The sealing of the gas path in a gas turbine engine at the blade tips is improved by maintaining a minimum clearance between the rotor blade tips and the gas path seal. This is accomplished by taking advantage of an increase in volume during controlled oxidation of certain intermetallic compounds which have high melting points. The increase in volume closes the clearance subsequent to a rub between the blades and the seal. Thus, these compounds re-form the tip seal surface to assure continued engine efficiency

Cawley, J. D.

English

NASA Technical Reports Server

United States Patent 1191 
Cawley 
[54] 
[75] Inventor: James D. Cawley, Cleveland Hts., 
[73] Assignee: The United States of America as 
OXIDIZING SEAL FOR A TURBINE TIP GAS 
PATH 
Ohio 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. NO.: 602,050 
[22] Filed: Apr. 19, 1984 
[51] Int. Cl.3 .............................................. FOlD 11/08 
[52] U.S. C1. ................................ 415/174; 415/170 R 
415/196; 415/200; 416/174 
[58] Field of Search ................... 415/174, 170 R, 196, 
415/200, 197,9, 172 A, 416/174; 277/53, DIG. 
6, 96.2; 428/614 
1561 References Cited 
U.S. PATENT DOCUMENTS 
3,883,147 5/1975 Troy et al. ......................... 217/96.2 
4,149,823 4/1979 Zboril .................................. 415/200 
[I 11 Patent Number:: 4,540,336 
[45] Date of Patent:: Sep. 10, 1985 
4,202,928 5/1980 Staebler ............................... 428/446 
4,336,276 6/1982 Bill et al. ............................. 415/174 
4,331,476 6/1982 Fraser et al. ........................ 428/446 
4,377,371 3/1983 Wisander et al. ................... 415/197 
Primary Examiner-Robert E. Garrett 
Assistant Examiner-John Kwon 
Attorney, Agent, or Firm-Gene E. Shook; John R 
Manning 
[571 ABSTRACT 
The sealing of the gas path in a gas turbine engine at the 
blade tips is improved by maintaining a minimum clear- 
ance between the rotor blade tips and the gas path seal. 
This is accomplished by taking advantage of an increase 
in volume during controlled oxidation of certain inter- 
metallic compounds which have high melting points. 
The increase in volume closes the clearance subsequent 
to a rub between the blades and the seal. Thus, these 
compounds re-form the tip seal surface to assure contin- 
ued engine efficiency. 
13 Claims, 4 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19850026089 2020-03-22T20:24:09+00:00Z
U.S. Patent Sep. 10,1985 Sheet 1 of2 4,540,336 
FIG. 1 
U.S. Patent Sep. io, 1985 Sheet 2 of 2 4,540,336 
FIG. 4 
4.540,3 3 6 
1 
OXIDIZING SEAL FOR A TURBINE TIP GAS 
PATH 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the U.S. Government and may be manufac- 
tured and used by or for the Government for govern- 
mental purposes without the payment of any royalties 
thereon or therefor. 
TECHNICAL FIELD 
In a gas turbine engine there is a pressure differential 
across the turbine rotor. The engine performance is 
improved if this differential is maintained at a high level. 
In order to achieve this desired result the clearance 
between the rotor tips and the gas path seal must be kept 
to a minimum. 
A consequence of these close clearances is that there 
is a high probability of a rubbing interaction between 
the turbine blades and the seal. Any wear that occurs 
during a rub increases the clearances and decreases the 
obtainable differential in pressure. 
Seals in present day systems utilize materials that are 
stable in an oxidizing atmosphere, and these materials 
wear preferably to the tubine blades. The result of a rub 
in this configuration is that of material being swept out 
of the seals during the rub. The penalty in this event is 
permanent and cumulative, though less than if the rotor 
had received the wear. 
It is, therefore, an object of the present invention to 
provide an improved gas path seal for a gas tubine en- 
gine having the ability to reform the tip seal surface 
subsequent to a rub to restore and maintain a minimum 
clearance. 
BACKGROUND ART 
Watkins, Jr. et al. U.S. Pat. No. 4,063,742 and Bill et 
al. U.S. Pat. No. 4,295,786 are directed to seals having 
compliant backing structures which provide for defor- 
mation to reduce abrasion. In the Watkins, Jr. patent the 
seal uses a series of thin narrow elongated metal strips to 
accommodate deformation while the Bill et al. patent 
discloses a gas path seal made of a thin layer of a de- 
formable, metallic material such as aluminum. 
Schilke et al. U.S. Pat. No. 3,817,719 discloses a po- 
rous abradable material used in gas turbine seals. The 
disclosed material is formed through an oxidizing step, 
but the resulting product is resistant to oxidation. 
Panzera U.S. Pat. No. 4,080,204 describes an abrad- 
able seal for a turbine made from various alloys. The 
Dreferred alloy includes 20-27% nickel, 18-22% chro- 
dation time, or oxide thickness, 
the tip seal surface to assure continued engine eff- 
cienc y. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The objects, advantages, and novel features of the 
invention will be more fully apparent from the follow- 
ing detailed description when read in connection with 
the accompanying drawings in which 
FIG. 1 is a schematic view of a transverse cross sec- 
10 tion of an arrangement for a turbine or a compressor 
shroud having an improved seal constructed in accor- 
dance with the present invention, 
FIG. 2 is a schematic view in transverse cross-section 
of the seal shown in FIG. 1 immediately after rubbing, 
FIG. 3 is a transverse cross-section of the seal shown 
in FIG. 2 after a period of time has elapsed subsequent 
to the rubbing showing the post rub oxide growth filling 
in the rub area, and 
FIG. 4 is a cross-section view illustrating an oxidizing 
20 intermetallic material. 
5 
15 
BEST MODE FOR CARRYING OUT THE 
INVENTION 
Referring now to the drawing a rotor blade 10 of a 
25 turbine rotates about an axis 12 in a counter-clockwise 
direction as shown in FIG. 1. The fluid in which it 
operates flows in a direction into the paper. A shroud 14 
surrounds the blade 10 and is substantially concentric 
with the axis 12. The shroud 14 includes an oxide layer 
30 16 of a material 18 that is oxidizable. The surface of the 
oxide layer 16 is aerodynamically smooth and closely 
spaced to the blade tips. 
A sprayed coating of the material 18 on the inside 
surface of the turbine housing or casing (not shown) has 
35 been found to be suitable for forming a seal between the 
tip of the rotor blade 10 and the shroud 14. The material 
18 may be deposited on the turbine housing by any of a 
number of conventional methods, such as spray coating, 
sintering, etc. to form a substrate that is subsequently 
It is further contemplated that the material 18 may 
not be deposited on the turbine housing in certain instal- 
lations. The entire shroud 14 may be fabricated from a 
sheet or sheets of the material 18. 
45 This invention is concerned with the material 18 
utilized to form the oxide layer 16. Many materials 
exhibit parabolic oxidation kinetics. In this invention 
the oxide thickness may be described by the equation 
(1)x2=Kpt or x = d K p t .  In this expression, x is the oxide 
50 thickness shown in the drawings, t is the time, and Kp is 
a material property termed the parabolic rate constant. 
The rate of oxide growth, dx/dt, decreases with oxi- 
40 oxidized. 
mium, and 9-i5% aluminum, with a trace of yttrium. 55 
The materials described in both the Panzera patent and 
the Schilke et al. patent can be operated only at relative 
low temperatures below about 2000" F. 
DISCLOSURE O F  INVENTION 
This invention is concerned with improving the seal- 
ing of the gas path at the tubine blade tip. This is accom- 
plished by taking advantage of an increase in volume 
during controlled oxidation of certain intermetallic 
compounds. 
These materials have iiigh melting points. The in- 
crease in volume is relied on to close the clearances 
subsequent to a rub. In effect, these materials re-form 
(2) 
A= dt $*= + 
Many materials exhibit a volume change upon oxida- 
60 tion. By way of example, a general intermetallic com- 
pound is identified as ApBq, where A and B represent 
metallic elements, and p and q are integers. In order to 
simplify the mathematics, the assumption is made that 
the oxide of one of the species is dominant; B forms 
65 BDt. This assumption is made only for convenience; the 
principle also holds for multi-phase oxidation. 
When the material is oxidized, dimensional changes 
may occur. These are dependent upon the relative den- 
3 
4,540,336 
sities of the oxide and the intermetallic compound. Such 
a system is shown in FIG. 4. Defining x as the thickness 
of the BsOt oxide, y as the thickness of intermetallic 
substrate, and X as the overall thickness of the system, 
a number of relationships may'be set forth. For exam- 
ple, the overall dimensional change is the sum of the 
changes in the thicknesses of oxide and substrate. 
(3) 10 
If there is no external source of B and no evaporation of 
B occurs, then the number of moles of B in the system 
is constant 15 
where Mi is the number of moles of B in the ith phase 20 
therefore 
or 
(5)  
30 
The amount of B entering the oxide is 
where Ni is the fractional molar density of B in the ith 
phase (Mol/M3). Vi is Ehe volume of ith phase (m3), 
while the amount leaving the substrate is 40 
dMsubsrrure d Vsubsrrare (7) t= N s u b s t r a r e T  
Therefore, it is possible to obtain an expression for the 45 
dimensional change of system in terms of the oxide 
growth rate. 
Equating equations 6 and 7 results in the expression 
50 
When the cross sectional area is constant, this may be 
rewritten as 
55 
Solving for substrate dimensional change, dy/dt, and 6o 
substituting into equation 3 results in the expression 
(9) 
65 
The fraction molar concentrations can, in turn, be writ- 
ten as 
N i  = Pi  C i  
where 
Ci is the fraction of B in the ith phase 
pi is the density of the ith phase (g/m3) 
oi is the molecular weight of the ith phase (g/mol) 
Therefore, equation 8 may be rewritten as 
Because these are all material constants, a composite 
constant may be defined 
p=-.-.- poxide wsubsrrare coxide 
psubsrrare woxide Csubsrrate 
From this definition it is apparent that when the com- 
posite constant is less than one, the overall thickness 
increases with increasing oxidation. When the compos- 
ite constant is greater than one, the overall thickness 
would decrease. 
According to the present invention the seal material 
18 is oxidized prior to installation for a given time, ti, at 
a given temperature, TI ,  to produce an oxide layer 16 
having a thickness of xi as shown in FIG. 1. In the event 
of a rub which abrades the oxide, a gradient in oxide 
thickness is produced as shown in FIG. 2. The region of 
the seal which lost material during the rub is an interme- 
tallic compound protected by a thinner amount of ox- 
ide. The result is that this material will oxidize faster, as 
can be seen in equation 2 above. 
Any volume change which oxidation produces will 
occur to a greater extent in the rub area. In the case of 
a positive volume change, the rubbed area will tend to 
be filled in, as schematically shown at 20 in FIG. 3. The 
amount that can be recovered is dependent on the value 
of the constant defined in equation 10. The rate at which 
recovery takes place is dependent on the value of the 
parabolic rate constant, Kp in equation 1. 
This type of post rub dimensional change can be 
effected through the use of any material which forms an 
adherent oxide film. One class of materials which have 
properties suitable for the oxidizing seal of the present 
invention is the silicides. These materials include sili- 
cides of molybdenum, tungsten, tantalum, titanium, and 
boron. All have high melting points and are silica for- 
mers. During oxidation an adherent layer of Si02 is 
formed through which diffusion must take place for 
further oxidation. 
A positive volume increase also occurs. For example, 
in MoSi2 the composite constant in equation 10 is 
Equation 9 for MoSi2 undergoing oxidation, then, is 
= 0.543 
d t  
Therefore, the overall thickness increases at over half 
the rate of oxidation. 
4,540,336 
5 6 
It should also be noted that the incorporation of po- 
rosity, as a second phase, in the oxide will decrease the 
value of the composite constant, p, because the effec- 
tive oxide density will be lower. Also, if the oxide is 
being formed On a 
seal of the type shown in FIG. 1, the radial change will 
be greater than equation 11 predicts because the cross- 
sectional area decreases with decreasing radial distance. 
While the preferred embodiment of the invention has 
been shown and described, it will be appreciated that 
various structural modifications may be made without 
departing from the spirit of the invention or scope of the 
subjoined claims. 
7. A gas path seal for a turbine or the like having a 
plurality of blades mounted for rotation about an axis 
comprising 
a stator shroud having an aerodynamic smooth sur- 
face on an oxide layer of a high melting point silica 
that is oxidizable, said oxide layer being wearable 
relative to said blade tips whereby a portion of said 
oxide layer is removed in a rub area of said surface 
thereby reducing the thickness of said layer in said 
rub area when said blades rub against said surface, 
and 
a substrate of an oxidizable intermetallic compound 
of a silicide in contact with said oxide layer at least 
in said rub area so that said intermetallic compound 
in said substrate adjacent to said rub area oxidizes 
to reform said surface. 
8. A gas path seal as claimed in claim 7 wherein the 
9. A gas path seal as claimed in claim 7 wherein the 
20 intermetallic compound is a silicide of tungsten. 
10. A gas path seal as claimed in claim 7 wherein the 
intermetallic compound is a silicide of tantalum. 
11. A gas path seal as claimed in claim 7 wherein the 
intermetallic compound is a silicide of titanium. 
12. A gas path seal as claimed in claim 7 wherein the 
intermetallic material is a silicide of boron. 
13. A gas Path seal for a turbine or the like having a 
plurality of blades mounted for rotation about an axis 
comprising 
a stator shroud having an aerodynamic smooth sur- 
face on an oxide layer of a high melting point mate- 
rial that is oxidizable, said oxide layer being wear- 
able relative to said blade tips whereby a portion of 
said layer is removed in a rub area of said surface 
thereby reducing the thicknless of said layer in said 
rub area when said blades rub against said surface, 
and 
a substrate of an intermetallic compound of a material 
whose oxide exhibits parabolic growth in contact 
with said oxide layer at least at said rub; area so that 
said material in said substrate adjacent to said rub 
area oxidizes to reform said surface. 
surface, as in a gas turbine 5 
10 
I claim: 
1. In a gas path seal for a turbine or the like having a 
plurality Of blades mounted for rotation about an axis, 
an improved shroud surrounding the tips of said blades 
in substantially concentric relationship to said axis, said 
shroud comprising 
an annular substrate of an oxidizable intermetallic 
compound of a silicide having a high melting point 
spaced from the tips of said blades, and 
an oxide layer of silica that is abradable relative to 
said blades and closely spaced to said blade tips 25 
covering said intermetallic compound to form a 
seal surface SO that said oxide layer abrades in a rub 
area on said surface when said blades contact said 
layer thereby reducing the thickness of said layer in 
said rub area whereby said 
adjacent to said rub area oxidizes to reform the seal 
surface. 
2. A gas path seal as claimed in claim 1 wherein the 
3. A gas path seal as claimed in claim 1 wherein the 
4. A gas Path seal as claimed in claim 1 Wherein the 
5. A gas path seal as claimed in claim 1 wherein the 40 
6. A gas path seal as claimed in claim 1 wherein the 
15 
intermetallic compound is a silicide of molybdenum. 
compound 30 
intermetallic compound is a silicide of molybdenum. 
intermetallic compound is a silicide of lungstun. 
intermetallic compound is a silicide of tantalum. 
intermetallic compound is a silicide of titanium. 
35 
intermetallic compound is a silicide of boron. * * * * *  
45 
50 
55 
60 
65 


Oxidizing seal for a turbine tip gas path

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