Thermal spray coating process for rotor blade tips using a rotatable holding fixture

A process for controllably applying thermal spray coating onto substrates is described. The process includes positioning rotor blades in a fixture rotatable about an axis, forming a spray of particles of softened coating medium in an apparatus for propelling the coating medium towards the blade tips and coating the blade tips by passing the blades through the spray of particles of coating medium. Various process details, including process parameters, are developed.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application relates to the following copending U.S. applications filed 
on even date herewith and commonly assigned to the assignee of the subject 
application: U.S. Application Number EH-10120, U.S. Ser. No. 08/994,926, 
filed Dec. 19,1997, entitled "Thermal Coating Composition", by Freling et 
al; U.S. Application Number EH-10095, U.S. Ser. No. 08/994,680, filed Dec. 
19, 1997, entitled "Tooling Assembly for Positioning Airfoils of a Rotary 
Machine", by Zajchowski and Diaz; U.S. Application Number EH-10117, U.S. 
Ser. No. 08/994,676, filed Dec. 19, 1997, entitled "Shield and Method for 
Protecting an Airfoil Surface", by Zajchowski and Diaz; and U.S. 
Application Number EH-10118, U.S. Ser. No. 08/994,662, filed Dec. 19, 
1997, entitled "Method for Applying a Coating to the Tip of a Flow 
Directing Assembly", by Zajchowski and Diaz. 
TECHNICAL FIELD 
This invention relates to a process for controllably applying thermal spray 
coating onto substrates and more particularly, to applying a plurality of 
coating layers onto gas turbine engine rotor blade tips. 
BACKGROUND ART 
Large gas turbine engines are widely used for aircraft propulsion and for 
ground based power generation. Such large gas turbine engines are of the 
axial type, and include a compressor section, a combustor section, and a 
turbine section, with the compressor section normally preceded by a fan 
section. An annular flow path for working medium gases extends axially 
through the sections of the engine. Each of the fan, compressor, and 
turbine sections comprises a plurality of disks mounted on a shaft, with a 
plurality of airfoil shaped blades projecting radially from the disks. A 
hollow case surrounds the various engine sections. A plurality of 
stationary vanes are located between the disks and project inwardly from 
the case assembly which surrounds the disks. 
During operation of the fan, compressor, and turbine sections, as the 
working medium gases are flowed axially, they alternately contact moving 
blades and the stationary vanes. In the fan and compressor sections, air 
is compressed and the compressed air is combined with fuel and burned in 
the combustion section to provide high pressure, high temperature gases. 
The working medium gases then flow through the turbine section, where 
energy is extracted by causing the bladed turbine disks to rotate. A 
portion of this energy is used to operate the compressor section and the 
fan section. 
Engine efficiency depends to a significant extent upon minimizing leakage 
of the gas flow to maximize interaction between the gas stream and the 
moving and stationary airfoils. A major source of inefficiency is leakage 
of gas around the tips of the compressor blades, between the blade tips 
and, the engine case. Accordingly, means to improve efficiency by 
reduction of leakage are increasingly important. Although a close 
tolerance fit may be obtained by fabricating the blade tips and the engine 
case to mate to a very close tolerance range, this fabrication process is 
extremely costly and time consuming. Further, when the assembly formed by 
mating the blade tips and the engine case is exposed to a high temperature 
environment and rotational forces, as when in use, the coefficients of 
expansion of the blade tips and the engine case parts may differ, thus 
causing the clearance space to either increase or decrease. A significant 
decrease in clearance results in contact between blades and housing, and 
friction between the parts generates heat causing a significant elevation 
of temperatures and possible damage to one or both members. On the other 
hand, increased clearance space would permit gas to escape between the 
compressor blade and housing, thus decreasing efficiency. 
One approach to increase efficiency is to apply an abradable coating of 
suitable material to the interior surface of the compressor housing, which 
when abraded allows for the creation of a channel between the blade tips 
and the housing. Leakage between the blade tips and the housing is limited 
to airflow in the channel. Various coating techniques have been employed 
to coat the inside diameter of the compressor housing with an abradable 
coating that can be worn away by the frictional contact of the compressor 
blade, to provide a close fitting channel in which the blade tip may 
travel. Thus, when subjecting the coated assembly to a high temperature 
and stress environment, the blade and the case may expand or contract 
without resulting in significant gas leakage between the blade tip and the 
housing. 
However, it is critical that the blade tips not degrade when contacted with 
the coatings applied to the interior surface of the compressor housing. To 
increase the durability of the blade tips which rub against the abradable 
seals, abrasive layers are sometimes applied to the blade tip surface by a 
variety of methods. See, for example, U.S. Pat. No. 4,802,828, of Rutz et 
al, which suggests several techniques for providing an abrasive layer on a 
blade tip, including powder metallurgy techniques, plasma spray 
techniques, and electroplating techniques; Schaefer et al, U.S. Pat. No. 
4,735,656, which teaches application of an abrasive comprising ceramic 
particulates in a metal matrix by controlled melting and solidification of 
the matrix metal; or, Schaefer et al, U.S. Pat. No. 4,851,188, which 
teaches a sintering operation for application of an abrasive layer to the 
tip of a superalloy gas turbine blade. 
Plasma spraying devices and techniques are well known in the art for 
depositing protective coatings on underlying substrates. One known device 
is illustrated in U.S. Pat. No. 3,145,287 to Siebein et al entitled 
"Plasma Flame Generator and Spray Gun ". In accordance with the teaching 
of the Siebein et al patent, a plasma-forming gas forms a sheath around an 
electric arc. The sheath of gases constricts and extends the arc part way 
down the nozzle. The gas is converted to a plasma state and leaves the arc 
and nozzle as a hot plasma stream. Powders are injected into the hot 
plasma stream and propelled onto the surface of the substrate to be 
coated. 
U.S. Pat. Nos. 3,851,140 to Coucher entitled "Plasma Spray Gun and Method 
for Applying Coatings on a Substrate" and 3,914,573 to Muehlberger 
entitled "Coating Heat Softened Particles by Projection in a Plasma Stream 
of Mach 1 to Mach 3 Velocity" disclose contemporaneous coating technology. 
This above art notwithstanding, scientists and engineers working under the 
direction of Applicant's assignee are seeking to improve the process of 
applying thermal spray coating to substrates in a gas turbine engine. In 
particular, they have sought to improve the application time of the 
thermal spray coating using the plasma spraying devices, and to produce a 
process that is tolerant of variations in flow parameters affecting the 
spray coating. 
DISCLOSURE OF THE INVENTION 
According to the present invention, a method for controllably applying 
spray coating to the tips of gas turbine engine rotor blades includes 
positioning the rotor blades in a holding fixture rotatable about an axis, 
forming a spray of particles of softened coating medium in an apparatus 
for propelling the coating medium, and coating the tips of the rotor 
blades by rotating the tips of the rotor blades about the axis of rotation 
and passing the blades through the spray of particles of softened coating 
material to deposit layers of the coating on each rotor blade sequentially 
with each pass of the blade tip through the spray such that variations in 
the coating process parameters are spread over a number of blades. 
In accordance with the present invention, the process includes forming a 
spray of particles of softened coating medium, the spray having a 
circumferential width at least the size of the circumferential width of 
the blades. 
In accordance with the present invention, the process includes the step of 
heating the tips of the rotor blades and the fixture to an elevated 
temperature during coating. 
In accordance with one particular embodiment of the invention, the process 
includes positioning the blades circumferentially in the holding fixture 
which is rotatable about an axis of rotation such that adjacent points on 
the blade tips will approximate a surface of rotation substantially 
parallel to the surface of rotation which the blade tips will experience 
in an operational engine. 
In accordance with one particular embodiment of the present invention, the 
process includes the step of translating the apparatus for forming and 
propelling the spray coating, such as a spray gun, between a first and 
second position in a direction substantially parallel to the plane of 
rotation of the fixture which allows for a thin layer of thermal spray 
coating to be deposited on the tips of the blades each time they pass in 
front of the spray gun which build up to a plurality of layers, resulting 
in a splat structure with vertical microcracks. 
A primary feature of the present invention is the relative motion between 
the rotating holding fixture for the blades and the apparatus for forming 
and propelling the coating medium. Another feature is disposing and 
rotating the blades in the holding fixture such that points receiving the 
coating on the blade tips describe a circle about the axis of rotation of 
the fixture. In one embodiment, the holding fixture is rotated such that 
adjacent points on the blade tips approximate a surface of rotation 
substantially parallel to the surface of rotation which the blade tips 
will experience in an operating engine. Another feature is the step of 
translating the apparatus for forming and propelling the spray coating 
between a first and second position. In one embodiment, the spray coating 
apparatus moves in a direction substantially parallel to the surface of 
rotation of the fixture. Another feature is heating the blades to an 
optimum temperature by passing the blades in front of the spray coating 
apparatus and through the plasma gas stream. Another feature is depositing 
the coating medium by passing the blades through the spray of particles of 
softened coating medium. Another feature is cooling the blades by moving 
them away from the spray gun, after an independent source of the 
deposition of each coating layer. Another feature is cooling the blades by 
directing an independent source of cooling air on the blade tips as the 
blades rotate away from the gun. Another feature is heating the blades 
using an independent source of heat before the blades re-enter the plasma 
gas stream. Another feature of the present invention is the use of control 
processing parameters to apply the plurality of coating layers. These 
parameters include the relative speed of the holding fixture to the 
apparatus for providing the coating (e.g. spray gun), distance from spray 
gun-to-blade tips, coating powder feed rate, plasma gas stream flow and 
the power of the spray gun. 
A primary advantage of the present invention is the quality of coating 
applied to the tips of rotor blades which results from using the process 
to distribute among a multiplicity of the rotor blades any variations in 
the process flow parameters affecting the stream of particles propelled 
against the tips. As a result, the coating process of the present 
invention has less sensitivity to process variations than a process which 
allows variations to occur only on one blade. Another advantage is the 
reproducible and reliable process that results due to the use of the 
control parameters. This process can be used to repetitively apply the 
coating on substrate surfaces. 
Another advantage is the ease and speed of application of the coating on 
the surfaces of a large number of blades at a given time which results 
from the size of the holding fixture and process which accommodates a 
number of blades. Using a holding fixture that accommodates a plurality of 
blades, the resultant fixturing time is minimized. Another advantage of 
one embodiment of the present invention is the application of coating to 
substrates without the use of additional heating apparatus for the 
substrates. During coating deposition, the optimum amount of heat required 
is transmitted to the substrates by the plasma gas and the molten coating 
powder. The rotor blade is not overheated during the coating process. As a 
result, a rotor blade can be coated without changing the microstructure or 
properties of the substrate. 
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 best mode for carrying out the invention and from the 
accompanying drawings which illustrate an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 2 shows a schematic representation of an apparatus for forming and 
propelling particles of coating medium and a holding fixture. A plurality 
of rotating blades such as compressor blades 10 are positioned in the 
cylindrical holding fixture 12. The holding fixture has an axis of 
rotation A,. The holding fixture can accommodate a large number of blades, 
up to a full stage of blades. The fixture diameter ranges from about 
eighteen to thirty-six inches (18 to 36") (457 to 914 mm), preferably 
about twenty to twenty-eight inches (20 to 28") (508 to 711 mm) to 
approximate the size of the flowpath of the engine. The large size of the 
fixture can accommodate an entire stage of blades. Selecting a fixture 
which positions the blades at a radius from the axis of rotation A, which 
is the same as the operative radius ensures the location of the blade tip 
approximates closely the radius in the engine. 
Each rotor blade has a root and a platform. An airfoil extends from the 
platform and terminates in a tip. Each airfoil has a leading edge and a 
trailing edge. A suction surface and a pressure surface extend between the 
edges. The blades are oriented such that points on the blade tips describe 
a circle about the axis of rotation of the holding fixture. The blade tips 
face in the outward direction from the holding fixture. 
The apparatus for propelling particles toward the blade tips, as 
represented by a spray coating apparatus 14, is in close proximity to the 
holding fixture. The spray coating apparatus includes a spray gun 16 
positioned at the outer diameter of the cylindrical fixture for depositing 
the layers. The spray gun is translatable in different directions with 
respect to the holding fixture. The spray coating apparatus forms a heated 
plasma including molten particles, such as molten zirconium oxide 
particles, which are propelled in the heated plasma gas stream toward the 
blades disposed in the fixture. 
In one embodiment, the blades are positioned in the holding fixture such 
that adjacent points on the blade tips approximate a surface of rotation 
substantially parallel to the surface of rotation which the blade tip will 
experience while in a working engine. As the blades are rotated, the gun 
moves up and down in a direction substantially parallel to the plane of 
rotation of the fixture, coating the blades in sequence. 
The thickness of the abrasive coating deposited depends on the application 
of the substrate. In compressor and brush seal applications, the abrasive 
layer may have a thickness ranging from five to forty mils (5 to 40 mils) 
(0.13 to 1.02 mm). 
FIG. 3 is an enlarged view taken along lines 3--3 of FIG. 2 showing the 
relationship between the plasma spray propelled from the apparatus for 
forming and propelling particles and the blade tips disposed in the 
holding fixture. The circumferential width of the spray can range from the 
size of the circumferential width of the blades to a width ten times 
(10.times.) that of the circumferential width of the blades. This enables 
the spray coating to be deposited uniformly onto the suction and pressure 
surfaces of the airfoil of the blade. The phenomenon of overspraying is 
known in the art, even in processes that spray coat straight onto blade 
tips that are stationary. However, the overspray that results from the 
present invention process coats more airfoil surface area and is applied 
uniformly as compared with prior art processes. The overspray onto the 
airfoil surfaces provides for better adhesion of the spray coating onto 
the blades. The coating is not subject to chipping at the leading and 
trailing edges as by overspraying and applying the coating to the leading 
and trailing edges of the blade and to contiguous areas of the suction and 
pressure surfaces, as well as to the tip itself, a more durable blade tip 
may be obtained. 
The processing steps of the invention are controlled to produce vertical 
microcracking (essentially perpendicular to the bond coat surface) and are 
specific to variables such as gun-type and fixture geometry. The vertical 
microcracks may extend through a top coating layer to a bond coating 
layer. The vertical microcracks do not extend to the substrate surface. 
The processing steps include the selection of certain parameters. These 
parameters include rotating the fixture at a preselected speed, angling 
the gun with respect to the substrate, moving the gun at a preselected 
traverse speed, heating the substrate to a preselected temperature, 
injecting the coating powder at a preselected rate, and flowing the 
carrier gas and plasma gases at preselected flow rates. These parameters 
all influence the structure of the coating and as such should be adjusted 
to provide uniform coating of compressor blades, or other substrates. In 
general, it has been found that a close gun-to-substrate spray distance 
coupled with relatively high spray gun power results in the desired 
vertical segmentation or microcracking of the coating structure. The 
parameters described herein were tailored for use with an F-4 model air 
plasma spray gun purchased from Plasma Technics, Inc., now supplied by 
Sulzer Metco having facilities in Westbury, N.Y., and various diameter 
cylindrical fixtures depending on substrate configuration. As will be 
realized, the parameters may vary with the use of a different spray gun 
and/or fixture. Accordingly, the parameters set forth herein may be used 
as a guide for selecting other suitable parameters for different operating 
conditions. 
The process for controllably applying spray coating as flow charted in FIG. 
1, includes a number of interrelated steps beginning with providing blades 
having clean, exposed blade tips and protected airfoil and root surfaces 
typically provided by masking. Conventional cleaning and preparation of 
the blade tip prior to application of the abrasive layer should be 
conducted. In the practice of the present invention, for example with a 
blade tip as shown in the figures, the surface of the blade tip is cleaned 
and roughened to enhance adherence of subsequently applied coating 
materials. Such cleaning can include mechanical abrasion such as through a 
vapor or air blast type process employing dry or liquid carried abrasive 
particles impacting the surface. 
Prior to cleaning the surface, blades are suitably masked as shown in U.S. 
Application Number EH-10117, U.S. Ser. No. 08/994,676, filed Dec. 19, 
1997, entitled "Shield and Method for Protecting an Airfoil Surface", by 
Zajchowski and Diaz, herein incorporated by reference. 
The process includes propelling a spray of particles of softened bond 
coating medium toward the blade tips. The step of propelling the coating 
medium includes the step of forming a spray of particles of softened bond 
coating medium in the spray coating apparatus. This step includes flowing 
bond coat powder and carrier gases into a high-temperature plasma gas 
stream. In the plasma gas stream, the powder particles are melted and 
accelerated toward the substrate. Generally, the powder feed rate should 
be adjusted to provide adequate consistency and amount of bond coating. 
The bond coat powder feed rate ranges from thirty to fifty-five grams per 
minute (30 to 55 grams/min). Carrier gas flow (argon gas) is used to 
maintain the powder under pressure and facilitate powder feed. The carrier 
gas flow rate ranges from four to eight standard cubic feet per hour (4 to 
8 scfh) (1.9 to 3.8 standard liters per minute (SLM)). Standard conditions 
are herein defined as about room temperature (77.degree. F.) and about one 
atmosphere of pressure (760 mmHg) (101 kPa). 
The gases that make up the plasma gas stream comprise of a primary gas 
(argon gas) and a secondary gas (hydrogen gas). Helium gas may also be 
used as a secondary gas. The primary gas flow rate in the gun ranges from 
seventy-five to one hundred and fifteen standard cubic feet per hour (75 
to 115 scfh) (35 to 54 SLM), while the secondary gas flow rate ranges from 
ten to twenty-five standard cubic feet per hour (10 to 25 scfh) (4.7 to 12 
SLM). Spray gun power generally ranges from thirty to fifty kilowatts (30 
to 50 KW). 
The process then includes the step of translating the spray of softened 
bond coating medium at a distance ranging between about four to six inches 
(4 to 6") (102 to 152 mm) from the blade tips, between a first and second 
position. In one embodiment, the spray gun is moved in a direction 
substantially parallel to the surface of rotation of the holding fixture. 
Spray gun traverse speed during bond coat deposition ranges from six to 
twelve inches per minute (6 to 12 in/min) (152 to 305 mm/min). 
Further, the process includes passing the blades through the spray of 
particles of softened bond coating medium by rotating the fixture about 
its axis of rotation. This step includes heating the blades to a 
temperature of two hundred to four hundred and fifty degrees Fahrenheit 
(200.degree. to 450.degree. F.) by passing the blades in front of the 
spray gun and hot plasma gas stream. The step of passing the blades 
through the spray of particles of softened bond coating medium also 
includes cooling the blades and the coating layer deposited by rotating 
them away from the spray gun. Additional cooling of the blades can be 
provided by directing a cooling air stream or cooling jet on the blades or 
the fixture. Independent sources of heating can also be provided to heat 
the blades prior to the blades entering the spray of particles of coating 
medium. The independent heating source would allow for control of blade 
temperature without adjusting the spray gun to provide heating. 
Specifically, during bond coat deposition, the cylindrical fixture rotates 
at a speed which ranges from twenty to seventy-five revolutions per minute 
(20 to 75 rpm), depending on substrate diameter. The surface speed of the 
blades ranges typically from one hundred and twenty-five to three hundred 
surface feet per minute (125 to 300 sfpm). 
The coating process then includes the step of forming a spray of particles 
of softened top coating medium. This step includes flowing top coat powder 
and carrier gases into the high-temperature plasma gas stream. Generally, 
the powder feed rate should be adjusted to provide adequate mix to cover 
the substrate, yet not be so great as to reduce melting and crack 
formation. Top coat powder feed rate ranges from fifteen to forty grams 
per minute (15 to 40 grams/min). Carrier gas flow (argon gas) is used to 
maintain the powder under pressure and facilitate powder feed. The flow 
rate ranges from four to eight standard cubic feet per hour (4 to 8 scfh) 
(1.9 to 3.8 SLM). As described hereinabove, standard conditions are herein 
defined as about room temperature (77.degree. F.) and about one atmosphere 
of pressure (760 mmHg) (101 kPa). 
The step of forming a spray of particles of softened top coating medium 
includes the injection of the top coat powder angled such that it imparts 
a component of velocity to the powder which is opposite to the direction 
of flow of the plasma toward the rotating fixture. The projection of the 
injection angle in a plane perpendicular to the axis of rotation of the 
holding fixture lies in a range from sixty-five to eighty-five degrees 
(65.degree. to 85.degree.). This injection angle serves to introduce the 
top coat powder further back into the plasma plume, thus increasing the 
residence time of the powder in the plasma gas stream. The increased 
residence time in the plasma gas stream provides for better melting of the 
powder particles. 
Primary gas flow (argon gas) in the gun ranges from fifty to ninety 
standard cubic feet per hour (50 to 90 scfh) (24 to 43 SLM). Similarly, 
secondary gas flow (hydrogen gas) in the gun ranges from ten to thirty 
scfh (10 to 30 scfh) (4.7 to 14 SLM). Spray gun power generally ranges 
from thirty to fifty kilowatts (30 to 50 KW). 
The process further includes the step of translating a spray of softened 
top coating medium at a distance ranging from three to four inches (3 to 
4") (76 to 102 mm) from the blade tips, between a first and second 
position in a direction substantially normal to the plane of rotation of 
the holding fixture. Spray gun traverse speed across each part during 
deposition ranges from two to ten inches per minute (2 to 10 in/min) (50.8 
to 254 mm/min). The gun-to-substrate distance may be varied with the 
intent of maintaining the appropriate temperature level at the substrate 
surface. A close gun-to-substrate distance is necessary for satisfactory 
vertical microcracking. 
The process further includes the step of passing blades through the spray 
of particles of softened top coating medium by rotating the fixture about 
its axis of rotation, wherein the step includes heating the blades by 
passing the blades in front of the spray gun. The temperature of top coat 
application is the temperature measured at the substrate at the time of 
applying the top coating. The temperature of application may vary from 
three hundred to eight hundred and fifty degrees Fahrenheit (300.degree. 
F. to 850.degree. F.). The actual temperature of application is preferably 
maintained at a relatively constant level varying from about .+-. five to 
ten percent (.+-.5% to 10%) of a predetermined temperature, depending upon 
the size of engine element coated, and the substrate on which the top 
coating is sprayed. 
The step of passing the blades through the spray of softened particles 
includes the step of cooling the blades. Additionally, external cooling 
may be used to control deposition temperature. 
This process results in layers of bond and top coating being sequentially 
deposited onto the blade tips in a surface of rotation substantially 
parallel to the surface of rotation which the blades describe when 
rotating in operating conditions. While the phenomenon is not well 
understood, it is believed that by depositing coating layers one at a time 
in an orientation substantially parallel to the surface of rotation that 
the coating layers will experience in an operating engine, the process 
confers an advantage as it provides relatively uniform microcracking of 
the coating in a radial direction. This results in relatively uniform 
stresses in the coating structure during operative conditions. 
The bond coating medium provides as oxidation resistant coating. Typically 
the bond coating material is a nickel-aluminum alloy. However, the bond 
coating medium may alternatively comprise of McrAIY or other oxidation 
resistive material. 
The top coating medium used consists essentially of from eleven to fourteen 
weight percent (11 to 14 wt. %) of yttria and the balance primarily being 
zirconia. This top coating composition with a high yttria content provides 
improved resistance to corrosion, as well as better temperature stability 
of the top coating ceramic material. The improved stability of the top 
coating material decreases the likelihood of spalling of the material. 
Thus, the substrate material remains protected from the corrosive effects 
of the sulfides and salts from the ambient environmental conditions. 
Further, the high yttria content of the top coating material provides for a 
material having a lower thermal conductivity as compared with material 
prepared with lower yttria content. The thermal conductivity for the 
eleven to fourteen weight percent (11 to 14 wt. %) yttria is approximately 
one point one five watts per meter Kelvin (1.15 watts/meter-k) as compared 
to a thermal conductivity of one point four watts per meter Kelvin (1.4 
watts/meter-k) for a coating consisting of seven to nine weight percent (7 
to 9 wt. %) of yttria. The lower thermal conductivity of the coating 
provides an advantage during rub events in an operational engine when the 
blade tips make contact with the inner surface of the engine case. The rub 
generates a step input of frictional heat in the contracting surface. This 
heat has to be removed. The lower thermal conductivity of the blade tip 
coating, comprising eleven to fourteen weight percent yttria, provides for 
heat transfer from the blade tip via convention and radiation. The process 
of conduction is not used for heat removal. Thus, it is believed that 
lower thermal conductivity would result in a lower substrate temperature 
as the coating does not conduct heat down to the bond coat and therefore 
to the substrate as compared with substrates coated with compositions 
containing a lower weight percent of yttria. The properties of the base 
metal substrate, thus are unaffected by heat as in the case of compressor 
blade tips, and thus retains the coating better in service. 
A primary advantage of the present invention is the quality of coating 
applied to the tips of rotor blades which results from using the process 
to distribute among a multiplicity of the rotor blades any variations in 
the process flow parameters affecting the stream of particles propelled 
against the tips. Due to the rotating fixture, a number of blades pass 
through the spray of softened coating medium. Any variations in the flow 
parameters such as variations in spray intensity, temperature, composition 
and feed of powders to the spray are distributed over a number of blades 
that pass through the spray during the period of variation. This ensures 
that one rotor blade tip does not receive all of the variations in 
coating. As a result, the coating process of the present invention 
provides for a more uniform coating and has less sensitivity to process 
variations than a process using a stationary fixture in which all 
variations are deposited only on a single blade. Further, the coating is 
applied in layers that are approximately parallel to the location of that 
part of the tip of the rotor blade about the axis. By selecting a fixture 
which positions the tips at a radius from the axis of rotation A.sub.R 
which is the same as the operative radius ensures the location of the tip 
approximates closely the radius in the engine. As a result, the coating is 
substantially parallel to the axis of rotation of the fixture and the 
coating layer follows approximately the surface of rotation which the 
coating layer will experience during operation of the engine. It is 
believed the orientation of the coating will enhance performance of the 
coating. 
Another advantage is the reproducible and reliable process that results due 
to the use of the control parameters. This process can be used to 
repetitively apply bond coating onto substrate surfaces or top coating 
onto bond coating layers. 
Another advantage is the ease and speed of application of the coating on 
the surfaces of a large number of blades at a given time which results 
from the size of the holding fixture and process which accommodates a 
multiplicity of blades. Using a holding fixture that accommodates a number 
of blades, the resultant fixturing time is minimized. In certain 
embodiments, an entire stage of blades can be coated. 
Another advantage of the present invention is the application of coating to 
substrates without the use of additional heating apparatus for the 
substrates. During coating deposition, the optimum amount of heat required 
is transmitted to the substrates through the plasma gas and the molten 
coating powder. The rotor blade is not overheated during the coating 
process. As a result, a rotor blade can be coated without changing the 
substrate microstructure or properties. 
In the following examples, the best mode practices, just described, are 
generally followed. An F-4 model air spray gun purchased from Plasma 
Technics, Inc., now supplied by Sulzer Metco, having facilities in 
Westbury, N.Y., is used for all the following examples. 
EXAMPLE I 
In this practice of the invention, small nickel rotor blades are positioned 
in a holding fixture measuring twenty-four inches (24") (610 mm) in 
diameter. 
For the bond coat application, the spray gun is powered to about 
thirty-five kilowatts (35 KW). The bond coat powder feed rate is 
forty-five grams per minute (45 gms/min). The primary gas (argon) flow 
rate is ninety-five scfh (95 scfh) (45 SLM) and secondary gas (hydrogen) 
flow rate is eighteen scfh (18 scfh) (8.5 SLM). The spray gun is 
positioned five and one-half inches (5.5") (140 mm) away from the blade 
tip surfaces. The holding fixture rotation speed is forty revolutions per 
minute (40 rpm) while the spray gun traverse rate is nine inches per 
minute (9"/min) (229 mm/min). 
For the top coat application, the plasma spray gun is powered to about 
forty-four kilowatts (44 KW). The top coat powder feed rate is twenty-two 
grams per minute (22 gms/min). The primary gas (argon) flow rate is 
sixty-seven scfh (67 scfh) (32 SLM) and secondary gas (hydrogen) flow rate 
is twenty-four scfh (24 scfh) (11 SLM). The spray gun is positioned three 
and one-quarter inches (3.25") (83 mm) away from the blade tip surfaces. 
The holding fixture rotation speed is thirty revolutions per minute (30 
rpm), while the spray gun traverse rate is six inches per minute (6"/min) 
(152 mm/min). The blade temperature during top coat application is six 
hundred plus/minus twenty-five degrees Fahrenheit 
(600.degree..+-.25.degree. F.). 
The bond coat composition is ninety-five weight percent nickel (95 wt. %) 
and five weight percent aluminum (5 wt. %). This composition results in an 
adherent bond coat on the blade tips. 
The top coat composition is twelve weight percent yttria (12 wt. %) and the 
balance essentially being zirconia. The process and the composition of the 
coatings results in a desired splat structure having vertical microcracks 
being deposited on the blade tips. The vertical microcracks extend through 
the top coating layer to the bond coating layer. 
EXAMPLE II 
In this practice of the invention, titanium rotor blades, twice the size of 
the blades used in Example I, are positioned in a holding fixture 
measuring twenty-four inches (24") (610 mm) in diameter. 
For the bond coat application, the spray gun is powered to about 
thirty-four kilowatts (34 KW). The bond coat powder feed rate is 
forty-five grams per minute (45 gms/min). The primary gas (argon) flow 
rate is ninety-five scfh (95 scfh) (45 SLM) and secondary gas (hydrogen) 
flow rate is eighteen scfh (18 scfh) (8.5 SLM). The spray gun is 
positioned five and one-half inches (5.5") (140 mm) away from the blade 
tip surfaces. The holding fixture rotation speed is forty revolutions per 
minute (40 rpm), while the spray gun traverse rate is nine inches per 
minute (9"/min) (229 mm/min). 
For the top coat application, the plasma spray gun is powered to about 
forty-four kilowatts (44 KW). The top coat powder feed rate is twenty-two 
grams per minute (22 gms/min). The primary gas (argon) flow rate is 
sixty-seven scfh (67 scfh) (32 SLM) and secondary gas (hydrogen) flow rate 
is twenty-four scfh (24 scfh) (11 SLM). The spray gun is positioned three 
and one-quarter inches (3.25") (83 mm) away from the blade tip surfaces. 
The holding fixture rotation speed is thirty revolutions per minute (30 
rpm), while the spray gun traverse rate is six inches per minute (6"/min) 
(152 mm/min). The blade temperature during top coat application is four 
hundred and twenty-five plus/minus twenty-five degrees Fahrenheit 
(425.degree..+-.25.degree. F.). 
The bond coat composition is ninety-five weight percent nickel (95 wt. %) 
and five weight percent aluminum (5 wt. %). This composition results in an 
adherent bond coat on the blade tips. 
The top coat composition is twelve weight percent yttria (12 wt. %) and the 
balance essentially being zirconia. The process and the composition of the 
coatings results in a desired splat structure having vertical microcracks 
being deposited on the blade tips. The vertical microcracks extend through 
the top coating layer to the bond coating layer. 
EXAMPLE III 
In this practice of the invention, large titanium rotor blades, three times 
the size of the blades used in Example I, are positioned in a holding 
fixture measuring thirty-four inches (34") (864 mm) in diameter. 
For the bond coat application, the spray gun is powered to about 
thirty-five kilowatts (35 KW). The bond coat powder feed rate is 
forty-five grams per minute (45 gms/min). The primary gas (argon) flow 
rate is ninety-five scfh (95 scfh) (45 SLM) and secondary gas (hydrogen) 
flow rate is eighteen scfh (18 scfh) (8.5 SLM). The spray gun is 
positioned five and one-half inches (5.5") (140 mm) away from the blade 
tip surfaces. The holding fixture rotation speed is thirty-two revolutions 
per minute (32 rpm), while the spray gun traverse rate is nine inches per 
minute (9"/min) (229 mm/min). 
For the top coat application, the plasma spray gun is powered to about 
forty-four kilowatts (44 KW). The top coat powder feed rate is twenty-two 
grams per minute (22 gms/min). The primary gas (argon) flow rate is 
sixty-seven scfh (67 scfh) (32 SLM) and secondary gas (hydrogen) flow rate 
is twenty-four scfh (24 scfh) (11 SLM). The spray gun is positioned three 
and one-quarter inches (3.25") (83 mm) away from the blade tip surfaces. 
The holding fixture rotation speed is twenty-two revolutions per minute 
(22 rpm), while the spray gun traverse rate is two inches per minute 
(2"/min) (51 mm/min). The blade temperature during top coat application is 
three hundred and twenty-five plus/minus twenty-five degrees Fahrenheit 
(325.degree..+-.25.degree. F.). 
The bond coat composition is ninety-five weight percent nickel (95 wt. %) 
and five weight percent aluminum (5 wt. %). This composition results in an 
adherent bond coat on the blade tips. 
The top coat composition is twelve weight percent yttria (12 wt. %) and the 
balance essentially being zirconia. The process and the composition of the 
coatings results in a desired splat structure having vertical microcracks 
being deposited on the blade tips. The vertical microcracks extend through 
the top coating layer to the bond coating layer. 
Although the invention has been shown and described with respect to 
detailed embodiments thereof, it should be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and the scope of the claimed invention.