[0001] The present invention concerns an article comprising a substrate having on its surface
a multiplicity of fiber segments in a matrix of layered thermal sprayed particles
and a method for forming a thermal sprayed article comprising a substrate having on
its surface a multiplicity of fiber segments in a matrix of layered thermal sprayed
particles. An article and a method of the type indicated above are known from FR-A-1
434 948.
[0002] In the last two decades there has been extensive development of plasma arc spraying
and many applications have been developed. Plasma spraying offers the ability to create
coatings and free standing structures of virtually any material which can be melted.
[0003] Of particular interest has been the adhering of ceramic surfaces to metal elements,
to protect them from thermal and abrasive environments. As is well known, substantial
problems of incorporating ceramic material with metal structures arise from the differences
in thermal expansion which exist between most ceramics and most metals. High temperature
structures generally utilize high temperature metals, such as superalloys of iron,
nickel, and cobalt. These materials characteristically have high thermal expansion
coefficients of the order of 10-14 x 10-
s per °C. The ceramics which are of most interest tend to be those containing alumina,
zirconia, magnesia, and like materials which have low thermal expansion coefficients,
of the order of 5-10 x 10-
6 per °C.
[0004] Several different approaches have been utilized to obtain good adhesion between a
low expansion ceramic structure and a high expansion metal structure. One approach
has been to form sprayed composite interlayers by mixing metal and ceramic powders
to provide a gradation in composition, starting with entirely metal powder at metal
surfaces, progressing through partial metal and partial ceramic, and ending with entirely
ceramic. Still another method described in U.S. Patent 4 273 824 of McComas et al.,
having common assignee herewith, has been to first adhere a fiber metal mat to a metal
surface, by brazing or diffusion bonding. Plasma spraying is used to build up a coating
of ceramic on the fiber mat. To improve bonding of the ceramic to the fiber mat, a
thin bond coating of a metal has been first sprayed on the mat. While success has
been met with these approaches, there are still improvements needed for lower cost
and improved performance.
[0005] The FR-A-1 434 948 discloses a process for fabricating articles provided with coatings
comprising a continuous metal phase having therein a discontinuous phase of glass
fibers. The process consists in projecting on the substrate by means of a spray gun
a composite rod consisting of said metal and glass. The FR-A-1 434 948 discloses an
article and process according to the precharacterizing portion of claims 1 and 9.
[0006] The US-A-4 141 802 discloses a process for producing a panel comprising the steps
of depositing a thin layer of a bonding metal or alloy on a metal substrate, positioning
a layer of spaced apart reinforcing fibers on the bonding metal or alloy layer and
plasma arc spraying a metal on to and through the fibers to form a metal matrix reinforced
fiber coating on the substrate.
[0007] Plasma spray coatings and free standing plasma sprayed structures, particularly when
they are accreted to relatively great thicknesses, tend to be materials which have
relatively low strength compared to materials which have been formed by other methods.
Thus, it is desirable to find convenient ways to include fibers within a built up
plasma sprayed structure since fibers will enhance their strengths. Boron fiber reinforced
aluminum composites are one known combination of fibers with plasma coatings. They
are made by laying fibers on thin metal foils and spraying with aluminum to bond the
fibers to the foil, to form laminae. Subsequently, many such fiber-foil laminae are
pressed together to form generally thin and wide articles, such as airfoils. But the
process is costly. Also, there is no feasible way of incorporating fibers transverse
to the nominal plane of the articles, owing to the mode of construction from laminae.
[0008] The article according to the present invention is characterized by said structure
being obtained in plasma arc spraying metal fiber segments to cause them to melt partially
and thereby to adhere to the surface of the substrate. The method according to the
present invention is characterized by thermal spraying metal fibers to melt portions
thereof, without altering their acicular nature, to cause them thereby to adhere to
the surface of a substrate, with a portion of the fibers projecting from the surface.
[0009] According to the invention, fibers are partially melted and adhered to one another
when they are deposited on a workpiece surface using a thermal spray process, such
as plasma spraying. In the principle embodiment of the invention, the fibers are adhered
to the workpiece surface, as well. The surface is optionally made more receptive by
the use of a preliminary bond coating. The deposited fibers may be caused to have
a random pattern or a more normally aligned pattern, according to the fiber aspect
ratios and the spraying parameters which are used. In both instances, a substantial
portion of the fibers project from the surface, as opposed to aligning generally parallel
to it. During spraying, only portions of the fibers are melted. Most of a typical
sprayed fiber remains intact, but partial melting, of the ends and exterior surface,
causes desirable bonds with the workpiece and between the fibers themselves. To obtain
the foregoing results, the fibers are injected into the hot plasma gas stream at a
point between the plasma generating nozzle and the workpiece.
[0010] Matrix material can be infiltrated among the fibers, after they are deposited on
the workpiece surface. The matrix may be applied by a variety of techniques, but the
invention will be found principally useful when the matrix is comprised of a layered
plasma sprayed coating. The fibers aid in holding the plasma sprayed matrix onto the
substrate. In addition, by projecting through the layers of the sprayed matrix, the
invention provides greater strength to the matrix. When the matrix material is a plasma
sprayed coating, a bonding coat may be deposited on the fibers, before the principal
matrix material is applied.
[0011] The invention is particularly suitable for forming a metal-ceramic airseal for a
gas turbine engine. In such instances, preferably the substrate is a superalloy and
the matrix material is a zirconia base ceramic material; the fibers are a metal having
high temperature strength and corrosion resistance. This embodiment is further improved
by the following practice of the invention: Afterthe fibers have been deposited, but
before the matrix is deposited, a fugitive material, such as a polymer, is placed
on the substrate so that it fully envelopes a portion of the fibers on the workpiece.
But the fiber portions which project furthest from the workpiece are not fully enveloped
by the polymer. Thus, when the matrix material is subsequently sprayed, it envelopes
the projecting ends of the fibers. Then, the fugitive polymer material is removed,
such as by combustion. This leaves a ceramic and metal fiber composite structure joined
to the substrate surface by a network of metal fibers which are not embedded in the
matrix material. This network of metal fibers has relatively good structural compliance.
That is, it is adapted to deform with relatively low resistance, to accommodate differences
in thermal expansion between the ceramic and the metal substrate. Thus, the ceramic
is held closely to the substrate, but is not subject to damaging strains.
[0012] Generally, the inclusion of fibers in coatings will increase strength or other properties,
such as thermal conductivity. The invention is felt useful with all manner coatings,
in addition to plasma coatings. The fibers may be of any material which may be plasma
sprayed. Fibers alone, without any matrix material, will be useful when adhered to
a substrate to increase its surface area.
[0013] 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.
Figure 1 illustrates the steps in forming certain inventive articles, by end views
of a substrate.
Figure 2 shows in a cross section a ceramic matrix surrounding metal fibers, both
on a metal substrate.
Figure 3 is similar to Figure 2, but the specimen has a purposeful gap between the
ceramic and the substrate.
Figure 4 shows the relationship of the plasma spraying apparatus and workpiece.
Figure 5 is a photograph of sprayed copper fibers, adhered to a workpiece.
Figure 6 is a high magnification photograph of the fibers of Figure 5.
Figure 7 is similar to Figure 6, but at higher magnification.
[0014] The invention is described in terms of the application of a zirconia ceramic coating
to a stainless steel substrate using stainless steel fibers. However, it will be seen
that the invention is equally applicable to other material combinations.
[0015] Figure 1 illustrates generally the preferred steps in the invention. A bond coat
22 is first plasma sprayed onto the clean surface of a metal substrate or workpiece
20, as shown in Figure 1 (a), to provide a particularly receptive surface 23 for the
later deposited materials. Next, fine metal fibers 24 are plasma sprayed so they adhere
to the bond coated workpiece surface. As illustrated by Figure 1 (b), many of the
fibers will project above the surface of the workpiece. The next step is to plasma
spray powders to form a typical layered ceramic structure 26, which will envelope
the projecting fibers, as shown in Figure 1(c). Prior to this step it may be preferred
to plasma spray a light bond coat of metal powder onto the adhered fibers, although
generally we have not found this necessary. Because of the uneven surface of the fibers,
the deposited ceramic surface will be uneven. Thus, an optional next step, is to remove
protuberances 28 from the surface of the ceramic, as by grinding, to provide a smooth
finish. The resultant article 27, seen in Figure 1(d), is comprises of a substrate
20 with a fiber and ceramic matrix coating 27 adhered to its surface 23.
[0016] An optional procedure, illustrated by Figures 1(e)-(g), is to produce an article
where the ceramic matrix-fiber composite material is separated from the substrate,
by a compliant low stiffness structure of fibers. As illustrated by Figure 1 (e),
a polymer layer 30 is plasma or otherwise sprayed onto the workpiece surface 23, so
that it envelopes a portion of the fibers which project from the surface. The thickness
of the layer 30 is chosen so that portions of the projecting fibers 24 protrude above
the mean surface of the layer. Then, the ceramic matrix material is sprayed onto the
polymer layer, as illustrated by Figure 1(f), using a procedure analogous to that
which resulted in the structure shown in Figure 1 (c). The layered ceramic material
26' will adhere to the polymer surface and envelope the portions of the fiber which
protrude above the polymer. Next, the surface 28' of ceramic is optionally ground
to produce a smooth and even finish. Then the article is placed in a furnace having
an oxidizing atmosphere to cause the polymer to combust, converting it to a gas which
is carried away. This leaves the article illustrated in Figure 1 (g) wherein the fiber
and ceramic matrix structure 26' is spaced apart from the bond coated surface 23'
of the substrate, but it is joined to it by many fibers. Thus, the polymer has functioned
as a fugitive material, to temporarily bar the infiltration of ceramic materials into
the said space. When its function has been fulfilled, it has been removed without
adverse effect on the workpiece or coating. It is seen that the coating on the substrate
can be characterized as having a first portion 26' comprised of fiber reinforced ceramic
matrix, and second portion 30' comprised of fibers substantially free of matrix particles.
[0017] Figure 2 shows in cross section an actual article corresponding to Figure 1(c) comprised
of fibers 24a, 24b of stainless steel, a substrate 20a also of stainless steel, and
a matrix 26a of predominantly zirconia. The matrix is about 2.5 mm thick. Nominally
normal fibers 24a are seen in combination with portions of fibers 24b which are either
parallel or inclined to the workpiece. Protuberances 28a are caused by plasma build
up on the fibers. Figure 3 shows in perspective and cross section an analogous specimen
corresponding with Figure 1(g), except the ceramic surface protuberances 28b have
not been removed. Between the composite structure of matrix 26b and fibers is a space
30b about 0.1 mm wide created by polymer which has been removed. A fiber 24c crossing
the space and holding the ceramic 26.
[0018] Specimens like those in Figures 2 and 3 were made as follows. A piece of AISI 304
stainless steel, was cleaned with solvent and grit blasted in a conventional manner.
The bond coat was a nickel chromium aluminum alloy powder sized 45-120 x 10-
6 m (Alloy 443, Metco, Inc., infra). The fibers were AISI 304 stainless steel, with
a 0.25 x 0.25 mm square cross section and a length of about 30 mm. The ceramic powder
was an admixture of 80% zirconia and 20% yttria, sized 10-90 x 10-
6 m (Metco Material 202NS). For plasma spraying, a conventional gun and power supply
were used, namely, a Metco Model 7M systems and gun with a style G tapered nozzle
having a 7.8 mm exit dia. (Metco, Inc., Westbury, New York). The gun was traversed
across the flat workpiece at a rate of about 0.3 m/s, with each successive pass being
offset about 3 mm from the preceding pass. Fibers were fed using a Thermal Arc P1-AOV-2
Feeder (Sylvester & Co., Cleve- land, Ohio). The fibers were injected into the plasma
stream outside the nozzle, as more particularly described below. The powders were
injected into the stream immediately downstream from the exit face of the conventional
manner, with feed rates at about 0.05 g/s.
[0019] The bond coat was applied to a thickness of about 0.05-0.14 mm. Next the fibers were
applied to the surface in a manner which caused them to adhere. When the fibers are
injected, they are entrained in the plasma stream and impelled toward the workpiece.
Only portions of the fibers are melted, and they adhere to the workpiece. The heat
transfer, a function of plasma gas enthalpy and residence time in the stream, must
be sufficient to melt a portion of the fibers, to cause them to adhere to the workpiece
and to each other. However, the heat transfer must not be so high as to cause complete
melting of the fibers, which because of surface tension forces, would cause them to
be converted into droplets. For the 0.25 mm stainless steel fibers, a relatively high
enthalpy was required to obtain the requisite melting. The technique is described
in more detail below. The density of sprayed fibers was estimated to be in the range
of 10-25% of the bulk metal density of 7.9 g/cc. Nominally it is characterized herein
as being of about 15% density.
[0020] The ceramic powders were sprayed in a conventional manner, with the gun nozzle oriented
90 degrees to the substrate. Parameters for spraying the powders were conventional,
generally comprising a gun to workpiece distance of about 64 mm, 700 amps, 70 volts,
about 62 cm
3/ s nitrogen in combination with 9 cm
3/s hydrogen. The same parameters were used for spraying the fibers, as described below.
For the aforementioned nominal 15% fiber density, the ceramic penetrated through to
the workpiece and gave a relatively uniform density. Usually, it is expectable that
there will be some shielding of the areas underneath fibers which project across the
plane of the workpiece. But this did not seem to cause significant voids in the particular
example. If excessive shielding is encountered, then the gun may be inclined at varied
oblique angles to the workpiece surface, to better deposit ceramic under the fibers,
and obtain higher density. However, there will be a density of the fibers sufficiently
high such that the ceramic will not be able to penetrate through, and lower density,
or not density, can result. In special circumstances this may be desired.
[0021] In most instances, the ceramic will be able to penetrate the fiber layer. Thus, as
described above, a polymer or other coating is used as a fugitive material, to produce
an absence of ceramic matrix near the substrate surface when this is desired. In the
example, the polyester (Metco 600 material), with particle size distribution between
44-106 x 10-
6 m, was sprayed in a conventional mode to a thickness of about 0.25 mm. It was removed
by furnace heating for 3 hr at 550°C. Other fugitive materials may be used, such as
Lucite 4F acrylic resin (Dupont Co., Wilmington, Delaware). Polymers are preferred
because they may be removed easily by oxidation and moderate heating. Also usable
will be soluble or meltable materials, such as salts, and other materials used to
coat mandrels when free-standing structures are created by plasma coating.
[0022] The foregoing description is for a demonstration specimen. To make an actual ceramic
airseal for a gas turbine engine, along the lines shown in U.S. Patent 4,273,824,
the substrate would be a nickel, iron or cobalt superalloy. The fibers would be a
material with strength and corrosion resistance at high temperature. They may have
a similar composition to the substrate, or another composition. One specific example
of another useful high temperature fiber is Hoskins 875 alloy (by weight, 22.5 Cr,
5.5 Al, 0.5 Si, 0.1 C, balance Fe) produced by the Hoskins Manufacturing Co., Detroit,
Michigan, USA. In an airseal, the previously described zirconia base ceramic would
be useful. Other ceramics which will be useful will be meltable refractory compounds
of metals with melting points over 1400°C, preferably oxides, but also including borides,
nitrides, carbides, as pure compounds or combinations. The spacing between the ceramic
and the substrate, where they are only fibers, may be varied over the range of about
0.25-12 mm, by applying sufficient fibers and sufficient fugitive material. The thickness
of the space having fibers only will depend on the particular application. Greater
spacings will provide greater capability for absorbing thermal mis-match strains.
[0023] The manner in which the fibers are deposited on the substrate is illustrated in part
by Figure 4. A plasma gun 32 is positioned a distance D from a workpiece or substrate
34. The plasma gas stream 36 issues from the opening 38 of the nozzle 39. Immediately
downstream, adjacent to the nozzle face 40, is the conventional powder injection conduit
42. Unlike powders, fibers 44 are injected by means of a separate conduit, tube 46,
spaced a distance from the nozzle face. Tube 46 is preferably positioned normal to
the centerline 47 of the plasma gas stream, although some inclination of the pipe
toward the workpiece may be used. The pipe outlet 48, through which the fibers 44
exit, is spaced apart from the centerline of the plasma stream a distance E, sufficient
to ensure that it will not be directly impacted by the stream. Fibers are conveyed
through the tube 46 by a carrier gas; e.g., a flow of about 10 cm
3/s was used to convey the aforementioned 0.25 mm stainless steel fibers through a
6 mm dia. tube 46. Upon exiting from the outlet 44 of the tube, the fibers become
entrained in the gas stream.
[0024] The exact position of the fiber injection tube may be varied, dependent on the specific
operating conditions, and fiber size and results desired. Generally, the tube axis
57 will approximately intersect the centerline 47 of the plasma stream. It is found
that the point of injection of fibers preferably is located downstream from the point
at which powders are ordinarily injected. This is reflective of the need for comparatively
less heating of the fibers, relative to powders, to carry out the objects of the invention
and have the fibers adhere to the workpiece with substantially an acicular configuration,
as described further herein. By example, the aforementioned 0.25 mm dia. steel fibers
were injected at a distance F of approximately 8 mm from the nozzle face when the
nozzle face to workpiece distance D was about 64 mm. The spacing E, off the centerline
47 was about 6 mm.
[0025] In our practice of the invention, we vary the distance F at which the fibers are
introduced, to control the precise degree of fiber melting which is needed. Generally,
fibers in which less energy is needed for melting will be introduced at points closer
to the workpiece surface. By following this practice, of varying the point of axial
introduction, the plasma stream power level may be set more independently. Thus, high
velocities associated with high power levels may be attained, but the fiber residence
time will not be so great as to cause undue melting. Further, our approach enables
the power setting of the gun to be set at that required by a powder being sprayed,
thus facilitating practice of various embodiments of our invention, especially, that
involving simultaneous introduction of powder and fibers. The fibers will be introduced
at distances E which are within 5-80% of the nozzle face to workpiece surface distance
D; preferably, the foregoing range will be 10-50%. This distance D will vary as it
does for spraying powders. Generally it will be in the range 50-175 mm, depending
on materials being sprayed, ambient environment, etc. Of course, if fibers are introduced
too close to the workpiece surface there will be insufficient residence time in the
stream to cause melting and obtain adherence of the fibers to the workpiece. (In such
circumstances, however, the fibers may still be included within a plasma coating if
powders are impinged on the surface simultaneously.)
[0026] Microscopic studies have been made of the fibers which are deposited on the workpiece.
Figure 5 shows 0.35 mm dia. by 3-6 mm long copper fibers deposited onto a Metco Alloy
443 coated workpiece. The fiber-density was estimated at about 40%. Figures 6 and
7 are higher magnification views from a 30 degree angle off surface perpendicular.
It is seen from Figure 5 that the fibers 50 have a variety of orientations with substantial
numbers of the fibers projecting, at various angles approaching normal, up to 3 mm
into space from the plane of the workpiece 52. This is in contrast to a 1.8 mm thick
fiber mat which might be brazed on the workpiece in accord with the prior art in U.S.
Patent 4,273,824, where all the fibers would lie approximately parallel to the plane
of the workpiece surface. Figures 6 and 7 show that portions 54 of the fibers are
melted. Also seen is some fiber fracture 56 and oxidation scale 58. Some of the bond
coated substrate surface 60 is visible. Mostly, the ends of the fibers are melted,
and applying force to the fibers shows they are mostly bonded to the workpiece surface.
There is also some surface melting along the length of the fibers, which provide bonding
between the fibers where they contact one another. While some are broken and some
excessively melted, the preponderance maintain an acicular shape, substantially of
their original diameter.
[0027] In our practice of the invention thus far, we have utilized metal fibers. Basically,
these have been chopped up pieces of commercial wrought wire or pieces of foil which
have been slit to very narrow widths (which results in a fiber with essentially a
square or rectangular cross section). When we refer herein to the diameter of our
fiber, for non-circular cross section fibers, we mean the diameter of the mean circle
which fits within the non-circular cross section. Presently, we believe that the diameters
between about 0.05 and 0.35 mm to be useful with conventional plasma spray equipment.
As pointed out earlier, the minimum fiber diameter will be determined by the minimum
plasma gun heat transfer conditions which result in an effective coating. When we
sprayed 0.01 mm dia. fibers, it was not possible to avoid entirely melting them with
our equipment. The maximum diameter will be a function of heat transfer condition
also, especially the residence time of the fiber in the plasma stream before it contacts
the workpiece. To obtain uniform results, the fibers should be of substantially uniform
diameters. If undersize fibers are included, they are likely to melt; too many would
defeat the objects of the invention. However, the fibers within a lot may vary in
length, since this parameter will not substantially affect the results, except regarding
the orientation, as discussed elsewhere.
[0028] Preferably the fibers will be incorporated into the matrix in a manner which provides
the strengthening or property improvement most desired. For strength, it is generally
known that a major limitation of plasma coatings is their bonding to the substrate.
The invention as described above, where the fibers are attached to both the substrate
and the matrix, provides an improvement in this respect. Plasma coatings are deposited
in successive passes, and thus are characterizable as layers of solidified particles.
There is a propensity for failure between the layers, and thus when the fibers are
incorporated so that they project through the layers, strengthening is provided. Typically,
a layer may have a thickness of the order of 0.08 mm, and thus a fiber would project
through at least half of two such abutting layers, for a total fiber length of about
0.08 mm, to provide a benefit. To strengthen a layered matrix, the fibers must be
adequately bonded to the matrix. The fiber length along which bonding must be present
to strengthen the matrix is a function of the shear strength of the bond. This will
vary with the composition of the fiber and matrix, but generally, we believe that
a fiber must be bonded along a length equal to about three fiber diameters to provide
adequate strength. Thus, for this application, the minimum fiber aspect ratio would
be 6:1.
[0029] The aspect ratio, the (ratio of the length to the nominal diameter of the fiber)
is an important parameter. First, it affects the pattern which the fibers form when
they adhere to the workpiece. Based on limited observation, it appears that if fibers
have high aspect ratios, e.g., about 20:1 for 0.25 mm dia. stainless steel fibers,
they will tend to be deposited in a random orientation fashion. However, when the
aspect ratio of such fibers is less than about 15: 1, they tend to be deposited in
a more aligned pattern, that is, more nearly normal to the surface of the workpiece.
Thus when one orientation or the other is preferred, the fiber aspect ratio would
be selected accordingly. It is not fully understood why the foregoing effects are
observed. But, it is believed that all fibers tend to become aligned parallel to the
flow direction of the plasma gas stream. However, when they impact the workpiece the
longer fibers will tend to bend over more, and thus become more randomly oriented.
[0030] When fibers are too long, difficulty will be encountered in feeding them. This, of
course, depends on the powder feeding device and the size of the nozzle, etc. For
most applications we believe that the useful lengths of fibers will range between
about 0.1-4 mm. Following along the lines of the discussion above, the aspect ratio
preferably will range from about 3:1 to 80:1. The foregoing ranges may change with
further development.
[0031] The density of the fibers which are deposited prior to the matrix may be varied by
selection of parameters, especially fiber size, feed rate, carrier gas flow, and stream
conditions. Generally, for fibers deposited independently, the bulk density will range
up to 60% of the solid metal density. The density of articles comprised of deposited
fibers and subsequently sprayed matrix will depend on the degree to which the matrix
is able to penetrate the fibers. (Of course the matrix will have an inherent density
of its own, irrespective of the presence of fibers.) Because our fibers tend to be
oriented in more nearly normal orientation, higher matrix-fiber composite density
can be obtained, compared to fiber mats in previous use, such as described in U.S.
Patent 4,273,824. Based on limited evidence, for fiber deposits such as shown in Figure
3, we are able to get approximately normal matrix density where fiber densities range
up to about 50%.
[0032] We have mentioned the use of a bonding coat at various points herein. Conventional
plasma coating underlayer materials such as nichrome, nickel aluminum, and the like
will be found useful. They will be deposited on the workpiece, in the manner which
is well known as being used for improving the adherence of conventional plasma coatings.
When the bonding coat is applied to the surface of fibers already deposited, or contemporaneously
with them, the quantity which will be deposited will be that which would produce a
coating of about 0.08 mm thick on a flat workpiece, were the fibers not present. Too
great a deposit would instead convert the bonding coat into a matrix.
[0033] While we contemplate that the major utility of our invention will be to strengthen
ceramic and other brittle coatings, we believe that further work will demonstrate
other improved materials. Thus, it is within our contemplation that the invention
will be useful with all kinds of plasma coatings.
[0034] An example of a plasma coating which can especially benefit from the inclusion of
metal fibers is a porous (40% density) metal coating, used as a relatively soft abradable
material, such as is made by spraying in combination a polymer and nichrome powder,
and subsequently removing the polymer. By including nichrome fibers in the porous
nichrome matrix, thermal conductivity of the metal article will be enhanced. In such
instances, the degree of bonding between fiber and matrix is of less importance, but
it is desired that the fibers be aligned to the best degree possible, along the direction
in which the heat transfer is desired. One application for such a material would be
as an abradable seal used in the compressor of a gas turbine. Heat will be transferred
from a local rub spot to adjacent areas of the seal, minimizing localized heating
which might degrade the seal or the structure with which it interacts. While be believe
the major initial use of our invention will be as an improvement for supplanting fiber
mats, in certain instances, our techniques will enable a direct substitution for fiber
mats. To do so, we would plasma spray using fibers and parameters which tended to
give a fiber orientation parallel to the surface. A hot or cold pressing step may
be subsequently used to deform the fibers after deposition, to cause them to become
more nearly parallel to the surface.
[0035] It is well known that plasma coatings can be used for forming free-standing articles,
such as crucibles, rocket nozzles, and the like. Our fiber spraying techniques may
be used to improve the properties of such articles, in accord with the foregoing embodiments
of the invention.
[0036] Further, we believe that our method of spraying fibers and adhering them to a metal
surface may be useful to hold and strengthen other coatings than plasma coatings such
as polymers, vapor depositions, electroless coatings, etc. It is also within contemplation
that fibers alone adhered to workpiece surfaces as shown in Figure 1(b), will provide
desirable high surface areas in electrical and chemical applications, or would be
useful as abradable materials in gas turbines.
[0037] We have described the best present mode of our invention, but other refinements are
expected to improve its practice. We have used plasma arc spraying because it is an
advanced method. But other thermal spraying processes, such as those which use products
of combustion or heat sources other than electric arcs, may suitably melt the fibers
and can be used to practice the invention.
[0038] Separate guns may be used for spraying the fibers and the powders when they are to
be sprayed simultaneously, to enable independent control of the parameters for each
material. As another alternative, a single gun with a single powder/fiber injection
port might be used, where the fibers and powders are mixed together. This would require
experiment to determine the compatibility of the parameters with the selected sizes
of powders and fibers, and the point of introduction.
1. An article comprising a substrate having on its surface a multiplicity of fiber
segments in a matrix of layered thermal sprayed particles, characterized by said structure
being obtained in plasma arc spraying metal fiber segments to cause them to melt partially
and thereby to adhere to the surface of the substrate.
2. The article according to claim 1 further characterized by a portion of the fibers
projecting transverse to the layers of plasma sprayed particles.
3. The article according to claim 2 further characterized by a matrix material enveloping
the fibers.
4. The article according to claims 1-3 further characterized by a bond coat on the
substrate surface, to improve bonding of the fibers to the substrate.
5. The article according to claims 1-4, further characterized by the fibers having
adhered to their surface a bond coat, to improve adherence of the matrix to the fibers.
6. The article according to claim 3, further characterized by a portion adjacent the
surface of the substrate having fibers which are substantially free of envelopment
by material, to space apart the matrix and fiber combination material from the substrate
surface.
7. The article according to claims 3 to 6, characterized by metal alloy fibers adhered
to a metal alloy substrate and a ceramic matrix material.
8. The article according to claim 7 further characterized by a superalloy substrate,
the article shaped as an airseal for a gas turbine engine.
9. The method for forming a thermal sprayed article comprising a substrate having
on its surface a multiplicity of fiber segments in a matrix of layered thermally sprayed
particles said method being characterized by thermal spraying metal fibers to melt
portions thereof, without altering their acicular nature, to cause them thereby to
adhere to the surface of a substrate, with a portion of the fibers projecting from
the surface.
10. The method according to claim 9, further characterized by spraying powders, to
envelope at least a portion of the projecting portions of the fibers in a layered
matrix.
11. The method according to claim 10, further characterized by spraying on the substrate,
after the fibers have been sprayed, a fugitive material which surrounds a portion
of the fibers, and then spraying the matrix powder material; removing the fugitive
material by means which does not adversely affect the properties of the substrate,
fibers or matrix, to thereby form an article with the fiber containing matrix spaced
apart from the substrate surface, but joined thereto by the fibers.
12. The method according to claim 10, characterized by the fibers, before spraying,
having a substantially uniform nominal diameter of less than about 0.5 mm and a length
to diameter ratio of at least about 3:1.
13. The method according to claim 10, further characterized by spraying at least a
portion of the matrix simultaneously with the fibers.
14. The method according to claim 10, further characterized by plasma spraying.
15. The method according to claim 13, further characterized by spraying the particles
and the fibers from separate plasma arc spray guns.
16. The method according to claim 11, further characterized by the use of a fugitive
material which is a polymer and the removal thereof by heating in an oxidizing atmosphere
at less than 500°C.
1. Gegenstand mit einem Substrat, das auf seiner Oberfläche eine Vielzahl von Faserabschnitten
in einer Matrix aus geschichteten, thermisch gespritzten Teilchen hat, dadurch gekennzeichnet,
daß die Struktur durch Plasmalichtbogenspritzen von Metallfaserabschnitten, durch
das diese teilweise zum Schmelzen und dadurch zum Haften an der Oberfläche des Substrats
gebracht werden, erzielt wird.
2. Gegenstand nach Anspruch 1, dadurch gekennzeichnet, daß ein Teil der Fasern quer
zu den Schichten von plasmagespritzten Teilchen vorsteht.
3. Gegenstand nach Anspruch 2, gekennzeichnet durch ein Matrixmaterial, das die Fasern
umhüllt.
4. Gegenstand nach den Ansprüchen 1-3, gekennzeichnet durch einen Verbindungsüberzug
auf der Substratoberfläche zum Verbessern der Verbindung der Fasern mit dem Substrat.
5. Gegenstand nach den Ansprüchen 1-4, dadurch gekennzeichnet, daß die Fasern einen
an ihrer Oberfläche haftenden Verbindungsüberzug zum Verbessern der Haftung der Matrix
an den Fasern haben.
6. Gegenstand nach Anspruch 3, dadurch gekennzeichnet, daß ein Teil nahe der Oberfläche
des Substrats Fasern hat, die im wesentlichen frei von einer Umhüllung durch Material
sind, um dem Matrix- und Faserkombinationsmaterial Abstand von der Substratoberfläche
zu geben.
7. Gegenstand nach Anspruch 3 oder 6, gekennzeichnet durch Metalllegierungsfasern,
die an einem Metalllegierungssubstrat und an einem Keramikmatrixmaterial haften.
8. Gegenstand nach Anspruch 7, gekennzeichnet durch ein Superlegierungssubstrat, wobei
der Gegenstand als eine Luftabdichtung für ein Gasturbinentriebwerk gestaltet ist.
9. Verfahren zum Herstellen eines thermisch gespritzten Gegenstands, beinhaltend ein
Substrat, das an seiner Oberfläche eine Vielzahl von Faserabschnitten in einer Matrix
aus geschichteten, thermisch gespritzten Teilchen hat, wobei das Verfahren gekennzeichnet
ist durch thermisches Spritzen von Metallfasern, um Teile derselben zum Schmelzen
zu bringen, ohne ihre nadelförmige Ausbildung zu verändern, um sie dadurch dazu zu
bringen, an der Oberfläche eines Substrats zu haften, wobei ein Teil der Fasern von
der Oberfläche vorsteht.
10. Verfahren nach Anspruch 9, gekennzeichnet durch Spritzen von Pulvern, um wenigstens
einen Teil der vorstehenden Teile der Fasern in einer geschichteten Matrix zu umhüllen.
11. Verfahren nach Anspruch 10, dadurch gekennzeichnet, daß, nachdem die Fasern gespritzt
worden sind, ein flüchtiges Material auf das Substrat gespritzt wird, welches einen
Teil der Fasern umhüllt, und daß dann das Matrixpulvermaterial gespritzt wird; daß
das flüchtige Material durch eine Maßnahme, die die Eigenschaften des Substrats, der
Fasern oder der Matrix nicht nachteilig beeinflußt, entfernt wird, um dadurch einen
Gegenstand herzustellen, bei dem die die Fasern enthaltende Matrix Abstand von der
Substratoberfläche hat, aber mit dieser durch die Fasern verbunden ist.
12. Verfahren nach Anspruch 10, dadurch gekennzeichnet, daß die Fasern vor dem Spritzen
einen im wesentlichen gleichmäßigen Nenndurchmesservon weniger als etwa 0,5 mm und
ein Verhältnis von Länge zu Durchmesser von wenigstens etwa 3:1 haben.
13. Verfahren nach Anspruch 10, gekennzeichnet durch Spritzen wenigstens eines Teils
der Matrix gleichzeitig mit den Fasern.
14. Verfahren nach Anspruch 10, gekennzeichnet durch Plasmaspritzen.
15. Verfahren nach Anspruch 13, gekennzeichnet durch Spritzen derTeilchen und der
Fasern aus getrennten Plasmalichtbogenspritzpistolen.
16. Verfahren nach Anspruch 11, gekennzeichnet durch die Verwendung eines flüchtigen
Materials, das ein Polymer ist, und Entfernen desselben durch Erhitzen in einer oxidierenden
Atmosphäre bei weniger als 500°C.
1. Article comprenant un substrat dont la surface est pourvue d'une multitude de segments
de fibres dans une matrice de particules projetées -en couches par voie thermique,
caractérisé en ce que cette structure est obtenue en projetant, à l'arc de plasma,
des segments de fibres métalliques pour en provoquer la fusion partielle et ainsi,
leur adhérence à la surface du substrat.
2. Article selon la revendication 1, caractérisé en ce qu'une partie des fibres ressortenttransversale-
ment par rapport aux couches des particules projetées au plasma.
3. Article selon la revendication 2, caractérisé en ce qu'il comporte une matière
de matrice enveloppant les fibres.
4. Article selon les revendications 1 à 3, caractérisé en ce qu'il comporte un revêtement
de liaison déposé sur la surface du substrat afin d'améliorer la liaison des fibres
à ce dernier.
5. Article selon les revendications 1 à 4, caractérisé en ce qu'un revêtement de liaison
est amené à adhérer sur la surface des fibres afin d'améliorer l'adhérence de la matrice
à ces dernières.
6. Article selon la revendication 3, caractérisé en ce qu'une partie adjacente à la
surface du substrat comporte des fibres qui ne sont pratiquement pas enveloppées de
matière afin d'espacer le matériau composite de matrice/fibres de la surface du substrat.
7. Article selon la revendication 3 ou 6, caractérisé en ce que des fibres d'alliage
métallique sont amenées à adhérer à un substrat constitué d'un alliage métallique
et à une matière de matrice céramique.
8. Article selon la revendication 7, caractérisé en ce qu'il comporte un substrat
en un superalliage, cet article étant façonné sous forme d'un joint étanche à l'air
pour une turbine à gaz.
9. Procédé en vue de former un article ayant subi une projection par voie thermique
et comprenant un substrat dont la surface est pourvue d'une multitude de segments
de fibres dans une matrice de particules projetées en couches par voie thermique,
ce procédé étant caractérisé en ce qu'on soumet des fibres métalliques à une projection
thermique pour en faire fondre certaines parties sans modifier leur nature aciculaire,
en les faisant ainsi adhérer à la surface d'un substrat, une partie de ces fibres
ressortant de cette surface.
10. Procédé selon la revendication 9, caractérisé en ce qu'on projette des poudres
pour envelopper au moins une partie des saillies des fibres dans une matrice constituée
de plusieurs couches.
11. Procédé selon la revendication 10, caractérisé en ce que, après que les fibres
ont été projetées, on projette, sur le substrat, une matière fugitive qui entoure
une partie des fibres, après quoi on projette la matière en poudre de la matrice,
on élimine la matière fugitive par un moyen n'exerçant aucune influence néfaste sur
les propriétés du substrat, des fibres ou de la matrice, afin de former ainsi un article
comportant une matrice contenant des fibres, espacée de la surface du substrat, mais
réunie à celle-ci par les fibres.
12. Procédé selon la revendication 10, caractérisé en ce que, avant la projection,
les fibres ont un diamètre nominal pratiquement uniforme inférieur à environ 0,5 mm
et un rapport longueur/ diamètre d'au moins environ 3:1.
13. Procédé selon la revendication 10, caractérisé en ce qu'on projette au moins une
partie de la matrice simultanément avec les fibres.
14. Procédé selon la revendication 10, caractérisé en ce qu'on procède à une projection
au plasma.
15. Procédé selon la revendication 13, caractérisé en ce qu'on projette les particules
et les fibres par des pistolets séparés de projection à arc au plasma.
16. Procédé selon la revendication 11, caractérisé en ce qu'on utilise une matière
fugitive qui est un polymère que l'on élimine par chauffage à moins de 500°C sous
une atmosphère oxydante.