Patent Publication Number: US-2018038276-A1

Title: Metallic microsphere thermal barrier coating

Description:
TECHNICAL FIELD 
     The present disclosure relates to a thermal barrier coating for an internal combustion engine. 
     BACKGROUND 
     Some vehicles include an engine assembly for propulsion. The engine assembly may include an internal combustion engine and a fuel injection system. The internal combustion engine includes one or more cylinders. Each cylinder defines a combustion chamber. During operation, the internal combustion engine combusts an air/fuel mixture in the combustion chamber in order to move a piston disposed in the cylinder. Maintaining temperature environments in engine assemblies may be limited based upon the configuration of the engine assembly and the functions of various components. 
     SUMMARY 
     A thermal barrier coating comprises insulating layer applied to a surface of a substrate. The insulating layer comprises a plurality of microspheres. A sealing layer is bonded to the insulating layer. The sealing layer is non-permeable such that the sealing layer seals against the insulating layer. The insulating layer may have a porosity of at least 80% and have a thickness of between about 100 microns and about 1 millimeter. 
     The insulating layer may further comprise a matrix material configured to bond with the plurality of microspheres. The plurality of microspheres may include a base surface formed of at least one of a metal alloy, polymer or ceramic. A first coating of nickel may be applied to the base surface of the plurality of hollow microspheres. One or more of a second coating and a third coating of at least one alloying element is applied to the first coating. The second coating may comprise nanoparticles applied to the first coating. The sealing layer may have a thickness of between about 1 micron and about 20 microns. 
     In another embodiment of the disclosure, a method for applying a thermal barrier coating to a component comprises placing an insulating layer of the thermal barrier coating on a substrate of the component. The insulating layer may include a matrix material configured to bond with a plurality of microspheres. A heat treatment is applied to the insulating layer on the surface of the substrate. A sealing layer of the thermal barrier coating is bonded to the insulating layer. The sealing layer is non-permeable such that the sealing layer seals against the insulating layer. 
     The insulating layer of the thermal barrier coating may be formed by providing a plurality of microspheres, wherein each of the plurality of microspheres includes a base surface. A first coating including a nickel alloy is applied to the base surface, while a second coating that includes one or more of aluminum, chromium and nanoparticles is applied to the first coating. The first and second coating may be applied by one or more of electroless plating, chemical vapor deposition, and physical vapor deposition. 
     The above features and advantages and other features and advantages of the present disclosure are readily apparent from the following detailed description of the best modes for carrying out the disclosure when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, diagrammatic view of a vehicle illustrating a side view of a single cylinder internal combustion engine having a thermal barrier coating disposed on a plurality of components; 
         FIG. 2  is a schematic cross-sectional side view of the thermal barrier coating disposed on the component; 
         FIGS. 3A-3C  are schematic cross-sectional side views of microspheres of the thermal barrier coating as formed in accordance with the present disclosure; 
         FIGS. 4A-4B  are schematic cross-sectional side views of microspheres of the thermal barrier coating bonded with a matrix material as applied to a substrate of the component; and 
         FIGS. 5A-5B  is a schematic cross-sectional side view of the thermal barrier coating disposed on the component illustrating the insulating and sealing layers of the thermal barrier coating applied to the substrate. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to several embodiments of the disclosure that are illustrated in accompanying drawings. Whenever possible, the same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. For purposes of convenience and clarity only, directional terms such as top, bottom, left, right, up, over, above, below, beneath, rear, and front, may be used with respect to the drawings. These and similar directional terms are not to be construed to limit the scope of the disclosure in any manner. 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several Figures, a portion of a vehicle  10  with a propulsion system  12  in accordance with an exemplary embodiment of the disclosure is shown schematically in  FIG. 1 . The propulsion system  12  may be any of an internal combustion engine, fuel cells, motors and the like. The propulsion system  12  may be part of the vehicle  10  that may include a motorized vehicle, such as, but not limited to, standard passenger cars, sport utility vehicles, light trucks, heavy duty vehicles, minivans, buses, transit vehicles, bicycles, robots, farm implements, sports-related equipment or any other transportation apparatus. For purposes of clarity, propulsion system  12  will be referred to hereinafter as an internal combustion engine or engine  12 . 
     The engine  12  of vehicle  10  may include one or more components  14 . The component  14  has a thermal barrier coating (TBC)  16  of the type disclosed herein, applied thereto. In one embodiment of the disclosure, TBC  16  may include a composite or multi-layer structure or configuration. While the vehicle  10  and the engine  12  of  FIG. 1  are a typical example application, suitable for the TBC  16  disclosed herein, the present design is not limited to vehicular and/or engine applications. 
     Any stationary or mobile, machine or manufacture, in which a component thereof is exposed to heat may benefit from use of the present design. For illustrative consistency, the vehicle  10  and engine  12  will be described hereinafter as an example system, without limiting use of the TBC  16  to such an embodiment. 
       FIG. 1  illustrates an engine  12  defining a single cylinder  18 . However, those skilled in the art will recognize that the present disclosure may also be applied to components  14  of engines  12  having multiple cylinders  26 . Each cylinder  18  defines a combustion chamber  22 . The engine  12  is configured to provide energy for propulsion of the vehicle  10 . The engine  12  may include but is not limited to a diesel engine or a gasoline engine. The engine  12  further includes an intake assembly  28  and an exhaust manifold  30 , each in fluid communication with the combustion chamber  22 . The engine  12  includes a reciprocating piston  20 , slidably movable within the cylinder  18 . 
     The combustion chamber  22  is configured for combusting an air/fuel mixture to provide energy for propulsion of the vehicle  10 . Air may enter the combustion chamber  22  of the engine  12  by passing through the intake assembly  28 , where airflow from the intake manifold into the combustion chamber  22  is controlled by at least one intake valve  24 . Fuel is injected into the combustion chamber  22  to mix with the air, or is inducted through the intake valve(s)  32 , which provides an air/fuel mixture. The air/fuel mixture is ignited within the combustion chamber  22 . Combustion of the air/fuel mixture creates exhaust gas, which exits the combustion chamber  22  and is drawn into the exhaust manifold  30 . More specifically, airflow (exhaust flow) out of the combustion chamber  22  is controlled by at least one exhaust valve  26 . 
     With reference to  FIGS. 1 and 2 , the TBC  16  may be disposed on a face or surface of one or more of the components  14  of the engine  12 , including, but not limited to, the piston  20 , the intake valve  24 , exhaust valve  26 , interior walls of the exhaust manifold  30 , and the like. In one embodiment of the disclosure, the TBC  16  may be applied onto high temperature sections or components of the engine  12  and bonded to the component  14  to form an insulator configured to reduce heat transfer losses, increase efficiency, and increase exhaust gas temperature during operation of the engine  12 . 
     The TBC  16  is configured to provide low thermal conductivity and low heat capacity to increase engine efficiency. As such, the low thermal conductivity reduces heat transfer losses and the low heat capacity means that the surface of the TBC  16  tracks with the temperature of the gas during temperature swings and heating of cool air entering the cylinder is minimized. In one non-limiting embodiment of the disclosure, the TBC  16  may be about 200 microns (μm) in thickness that is applied to a surface  42  of the component  14  which exhibits a calculated thermal conductivity of about 0.36 W/mK and heat capacity of 289 kJ/m3K, a porosity of about 92.5%, crushing strength of about 10 MPa to minimize heat losses and could increase engine efficiency by 5-10%. 
     For example, a TBC  16  for the engine  12  may be desired that insulate the hot combustion gas from the lower temperature water-cooled engine block to avoid energy loss by transferring heat from the combustion gas to the cooling water. Further, during the intake cycle, the insulation material should cool down rapidly in order to not heat up the fuel-air mixture before ignition to avoid abnormal combustion caused by heat being retained within the combustion chamber  22 . It should be appreciated that the TBC  16  may be applied to components other than present within the engine  12 . More specifically, the TBC  16  may be applied to components of spacecraft, rockets, injection molds, and the like. 
     Referring now to  FIG. 2 , each component  14  includes a substrate  40  having at least one exterior or presenting surface  42 . The TBC  16  may include at least one layer  44  that is applied and/or bonded to the surface  42  of the substrate  40 . The at least one layer  44  of the TBC  16  may include multiple layers, such as a first or insulating layer  46 , and a second or sealing layer  48 . 
     The insulating layer  46  may include a plurality of hollow microspheres  50 , sintered together to create a layer having an extremely high porosity and mostly closed celled structure. Preferably, the porosity of the insulating layer  46  may be at least about 80%. The high porosity of the insulating layer  46  provides for a corresponding volume of air and/or gases to be contained therein, thus providing the desired insulating properties of low effective thermal conductivity and low effective heat capacity. 
     It is contemplated that the higher the volume fraction of porosity in the first coating  62 , the lower the thermal conductivity and capacity. The porosity level needs to be balanced with the mechanical requirements, such as compressive strength, which is required to withstand the high pressure levels in the engine  12 . The thickness T1 of the insulating layer may be between about 50 microns or micrometers (μm) and 1000 μm or 1 millimeter (mm). More preferably, the thickness T2 of the sealing layer may be between about 1 μm and about 20 μm. The insulating layer  46  is configured to withstand pressures of around 100 bar and withstand surface temperatures of around 1,100 degrees Celsius (° C.). 
     The hollow microspheres  50  may be comprised of a combination of polymeric, metal, glass, and/or ceramic materials. In one non-limiting embodiment, the hollow microspheres  50  are comprised of metal, such as Nickel (Ni), nickel alloy compounds, Iron-Chromium-Aluminum (FeCrAl) alloys, Cobalt (Co) alloys and the like for durability and resistance to oxidation and corrosion at high temperatures of around 1,000 degrees Celsius (° C.). The hollow microspheres  50  may have a diameter D1 of between about 10 μm and about 100 μm. The wall thickness of the hollow microspheres may be between about 0.5 micron and 5 microns. 
     Referring now to  FIG. 3 , microspheres  50  are illustrated that may be formed by a variety of processes. A microsphere  50  may be formed with a base surface, generally referenced by numeral  60 . The base surface  60  may be formed of a polymeric material to provide a spherical shaped template for the microsphere  50 . The polymeric material may be advantageous for the base surface  60  to limit conductivity and heat capacity as part of the completed microsphere  50 . 
     The base surface  60  may be formed using a variety of materials, including, but not limited to, polyvinylidene chloride copolymer for a hollow microsphere  50 , a polystyrene for a solid microsphere  50  that may be removed at a later step in the formation process. Alternatively, hollow spheres formed using ceramics such as glass bubbles or cenospheres such as fillite, can also be used but may not be removed in the formation process. 
     A first coating  62  is applied to at least a portion of the base surface  60 . In one embodiment of the disclosure, the first coating  62  may comprise a material such as nickel that is applied or deposited over substantially the entire base surface  60  via electroless plating or a chemical vapor deposition (CVD) process. It is also appreciated that another material, such as iron or cobalt could be used as the first coating  62  material in place of nickel. 
     The thickness of the first coating  62  may be tailored by adjusting the amount of time of the plating process at a specified temperature, for example between about 0.2 μm and about 2 μm of nickel may be deposited depending on the diameter D1 of the base surface  60  and the target density of the insulating layer  46 . In one embodiment, the TBC  16  with a higher porosity will exhibit a lower thermal conductivity and heat capacity, while decreasing the strength and robustness of the insulating layer  46 . As such, a porosity between about 90% and about 97% of the insulating layer  46  is preferred. 
     A second coating  64  may then be applied and/or deposited over at least a portion of the first coating  62 . The second coating  64  may be a material that forms an alloy with the first coating  62 . In one embodiment the first coating contains nickel and the second coating contains at least one or more elements, including, but not limited to, Zinc (Zn), Copper (Cu), chromium (Cr), aluminum (Al), cobalt (Co), Molybdenum (Mo), Tungsten (W), Tantalum (Ta), Titanium (Ti), Zirconium (Zr), Hafnium (Hf) and/or Yttrium (Y). It is advantageous for the second coating  64  to form an alloy with the first coating, as pure nickel provides limited strength and oxidation and corrosion resistance at elevated temperatures. 
     The alloying material of the second coating  64  may be applied to at least a portion of the first coating  62  by an electroless plating, CVD, vapor phase deposition process or dry sputtering. Referring to  FIGS. 3A-3C , various configurations of microspheres  50  for use in the TBC  16  are illustrated.  FIG. 3A  illustrates microsphere  50  including a base polymeric surface  60  at least partially covered by a first coating  62  comprising nickel. The second coating  64  comprising one alloying element, such as chromium or aluminum, which at least partially covers the first coating  62 . 
     It is understood that the materials used with the base surface  60  of microsphere  50 , first coating  62  and second coating  64  may be adjusted without affecting the functionality of the microsphere  50 . In one embodiment of the disclosure, the second coating  64  may be chromium that is about 5% to about 30% of the thickness of the first coating  62 . In another embodiment of the disclosure, the second coating  64  may be aluminum that is about 5% to about 30% of the thickness of the first coating. 
       FIG. 3B  illustrates an alternative configuration for microsphere  50 . Microsphere  50  includes a base polymeric, glass or ceramic surface  60  at least partially covered by a first coating that comprises mostly nickel or cobalt or iron and is deposited by electroless plating or CVD. The second coating  64  comprises a first alloying element, such as chromium or aluminum, which at least partially covers the first coating  62 . A third coating  66  of a second alloying element at least partially covers the second coating  62 . In one embodiment of the disclosure, the coating thicknesses are configured to yield the ratio of elements of the target alloy. One embodiment of the ratio of elements may be a nickel alloy with about 22% by weight of chromium and about 10% by weight of aluminum to produce hollow microspheres  50  with a 50 μm diameter and 1 μm shell thickness. 
     In this embodiment, a first coating  62  of about 0.53 μm of nickel is deposited on the base surface  60 , followed by a second coating  64  of about 21 μm chromium and then a third coating  66  of about 26 μm aluminum. After application of the first coating  62 , second coating  64  and third coating  66 , microspheres  50  may be subjected to a homogenization heat treatment of about 1200 degrees Celsius (° C.) for 48 hours to interdiffuse the elements in the three coatings and form a homogeneous alloy. An optional ageing heat treatment of about 900 degrees Celsius (° C.) for 8 hours or a similar time and temperature may be performed to form precipitates that strengthen the nickel alloy. 
     In another embodiment the outer coating, either the second or third coating depending on how many coatings are deposited, is selected from a group of materials including Zinc (Zn), Copper (Cu), Silver (Ag) and Aluminum (Al) that exhibit a lower melting point than the first coating and therefore promote sintering of the microspheres to each other and to the substrate and sealing layer. 
     Alternatively, as is shown in  FIG. 3C , the second coating  64  may include nanoparticles containing the alloying elements with diameters of about 20 nanometers (nm) to about 500 nm may be applied to the first coating  62 . The nanoparticles, which may contain Inconel® alloys, nickel base superalloys or stainless steel, may be diffused into the first coating  62  using heat treatments of between about 1000 degrees Celsius (° C.) and about 1100 degrees Celsius (° C.) for a period of about 10 hours to about 20 hours. The first coating may comprise mostly nickel, cobalt or iron deposited by electroless plating of CVD. The heat treatments may be performed after a TBC  16  coating has been applied to a substrate, but they could also be performed before application to the substrate. In one embodiment of the disclosure, the second coating  64  of nanoparticles may be comprised of Inconel® alloy or nickel based superalloy particles having a diameter of about 20 nm to about 200 nm with the coating being about 5% to about 30% of the thickness of the first coating  62 . 
     Referring back to  FIG. 2 , application of the first or insulating layer  46  to the surface  42  of the substrate  40  is described in greater detail. In one embodiment the microspheres  50  are placed on the substrate  40  and sintered at an elevated temperature that ensures diffusion between the microspheres themselves and the substrate. In another embodiment, microspheres  50  are placed in a slurry. The slurry may be formed of a solvent, such as water, and a water soluble binder, for example polyvinyl-alcohol, polyvinyl-pyrrolidone or cellulose polymer derivatives. An organic solvent such as isopropanol or acetone can also be added to water or fully substituted for the solvent in which case the binder must be suitably soluble in the mixture, such as a polyvinyl butyral resin. Other slurry additives, for example polyethylene-glycol and glycerol, may be used for rheological adjustments such as deflocculation, lubrication, and antifoaming to maximize the packing efficiency upon slurry application. 
     Preferably the slurry is fluidized for application by addition of just enough solvent to flow smoothly, for example about 10 milliliters (ml) for 10 grams (g) of dry microspheres  50  and a minimum amount of binder is also added to reduce residual carbon after burnout. The first or insulating layer  46  may be formed by applying a slurry of the microspheres  50  to the surface  42  of substrate  40  by spray coating, dipping, painting, doctor-blading or other methods. 
     After application, the coating is dried to remove the solvent and then sintered at a temperature that ensures diffusion between the microspheres  50  themselves and between the microspheres  50  and the substrate  40 . Sintering is typically carried out in an inert or reducing atmosphere. The organic components of the slurry can either be removed during a separate burn-out heat treatment in air at 400-600 degrees Celsius (° C.) before sintering or during the sintering step. 
     Referring to  FIGS. 4A and 4B , in one embodiment of the disclosure, microspheres  50  including at least one coating such as the first coating  62  and the second coating  64  may be combined with particles  54  of a matrix forming alloy, generally referred to by numeral  56  to be applied to the surface  42  of substrate  40 . FIG.  4 A 4 A illustrates a portion of the TBC  16  prior to heating, wherein particles  54  are positioned in cavities between adjacent microspheres  50 . Particles  54  combine in matrix  56  with microspheres  50  to increase structural durability and robustness of the insulating layer. It is contemplated that particles  54  may be added to the slurry to form the matrix  56 . 
     To increase the strength of the first coating  62  and/or second coating  64 , either the thickness of the microspheres  50  or the volume fraction of matrix  56  may be increased. Upon heat treatment, the matrix forming particles  54  generate the matrix  56 , the density of which may be dependent upon the volume fraction of matrix material  56 . The matrix forming particles  54  may be less than 50 μm in diameter and may represent no more than about 10% by weight to about 20% by weight of the insulating layer  46 . Further particles  54  and matrix  56  may exhibit a lower melting point, for example, less than 1100 degrees Celsius (° C.), than the microspheres  50  to enable sintering of the matrix  56  or create a small amount of liquid phase to fuse adjacent microspheres together and distribute the liquid throughout the first coating  62  and/or second coating  64 . 
     Non-limiting examples of materials for particles  54  include, but are not limited to, aluminum alloys, pure aluminum, nickel alloys with about 1% by weight to about 10% by weight of Boron (B), nickel alloys with about 1% by weight to about 10% by weight of Phosphorous (P), nickel alloys with about 1% by weight to about 15% by weight of Silicon (Si) or mixtures thereof. It is also contemplated that the particles  54  may contain additional alloying elements including chromium, aluminum, cobalt, molybdenum, tungsten, tantalum, titanium, zirconium, hafnium and/or yttrium. 
     A coating of the slurry is applied to the surface  42  of the substrate  40  of the component  14 , such as a piston head, valve or an exhaust port. The coating may be applied by a number of non-limiting methods, including spray coating, dipping, painting, doctor-blading and the like to a coating thickness of between about 100 μm and 1 mm. The slurry coating  52  may be heated at a temperature of about 100 degrees Celsius (° C.) to about 300 degrees Celsius (° C.) for about 1 hour to about 5 hours to dry the coating. 
     The slurry coating of hollow microspheres  50  may be molded or sintered under pressure, while being heated, over a molding time, until the insulating layer  46  is formed. For example, the slurry may be sintered at a temperature of about 800 degrees Celsius (° C.) to about 1100 degrees Celsius (° C.) for about 2 to about 20 hours. During the sintering heat treatment, microspheres  50  fuse together with the substrate to improve structural integrity. Diffusional mixing of the alloying elements and the nickel base metal may result in a nickel alloy with more than 10% by weight Chromium and more than 4% by weight aluminum and a ratio of aluminum to Chromium greater than 0.25 to form an aluminum oxide for oxidation resistance at temperatures above 900 degrees Celsius (° C.). If iron or cobalt is chosen as a base material in place of nickel, similar Fe—Cr—Al or Co—Cr—Al alloys may be used to achieve similar results. 
     Referring now to  FIGS. 5A and 5B , the sealing layer  48  is disposed over the insulating layer  46 , such that the insulating layer  46  is disposed between the sealing layer  48  and the surface  42  of the substrate  40  of the component  14 . The sealing layer  48  may be a high temperature thin film. More specifically, the sealing layer  48  comprises material that is configured to withstand temperatures of at least 1100 degrees Celsius (° C.). The sealing layer  48  may be configured to be a thickness of about 1 μm to about 20 μm. 
     The sealing layer  48  may be non-permeable to combustion gases, such that a seal is provided between the sealing layer  48  and the insulating layer  46 . Such a seal prevents debris from combustion gases, such as unburned hydrocarbons, soot, partially reacted fuel, liquid fuel, and the like, from entering the porous structure defined by the hollow microspheres  50 . If such debris were allowed to enter the porous structure of the insulating layer  46 , air disposed in the porous structure would end up being displaced by the debris, and the insulating properties of the insulating layer  46  would be reduced or eliminated. 
     The sealing layer  48  may be configured to present an outer surface  68  that is smooth. Having a smooth sealing layer  48  may be important to prevent the creation of turbulent airflow as the air flows across the outer surface  68  of the sealing layer  48 . Further, having a sealing layer  48  with a smooth surface will prevent an increased heat transfer coefficient. In one non-limiting example, the sealing layer  48  may be applied to the insulating layer  46  via electroplating. In another non-limiting example, the sealing layer  48  may be a thin film comprised of metals including nickel, nickel alloy, cobalt alloy, iron alloy or steel that is applied to the insulating layer simultaneously with or after sintering the insulating layer  46 . 
     The sealing layer  48  is configured to be sufficiently resilient so as to resist fracturing or cracking during exposure to debris. Further, the sealing layer  48  is configured to be sufficiently resilient so as to withstand any expansion and/or contraction of the underlying insulating layer  46 . Further, the insulating and sealing layers  46 ,  48  are each configured to have compatible coefficient of thermal expansion characteristics to withstand thermal fatigue. 
     The detailed description and the drawings or figures are supportive and descriptive of the disclosure, but the scope of the disclosure is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed disclosure have been described in detail, various alternative designs and embodiments exist for practicing the disclosure defined in the appended claims. Furthermore, the embodiments shown in the drawings or the characteristics of various embodiments mentioned in the present description are not necessarily to be understood as embodiments independent of each other. Rather, it is possible that each of the characteristics described in one of the examples of an embodiment can be combined with one or a plurality of other desired characteristics from other embodiments, resulting in other embodiments not described in words or by reference to the drawings. Accordingly, such other embodiments fall within the framework of the scope of the appended claims.