Method for making a turbine blade having a wear resistant layer sintered to the blade tip surface

An abrasive, wear resistant layer is applied to the tip surface of a superalloy gas turbine blade by high temperature sintering operation which produces a high strength bond between the layer and the blade, minimizes gamma prime phase growth, and prevents recrystallization in the blade. Important features of the invention include removing plastic strain damage from the tip surface prior to the sintering operation, using induction heating techniques to sinter a layer of metal powder particles and ceramic particulates to the blade tip surface, and shielding the airfoil and root portion of the blade from the radiant heating source during the sintering operation while at the same time, conductively removing heat from the blade.

CROSS REFERENCE 
Attention is directed to the copending and commonly assigned patent 
application entitled "Apparatus for Radiantly Heating Blade Tips", U.S. 
Ser. No. 135,955, filed by J. D. Formanack et al concurrently with this 
application. 
TECHNICAL FIELD 
This invention relates to sintering, and in particular, to methods for 
sintering a mixture containing ceramic particulates and metal powder 
particles to the tip surface of a gas turbine engine blade. 
BACKGROUND 
Gas turbine engines and other similar types of turbomachines include 
axially spaced apart stages of disks which rotate within a generally 
cylindrical engine case. Attached to each disk are blades which extend 
radially outwardly from the engine axis of rotation, towards the case wall 
and across a gas flowpath. In order to increase the operating efficiency 
of these types of engines, the amount of gas in the flowpath which leaks 
through the space between the radially outer end of the blade (its blade 
tip) and the case wall should be minimized. In some engines, this is 
accomplished by a design in which the blade tips rub against the case wall 
as the disks rotate. To make the blade tips less prone to wear, abrasive 
particulates are sometimes incorporated in a metal matrix which is bonded 
or otherwise attached to the tip surface. Such wear resistant layers are 
shown in, for example, U.S. Pat. Nos. 4,249,913 to Johnson et al; 
4,227,703 to Stalker et al; 4,232,995 to Stalker et al; 4,390,320 to 
Eiswerth; 4,589,823 to Koffel; and 4,610,698 to Eaton et al. These patents 
describe numerous techniques for making a wear resistant layer and 
applying it to the blade tip surface. Powder metallurgy, electroplating, 
brazing, and plasma spray techniques are among those mentioned as being 
useful. 
The blades used in modern gas turbine engines are fabricated from high 
temperature nickel base super alloys and have either a columnar grain or 
single crystal microstructure. See, e.g., U.S. Pat. No. 3,711,337 to 
Sullivan and 4,209,348 to Duhl et al. These blades owe their desirable 
high temperature properties to an optimum microstructure, characterized in 
part by cubical gamma prime phase particles uniformly distributed in a 
gamma phase matrix. When a wear resistant layer is applied to the tip 
surface of such blades, the processes used to apply the layer must not 
adversely affect this optimum microstructure. 
In particular, the process shall not substantially alter the size, shape, 
or distribution of the gamma prime phase particles, and should not 
introduce extraneous grains (or crystals) in the microstructure, such as 
by recrystallization. Recrystallization is especially undesired, since the 
boundaries of the new grains it produces can be perpendicular to the 
primary stress axis of the blade, and are prone to cracking during 
service. 
Because of the usefulness of the wear resistant layers in gas turbine 
engines, engineers continually search for improved fabrication methods. 
This invention describes a technique which offers several advantages over 
those of the prior art. 
SUMMARY OF THE INVENTION 
This invention relates to a method for applying a wear resistant layer 
containing ceramic particulates uniformly distributed in a metal alloy 
matrix to the surface of an engine component. The metal matrix and the 
engine component are each made of high temperature alloys, each different 
from the other. The method includes the steps of: removing plastic strain 
damage from the surface of the component which is to receive the wear 
resistant layer; disposing a mixture of the ceramic particulates and 
powder particles having the matrix alloy composition onto the component 
surface, the particulates uniformly distributed throughout the powder 
particles; radiantly heating the mixture of ceramic particulates and 
powder particles at a controlled rate to a temperature sufficient to melt 
most of the powder particles and to cause interdiffusion between the 
powder and component alloys, while simultaneously removing heat from the 
component to prevent substantial melting or other metallurgical changes 
therein; and then cooling the layer to cause the melted powder particles 
to solidify and sinter to each other and to the component surface to form 
a wear resistant layer metallurgically bonded to the component surface, 
the ceramic particulates being entrapped in the solidified metal matrix. 
The term "plastic strain damage" is used in the conventional sense, and is 
meant to describe plastic as opposed to elastic deformation which, when 
heated to a sufficiently high temperature, recrystallizes to form stress 
free grains. When the surface of the component which is to receive the 
wear resistant layer is free of plastic strain damage, it is in a 
"stress-free condition", and no recrystallization is produced when the 
component is heated to high temperatures. 
The term "sintering" and "liquid phase sintering" are both meant to 
describe melting of a substantial amount, but not all, of the matrix metal 
powder particles, followed by cooling below the matrix solidus temperature 
to solidify the molten metal. 
In its most preferred form, the invention is used to apply a wear resistant 
layer to the tip surface of a directionally solidified, single crystal 
turbine blade made from a nickel base superalloy. The wear resistant layer 
contains nickel base superalloy powder particles and silicon carbide 
abrasive particulates coated with a thin layer of aluminum oxide. The 
powder particles are mixed with the abrasive particulates and with a 
thermoplastic resin to form a tape-like material in which the particulates 
are uniformly distributed throughout the thickness of the tape. The tape 
is placed upon the tip surface of the blade which has previously been 
electropolished to remove any plastic strain damage which might 
recrystallize during the radiant heating step. Then, the airfoil and root 
portions of the blade are wrapped in a heat insulating material, the blade 
placed upon a water cooled copper chill plate, and the chill plate moved 
upwards to position the tip of the blade into an inductively heated 
graphite susceptor. At the completion of a predetermined heating cycle 
which melts a majority of the powder particles, the chill plate is moved 
out of the susceptor, causing the blade to cool and the melted powder 
particles on the blade tip to solidify. The rate at which the tape is 
heated within the susceptor, in conjunction with the electropolished tip 
surface, and the insulated and conductively cooled blade, controls the 
resulting microstructure of the wear resistant layer and of the blade 
itself. In particular, process parameters are chosen to control the amount 
of powder particles which melt, and the temperature to which the blade 
reaches during the sintering process. Most but not all of the powder 
particles melt, while at the same time, interdiffusion between the 
particles and the blade tip occurs. The melted powder particles wet the 
surface of the blade tip, and some melting of the tip occurs, which can be 
likened to the surface dissolution which takes place during a conventional 
brazing process. As a result, a braze-like bond forms between the metal 
powder particles and blade tip surface. 
Because not all of the metal powder particles melt during the heating step, 
the process for forming the wear resistant layer of this invention is 
considered to be a type of liquid phase sintering which does not 
substantially alter the distribution of abrasive particulates within the 
layer. Wear resistant layers made according to this invention have high 
strength, and the properties of the blade are not diminished during the 
high temperature sintering process. 
The details of the present invention will become more apparent from the 
following description of preferred embodiments and the accompanying 
drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
The invention is described in terms of sintering a wear resistant layer to 
the tip surface of a nickel base superalloy gas turbine engine blade. 
However, it should be noted that other components which require a durable, 
wear resistant surface layer can be fabricated according to the methods 
described below. 
FIG. 1 shows a wear resistant layer 10 fabricated according to this 
invention, on the tip surface 12 of a gas turbine engine blade 14. The 
blade has a root portion 16, an airfoil portion 18, and a tip portion 20 
at the radially outer end of the blade 14. 
The blade 14 preferably has a single crystal microstructure, although the 
invention is equally useful with blades that have a columnar grain or 
equiaxed grain microstructure. Single crystal blades are preferred, as 
they have the levels of strength which are required for use in advanced 
gas turbine engines. Single crystal components derive their high strength, 
in part, from an optimized distribution of cuboidal gamma prime phase 
particles in a gamma phase matrix, and from the absence of grain 
boundaries. The optimum size of the gamma prime phase should be in the 
range of about 0.2-0.5 microns (0.008-0.020 mils). The tip portion of the 
blade 14 is usually subject to lower operating stresses than the airfoil 
portion 18 of the blade 14, and some deviation in the optimum gamma prime 
size at the blade tip portion 20 is permitted. In particular, the size of 
the gamma prime near the blade tip 20 may be somewhat larger, perhaps up 
to about 0.7 microns (0.028 mils). However, because of the adverse effects 
of recrystallization on the mechanical properties of single crystal 
components, there should be no recrystallized grains in the 
microstructure. 
The wear resistant layer 10 of this invention is characterized by a uniform 
distribution of abrasive particulates 22 within a high temperature metal 
alloy matrix 24. The preferred abrasive particulate is alumina coated 
silicon carbide, of the type described in the aforementioned patent to 
Johnson et al. The alumina coating prevents chemical interaction between 
the silicon carbide particulates 22 and the metal matrix 24 during 
fabrication of the wear resistant layer 10 and during service use. The 
ceramic particulates have a nominal diameter of about 25-625 microns (1-25 
mils), depending on the operating requirements of the layer 10. In most 
cases, the preferred particulate size is about 125-500 (5-20 mils), most 
preferably about 375 microns (15 mils). Particulates such as those 
described in U.S. Pat. No. 4,424,066 to Sarin et al (alumina coated 
SiAlON) may also be used, as long as they do not react with the metal 
matrix 24 and have the necessary abrasive characteristics and high 
temperature stability. To fabricate a wear resistant layer having high 
strength, the matrix 24 should have a nickel or cobalt base superalloy 
composition, as described in copending and commonly assigned application 
Ser. No. 947,067 to Schaefer et al. 
The initial step in making the layer 10 is to mix the abrasive particulates 
with metal powder particles having the matrix composition, and with a 
volatilizable resin, and then to form the mixture into a sheet of transfer 
tape, or any other tape-like material. Techniques for making such 
materials are described in U.S. Pat. Nos. 4,596,746 and 4,563,329, both to 
Morishita et al, as well as in the aforementioned application to Schaefer 
et al; all are incorporated by reference. The abrasive particulates should 
be uniformly distributed throughout the powder metal matrix for optimum 
wear resistance, and should make up about 10-35 volume percent of the 
abrasive layer. One advantage of using a tape-like product is that it is 
easily cut into the size and shape which corresponds to the blade tip 
surface. The tape is then placed on the blade tip surface 12 and sintered 
to the surface 12 in a high temperature sintering process, which is 
described in more detail below. The sintering process must be closely 
controlled to obtain the necessary properties in the wear resistant layer 
10, and to prevent degradation of the base metal blade properties. Of 
particular concern regarding base metal properties is that there be no 
recrystallization in the blade during sintering and that the amount of 
gamma prime phase growth in the blade must be minimized. 
The most important properties which the sintered abrasive layer 10 must 
have, besides wear resistance, are creep strength and oxidation 
resistance. Both of these properties are significantly influenced by the 
composition and microstructure of the layer 10, the latter being 
particularly dependent upon the way in which the tape is sintered to the 
blade tip surface 12. Porosity should be minimized in the sintered layer 
10, and the ceramic particulates 22 should be uniformly distributed 
throughout the thickness of the layer 10. Of course, the layer 10 must be 
securely bonded to the blade tip surface 12. For optimum creep strength, 
the matrix 24 must have a large grain size, meaning that the grains in the 
solidified matrix are larger than the size of the starting (unmelted) 
metal powder particles. 
In order to achieve a dense matrix having a large grain size, some melting 
of the metal powder particles must take place during the sintering 
process. Thus, there must be liquid phase sintering, which must take place 
under closely controlled conditions. Tests have shown that if too little 
melting of the metal powder particles take place, the sintered metal 
matrix will contain some porosity, due to the inability of the melted 
metal to fill in the interstices between all of the unmelted powder 
particles. And if too much melting takes place, the ceramic particulates 
will have a tendency to float in the liquid, because the ceramic is less 
dense than the metal. If the particulates are able to float or otherwise 
move around to a considerable extent during the sintering operation, the 
desired uniform distribution of ceramic particulates within the sintered 
layer will not be achieved. Between about 50 and 90% of the powder 
particles should melt during the sintering process. Melting of the powder 
particles also results in interdiffusion between the particles and the 
blade tip surface, and produces a braze-type bond between the layer and 
blade tip on cooling. 
The rate at which the temperature is raised during the sintering operation 
must be closely controlled, especially to control the rate at which the 
binder in the tape volatilizes. If the binder volatilizes too rapidly, as 
a result of too rapid a rate of temperature increase, the powder particles 
and abrasive particulates will be violently expelled from the tape, 
resulting in a non-useful layer. 
To prevent recrystallization during the sintering operation, plastic strain 
damage which would otherwise recrystallize during the sintering process 
must be removed from the surface of the blade before sintering, especially 
strain damage present in the radially outer portion of the blade, 
identified as Region A in FIG. 2. (Region A includes the blade tip surface 
12.) Conventional electrolytic techniques such as electropolishing may be 
used; see, e.g., U.S. Pat. Nos. 3,057,764 to LaBoda et al and 3,873,512 to 
Latanision. Since plastic strain damage is typically confined to very near 
the surface, no more than only about 12-25 microns (0.5-1 mils) of surface 
material usually needs to be removed. 
To control the temperature to which the blade is heated during the 
sintering operation, special methods and apparatus are used. These methods 
and apparatus are best described with reference to FIGS. 2-3. 
FIG. 2 shows a blade 14 positioned within a heating chamber 26 which 
sinters the abrasive carrying tape 28 to the tip surface 12 according to 
this invention. As described above, the tape 28 comprises abrasive 
particulates uniformly distributed within a matrix of superalloy powder 
particles. A resin type material binds the particulates and powder 
particles to each other. 
The heating chamber 26 is defined by slot walls 34 which extend through the 
thickness of a graphite susceptor 36. The susceptor 36 is inductively 
heated by low frequency (2,500-3,000 Hz) induction coils which are not 
shown in the Figure. Preferably, the susceptor 36 is cylindrical in shape, 
having a plurality of circumferentially spaced apart heating chambers 26, 
as shown in FIG. 3. To prevent oxidation or other such adverse reactions 
during the high temperature sintering process, the susceptor 36 is 
enclosed within a protective atmosphere chamber, preferably a vacuum 
chamber which is not shown in the Figures. 
The blade 14 rests upon a support fixture 30 which is machined to receive 
the root portion 16 of the blade. The fixture 30 is disposed upon a heat 
sink 32 such as a water cooled copper chill plate. While the abrasive 
carrying tape 28 and blade tip portion 20 are heated by the susceptor 36 
as described below, the heat sink 32 conducts heat from the blade, which 
maintains the blade 14 within a desired temperature range. 
The temperature of the blade 14 during the sintering process is further 
controlled by insulation which shields the airfoil and root portions 18, 
16, respectively, from the radiant heat source 36. In particular, a 
tantalum metal shield 38 surrounds the airfoil portion 18 of the blade 14, 
and rests upon a support 40 which extends from the blade root fixture 30. 
The shield 38 is a box-like structure having an airfoil shaped cut-out 39 
in its top surface 41 through which the blade tip 12 extends. The shield 
38 acts as a heat reflector, and also is stuffed with a material 43 which 
insulates the airfoil and root portions of the blade 14 from the radiant 
source 36. FIBERFAX.RTM. insulation (the Carborundum Company, Niagara 
Falls, N.Y.) is the preferred material used in this instance; other 
suitable insulating materials and shield designs will be apparent to those 
skilled in the art. Layers of rigid insulating materials 44, 46 also 
shield the blades 14, as well as the fixture 30 and heat sink 32 from the 
heat source 36. A first insulation layer 44 is secured to the lower 
surface 48 of the graphite susceptor 36, and second insulation layer 46 
rests upon the support 40. A layer of rigid insulation 50 rests upon the 
top surface 52 of the susceptor 36. 
At the beginning of the sintering process, the susceptor 36 is inductively 
heated to the maximum sintering temperature, and then the chill plate 32 
is moved into proximity with the susceptor 36, so that the tip portion 20 
of each blade 14 extends into its respective heating chamber 26, as shown 
in FIG. 2. The rate at which the blades 14 are raised into their heating 
chamber 26 is controlled, so that each blade 14 gradually reaches the 
maximum sintering temperature. A controlled rate is particularly important 
at the beginning of the process, to avoid an excessive rate of binder 
volatilization, as discussed above. After the tape 28 and blade tip 12 
have been heated to melt between about 50-90% of the powder particles, the 
chill plate 32 is moved vertically downward, so that the blades 14 are 
removed from their respective chamber 26. As the blades 14 cool, the 
molten metal powder particles solidify to each other and to the blade tip 
surface 12, to form a dense, wear resistant layer. The abrasive 
particulates are entrapped within the layer 10 as the metal powder 
particles solidify. 
During the sintering process, the melted powder particles wet the tip 
surface 12 of the blade 14. Some melting and/or dissolution of the surface 
12 likely occurs, similar to that which occurs during brazing processes. 
The resulting joint between the layer 10 and tip surface 
metallographically appears as a braze-like bond. 
Process parameters which influence the success of the sintering process 
include the position of the blade 14 within the heating chamber 26 and the 
location of the various types of insulating material relative to the blade 
14 and the heat source 36; and the rate at which the temperature is raised 
during the sintering operation. For example, if too much of the airfoil 
portion 18 of the blade 14 is directly exposed to the heat source 36, the 
blade may recrystallize or undergo excessive gamma prime phase growth. 
Alternatively, if too little of the blade 14 is radiantly heated, an 
insufficient amount of powder particle melting will take place, and the 
wear resistant layer 10 will not have the requisite properties. 
Use of radiant heat source in this invention, and in particular, an 
inductively heated graphite susceptor, allows for close control over the 
actual sintering temperature, and for close control over the rate at which 
the temperature of the tape 28 and blade tip 12 are increased up to the 
maximum sintering temperature. Both are necessary for the successful 
practice of this invention. Tests have shown that when a mixture of 
ceramic particulates and nickel base superalloy powder was heated by 
sources such as plasmas (e.g., with plasma spray apparatus), lasers, and 
electric arcs (e.g., with a tungsten inert gas welding apparatus), the 
amount of melting was virtually uncontrollable, and the distribution of 
particulates within the matrix was destroyed. Thus, techniques such as 
those described in U.S. Pat. No. 4,627,896 to Nazmy et al are not useful 
in the fabrication of a wear resistant layer. 
While the exact relationships between the blade, the heat source, and the 
insulation will vary depending upon the particular materials and 
facilities used in making a wear resistant layer on a gas turbine engine 
component, the example discussed below is provided to illustrate one 
particular fabrication sequence. This example is meant merely to 
illustrate several features of the invention, as applied to a particular 
blade alloy, transfer tape composition, and radiant heating equipment. 
The nickel base superalloy composition described in the aforementioned U.S. 
Patent to Duhl et al was melted and solidified to form a single crystal 
casting. The casting was removed from its investment casting mold, 
cleaned, and then machined to the desired length. After machining, the tip 
surface and airfoil walls within about 12 millimeters (1/2 inch) from the 
tip were electrolytically polished to remove plastic strain damage 
produced during the cleaning and machining operations; about 12 microns 
(0.5 mils) of material was removed. Then, a tape containing about 25 
volume percent of 375 micron (15 mil) alumina coated silicon carbide 
particulates distributed throughout a nickel base superalloy powder matrix 
and METHOCEL.RTM. binder (Dow Chemical Company, Midland, Mich.) was 
affixed to the blade tip surface with NICROBRAZ.RTM. binder (Wall Colmony 
Corp., Detroit, Mich.). The nominal composition of the powder matrix was, 
on a weight percent basis, about 25 Cr, 8 W, 4 Ta, 6 Al, 1.2 Si, 1 Hf, 0.1 
Y, balance Ni. The powder particles were about minus 80 mesh, U.S. Sieve 
Series. 
The blade was placed within a broach block upon a water cooled copper chill 
plate, and then a Fiberfrax filled tantalum shield placed over the airfoil 
portion of the blade, substantially as shown in FIG. 2. About 9 mm (3/8 
inch) of the blade tip protruded above the top of the shield. 
The blade was then raised at a controlled rate into an evacuated heating 
chamber (about 10.sup.-6 mm Hg) within an inductively heated graphite 
susceptor; the temperature within the chamber was about 1,270.degree. C. 
(2,320.degree. F.). The blade was raised partially into the chamber, such 
that the temperature of the blade near the blade tip increased at a rate 
of about 15.degree. C. (27.degree. F.) per minute between ambient 
conditions and about 480.degree. C. (900.degree. F.). This relatively slow 
temperature increase allowed the Methocel binder in the tape to slowly 
volatilize. Then, the blade was raised further into the chamber, until 
about the outer 9 mm was directly adjacent to the susceptor walls 34; the 
tip portion temperature then increased at a rate of about 250.degree. C. 
(45.degree. F.) per minute. After the blade tip reached 1,270.degree. C. 
and was held at 1,270.degree. C. for 15 minutes, the blade Was removed 
from the chamber, which caused the melted powder particles to solidify. 
Metallographic examination of the sintered layer and the blade itself 
revealed the structure shown in FIG. 4: At the interface between the layer 
and the blade tip surface was a braze-like bond joint, which indicated 
that some amount of interdiffusion between the elements in the metal 
matrix and the elements in the blade alloy took place. It also indicated 
that a small amount of melting took place at the tip surface. The amount 
of such melting was considered acceptable, and is believed to be 
preferable for optimum bond strength. Metallographic examination also 
revealed a uniform distribution of silicon carbide particulates within the 
layer, and some remnants of unmelted metal powder particles. It appeared 
that less than about 10% of the powder particles did not melt during the 
sintering process. 
No recrystallization of the blade was evident. Some of the gamma prime 
phase near the blade tip was larger than that found in the airfoil portion 
of the blade; however, the size of this enlarged gamma prime phase was 
considered to be acceptable. 
Although this invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that various changes and omissions in the form and detail thereof 
may be made without departing from the spirit and scope of the invention.