Abstract:
A method and apparatus ( 2 ) for masking a portion ( 36 ) of a component ( 30 ), the apparatus ( 2 ) comprising: a sacrificial masking element ( 4 ) for masking the portion ( 36 ); and a locating jig ( 6 ) detachably connectable to the sacrificial masking element ( 4 ) and configured to position the sacrificial masking element ( 4 ) over the portion ( 36 ) of the component ( 30 ).

Description:
The present invention relates to a method and apparatus for masking a portion of a component and particularly, but not exclusively, to a method and apparatus which allows a plurality of components to be masked both quickly and accurately. 
     BACKGROUND 
     It is known to apply a Thermal Barrier Coating (TBC) to a surface of a component which operates at an elevated temperature. Thermal barrier coatings typically have low thermal conductivity, and therefore, in use, display a large temperature gradient across the thickness of the coating. Accordingly, thermal barrier coatings provide thermal insulation to components and thus allow the components to operate under large and prolonged heat loads. Furthermore, thermal barrier coatings may extend the life of the component by reducing oxidation and by reducing cyclic loading caused by temperature variations which may result in thermal fatigue. 
     Thermal barrier coatings are commonly applied to metallic components which are subjected to high-temperature conditions. For example, thermal barrier coatings are widely used within gas turbine engines, particularly on combustor rings, nozzle guide vanes, turbine blades, etc. 
     Thermal barrier coatings may be applied to a component using a number of techniques. For example, a thermal barrier coating may be applied using a physical vapour deposition technique (e.g. electron beam or laser beam deposition), direct vapour deposition, plasma spraying, electrostatic spray assisted vapour deposition, etc. 
     It is known to provide an interior surface of a circular combustor ring with a thermal barrier coating. To achieve this, the combustor ring is located within a TBC spray-chamber with the TBC spray nozzle located approximately at the centre of the circular combustor ring and in alignment with the interior surface. The TBC spray nozzle and combustor ring may be rotated relative to one another, such that the thermal barrier coating from the TBC spray nozzle covers the entire circumferential interior surface. This may be achieved by rotating the combustor ring using a rotary table or by rotating the TBC spray nozzle. 
     However, for the purposes of assembling the combustor a circumferential portion of the interior surface of the combustor ring is left uncoated. Accordingly, this portion of the interior surface is covered by a special adhesive TBC-proof tape, which masks the portion from the thermal barrier coating. The application and subsequent removal of the adhesive tape is carried out by hand and as a result is a slow and correspondingly expensive procedure. Furthermore, the current method is wasteful since the tape (approximately 5 m of tape per combustor ring) is discarded after every use. 
     The present invention seeks to overcome some or all of the problems associated with the prior art method described above. 
     STATEMENTS OF INVENTION 
     In accordance with an aspect of the invention there is provided a method of masking a portion of a component, the method comprising: attaching a sacrificial masking element to a locating jig; and disposing the locating jig against a surface of the component such that the sacrificial masking element is positioned over the portion of the component. 
     The sacrificial masking element may be attached to the locating jig by a sacrificial connector; and the method may further comprise: inserting a deformable portion of the sacrificial connector through the sacrificial masking element, wherein the deformable portion is in an unlocked position, and deforming the deformable portion from the unlocked position to a locked position to lock the sacrificial masking element to the locating jig. 
     The deformable portion may be a pair of legs which are bendable from the unlocked position to the locked position. 
     The method may further comprise stacking a second component on top of the locating jig. 
     Further components may be stacked on top of one another with a locating jig disposed between adjacent components. This may allow a coating to be applied to a plurality of components in a single operation. This may be particularly advantageous where the coating is applied in a vacuum chamber (e.g. for electron beam deposition), since the invention reduces the number of times the chamber must be evacuated. 
     The sacrificial masking element may also mask a portion of the second component. 
     The method may further comprise applying a coating to the component. The sacrificial masking element may prevent the coating from contacting the masked portion of the component. 
     The coating may be a thermal barrier coating. 
     In accordance with another aspect of the invention there is provided a masking apparatus, the apparatus comprising: a sacrificial masking element for masking a portion of a component; and a locating jig detachably connectable to the sacrificial masking element and configured to position the sacrificial masking element over the portion of the component. 
     The method and apparatus of the present invention may allow a portion of a component to be quickly and accurately masked. Furthermore, the present invention may reduce costs by using a sacrificial masking element which can be replaced without needing to replace the locating jig. 
     The locating jig may be detachably connectable to the sacrificial masking element via a sacrificial connector. 
     The locating jig may comprise a hole or slot for receiving the sacrificial connector. 
     The sacrificial connector may comprise a deformable portion which may be deformable from an unlocked position to a locked position to lock the sacrificial masking element to the locating jig. 
     This may allow the sacrificial masking element to be quickly and easily connected to and disconnected from the locating jig. The sacrificial connector may be deformed from the unlocked position to the locking position, and vice versa, by hand or using readily available tools. 
     The deformable portion may comprise a pair of legs which may be bendable between the unlocked position and locked position. 
     The locating jig may comprise a recess for receiving the sacrificial connector. 
     The recess may comprise a shoulder which abuts an end surface of a shoulder of the sacrificial connector. The shoulder of the recess may be gripped between the shoulder of the sacrificial connector and the sacrificial masking element when the sacrificial connector is in the locked position. 
     The sacrificial masking element may comprise a first layer and a second layer. The first layer may be disposed against the component and the second layer may be spaced away from the component by the first layer. The second layer may protrude over an edge of the first layer and thus may define the extent of the masked portion of the component. 
     This configuration may prevent a continuous film from forming between the second layer and the component which may otherwise cause the sacrificial masking element to become joined to the component. 
     The sacrificial masking element may be substantially perpendicular to the locating jig. 
     The locating jig and sacrificial masking element may be circular. 
     The locating jig may be formed from a plurality of arcuate sections. 
     This may allow individual arcuate sections to be replaced. Furthermore, the locating jig may be disassembled to reduce its size and may be assembled around the component where it is difficult to install the locating jig once it is fully assembled. 
     The locating jig may be formed from first and second layers of arcuate sections which may be rotated relative to one another and connected to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a section of a masking apparatus in accordance with an embodiment of the invention prior to assembly; 
         FIG. 2  is an enlarged perspective view of a sacrificial connector of the masking assembly; 
         FIG. 3  is a perspective view of the masking apparatus during assembly; 
         FIG. 4  is a perspective view of the masking apparatus after assembly; and 
         FIG. 5  is a cross-sectional view of a stack of combustor rings during a thermal barrier coating operation, where a portion of each combustor ring is masked by the masking apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a masking apparatus  2  in accordance with an embodiment of the invention comprises a circular sacrificial masking element  4 , a circular locating jig  6 , and a plurality of sacrificial connectors  8  which detachably connect the sacrificial masking element  4  to the locating jig  6 . 
     The sacrificial masking element  4  is a circular ring of material having a plurality of circumferential slots  10  which pass through the sacrificial masking element  4  and are spaced around its circumference. The sacrificial masking element  4  is formed from an inner ring  5   a  and an outer ring  5   b . The inner ring  5   a  has a slightly smaller diameter than the outer ring  5   b . Furthermore, the inner ring  5   a  is longer than the outer ring  5   b . The inner ring  5   a  is received within the outer ring  5   b  with the outer ring  5   b  located at the axial centre of the inner ring  5   a . Accordingly, the inner ring  5   a  projects either side of the outer ring  5   b  and the inner and outer rings  5   a ,  5   b  contact one another (as shown in  FIG. 5 ). 
     The locating jig  6  is formed from first and second locating rings  12   a ,  12   b . The first and second locating rings  12   a ,  12   b  are in turn formed from a plurality of arcuate sections  14 . The arcuate sections  14  of the first locating ring  12   a  are rotated relative to the arcuate sections  14  of the second locating ring  12   b , such that joints between adjacent arcuate sections  14  of the first locating ring  12   a  are not aligned with the joints of the second locating ring  12   b . Adjacent arcuate sections  14  of the first locating ring  12   a  are connected to one another by passing a connector, such as a pin, screw or rivet, through an adjacent end of each of the adjacent arcuate sections  14  and into the second locating ring  12   b . The connection of the adjacent arcuate sections  14  to the second locating ring  12   b  locks the arcuate sections  14  together. Similarly, the arcuate sections  14  of the second locating ring  12   b  are connected via the first locating ring  12   a  using connectors  16 . 
     Each of the first and second locating rings  12   a ,  12   b  has a plurality of recesses  18  spaced around the circumference of the ring. As shown, each arcuate section  14  may have four recesses  18  formed in it. The recesses  18  of the first and second locating rings  12   a ,  12   b  are located so that when the first and second locating rings  12   a ,  12   b  are connected to one another using the connectors  16 , the recesses  18  of the first locating ring  12   a  are aligned with the recesses  18  of the second locating ring  12   b.    
     The recesses  18  are shaped to receive the sacrificial connectors  8 . As shown in  FIG. 2 , each sacrificial connector  8  comprises a pair of lateral shoulders  20  which protrude from each side of the sacrificial connector  8 . Each sacrificial connector  8  further comprises a pair of legs  22  extending from between the shoulders  20 . An indentation  24  is formed in each side of the recesses  18 . The indentations  24  are defined by a pair of shoulders  26  located at the opening to each recess  18 . The indentations  24  are complementary to the shoulders  20  of the sacrificial connectors  8 . In use, the shoulders  20  of the sacrificial connectors are received within the indentations  24  with an end surface  28  of each shoulder  20  abutting a respective shoulder  26  of the recess  18 . 
     As shown in  FIG. 3 , the legs  22  of each sacrificial connector  8  are received through one of the slots  10  in the sacrificial masking element  4  with the legs in an unlocked position where they lie in the plane of the remainder of the sacrificial connector  8 . The legs  22  may then be deformed from the unlocked position to a locked position to connect the masking element to the locating jig  6  and to prevent disengagement. This is achieved by bending the legs  22  over towards the masking element  4  so that they no longer lie in both the plane of the remainder of the sacrificial connector  8  and the plane of the slots  10  of the sacrificial masking element  4 . As shown in  FIG. 4 , one leg  22  may be bent downwards and the other leg  22  may be bent upwards. In doing so, the end surface  28  of each shoulder  20  of the sacrificial connector  8  is urged into contact with each shoulder  26  of the recess  18 . This locks the shoulder  26  of the recess  18  between the end surface  28  of the sacrificial connector  8  and the sacrificial masking element  4 . Consequently, the locating jig  6  is locked to the sacrificial connector  8 , which is in turn locked to the sacrificial masking element  4 . The legs  22  are preferably deformed by hand or using readily available tools, such as pliers. 
     In use, the masking apparatus  2  may be used to mask a portion of a component, such as a combustor ring  30 , from the application of a thermal barrier coating. The combustor ring  30  has a circular cross-section which has a widened portion at one end. Accordingly, the combustor ring  30  has a narrow end  32  and a wide end  34 . The combustor ring  30  has a circumferential portion  36  at both the narrow end  32  and wide end  34  which must remain uncoated. 
     As shown in  FIG. 5 , a plurality of combustor rings  30  are stacked on top of one another. The combustor rings  30  are arranged so that adjacent combustor rings  30  have their like-ends adjacent to one another (i.e. the wide end  34  of one combustor ring  30  is adjacent to the wide end  34  of the next combustor ring  30  in the stack, and the narrow end  32  of one combustor ring  30  is adjacent to the narrow end  32  of the next combustor ring  30  in the stack). 
     A masking apparatus  2  is disposed between adjacent combustor rings  30 . The masking apparatus  2  is provided in two different diameters; one sized for masking the wide ends 
       34  of adjacent combustor rings  30  and the other sized for masking the narrow ends  32  of adjacent combustor rings  30 . The outer ring  5   b  of the sacrificial masking element  4  contacts the circumferential portion  36  of the adjacent combustor rings  30  which centralises the masking apparatus in the combustor rings  30 . Consequently, the inner ring  5   a  is spaced away from the circumferential portion  36  and a gap is left between the inner ring  5   a  and the circumferential portion  36  where the inner ring  5   a  overhangs the outer ring  5   b . The locating jig  6  is sandwiched between radial surfaces of the adjacent combustor rings  30  which correctly aligns the masking apparatus  2  in an axial direction so that the inner ring  5   a  fully covers the circumferential portion  36  which should remain uncoated. 
     A spray nozzle (not shown) is located at a centre of the combustor rings  30  and is translatable in an axial direction along a path  38  from the combustor ring  30  at the bottom of the stack to the combustor ring  30  at the top of the stack. The spray nozzle has a number of discrete positions  40  which are each aligned with one of the combustor rings  30 . In these positions  40  the spray nozzle can apply the thermal barrier coating to the combustor ring  30  over a region between the circumferential portion  36  at the narrow end  32  and the circumferential portion  36  at the wide end  34 . The stack of combustor rings  30  are located on a rotary table (not shown) and the spray nozzle sprays the thermal barrier coating onto the combustor rings  30  whilst they are being rotated by the rotary table. This ensures that the whole circumference of the combustor rings  30  is coated. This is repeated for each position  40  of the spray nozzle so as to coat every one of the combustor rings  30 . 
     The spray nozzle ejects a thermal barrier coating in a radial direction with little or no spread. Consequently, the inner ring  5   b  of the sacrificial masking element  4  casts a shadow over the circumferential portion  36 . The sacrificial masking element  4  therefore prevents the thermal barrier coating from contacting the circumferential portion  36 . 
     Furthermore, as the outer ring  5   b  spaces the inner ring  5   a  away from the circumferential portion  36 , the coating does not create a continuous film between the inner ring  5   a  and the combustor ring  30 , which would otherwise cause the sacrificial masking element  4  to become joined to the combustor ring  30 , thus hindering disassembly of the stack of combustor rings  30 . 
     Once all of the combustor rings  30  have been coated, they may be easily disassembled, simply by unstacking the combustor rings  30  and removing the masking apparatuses  2 . The sacrificial masking element  4  can then by disconnected from the locating jig  6  by bending the legs  22  of the sacrificial connectors  8  back from the locked position to the unlocked position where the legs  22  lie in the plane of the slots  10  of the sacrificial masking element  4 . This also releases the sacrificial connectors  8  from the locating jig  6 . The sacrificial masking element  4  and/or sacrificial connectors  8  may then be replaced before the next thermal barrier coating operation. 
     Although the masking apparatus  2  has been described with reference to combustor rings  30 , it may be used to mask other components. Such components are not limited to circular components. Accordingly, the sacrificial masking element  4  and locating jig  6  need not be circular, but instead may be appropriately configured for the component for which it is intended to mask. 
     Furthermore, it is not necessary for the masking apparatus  2  to be disposed between adjacent components. The locating jig  6  of the masking apparatus  2  may instead rest on a top or bottom surface of a single component. The weight of the component forces the locating jig  6  against the bottom surface. The locating jig  6  may be fixed to the top surface of the component using a weight or clamping arrangement to ensure that it does not move. 
     Although the inner ring  5   a  has been described as overhanging the outer ring  5   b  on either side, it may only overhang on one side of the outer ring  5   b . Furthermore, the inner ring  5   a  need not be ring shaped and may instead be configured to mask the desired portion of the component. For example, where it is desired that the portion to be masked has a crenulated edge, the inner ring  5   a  may have a correspondingly shaped edge to provide such a shadow over the component. 
     The sacrificial masking element  4  and sacrificial connectors  8  may be discarded after every thermal barrier coating operation or may be used for several separate operations. The sacrificial masking element  4  and sacrificial connectors  8  are intended to be replaced more frequently that the locating jig  6 . 
     The masking apparatus  2  is not limited to providing a mask from a thermal barrier coating, as described above. The masking apparatus  2  may mask a component from other coatings which are sprayed, such as paint. Furthermore, the masking apparatus  2  could be used as a sacrificial beam stopper in electron beam welding or laser trimming operations. The sacrificial masking element  4  may be formed from a material which is suitable for the intended purpose of the masking apparatus  2 . For example, where the sacrificial masking element  4  is used as a beam stopper, it may be made from iron and have an appropriate thickness to prevent the beam from penetrating through the sacrificial masking element  4 .