Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a component of a basic
material that is based on nickel or cobalt, which has a
coating to protect it against oxidation, corrosion, and
thermal fatigue.
Super alloys that are resistant to high temperatures, and are
based on nickel or cobalt, were developed for use in turbine
construction. The material used in the blades is exposed to
particularly high stress levels; the blades have to be able
to withstand not only the high temperatures (in excess of
950C) within the turbine, but must also possess a high level
of creep resistance. In order to ensure a higher order of
creep resistance, the material used for the blades is grown
with large crystals and in part with a columnar structure,
from super alloys, by suitable casting and crystallization
techniques. It is a disadvantage for corrosion resistance
that during this growth, grain boundary deposits of easily
oxidized alloy additives, such as vanadium or titanium, for
example, are formed. This causes a disadvantageous
deterioration of the surface properties such as resistance to
oxidation and corrosion, as well as resistance to thermal
fatigue. For this reason, coatings such as the MCrAlX,Y
family have been developed (metal, chromium, aluminum, X =
rare earths, Y = yttrium); these improve the surface
properties by virtue of their high chromium and aluminum
contents which, for their part, form stable oxides during
operation of the turbine, and which, as a result of the rare
earth metals, improve the adhesion of the oxide layer to the
surface coating.
Diffusion processes are disadvantageous because of the
different concentrations on both sides of the boundary layer
between the coating surface and the coating, which lead to
diffusion pores in the area close to the boundary layers, so
that the protective coating bursts when thermal stresses are
superimposed on points with higher densities of diffusion
pores. Furthermore, the MCrAlX,Y coatings have a tendency to
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thermal fatigue since there is a disparity in the thermal
expansion behaviour between the basic material alloy and the
MCrAlYX coating, and the MCrAlX,Y coatings are extremely
ductile in comparison to the basic material.
A further known technical solution is the formation of
chromium and/or aluminum-rich diffusion coatings on the
surface of the basic material by powder pack cementing and/or
gas diffusion coating. Coatings of this kind form oxidation-
resistant intermetallic phases with the basic material.
Because of the increased hardness of these coatings with
intermetallic phases, the fatigue limit of the components is
disadvantageously reduced by up to 30 per cent. Since the
thermal expansion behaviour is not adapted to the basic
material, there is an increased risk of microcracks forming
in the component, and this increases with increased thickness
of the coating. For this reason, and most disadvantageously,
the thickness of the coatings must be kept below 100 ~m.
In the case of the known coatings, components of the basic
material that are sensitive to oxidation and corrosion, such
as vanadium and titanium, are not used, and stable oxide-
forming substances such as aluminum, up to 20 per cent, for
example, and chromium up to 40 per cent, are added to form
alloys. Matching the composition of the coating to the
super-alloy based on cobalt or nickel that is to be coated
becomes increasingly involved and more extensive in order to
overcome problems of adhesion or to minimize diffusion
processes, or to build up protective stable oxides on the
surface.
It i~ an object of the present invention to describe a
component produced from a basic material based on cobalt or
nickel and with a protective coating, that displays greater
resistance to thermal fatigue, oxidation, and corrosion at
temperatures above 800C, than formerly known coatings and
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which overcomes the disadvantages of such coatings, and a
process for producing such components.
This has been achieved in that the basic material and the
protective coating are of the same chemically equal material,
and the protective coating is structured with a much finer
grain.
According to the present invention there is provided in a
component of a basic material based on nickel or cobalt, with
a protective coating of the same chemical composition to
ensure protection against oxidation, corrosion, and thermal
fatigue, the improvement wherein the protective coating is
essentially structure with a finer grain than the basic
material, and the lowest layer of the fine-grain coating
exhibits the same crystal orientation as the large-volume
crystallite of the basic material on the boundaries of the
coating.
The present invention solves the problems and the
disadvantages as they exist in the prior art, in that the
substance for the basic material is used for a coating of a
similar type, so that there are no diffusion processes and
problems of adhesion do not occur when the surface of the
basic material is free of oxide. Thus there is no bursting
of the particles in the protective coating.
Because of a constant composition of the alloy in the grain
volume, an even, stable and protective oxide layer will
advantageously be formed on the grain surface when such
components are used in an oxidizing flow of hot gas, as in a
turbine, for example. Since the grain boundaries of this
coating exhibit fewer grain boundary deposits than the basic
material, grain boundary corrosion is advantageously reduced. -
The preferred corrosion attack at the grain boundaries and
the associated tendency to crack, is hindered by the
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which overcomes the disadvantages of such coatings, and a
process for producing such components.
This has been achieved in that the basic material and the
protective coating are of the same chemically equal material,
and the protective coating is structured with a much finer
grain.
According to the present invention there is provided in a
component of a basic material based on nickel or cobalt, with
a protective coating of the same chemical composition to
ensure protection against oxidation, corrosion, and thermal
fatigue, the improvement wherein the protective coating is
essentially structure with a finer grain than the basic
material, and the lowest layer of the fine-grain coating
exhibits the same crystal orientation as the large-volume
crystallite of the basic material on the boundaries of the
cvating.
The present invention solves the problems and the
disadvantages as they exist in the prior art, in that the
substance for the basic material is used for a coating of a
similar type, so that there are no diffusion processes and
problems of adhesion do not occur when the surface of the
basic material is free of oxide. Thus there is no bursting
of the particles in the protective coating.
Because of a constant composition of the alloy in the grain
volume, an even, stable and protective oxide layer will
aclvantageously be formed on the grain surface when such
components are used in an oxidizing flow of hot gas, as in a
turbine, for example. Since the grain boundaries of this
coating exhibit fewer grain boundary deposits than the basic
material, grain boundary corrosion is advantageously reduced. ~-
The preferred corrosion attack at the grain boundaries and
the associated tendency to crack, is hindered by the
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this reason, the grain volume of the coating is preferably
three powers of ten smaller than the grain volume of the
basic material.
The grain boundaries of the preferred basic material IN 100
exhibit grain boundary deposits that contain titanium and
vanadium, and these form unstable or low-melting point
oxides. For this reason, the coating has fewer deposits on
the grain boundaries than the basic material, and this
advantageously improves resistance to oxidation and
corrosion.
A preferred formation of the protective coating is such that
the protective coating is a plasma-spray coating that
crystallizes with an extremely fine grain and with small
number of deposits because of its very high hardening speed.
In addition, present invention provides a process for the
production of a component by the following process steps:
a) Surface preparation by removal of the surface of the
basic material, this being done to improve adhesion;
b) Coating applied to the basic material by means of plasma
spraying with plasma spra~ing material with the chemical
composition of the basic material;
c) Epitactic recrystallisation by means of solution heat
treatment at temperatures between 1150 and 1250C;
d) After-treatment of the surface of the protective coating
2S by mechanical consolidation for smoothing and strengthening
of the surface and/or diffusion coatings to increase
resistance to oxidation.
The process has the advantage that it is suitable for mass-
production processes.
When increased demands made of the quality of the coating,
the surface preparation is effected by plasma etching with
argon plasma. This preparation entails the advantage of
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freedom from contamination, and is compatible with a low-
pressure plasma spraying process, so that both surface
preparation and coating of the basic material can be effected
on a component element with an assembly procedure. This
enhances the quality so no move to another plant is needed,
and no time is spent in a normal atmosphere.
In the event that there are increased demands for economy,
the surface preparation can be effected by chemical removal,
so that a higher throughput can be achieved.
An abrasive jet preparation is advantageously used as surface
removal since large area components such a rotor disks can be
prepared for subsequent coating by using this process.
In the case of increased demands for quality, coating can be
effected by plasma spraying with plasma spraying material of
the same chemical composition as the basic material; in the
case of large components and/or in the event of higher
demands for economy, this can be effected by plasma spraying
in an atmosphere of protective gas.
An optimal accumulation of the coating on the basic material
is achieved by epitactic recrystallisation at a solution heat
treatment temperature between 1150 and 1250C. When this is
done, the lowest position of the fine-grain coating
recrystallises in the transition zone between the basic
material and the coating, in the same crystal orientation as
the large volume crystallite of the basic material on the
coating boundary, so that intensive denticulation results
between the fine-grain coating and the coarse-grain basic
material, which greatly increases adhesion compared to
conventional coatings of different kinds. Then, the coated
components can be cooled from 1000C to 800C at 30C/minute
to 80C/minute and subjected to a multi-stage thermal aging
processingO
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For cast components of super-alloys based on nickel or cobalt
a two-stage aging process for forming a suitable ~
structure at 1080C to 1120C for 2 hours to 6 hours followed
by 900 to 980C for 10 hours to 20 hours, with intermediate
cooling at 750C to 800C. This type of thermal treatment
regenerates the properties of the basic material that have
been altered by the solution heat treatment, and the strength
values of the coating are advantageously enhanced thereby.
Mechanical after-treatment of the surface of the protective
coating improves the hardness by preferably shot-blasting, as
serves to smooth the surface. The smoothing of the surface
can also be effected by means of vibratory grinding or
Druckfliess processing.
Diffusion coating as an after-treatment of the surface, as is
usually applied to the basic material of super-alloys that
are based on nickel or cobalt in order to increase long-term
resistance to oxidation can advantageously be effected on the
fine-grain coating. This entails the advantage that deep
diffusions, as they occur along the grain boundary deposits
of the basic material, do not occur in the fine-grain
coatings with fewer grain-boundary deposits. The diffusion
zone in the fine-grain coating is thus more even and
homogeneous when doped, for example, with aluminum or
chromium, than is possible on the coarse crystalline basic
material. When this is done, the chromium doping improves
the resistance to oxidation up to temperatures of 850C, and
at the same time, brings about enhanced resistance to
corrosion caused by sulfidation. Doping with aluminum, for
example, increas~s resistance to oxidation at temperatures of
up to 1250C.
The following examples of applications for a component and a
process represent preferred embodiments of the present
invention.
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Example of a component:
A low-pressure plasma coating of the same chemical
composition, which has a 3-103-times smaller grain volume
than the basic material, was applied on a coarse-crystalline
turbine blade of IN 100 as the basic material, which was
composed as follows:
13 to 17 %-wt Co
8 to 11 %-wt Cr
to 6 %-wt Al
4.5 to 5 %-wt Ti
2 to 4 %-wt Mo
0.~ to 1.2 %-wt V
0.15 to 0.2 %-wt C
0.01 to 0.02 %-wt B
0~03 to 0.09 %-wt Zr
Remainder Ni
During thermal-fatigue testing (test temperature 1050DC) the
coated component exhibited a temperature-endurance three
times greater than that of the uncoated basic material.
Example of a process
In a coarse-crystalline turbine blade of IN 100 as the basic
material, composed of the following elements
13 to 17 %-wt Co
8 to 11 %-wt Cr
5 to 6 %-wt ~1
4.5 to 5 %-wt Ti
2 to 4 %-wt Mo
0.7 to 1.2 %-wt V
0.15 to 0.2 % w~ C
0.01 to 0.02 %-wt B
0.03 to 0.09 %-wt Zr
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Remainder Ni
the surface of the basic material was removed on average to a
depth of 0.5 to 10 ~m by means of argon~plasma etching at a
pressure of 2 kPa to 4 kPa.
Next, the basic material was coated with plasma spray
material of the same chemical composition as the basic
material, using plasma spray technology; this was done at a
pressure of 4 kP~ and at a temperature of the basic material
of 900C, for a period of 120 seconds.
After the removal of the coated turbine blade, epitactic
recrysallisation was effected in a high-vacuum oven. To this
end, the component was maintained at a solution heat
treatment temperature of 1200C for 4 hours, and cooled to
80C at a rate of 60C/minute.
In order to regenerate the strength characteristics of the
basic material and to enhance the strength of the coating, a
two-state heat treatment was completed in a high vacuum at
1100C for 4 hours and at 950C for 16 hours, with
intermediate cooling to 800C at 60C/minute.
After cooling to room temperature, the surface of the
component was smoothed and consolidated by shot peening with
zirconium oxide pellets 0.5 to 1.0 mm diameter.
