Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Description
Co-Spray A~rasive Coating
Technical Field
This invention relates to abrasive coatings and
more specifically to grit containing coatings applied
by plasma spray process techniques.
The concepts were developed in the gas turbine
engine field for the application of abrasive coatings
to parts in that industry, but have wider applica-
bility to components and structures in other industriesas wellO
Background Art
Grit type materials are used in the gas turbine
engine industry to impart abrasive qualities to one
of two opposing surfaces which are susceptible to
rubbing contact. The avoidance of destructive
interference at contact between the two surfaces is
sought by causing the abrasive surface to cleanly
cut- material from the opposing surface until non-
interfering movement results.
The above technique is representatively appliedat the interstage gas path seals between rotor and
stator assemblies. Both inner diameter and outer
diameter seals are capabl~ of employing the concept.
At the outer diameter air seals the tips of the
rotor blades are provided with an abrasive quality
such that during rotor excursions of greater relative
growth than the circumscribing stator, the rotor
blades cut cleanly into the opposing shroud. Once
the seals are "run in" a minimum or zero clearance is
established at the point of maximum rotor excursion.
Subsequent excursions do not wear away additional
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material. Representative prior art methods of
manufacturing abrasive tipped rotor blades are
discussed in U.S, Paten-t Nos. 3,922,207 to Lowrey
et al entitled "Method for Plating ~rticles with
Particles in a Metal ~latrix" and 4,169,020 to
Stalker et al entitled "Method for Making an Improved
Gas Seal".
Similarly, abrasive coatings are utilized in
other sealing applications, such as at labyrinth
seals internally of an engine. UOS. Patent No~
4,148,494 to Zelahy et al entitled "Rotary Labyrinth
Seal Member'l is representative of such a construction.
As the desira~ility of abrasive grit coating in
the gas turbine engine industry has increased,
scientists and engineers in that industry have sought
yet improved structures and deposition techniques,
particularly techniques capable of maint~; n; ng
angularity of the grit particles and good adherence
to the surface on which the particles are deposited.
Disclosure of Invention
According to the present invention abrasive
grit particles and matrix material for adhering the
grit particles to the surface of a substrate are co-
deposited at the surface of the substrate in a process
causing simultaneous incidence of the metal matrix
material with abrasive grit at the surface of the
substrate.
In accordance with a detailed deposition method
a plasma gas stream is generated in a plasma gun, metal
matrix particles are injected into a plasma stream,
abrasive grit particles are subsequently injected into
that stream at the point of incidence of the stream
with the surface of the substrate to be coated, and
the gun is traversed across the surface of the
substrate.
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A principal feature of the co~deposition method
is the simultaneous incidence of the abrasive grit
particles with the heated matrix material carried by
the plasma stream at the surface of the substrate
to be coatedO Powders of metallic matrix material
are injected into the plasma stream at a location
spaced from the surface to be coated and the grit
particles are ;njected ;nto the plasma stream at a
location nearer the substrate to be coated than the
point of injection of matrix particles. The abrasive
grit particles injected into the stream come into
contact with the metal matrix materials at the
surface to be coated. In one detailed apparatus the
grit injector and the matrix injector are oriented
one hundred eighty degrees (180) apart at the perimeter
of the plasma stream~
A principal advantage of the present invention
is the capability of depositing economical coatings
with good adherability and angularity of the grit
~0 particles. Good adherability is achieved by trapping
the grit particles in the molten metal matrix material
as the metal matrix material solidifies at the
surface of the substrate to be coated. Good angularity
of the grit particles is preserved by avoiding
prolonged contact of the gxit particles with the
high temperature portion of the plasma stream. The
deposition process has good flexibility in the
ability to deposit grit particles of varying size
and in the ability to utilize matrix materials
having wide]y varying characteristics. Good abrasive
quality of the coating is maintained throughout the
application process. Grit particles may be deposited
through the full depth of the coating, or merely
at the surface by delaying grit injection -to one or
more subsequent passes over the substrate to be coated.
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The coating process described is well suited to the
reburbishment of coated parts after initial use. The
process can be employed to apply abrasive coatings
to surfaces of complex geometry.
In one aspect of the present invention there
is provided a method utilizing a plasma spray gun for
depositing an abrasive grit coating on a substrate,
including the steps of generating a high temperature
plasma stream; injecting particles of matrix material
into the plasma stream; injecting particles of abrasive
grit into the plasma stream at a location downstream
of the location at whi.ch the particles of matrix
material are injected, in a direction approximately
one hundred eighty degress (180) apart at the cir-
cumference of the plasma stream from the directionof injection of the matrix material particles, and
at a distance from the substrate to be coated such
that the matrix particles and the grit particles come
into simultaneous contact with t:he surface of the
substrate to be coated and with each other; and travers-
ing the plasma spray gun across the substrate to be
coated.
In a further aspect of the present invention
there is provided a method for applying a grit containing
2S coating by plasma spray techniques to a narrow substrate
wherein the improvement comprises offsetting the narrow
substrate f~om the axis o~ the plasma spray stream
during application of the coating to avoid the erosive
zone at tne axis of the spray.
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The foregoing, and other objects, features and
advantages of the present invention will become more
apparent in the light of the following detailed
description of the preferred embodiment thereof as
shown ln the accompanying drawing.
Brief Description of Drawing
Fig. 1 is a simplified side elevation view of a
portion of a gas turbine engine including sections
broken away to reveal opposing components of the
stator and rotor assemblies;
Fig. 2 is a simplified illustration of the tip
of a rotor blade with abrasive coating adhered thereto;
Fig. 3 is a simplified representation of a portion
of the rotor assembly drum with abrasive coating
adhered thereto;
Fig. 4 is a simplified illustration of the knife-
edge portion of a labyrinth type seal with abrasive
coating adhered thereto;
Fig. S is a simplified representation of plasma
spray apparatus depositing an abrasive coating in
accordance with the concepts of the present invention;
Fig. 6 is an enlarged view illustrating simul~
taneous impact of the grit particles with the matrix
paxticles at the surface of the substrate being coated,
Fig. 7 is a sectional view taken along the line
7-7 of Fig. 6;
Fig. ~ is a cross section photograph (lOOx) of
an abrasive coating applied to a rotor blade tip
under the Example I parameters; and
Fig. 9 is a cross section photograph l200x) of an
abrasive coating applied to the knife-edge of a labyrinth
type seal under the Example II parameters.
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Best Mode for Carrying Out the Invention
Coatings applied by the present method have
utility in the gas turbine engine industry. Fig. 1
is a simplified cross section illustration of a portion
of the compressor section of an engine in that
industry. A rotor assem~ly 12 extends axially through
the engine and is encased by a stator assembly 14.
flow path 16 for working medium gases extends
axially through the engine. Rows o~ rotor blades, as
represented by the single blades 18, extend outwardly
from a rotor drum 20 across the flow path 16. Rows
of stator vanes, as represented by the single vanes
22, are cantilevered inwardly from an engine case 24
across the flow path~ An outer air seal 26 circum-
scribes each row of rotor blades 18. An inner airseal 28 is formed by the rotor drum 20 inwardly of
each vane row 22. Abrasive coatings are applied,
for example, at the interface between the tips of
the rotor blades 18 and the outer air seal or at the
interface between the tips o the vanes 22 and the
inner air seal 28. The elimination of destructive
interference at such interfaces upon the occurrence
of rotor excursions during transient conditions is
sought. Providing an abrasive coating on one of said
opposing surfaces wears material cleanly away from
the corresponding surface without destroying the
structural integrity of either part.
The compressor structure of Fig. 1 illustrates
components to which abrasive coatings may be
applied-~tips of the rotor blades 18 and inner air
seals 28 on the xotor. Such components and
their coatings are illustrated in Figs. 2 and 3
respectively. Other applications might include the
solid land 30 of a wide channel type seal 32 such as
that illustrated in Fig. 1 or the knife edge, Fig. 4,
of a labyrinth type seal.
In one detailed aspect such abrasive coatings
have particular utility when used in conjunction with
components fabricated of titanium alloy. The large
heat of reaction released on oxidation of such alloys
renders the components susceptible to fires upon the
occurrence of rubbing interference. An abrasive coating
on one of such rubbing components causes material to
be cut from the opposing component without generating
excessive heat loads.
lQ A method of applying abrasive coatings by the
present techniques is illustr~ted by Fig. 5. A stream
34 of plasma gases is formed within a plasma generator
36 and is discharged toward the surface of the substrate
38 to be coated, Particles 40 of matrix material are
injected into the plasma stream remotely from the
surface of the substrate and are plasticized or melted
within the plasma stream. Particles 42 of grit
material are injected into the plasma stream in close
proximity to the surface of the substrate~ Both the
grit particles and the matrix particles are preferably
injected parallel to the direction of the motion vector
of the gun across the substrate. The mass ratio of
matrix material to deposited grit particles may be widely variable.
- Ratios between 1:1 and 100 1 are typical. In at least
one detailed method, the matrix ~articles and the grit
particles are injected into the plasma stream at
relative locations around the perimeter of the plasma
stream which are approximately one hundred eighty
degrees (180) apart. In a further detailed method
the matrix particles and the grit particles are injected
into the plasma stream from directions substantially
perpendicular to the axis A of the plasma stream.
The plasma sprayed coating is cooled at the
substrate by cooling jets 44 which emanate from no~zles
46 on opposing sides of the plasma gun. The jets 44
are directed in the illustration so as to intersect at
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a point P above the surface of the substrate.
The spacings of the matrix particle injection
point and of the grit particle injection point from
the surf~ce of the substrate are important factors
to successful application of the abrasive coa-ting.
In pxinciple, the matrix particle injection point
must be spaced at a suficient distance from the
substrate to enable softening or melting of the
particles in the plasma stream. The grit particle
injection point must be su~fîciently close to the
substrate so as to ena~le entrapment of the grit
in the matrix material at the surface of te substrate
without melting o the angular cutting edges on the
gritO Additionally, spacing the grit particle
injection point close to the substrate minimizes
acceleration of the grit particles by the plasma
stream, and reduces the tendency of the grit to
bounce from the substrate before the grit becomes
entrapped in the matrix. Actual spacings of the
grit and matrix injection points from the substrate
will depend upon the composition and particle size
of the materials selected.
Another important aspect considered in location
of the grit injection point is the effect of location
on the incidence between the matrix particles and
the yrit particles. The optimum point o~ incidence
occurs at the surface o~ the substrate. Simultaneous
contact of the grit particles with matrix particles
and the surface of the substrate is desired. Incidence
of the grit particles with the matrix material above
the substrate surface results in premature cooling
of the ma-trix and low retention ratio of the grit
particles by the matrix since only molten or plasticized
matrix material will deposit at the surface.
Additionally, prolonged contact of the grit particles
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with the high temperature plasma gas may reduce the
angularity of th~ grit particle cutting edges.
Another factor in achieving high probability
of grit particle entrapment is the lnjection angle
of the grits into the plasma stream. The optimum
angle is as close to ninety degrees (90) as is
practicable such that the dwell time of the
particles in proximity to the substrate is maximized.
Particles injected in the downstream direction have
an increased tendency to bounce off the substrate;
particles injected in the upstream direction are
ultimately accelerated by the plasma stream and also
have a tendency to bounce off of the substrate.
Multiple coating runs have been made with a
wide variety of material selections and application
parameters. The examples shown below are representa-
tive of the most successful runs.
Example I
The tip of a compressor rotor blade, such as the
blade 18 illustrated in Fig. 2 was coated to a depth
on the order of ten thousandths of an inch (0,010 iIl . )
in a single pass of the plasma gun across the blade
tip. Plasma spray parameters were as indicated below:
Plasma Gun -- Metco 7M Gun with type G nozzle
Nozzle Distance from Substrate -- 2-3/8 inches
Matrix Injection Point from -- 2-5/16inches
Substrate
Grit Injection Point from -- 1/16 inch
Substrate
Cooling Jet Crossing Distance -- 3/8 inch
from Substrate
Plasma Gun Current -- 540 amps
Plasma Gun Voltage -- 70 volts
Relative Velocity between Gun -- 3 feet per second
and Substrate
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Primary Plasma Arc Gas-- Nitrogen
- 130 cu. ft./hr.
- 50 psi
Secondary Plasma Arc Gas -- Hydrogen
- approx, lO
cu. ft.~hr.
- 50 psi
Matrix Material-- Metco 443 (Nickel
Chromium Alloy
plus Aluminum)
- particle size
(-15~/+38
microns)
- flow rate
(25 grams/min.)
Grit Material -- Silicon Carbide
- particle size
(140 grit)
- flow rate
(100 grams/min.)
Matrix Carrier Gas -- Nitrogen
ll cu. ft./hr.
- SO psi
G t C rrier Gas -- Argon
rl a
- 15 cu. ft./hr.
- 50 psi
Matrix Injector Port -- Metco $2 Powder Port
Grit Injector Port -- l/4 inch O.D. tubing
Substrate Material -- Titanium Alloy
Substrate Pxeparation -- Grit blast/Metco 443
bond coat
Substrate Offset from Plasma - l/16 inch
Spray Axis
Gri~ ïnjector Distance from -- 7/8 inch
Plasma Spray Axis
Direction of Grit Injection -- Perpendicular to
Plasma Spray Axis
Relationship of Matrix and - 180
Grit Injectors
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Example XI
The knife edge of a labyrinth type seal, such
as the knife edge illustrated in Fig. 4, was coated
to a depth on the order of ten thousandths of an inch
(0.010 in~) in a single pass of the plasma gun
across the $ubstrate, Plasma spray parameters were
as indicated below:
Plasma Gun -- Metco 7M Gun with type G nozzle
~ozzle Distance from Substrate -- 2-l/4 inches
Matrix Injection Point from -- 2-3/16 inches
Substrate
Grit Injection Point from -- l/4 inch
Substrate
Cooling Jet Crossing Distance ~- -0- inch
from Substrate
Plasma Gun Current -- 480 amps
Plasma Gun Voltage -- 65 volts
Relative Velocity between Gun -- 5 feet per second
and Substrate
Primary Plasma Arc Gas -- Nitrogen
- lO0 cu. ft./hr.
50 psi
Secondary Plasma Arc Gas -- Hydrogen
- approx. lO
cu. ft./hr.
- 50 psi
Matrix Material -- Metco 4~3 (Nickel
Chromium Alloy
plus Aluminum)
- particle size
(-150/+38
microns)
-- flow rate
~25 grams/min.)
Grit Material -- Silicon Carbide
320 grit
Matrix Carrier Gas -- Nitrogen
~ ll cu. ft./hr.
- 50 psi
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Grit Caxrier Gas -- Argon
- 15 cu. ft./hr.
- 50 psi
Matrix Injector Port -- Metco #2 Powder Port
Grit Injector Port -- 3/6 inch O.D. Tubing
Substrate Material -- Titanium Alloy
Substrate Preparation - Grit blast/Metco 443
bond coat
Substrate Offset from Plasma -~ 1/16 inch
Spray Axis
Grit Injector Distance from -- 7/8 inch
Plasma Spray Axis
Direction of Grit Injector -- Perpendicular to
Plasma Spray Axis
Relation~hip of Matrix and -- 180
Grit Iniectors
The Fig. 7 sectional view illustrates an important
concept in the coating of very narrow substrates,
particularly compressor blade tips which may be
coated in accordance with the Example I parameters
or knife edges which may be coated in accordance
with the Example II parametersD Typical compressor
blade tips may be as narrow as forty thousandths of
an inch (0.040 inch); typical knife edges are tapered
25. to a width on the order of ten thousandths of an
inch (0.010 inch). Note that the narrow substrate 3
to be coated in Fig, 7 is offset a distance X from
the axis A of the plasma stream. In spraying
abrasive materials it has been empirically discovered
that a highly eroslve ~one precisely at the axis A
of the plasma stream inhibits the buildup of coating
material in that region. Offsetting the substrate
from the erosive zone at the axis greatly increases
the rate at which entxapped grit particles build up
on the substrate.
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Although the invention has been shown and
described with respect to preferred embodiments
t~ereof, it should be understood by those skilled
in the axt that various changes and omissions in
the form and detail thereo may be made therein
without departing from the spirit and the scope of
the invention.
