Abstract:
A method of removing a deposit from a passage extending through a component includes immersing the passage within a liquid, and directing a liquid jet at the passage from a source within the liquid bath and with a sufficient velocity so as to remove at least a portion of the deposit.

Description:
BACKGROUND 
     The present invention relates generally to a method of removing material from a component having holes through its walls. In particular it relates to a method of removing excess coating material from within cooling holes of coated gas turbine components. 
     Gas turbine engines operate at extremely high temperatures for increased performance and efficiency. A limiting factor in most gas turbine engine designs, however, is the maximum temperature that various components of the engine can tolerate. One such particular component area which is so limited is the combustion chamber of a gas turbine engine. 
     One method to increase the maximum allowable temperature and/or decrease the component metal temperature is to provide cooling holes in the walls of the component. These holes allow cool air to flow through and along the walls of the component exposed to the high gas temperatures. As the air flows along the surface of the walls it forms a cool layer. This cool layer reduces the temperature of the wall surface and physically keeps the hot gases from contacting the walls of the component, thereby permitting the component to withstand higher gas temperatures than would otherwise be possible. 
     Another method of allowing higher gas temperatures to be used is to apply a protective thermal barrier coating to the walls of the component that are exposed to the hot gases. In the case of combustors this is, in particular, the inner walls of the flame tube, the outer walls being exposed to cooler compressor delivery air. Such coatings conventionally comprise, for example a MCrAlY material which offer thermal and corrosion protection. MCrAlY refers to known coating systems in which M denotes nickel, cobalt,iron or mixtures thereof; Cr denotes chromium; Al denotes aluminium; and Y denotes yttrium. A further ceramic layer is also often applied on top of the MCrAlY layer to give improved thermal protection. In such an arrangement the MCrAlY layer acts as a bond coat for the ceramic coating layer. An example of such a ceramic coating material is yttria stabilised zirconia which is applied on top of an MCrAlY layer. 
     The MCrAlY and ceramic protective coatings are typically applied by physical vapour deposition (PVD), chemical vapour deposition (CVD) or plasma spraying means. Examples of such protective coatings and the methods of applying them are well known and are described in: U.S. Pat. No. 4,321,311, U.S. Pat. No. 5,514,482, U.S. Pat. No. 4,248,940 among many others. 
     Cooling holes and protective coatings can, and are, used in conjunction to allow operation of a component at a high temperature. There are two basic methods for producing such components that have cooling holes and a protective coating. In the first method the coating is applied to the component and then the holes are drilled through the coated component. Examples of this method are described in EP0826457 in which laser drilling is used to penetrate a thermal barrier coating and the metal of the component. A problem with this method is that, by design, the thermal barrier coating is resistant to heating produced by the laser to drill through the material. Consequently drilling of the coating requires a high power laser, a prolonged operation, and results in considerable heating of the surrounding area which can be undesirable. Problems also exist if mechanical drilling techniques are used since the thermal barrier coatings are generally brittle. Mechanical drilling can crack and damage the coating in the region around the holes causing the coating to fall off the component either during the machining operation or prematurely during service. 
     In the second method holes are drilled in the component and then the coating is applied to drilled component. This method does not have any of the problems associated with drilling/machining through the coating described above. However application of the coating after the holes have been drilled does tend to at least partially block some or all of the holes. This restricts the flow of cooling air through the holes and can result inadequate cooling of the component producing hot spots, overheating and possible failure of the component. Furthermore the blocking of the cooling holes is unpredictable and so designing the holes to accommodate a degree of blockage is problematic and also, if it is possible will reduce the efficiency of the engine. 
     Consequently any coating material blocking the cooling holes has to be removed. The problem of cooling hole blockage and a method of removing the coating from a cooling hole is described in EP0916445. A fluid jet at a high pressure is directed at the opposite face of the component to that to which the coating has been applied. The through holes then act as a mask and protect the coating from damage. 
     Fluid jets operating at high pressure, up to 60000 psi, are noisy at levels of the order 120 dB. Additionally, careful control is required to prevent damage to the surface of the component against which the fluid jet is directed. 
     It is therefore desirable to provide an improved method of removing material from holes within a component that addresses the above mentioned problems and/or offers improvements generally to such methods. 
     SUMMARY 
     According to the present invention there is provided a method of removing a deposit from a passage extending through a component comprising the steps, immersing the passage within a liquid, directing a liquid jet at the passage from a source within the liquid and with a sufficient velocity so as to remove at least a portion of the deposit. 
     In this method the liquid within which the passage is immersed acts to attenuate the noise of the process. 
     The deposit may be contiguous with a coating provided on a first surface of the component, with the liquid jet preferably being directed at the passage from a second surface of the component opposite the first surface. 
     In this method the component itself is used as a mask to direct a high pressure fluid jet though a cooling hole, whereupon it machines away and removes any material blocking the hole. This has the advantage that the jet does not have to be accurately directed at a particular hole allowing a less accurate, cheaper and simpler machine to be used. In addition the remainder of the coating which is not blocking the hole is protected from the jet by the component itself. Any damage to the coating is therefore reduced. The use of the component itself as a mask also means that the process is simple and relatively rapid. 
     The liquid jet may further comprise solid particles disposed therein. The solid particles may be glass beads. 
     The passages in the component preferably have an axis that is angled relative to the first and second surface. The jet preferably has a negative rake relative to the axis of the passage of between 0 and 5 degrees in this way the jet may be directed towards the debris and can remove the debris from the hole without damaging the hole entrance. 
     Preferably the method further comprises the step of subsequently directing a second liquid jet at the passage from a second source within the liquid bath. Preferably the second liquid containing jet further comprises gas admixed therein. 
     The second jet is preferably directed at the passage with a positive rake angle of 0 to 5 degrees relative to the axis of the passage. The second jet cleans the passage and the positive rake angle helps to prevent chipping of the coating at the hole exit 
     The component may have a plurality of passages and the method comprise directing the first liquid jet at a first one of the plurality of passages and then traversing the jet across the second surface of the component to a second passage and directing the first liquid jet at a second one of the plurality of passages. 
     The second liquid jet may be traversed with the first liquid jet to direct the second liquid jet at the first one of the plurality of passages after the first liquid jet has been directed thereat. 
     Preferably the first liquid jet is traversed at a constant rate over a region of the second surface of the component in which the passages are located 
     Substantially all the deposit within the first one of the plurality of passages may be removed before the first liquid jet is traversed to the second one of the plurality of passages. 
     The component may be rotated about an axis and relative to the first liquid jet such that the fluid jet is intermittently directed at the passage. Preferably the axis is orthogonal to the direction in which the first liquid jet traverses. 
     Preferably the fluid within which the passage is immersed has a current which flows towards the component. Preferably the current flows towards the first surface of the component. 
     Preferably the passage is arranged to provide, in use, a cooling flow for the component. The component may be made from metal and the deposit ceramic. The component may be a combustor flame tube, or a turbine blade. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying drawings in which; 
         FIG. 1  is a sectional view of a part of an annular combustor section of a gas turbine engine, 
         FIG. 2  is an illustrative view of a fluid jet operating on a part of a combustor flame tube wall in accordance with the present invention, 
         FIGS. 3   a,b,c  are diagrammatic views showing the combustor flame tube wall and cooling hole at various stages during manufacture. 
         FIG. 4  illustrates a second embodiment method of machining holes in a combustor flame tube according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring to  FIG. 1  there is shown a combustor section  20  of a gas turbine engine. Inner and outer annular casing walls  2  and  4  respectively define an annular duct  11 . Within this annular duct  11  there is provided an annular flame tube  6 . Compressed air from a compressor section (not shown) of the gas turbine engine flows, as shown by arrow A, into this duct  11  through an inlet  14 . A portion of this air flows into the interior  7  of the flame tube  6  as shown by arrow G, through an upstream annular flame tube inlet  8 . The remainder of the air flows around the outside  9  of the flame tube  6 , as shown by arrows H. The air entering the flame tube  6  is mixed with fuel, which is supplied from a number of fuel nozzles  18  within the flame tube  6 . The resulting fuel/air mixture in the interior  7  of the flame tube  6  is then burnt to produce a high temperature gas stream. This high temperature gas stream flows along the flame tube  6  as shown by arrow B, through an annular outlet  10  and series of outlet guide vanes  12  at the downstream end of the flame tube  6  and combustor  20 , into the turbine section and/or the exhaust of the gas turbine engine. 
     The walls  44  of the annular flame tube  6  are pierced by a number of cooling holes  16 . The cooling holes  16  act as passages through the walls  44  of the flame tube  6 . Cool compressed air flowing around the flame tube  6  flows through these holes  16  into the interior  7  of the flame tube  6  and along the walls  44  of the flame tube  6 . This flow of cool air through the walls  44  of the flame tube  6  cools the walls  44  of the flame tube  6 . The flow of air along the inside walls  22  of the flame tube  6  produces a layer of relatively cool air adjacent to these walls  22  which provides a thermal barrier between the wall  44  of the flame tube  6  and the hot combustion gases within  7  the flame tube  6 . A thermal barrier coating  28 , generally comprising a layer of ceramic material is also provided on the inside walls  22  of the flame tube  6  which also protects the walls  44  of the flame tube  6  from the hot combustion gases. 
     The flame tube  6  may also have a number of other, larger, openings  26  within the walls  44  to admit additional compressed air to the interior  7  of the flame tube  6 . This additional air being provided to aid further, and more complete combustion within the interior  7  of the flame tube  6 . 
     The flame tube  6  is made from sheet metal, generally a high temperature alloy for example a nickel cobalt or iron superalloy, which is fabricated into the required shape of the flame tube walls  44 . The thickness of the metal walls is typically between 1-1.6 mm. Alternatively the metal flame tube  6  can be fabricated from forged rings or even cast. 
     The cooling holes  16  in the flame tube walls  44  are conventionally produced by such methods as electrical discharge machining (EDM) or laser drilling.  FIG. 3   a , shows a detailed view of a hole  16  produced in the flame tube wall  44 . As shown the cooling holes  16  are generally angled in the flow direction and act in effect as passages through the walls  44  of the component. Such angling promotes the formation of a layer of cool air along the inside  22  of the flame tube walls  44 . The diameter of the cooling holes  16  is typically between about 0.25 mm and about 0.76 mm. 
     After production of the cooling holes  16 , the first (interior) surfaces  22  of the flame tube walls  44 , which define the interior  7  of the flame tube  6 , are coated with a thermal barrier coating  28 . This coating  28  on the first (interior) surfaces  22  provides the flame tube walls  44  with protection from the high temperature combustion gases. The second (exterior) surfaces  24  of the flame tube  7 , being exposed to relatively cool compressor air  9 , do not require thermal protection and are accordingly not coated. Typically the coating  28  comprises a MCrAlY, and/or an aluminide bond coat that is first applied to the wall. On top of this bond coat a ceramic coating, for example yttria stabilised zirconia, is deposited. Such coatings are well known in the art and are applied by conventional techniques for example sputtering, electron beam physical vapour deposition (EBPVD), and plasma spraying. An example of such a coating  28  and method of application is described in U.S. Pat. No. 4,321,311, which describes an MCrAlY bond coat and alumina layer and an EBPVD columnar grain ceramic layer. U.S. Pat. No. 5,514,482 describes a diffusion aluminide bond coat with an alumina layer and then an EBPVD ceramic layer. U.S. Pat. No. 5,262,245 describes an MCrAlY bond coat with a plasma sprayed ceramic layer. Further examples are described in U.S. Pat. No. 4,248,940, U.S. Pat. No. 5,645,893 and U.S. Pat. No. 5,667,663. 
     The thickness of these coatings  28  is typically between about 0.3 mm to about 0.5 mm depending upon the particular requirements of the combustor  20 , or component being protected. 
     Application of the coating  28  often results in an undesirable accumulation  30  of the coating material within and over the cooling holes  16 , as shown in  FIGS. 2 and 3   b . This accumulation may either partially or totally block the cooling hole  16 , thereby restricting or preventing the flow of cooling air through the hole  16  during engine operation. This, if not removed, may result in inadequate cooling of the flame tube wall  44  and a reduction or elimination in the thickness of the cooling layer adjacent the flame tube walls  44 . In turn this may then lead to local hot spots on the flame tube wall  44  which may cause the flame tube material to fail and will reduce the service life of the component. 
     Accordingly after application of the coating  28  the accumulation  30  of coating material within and over the holes  16  is removed. The component, or at least the hole to be cleared is immersed in water  101 . Cleaning of the hole is achieved using a high pressure water jet  38  as shown in  FIG. 2 . High pressure water jet machining and machines capable of carrying out the process are generally known. Examples of such machines are produced and available from Flow Europe GmbH, Germany. Such machines have a nozzle  32  which is supplied with high pressure water, typically between about 10,000 psi (689 bar) and about 60,000 psi (4136 bar). This exits the nozzle  32  through a circular orifice producing a generally circular jet  38  of high pressure water. The diameter of the jet  38  is generally between 0.7 mm and 1.7 mm, and is typically about 1 mm. The nozzle  32  is mounted on a suitable support means (not shown), for example a robot arm, that is capable of moving the nozzle  32 , and jet  38 , relative to a workpiece, for example the flame tube  6 . 
     The nozzle  32  of the high pressure water jet is immersed within the water  101  along with the component  44 . The jet  38  is directed against the exterior surfaces  24  of the flame tube  6  in the region of the holes  16 . The jet  38  is angled so that it impinges the walls  44  of the flame tube  6  at substantially the same angle, between 0 to 5 degrees of the axis of the cooling holes  16  and is traversed over the holes  16  in the flame tube walls  44 , as shown generally by arrows C. The pressure of the water jet  38 , the distance  49  (sometimes call standoff) between the nozzle  32  and the flame tube walls  44 , and the length of time that the jet  38  impinges on the surface are all controlled such that there is substantially no machining of the metal of the uncoated exterior surface  24  of the flame tube walls  44 . Typically a standoff distance  49  of up to about 20 mm is used. 
     As the jet  38  is traversed across uncoated side  24  of the flame tube walls  44  it encounters a cooling hole  16 . The cooling hole is in effect a passage and once the jet  38  enters the hole  16  it is guided and channelled by the unblocked metal sides of a first portion  42  of the cooling hole  16 . At the hole exit the jet  38  encounters the coating accumulation  30 , or other debris, blocking or partially restricting the hole  16 . The coating  28  material, for example a ceramic, is less resistant to the water jet  38  than the metal of the flame tube wall  44 . The water jet  38  therefore machines away, by particle erosion, the coating accumulation  30  within the cooling hole  16  until the jet  38  can pass freely through the cooling hole  16 . An illustration of a cleared hole  16  is shown in  FIG. 3   c . As can be seen, by this method, a clear well defined hole exit  48  is produced through the coating  28 . The jet  38  is then traversed to the next cooling hole  16  and the process repeated until all of the cooling holes  16  have been cleared. By this method each of the cooling holes  16  are cleared in succession. 
     The submerged water jet contains 12 mesh glass beads as a media in the clearing process, which is delivered at about 50,000 psi. The beads are delivered to the passageway at a rate equivalent to 60 g every minute. 
     Since the jet  38  is guided by the first portion  42  of the hole  16  accurate alignment of the jet  38  with the hole  16  is not required using this method. Additionally since, in this case, the cooling holes  16  are of a smaller diameter than the water jet  38 , the jet  38  will still overlap the hole  16  even when not fully aligned. Furthermore since the water jet  38  is directed against the exterior side  24  of the flame tube  6 , the coating  28  on the interior surface  22  that is not within the hole  16  is not exposed to the water jet  38 . Consequently the possibility of the remainder of the coating  28 , on the interior side  22  of the wall  44 , being damaged by the water jet  38  is substantially eliminated. This is not the case in the prior art methods where a machining jet or abrasive fluid is supplied from the coated interior side  22  of the component. 
     A second water jet nozzle is directed towards the passage after the passage has been addressed by the first jet. The second water jet, unlike the first water jet, does not contain glass beads. Instead, the flow is water and air. 
     Beneficially, the two jets clean using slightly different mechanisms. Where particles are present in the water jet the jet cleans using an abrasive action and therefore erodes the coating overspray, but typically will work with difficulty on a fully blocked hole. By contrast the water jet containing air removes the coating using a fracture mechanism, which works well even when the hole is fully blocked. 
     The inclination of the nozzle angle to hole during the glass bead cleaning has to be in a negative rake condition between 0 and 5 degrees. This removes all bond coat or TBC from the hole without damaging the hole entrance. If a negative rake condition is not applied then hole shape and angle can be adversely affected. When water/air cleaning is applied then a positive angle is required to reduce chipping of TBC at hole exit. 
     By cleaning the component under water, with the source of the cleaning jets also being located under water, the noise of the jets is attenuated. Use of high pressure jets in air produces a level of noise in excess of 120 dB. Where the jet does not contain solid particles the noise level is further increased. 
     When the cleaning operation is performed while the component and jet source is submerged in water  101  the level of noise is reduced to around 90 dB. 
     In the preferred embodiment a current  103  is induced in the water  101  that flows around the component  44 . The flow has a low pressure but a high volume. Beneficially, the flow further reduces the noise perceived by a sensor located out of the water  101  to around 70 dB. The direction of water flow  103  has minimal bearing on the noise reduction however, a flow towards the surface of the component having the coating  28  is preferred. 
     In an alternative method the water jet  38  is traversed repeatedly across the uncoated side of the flame tube wall  44  containing the holes  16 . During each traverse, or pass, the jet periodically encounters the cooling holes  16  and flows through them. Generally a traverse rate of between about 0.5 m/minute and 10 m/minute, and typically of 2 m/minute is used for a substantially linear traverse of the jet  38 . At such a rate there is not sufficient time for the jet to remove all of the coating  30  from within the hole  16  in a single pass. Consequently only a portion of the material  30  is removed from within the hole  16  during a single pass of the jet  38  over, and through, a hole  16 . The hole  16  is fully cleared after a number of individual passes of the jet  38  over and through the hole  16 . 
     The advantage of this method is that a large number of holes  16 , within a single pass of the jet, can be cleared at substantially the same time. The jet  38  also does not have to be paused and directed individually at each hole  16 . Consequently this alternative method requires even less alignment of the jet  38  with the holes  16  and provides an even faster method of clearing the holes  16 . Furthermore since accurate control of the water jet  38  is not critical in this method, less accurately controlled water jet machines that are simpler and cheaper can be used. 
     A further variation of the above method is shown in  FIG. 4 . The flame tube  6 , as described previously with reference to  FIG. 1 , has a coating  28  on the inside  22  walls of the annulus defined by the flame tube  6 . The flame tube  6  is submerged in a bath  105  containing water  101 . A radially directed water jet  38 , preferably containing a solid media, is traversed across the cooling holes  16  by rotating the flame tube  6  about its longitudinal axis  50 , as shown by D. The jet  38  thereby acts on an entire circumference of the flame tube wall  44 , in which the holes  16  have been drilled, during rotation of the flame tube  6 . The water jet  38  is then axially translated, as shown by arrow E, to impinge a further circumference, and series of holes  16 , axially along the flame tube  6 . The jet  38  is also moved radially, shown by arrow F, relative to the flame tube walls  44  to achieve the required standoff distance  49 . Rotation of the flame tube  6  is carried out by any conventional means, for example by mounting the flame tube  6  upon a rotary table. The above rotary system provides a simpler and easier method of traversing the jet  38  over the surface of the component, and higher traverse rates than can be easily achieved with a linear system can be produced. In a rotary system a traverse rate of the jet  38  over the surface of the component of 5 m/s can be used. It will be appreciated that with such rapid traverse rates only a very small amount of coating  28  material will be removed in any pass of the jet over the hole  16 . 
     In the arrangement shown in  FIG. 4  the jet  38  is shown being used to clear the holes  16  in the inner walls  52  of the flame tube. It will be appreciated that to clear the holes  16  in the outer walls  54  the jet  38  is mounted outside of the outer wall  54  of the flame tube  6 , with the jet  38  being directed radially inward. By this method the holes  16  that have been drilled within the flame tube walls  44  are cleared by repeated passes of the water jet  38  as the flame tube  6  rotates. 
     Additional jet heads  32  may be mounted on the same support to increase the speed at which the component is cleaned. These heads are spaced at an equivalent spacing to the passages in the component to be cleaned. 
     As discussed with the embodiment described with respect to  FIG. 2 , the water jets that include the solid media beads are followed by a second head that ejects a water and air jet towards the passages. This head may be duplicated to decrease the time required to process the component. 
     In a specific illustrative test of the basic method of the invention a 1 mm thick piece test piece of C263, a nickel cobalt superalloy, was laser drilled with a number of 0.5 mm holes, in a row, with each hole inclined at an angle of 30°. One side of the test piece was then then coated with a 0.4 mm thick layer of a standard ceramic thermal barrier coating. In this test the coating comprised a 0.1 mm layer of MCrAlY bond coat, applied by plasma spraying, with a 0.3 mm layer of yttria stabilised zirconia ceramic deposited by plasma spraying on top of the bond coat. This coating at least partially blocked the pre drilled holes. A 1 mm circular water jet at a pressure of 50,000 psi and containing 12 mesh glass beads, oriented at the same 30° angle as the holes, was directed at the metal side of the test piece with the water jet nozzle approximately 10 mm from the test piece. This jet was traversed across the row of holes at a constant rate of 2 m/minute. A second 1 mm circular water jet at a pressure of 50,000 psi, oriented at the same 30° angle as the holes, traversed with the first water jet at a constant rate of 2 m/minute and was directed at the metal side of the test piece after the first with the water jet nozzle approximately 10 mm from the test piece. The second water jet contained entrained air bubbles. 
     Inspection of the holes showed that they had been adequately cleared of the ceramic coating previously deposited within them. The coating around the holes was also substantially unaffected with a clean hole having been machined through the coating by the water jet. There was also no significant damage to the surface of the test piece that was exposed to the water jet during traversal of the jet between holes. Although this method has been described in relation to clearing holes in annular flame tubes  6  it will be appreciated that it can be applied to other known types of combustors which incorporate cooling holes, or other small holes, and have a coating material applied to one side of their walls in the region of the holes. For example it can be used with cannular combustors that comprise a number of individual cylindrical combustion cans disposed around the engine. The method of the invention can also be applied to clearing cooling holes within the combustor tiles of a tiled combustor. On side of the tiles being generally coated with a thermal barrier coating. Such tiled combustors also being well known in the art. 
     The method of the invention can also be applied to other components both within the combustion section  20  of a gas turbine engine and more generally. Indeed it is envisaged that it can be used to manufacture any component which, during manufacture, may have holes that are blocked, or partially blocked, by a coating material. For example it can also be applied to the manufacture of turbine blades which have cooling holes and are coated, on their outside, with a thermal barrier coating. A restriction on the application of the method though is that there must be sufficient access for the jet to be directed at the cooling holes. This could possibly be a problem for some, in particular small, turbine blades where there must be sufficient room for the nozzle and jet to be inserted and operate inside of the blade. 
     The method is not limited to use in removing thermal barrier protective coatings from within cooling holes. Other coatings may similarly block, or partially block any holes in the flame tube  6 , or any other component. Such coatings could be applied, for example, to offer corrosion protection of the component. 
     It will also be appreciated that the method can be applied to the repair of components as well as in their original manufacture. During repair and overhaul of used components and coating material is usually removed. A new coating is then applied which will generally block or partially block the original cooling holes in the component. Accordingly the method of the invention can then be applied to remove this excess coating material from these cooling holes. 
     In the embodiments of the invention a water jet  38  has been described as being used to clear the holes. In alternative embodiments though other fluids could be used. 
     Various modifications may be made without departing from the scope of the invention.