Patent Publication Number: US-2019194812-A1

Title: Gap-filling sealing layer of thermal barrier coating

Description:
STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under contract no. DE-EE0007754 awarded by the United States Department of Energy. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to a thermal barrier layer, which may be referred to as a thermal barrier coating (TBC), for protecting components subject to high-temperature gasses, and a method of forming the same. 
     INTRODUCTION 
     Internal combustion engines include a plurality of cylinders, a plurality of pistons, at least one intake port, and at least one exhaust port. The cylinders each include surfaces that define a combustion chamber. One or more surfaces of the internal combustion engine may be coated with thermal barrier coatings, or multi-layer thermal barriers, to improve the heat transfer characteristics of the internal combustion engine and minimize heat loss within the combustion chamber. For example, such a coating system is desired for insulating the hot combustion gasses from the cold, water-cooled engine block, to avoid energy loss by transferring heat from the combustion gasses to the cooling water. 
     A sealing layer may be provided over an insulating layer to effectively seal the component from the particles that may be present in the combustion gasses. In addition, the surface of the coating system should follow the temperature of the combustion gasses, including cooling down rapidly, to avoid heating up the fuel-air mixture before ignition to avoid knocking. Therefore, the sealing layer is provided as a very thin layer that can follow the temperature of the adjacent gasses. However, given the porosity of the insulating layer, the very thin sealing layer only bonds to some of the surface of the insulating layer. Further, given the thinness of the sealing layer, the sealing layer may break off when subject to extreme conditions within the combustion chamber. 
     SUMMARY 
     The present disclosure provides a sealing layer that fills in gaps or crevices along an outer edge of the insulating layer. For example, the sealing layer may be made of a fine powder that fills in the gaps and/or crevices along the edge of the insulating layer, providing a more robust surface contact between the insulating layer and sealing layer, so that the sealing layer effectively bonds to a substantial majority of the outer surface of the insulating layer. 
     In one example, a thermal barrier coating is provided that may be applied to a surface of one or more components within an internal combustion engine. The thermal barrier coating is bonded to the component(s) of the engine to provide low thermal conductivity and low heat capacity insulation that is sealed against combustion gasses. The thermal barrier coating includes an insulating layer and a sealing layer disposed on the insulating layer, wherein the sealing layer fills in crevices along the edge of the insulating layer. 
     The thermal barrier coating, or multi-layer thermal barrier coating, may include two, three, four, or more layers, bonded to one another, with at least an insulating layer and a sealing layer. A bonding layer may also be provided under the insulating layer, in which case, the insulating layer would be disposed between the bonding layer and the sealing layer. The innermost layer is bonded to the component. 
     The thermal barrier coating has a low thermal conductivity to reduce heat transfer losses and a low heat capacity so that the surface temperature of the thermal barrier coating tracks the gas temperature in the combustion chamber. Thus, the thermal barrier coating allows surface temperatures of the component to swing with the gas temperatures. This reduces heat transfer losses without affecting the engine&#39;s breathing capability and without increasing knocking tendency. Further, heating of cool air entering the cylinder of the engine is reduced. Additionally, exhaust temperature is increased, resulting in faster catalyst light off time and improved catalyst activity. 
     In one form, which may be combined with or separate from the other forms described herein, a multi-layer thermal barrier coating is provided that includes at least an insulating layer and a sealing layer. The insulating layer comprises a plurality of hollow round microstructures bonded together and defining an outer layer of microstructures disposed along an outer edge of the insulating layer. The outer layer of microstructures defines a plurality of crevices between adjacent microstructures along the outer edge. The sealing layer is bonded to the outer layer of microstructures, the sealing layer being substantially non-permeable and configured to seal against the outer layer of microstructures. The sealing layer fills in at least a portion of the crevices. 
     In another form, which may be combined with or separate from the other forms disclosed herein, a multi-layer thermal barrier coating is provided that includes a bonding layer, an insulating layer, and a sealing layer. The bonding layer is configured to be bonded to a metal substrate. The insulating layer is bonded to the bonding layer, the insulating layer having an outer surface defining a plurality of crevices therein. The sealing layer is bonded to the outer surface of the insulating layer. The sealing layer is substantially non-permeable and configured to seal against the insulating layer. The sealing layer fills in at least a portion of the crevices. 
     In yet another form, which may be combined with or separate from the other forms disclosed herein, a method of forming a thermal barrier coating is provided. The method includes a step of providing a plurality of hollow round microstructures bonded together, each having a diameter in the range of 10 to 100 microns, to create an insulating layer. The method further includes a step of depositing a plurality of metal particles onto the insulating layer, and the method includes a step of heating the plurality of metal particles to form a substantially non-permeable sealing layer over the insulating layer. 
     Additional features may optionally be provided, including but not limited to the following: the sealing layer being formed of a plurality of metal particles; the sealing layer having a sealing layer melting point and the insulating layer having an insulating layer melting point, the sealing layer melting point being lower than the insulating layer melting point; each microstructure consisting essentially of nickel; the sealing layer being comprised of an alloy formed of nickel and copper; wherein each metal particle is smaller than each microstructure of at least a substantial majority of the microstructures; the sealing layer extending outward from the insulating layer by no more than 5 microns; the insulating layer having a thickness between 75 and 300 microns; each microstructure having a width or diameter not greater than 100 microns; each microstructure having a width or diameter in the range of about 40 to about 50 microns; a bonding layer configured to be bonded to a metal substrate; the insulating layer being bonded to the bonding layer; the bonding layer comprising a copper-based material, a zinc-based material, an alloy comprising copper and zinc, or any other desirable material, preferably having a lower melting temperature than the insulating layer and that improves bonding to the substrate; each microstructure comprising a nickel-based material and/or an iron-based material; and the insulating layer having a porosity of at least 90%. 
     Further additional features may be provided, including but not limited to the following: providing the plurality of hollow round microstructures to define an outer layer of microstructures disposed along an outer edge of the insulating layer; the outer layer of microstructures defining a plurality of crevices between adjacent microstructures along the outer layer; disposing at least a portion of the plurality of metal particles within the crevices; providing a bonding layer configured to be bonded to a metal substrate; bonding the insulating layer to the bonding layer; and performing the step of heating the sealing layer by laser scanning, laser welding, radiation, or inductive heating. 
     Furthermore, a component comprising a metal substrate presenting a surface may be provided, with a version of the thermal barrier coating being bonded to the surface of the substrate. The component may be a valve face or a piston crown, by way of example. In addition, the present disclosure contemplates an internal combustion engine comprising such a component having any version of the thermal barrier coating disposed thereon or bonded thereto, wherein the component is configured to be subjected to combustion gasses. 
     The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description for carrying out the present teachings when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic side cross-sectional view of a portion of a propulsion system having a cylinder of an internal combustion engine including a thermal barrier coating disposed on a plurality of components, in accordance with the principles of the present disclosure; 
         FIG. 2  is a schematic side cross-sectional view of one example of the thermal barrier coating disposed on the components of  FIG. 1 , according to the principles of the present disclosure; 
         FIG. 3  is a close-up schematic cross-sectional side view of a portion of the thermal barrier coating of  FIG. 2 , taken along the line  3 - 3 , in accordance with the principles of the present disclosure; 
         FIG. 4  is a schematic cross-sectional side view of another example of the thermal barrier coating disposed on the components of  FIG. 1 , according to the principles of the present disclosure; and 
         FIG. 5  is block diagram illustrating a method of forming a thermal barrier coating, according to the principles of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Those having ordinary skill in the art will recognize that terms such as “above,” “below,” “upward,” “downward,” “top,” “bottom,” etc., are used descriptively for the figures, and do not represent limitations on the scope of the disclosure, as defined by the appended claims. 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the views,  FIG. 1  shows a portion of an example vehicle propulsion system  10  that includes an engine  13  having a component  12 . The component  12  has a thermal barrier “coating” (TBC)  14  of the type disclosed herein, applied thereto. The thermal barrier coating  14  may be referred to as a composite thermal barrier coating or multi-layer thermal barrier in forms that have multiple layers bonded together. For example, the TBC  14  may be an engineered surface comprised of a plurality of layers, which is described in further detail below. 
     While the engine  13  of  FIG. 1  is a typical example application suitable for the thermal barrier coating  14  disclosed herein, the present design is not limited to vehicular and/or engine applications. Stationary or mobile, machine or manufacture, in which a component thereof is exposed to heat, may benefit from use of the present design. 
       FIG. 1  illustrates an engine  13  defining a single cylinder  26 . However, those skilled in the art will recognize that the present disclosure may also be applied to components  12  of engines  13  having multiple cylinders  26 . Each cylinder  26  defines a combustion chamber  30 . The engine  13  is configured to provide energy for the propulsion system  10  of the vehicle. The engine  13  may include but is not limited to a diesel engine or a gasoline engine. 
     The engine  13  further includes an intake assembly  36  and an exhaust manifold  38 , each in fluid communication with the combustion chamber  30 . The engine  13  includes a reciprocating piston  28 , slidably movable within the cylinder  26 . 
     The combustion chamber  30  is configured for combusting an air/fuel mixture to provide energy to the propulsion system  10 . Air may enter the combustion chamber  30  of the engine  13  by passing through the intake assembly  36 , where airflow from the intake manifold into the combustion chamber  30  is controlled by at least one intake valve  32 . Fuel is injected into the combustion chamber  30  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  30 . Combustion of the air/fuel mixture creates exhaust gas, which exits the combustion chamber  30  and is drawn into the exhaust manifold  38 . More specifically, airflow (exhaust flow) out of the combustion chamber  30  is controlled by at least one exhaust valve  34 . 
     With reference to  FIGS. 1 and 2 , the thermal barrier coating  14  may be disposed on a face or surface of one or more of the components  12  of the engine  13 , e.g., the piston  28 , the intake valve  32 , exhaust valve  34 , interior walls of the exhaust manifold  38  and/or the combustion dome  39 , and the like. The thermal barrier coating  14  is bonded to the component  12  to form an insulator configured to reduce heat transfer losses, increase efficiency, and increase exhaust gas temperature during operation of the engine  13 . The thermal barrier coating  14  is configured to provide low thermal conductivity and low heat capacity. The low thermal conductivity reduces heat transfer losses, and the low heat capacity results in the surface of the thermal barrier coating  14  tracking with the temperature of the gas during temperature swings, and heating of cool air entering the cylinder is minimized. 
     Referring to  FIG. 2 , each component  12  includes a substrate  16  presenting a surface  18 , and the thermal barrier coating  14  is bonded to the surface  18  of the substrate  16 . The thermal barrier coating  14  may include two, three, four, or more layers, by way of example. In  FIG. 2 , the thermal barrier coating  14  includes three layers, e.g., an optional first (bonding) layer  20 , a second (insulating) layer  22 , and a third (sealing) layer  24 . 
     The bonding layer  20  is configured to bond to the surface  18  of the substrate  16  and to the insulating layer  22 , such that the insulating layer  22  is attached to the substrate  16 . In one non-limiting example, the bonding layer  20  is configured to diffuse into the surface  18  of the substrate  16  and into the insulating layer  22  to form bonds therebetween. 
     In one non-limiting example, the substrate  16  comprises an alloy of aluminum, the insulating layer  22  comprises nickel or iron, and the bonding layer  20  comprises copper and/or brass (a copper-zinc (Cu—Zn) alloy material). Copper and/or brass create optimum bonding strength, optimum thermal expansion characteristics, heat treatment processes, fatigue resistance, and the like. In addition, copper and/or brass have good solid solubility in aluminum, nickel, and iron, while iron and nickel have very low solid solubility in aluminum. Thus, a bonding layer  20  having copper and/or brass combinations provides an intermediate structural layer that promotes diffusion bonding between the adjacent aluminum substrate  16  and the adjacent nickel or iron insulating layer  22 . It should be appreciated, however, that the substrate  16 , insulating layer  22 , and bonding layer  20  are not limited to aluminum, nickel, iron, copper, and brass, but may comprise other materials. For example, in another variation, the substrate  16  includes or is substantially comprised of iron. 
     One side of the bonding layer  20  may be disposed across the surface  18  of the substrate  16 , such that the bonding layer  20  is disposed between the substrate  16  and the insulating layer  22 . A compressive force may be applied to the insulating layer  22  and the substrate  16 , at a bonding temperature, for at least a minimum apply time. The melting temperature of the material of the bonding layer  20  is less than the melting temperature of each of the substrate  16  and the material of the insulating layer  22 . In another example, the melting temperature of the material of the bonding layer  20  is between the melting temperature of each of the substrate  16  and the material of the insulating layer  22 . Further, the required bonding temperature may be less than the melting temperature of the material of the substrate  16  and the material of the insulating layer  22 , but sufficiently high enough to encourage diffusion bonding to occur between the metallic material of the substrate  16  and the metallic material of the bonding layer  20  and between the metallic material of the bonding layer  20  and the metallic material of the insulating layer  22 . 
     It should be appreciated that the bonding layer  20  may be bonded to an inner surface of the insulating layer  22  prior to bonding the bonding layer  20  to the surface  18  of the substrate  16 . Additionally, the bonding layer  20  is not limited to being bonded to the surface  18  of the substrate  16  and/or the insulating layer  22  with solid-state diffusion, as other methods of adhesion may also be used, such as by wetting, brazing, and combinations thereof. It should be appreciated that any desired number of bonding layers  20  may be applied, providing the desired characteristics, so long as the bonding layer  20  as a whole bonds to the insulating layer  22  and to the substrate  16 . 
     The insulating layer  22  may comprise a ceramic material, such as zirconia, stabilized zirconia, alumina, silica, rare earth aluminates, oxide perovskites, oxide spinels, and titanates. In other variations, the insulating layer  22  may be formed of porous aluminum oxide, or the insulating layer  22  may be formed of a metal, such as iron or nickel. In some variations, the insulating may comprise a plurality of hollow microstructures bonded together, which is shown and described with greater detail with reference to  FIG. 4 . 
     The insulating layer  22  may have a porosity in the range of 50% to 90%, and in some cases, the porosity of the insulating layer exceeds 90%, or even 95%. Preferably, the porosity of the insulating layer  22  is at least 80%, in some cases it is preferable that the porosity of the insulating layer  22  is at least 90%, and furthermore, in some cases, it is preferable that the porosity of the insulating layer  22  is at least 95%. The high porosity provides for a corresponding volume of air and/or gasses to be contained therein, thus providing the desired insulating properties of low effective thermal conductivity and low effective heat capacity. The insulating layer  22  is preferably formed of a material having a low effective thermal conductivity, such as in the range of 0.1 to 5 W/mK, and from a material having a coefficient of thermal expansion similar to that of the substrate  16 . 
     The insulating layer  22  could be applied by thermal spray techniques, such as air plasma spray or high velocity oxy-fuel plasma spray. In the case of a porous aluminum oxide insulating layer  22 , the insulting layer  22  may be formed by anodizing. 
     To achieve the desired thermal barrier performance, the thickness of the insulating layer  22  may be tailored for specific applications. For example, a greater thickness T 2  could be used if the insulating layer  22  is comprised of a material having a higher thermal conductivity, and a lesser thickness T 2  could be used if the insulating layer  22  is comprised of a material having a lower thermal conductivity. In some examples, the insulating layer  22  has a thickness T 2  in the range of 50 to 1000 microns, or in the range of 50 to 500 microns, or in the range or in the range of 75 to 300 microns. In some variations, the insulating layer  22  is not greater than 250 microns. 
     The insulating layer  22  is configured to withstand pressures of at least 80 bar, and in some cases at least 100 bar, or even at least 150 bar. Additionally, with respect to temperature, the insulating layer  22  is configured to withstand surface temperatures of at least 500 degrees Celsius (° C.), or at least 800° C., or even at least 1,100° C. The heat capacity of the thermal barrier coating  14  may be configured to ensure that the surface  18  of the substrate  16  does not get above 300° C. 
     The sealing layer  24  is disposed over the insulating layer  22 , such that the insulating layer  22  is disposed between the sealing layer  24  and either the bonding layer  20  of the surface  18  or of the substrate  16 . The sealing layer  24  is a high temperature, thin film. More specifically, the sealing layer  24  comprises material that is configured to withstand temperatures of at least 1,100° C. In some forms, the sealing layer  24  may be formed of a metallic material, such as stainless steel, nickel, iron, nickel alloy, cobalt alloy, refractory alloy, a nickel-copper alloy, or any other desired metal or other desired material. 
     The sealing layer  24  is substantially non-permeable (or has very low permeability) to combustion gasses, such that a seal is provided between the sealing layer  24  and the insulating layer  22 . For example, the sealing layer  24  may be no more than 10% porous. Such a seal prevents debris from combustion gasses, such as unburned hydrocarbons, soot, partially reacted fuel, liquid fuel, and the like, from entering the porous structure of the insulating layer  22 . If such debris were allowed to enter the porous structure, air disposed in the porous structure could end up being displaced by the debris, and the insulating properties of the insulating layer  22  could be reduced or eliminated. Also, if gases are able to penetrate during each combustion cycle, the insulating quality of the insulating layer  22  is much less. Therefore, the sealing layer  24  is preferably substantially impermeable to gases, as well as to solids. 
     In one non-limiting example, the sealing layer  24  may be applied to the insulating layer  22  via electroplating or vapor deposition, or by another process of applying a powder material. In another non-limiting example, the sealing layer  24  may be applied to the insulating layer  22  simultaneously with sintering the insulating layer  22 . 
     The sealing layer  24  is configured to be sufficiently resilient so as to resist fracturing or cracking during exposure to combustion gasses, thermal fatigue, or debris. Further, the sealing layer  24  is configured to be sufficiently resilient so as to withstand expansion and/or contraction of the underlying insulating layer  22 . 
     In some forms, the sealing layer  24  is thin, with a thickness T 3  not greater than 20 microns, and in some cases, not greater than 5 microns. However, in some cases, the thickness T 3  of the sealing layer  24  may be as great as 50 microns, by way of example. Thus, for example, T 3  may be in the range of 3 to 50 microns. 
       FIG. 3  provides a close-up cross-sectional view taken along the lines  3 - 3  in  FIG. 2 . Referring to  FIGS. 2 and 3 , as explained above, the insulating layer  22  is formed of a porous material. As such, the insulating layer  22  has a plurality of pores  27  formed therein, and the insulating layer  22  has an outer surface  46  defining a plurality of crevices  48  therein. The crevices  48  are gaps or cracks in the outer surface  46  of the insulating layer  22 , which may be formed by the existence of pores  27  at the surface  46 . 
     The sealing layer  24  fills in at least a portion of the crevices  48  in the outer surface  46  of the insulating layer  22 . For example, the sealing layer  24  may be formed of metal particles  51 , such as a metal powder. The metal particles  51  may be deposited in the crevices  48  of the outer surface  46  of the insulating layer  22 . To form the sealing layer  24 , the sealing layer  24  may be heated to melt an outer portion  52  of the metal particles  51 , forming the outer portion  52  of the metal particles into a continuous surface  54  at the outer edge  56  of the sealing layer  24 . 
     Referring now to  FIG. 4 , the component of  FIG. 1  (labeled as  12 ′ here) is illustrated again with another variation of the thermal barrier coating  14 ′ disposed thereon. Again, the component  12 ′ includes a substrate  16 ′ presenting a surface  18 ′, and the thermal barrier coating  14 ′ is bonded to the surface  18 ′ of the substrate  16 ′. In this example, the thermal barrier coating  14 ′ includes two layers: an insulating layer  22 ′ and a sealing layer  24 ′. The bonding layer  20  is omitted, and the insulating layer  22 ′ is bonded directly to the surface  18 ′ of the substrate  16 ′; however, it should be understood that the bonding layer  20  shown in  FIG. 2  could be included between the insulating layer  22 ′ and the substrate  16 ′, if desired. 
     In the variation of  FIG. 4 , the insulating layer  22 ′ includes a plurality of hollow microstructures  40 , bonded or sintered together to create a layer having an extremely high porosity. Preferably, the porosity of the insulating layer  22 ′ is at least 80%. More preferably, the porosity of the insulating layer  22 ′ is at least 90%, or even 95%. The high porosity 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. 
     In one example, the hollow microstructures  40  may be comprised of hollow polymer, metal, glass, and/or ceramic centers  45 , which may be, or may start off as being, spherical, elliptical, or oval in shape. Thus, in some examples, the microstructures  40  are round. At least one metallic coating layer  44  may be disposed on an exterior surface of each hollow center  45 ; in some cases, a first metal coating may be overcoated with a second metal coating. The metallic coating layer  44  may include nickel (Ni), iron, or the like, alone or in combination. The metallic coating layer  44  may be disposed on the exterior surface of the microstructures  40  via electroplating, flame spraying, painting, electroless plating, vapor deposition, or the like. 
     It should be appreciated that during the bonding or sintering of the metallic coated microstructures  40 , the hollow centers  45  that are comprised of polymer, metal, and glass having a melting temperature that is less than that of the metallic coating layer  44 , and therefore, the hollow centers  45  may melt or otherwise disintegrate to become part of the metallic coating layer  44  itself, or melt and turn into a lump of material within the hollow microstructure  40 . However, when the melting temperature of the hollow center  45  is higher than the melting temperature of the material of the metallic coating layer  44 , such as when the hollow center  45  is formed from a ceramic material, the hollow center  45  remains intact and does not disintegrate or become absorbed. 
     In instances where the hollow centers  45  are formed from polymer, metal, and glass, the hollow center  45  may melt as a function of a material properties of the hollow center  45  and a sintering temperature applied to the microstructures  40 . Therefore, when melting of the hollow centers  45  occurs, the metallic coating layer  44  is no longer a “coating”, but rather becomes an inner wall of the microstructure  40 . Thus, in some examples, the microstructures  40  may be thin-walled hollow metal structures without anything in their centers. 
     In examples where the microstructures  40  are round or elliptical, such as shown in  FIG. 4 , the hollow microstructures  40  may have a diameter D 1  of between 5 and 100 microns, between 20 and 100 microns, or between 20 and 40 microns, by way of example. In another example, the diameter D 1  is between about 40 and about 50 microns. It should be appreciated that the microstructures  40  do not necessarily have the same diameter, as a mixture of diameters may be configured to provide a desired open porosity, e.g., packing density, to provide a desired amount of strength to the insulating layer  22 ′. 
     The plurality of the hollow microstructures  40  may be molded or sintered at a sintering temperature, under pressure, for a molding time, until bonds are formed between the coating layers  44  of adjacent hollow microstructures  40  forming the insulating layer  22 ′. 
     The insulating layer  22 ′ defines an outer layer  23  of microstructures  40  disposed along an outer edge  46 ′ of the insulating layer  22 ′. The outer layer  23  of microstructures  40  defines a plurality of crevices  48 ′ between adjacent microstructures  40  along the outer edge  46 ′. The crevices  48 ′ are gaps between adjacent microstructures  40  along the outer edge  46 ′ of the insulating layer  22 ′. Thus, the crevices  48 ′ are located between walls  44  of adjacent microstructures  40  and extend into the insulating layer  22 ′ from an outermost part of the outer edge  46 ′. 
     The sealing layer  24 ′ fills in at least a portion of the crevices  48 ′ along the outer edge  46 ′ of the outer layer  23  of the microstructures  40  of the insulating layer  22 ′. For example, the sealing layer  24 ′ may be formed of metal particles  51 ′, and the metal particles  51 ′ may be deposited in the crevices  48 ′ of the outer layer  23  of microstructures  40 . Each of the metal particles  51 ′ may be smaller than each, or most, of the microstructures  40 , so that the metal particles  51 ′ can fill in the crevices  48 ′ between the microstructures  40 . An outer portion  52 ′ of the sealing layer  24 ′ may be melted together into a continuous surface  54 ′ at an outer edge  56 ′ of the sealing layer  24 ′. 
     To form the sealing layer  24 ′, the metal particles  51 ′ may be deposited along the outer edge  56 ′ of the insulating layer  22 ′, including in the crevices  48 ′. Then, the metal particles  51 ′ may be heated to melt an outer portion  52 ′ of the metal particles  51 ′, thereby forming the outer portion  52 ′ of the metal particles  51 ′ into a continuous surface  54 ′ at the outer edge  56 ′ of the sealing layer  24 ′. 
     The sealing layer  24 ′ is bonded to the outer layer  23  of microstructures  40 . The sealing layer  24 ′ is substantially non-permeable and is configured to seal against the outer layer  23  of microstructures  40 , and the sealing layer  24 ′ fills in at least a portion of the crevices  48 ′. 
     Referring to the versions of the thermal barrier coating  14 ,  14 ′ shown in both of  FIGS. 3 and 4 , the sealing layer  24 ,  24 ′ has a sealing layer melting point and the insulating layer  22 ,  22 ′ has an insulating layer melting point. The sealing layer melting point is lower than the insulating layer melting point. Therefore, during the heating of the outer portion  52 ,  52 ′ of the metal particles  51 ,  51 ′, the metal particles  51 ,  51 ′ may be melted to form the continuous surface  54 ,  54 ′ without melting the microstructures  40  or other configuration (as in  FIG. 2 ) of the insulating layer  22 ,  22 ′. For example, the microstructures  40  or other configuration (as in  FIG. 2 ) of the insulating layer  22 ,  22 ′ may consist essentially of nickel, which has a melting point of about 1453° C. A nickel-copper alloy may be used for the metal particles  51 ,  51 ′, and thus, the sealing layer  24 ,  24 ′ may have a melting point of between 1085 and 1452° C., depending on the amount of copper included. Accordingly, the heating of the sealing layer  24 ,  24 ′ may be performed at a temperature between the melting points of the insulating layer  22 ,  22 ′ and the sealing layer  24 ,  24 ′ to melt the outer portion  23 ,  23 ′ of the metal particles  51 ,  51 ′ of the sealing layer  24 ,  24 ′ without melting the insulating layer  22 ,  22 ′. 
     Other materials may alternatively be used for the insulating layer  22 ,  22 ′ and the sealing layer  24 ,  24 ′. For example, the insulating layer  22 ,  22 ′ may be formed of a nickel alloy containing cobalt, chromium, molybdenum, tungsten, iron, and magnesium, as well as small amounts of other elements, such as the nickel alloy sold under the registered trademark Hastelloy® and labeled as a C-276 composition. In other forms, stainless steel, tungsten, Mo, Mn, Cr, and alloys of these may be used to form the insulating layer  22 ,  22 ′. Preferably, the material of the sealing layer  24 ,  24 ′ compliments the insulating  22 ,  22 ′ by having a lower melting point than the material of the insulating layer  22 ,  22 ′. Therefore, in one example, the insulating layer  22 ,  22 ′ (or the microstructures  40  comprising it) may be formed of chromium, and the sealing layer  24 ,  24 ′ may be formed of a manganese/chromium alloy. In another example, a molybdenum insulating layer  22 ,  22 ′ may be used with a molybdenum/titanium sealing layer  24 ,  24 ′. In another example, a molybdenum insulating layer  22 ,  22 ′ may be used with a molybdenum/nickel sealing layer  24 ,  24 ′. These are just a few possible examples; other combinations of materials are possible, such as ternary and many other multi-component alloys. 
     The sealing layer  24 ,  24 ′ may extend outward from the insulating layer  22 ,  22 ′ by a relatively short distance, such as no more than 5 microns, while the entire depth of the sealing layer  24 ,  24 ′ may extend down much deeper into the crevices  48 ,  48 ′. Thus, the sealing layer  24 ,  24 ′ may be strengthened with more material in the crevices  48 ,  48 ′, and with more material being bonded to the surfaces of the outer layer  23  of microstructures  40 , without adding to the thickness of the sealing layer  24  at the peaks  50 ′ of the microstructures  40 , or at the outermost parts  50  of the outer surface  46  of the insulating layer  22  of  FIG. 3 . 
     Though not shown, the sealing layer  24 ,  24 ′ could also include more than one layer to provide desired properties, e.g., super-high temperature resistance and corrosion resistance. For example, a separate top layer could form the continuous portion  52 ,  52 ′ over the rest of the metal particles  51 ,  51 ′, if desired. 
     It should be understood that any of the variations, examples, and features described with respect to one of the thermal barrier coatings  14 ,  14 ′ described herein may be applied to one of the other thermal barrier coatings  14 ,  14 ′ described herein. The thermal barrier coatings  14 ,  14 ′ may be formed in any suitable way, which may include heating the insulating layer  22 ,  22 ′, the bonding layer  20 , and the sealing layer  24 ,  24 ′, such as by sintering. 
     Referring to  FIG. 5 , and with continued reference to  FIG. 4 , one method of forming the thermal barrier coating  14 ′ is illustrated and generally designated at  100 . It should be understood that some portions of the described method  100  may also be used to form the thermal barrier coating  14  shown in  FIGS. 2-3 . 
     The method  100  includes a step  102  of providing a plurality of hollow round microstructures bonded together, each having a diameter in the range of 10 to 100 microns, to create an insulating layer, such as the insulating layer  22 ′ shown and described with respect to  FIG. 4 . Further, the insulating layer  22 ′ is preferably provided having a porosity of at least 90%, as explained above. A bonding layer may also be optionally provided, such as the bonding layer  20  shown in  FIG. 2 , where the bonding layer  20  is configured to be bonded to a metal substrate  16 . If the bonding layer  20  is included, the method  100  may include bonding the insulating layer  22 ,  22 ′ to the bonding layer  20 . 
     The method  100  includes another step  104  of depositing a plurality of metal particles, such as the metal particles  51 ′ shown in  FIG. 4 , onto the insulating layer, such as the insulating layer  22 ′. The metal particles  51 ′ are preferably smaller than the hollow round microstructures  40 , so that the metal particles  51 ′ at least partially fill in the gaps, and are disposed in the crevices  48 ′, defined between the microstructures  40  along the outer edge  46 ′ of the insulating layer  22 ′. 
     The method  100  further includes a step  106  of heating the plurality of metal particles  51 ′ to form a substantially non-permeable sealing layer  24 ′ over the insulating layer  22 ′. The sealing layer  24 ′, and the metal particles  51 ′ that the sealing layer  24 ′ is made from, is preferably provided having a sealing layer melting point that is lower than a melting point of the insulating layer  22 ′. For example, the hollow round microstructures  40  could be formed of pure nickel having a melting point of 1453° C., and the metal particles  51 ′ could formed of a nickel-copper alloy, which has a melting point between 1085 and 1453° C., depending on the copper content of the alloy. Accordingly, when heat is applied to the outer side  56 ′ of the sealing layer  24 ′, the metal particles  51 ′ melt together to form the continuous surface  54 ′ without damaging or melting the hollow round microstructures  40 . The continuous surface  54 ′ may then be similar to a weld or clad microstructure. The heating of the metal particle  51 ′ can be applied via laser scanning, laser welding, radiation, inductive heating, and/or additive manufacturing techniques. The sealing layer  24 ′ is preferably melted quickly and solidified before the hollow microstructures  40  are damaged or destroyed, though the outermost microstructures  40  may have signs of melting and solidification. Within the crevices  48 ′, some of the metal particles  51 ′ may remain unmelted and keep their original form. Additional diffusion bonding to the underlying structure  16  can be carried out at lower temperatures. 
     It should be appreciated that the thermal barrier coatings  14 ,  14 ′ described herein may be applied to components other than those present within an internal combustion engine. More specifically, the thermal barrier coatings  14 ,  14 ′ may be applied to components of space crafts, rockets, injection molds, and the like. 
     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 examples for carrying out the claimed disclosure have been described in detail, various alternative designs and examples exist for practicing the disclosure defined in the appended claims. Furthermore, the examples shown in the drawings or the characteristics of various examples mentioned in the present description are not necessarily to be understood as examples independent of each other. Rather, it is possible that each of the characteristics described in one example can be combined with one or a plurality of other desired characteristics from other examples, resulting in other examples not described in words or by reference to the drawings. Accordingly, such other examples fall within the framework of the scope of the appended claims.