Abstract:
Layered systems in prior art are inefficient at cooling an external hot gas. The inventive layered system comprises an external porous layer, in which the pore walls of the pores have differing thicknesses. This improves the cooling action by preventing too much heat from entering the layered system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is the U.S. National Stage of International Application No. PCT/EP2004/011429, filed Oct. 12, 2004 and claims the benefit thereof. The International Application claims the benefits of European application No. 03026281.0 EP filed Nov. 14, 2003, both applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a layer system and a process for producing a layer system.  
       BACKGROUND OF THE INVENTION  
       [0003]     U.S. Pat. No. 3,825,364 shows an outer wall which is completely porous in form. A cavity is present between this supporting wall and a substrate.  
         [0004]     U.S. Pat. No. 5,080,557 shows a layer structure comprising a substrate, a porous interlayer and an absolutely impervious outer layer.  
         [0005]     U.S. Pat. No. 4,318,666, compared to U.S. Pat. No. 5,080,557, shows additional cooling passages in the substrate, to which a porous interlayer and an impervious outer layer have been applied.  
         [0006]     JP 10-231 704 shows a substrate with cooling passages and a porous interlayer.  
         [0007]     WO03/006883 and U.S. Pat. No. 6,412,541 show a porous structure within a supporting wall, the wall once again having a coating on the outer side. The wall and the coating have cooling passages.  
         [0008]     However, the layer structures are inadequately cooled.  
       SUMMARY OF THE INVENTION  
       [0009]     Therefore, the object of the invention is to improve the cooling in a layer structure.  
         [0010]     The object is achieved by the layer structure and the process for producing a layer structure as claimed in the claims.  
         [0011]     Further advantageous measures are listed in the subclaims. The measures listed in the subclaims can be combined with one another in advantageous ways. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Exemplary embodiments of the invention are explained in the figures, in which:  
         [0013]     FIGS.  1  to  6  show examples of layer systems in cross section,  
         [0014]     FIGS.  7  to  17  show an enlarged view of a porous layer,  
         [0015]     FIGS.  18  to  24  show process steps for producing a layer system according to the invention,  
         [0016]      FIG. 25  shows a gas turbine, and  
         [0017]      FIG. 26  shows a combustion chamber. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1  shows a first exemplary embodiment of a layer system  1  according to the invention.  
         [0019]     The layer system  1  comprises a substrate  4 . The substrate  4  may be metallic and/or ceramic. In particular when used for turbine components of a turbomachine, such as a gas turbine  100  ( FIG. 25 , although an aircraft turbine is also possible) or a steam turbine, such as for example turbine blades or vanes  120 ,  130  ( FIG. 25 ) or combustion chamber linings  155  ( FIG. 26 ), the substrate  4  is an iron-base, nickel-base or cobalt-base superalloy.  
         [0020]     The substrate  4  of the layer system  1 , at a surface  9 , directly or indirectly adjoins a region  110 ,  111  ( FIG. 25 ) which is exposed to a hot medium. This region  110 ,  111  is, for example, a combustion chamber  110  or a hot-gas duct  111  of a gas turbine  100  ( FIG. 25 ). Layers (MCrAlX) which protect against oxidation and corrosion and/or thermal barrier coatings (ZrO2) may be present on the surface  9  in a known way.  
         [0021]     A radial direction  11  runs perpendicular or virtually perpendicular to the surface  9  of the substrate  4 .  
         [0022]     Despite these measures protecting against excessive introduction of heat, the substrate  4  is also additionally cooled at a surface  14  which lies on the opposite side from the surface  9 . In this case, therefore, the layer system  1  is, for example, a hollow component (e.g. a hollow turbine blade or vane) with an inner surface  14 . The substrate  4  is cooled by a cooling medium KM being passed through the hollow component so as to dissipate the heat from the substrate  4  at the surface  14 . According to the invention, this takes place through a porous layer  10  which is present on the surface  14  of the substrate  4  in order to allow better dissipation of this heat to the cooling medium.  
         [0023]     A, for example, metallic bonding layer may be present between the porous layer  10  and the substrate  4 .  
         [0024]     The statements made above in connection with the layer system having the substrate  4  and the layer  10  also apply analogously to the layer system having the substrate  4 /interlayer and layer  10 .  
         [0025]     The cooling medium KM can flow past the free surface of the porous layer  10  or can at least partially flow through the porous layer  10  ( FIGS. 2, 3 ,  4 ).  
         [0026]      FIGS. 2, 3 ,  4  show how a cooling medium KM can flow through this porous layer  10 .  
         [0027]     In  FIG. 2 , the cooling medium flows in an axial direction  17  (flow of a hot gas in  110 ,  111 , perpendicular to the radial direction  11 ) through the entire porous layer  10 .  
         [0028]     In the case of the combustion chamber  110 , the cooling medium KM is supplied at one end and flows from one axial end  161  to the other end  164  ( FIG. 26 ). In this case, the porous layer comprises, for example, tubes extending in the axial direction  17 .  
         [0029]     Other arrangements are conceivable.  
         [0030]     The same applies to the hot-gas duct  111 .  
         [0031]     It is also possible for the porous layer  10  to be split in the axial direction  17  into a number of segments  15  ( FIG. 3 ), in which case the cooling medium KM is in each case fed to each segment  15  separately and then flows through the said segment.  
         [0032]     In the case of the combustion chamber  110  ( FIG. 26 ), by way of example the heat shield element  155  ( FIG. 26 ) corresponds to the segment  15 .  
         [0033]     The segments  15  prevent the cooling medium KM from flowing through the porous layer  10  horizontally (in the axial direction  17 ) and being excessively heated on account of the pressure difference in the hot-gas duct  111  or in the combustion chamber  110 . Chamber walls can be formed by filling pores  25  ( FIG. 7 ) in the radial direction  11 , or alternatively a perpendicular flow through the porous layer  10  is achieved by a suitable arrangement of the passages  26  ( FIG. 7 ). This is also shown in WO03/006883, which forms part of the present disclosure with regard to the arrangement of segments or chambers and the flow through them.  
         [0034]     In the arrangement shown in both  FIG. 2  and  FIG. 3 , it is possible for cooling passages to be provided in the substrate  4 , allowing a cooling medium KM to flow out of the porous layer  10  through the substrate  4  ( FIG. 4 ). In this case, film cooling can be produced on the surface  9  of the substrate  4  or of a layer on the substrate  4  by virtue of the cooling medium KM flowing out of the surface  9 .  
         [0035]      FIG. 5  shows a further exemplary embodiment of a layer system  1  according to the invention.  
         [0036]     The layer system  1  comprises a substrate  4 . The substrate  4  may be metallic and/or ceramic. In particular when used for turbine components of a gas turbine  100  ( FIG. 25 ) or a steam turbine, such as for example turbine blades or vanes  120 ,  130  ( FIG. 25 ) or combustion chamber linings  155  ( FIG. 26 ), the substrate  4  is an iron-base, nickel-base or cobalt-base superalloy.  
         [0037]     By way of example, at least one interlayer  7  is present on the substrate  4 . The interlayer  7  may be metallic and/or ceramic. The outer porous layer  10  is applied to the surface  8  of the interlayer  7 . This porous layer  10  may likewise be metallic and/or ceramic.  
         [0038]     By way of example, a cooling passage  13 , through which a cooling medium (air and/or steam or another cooling medium) can be supplied, leads through the substrate  4  and the interlayer  7 . The cooling medium, which flows into the porous layer  10  via the cooling passage  13 , can flow within the layer  10  or can emerge from the surface  16  of the outer layer  10 . If the cooling medium emerges from the surface  16 , effusion cooling takes place.  
         [0039]     In terms of the supply and flow of the cooling medium KM, the statements which have been made in connection with  FIGS. 2, 3  and  4  apply accordingly.  
         [0040]     The interlayer  7  is, for example, an oxidation-resistant or corrosion-resistant layer, for example having the composition MCrAlX, where M stands for at least one element selected from the group consisting of iron, cobalt or nickel. X stands for at least one element selected from the group consisting of yttrium and/or the rare earths or an active element. The interlayer  7  may also be a platinum layer or a platinum-enriched MCrAlX layer  
         [0041]      FIG. 6  shows a further exemplary embodiment of a layer system  1  formed in accordance with the invention. Compared to  FIG. 1 , there is no interlayer  7 , but rather the outer porous layer  10  rests directly on the surface  9  of the substrate  4 .  
         [0042]     The material for the layer  10  is, for example, formed from silicon carbide (SiC), silicon oxide (SiO 2 ), silicon nitride (Si 3 N 4 ) or fiber materials (CMC) or mixtures thereof.  
         [0043]     The layer  10  may be formed integrally with the substrate  4  or the interlayer  7 , so that there are no bonding problems between layer  10  and substrate  4 .  
         [0044]     The porous layer  10  may, for example, have been produced together with the substrate  4  in a single casting operation or by other melt metallurgy processes (epitaxial growth). This effects an ideal join between substrate  4  and porous layer  10  in terms of heat transfer and mechanical strength between substrate  4  and layer  10  or interlayer  7 .  
         [0045]      FIG. 7  shows an enlarged illustration of the outer layer  10 , which extends in a radial direction  11  (perpendicular to the substrate  4 ). Here, the porous layer  10  adjoins the region  110 ,  111 , i.e. it rests on the surface  9  of the substrate  4 . The following statements relating to the formation of the porous layer  10  and the bonding to the substrate  4 , however, also apply to the arrangement of the porous layer  10  on the substrate  4  in accordance with  FIGS. 1, 2 ,  3  and  4 .  
         [0046]     The outer porous layer  10  comprises a large number of pores  25 . The pore size varies from approximately 0.5 millimeter to several millimeters (≧2 mm). A pore  25  is in each case surrounded by pore walls  22 . The pore walls  22  of the individual adjacent pores  25  meet at a wall section surface  19 . A cooling medium KM can flow through the porous layer  10  by virtue of being supplied from the interior of the layer system (hollow turbine blade or vane, inside a combustion chamber)  1 , although it does not have to do so, since the porous layer  10  acts as a thermal barrier coating and therefore already makes a contribution to relieving the thermal stresses in the substrate.  
         [0047]     Between the pores  25 , passages  26  may be present in the pore walls  22 , through which a cooling medium KM can flow. A meandering arrow line shows how a cooling medium can flow from the substrate  4  out through the porous layer  10 . For example, the cooling medium KM may flow through the porous layer  10  in the radial direction  11 .  
         [0048]     If the cooling medium KM is to flow through the porous layer  10  in the axial direction  17 , the passages  26  are not required.  
         [0049]     The pores  25  have a pore diameter or pore width  28 . In this exemplary embodiment, in cross section perpendicular to the radial direction  11  the pores  25  have a virtually square or right-angled cross section.  
         [0050]     The porous layer  10  is, for example, arranged on the substrate  4  or an interlayer  7  in such a way that a plurality of wall section surfaces  19  form a contact surface  37  with the substrate  4 . Therefore, the substrate  4  or the interlayer  7  adjoins wall section surfaces  19  and pores  25 .  
         [0051]     Edges of the pore walls  22 , of the passages  26  or of the wall section surfaces  19  are at least rounded, so that dirt particles which manage to enter the porous layer  10  with the cooling medium cannot become snagged at the edges.  
         [0052]     The pore size  28  is, for example, designed to be sufficiently large for foreign particles which flow through the layer  10  with the cooling medium not to block the porous layer  10 , i.e. the pore size  28  is larger than the size of the foreign particles. In particular, the porous layer  10  has a honeycomb structure.  
         [0053]     A transition  20  between a pore wall  22  or wall section surface  19  and the substrate  4  or the interlayer  7  is widened and designed to have the largest possible area with large rounding radii, in order to reduce thermal stresses and notch effects and in order to increase the size of the contact surface  37  between layer  10  and substrate  4  or interlayer  7 , so as to produce good mechanical bonding and heat transfer between the porous layer  10  and the substrate  4  or the interlayer  7 .  
         [0054]     In particular, the contact surface  37  of the layer  10  with the substrate  4  or the interlayer  7  is produced by the wall section surface  19  ( FIG. 12 , Prior Art). The size of the wall section surface  19  according to the invention is correspondingly widened at the transition  20  compared to the cross section of the wall section surface of this pore  25  above the transition  20  ( FIG. 13 ).  
         [0055]     If the pore walls  22  form the contact surface to the substrate  4  or the interlayer  7  ( FIG. 14 , Prior Art), the cross section of the transition  20  is correspondingly widened compared to the thickness of the pore wall  22  above the contact surface ( FIG. 15 ).  
         [0056]     The widening of the transition  20  is designed in such a way as to produce a discontinuous transition ( FIG. 16 ) (a defined angle α) or a continuous transition ( FIG. 17 )  
         [0057]     The surface  9  of the substrate  4 , which is covered by the porous layer  10 , is therefore largely (&gt;10%, in particular&gt;20% or&gt;30%) in contact with the wall section surfaces  19  or the pore walls  22 .  
         [0058]      FIG. 9  shows a further cross-sectional form of the pores  25 . The cross section of the pores  25  is, for example, triangular in form. Further cross-sectional forms are conceivable.  
         [0059]      FIG. 8  shows a further exemplary embodiment of a layer system  1  according to the invention.  
         [0060]     Along the radial direction  11 , the pore walls  22  are designed to be thicker in the vicinity of the substrate  4  or the interlayer  7  (thickness, diameter d) than in the vicinity of the outer surface  16  of the porous layer  10 . Therefore, the pore width  28  also changes in the radial direction  11 ; specifically, the pore width  28  is smaller in the region close to the substrate  4  than in the region close to the outer surface  16  of the substrate  4 . The thicker pore walls  22  in the vicinity of the substrate  4  or the interlayer  7  produces a larger contact surface  37  between the porous layer  10  and the substrate  4  (&gt;10% of the area covered by the porous layer  10 ). This increases the mechanical bonding and the heat transfer between the porous layer  10  and the substrate  4  or the interlayer  7 .  
         [0061]     The transition  20  between a pore wall  22  and the substrate  4  or the interlayer  7  is, for example, likewise widened ( FIGS. 13, 15 ,  16 ,  17 ).  
         [0062]     The porous layer  10  can be produced separately in a known way and joined to the substrate  4  for example by soldering.  
         [0063]     However, it is also possible for the porous layer  10  to be built up directly on the substrate  4 .  
         [0064]     The following statements apply to the arrangement of the layer  10  on the substrate  4  in accordance with FIGS.  1  to  4 . In general, the transition  20  between the pore walls  22  or the wall section surfaces  19  of the layer  10  and the solid load-bearing substrate  4  represents a mechanical weak point. In particular in the event of sudden temperature fluctuations, as are inevitable when operating gas or steam turbines, a uniformly thin, porous structure adopts the new temperature very much more quickly than the solid substrate  4 . This is associated with different thermal expansions of these regions, which can lead to extremely high stresses in the transition region between layer  10  and substrate  4 . On account of the large-area, more solid configuration of the pore walls  22  in the transitions  20 , such effects no longer occur or only occur to a greatly reduced extent.  
         [0065]     A constant cross section of the pore walls  22  along the radial direction  11  would also reduce the heat conduction cooling efficiency. All of the heat being produced has to flow from the hot-gas duct  110  via the substrate  4  into the porous structure  10 , where it is uniformly dissipated to the cooling air. As a result, heat flows mostly through the pore walls  22  at the surface  14  of the substrate  4  and less so at the free surface  16 . If the cross section of the pore wall  22  remains constant, the associated temperature gradient in the porous layer  10  changes analogously to the heat which is flowing, i.e. it is high in the vicinity of the surface  9  and decreases toward the free surface  16 . However, since the heat transfer to the cooling air is directly dependent on the temperature difference with respect to the porous structure  10 , it is overall only possible for a more restricted amount of heat to be transferred to the cooling air.  
         [0066]     The thicker pore walls  22  in the vicinity of the substrate  4  increases the cross-sectional area for heat conduction, so that the temperature gradient in the radial direction  11  flattens out. As a result, it is possible to keep the temperature gradient between the pore walls  22  and a coolant passed through the pores  25 , which is a requirement for effective cooling, at as high a level as possible in wide regions at the porous layer  10 .  
         [0067]     Proceeding from  FIG. 8  (but also analogously to  FIGS. 7, 9 ),  FIG. 10  shows a further exemplary embodiment of a layer system  1  according to the invention.  
         [0068]     A protective layer  12  has been applied to the porous layer  10 . In particular if the porous layer  10  is a metallic layer, for example an MCrAlX layer, it is necessary to provide the layer  10  with additional protection against further introduction of heat. In this case, the protective layer  12  is a ceramic thermal barrier coating. The ceramic thermal barrier coating can be applied to the porous layer  10  by means of known coating processes.  
         [0069]     The protective layer  12  may also have holes (not shown), out of which a cooling medium can flow (film cooling).  
         [0070]     The protective layer  12  may also form a wearing layer.  
         [0071]      FIG. 11  shows a further exemplary embodiment of a layer system according to the invention proceeding from  FIG. 8  (but also analogously to  FIGS. 7, 9 ).  
         [0072]     A protective layer  12  has been applied to the pore walls  22 . The protective layer  12  does not constitute a layer which has only been applied to the outside of the layer  10 , but rather covers the outer surfaces and inner surfaces of the pores  25 . The protective layer  12  may be applied in an outer part of the porous layer  10  or may extend all the way to the substrate  4 .  
         [0073]     If appropriate, gas-permeable connections passing through the protective layer  12  may also be present, so that a cooling medium can also continue to pass out of the porous layer  10  into a hot-gas duct.  
         [0074]     The porous layer  10  of the above exemplary embodiments can be produced as follows.  
         [0075]     By way of example by means of laser stereolithography, a first negative form of the porous layer  10  is produced in a first layer  10 ′, for example from plastic particles. Since the structure of the porous layer  10  is present for example in a CAD model, the layer  10  can be broken down in virtual form into an appropriate number of layers. A first layer of this type is then produced by means of the laser stereolithography, which joins the plastic particles to one another by curing precisely where pores  25  and passages  26  between pores  25  are present.  
         [0076]     It is then possible for the further layers to be produced separately and joined to one another, or alternatively plastic particles are once again applied to the first laser-treated layer, resulting in a second layer on the first layer. The second layer is then likewise treated in a targeted manner using the laser, so that the plastic particles are joined to one another where the laser impinges on them.  
         [0077]     In this way, the entire model of the negative form of the porous structure  10  is built up from plastic by means of the CAD model layer by layer. Materials other than plastic are also conceivable.  
         [0078]     The negative produced in this way can be filled (if appropriate by casting) with the material of the porous layer  10  and densified. The plastic is then removed by being burnt or leached out.  
         [0079]     It is also possible to use laser stereolithography to build up a model of the porous layer  10  ( FIGS. 7, 8 ) in such a way that a casting mold  46  ( FIG. 23 ) is formed by duplicate molding of the model, and then the porous layer  10  is formed by casting into the casting mold  46 . The casting mold  46  comprises volume bodies  43 , the filled pores  25  and if appropriate webs  40  which correspond to filled passages  26 .  
         [0080]     Further processes for producing the porous layer  10  are conceivable.  
         [0081]     In particular, the porous layer  10  can be produced in layers ( FIG. 18  to  FIG. 22, 24 ).  
         [0082]     In a first process step ( FIG. 18 ), the wall section surfaces  19  which form the contact surface  37  with the substrate  4  are applied to the substrate  4 . In this way, a first part  10 ′ of the layer  10  is formed.  
         [0083]     A plan view onto the component  1  from  FIG. 18  ( FIG. 19 ) shows that the substrate  4  has been only partially coated with the material of the layer  10 . The substrate  4  is uncovered at the locations  23  at which pores  25  are to be formed.  
         [0084]     In a further process step, further material is applied to a layer system  1  as shown in  FIG. 18  ( FIG. 20 ). The locations  23  can, for example, be filled with a material other than the material of the layers  10 ′,  10 ″, in order to prevent them from being filled. This other material for the locations  23  can be leached or burnt out, whereas the material of the pore walls  22  cannot be removed in this way.  
         [0085]     The uncoated locations  23  in accordance with the treatment step presented in  FIG. 18  are now closed, so that first pores  25  have formed adjacent to the substrate  4 . A further layer region  10 ″ has been added to the layer region  10 ′.  
         [0086]     A plan view ( FIG. 21 ) onto a layer system  1  as shown in  FIG. 20  reveals holes in such a surface, which result in the formation of pores  25  following a further layer application. The pores  25  which have by now been closed up are indicated by dashed lines. This procedure is continued in steps ( FIG. 22 ) until a porous layer  10  for example as shown in  FIG. 4  results.  
         [0087]      FIG. 4  diagrammatically depicts how it is possible to produce a porous layer  10 , namely by printing the porous structure.  
         [0088]     In this case, similarly to in the case of stereolithography, the structure is built up from individual layers in succession, except that in this case there is no laser fusing together plastic particles, but rather an ultrathin printing paste which contains material of the layer  10 , such as for example dye, is printed layer by layer over the layer sequence  49 . This process allows the material of the porous layer to be used directly for printing. In this case, the material, for example in the form of a fine powder, is mixed with a binder.  
         [0089]     Once the porous layer  10  has been fully printed, the binder is evaporated off in a furnace and then the material of the porous layer  10  is sintered together. There is in this case no need to use a plastic core or to produce a casting mold.  
         [0090]      FIG. 25  shows a partial longitudinal section through a gas turbine  100 .  
         [0091]     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 duct  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . Each turbine stage  112  is formed from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas duct  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 .  
         [0092]     The guide vanes  130  are secured to the stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  by means of a turbine disk  133 . A generator (not shown) is coupled to the rotor  103 .  
         [0093]     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 duct  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.  
         [0094]     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. The blades or vanes  120 ,  130  may also have coatings which protect against corrosion (MCrAlX; M=Fe, Co, Ni, X=Y, rare earths) and heat (thermal barrier coating, for example ZrO 2 , Y 2 O 4 —ZrO 2 ).  
         [0095]     A porous layer  10 , for example as shown in FIGS.  1  to  4 , may be applied in the interior of the turbine blades or vanes  120 ,  130 . The porous layer  10  can also delimit the blade or vane  120 ,  130  in the hot-gas duct  111 .  
         [0096]     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 .  
         [0097]     The combustion chamber  110  in  FIG. 26  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 .  
         [0098]     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 their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 .  
         [0099]     A porous layer  10 , for example as shown in FIGS.  1  to  4 , is then applied in the interior of the holding elements. It is also possible for the porous layer  10  to be arranged on the outside toward the combustion chamber  111 .  
         [0100]     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 .