Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Porous Metal Structures Made by Thermal
SprayLng Fugitive Ma-teriaL ancl Metal
Technical Field
The present invention relates -to a method of
making porous metal structures by thermal spraying,
such as plasma arc spraying.
BackyrOund
Porous metal structures may be made by a variety
of processes and used in a variety of situations.
It has been found that porous metal structures are
particularly useful for abradable seals, which are
structures that readily wear at a rapid rate when
contacted by a high velocity part, but which other-
wise have integrity. They are especially used in
turbomachinery. See for instance Elbert et al
U.S. Pat. No. 4,049,428 and Ball U.S. Pat. No.
3,111,396.
One of the favored methods for making porous
metal structures is to form a compact of metal and
fugitive material, and then cause the fugitive
material to disappear, thus leaving a metal structure
with less than full density. Conventional powder
metallurgy techniques which involve making an
admixture, pressing and sintering, have been used.
See Breton et al U.S. Pat. No. 3,864,124, Salyer et al
U.S. Pat No. 3,897,221 and the Bail and Elbert et al
patents. See also U.S. Pat. No. 3,350,178 to Miller.
Because of the substantlal shrinkage which
sintering causes in a powder metal article, we have
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previously pre~errecl using plasma arc sprayiny to
make porous metal structures. Duriny plasma spraying
of a metal-polyrner mixture, the metal particulates
are bonded to one another. Thus, a subsequent
sintering is either not needed or, if used, does not
cause excessive shrinkage. Generally, a composite
has been first macle according to the teachings of
Longo et al in U.S. Pat. No. 3,723,165 wherein a
rnetal powder such as nichrome is sprayed along with
a high temperature polyester powder, such as
poly(paraoxybenzoyl), having a high melting point.
The polyester softens but does not melt during
spraying. While the particular step of making a
porous structure by subsequently oxidizing the polymer
is not mentioned in the Longo et al patent, we and others
have done so in the making of experimental porous seal
structures for gas turbine engines. Since the poly-
ester used is a high temperature material, temperatures
in the range of 540C are needed to oxidize away the
fugitive. However, while the abradable seals so made
are effective for yas turbine engine use, the cost of
the polyester resin particulate is high. Therefore,
improvements have been sought, both to reduce costs
and to improve the performance of abradable seals of
porous metals by changing their physical and chemical
characteristics.
Disclosure of the Invention
An object of the invention is to provide an im-
proved method for making porous metal structures, in
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particular Eor makLng an abradable .seal for a turbine
engine where the seal has improved per~orrnance and
lower cos-t.
According to -the invention, a polymer which is
meltable and which has a low temperature of fleeing
from a sprayed deposit is thermally sprayed together
with a metal powder onto a substrate. The polymer is
made to flee from the deposi-t by heating to a
temperature less than that which causes more than 30
lG weight percent oxide to be present in the porous
metal deposit which remains. It has been discovered
that a low oxide content in an abradable seal material
enhances its performance, particularly with respect to
avoiding glazing during interaction with a com-
pressor blade.
Preferably, plasma arc spraying, or anotherspraying process which avoids excess oxidation of
the particulate, is used to make an initial deposit of
polymer and metal mixture on a substrate. Then the
polymer is removed by heating to a relatively low
temperature. The polymer may disappear by de-
polymerizing to a volatile, cherrlically combining to
form a volatile, by dissolution, or by another
process which does not involve excess heat generation.
Depolymerization is preferred because it is an
endothermic process and thus contrasts favorably with
an oxidation process which exothermically can raise
temperatures and cause unwanted oxidation or other
reaction. The invention applied to nichrome alloys
produces oxide contents of 4-30 weight percent in
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structures which are 26-~0% of the density of solid
metal. Such levels compare with a typical 30-~0
weight percent oxicle con-tent characteristic of
nichrome porous metals made by the earlier technique
using a high temperature polymer. Preferably, the
polymer and metal deposit is heated in air to reduce
cost. For nichrome alloys this essentially means
conducting the removal process at a low temperature,
less than 5~0C, preferably 250-~30C.
In a preferred practice of the invention, 14
weight percen-t polymethylmethacrylate powder is
sprayed with 86 weight percent 80Ni-20Cr powder. The
deposit so created is heated to about 315C to con-
vert the polymer to a volatile monomer. The
resultant porous metal structure has an apparent
specific gravity of 2.7 g/cc, or about 32% of
theoretical for the metal
By using plastics which are meltable, compared
to those which only heat soften during spraying, an
improved character of pore structure results. This
pore structure is associated with better performance
in abradable seals, as is the lower oxide content.
The foregoing and other objects, features and
advantages of the present invention will become more
apparent from the following description of preferred
embodiments and accompanying drawings.
Brief Description of Drawings
Figure 1 is a photomicrograph showing the cross
section of a porous nichrome metal structure made with
polymethylmethacrylate in accord with the invention.
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Figure 2 ls similar -to Figure 1 but shows a
nichrome structure made with poLyester according to
the prior art, having a differen-t character of porosity
from Figure 1.
Figure 3 is a graph showing how oxide content
influences the glazing character of a 80-20 nichrome
structure, where the absence of glazing is good.
Figure A is a photograph showing the shape
o polymethylmethacrylate (Lucite) particula-te after
thermal spraying into free air.
Figure 5 is similar to Figure 4 showing
polyparaoxybenzoyl (EkonolTM) particulate.
Best rode for Carrying Out the Invention
The invention is described in terms of the
manufacture of an abradable seal structure of a nickel
chrome alloy, 80NI-20Cr. The spraying of a mixture
of metal and polymer is described in U.S. Pat. Jo. 3,
179,784 to Johnson and U.S. Pat. No. 3,723,165 to
Longo et al. In the preferred practice of the present
invention a polymer powder is used which has a depoly-
merisation temperature of less than 430C. Preferably,
the powder is a polymethylmethacrylate, such as Lucite
Grade 4FNC-99 powder (Dupont Company). powder
mixture by weight of 14~ Lucite and 86% nichrome is
sprayed. The nichrome is nominally -250 ~500 Tyler
Sieve Series mesh size (hereinafter called "-250 nichrome")
while the Luclte is -80 +400 mesh. About 0.2 weight percent
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TM
of submicron silica particulate, such as Cab-o-Sll
powder (Cabo-t Corporation), is added as needed to
improve Elow properties of the mixture. The mixture
is passed through a plasma arc -torch and is de-
posited to a thickness of about 2 mm on a suitablyprepared substrate such as a nickel superalloy work-
piece, using practices commonly lcnown for plasma
arc spraying.
After the desired thickness of sprayed deposit
has been accumulated, it is heated in air, vacuum,
or inert atmosphere. Preferably, air is used for
simplicity. The article is heated to a temperature
of about 315C for about 2 hr to cause the polymer
to depolymerize and to form the monomer which
volatilizes from the articlel thereby leaving on the
substrate a porous metal structure. Figure l is a
photomicrograph of the cross section of the re-
sultant nichrome metal layex which has an apparent
density of 2.7 g/cc, about 32% of theoretical nichrome
metal density. (The structures of Fig. 1 and ~iq. 2
appear essentially the same as shown here when viewed
perpendicular to the cross section.) Densi-ty is
measured in the conventional manner by dividing the
weight of a specimen by the volume its exterior
surface encompasses. sut the nichrome porous
structure described above is comprised of metal and
about 25% metal oxide. The Ni-Cr oxides, which have
jot been accurately characterized, are lower in
specific gravity than thy metal. Thus the actual
porosity of a structure made in the invention is less
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than the ratio o:E struc-ture speci.fic gravity to fully
dense metal specific gravi-ty i.ndicates; e.g., the
2.7 g/cc porous structure of 8.~ y/cc nichrome,
having an apparent 32~ density, is somewhat less
than 68% void.
To appreciate the invention's advantage, it must
be understood that the desired function of an
abradable material is that it remain intact under
particulate erosion and other mechanical stresses.
Yet it must easily disintegrate in a friable mode
when it is contacted by a high speed moving part,
such as a blade tip. This characteristic is called
abradability. In the absence of such easy disin
tegration behavior, the tip of the blade will be
excessively heated and desraded itself. In per-
formance tests simulating operation in a gas turbine
engine, the structure produced using the polymethyl-
methacrylate resin has been found surprisingly
different and superior to those produced with the
polyester resin. Most notably the new abradablematerial has less tendency to smear or glaze over
when contacted with the blade tip, compared to the
same structure made with a polyester resin. Glazing
is symptomatic of inadequate abradability. We have
discovered some of the phenomena which underlie this
improved performance and which are peculiar to the
utilization ox meltable low temperature polymer
powders such as polymethylmethacrylate.
First, the oxide content of the improved abradable
seals has been found to be about 25 weight percent
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when using the -250 nichrome, compared to the same
seal made using t:he polyester resin, wherein an oxide
content of about 35 weight percen-t results. Whereas
related experience would seem to suggest thak higher
oxide contents would he desirable in that they
generally are associated with embrittling of metals,
we have discovered that seals with lower oxide content
produce improved performance. Figure 3 illustrates
how glazing is dependent on oxide content over a
particular useful range of density of nichrome. The
data are based on vlsual observation of the rubbed
surface of a porous metal structure which was con-
tacted at a temperature of about 24C by six simulated
AMS 4928 titanium alloy blade tips at rubbing speeds
of about290 m/s. Glazing is evident if the rubbed
surface of the sprayed structure is shiny and metallic,
as opposed to dull, after the rubbing test. In
addition, glazing is evidenced by significant wear of
the rubbing blade, compared to a no-glazing condition
where the aggregate volume of titanium lost from a
blade will be less than 0.5-2% of the aggregate volume
of material removed from the seal structure during a
rub test. Oxide content of a seal structure is cal
cuable using conventional digestion techniques. For
nichrome, we use hot methanol-5~ bromine and chacter-
ize the insoluble residue as oxide.
In the process of removing a prior art polyester
resin from a sprayed deposit, owing to the high
temperature characteristics of the resin, furnaces
set at about 540C must be used. Oxidation of the
polymer during such removal has been found to
exothermically further raise the temperature of the
porous metal structure to about 620C. Owing to the
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high surface area of a porous metal structure, the ln--
dicated relatively high degree of oxidation results,
even -through nichrome is an oxidation resistant alloy.
Thus, the polymer removal process is revealed to be
critical and -the use of a fugitive material which de-
polymerizes or otherwise flees at a low temperature
is necesitated. The oxide is generally dispersed
through the metal of the struc-ture and is no-t as might
be expected concentrated on the surfaces surroundi.ng
the visible pores. Table 1 shows the oxide content
of 2.7 g/cc 80-20 NiCr porous material resulting from
100 hr exposure to certain baking temperatures which
may be used to drive off various fugitive material.
The data show an unexpected oxidation resistance
superiority of a deposit made with polymethylmethacrylate
(Lucite), compared to the same deposit made with
polyester (Ekonol). This may be due to the more
favorable pore s-tructure which the meltable polymer
provides. Our method enables achievement of a desired
oxide content of less than 35~, usually in the 20-30
range. With metals having oxidation characteristics
similar to 80-20 nichrome, this necessitates generally
that the polymer decompose orconvert to a volatile
constituent such as a monomer or gas at a temperature
of less than about 540C. With less oxidation re-
sistant metals lower temperatures will be
required.
Second, the physical structure of porous metal
produced by the use of a meltable low temperature
polymer is improved insofar as abradability over that
produced ~ith a higher temperature heat softening
polymer. This can be appreciated by comparing
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Figure 2 with Figure 3 which is a pho-toyraph oE a
cross section made by spraying a mixture by weight
percent 75 NiCr and 25 Ekonol polyparaoxybenzoyl
polyester resin (Carborundum Co. and ~etco Inc.) of
-150 ~325 mesh. The specimen shown in Figure 2 has
a density of 2.8 g/cc, essentially the same as that
of -the Figure 1 specirnen. (A lower deposit
efficiency necessita-tes using more volume percent
Ekonol than Lucite to obtain the same porosity.) Yet,
it is seen that the structure in Figure 1 has more
openness to it. We attribute this to the behavior
of the Lucite particulate since it is melted and
apparently agglomerates during its transit to the
substrate, whereas the Ekonol does not. The melting
type of polymer produces a wider range of pore sizes,
and the greater amount of large pores creates the
more open appearance in the structure.
Figures 5 and 6 show the difference in behavior
between the Lucite and Ekonol resins when they are
plasma arc sprayed by themselves into free air and
collected. The Ekonol material in Figure 5 exhibits
no sign of melting and remains as individual par-
ticulate, whereas thy Lucite has welted into spheres
and agglomerated.
Generally, the invention involves the use of a
poiymer which melts and;which volatilizes at a
temperature less than that which càuses more than
30~ oxide in the metal. For nichrome, various poly-
mers will be suitable, including those selected from
the general group comprised of polystrenes, poly-
-12-
ethylenes, polypropylenes and polyacrylates, all
f]eeing the substra-te at atmospheric pressure under
temperatures of less than 5~0C. For example, poly
methylmethacrylate decomposes at about 250C and we
heat it -to about 315C for convenience and speed.
A further reason to prefer the last mentioned
material is because of its ready availability as a
particulate at a low cost.
Of course, the primary cause of oxidation of the
porous structure which our procedure addresses in-
volves the step for removing the fugitive polymer.
But care must be taken to prevent oxidation during
the thermal spraying process as well. For nichrome-
polymer, plasma arc spraying in air with 50-50
argon-helium at an enthalpy of about 7 kwhr/m3 pro-
duces good results. Plasma arc spraying in air in
general will be useful since it involves the use of
non-oxidizing gases. But other thermal spraying
processes such as combustion spraying and detonation
gun processes can be useful as well, where they are
known to produce deposits with relatively low oxide
contents of less than about 25%. And of course, the
oxide content of a deposit will vary according to the
metal powder size which is used with coarser powders
producing less oxide content With -250 nichrome the
oxide contents will be in -the 4-10~ range after the
polymer is removed. Not only does the as sprayed
deposit of a coarser metal powder have less oxide, but
we have discovered the rate of oxidation at a con-
stant temperature in the 200-650C range is less,
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apparently due to a diEEerence in the character oE
-the sprayed structure. See again Table 1 where -325
mesh 500 mesh powder (" 325 mesh 80Ni 20Cr") produces
high oxygen content compared to -240 mesh 80Ni 20Cr.
When an abradable seal is made for use at
elevated temperatures, the spray deposit will be
applied onto a curved piece of nickel or iron super-
alloy, e.g., IN 718 or AISI 410 alloys. After the
fugitive material is removed the metal porous structure
will be well bonded to the superalloy substrate, by
which means it is affixed in the engine. To make
such structures of nichrome we spray from 80 to 90
weight percent nichrome with 20 to 10 percent
polymethylmethacrylate. These mixtures produce de-
posits of from 35 to 45 volume percent polymer,
preferably 37-43 percent. The resultant metal
deposits, after the polymer is caused to flee have
void contents of 50-70 volume percent, i.e., the
apparent density is 30-50 percent.
us a specific example, the nichrome specimen
mentioned above, created by spraying 86 nichrome
with 14 polymethylmethacrylate, has a density of
about 2.7 gm/cc and an oxide ¢ontent of 7~. Weight
loss measurement during removal of the fugitive
polymer shows the deposit was 43 volume percent
polymer. The apparent density is 32~, so without
the small adjustment for the volume of metal oxide
(compared to pure metal alloy), the apparent void
content is 63~. Thus, the void content of a finished
deposit is greater than that provided by the polymer.
It is well known plasma coatings are porous as
sprayed and this i.nherent void creation and other
phenomena associa-ted with -the process described are
apparently operable in providing the finished
product.
To obtain comparable results with other polymers
mentioned above, due adjustment in deposit compo-
SitiOIl must be made to account for change in density
to obtain the desired porosity. And :Eurther adjusk-
ment in the composition of the mixture sprayed wouldbe made to account for the efficiency of deposition
of the polymer and metal constituents.
The invention is especially meaningful for
porous abradable seal structures made of nichrome
alloys generally, as they are commonly known in
diverse compositions, since such materials have
exhibited good performance heretofore in seals made
by older techniques. Nonetheless, the invention will
be applicable to other materials as well, including
other alloys of nickel, and alloys based on iron,
cobalt and aluminum.
Although this invention has been shown and
described with respect to a preferred embodiment,
it will 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 scope of the
claimed invention.
