US20140141173A1

US20140141173A1 - Method of applying a coating to a perforated substrate

Info

Publication number: US20140141173A1
Application number: US13/679,151
Authority: US
Inventors: Dennis Michael Gray; Jeremy Asher Glaun; Kenneth Thaddeus Juncewicz
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2012-11-16
Filing date: 2012-11-16
Publication date: 2014-05-22

Abstract

The present invention provides a method of applying a coating to a perforated substrate, the method comprising: (a) providing a perforated substrate having a substrate first surface, a substrate second surface and one or more perforations traversing the substrate from the first surface to the second surface; (b) bringing the substrate first surface into contact with a perforation blocking solid; (c) applying a metallic thermal spray coating composition to the substrate second surface using a thermal spray technique; and (d) separating the perforation blocking solid from the substrate first surface to provide a substrate having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating. Among its many other uses, the method is also useful for applying thermal spray coatings to the interior surface of valve cages used in flow control valves.

Description

BACKGROUND

The present invention relates generally to applying a coating a perforated substrate. In particular, the invention relates to methods and techniques of applying a metallic thermal spray coating to a perforated substrate, and wherein the perforations are not occluded by the metallic thermal spray coating.
In numerous areas of science and commerce it is desirable to apply a metallic thermal spray coating to a perforated surface in such a way that the surface is rendered more resistant to erosive and corrosive agents, while at the same time not occluding the perforations. Various schemes for achieving this desirable outcome have been advanced and/or employed. These schemes include, for example, coating the substrate prior to the creation of the perforations, and thereafter machining the perforations through the metallic thermal spray coating and the substrate. Such machining can reduce the overall structural integrity of less robust metallic thermal spray coatings. In the case of very hard metallic thermal spray coatings, such machining can damage the machining tools themselves and be time intensive. Other schemes include filling the perforations with a readily removable filler to which the metallic thermal spray coating has a low adhesive affinity. This necessitates identification of the perforation-filling material, its introduction into the perforations of the perforated substrate, and the removal of such perforation-filling material from the perforations following the application of the metallic thermal spray coating to the substrate. The considerable expenditure of human ingenuity and effort in this area notwithstanding, further improvements are both desired and needed, particularly in the field of control valves in which a simple and reliable method of applying a metallic thermal spray coating to a perforated substrate would be especially prized.
The present invention provides a simple and surprisingly effective method of applying a metallic thermal spray coating to a perforated substrate.

BRIEF DESCRIPTION

In one embodiment, the present invention provides a method of applying a coating to a perforated substrate, the method comprising: (a) providing a perforated substrate having a substrate first surface, a substrate second surface and one or more perforations traversing the substrate from the first surface to the second surface; (b) bringing the substrate first surface into contact with a perforation blocking solid; (c) applying a metallic thermal spray coating composition to the substrate second surface using a thermal spray technique; and (d) separating the perforation blocking solid from the substrate first surface to provide a substrate having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating.
In an alternate embodiment, the present invention provides a method of applying a coating to a perforated article, the method comprising: (a) providing a perforated article having a first surface, a second surface and one or more perforations traversing the article from the first surface to the second surface; (b) bringing the first surface into contact with a perforation blocking solid; (c) applying a metallic thermal spray coating composition to the second surface using a thermal spray technique; and (d) separating the perforation blocking solid from the first surface to provide an article having a metallic thermal spray coating disposed upon the second surface and wherein the perforations are not occluded by the metallic thermal spray coating.
In yet another embodiment, the present invention provides a method of applying a coating to a perforated cylindrical metal article, the method comprising: (a) providing a perforated cylindrical metal article having an outer first surface and an inner second surface and one or more perforations traversing the substrate from the first surface to the second surface; (b) wrapping the outer first surface of the perforated cylindrical metal article with a perforation blocking solid; (c) applying a metallic thermal spray coating composition to the inner second surface using a thermal spray technique; and (d) separating the perforation blocking solid from the first surface to provide a cylindrical metal article having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Various features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters may represent like parts throughout the drawings. Unless otherwise indicated, the drawings provided herein are meant to illustrate key inventive features of the invention. These key inventive features are believed to be applicable in a wide variety of systems which comprising one or more embodiments of the invention. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the invention.

FIG. 1 illustrates a valve cage and valve plug combination used according to one or more embodiments of the present invention.

FIG. 2 illustrates a method used according to one or more embodiments of the present invention.

FIG. 3 illustrates a method step used according to one or more embodiments of the present invention and an illustration of one theory of operability of the present invention.

DETAILED DESCRIPTION

In the following specification and the claims, which follow, reference will be made to a number of terms, which shall be defined to have the following meanings.
The singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.
In one or more embodiments, the present invention provides methods and techniques for applying a metallic thermal spray coating to one or more surfaces of a perforated substrate, wherein the perforations are not occluded by the metallic thermal spray coating being applied to the substrate. The method depends upon the inventors' rather remarkable finding if that a substrate traversed by one or more perforations extending from a first surface of the substrate to a second surface of the substrate is contacted with a metallic thermal spray at the first surface, the simple expedient of blocking the outlets of the perforations at the first surface results in the perforation openings at the second surface being remarkably resistant to becoming occluded by the metallic thermal spray coating as it builds upon on the second surface of the substrate. This disclosure provides details of this discovery, guidance on how it may be practiced and exploited, and data illustrating the utility of the discovery in the field of control valve manufacture.
The perforated substrate may be any substrate containing perforations extending from a substrate first surface, through the substrate and to a substrate second surface and susceptible to being coated using a thermal spray technique, such as the HVAF (High Velocity Air Fuel), AC-HVAF (Activated Combustion High Velocity Air Fuel) and HVOF (High Velocity Oxygen Fuel) thermal spray techniques. By being susceptible to being coated by a thermal spray technique, it is meant that the substrate is not destroyed or rendered useless by the coating technique. Typically, the substrate is a metallic substrate which can withstand the high temperatures inherent in thermal spray techniques. In one embodiment, the perforated substrate is essentially metallic. In an alternate embodiment, the perforated substrate may be a non-metallic material, for example a ceramic or a composite, which is stable under the conditions of the thermal spray technique. A wide variety of substrate cooling techniques are known to those of ordinary skill in the art, and such cooling techniques may be used to mitigate the sometimes negative effects of substrate temperature build-up during the application of the metallic thermal spray coating to the substrate.
In one or more embodiments the substrate is a sheet of metal defining a substrate first surface and a substrate second surface and containing a plurality of perforations extending from the first surface to the second surface. In one embodiment the first surface is substantially parallel to the second surface, meaning that the first surface and the second surface define two substantially parallel planes. When this condition is met, the substrate first surface and the substrate second surface are said to be substantially parallel surfaces. In one embodiment the first surface is substantially non-parallel to the second surface, meaning that the first surface and the second surface do not define two substantially parallel planes. When this condition is met, the substrate first surface and the substrate second surface are said to be substantially non-parallel surfaces. A flat, sheet-like perforated substrate in which the first surface and the second surface have essentially the same surface area and define substantially parallel planes is an example of the perforated substrate wherein the substrate first surface and the substrate second surface are substantially parallel surfaces. See for example perforated substrate 530 in FIG. 3 of this disclosure. If the same sheet-like structure were made to bend and/or twist into a structure having curved surfaces, the bent and/or twisted structure would qualify as a perforated substrate in which the substrate first surface and the substrate second surface are substantially non-parallel surfaces. The foregoing discussion of substantially parallel and substantially non-parallel substrate first surfaces and substrate second surfaces ignores edge surfaces. In one embodiment, the perforated substrate is a perforated cylindrical metal article such as a cage for a flow control valve.
As noted, blocking the outlets of the perforations at the substrate first surface results in being able to apply a metallic thermal spray coating to the substrate second surface such that the perforations are not occluded by the metallic thermal spray coating as the coating is deposited on the substrate second surface. This means that the openings of the perforations at the substrate second surface remain substantially unchanged as the substrate second surface is transformed from an initial uncoated stage to a coated surface. This effect is demonstrated experimentally herein (See Experimental Part). The inventors themselves have proposed various theories to account for this effect and while not wishing to be bound by any particular theory, it is possible that blocking the outlets of the perforations at the substrate first surface results in the creation of a relatively high local pressure within the perforations during the application of the metallic thermal spray coating as gas molecules present between the thermal spray and the substrate second surface are forced by the moving thermal spray into the perforations. As the thermal spray is applied in a zone containing an opening of a perforation at the substrate second surface, this relatively higher local pressure within the perforation creates a bias away from the opening and directs the thermal spray particles onto the substrate second surface adjacent to the opening. An alternative theory, illustrated in FIG. 3, posits that incident thermal spray particles simply ricochet off the closed end of the perforation.
Blocking the outlets of the perforations at the substrate first surface can be effected simply by bringing the substrate first surface into contact with a perforation blocking solid which conforms to the substrate first surface. In the case of a planar substrate, the perforation blocking solid can be a strip of sheet metal of sufficient breadth to block the outlets of the perforations at the substrate first surface. In another embodiment, the perforation blocking solid is non-planar and conforms to a non-planar substrate first surface. For example, in one embodiment, the substrate first surface is the outer surface of a perforated cylinder and the perforation blocking solid is a metal sleeve which conforms to the substrate first surface. Those of ordinary skill in the art will understand that the perforation blocking solid must be in close physical contact with the perforation outlet at the substrate first surface, and that in certain embodiments the perforation blocking solid may comprise surface features which complement and couple with the perforation outlets at the substrate first surface. It should be stressed, however, that it is unnecessary to completely fill the perforation volume in order to prevent occlusion of the perforation at the substrate second surface. Simply blocking the perforation outlet at the substrate first surface is sufficient.
As noted, the perforation blocking solid conforms to the substrate first surface and preferably has a low surface roughness such that if one attempted to apply a metallic thermal spray coating using a standard thermal spray technique, adhesion of the coating to the perforation blocking solid would be poor. In one embodiment, the perforation blocking solid has a surface roughness less than 32 R_a. In one embodiment, the perforation blocking solid comprises stainless steel. In an alternate embodiment, the perforation blocking solid comprises aluminum. In yet another embodiment, the perforation blocking solid comprises chromium.
In contrast to the perforation blocking solid, the substrate second surface to which the metallic thermal spray coating is to applied should have a suitable surface roughness to promote adhesion of the metallic thermal spray coating. Various techniques are known to those of ordinary skill in the art for increasing or decreasing the surface roughness of an article's surface. In one embodiment, the substrate second surface has a surface roughness of about 100 R_a. In another embodiment, the substrate second surface has a surface roughness greater than 100 R_a.
As noted, following the application of the metallic thermal spray coating to the substrate second surface, the perforation blocking solid is separated from the substrate first surface to provide a substrate having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating. As used herein, the term “not occluded” means that the cross-sectional area of the opening of the perforation at the substrate second surface after the application of the metallic thermal spray coating is at least eighty-five (85) percent of the cross-sectional area of the opening of the perforation at the substrate second surface prior to the application of the metallic thermal spray coating. In one embodiment, the cross-sectional area of the opening of the perforation at the substrate second surface after the application of the metallic thermal spray coating is at least ninety (90) percent of the cross-sectional area of the opening of the perforation at the substrate second surface prior to the application of the metallic thermal spray coating. In another embodiment, the cross-sectional area of the opening of the perforation at the substrate second surface after the application of the metallic thermal spray coating is at least ninety-five (95) percent of the cross-sectional area of the opening of the perforation at the substrate second surface prior to the application of the metallic thermal spray coating.
The perforations may be of uniform or non-uniform size within the perforated substrate. In one embodiment, the perforated substrate comprises perforations of uniform size, meaning that each perforation has substantially the same shape and dimensions as all other perforations in the perforated substrate. In another embodiment, the perforated substrate comprises perforations of non-uniform size, meaning that at least two perforations differ significantly in their size and/or shape.
As noted, the perforations traverse the perforated substrate from a perforation opening at the substrate second surface to a perforation outlet at the substrate first surface. In one embodiment, the perforations may be substantially orthogonal to the substrate first surface and substrate second surface See, for example, perforated substrate 530 in FIG. 3. In another embodiment, the perforations may be substantially non-orthogonal to the substrate first surface and the substrate second surface. In an alternate embodiment, the perforated substrate comprises perforations some of which are substantially orthogonal to the substrate first surface and substrate second surface and some of which are substantially non-orthogonal to the substrate first surface and the substrate second surface. In one or more embodiments, the perforations cylindrical in shape and have average diameter in a range from about 0.1 mm to about 10 mm.
The perforated substrate used according to one or more embodiments of the invention may be prepared from an un-perforated substrate by various means known to those of ordinary skill in the art, for example machining and laser ablation techniques. In one embodiment, the perforated substrate is prepared from an un-perforated substrate by drilling perforations from an inlet at the substrate second surface to an outlet at the substrate first surface.
The metallic thermal spray coating composition employed according to the invention may be any metallic coating composition suitable for application by a thermal spray technique. In one embodiment, the thermal spray coating composition employed comprises tungsten carbide, cobalt and chromium and is characterized by an average particle size (prior to application) in a range from about 1 micron to about 100 microns. Additional suitable metallic thermal spray coating compositions are known to those of ordinary skill in the art and include tungsten carbide/nickel, tungsten carbide/nickel-chromium, chromium carbide/nickel-chromium, and cobalt/chromium/carbon alloys.
Those of ordinary skill in the art will understand that the term “perforated substrate” may be applied to perforated articles, a perforated article being a subset of the set which includes all perforated substrates. The distinction between a perforated substrate and a perforated article is that the term “article” refers to (1) substrates which can be recognized as independent components of a system or device, or (2) substrates capable of independently performing a function. According to this definition, all articles are substrates, but not every substrate qualifies as an article.
As noted, in one embodiment, the present invention provides method of applying a coating to a perforated article, the method comprising: (a) providing a perforated article having a first surface, a second surface and one or more perforations traversing the article from the first surface to the second surface; (b) bringing the first surface into contact with a perforation blocking solid; (c) applying a metallic thermal spray coating composition to the second surface using a thermal spray technique; and (d) separating the perforation blocking solid from the first surface to provide an article having a metallic thermal spray coating disposed upon the second surface and wherein the perforations are not occluded by the metallic thermal spray coating.
In one embodiment, the article is a cage for a flow control valve. Flow control valves are exemplified by flow control valves known to those of ordinary skill in the art and include, for example the 40005 series (e.g. Models 41305, 41505, 41605 and 41905) of flow control valves available from Dresser Flow Control an affiliate of the General Electric Company. As used herein a valve cage for a flow control valve may in certain embodiments qualify as a perforated cylindrical metal article (See for example FIGS. 1 and 2 of this disclosure.)
In one embodiment, the present invention provides a method coating a cage (at times herein referred to as a “valve cage”) for a flow control valve with an erosion resistant metallic thermal spray coating wherein the coating can be applied according to one or more embodiments of the present invention and without occluding the perforations (holes) present in the valve cage structure.
Turning now to the figures, FIG. 1 illustrates a valve cage and valve plug combination shown in cutaway view 300 and three dimensional view 302 the performance of which may be enhanced by applying metallic thermal spray coating on the interior surface 320 of valve cage 212. The valve cage and valve plug combination shown in FIG. 1 is contained within a control valve coupled to a working fluid inlet conduit and a working fluid outlet conduit. As shown in the figure valve cage 212 consists of an upper un-perforated portion 420 (FIG. 2) and a lower perforated portion 410 (FIG. 2). Perforations 213 traverse the valve cage from the inner surface 320 to the outer surface of the valve cage. Valve plug 310 is disposed within a central cavity defined by the valve cage and is sized such that it may move freely within the valve cage cavity and yet free space between the main body of the valve plug and the interior surface of the valve cage is minimized in order to limit leakage of a working fluid through said free space and into other components of the valve. The valve is opened by moving the valve plug in the cavity defined by the valve cage so as to expose one or more of the cage perforations to the working fluid which may then enter and pass through the perforation and into a suitable exit conduit. In one embodiment, the working fluid may enter the interior cavity of the valve cage from a fluid inlet conduit (not shown) connected to the lower end of the valve and exit the valve through perforations 213 which empty into a working fluid outlet conduit (not shown). Such valves are typically not self-actuating, and an active control mechanism is typically employed to open and close the valve. Thus a drive rod (not shown) may be coupled with valve plug stem 330 to open and close the valve to the extent required to achieve a desired flow rate of the working fluid across the valve.
The foregoing discussion of control valve mechanics highlights the need for erosion and corrosion resistant coatings, especially at the interface between the valve plug and the valve cage. The working fluid frequently contains hard particles such as sand and metallic particles which can erode the surfaces of the valve plug and the surface of the valve cage. In the embodiment illustrated in FIG. 1 this interface between the valve plug and the valve cage includes the interior surface 320 of the perforated portion 410 of the valve cage 212. While many metallic thermal spray coatings are known to enhance surface hardness, wear resistance and resistance to corrosive chemicals, the application of such metallic thermal spray coatings to the surfaces of perforated articles without occluding the perforations has been notoriously difficult.
FIG. 2 illustrates an embodiment of the present invention which is a method 400 of applying a metallic thermal spray coating to the inner surface of a control valve cage 212. In a first method step 440, a sheet of flexible metal 430 is wrapped around the outer surface (substrate first surface) of the perforated portion 410 of the valve cage in such a way that the surface of the sheet of flexible metal conforms to the outer surface of the valve cage. The sheet of flexible metal is held in place by securing clamps 450. In a second method step 442 a metallic thermal spray coating is applied to the inside surface 320 (substrate second surface) of the valve cage using a thermal spray technique. When the desired thickness of the coating has been achieved, the sheet of flexible metal 430 may be removed in a third method step 444 to provide the valve cage comprising an metallic thermal spray coating on the interior surface of the valve cage and wherein the perforations are not occluded by the metallic thermal spray coating. In alternate embodiments, a metal sleeve closely conforming to the dimensions of the outer surface of the perforated portion of the valve cage may be used instead of a sheet of flexible metal.
Referring now to FIG. 3 the figure represents the application of a metallic thermal spray coating onto a substrate second surface 535 of a perforated substrate 530. A robot controlled thermal spray nozzle 510 moves above the substrate second surface 535 of the perforated substrate 530 along axes of motion 512 while directing thermal spray 520 toward the perforated substrate. The figure shows a perforation blocking solid 540 conforming to a substrate first surface 536 and serving to block the lower end of the perforations. The figure also illustrates one theory of how the present invention works. Under this theory, incident thermal spray particles 522 collide with the surface of the perforation blocking solid 545 at the bottom of perforation in ricochet events 550, producing thereby a stream of ricocheting particles 524 exiting the perforation. Although, not wishing to bound by this or any other theory of operability, this model accounts for the limited occlusion of the perforations observed, since the ricocheting thermal spray particles 524 exiting the perforation will tend to scour the edges of the perforation at its end adjacent to substrate second surface 535. It should be noted that incident thermal spray particles impacting substrate second surface 535 will tend adhere to that surface, since the substrate is selected such that it has a high affinity for metallic thermal spray particles. In addition, surface treatments known to those of ordinary skill in the art may be employed to enhance the affinity of a substrate second surface such as 535 for thermal spray particles such as 522.

Experimental Part

General Methods

The thermal spray equipment and metallic thermal spray coating precursor compositions employed in the application of metallic thermal spray coatings are commercially available from suppliers such as Kermetico, Inc.; Sulzer Metco, Inc.; and Praxair, Inc. Commercially available cylindrical valve cages having a plurality of substantially round perforations extending from the interior surface of the valve cage to the exterior surface of the valve cage were obtained from Dresser Flow Control, Inc. All of the perforations had a diameter of about 0.12 inches. The valve cage had a perforated portion extending from a first end of the valve cage to approximately the midpoint along the length of the valve cage. In addition, the valve cage had an un-perforated portion extending from a second end of the valve cage to approximately the midpoint along the length of the valve cage.
A recessed zone approximately 0.010 inch deep and several centimeters wide (also referred to as “the machined zone”) was machined into the inside surface of the valve cage near the interface between the perforated portion and the un-perforated portion of the valve cage and extending several centimeters into the un-perforated portion of the valve cage to compensate for additional metallic thermal spray coating thickness in the region of the recessed zone that resulted from the thermal spray protocol used. In this thermal spray protocol, the thermal spray gun was, of necessity, positioned outside of the interior cavity of the valve cage but along the central axis of the valve cage during the application of the metallic thermal spray coating. It should be noted that the thermal spray gun apparatus was too large to be inserted into the interior cavity of the valve cages studied. The metallic thermal spray coating was applied to the interior surface of the valve cage in two steps. In a first step, the metallic thermal spray coating was applied through a first end of the valve cage continuously onto the valve cage interior surface from the portion of the valve cage interior surface closest to first end, and up to a point just beyond the machined zone along the inside diameter of the valve cage. In a second step, the metallic thermal spray coating was applied through a second end of the valve cage (opposite the first end) continuously onto the valve cage interior surface from the portion of the valve cage interior surface closest to second end, and up to a point just beyond the machined zone along the inside diameter of the valve cage. As such, the inside diameter of the valve cage in the region of the machined zone of the valve cage was susceptible to being coated from each end of the valve cage, and the resultant metallic thermal spray coating was approximately twice as thick on the interior surface of the valve cage in the region of the machined zone relative to other regions of the interior surface of the valve cage. The machined zone along the interior diameter was included to ensure that this additional thickness did not interfere with the performance of the valve cage when in combination with a movable valve plug. As an additional precaution against further overlap of metallic thermal spray coatings applied from either end of the valve cage, a removable metallic mask cylinder was inserted into the portion of the valve cage interior cavity not being subjected to the thermal spray technique at a given time, such that the interior surface of the valve cage as measured from a first end of the valve cage opposite a second end of the valve cage through which the thermal spray was being applied, and up to the edge of the machined zone closest first end of the valve cage was covered by the removable metallic mask cylinder and protected from the incident thermal spray.
The interior surface of the valve cage, including the machined zone, was grit blasted prior to the application of the metallic thermal spray coating. Following grit blasting the exterior surface of the perforated portion of the valve cage was brought into contact with a piece of flexible rolled stainless steel sheet metal of appropriate length and breadth to cover all of the valve cage perforations and was estimated to have a surface roughness of about 16 R_a. The sheet metal was secured by hose clamps which assured close contact between the perforation blocking sheet metal and the openings of the perforations at the valve cage outer surface.
The thermal spray gun was oriented such that the thermal spray was directed towards the interior surface of the valve cage at an angle of incidence of about 45 degrees. The valve cage was rotated around its axis while the thermal spray gun applied the metallic thermal spray coating and moved axially at a translation speed of about 1000 millimeters per second through a continuous series of axial positions over a distance of about 75 millimeters which maintained a working distance between the thermal spray gun and the portion of the interior surface being coated at any given time of about 15 inches. After its closest approach to the work piece, the thermal spray gun reversed directions and returned to its original position while continuing to apply the metallic thermal spray coating. The process was repeated, typically through about 18 such cycles (also referred to at times herein as “reps”), until the metallic thermal spray coating disposed upon valve cage interior surface was judged to be of sufficient thickness, typically 9 to 10 mils as measured using a micrometer at multiple points along the valve cage walls. This provided a valve cage in which slightly more than half of the interior surface (the first portion) was coated with the metallic thermal spray coating. The remaining uncoated portion of the interior surface of the valve cage was coated using the same technique used to coat the first portion except ingress of the thermal spray was from the opposite end of the valve cage.
In one or more experiments a cermet powder was used as the metallic thermal spray coating precursor and consisted of tungsten carbide (WC) particles that were primarily less than 1 micron in size together with a cobalt-chromium (CoCr) matrix. The powder composition was 84 weight percent WC, 10 weight percent Co and 4 weight percent Cr. Other suitable metallic thermal spray coating precursor include compositions comprising about 85 weight percent WC and about 15 weight percent Co; compositions comprising about 85 weight percent WC and about 15 weight percent Ni; compositions comprising about 80 weight percent Cr₃C₂and about 20 weight percent Ni20Cr alloy; and compositions comprising about 50 weight percent Cr₃C₂and about 50 weight percent Ni 20Cr alloy.

EXAMPLES 1-2 AND COMPARATIVE EXAMPLES 1-2

Data are gathered in Table 1 below which demonstrate the effectiveness of the method of the present invention at prevention of perforation occlusion (closure) during the application of a metallic thermal spray coating to the interior surface of valve cages. Four control valve cages; two 5.25 inch inner diameter valve cages (Dresser) and two 3.25 inch inner diameter valve cages (Dresser) were used in Example 1 and Comparative Example 1, and Example 2 and Comparative Example 2 respectively. Each valve cage was prepared for thermal spray treatment as described in the General Methods section. The difference between the Examples and the Comparative Examples being the absence of the perforation blocking solid in the Comparative Examples. Grit blasting of the interior surface of each of the valve cages was carried out using 60 mesh alumina at 80 psi. Following grit blasting, one of each size of the valve cages was wrapped with a flexible stainless steel sheet in contact with the entire outer surface of the perforated portion of the valve cage such that all perforation outlets (hole outlets) were blocked. These wrapped valve cages were used in Examples 1 and 2, and were coated using the thermal spray technique described in the General Methods section using a dry metallic thermal spray coating precursor composition comprising WCCoCr (WC760) which was applied as received from a commercial supplier (PRAXAIR). Comparative Examples 1 and 2 were essentially identical to Examples 1 and 2, with the exception that the perforations were not blocked in any way during the application of the metallic thermal spray coating to the interior surfaces of the two valve cages used in the Comparative Examples.

TABLE 1

	cage inner		R_a	Starting	After coating
	diameter	thickness	(micro-	hole size	hole size
Example	(inches)	(mils)*	inch)	(mils)	(mils)†

Example 1	5.25	11	121	113
Comparative	5.25	11	121	98
Example 1
Example 2	3.25	9.95	121	110
Comparative	3.25	9.95	121	97
Example 2

*Measured with a micrometer at least three locations on the valve cage.
†As determined using a set of pin gages.

The data in Table 1 compare the valve cage hole sizes before and after coating. Results for Examples 1-2 when compared Comparative Examples 1-2 demonstrate that when valve cage perforations (holes) were in blocked at the outer surface of the valve cage, the perforations showed significantly less occlusion than unblocked perforations (Comparative Examples 1 and 2) following the application of the metallic thermal spray coating to the interior surface of the valve cage. Pin gauges were used to measure hole sizes and the size of each of the hundreds of holes defined by the valve cage was measured.
The foregoing examples are merely illustrative, serving to illustrate only some of the features of the invention. The appended claims are intended to claim the invention as broadly as it has been conceived and the examples herein presented are illustrative of selected embodiments from a manifold of all possible embodiments. Accordingly, it is Applicants' intention that the appended claims are not to be limited by the choice of examples utilized to illustrate features of the present invention. As used in the claims, the word “comprises” and its grammatical variants logically also subtend and include phrases of varying and differing extent such as for example, but not limited thereto, “consisting essentially of” and “consisting of.” Where necessary, ranges have been supplied, those ranges are inclusive of all sub-ranges there between. It is to be expected that variations in these ranges will suggest themselves to a practitioner having ordinary skill in the art and where not already dedicated to the public, those variations should where possible be construed to be covered by the appended claims. It is also anticipated that advances in science and technology will make equivalents and substitutions possible that are not now contemplated by reason of the imprecision of language and these variations should also be construed where possible to be covered by the appended claims.

Claims

What is claimed is:

1. A method of applying a coating to a perforated substrate, the method comprising:

(a) providing a perforated substrate having a substrate first surface, a substrate second surface and one or more perforations traversing the substrate from the first surface to the second surface;

(b) bringing the substrate first surface into contact with a perforation blocking solid;

(c) applying a metallic thermal spray coating composition to the substrate second surface using a thermal spray technique; and

(d) separating the perforation blocking solid from the substrate first surface to provide a substrate having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating.

2. The method according to claim 1, wherein the perforated substrate is a metallic substrate.

3. The method according to claim 1, wherein the substrate first surface and the substrate second surface are substantially parallel surfaces.

4. The method according to claim 1, wherein the substrate first surface and the substrate second surface are substantially non-parallel surfaces.

5. The method according to claim 1, wherein the perforations are substantially orthogonal to the substrate first surface and the substrate second surface.

6. The method according to claim 1, wherein the perforations are substantially non-orthogonal to the substrate first surface and the substrate second surface.

7. The method according to claim 1, wherein the perforation blocking solid is made of sheet metal.

8. The method according to claim 7, wherein the perforation blocking solid is characterized by a surface roughness of less than 32 Ra.

9. The method according to claim 1, wherein the thermal spray technique is an HVAF thermal spray technique.

10. The method according to claim 1, wherein the thermal spray technique is an HVOF thermal spray technique.

11. The method according to claim 1, wherein metallic thermal spray coating composition comprises tungsten carbide, cobalt and chromium and is characterized by an average particle size in a range from about 1 micron to about 100 microns.

12. A method of applying a coating to a perforated article, the method comprising:

(a) providing a perforated article having a first surface, a second surface and one or more perforations traversing the article from the first surface to the second surface;

(b) bringing the first surface into contact with a perforation blocking solid;

(c) applying a metallic thermal spray coating composition to the second surface using a thermal spray technique; and

(d) separating the perforation blocking solid from the first surface to provide an article having a metallic thermal spray coating disposed upon the second surface and wherein the perforations are not occluded by the metallic thermal spray coating.

13. A method of applying a coating to a perforated cylindrical metal article, the method comprising:

(a) providing a perforated cylindrical metal article having an outer first surface and an inner second surface and one or more perforations traversing the substrate from the first surface to the second surface;

(b) wrapping the outer first surface of the perforated cylindrical metal article with a perforation blocking solid;

(c) applying a metallic thermal spray coating composition to the inner second surface using a thermal spray technique; and

(d) separating the perforation blocking solid from the substrate first surface to provide a cylindrical metal article having a metallic thermal spray coating disposed upon the substrate second surface and wherein the perforations are not occluded by the metallic thermal spray coating.

14. The method according to claim 13, wherein the article is a cage for a flow control valve.

15. The method according to claim 13, wherein the perforations are essentially round and are characterized by a perforation average diameter in a range from about 0.1 mm to about 10 mm.

16. The method according to claim 13, wherein the perforation blocking solid is made of sheet metal.

17. The method according to claim 16, wherein the perforation blocking solid is characterized by a surface roughness of less than 32 Ra.

18. The method according to claim 13, wherein the thermal spray technique is an HVAF thermal spray technique.

19. The method according to claim 13, wherein the thermal spray technique is an HVOF thermal spray technique.

20. The method according to claim 13, wherein metallic thermal spray coating composition comprises tungsten carbide, cobalt and chromium and is characterized by an average particle size in a range from about 1 micron to about 100 microns.

21. The method according to claim 13, wherein metallic thermal spray coating composition comprises chromium carbide, nickel and chromium and is characterized by an average particle size in a range from about 1 micron to about 100 microns.