A method for minimising thermal gradients in an object

Abstract: Layers (14, 16) of thermally conductive material are applied to an object (10) such that second layers (16) of thermally conductive material are distributed amongst first layers (14) of thermally conductive material. A first layer (14) of thermally conductive material is less thermally conductive than a second layer (16) at any particular temperature. Thermal energy travelling through the layers to the object is impeded by the lower thermally conductive of the first layers, and substantially dispersed in a lateral direction by the higher thermally conductive of the second layers, thereby minimizing any thermal gradients that might otherwise exist within the object.

Description: The present invention pertains to thermal coatings in general, and to methods for minimizing thermal gradients along the surface of an object using a thermal coating in particular. It is often advantageous to minimize an object's exposure to thermal energy. In sufficient quantities, thermal energy can cause thermal expansion of an object and in some cases undesirable thermal stress within the object. Thermal stress, in turn, can manifest as stress cracks, reduced ductility, or distortion of the object. This is especially true in instances where the thermal energy is concentrated in a particular region of the object, resulting in a steep thermal gradient within the object. Insulative coatings may be bonded to the exterior surface of an object to minimize the object's absorption of thermal energy. Typically, the coating has a thermal conductivity value lower than that of the object (or a higher thermal resistance value) and/or a high degree of thermal reflectivity. In many cases, an insulative coating alone is sufficient to prevent or minimize thermal stress within the object. In instances where the thermal energy is concentrated, however, coatings or other insulative means may not be sufficient to prevent the growth of a thermal gradient capable of creating thermal stress within the object. It is known from an article entitled "Sputter-Deposited Multilayered Ceramic/Metal Coatings", Thin Solid Films, 64(1979) pp 337-343, to provide a coating comprising alternate layers of Ni/ZrO 2 +Y or Ni-50Cr/ZrO 2 +Y to increase the erosion/hot corrosion resistance of an airfoil. It is also known from an article entitled "Microlaminate Composites as Thermal Barrier Coatings", Surface and Coatings Technology, 36 (1988), pp 143-150, to provide a coating consisting of alternating layers of two materials as a thermal barrier layer. In this document, it is observed that the thermal conductivity of the composite can be increased by increasing the number of interfaces which act as a barrier to the transfer of heat across them. From a first aspect, the invention provides a method for minimizing thermal gradients in an object, comprising the steps of: applying to the object a plurality of first layers of thermally conductive material and a plurality of second layers of thermally conductive material such that said second layers of thermally conductive material are distributed amongst said first layers of thermally conductive material; wherein said first layers of thermally conductive material are less thermally conductive than said second layers at any particular temperature so that thermal energy travelling through said layers to the object is impeded by the lower thermal conductivity of said first layers, and substantially dispersed in a lateral direction by the higher thermal conductivity of said second layers, thereby minimizing the thermal gradient of the object. According to the invention, the layers are applied so that the distribution of one of the layers is more frequent closer to the object surface or to the outermost layer than it is otherwise within the coating, or the thicknesses of the first and second layers are selectively varied to alter the thermal conductivity of the layers. From a second aspect, the invention provides a coating for minimizing thermal gradients in an object, comprising: a plurality of first layers of thermally conductive material; a plurality of second layers of thermally conductive material; said first layers of thermally conductive material being less thermally conductive than said second layers at any particular temperature; wherein said layers are applied to the object such that said second layers of thermally conductive material are distributed amongst said first layers of thermally conductive material, whereby thermal energy travelling through said layers to the object is impeded by the lower thermal conductivity of said first layers, and substantially dispersed in a lateral direction by the higher thermal conductivity of said second layers, thereby minimizing the thermal gradient within the object. According to the invention, the layers are applied so that the distribution of one of the layers is more frequent closer to the object surface or to the outermost layer than it is otherwise within the coating, or the thicknesses of the first and second layers are selectively varied to alter the thermal conductivity of the layers. Preferably the thermal conductivity of the first layers is not greater than 20 Btu-in/hr.ft 2 .°F (2.88 Wm -1 °C -1 ) and that of the second layers is not less than 100 Btu-in/hr.ft 2 .°F (14.42 Wm -1 °C -1 ). An advantage of the present invention is that by minimizing thermal gradients in an object, thermal stress and distortion within an object can also be minimized. A further advantage is that nonuniform heating of an object can be minimized. A still further advantage is that undesirable localized cooling of a heated object can be minimized. A still further advantage is that thermal gradients can be minimized along surfaces having complex shapes. Under the prior art, it has not always been possible to minimize thermal gradients along surfaces having inwardly or outwardly sharp edges, or inaccessible regions, for example. The present invention provides a coating for minimizing thermal gradients that may be applied to complex surface geometries. A still further advantage is that a method for reducing thermal gradients in an object is provided which does not significantly add to the mass of the object. Mass added to high speed rotating objects adds to the mechanical load that must be supported, and therefore the stress within the object. Some preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIG. 1A is a diagrammatic view of an airfoil having a multilayer coating applied to an exterior surface. An edge of the coating is shown peeled away from the airfoil for illustrative purposes. FIG.1 is an enlarged sectional diagrammatic view of a thermal coating shown in FIG. 1A. FIG.2 shows a modification of the thermal coating shown in FIG. 1A. FIG.3 shows an embodiment of the thermal coating shown in FIG. 1A. FIG.4 shows another embodiment of the thermal coating shown in FIG.1A. FIG. 1A shows an exaggerated view of an airfoil 10 having a very thin thermal coating 12 applied to an exterior surface 18. In FIGS. 1-4, an enlarged sectional view of the airfoil 10 is shown with the thermal coating 12 which consists of a plurality of first 14 and second 16 layers. In the preferred embodiment, each first layer 14 comprises a material having a relatively low thermal conductivity value, such as yttria stabilized zirconia. Each second layer 16 comprises a material having a relatively high thermal conductivity, such as nickel aluminide. The first 14 and 16 second layers are applied to the airfoil 10 by a process called Electron Beam Physical Vapor Deposition, or "EBPVD". The EBPVD process consists of placing the object to be coated in a vacuum chamber (not shown) along with "targets" of the material to be deposited. An electron gun excites the molecules of the target and causes a plasma cloud to develop consisting of the target material. The plasma cloud, in turn, deposits a very thin layer of the target material on the object. The thickness of the layer can be adjusted by altering the parameters of the process, such as the temperature, the length of time the target is exposed, etc. FIG. 1 shows one coating not according to the claimed invention where the first 14 and second 16 layers are alternated by changing target material every layer. Specifically, a first layer 14 of zirconia having a particular thickness 20 is bonded to the airfoil 10 initially, followed by a second layer 16 of nickel aluminide having a thickness 21 equal to that of the first layer 14. As many alternating layers 14,16 as is advantageous for the particular application may be added subsequently. In one coating not according to the claimed invention, the order of the layers 14,16 is not alternated, but rather is customized for the application. For example, in a particular application, it may be advantageous to have the greater thermal conductivity of the second layers 16 every "n th " layer, where "n" is an integer. FIG.2 shows a coating 12 where the second layers 16 are distributed every third layer. In one embodiment, distribution frequency of one of the layers 14,16 is greater in the area closer to the airfoil surface 18 or, alternatively, in the area closer to the outermost layer. For example, the distribution frequency of second layers 16 may be greater towards the outermost layer and decrease in the direction of the airfoil surface 18. FIG.3 shows an example of this where the first 14 and second 16 layers alternate for the outermost four layers and then the frequency of the second layer 16 decreases to every third layer for the remaining layers. In a further embodiment, the thicknesses 20,21 of the first 14 and second 16 layers are selectively varied to alter the thermal conductivity of the layers 14,16. FIG.4 shows an example of a coating 12 where the thickness of the two outermost second layers 16 are twice that of the first layers 14. In both of these embodiments, the lower thermal conductivity of the first layers 14 resists the passage of thermal energy through the coating 12. The higher thermal conductivity of the second layers 16, in contrast, provides an avenue for the thermal energy of a "hot spot" to transfer laterally to cooler regions within the coating 12 and/or airfoil 10. Lateral transfer may be defined as thermal energy traveling substantially perpendicular to the direction of the thermal energy traveling through the coating toward the object. Alternatively, the second layers 16 can provide an avenue for the thermal energy of hotter regions to transfer laterally to a "cold spot" within the coating 12 and/or airfoil 10. A "hot spot" or a "cold spot" may be defined as a particular region of an object where there is a difference in thermal energy relative to the surrounding regions. In the case of a "hot spot", the thermal energy may be in excess due to a thermal source which exposes a particular region of the object to a greater thermal load. Alternatively, the excess of thermal energy could be caused by a lack of cooling in that particular region of the object. In the case of a "cold spot", an excessive cooling source could cause the thermal energy in a particular region to be significantly less relative to surrounding regions. Regardless of whether there is an excess of thermal energy or a lack of thermal energy relative to surrounding regions, the difference in thermal energy will cause a thermal gradient between regions of the object. The present invention minimizes the thermal gradient by first minimizing the thermal path through the coating 12 via the low thermal conductivity of the first layers 14 and second by laterally dispersing the thermal energy through the high thermally conductive second layers 16. Dispersing the energy causes the object and the coating 12 to experience a more spatially uniform thermal energy source, and therefore less of a thermal gradient. Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the scope of the claimed invention.

Claims: A method for minimizing thermal gradients in an object, comprising the steps of: applying a plurality of first and second layers (14,16) of thermally conductive material to an object, wherein the plurality of second layers (16) are distributed amongst the plurality of first layers (14) of thermally conductive material, and wherein said first layers (14) are less thermally conductive than said second layers (16) at any particular temperature so that thermal energy travelling through said layers to the object is impeded by the lower thermal conductivity of said first layers, and substantially dispersed in a lateral direction by the higher thermal conductivity of said second layers, thereby minimizing thermal gradients within the object, characterised in that the layers are applied so that the distribution of one of said layers (14, 16) is more frequent closer to the object surface or to the outermost layer than it is otherwise within the coating. A method as claimed in claim I, wherein the thicknesses (20, 21) of said first and second layers (14, 16) are selectively varied to alter said thermal conductivity of said layers. A method for minimizing thermal gradients in an object, comprising the steps of: applying a plurality of first and second layers (14,16) of thermally conductive material to an object, wherein the plurality of second layers (16) are distributed amongst the plurality of first layers (14) of thermally conductive material, and wherein said first layers (14) are less thermally conductive than said second layers (16) at any particular temperature so that thermal energy travelling through said layers to the object is impeded by the lower thermal conductivity of said first layers, and substantially dispersed in a lateral direction by the higher thermal conductivity of said second layers, thereby minimizing thermal gradients within the object, characterised in that the thicknesses (20, 21) of said first and second layers (14, 16) are selectively varied to alter said thermal conductivity of said layers. A method as claimed in any of claims 1 to 3, wherein said first layers (14) of thermally conductive material have a thermal conductivity value of not greater than 20 Btu-in/hr.ft 2 .°F (2.88 Wm -1 °C -1 ). A method as claimed in any preceding claim, wherein said second layers (16) of thermally conductive material have a thermal conductivity value of not less than 100 Btu-in/hr.ft 2 . °F (14.42 Wm -1 °C -1 ). A method as claimed in any preceding claim, wherein said first layers (14) of thermally conductive material comprise yttria stabilized zirconia. A method as claimed in any preceding claim, wherein said second layers (16) of thermally conductive material comprise nickel aluminide. A method as claimed in any preceding claim wherein said object is an airfoil. A coating for minimizing thermal gradients in an object, comprising: a plurality of first layers (14) of thermally conductive material; a plurality of second layers (16) of thermally conductive material; wherein said first layers (14) of thermally conductive material are less thermally conductive than said second layers (16) at any particular temperature; wherein said layers (14, 16) are applied to the object such that said second layers (16) of thermally conductive material are distributed amongst said first layers (14) of thermally conductive material; whereby thermal energy travelling through said layers to the object is impeded by the lower thermal conductivity of said first layers, and substantially dispersed in a lateral direction by the higher thermal conductivity of said second layers, thereby minimizing the thermal gradient within the object, characterised in that the distribution of one of said layers (14, 16) is more frequent closer to the object surface or to the outermost layer than it is otherwise within the coating. A coating as claimed in claim 9, wherein the thicknesses (20, 21) of said first and second layers (14, 16) are selectively varied to alter said thermal conductivity of said layers. A coating for minimizing thermal gradients in an object, comprising: a plurality of first layers (14) of thermally conductive material; a plurality of second layers (16) of thermally conductive material; wherein said first layers (14) of thermally conductive material are less thermally conductive than said second layers (16) at any particular temperature; wherein said layers (14, 16) are applied to the object such that said second layers (16) of thermally conductive material are distributed amongst said first layers (14) of thermally conductive material; whereby thermal energy travelling through said layers to the object is impeded by the lower thermal conductivity of said first layers, and substantially dispersed in a lateral direction by the higher thermal conductivity of said second layers, thereby minimizing the thermal gradient within the object, characterised in that the thicknesses (20, 21) of said first and second layers (14, 16) are selectively varied to alter said thermal conductivity of said layers. A coating as claimed in any of claims 9 to 11, wherein: said first layers (14) of thermally conductive material have a thermal conductivity value of not greater than 20 Btu-In/hr.ft 2 .°F (2.88 Wm -1 °C -1 ); and said second layers (16) of thermally conductive material have a thermal conductivity value of not less than 100 Btu-in/hr.ft 2 .°F (14.42 Wm -1 °C). A coating as claimed in any of claims 9 to 12, wherein said first layers (14) of thermally conductive material comprise yttria stabilized zirconia. A coating as claimed in any of claims 9 to 13, wherein said second layers (16) of thermally conductive material comprise nickel aluminide. An object having a coating as claimed in any of claims 9 to 14. An airfoil having a coating as claimed in any of claims 9 to 14.

Inventors: DOWNS JAMES P, RUANO EDUARDO, DOWNS, JAMES P., RUANO, EDUARDO
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1995
Main classification: C23C14
IPC codes: F16L59, C23C14, F01D5, C23C28