Abstract:
The invention relates to a compound with the nominal chemical composition Al w Co x M y  wherein M represents at least one of the elements selected from the group Ni, Cr, and at least 30 mass percent of the compound is a quasicrystalline structure or similar. The invention is characterized in that 70≦w≦76 and w+X+Y=100.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2007/052732, filed Mar. 22, 2007 and claims the benefit thereof. The International Application claims the benefits of European application No. 06006053.0 filed Mar. 23, 2006, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a compound of the nominal atomic composition Al w CO x M y  where M is at least one element selected from the group consisting of Ni, Cr and at least 30 percent by mass of the compound is in the form of a quasi-crystalline structure or in approximate form, to a coating which consists of or contains the compound, to a layer system which comprises the coating and a metallic layer, and to the use of the compound as a thermal barrier coating for a component that is exposed to high temperatures. 
       BACKGROUND OF THE INVENTION 
       [0003]    When components are used at high temperatures and under corrosive conditions, it is in many cases necessary for them to be provided with protective coatings. For example, the use of thermal barrier coatings can not only increase the service life of the components but in some cases also allow the operating temperature to be raised, leading to efficiency control. This applies in particular to components that are used in gas or steam turbines. 
         [0004]    Zirconium oxides, for example stabilized by yttrium oxides, are normally used for thermal barrier coatings of this type. Ceramic thermal barrier coatings of this type can be applied to a metallic substrate using processes such as plasma spraying. However, since the ceramic layers do not adhere sufficiently well to the metallic substrate, it is necessary first of all to apply a bonding base MCrAlY to be component, where M is at least one element selected from the group consisting of iron, cobalt, nickel and Y is an active element and stands for yttrium and/or silicon and/or a rare earth element or hafnium. 
         [0005]    Applying two layers to the component that is to be protected is a complex process, and consequently efforts have been made to find alternative materials to the ceramic compounds. In the context of these efforts, quasi-crystalline materials have proven suitable, since they have a high resistance to corrosion and oxidation, a low thermal expansion, are suitable for processing to form coatings and in particular have a low thermal conductivity. 
         [0006]    Quasi-crystals, in the narrow sense of the term, are phases which have a 5-, 10- or 12-way rotational symmetry, not compatible with the symmetry of the translation lattice of classic crystal phases. Approximates of quasi-crystals is the term used to describe translational periodic intermetallic compounds which have diffraction patterns with a 5, 8; 10 or 12 times absolute symmetry. 
         [0007]    Quasi-crystalline alloys are described for example in U.S. Pat. No. 5,432,011. The alloys mentioned therein are used inter alia for coatings, which in turn are employed as thermal barrier coatings. However, the patent discloses a large number of possible alloys which may contain a very wide range of elements in numerous possible compositions. 
         [0008]    DE 103 58 813 A1 also mentions quasi-crystalline alloys and their use as a coating. Said document provides extensive references to the prior art on quasi-crystalline alloys. 
         [0009]    The quasi-crystalline alloys described contain rare and in particular also expensive metals, such as ruthenium, platinum or palladium. A further problem in producing the alloys is that in some cases they contain more than six different metals, which makes accurate weighing of the components difficult and in particular increases costs. Moreover, it is not always the case that a sufficient proportion of the alloy is in quasi-crystalline form or in approximate form, and the thermal conductivity is in some cases still too high for use as a thermal barrier coating. 
       SUMMARY OF INVENTION 
       [0010]    It is an object of the present invention to provide a compound which consists of a small number of inexpensive metallic constituents and in which at least 30 percent by mass is in the form of a quasi-crystalline structure or in approximate form. A further object of the invention is to develop a coating or a layer system using the compound and to employ the compound as a thermal barrier coating for a component. 
         [0011]    According to the invention, the object is achieved by providing a compound of the nominal atomic composition Al w Co x M y , in which 70≦w≦76 and w+x+y=100 and M represents one or two metals. 
         [0012]    Therefore, the basic concept of the invention is that of providing a compound which consists of at most three or four metallic elements, which are all relatively inexpensive to purchase and in which aluminum forms the main constituent, in a range between 70 and 76 atomic percent. 
         [0013]    Three Metallic Elements 
         [0014]    In one configuration of the invention, M=Ni, 10&lt;x≦15 and 10&lt;y≦20. This compound consists of just three elements and in addition has a very good thermal stability. 
         [0015]    Moreover, tests have shown that a compound with a particularly low thermal conductivity is obtained if M=Cr and 70≦w≦75, 10≦x≦15 and 10≦y≦20. 
         [0016]    Four Metallic Elements 
         [0017]    In a further embodiment of the invention, a compound, in addition to aluminum and cobalt, also comprises both chromium and nickel (M=Ni, Cr), where 70≦w&lt;75. 
         [0018]    Coating and Layer Systems 
         [0019]    The compound can be applied to a substrate in the form of a coating. It may also be included as one of a plurality of constituents in a coating. 
         [0020]    Moreover, it is possible to form a layer system with the aid of the coating according to the invention. It is preferable for a metallic layer to be arranged beneath the coating comprising the compound according to the invention. 
         [0021]    Here, it has proven advantageous for the metallic layer to contain nickel and aluminum, preferably in an atomic ratio of 95:5. 
         [0022]    The metallic layer may also be formed as a thin bonding layer, improving the adhesion of the coating. 
         [0023]    If the layer system comprising coating and metallic layer is applied a number of times in succession, the result is a multiple layer system, which has a particularly good corrosion resistance and low thermal conductivity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  identifies a steam turbine in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0025]    The compound according to the invention can also be used as a thermal barrier coating for a component  333 ,  357  ( FIG. 1 ) that is exposed to high temperatures. 
         [0026]    In the same way, it is also possible to use a compound which contains aluminum and manganese and in which at least 30 percent by mass is in the form of a quasi-crystalline structure or in approximate form. 
         [0027]    The use according to the invention is suitable in particular for parts of turbines, in particular of a steam turbine  300 ,  303 , such as turbine blades or vanes  357  ( FIG. 1 ). 
         [0028]      FIG. 1  illustrates a steam turbine  300 ,  303  with a turbine shaft  309  extending along an axis of rotation  306 . 
         [0029]    The steam turbine has a high-pressure part-turbine  300  and an intermediate-pressure part-turbine  303 , each having an inner housing  312  and an outer housing  315  surrounding the inner housing. The high-pressure part-turbine  300  is, for example, a part-like design. The intermediate-pressure part-turbine  303  is for example of two-flow design. It is also possible for the intermediate-pressure part-turbine  303  to be of single-flow design. 
         [0030]    Along the axis of rotation  306 , a bearing  318  is arranged between the high-pressure part-turbine  300  and the intermediate-pressure part-turbine  303 , the turbine shaft  309  having a bearing region  321  in the bearing  318 . The turbine shaft  309  is mounted on a further bearing  324  next to the high-pressure part-turbine  300 . In the region of this bearing  324 , the high-pressure part-turbine  300  has a shaft seal  345 . The turbine shaft  309  is sealed with respect to the outer housing  315  of the intermediate-pressure part-turbine  303  by two further shaft seals  345 . Between a high-pressure steam inflow region  348  and a steam outlet region  351 , the turbine shaft  309  in the high-pressure part-turbine  300  has the high-pressure rotor blading  357 , which preferably includes the compound according to the invention as a coating. This high-pressure rotor blading  357 , together with the associated rotor blades (not shown in more detail), constitutes a first blading region  360 . 
         [0031]    The intermediate-pressure part-turbine  303  has a central steam inflow region  333 , which preferably includes a compound according to the invention as a coating. Assigned to the steam inflow region  333 , the turbine shaft  309  has a radially symmetrical shaft shield  363 , a cover plate, on the one hand for dividing the flow of steam between the two flows of the intermediate-pressure part-turbine  303  and also for preventing direct contact between the hot steam and the turbine shaft  309 . The turbine shaft  309  has a second blading region  366  having the intermediate-pressure rotor blades  354  in the intermediate-pressure part-turbine  303 . The hot steam flowing through the second blading region  366  flows out of the intermediate-pressure part-turbine  303  from an outflow connection piece  369  to a low-pressure part-turbine (not shown) which is connected downstream in terms of flow. 
         [0032]    The turbine shaft  309  is composed for example of two turbine part-shafts  309   a,    309   b,  which are fixedly connected to one another in the region of the bearing  318 . Each turbine part-shaft  309   a,    309   b  has a cooling duct  372 , which is formed as a central bore  372   a  along the axis of rotation  306 . The cooling duct  372  is connected to the steam exit region  351  via an inflow duct  375  having a radial bore  375   a.  In the intermediate-pressure part-turbine  303 , the coolant duct  372  is connected to a cavity (not shown in more detail) beneath the shaft shield. The inflow ducts  375  are designed as a radial bore  375   a,  with the result that “cold” steam can flow out of the high-pressure part-turbine  300  into the central bore  372   a.  Via the outflow duct  372 , which in particular also forms a radially oriented bore  375   a,  the steam passes through the bearing region  321  into the intermediate-pressure part-turbine  303  and there passes on to the lateral surface  330  of the turbine shaft  309  in the steam inflow region  333 . The steam flowing through the cooling duct is at a significantly lower temperature than the reheated steam flowing into the steam inflow region  333 , so that effective cooling of the first rotor blade rows  342  of the intermediate-pressure part-turbine  303  and of the lateral surface  330  in the region of these rotor blades rows  342  is ensured.