Note: Descriptions are shown in the official language in which they were submitted.
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THERMAL SPRAY USING AC)JUSTED NOZZLE
Backqround of the Invention
The present invention relates generally to the thermal
spraying of powdered materials, as well as their application to
surfaces as protective coatings.
A variety of thermal spray coatings have long been used
to protect various components. A principal variety of thermal
spray coatings to which the subject matter of the present
invention pertains includes plasma sprayed coatings, although the
improvements of the present invention will also pertain to other
coatings and processes such as high velocity oxy-fuel (HVOF),
having similar uses and properties. Plasma spray processes have
been used to apply many different types of coatings to a variety
of substrates, and find utility in numerous industries. One such
application is responsive to conditions where a high degree of
stress and wear is prevalent. In such a case, protective
coatings containing carbides are often used. For example, the
mid-span stiffeners used in the fan blades of aircraft gas
turbine engines are commonly coated with a highly wear resistant
tungsten carbide-cobalt (WC-Co) coating.
Popular techniques for the application of such coatings
~ould include plasma spraying and high velocity oxy-fuel
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spraying. In the implementation of such coatings, hardness is
often a factor of primary concern. However, both plasma sprayed
coatings and high velocity oxy-fuel coatings have proven to be
limited in their a~ility to meet the minimum mechanical property
requirements for certain applications, particularly the
requirements for minimum hardness.
In the practice of such processes, the current trend in
the industry has been to develop processes that increase particle
velocity and deposition rates. For example, an article by M.L.
Thorpe and H.J. Richter, entitled "A Pragmatic Analysis and
Comparison of the HVOF Process", Proceedinqs of the International
Thermal SPraY Conference & Exposition, Orlando, Florida (May 28
to June 5, 1992), at pages 137-147, discusses the increase of
particle velocities in thermal spray processes. However, these
known techniques have remained somewhat limited in terms of the
overall hardnesses which could be achieved.
U.S. Patents No. 5,082,17g (simm et al.), 4,741,286
~Itoh et al.) and 4,236,059 (McComas et al.) disclose flame
sprayed coatings applied using a plasma spray process. Simm et
20 al. disclose relatively low velocity, low temperature spray
methods, which are generally not suited to severe wear and
erosion applications. Itoh et al. similarly disclose coatings
applied with a low rate plasma gas flow. Generally speaking, the
disclosed systems provide no specific guidance relative to the
~5 development of plasma sprayed coatings of a hardness commensurate
with many present applications.
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U.S. Patent No . 5, 330, 798 (Browning) discloses ~lame
sprayed coatings applied with a high velocity oxy-fuel spray
process. The disclosed process uses the ~inetic energy o~
impacting particles to obtain dense coatings. However, again,
the resulting coatings are not of a hardness commensurate with
many present applications.
SummarY of the Invention
It is therefore the primary object of the present
invention to provide thermal spray coatings for application to
substrates which are improved in terms of their hardness
characteristics.
It is also an object of the present invention to
provide thermal spray coatings of improved hardness that employ
traditional wear resistant materials.
It is also an object of the present invention to
provide thermal spray coatings of improved hardness which can be
implemented with existing thermal spray apparatus, and at a
reasonable cost.
It is also an object of the present invention to
provide an apparatus for applying thermal spray coatings of
improved hardness to various substrates.
It is also an object of the present invention to
provide a nozzle assembly for use with thermal spray apparatus
for applying thermal spray coatings of improved hardness to
various substrates.
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It is also an object of the present invention to
provide substrates with thermal spray coatings of improved
hardness.
These and other ob~ects which will be apparent are
achieved in accordance with the present invention by modifying
known thermal spray apparatus to achieve the thermal spray
coatings of improved hardness which are desired.
For example, U.S. Patents No. 4,256,779 (Sokol et al.)
and 4,235,943 (McComas et al.) disclose a plasma spray method and
apparatus which is known in the industry as the "Gator-Gard~"
System, offered by Sermatech International, Inc. of Limerick,
Pennsylvania. Generally speaking, the disclosed system includes
a plasma-producing torch coupled with a nozzle for directing the
resulting plasma stream to develop a plasma jet of improved
characteristics. The disclosed system is useful in applying
thermally sprayed coatings to desired substrates.
Upon entering the nozzle of the apparatus, the plasma
stream is passed through a plasma cooling zone defined by a
plasma cooling passageway, to a plasma accelerating zone defined
by a narrowed passageway that expands into a plasma/particle
confining zone for the discharge of material from the nozzle, and
the thermal spray apparatus. The narrowed passageway of the
apparatus is cooled, and the powder material to be applied to the
substrate by the apparatus is introduced into the plasma stream
along the cooled, narrowed passageway. This results in
appropriate heating (melting) and acceleration of the powder
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particles, for application to the substrate which is to receive
the thermal spray coating.
Such apparatus has worked well for applying coatings of
various ~ypes to appropriate su~strates. A variety of wear
resistant coa~ings such as wc-co and Cr3Cz-NiCr have been
effectively applied with such an apparatus. ~owever, for
purposes o~ improving the plasma spray coatings applied with such
apparatus, and in accordance with the present invention, it has
been ~ound that plasma spray coatings of increased hardness can
be applied to desired substrates by extending the distance at
which the apparatus can spray the plasma/particle stream,
preferably by a factor of two to three times the distance
normally used.
Tt has been found that this is achievable by
lengthening the passageway which defines the plasma/particle
confining zone of the nozzle of the thermal spray apparatus.
This, in turn, has an effect upon the ratio of this length to the
diameter of the plasma/particle confining zone of the nozzle
(generally referred to as the "L/D" ratio). For example, it has
been found that an increase in this L/D ratio from a conventional
value of about 5:l to an increased value o~ about 12.5:l will
achieve significantly improved results for certain WC coatings
which are conventionally used in the industry, as will be
discussed more fully below. Variations in this ratio of ~rom 7:1
to 16.5:1 will find utility in conjunction with other materials,
for implementing similar coatings. Ratios of from lO:1 to 13:l
have been found to be particularly useful.
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For further detail regarding such improvements,
reference is made to the detailed description which is provided
below, taken in conjunction with the following illustrations.
Brief Description of the Drawinqs
Figure 1 is a schematic illustration of a plasma spray
apparatus for implementing the improvements o~ the present
invention.
Figure 2 is a graph showing variations in microhardness
responsive to variations in gas flow rate for two different
nozzles including a conventional nozzle and a nozzle
incorporating the improvements of the present invention.
Figure 3 is a graph showing variations in microhardness
at different spray distances for different nozzles including a
conventional nozzle and nozzles incorporating the improvements of
the present invention.
Figure 4 is a graph similar to that of Figure 3,
showing results for different operating parameters.
Figure 5 is a graph similar to that of Figure 2,
showing results at an increased spray distance.
Figures 6A and 6~ are photomicrographs comparing
conventional coatings (Figure 6A) with coatings produced in
accordance with the present invention (Figure 6B).
Figures 7A and 7B show X-ray diffractions comparing
conventional coatings (Figure 7A) with coatings produced in
accordance with the present invention (Figure 7~).
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Detailed DescriPtion of Preferred Embodiments
Figure l is a schematic representation of a thermal
spray apparatus l corresponding to the thermal spray apparatus
disclosed in U.S. Patent No. 4,256,779 and incorporating the
improvements of the present invention. The thermal spray
apparatus l is generally comprised of a nozzle assembly 2 (i.e.,
an insert) whicA is mated to a plasma gun 3. The plasma gun 3
employs a cooperating cathode 4 (preferably ~ormed of tungsten)
and anode 5 (preferably formed o~ copper). The cathode 4 and
anode 5 are electrically excited to produce an arc at 6, for
igniting a plasma-forming gas (e.g., an inert gas such as helium)
which is introduced at 7, between the cathode 4 and the anode 5.
The plasma gun 3 is mated with the nozzle assembly 2 so
that the resulting plasma stream is introduced into an inlet
passageway lO of the nozzle assembly 2. The inlet passageway lO
communicates with a narrowed passageway ll, which thereafter
expands outwardly into a ceramic nozzle 12. In operation, the
plasma stream produced by the plasma gun 3 enters the inlet
passageway lO. The inlet passageway lO is surrounded by a
cooling medium, such as water, to define a plasma cooling zone
13. In passing from the inlet passageway lO to the narrowed
passageway ll, the plasma stream is constricted along a zone 14.
Thereafter, the plasma stream passes through a particle
introduction zone 15 which incorporates one or more conduits 16
for receiving a powder to be introduced into the plasma stream
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through one or more ports 17. upon entering the particle
introduction zone 15, powder introduced through the port 17
enters the narrowed passageway 11, where it is heated to a
plasticized state and accelerated in the ceramic nozzle 12. The
plasticized and accelerated powder particles are then discharged
from this plasma/particle confining zone 18, exiting the no~zle
assembly 2 as a spray 19 for application to an appropriate
substrate 20. The result is a thermal spray coating 21 applied
to the surface 22 of the substrate 20.
It has been ~ound that the characteristics of the
thermal spray coating 21 can be affected by varying the dimension
of the passageway 12. As an example, the passageway 12 of a
convention nozzle assembly 2 typically has a length of about 1.2
inches. In accordance with the present invention, it has been
found that the applied coating 21 can be improved, particularly
in terms of its hardness, by extending this length.
A more common parameter which is conventionally used in
the industry is the "L/D" ratio, which is a ratio of the length
of the passageway 12 relative to the inner diameter of the
passageway 12. In accordance with the present invention, this
ratio is preferably increased from a conventional value of about
5:1 to values in a range of from 7:1 to 16.5:1. Particularly
useful results are obtained with ratios of from 10:1 to 13:1.
The L/~ ratio which is employed will vary depending upon the
particular application involved (i.e., the substrate to receive
,the coating, the coating materials used, etc.). Although L/D
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ratios of between 5:1 and 7:1 can be effectively employed in
developing hardened plasma spray coatings, it has been found that
~etter results are achieved for coatings produced with L/D ratios
of from 7:1 to 16.5:1, and particularly for ratios of from lo:l
to 13:1.
As an example, the passageway 12 of a conventional
nozzle assembly 2 has a typical inner diameter of 0.25 inches (a
typical outer diameter for the nozzle 12 would be 0.370 inches)
and a typical length of 1.25 inches, yielding an L/D ratio of
5:1. Increasing the length of the passageway 12 to 3.1~5 inches
(e.g., for a WC coating) will increase the L/D ratio to 12.5:1.
L/~ ratios in the pre~erred range of 7:1 to 16.5:1 will
correspond to lengths ranging from 1.75 inches to 4.125 inches.
L/D ratios in the optimum range o~ 10:1 to 13:1 will correspond
to lengths ranging from 2.50 inches to 3.25 inches.
As previously indicated, the improvements of the
present invention are useful with coatings formed of a variety of
different materials, for application to various substrates, as
desired. However, for purposes of illustration, the improvements
of the present invention will be further discussed with reference
to a particular class of coatings, specified as "GG-WC-102"
coatings by Sermatech International, Inc. Such coatings are
similarly specified as l'PWA 256-4" Coatings by Pratt & Whitney
Aircraft (United Technologies Corp.). The specified system uses
a tungsten carbide-cobalt coating, which is widely applied in the
aircraft industry and which has been found to ~e particularly
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responsive to improvement in accordance with the present
invention.
One indication of the integrity of such a coating
(applied, for example, using Sermatech's "Gator-Gard~" System) is
the microhardness of the resulting coating. Typically, the value
for microhardness which is required to yield an acceptable
coating is on the order of 950 units (DPH300). This value is a
~unction of many process parameters, some of the more important
variables being the gas flow rate, the current applied to the
electrodes and the spray distance relative to the substrate.
Since the extended passageway 12 of the present
invention ~orms part of the nozzle assembly 2, which can be
separated from the plasma gun 3, this independently affixed
component can be varied in configuration (in particular, its
length) in straightforward fashion. These nozzle assemblies,
which are commonly referred to in the industry as "blocks", are
conventionally formed of copper (so-called "Cu-blocks").
Various nozzle assemblies having passageways 12 of a
different length have been studied. For purposes of illustrating
the improvements of the present invention, nozzle assemblies 2
having passageways 12 of five different lengths were compared.
This included a "blockless" nozzle (length = O inches), a
standard nozzle (length = 1.25 inches), and three lengthened
nozzles including a "long" nozzle (length = 1.75 inches), a "P"
nozzle (length = 3.125 inches) and a "P~" nozzle (length = 4.125
~inches). As a point of reference, and under typical conditions,
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a standard copper block was employed with a gas flow rate o~ 27
(arbitrary units) and a current of 800 to 840 amperes, for the
application of coatings at a conventional spray distance of
2 inches.
Figure 2 is a graph showing variations in mircohardness
responsive to variations in gas flow rate for two di~ferent
blocks including a standard block and a P-block operating at two
different current levels (800 and 840 amperes, respectively), at
a spray distance of 2 inches. From this graph it is seen that
the P-block yields coatings of increased hardness relative to
coatings applied with the standard block, particularly at the
higher gas flow rates.
Figure 3 is a graph showing variations in microhardness
as a function of block type, at various spray distances including
2, 3 and 4 inches, respectively. The spray apparatus was
operated at 840 amperes, with a gas flow rate of 300 units. From
this graph it is seen that in all cases, the hardness of the
resulting coating tends to ~;~ize for the P-block, rolling off
for both longer and shorter passageways.
Figure 4 is a graph similar to the graph of Figure 3,
except that the spray apparatus was in this case operated at 800
amperes, with a gas ~low rate of 300 units. Again, it is seen
that in all cases, the hardness of the resulting coating tends to
maximize for the P-block, rolling off for both longer and shorter
passageways.
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Figure 5 is a graph similar to the graph of Figure 2,
except that the spray distance was in this case increased to 4
inches. The spray apparatus was operated at both 800 and 840
amperes, respectively. From this graph it is seen that at a
spray distance of 4 inches, only the P-block meets an acceptable
minimum microhardness level of 950 units. This illustrates that
the P-block can be used to substantially double the spray
distance which can be used in coating a desired substrate,
relative to the spray distance used for a standard block (from 2
to 4 inches). Spray distances of from 3 to 6 inches (multiples
of from l.S to 3 times the conventional spray distance of 2
inches) can be used in appropriate applications, responsive to
suitable adjustment of the operating parameters ~or such
applications.
From a practical standpoint, this leads to various
improvements. First, increasing the spray distance permits parts
that are not easily accessible to be effectively sprayed. This
would include parts such as vanes and impellers, which are
limited in terms of their accessibility due to the cables and
~0 hoses which are attached to the thermal spray apparatus (the gun
3) and which tend to obstruct the thermal spray procedure which
is to take place. Second, increasing the spray distance causes
less of the heat produced by the ignited gas to be transferred to
the part which is being coating, which might otherwise have a
deleterious effect on the part and/or the coating applied to the
part. For example, parts formed of materials which can adversely
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respond to heat (such as steels, titaniums and the like) must be
kept to a relatively low temperature te.g., under 300~F) if they
are to maintain their desired physical properties. Increasing
the spray distance, in accordance with the present invention, is
useful in meeting such requirements.
In addition to the foregoing improvements, relative to
the flexibility of the resulting thermal spray process, coatings
produced with a P-block have been found to exhibit other
improvements, relative to the coating characteristics which
result.
To illustrate such improvements, coatings produced
according to the above discussed specifications (Sermatech
"GG-WC-102", Pratt & Whitney "PWA 256-4", or equivalent) were
developed. Previously, it had been found that such coatings
could not be consistently applied at working distances in excess
of 2 inches. In accordance with the present invention, such
coatings were applied using a P-~lock nozzle, at spray distances
of up to 4 inches from the substrate. Mechanical properties and
microstructures of the resulting coatings were then compared
(standard block vs. P-block). More specifically, standard
nozzles operating at a spray distance of 2 inches were compared
with P-block nozzles operated at a spray distance of 3.5 inches,
using process parameters appropriate to each design. Various
specimens were sprayed for comparative purposes including Almen
strips, SNECMA Drop Test specimens, and panels for
microstructure, microhardness and X-ray diffraction studies. The
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Pratt & Whitney specification "PWA 256-4" was used as the
standard for such testing. The following results were obtained.
Photomicrographs of typical structures were obtained at
a magnification of 200 (200X) for coatings achieved with both a
standard block nozzle (Figure 6A) and a P-block nozzle (Figure
6B). No significant differences are observable from these
photomicrographs, except that the coatings produced with the
P-block nozzle appear slightly more dense. However, both
coatings are within specified limits. Such coatings were further
~;ned for cracks, oxides, carbide content and cobalt islands.
In each case, the coatings produced with the P-block nozzle were
found to be acceptable.
Almen strips were coated using both the standard bloc~
nozzle and the P-block nozzle, and the height of curvature was
measured for such coatings. After subtracting the effects of
grit blasting on curvature, intensities achieved for the standard
block and P-block coatings were measured at -llN and -22N,
respectively. These numbers provide an indication of the
relative compressive stress imposed by the coating on the
su~strate. It is known that compressive stresses are helpful in
offsetting fatigue debit due to the application of hardface
coatings. The P-block coating would be expected, if anything, to
enhance performance under fretting wear conditions.
Five specimens coated using a P-block nozzle were drop
tested using a SNECMA Drop ~est apparatus. All five specimens
passed this test at a drop height of loOo mm. Coatings applied
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with the standard block nozzle could also pass this test.
Consequently, the impact resistance of coatings applied with the
P-block nozzle can be expected to match those applied with a
standard block nozzle.
Coatings applied with a P-block nozzle were subjected
to numerous runs over various iterations, and measurements of
microhardness were obtained. In all cases, the measured
microhardness was over lOoO DPH300. This would satisfy
"PWA 256-4" specifications, which limit microhardness to 950 to
1200 units. Experimentation has yielded hardness values for
coatings applied at a distance of 3.5 inches from th,e substrate
of between 1003 and 1071 DPH300. Although the exact nature of the
correlation between hardness and wear resistance has not been
established, it is widely accepted that a minimum of 1000 DPH300
is required for acceptable wear resistance under the conditions
to which a ~an blade mid-span area will be exposed. In the
course of such testing, the resulting su~strate temperatures were
found to be lower for coatings applied with a P-~lock nozzle,
primarily because the thermal spray apparatus was ~urther away
from the part being coated.
X-ray diffraction is typically used to measure the
relative content of various phases present in a coating. Many
such phases may be present in a tungsten carbide-cobalt coating
including, for example, WC, W2C, Co3W3C, Co2W4C, and other
combinations of such elements. Previous studies relative to such
coatings have shown that the primary phases present are WC, W2C
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and Co3W3C. The phase constituted of W2c is generally not present
in the powder, in the form received, but rather forms as a result
of the decarburization that occurs as a result of the thermal
spray process. Although the W2C phase is harder than the WC
5 phase, the former phase is not particularly desirable because it
is a more brittle phase than the WC phase.
Coatings produced with both the standard block nozzle
and a P-block nozzle were analyzed under a standard set of X-ray
diffraction parameters. The resulting diffractograms are shown
in Figure 7A (for a standard block nozzle) and Figure 7B (for a
P-block nozzle~. For the purposes of this study, a comparison
was made relative to the ratio of the WC peak to that of the W2C
peak. While a comparison of the area under each curve would
generally be considered a more accurate method, this is rather
? 5 difficult to implement, and a comparison of peak height ratios is
considered to be an acceptal~le method for making such
comparisons.
The diffractogralns of Figures 7A and 7B show the WC and
W2C peaks between 2~ angles normally used for such coatings. For
20 the purpose of comparison, only the primary WC and WzC peaks are
labeled in the diffractograms of Figures 7A and 7B. The
resulting measurements indicate ratios of l.6 and l.8 for
coatings produced with the standard block nozzle and the P-block
nozzle, respectively. This would suggest that coatings produced
25 with the P-block nozzle cause less decarburization of the powder.
In other words, coatings produced with the P-block nozzle had
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more of the desired phase, namely WC, than coatings produced with
the standard block nozzle. This should also lead to enhancement
of the coatings produced with the P-block nozzle in terms of
their impact wear properties.
~rom the above, it is concluded that coatings produced
with the P-block nozzle of the present invention will exhibit no
detrimental properties relative to coatings produced with a
standard block nozzle. However, the resulting coatings, when
produced with a P-block nozzle, will benefit from the enhanced
microstructural properties which are observed during the coating
process including higher compressive stress on the substrate,
lower W2C phase formation, and lower substrate temperature.
In addit~on to the above described WC-Co coatings,
coatings produced with a P-block nozzle and comprised of
Cr3Cz-NiCr or comprised of Cr3Cz have been found to yield improved
results. For example, chromium carbide coatings sprayed with a
P-block nozz}e, in accordance with the present invention, were
found to be harder and more dense structures than similar
coatings produced with a standard block nozzle. Again, this is
important since the industry standard is to obtain ]narder
coatings, which are considered to be more resistant to wear.
It will be understood that various changes in the
details, materials and arrangement of parts which have been
herein described and illustrated in order to explain the nature
of this invention may be made by those skilled in the art within
the principle and scope of the invention as expressed in the
following claims.
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