Abstract:
Cracks are conventionally difficult to clean which often leads to damage to other regions of the component for cleaning. According to the invention, a plasma cleaning method is used, whereby a pressure and/or a separation of an electrode to the component are varied, in order to achieve a plasma cleaning in the crack.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2005/001301, filed Feb. 9, 2005 and claims the benefit thereof. The International Application claims the benefits of European Patent application No. 04004892.8 filed Mar. 2, 2004. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a process for the plasma cleaning of a component as described in the claims. 
     BACKGROUND OF THE INVENTION 
     Surfaces of components often have to have contaminants removed from them for application of or in intermediate steps of various processes. The contaminants may be grains of dust, oil or grease films or corrosion products on the surface of the component. Simple wiping or dry ice blasting processes are known as prior art. However, if a recess or a crack is to be cleaned, it is necessary to employ more complex processes. This is done for example by fluoride ion cleaning (FIC), hydrogen annealing or salt bath cleaning. In these processes, which entail considerable outlay on apparatus, the surfaces which are not to be cleaned are in some cases also adversely affected to a significant extent. 
     Plasma-enhanced vacuum etching processes carried out on components as part of known PVD or CVD coating processes immediately prior to the vapor deposition are known. The basic principle of this surface treatment is the atomization or sputtering of adhering contaminants and of the upper atom layers of the material to be removed to form particles of atomic size by bombardment with inert gas ions. The very finely atomized contaminant has, as it were, passed into the vapor phase and can be sucked out. Plasmas of this type can be achieved by coupling suitable electrode arrangements to high-voltage/radiofrequency generators. However, these processes are only employed to clean planar surfaces. 
     EP 0 313 855 A2 discloses a process for generating a gas plasma in which the voltage is controlled to a specific value. 
     EP 0 740 989 A2 discloses a method for cleaning a vulcanization mold, in which a plasma flow is generated. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the invention to provide a process which allows a crack to have contaminants removed from it more easily and more quickly without other regions of the component being adversely affected. 
     This object is achieved by the plasma cleaning process as claimed in the claims. 
     The subclaims list further advantageous process steps of the process according to the invention. The measures listed in the subclaims can be combined with one another in advantageous ways. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIGS. 1 ,  2  show apparatuses for carrying out the process according to the invention, 
         FIG. 3  shows a turbine blade or vane, 
         FIG. 4  shows a combustion chamber, and 
         FIG. 5  shows a gas turbine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an example of an apparatus  25  for carrying out the process according to the invention. It comprises a chamber  13  in which a vacuum p is present. The vacuum p is generated by a pump  16 , which is connected to the chamber  13 . In the chamber  13  there is a component  1 , which has a crack  4  starting from a surface  22 . 
     There is also an electrode  10  arranged above the surface  22  of a component  1  in order to initiate and maintain a plasma  7 . This electrode  10  is at a certain distance d from the surface  22  of the component  1 . The condition that the product of distance times pressure must be constant (d×p=const.) is required to maintain a plasma  7 . 
     Since the crack  4  has a certain depth t down to the crack tip  34 , the inner surface  28  of the crack  4  is not completely covered by the plasma  7 , since the distance from the electrode  10  to the outer surface  22  of the component  1  and the distance to the crack tip  34  of the crack  4  differ. 
     Therefore, by way of example, the distance d from the electrode  10  to the surface  22  is varied, so that the plasma  7  migrates from the crack tip to the surface  22  or from the surface  22  of the component  1  to the crack tip  37  of the crack  4 . In this way, the distance d can be reduced, in particular continuously, so that the plasma  7  migrates from the surface  22  into the crack  4 . 
     A reactive gas  31 , which for example reacts with a corrosion product in the crack  4  and thereby promotes cleaning of the crack  4 , may likewise be present in the chamber  13 . 
     The component  1  may be metallic or ceramic. In particular, the component  1  is an iron-base, cobalt-base or nickel-base superalloy, which serves for example to produce a turbine blade or vane  12 ,  130  ( FIGS. 3 ,  5 ) or combustion chamber lining  155  ( FIG. 4 ) of a turbine  100  ( FIG. 5 ). Further components of a gas or steam turbine can be cleaned using this process. Cracks  4  in the component  1  may be present immediately after production or may have formed after the component  1  has been in operational use. 
     Worn components  1 ,  120 ,  130 ,  155  of this type are often refurbished. In this case, corrosion products are removed from the surface  22 . Corrosion products in the crack  4  are more difficult to remove. 
     After the crack  4  has been cleaned using the process according to the invention, the crack  4  can be welded or soldered up, since the solder can bond very well to a cleaned surface. 
       FIG. 2  shows a further apparatus  25 ′ which can be used to carry out the process according to the invention. The apparatus  25 ′ has a control unit  19  which regulates the pressure p in the chamber  13 . Since the condition “distance times pressure equals constant” applies to the maintaining of a plasma  7 , it is also possible to vary the pressure p in order to initiate and maintain a plasma  7  in the crack  4  if the distance d between electrode  10  and surface  22  is fixed. By, for example, continuously reducing the pressure p, the plasma  7  is made to migrate ever deeper toward the crack tip  34  of the crack  4 . 
     A reactive gas  31 , which for example reacts with a corrosion product in the crack  4  and thereby promotes cleaning of the crack  4 , may likewise be present in the chamber  13 . 
     Another possibility is for pressure and distance to be varied simultaneously, in such a way that the plasma  7  is maintained, although it is still necessary to comply with the condition for maintaining a plasma  7  (distance times pressure equals constant). 
     The distance d and the pressure p can be varied simultaneously or alternately. 
     An inert gas (Ar, H 2 , N 2 , etc.) may be present in the chamber  13 . 
       FIG. 3  shows a perspective view of a blade or vane  120 ,  130  which extends along a longitudinal axis  121 . 
     For generation of plasma, the blade  120  may be a rotor blade  120  or a guide vane  130  of a turbomachine. The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
     The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406 . As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
     A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
     In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . The blade or vane  120 ,  130  may in this case be produced by a casting process, also by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
     Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
     Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
     Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1. 
     Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused. 
     The blade or vane  120 ,  130  may be hollow or solid in form. If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes (not shown). 
     To protect against corrosion, the blade or vane  120 ,  130  has, for example, corresponding, generally metallic coatings, and to protect against heat it generally also has a ceramic coating. 
       FIG. 4  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  102  arranged circumferentially around the turbine shaft  103  open out into a common combustion chamber space. For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the turbine shaft  103 . 
     To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistant protective layer or is made from material that is able to withstand high temperatures. Moreover, a cooling system is provided for the heat shield elements  155  and/or for their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . 
     The materials of the combustion chamber wall and their coatings may be similar to those of the turbine blades or vanes. 
     The combustion chamber  110  is designed in particular to detect losses of the heat shield elements  155 . For this purpose, a number of temperature sensors  158  are positioned between the combustion chamber wall  153  and the heat shield elements  155 . 
       FIG. 5  shows, by way of example, a partial longitudinal section through a gas turbine  100 . In the interior, the gas turbine  100  has a rotor  103  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber  106 , with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . The annular combustion chamber  106  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
     The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . A generator (not shown) is coupled to the rotor  103 . 
     While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
     While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield bricks which line the annular combustion chamber  106 , are subject to the highest thermal stresses. To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant. Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). By way of example, iron-base, nickel-base or cobalt-base superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . Superalloys of this type are known, for example, from EP 1 204 776, EP 1 306 454, EP 1 319 729, WO 99/67435 or WO 00/44949; these documents form part of the disclosure. 
     The blades or vanes  120 ,  130  may also have coatings which protect against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and represents yttrium (Y) and/or silicon and/or at least one rare earth element) and against heat by means of a thermal barrier coating. The thermal barrier coating consists for example of ZrO 2 , Y 2 O 4 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
     Columnar grains are produced in the thermal barrier coating by suitable coating process, such as for example electron beam physical vapor deposition (EB-PVD). 
     The guide vane  130  has a guide vane root (not shown here) which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 .