxide can only form by either the
aluminium diffusing through the oxide to react with further oxygen
at the surface, or oxygen diffusing through the oxide to react with
i: ~
5~i
aluminium to form more oxide at the metal oxide interface. ~ne
further possibility may be that both mechanisms are operating and
fresh oxide is also formed within that already existing.
In general, if a metal atom diffuses outwards through the
oxide it may leave behind a vacancy. In the case of known castings
it is believed that during the course of time the vacancies would
normally coalesce at the interface between the metal and oxide to
form voids such that the oxide would only be attached to the parent
metal at a few places. Furthermore, the oxide formed might have a
greater volume than the original metal from which it had been formed
and so a compressive stress would develop in the growing oxide layer.
This might give rise to a complicated stress situation at the oxide-
-metal interface which is believed to consist mainly of shear
stresses and also transverse stress acting to pull the oxide away
from the metal. The net result of these stresses coupled with the
lack of coherency at the interface would be that the oxide would
spall off and expose fresh metal to the hot gas. This process would
repeat itself and so the metal would be progressively lostr
A problem which may arise if oxide spalling does not occur
is that the oxide simply gets thicker as time goes on and the compon-
ent gets weaker as the cross section of the pure metal decreases.
~.~ 7 ~ S ~ ~
O~e method by which these problems may be
overcome is to reduce simultaneously the rates
at which the oxygen and metal diffuse through
the oxide and to provide a means by which the
5. vacancies may be annihilated and so prevent void
formation. It is in this way that the inclusion
of the oxide particles, which are both thermo-
dynamically stable and chemically stable in an
oxidising atmosphere, may greatly improve the
10. adhesion of the aluminium oxide to the matrix
metal at the surface of the coating. It is
~7
6.
believed that the effect is achieved in three ways,
either singly or in combination.
The first of these is the "stress relief"
mechanism in which the pre~ence of oxide particles
5. at the coating's surface pro~ides "dead spots"
over which the developing oxide film can grow
laterally thereby relieving any stresses in the film.
The second is the "vacancy sink" mechanism in which
the vacancies left by aluminium atoms diffusing
~10. to the surface to react with oxygen are at least
partially ~illed by oxide molecules so that the oxide
particles undergo a reararrangement to occupy
the voids which effectlvely "diffuse" through the
matrix. The third is simply a reduction in the
15. diffusion coefficients through the growing surface
oxide layer of either the metal ions or the oxygen
atoms or both.
The above enhancement in adhesion is described
with reference to aluminium and its oxide;
20. however it is to be understood that the description
is also applicable to chromium and its oxide,
and therefore also to aluminium/chromium alloy
and both oxides. It has been found in practice
that an increase in the relative amount of aluminium
25. results in a predominance of aluminium oxide in
the outer layers of the coating which is then
more resistant in an oxidising en~ironment while
an increase in the relative amount of chromium
results in a predominance of chromium oxide which
30. is more resistant to a sulphur-contair,ing environment.
Thus, by adjusting the relative quantities of
aluminium and chromium the usability of the
coating can be varied.
According to a further aspect of the invention,
5. a mèthod of ~orming a composite coating comprises
electrodepositing a metal matrix from a plating
bath and simultaneously depositing particles
which are substantially insoluble in the plating
bath, the particles including a chromium-containing
10. substance, and further including aluminium oxide
and/or an oxide of a rare earth metal and/or
an oxide of a metal from Group I~ of the Periodic
Table of the Elements, and subsequently aluminising
the coating.
15. Preferably the particles also include an
aluminium-containing substance and, preferably, the
aluminising step comprises a pack-aluminising
process. In this process the material used may
be aluminium powder, or a mixture of aluminium and
20. aluminium oxide. Pre~erably, the aluminium is in
in the ~orm of an alloy with chromium.
The pack-alumini~ing is carried out at a high
temperature. The aluminium is believed to diffuse
into the surface of the electrodeposited coating
25. and thus form a bond. The coating may be
subsequently heat treated, for example by being
held at 1000C for about 4 hours, or may be heat-
treated in use.
The substances described with reference to
30. the first method may also be used in the second
~:~7~5gti
method with the same effect.
The coating process may involve an electrolytic or an
electroless method and may be carried out using the apparatus and oper-
ating conditions described in the Applicants' British Patents Nos.
1 218 179, 1 224 166, 1 329 081 and 1 347 184.
One specific example of a process applying a coating in
accordance with the invention will now be described.
EXAMPLE 1
Using the apparatus described in our co-pending Canadian
Patent No. 1,133,848, a stainless steel panel two inches (50.8 mm) by
one inch (25.4 mm) by one-eighth of an inch ~3.2 mm) thick was pro-
vided with a composite coating comprising a cbbalt matrix including
particles of lime-stabilised zirconia and particles of aluminium/
chromium alloy. The tank was filled with a solution comprising 450
grams per litre of cobalt sulphate, 30 grams per litre of boric acid
and 2.5 grams per litre of sodium chloride. To 125 litres of this
solution contained in the tank was added 10 millilitres of Canning's
anti-pit liquid.
The pane:L to be coated was given a pretreatment comprising
immersion in a cyanide cleaner for two minutes followed by a water
rinse~ etching by immersion for 30 seconds in a 50% sulphuric acid
followed by a water rinse, and a nickel strike by plating in a nickel
bath for three minutes at a
current density of 3.9 amps per square decimetre.
The panel was secured in a plating barrel and
connected to a cathode contact. 40 grams per litre
of barrel capacity of lime-stabilised zirconia powder
5. and 600 grams per litre of barrel capacity of
chromium/aluminium alloy powder, both with a mean
particle size of 2 io 5~m~ were added to the barrel
and the o~ening in the barrel through which the
panel to be coated and the powder were admitted was
10. closed. The barrel was then completely submerged in
the solution in the tank and was rotated at three
revolutions per minute while composite plating took
place at a voltage of between 2.5 and 3 volts with
a current density of approximately 2.7 amps per
15. s~uare decimetre. The solution temperature was
maintained at 50C and the solution had a pH of
between 4.5 and 5. After plating has proceeded for
a time sufficient to give a thickness of plating
of 0.05 mm, plating was stopped. The panel was then
20. held at 100CC for 4 hours, cooled and examined. It
was found that the panel had been given a tenacious
coating having an even distribution of pzrticles.
The coating had a particle content of approximately
2Si by volume of zirconia and 30% by volume of
25. chromium/aluminium alloy.
