THERMAL SPRAY DEPOSITION OF HOLLOW MICROSPHERES

Abstract

Methods of forming an insulating coating from thermal spraying are provides. In one variation, the method includes thermally spraying a jetted stream having a maximum temperature of greater than or equal to about 900° C. towards a substrate to form the insulating coating on the substrate. The thermal spraying may be a high-velocity oxygen flame (HVOF) process. The jetted stream comprises a plurality of hollow microspheres, which may comprise a metal, such as nickel or iron. The insulating coating as formed has a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions and may have a thermal capacity (cv) of greater than or equal to about 100 kJ/m3·K.

Description

INTRODUCTION

This section provides background information related to the present disclosure which is not necessarily prior art.

The present disclosure pertains to methods for thermally spraying hollow microparticles onto substrates to form insulating thermal barrier coatings.

Insulating and thermal barrier coatings are used in various applications to reduce heat transfer. Such coatings desirably have low heat capacity and low thermal conductivity. In certain aspects, a thermal barrier coating may comprise an insulating material including one or more hollow microspheres. Thus, the insulating or thermal barrier coating can be used in a variety of applications, including by way of non-limiting example, on surfaces of components within an internal combustion engine to reduce heat transfer losses and increase performance and efficiency. New methods of forming robust thermal barrier coatings on a variety of complex components are desirable.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

The present disclosure relates to thermal spray deposition of hollow microstructures, such as microspheres. In one variation, the present disclosure provides a method of forming an insulating coating including thermally spraying a jetted stream having a maximum temperature of greater than or equal to about 900° C. towards a substrate to form the insulating coating on the substrate. The jetted stream includes a plurality of hollow microspheres. The insulating coating that is formed has a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions.

In one aspect, the insulating coating includes a plurality of hollow microstructures having intact void regions after the thermal spraying.

In another aspect, the insulating coating has a net porosity of greater than or equal to about 80 volume %.

In yet another aspect, the insulating coating has a thickness of less than or equal to about 200 micrometers (μm).

In certain aspects, the jetted stream has a maximum temperature of less than or equal to about 1,400° C.

In another aspect, the plurality of microspheres includes a metal selected from the group consisting of: nickel, iron, combinations, and alloys thereof.

In other aspects, the plurality of microspheres includes the metal in a first layer and further includes a second layer of a second metal selected from the group consisting of: copper, zinc, tin, nickel, and combinations thereof.

In yet another aspect, the substrate includes at least one metal selected from the group consisting of: nickel, iron, copper, zinc, aluminum, combinations, and alloys thereof.

In another aspect, the thermal conductivity (K) of the insulating coating is less than or equal to about 100 mW/m·K at standard temperature and pressure conditions.

In further aspects, a thermal capacity (c_v) of the insulating coating is less than or equal to about 100 kJ/m³·K.

In another variation, the present disclosure provides a method of forming an insulating coating including jetting a stream including a plurality of hollow microspheres from a high velocity oxygen fuel (HVOF) device towards a substrate. The stream includes the plurality of microspheres that include a first metal layer having a first metal selected from the group consisting of: nickel, iron, combinations, and alloys thereof and a second metal layer having a second metal selected from the group consisting of: copper, zinc, tin, nickel, combinations, and alloys thereof. Further, the stream has a maximum temperature during the jetting that is at least about 50° C. below a melting point of the first metal layer, but at or above a melting point of the second metal layer. The method also includes forming the insulating coating on the substrate having a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure.

In one aspect, the insulating coating includes a plurality of hollow microstructures having intact void regions after the thermal spraying.

In another aspect, the insulating coating has a net porosity of greater than or equal to about 80 volume %.

In yet another aspect, the insulating coating has a thickness of less than or equal to about 200 micrometers (μm).

In yet other aspects, the insulating coating may have a thickness of less than or equal to about 200 micrometers (μm)

In a further aspect, the maximum temperature is greater than or equal to about 900° C. to less than or equal to about 1,400° C.

In yet another aspect, the substrate includes at least one metal selected from the group consisting of: nickel, iron, copper, zinc, tin, nickel, aluminum, combinations, and alloys thereof.

In other aspects, the substrate includes a first metal selected from the group consisting of: nickel, iron, combinations, and alloys thereof and further includes a surface coating of a second metal selected from the group consisting of: copper, zinc, tin, nickel, combinations, and alloys thereof.

In yet other aspects, the plurality of microspheres includes the first metal layer having nickel and the second metal layer having copper.

In still further aspects, the thermal conductivity (K) is less than or equal to about 100 mW/m·K at standard temperature and pressure conditions.

In another aspect, the insulating coating has a thermal capacity (c_v) of less than or equal to about 100 kJ/m³·K.

In yet another aspect, the method further includes sintering the insulating layer after the jetting.

In yet another variation, the present disclosure provides a method of forming an insulating coating that includes jetting a stream including a plurality of hollow microspheres from a high velocity oxygen fuel (HVOF) device towards a substrate to form a layer of deposited hollow microstructures. The stream has a maximum temperature during the jetting that is greater than or equal to about 900° C. to less than or equal to about 1,400° C. Each of the plurality of hollow microspheres includes a first metal layer and a second metal layer. The first metal layer has a first metal selected from the group consisting of: nickel, iron, combinations, and alloys thereof and the second metal layer has a second metal selected from the group consisting of: copper, zinc, tin, nickel, combinations, and alloys thereof. The method further includes sintering the layer of deposited hollow microstructures to form the insulating coating on the substrate having a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions and a thermal capacity (c_v) of less than or equal to about 100 kJ/m³·K.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 shows a high velocity oxygen flame (HVOF) thermal spraying device that can be used in accordance with certain aspects of the present disclosure for depositing hollow microspheres to form an insulating coating.

FIG. 2 shows an example of a hollow microsphere having a single metal coating layer.

FIG. 3 shows another example of a hollow microsphere having two distinct metal coating layers for use in the thermal spraying processes according to certain aspects of the present disclosure.

FIG. 4 shows an insulating coating including hollow microstructures deposited on a substrate in accordance with certain aspects of the present disclosure via thermal spraying.

FIG. 5 shows a composite thermal barrier coating including an insulating coating including hollow microstructures deposited on a substrate in accordance with certain aspects of the present disclosure via thermal spraying.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, elements, compositions, steps, integers, operations, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Although the open-ended term “comprising,” is to be understood as a non-restrictive term used to describe and claim various embodiments set forth herein, in certain aspects, the term may alternatively be understood to instead be a more limiting and restrictive term, such as “consisting of” or “consisting essentially of.” Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and/or process steps, the present disclosure also specifically includes embodiments consisting of, or consisting essentially of, such recited compositions, materials, components, elements, features, integers, operations, and/or process steps. In the case of “consisting of,” the alternative embodiment excludes any additional compositions, materials, components, elements, features, integers, operations, and/or process steps, while in the case of “consisting essentially of,” any additional compositions, materials, components, elements, features, integers, operations, and/or process steps that materially affect the basic and novel characteristics are excluded from such an embodiment, but any compositions, materials, components, elements, features, integers, operations, and/or process steps that do not materially affect the basic and novel characteristics can be included in the embodiment.

Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated.

When a component, element, or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other component, element, or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,” “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. For example, “about” may comprise a variation of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in certain aspects, optionally less than or equal to 0.1%.

In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges.

Example embodiments will now be described more fully with reference to the accompanying drawings.

In various aspects, the present disclosure describes methods of forming an insulating coating on a substrate. In certain aspects, the method may include thermally spraying a jetted stream towards a substrate. By thermal spraying, it is meant that a process is employed where a precursor material is heated and propelled as individual particles at a surface of the substrate to form a robust and cohesive coating. Thermal spraying thus relies on heat and momentum to cause the coating material to conform and bond to the surface being coated.

In certain aspects, the thermal spraying process has a maximum temperature of greater than or equal to about 900° C., optionally greater than or equal to about 1,000° C., optionally greater than or equal to about 1,100° C., and in certain aspects, optionally greater than or equal to about 1,200° C. In certain aspects, the thermal spraying process has a temperature of greater than or equal to about 900° C. to less than or equal to about 1,400° C., and in certain variations, optionally greater than or equal to about 1,100° C. to less than or equal to about 1,200° C. As will be appreciated by those of skill in the art, some cooling occurs as the material being sprayed exits the thermal spraying device or gun, thus desirably the temperature in the thermal spray device is high enough to promote softening or melting of at least one material in a hollow precursor, while avoiding overheating that could cause structural collapse, as will be described further below.

For all thermal spray coating processes, material is heated, accelerated, and shot at a target surface. Particle velocities vary in different thermal spray processes, for example, velocities of jetted streams are highest in high-velocity oxygen flame processes and lower in low velocity spraying processes, such as subsonic oxygen fuel powder processes. By a jetted stream, it is meant that the stream has a relatively high velocity and creates a jet, while still desirably avoiding velocities that would promote collapse of a hollow precursor under the select conditions used during thermal spraying (e.g., select temperatures and pressures). For example, a maximum velocity of a jetted stream for use in accordance with certain aspects of the present disclosure is less than or equal to about 400 m/s, optionally less than or equal to about 100 m/s, and in certain aspects, optionally less than or equal to about 10 m/s. In certain variations, the jetted stream may have supersonic velocity of greater than about 343 m/s but less than or equal to about 400 m/s.

With the high-velocity oxygen flame coating process, the gas stream is produced by mixing and igniting oxygen and fuel (gas or liquid) in a combustion chamber and allowing the high pressure gas to accelerate through a nozzle. Microparticles are introduced into this stream where it is heated and accelerated towards a target surface. In one variation according to certain aspects of the present disclosure, a method of forming an insulating coating is shown in FIG. 1 that includes jetting a stream comprising a plurality of hollow microparticles or microspheres 102 from a high velocity oxygen fuel (HVOF) device 100 towards a substrate 110. The hollow microparticles define an enclosed void region in the core that may be filled with an insulating material. Thus, the void region in the core may be filled with a gas, such as air or an inert gas, or may have vacuum conditions, by way of example. In certain aspects, the microparticles have at least one spatial dimension that is less than about 100 μm optionally less than or equal to about 50 μm and in certain aspects, less than or equal to about 10 μm.

The microparticles may be microspheres having a substantially round shape. “Substantially round-shaped” includes microparticles having a shape including spherical, globular, spheroidal, disk, cylindrical, discoid, domical, egg-shaped, elliptical, orbed, oval, and the like, so long as at least a portion of the center of the microparticle defines an enclosed void region. Thus reference herein to microspheres can encompass any of these substantially round shapes. Notably, while the precursor may be a microsphere, after the thermal spraying process, the microsphere may be distorted from a spherical or substantially round shape.

In certain aspects, the microspheres used as a precursor during the thermal spraying may have an average particle size diameter of less than about 100 micrometers (μm), optionally greater than or equal to about 10 μm to less than or equal to about 80 μm, optionally greater than or equal to about 20 μm to less than or equal to about 60 μm, and in certain variations, optionally greater than or equal to about 30 μm to less than or equal to about 40 μm. It should be appreciated that the microspheres have an average diameter within these ranges, but the plurality of microspheres do not necessarily all have the same diameter, as a mixture of microspheres having distinct diameters may be employed to provide a desired porosity or packing density, which can vary strength within the insulating coating formed. It should be further noted that the smaller the diameter of the microspheres used, the greater the particle density and therefore mass of the coating formed from such particles. Thus, relatively large microspheres form lighter coatings than smaller microspheres.

The plurality of microspheres comprises at least one metal. In certain variations, the metal may be selected from the group consisting of: nickel, iron, combinations, and alloys thereof. In one variation, the metal is nickel or a nickel alloy. In another variation, the metal is iron and microsphere may comprise an iron alloy, such as steel or stainless steel. Any alloy may contain additional elements as appreciated by those of skill in the art, such as carbon, manganese, chromium and nickel, molybdenum, and the like, by way of non-limiting example.

Generally, hollow microspheres 20, such as that shown in FIG. 2, may include a structural material 22 that defines the enclosed void region 24. The structural material 22 may be selected from the group consisting of: metal, glass, ceramic, polymers, and combinations thereof, so long the structural material 24 defines a hollow structure having the enclosed void region 24. The hollow microspheres 20 may then be coated with one or more conductive materials, such as metals like nickel, iron, nickel alloy compounds, iron alloy compounds, and the like. A metal coating 26 is thus formed over the structural material 22. The application of the metal on the surface of the microspheres 20 may be conducted via a process such as electroplating, vapor deposition, electroless plating, flame spraying, painting, and the like. The microsphere 20 may be used in thermal spraying, but in certain aspects is used as a precursor for forming a multilayered hollow microsphere like that shown in FIG. 3.

In FIG. 3, a hollow microsphere 30 used as a precursor in the thermal spraying process according to certain aspects of the present disclosure may have multiple distinct metallic layers. In one variation, the hollow microspheres 30 have a structural material 32 that defines a void region 34 in the center core region. The structural material 32 may be formed of the same materials as structural material 22 above. A first metallic material comprising a first metal may be applied to the surface of the structural material 32 to form a first metal layer or coating 36. A second metallic material may comprise a second distinct metal that be may applied over the first metal coating 36 to form a second metal layer or coating 38.

The first metal coating 36 may comprise nickel, iron, combinations, and alloys thereof. In certain aspects, the first metal coating 36 comprises nickel or a nickel alloy. The second metal coating 38 may comprise copper, zinc, tin, nickel, combinations, and alloys thereof. Notably, while the first metal coating 36 and the second metal coating 38 may contain one or more of the same metals, each layer/coating has a distinct composition and thus distinct melting points. In certain aspects, the second metal coating 38 comprises copper or a copper alloy. In other aspects, the second metal coating 38 may comprise a combination of copper and zinc, for example, a brass alloy. In certain aspects, a brass alloy has zinc present in the composition at less than or equal to about 32% by weight (to avoid formation of undesirable phases), while the balance may include copper and impurities. In this manner, the first metal coating 36 and second distinct metal coating 38 have different melting point temperatures, which can be beneficial for certain thermal spraying processes as described further below. More specifically, the first metal layer or coating 36 may have a higher melting point than the second metal layer or coating 38. Thus, in certain variations, the second metal coating 38 may comprise copper and nickel, where nickel is present in the composition up to about 30% by weight and the balance is copper and impurities. In other aspects, the second metal coating 38 may comprise nickel and tin, where tin is present in the composition up to about 30% by weight and the balance is nickel and impurities. Such a nickel-tin alloy has a low melting point/eutectic point of about 1130° C. In certain other aspects, the second metal coating 38 may comprise nickel and zinc, where zinc is present in the composition up to about 40% by weight and the balance is nickel and impurities.

In certain aspects, the first metal coating 36 may have a thickness of about 1 micrometer, while the second metal coating 38 may have a thickness of less than or equal to about 1 micrometer. Thus, the thickness of the second metal layer or coating 38 is less than the thickness of the first metal layer or coating 36. Where the first metal coating 36 comprises nickel and the second metal coating 38 comprises copper, the copper can diffuse into the nickel. The more copper that diffuses from the second metal coating 38 into the first metal coating 36, the lower the maximum temperature that can be used while thermally spraying while still maintaining the hollow structures (e.g., so that the first metal coating 36 remains structurally intact as a hollow shape, especially when the structural material 32 has been removed from the hollow microsphere 30). However, most diffusion of copper into nickel takes place after the thermal spray process, if a subsequent heat treatment (e.g., for sintering) is conducted. The final alloy composition (the Ni—Cu alloy) can potentially limits a maximum temperature of use in the final application (e.g., as a thermal barrier material in an engine).

With renewed reference to FIG. 1, the HVOF process uses combustion to generate heat and impart velocity to the jetted stream. The HVOF device 100 includes two fuel inlets 120 for a fuel stream and two oxidant inlets 122 for introducing an oxygen-containing stream (or other oxidant-containing stream). It should be noted that the fuel inlets 120 and oxidant inlets 122 are not restricted to the number of inlets or placement shown in FIG. 3. The fuel may comprise propane, propylene, and/or hydrogen, by way of non-limiting example. Further, while not shown, cooling inlets and channels that circulate a coolant may also be provided. A central inlet 130 receives a stream of a carrier gas and the plurality of hollow microspheres 102 insufflated therein. The carrier gas may be an inert gas, such as nitrogen and/or argon, by way of non-limiting example, or the carrier gas may have the same composition as the oxidant-containing stream or fuel stream. In certain alternative designs not shown, the central inlet 130 could be eliminated or modified so that the hollow microspheres are directly introduced into the fuel stream(s) or oxidant stream(s).

In the HVOF device 100, the oxygen-containing stream and fuel combine in a mixing region 132. The mixed stream is introduced into a combustion chamber 134 where an exothermic combustion reaction takes place. The carrier gas and hollow microspheres 102 pass into and through the combustion chamber 134 where they are heated and then enter a nozzle 136. A high temperature, high velocity jetted stream 140 exits the nozzle 136. The jetted stream 140 may be a supersonic spray flame in certain variations. The parameters of operation for the HVOF device 100 are selected in accordance with certain aspects of the present disclosure to promote softening, adhesion, and bonding of the microspheres onto the substrate, while minimizing collapse or rupture of the interior void regions.

For example, in certain aspects, a maximum temperature of the jetted stream 140 (and within the HVOF device 100) is selected to be at least about 50° C. below a melting point of a select metal forming the hollow microspheres 102, for example, a first metal in the first metal layer/coating. Thus, in certain aspects, where the metal is nickel, a maximum temperature is at least 50° C. below the melting temperature of nickel of 1,455° C., so that a maximum temperature in the HVOF process and the jetted stream is less than about 1,405° C. (or about 1,400° C.). In certain variations, throughout the process, the stream (and microspheres) encounter a maximum temperature during the jetting that is greater than or equal to about 900° C. to less than or equal to about 1,400° C.; optionally greater than or equal to about 1,000° C. to less than or equal to about 1,300° C., and in certain aspects, optionally greater than or equal to about 1,100° C. to less than or equal to about 1,200° C. The operating temperature and pressures used in HVOF can be tuned to allow the deposition of various microspheres, such as those comprising nickel or iron (e.g., steel or stainless steel) microspheres onto a surface without melting at least one layer of the microspheres. By using HVOF and other similar thermal spray technologies, hollow microspheres can be quickly deposited onto surfaces. Temperatures in the spray device and microspheres can be matched so that the microspheres do not collapse, but develop an initial bond on impact with a surface of the target substrate.

In certain variations, where a first metal is present in a first layer along with a second metal as part of a second layer of the hollow microspheres, the maximum temperature in the HVOF process may be at least 50° C. below the melting point of the first metal, but may approach or exceed the melting point of the second metal on the outer coating. The second metal and/or second layer thus softens and partially or fully melts to enhance adhesion and bonding as the hollow microspheres are deposited on the substrate 110. Thus, in certain variations, the first metal may be nickel with a melting point of about 1,455° C. and the second metal may be copper having a melting point of about 1,084° C., so that the maximum temperature of the jetted stream may be below the melting point of nickel, but above the melting point of copper. In such a variation, the maximum temperature may be greater than or equal to about 1,110° C. to less than or equal to about 1,400° C. For example, the temperature of the jetted stream may be above copper's melting point of about 1,084° C. as the microspheres hit the target, but the maximum temperature during the thermal spraying does not reach 1,455° C. (the melting point of nickel). Notably, the hollow microparticles cool down some as they leave the gun, so higher maximum temperatures in the gun/thermal spray device can be above 1,100° C. and cool as the hollow microparticles approach and contact the target. Where the second layer comprises copper and zinc, the melting point temperature is lower. For example, a brass alloy comprising copper and about 32 weight % zinc has a melting point of about 903° C., thus the thermal spraying temperatures above may be adjusted accordingly. The temperature of the jetted stream may be near or above the brass alloy's melting point of about 903° C. as the microspheres hit the target, but the maximum temperature of the jetted stream during the thermal spraying remains below 1,455° C. (the melting point of nickel).

The jetted stream is directed towards a surface of the substrate 110, where a plurality of hollow microstructures 142 is deposited in a cohesive, high porosity insulating coating 144. The substrate may be formed of a variety of materials capable of withstanding high temperatures, including metals, ceramics, and the like. In certain aspects, the substrate comprises at least one metal selected from: nickel, iron, copper, zinc, tin, aluminum, magnesium, combinations, and alloys thereof. The substrate in certain variations may include steel, superalloys, such as inconel nickel superalloys, aluminum alloys and magnesium alloys, by way of non-limiting example.

The substrate 110 may comprise a coating or be formed of a material that promotes adhesion of the hollow microstructures. The surface may comprise at least one metal selected from: copper, zinc, tin, nickel, aluminum, combinations, and alloys thereof. The surface may have a metallic surface coating or be formed of a metal comprising copper and/or zinc and/or alloys thereof. The coating comprising copper and/or zinc may be applied to any heat resistant substrate via electroplating, electroless plating, vapor deposition, flame spraying, painting, and the like. In one variation, the substrate 110 may be formed of a copper-containing material or may comprise a copper-containing coating. Where the substrate 110 is coated, the substrate may comprise any variety of heat resistant materials, including steel, superalloys, such as inconel nickel superalloys, by way of non-limiting example.

The resulting thermal spray coating comprises adjacent and/or overlapping hollow microstructures. After thermal spraying onto the substrate 110, a substantial portion of the hollow microstructures 142 desirably still have the enclosed void regions intact, for example, greater than about 80% of the hollow microstructures have intact voids, optionally greater than about 90%, optionally greater than about 95%, optionally greater than about 97%, and in some aspects, optionally greater than about 98% of the voids remain intact within the hollow microstructures after deposition. In certain variations, the deposited hollow microstructures 142 may have the same microsphere shape as the precursors introduced into the HVOF device 100, but it is also possible that they may also deform or distort into other shapes.

Accordingly, after thermal spraying the deposited microparticles may have some distortion in shape, but desirably retain an internal enclosed void region thus enhancing and retaining insulating properties of the deposited coating. As such, an insulating coating 200 formed on a substrate 210 via thermal spraying as shown in FIG. 4 comprises hollow microstructures (formed by deposition of the microspheres) with intact void regions. The insulating coating having such hollow microstructures desirably exhibits a thermal conductivity (K) of less than or equal to about 1,000 mW/m·K at standard temperature and pressure conditions, optionally less than or equal to about 500 mW/m·K, optionally less than or equal to about 250 mW/m·K, optionally less than or equal to about 200 mW/m·K, optionally less than or equal to about 100 mW/m·K, optionally less than or equal to about 50 mW/m·K, and in certain variations, optionally less than or equal to about 20 mW/m·K. Standard temperature and pressure conditions are about 32° F. or 0° C. and an absolute pressure of about 1 atm or 100 KPa.

Further, the insulating coating having such hollow microstructures desirably exhibits a thermal capacity (c_v-volumetric heat capacity) of less than or equal to about 5,000 kJ/m³·K, optionally less than or equal to about 1,000 kJ/m³·K, optionally less than or equal to about 500 kJ/m³·K, optionally less than or equal to about 100 kJ/m³·K, and in certain variations, optionally less than or equal to about 50 kJ/m³·K. In one variation, the insulating coating exhibits a thermal conductivity (K) of less than or equal to about 100 mW/m·K and a thermal capacity (c_v) of less than or equal to about 100 kJ/m³·K.

The insulating coating 200 deposited via thermal spraying, such as HVOF, may have densely packed hollow microstructures 212. In certain aspects, the insulating coating 200 comprising a plurality of hollow microstructures 212 deposited via thermal spraying (e.g., HVOF deposition) has a high open porosity, for example, having a net porosity of greater than or equal to about 80 volume % of the total volume of the coating. By net porosity, it is meant a total porosity volume includes both a volume of void spaces within the nanostructures and a volume of pores defined between nanostructures. In certain variations, such a net porosity is greater than or equal to about 85 volume %, optionally greater than or equal to about 90 volume %, and in certain variations, optionally greater than or equal to about 95 volume %.

In certain variations, the insulating coating 200 may have an average thickness of less than or equal to about 4,000 micrometers (4 mm), optionally less than or equal to about 2,000 micrometers (2 mm), optionally less than or equal to about 1,000 micrometers (1 mm), optionally less than or equal to about 500 micrometers, optionally less than or equal to about 400 micrometers, optionally less than or equal to about 300 micrometers, optionally less than or equal to about 200 micrometers, optionally less than or equal to about 100 micrometers, optionally less than or equal to about 75 micrometers, and in certain variations, optionally less than or equal to about 50 micrometers. In certain aspects, an average thickness of the insulating coating 200 is greater than or equal to about 100 micrometers to less than or equal to about 4,000 micrometers, optionally greater than or equal to about 100 micrometers to less than or equal to about 500 micrometers, optionally greater than or equal to about 100 micrometers to less than or equal to about 300 micrometers. In one variation, the insulating coating has a thickness of less than or equal to about 200 micrometers (μm). It should be noted that the desired thickness of the coating may depend on the application in which the insulating coating is used, so that a thicker coating and/or coating having greater mass may be appropriate in applications where slower thermal response is acceptable, while a thinner coating or lighter coating may be selected where faster thermal responses are desirable.

In certain aspects, the insulating layer 200 is capable of withstanding pressures of greater than or equal to about 8 MPa, optionally greater than or equal to about 10 MPa, optionally greater than or equal to about 15 MPa, and in certain aspects, greater than or equal to about 20 MPa without failure. With respect to high temperature performance, the insulating layer in certain variations is configured to withstand surface temperatures of greater than or equal to about 200° C., optionally greater than or equal to about 250° C., optionally greater than or equal to about 300° C., optionally greater than or equal to about 500° C., optionally greater than or equal to about 700° C., optionally greater than or equal to about 1,000° C., and optionally greater than or equal to about 1,300° C. without failure. The heat capacity may ensure the surface of the substrate 210 on which the coating 200 is disposed does not rise above about 250° C., for example.

As appreciated by those of skill in the art, the insulating layer 200 may in fact have multiple microstructures 212 having distinct compositions, sizes, or shapes. Such microstructures may be mixed together during thermal spraying or sequentially applied as distinct layers (e.g., different compositional layers within the insulating coating) over one another.

With renewed reference to FIG. 1, in certain aspects, the thermal spraying may optionally be conducted at ambient conditions or at a pressure of greater than or equal to about 0.5 MPa. In this manner, where a positive pressure is applied in the area surrounding the jetted stream 140 and near the substrate 110, volumetric expansion and contraction due to changes in temperature of the hollow microspheres 102 and/or deposited hollow microstructures 142 can be suppressed and minimized as the pressure is increased.

In certain alternative aspects, after the thermal spraying, the deposited microparticles may be cooled to ambient conditions and then further processed. For example, the hollow microstructures 212 in the insulating coating 200 may be further heat treated to promote additional bonding and sintering to enhance robustness of the coating. An exemplary heating process for sintering may include heating a deposited layer of microspheres on the substrate (having both a first metal in a first layer and a second metal in a second layer) to a temperature below the solidus temperature of the second metal. For example, the second metal layer may comprise Cu or a Cu—Zn alloy. Pure Cu can thus be heated to below 1,084° C. (copper solidus temperature), while a Cu—Zn alloy with less than 32% by weight Zn can be heated to below about 900° C. In one example, sintering can be conducted at a temperature of about 800° C. in an inert atmosphere, such as argon. The heat treatment for sintering can be conducted for greater than or equal to about 1 hour, optionally greater than or equal to about 2 hours, optionally greater than or equal to about 4 hours, optionally greater than or equal to about 6 hours, and in certain variations, greater than or equal to about 8 hours. In another variation, the temperature can be raised slowly above the melting temperature of the second metal (e.g., Cu) provided all diffused the second metal diffused into the first metal layer (e.g., Ni) where the alloy including both the first metal and the second metal have a higher melting temperature than the second metal alone.

Further, additional layers may be deposited over the insulating coating 200 after deposition, for example, ceramics, nickel, vanadium, molybdenum, or other high temperature metals.

In certain variations, the substrate may be formed of a substrate that may have lower heat resistance, such as aluminum, which is typically not heated to temperatures above 800° C. In such an application, a surface coating may be disposed on the surface of the aluminum and the deposited microparticles may be disposed on the surface coating. The deposited microparticles may be heated from an exterior side, while keeping the aluminum substrate itself cool. Alternatively, an intermediate substrate, such as a graphite wafer having electroplated nickel, may be used. The hollow microparticles are deposited onto the nickel wafer and then they are sintered. These materials may be added to a mold and the aluminum or other low temperature alloy may be cast around it. Yet another variation is using an intermediate substrate, such as the nickel wafer described above. The hollow microparticles are deposited onto the nickel wafer and then they are sintered. The low temperature substrate may have a surface coating as a bonding layer, for example, an aluminum substrate may have a copper and/or zinc surface coating for a bonding layer. The wafer having sintered hollow microparticles may then be sintered to the piston. This secondary sintering temperature is much less than the initial sintering temperature for the hollow microparticles (e.g., comprising nickel). Thus, the substrate may optionally comprise a nickel-containing or iron-containing sealing layer and this sealing layer could also have a fine copper or copper and nickel coating to promote bonding.

In one variation, the insulating coating may be integrated into a thermal barrier composite assembly 250 as shown in FIG. 5. A thermal barrier coating 260 is disposed on substrate 262 to define the thermal barrier composite assembly 250. In one non-limiting embodiment, the substrate 262 may comprise any variety of heat resistant materials, including steel, superalloys, such as inconel nickel superalloys, aluminum alloys, and magnesium alloys, by way of non-limiting example.

The thermal barrier coating thermal barrier coating 260 includes multiple layers (and may have more than 3 layers than those shown in FIG. 5). An optional first layer 264 (if used) is a bonding layer disposed on a surface of the substrate 262. A second layer 270 is an insulating layer comprising a plurality of hollow microstructures formed in accordance with certain aspects of the present disclosure. The first layer 264 facilitates bonding of the substrate 262 and the second layer 270. A third layer 272 disposed over the second layer 270 serves as a sealing layer disposed over the second layer 270. The third layer may be a thin film that is configured to resist the high temperatures and serves as a sealing layer that is impermeable to gasses and presents a smooth surface.

The optional first layer 264 that serves as a bonding layer may be formed of a metal comprising copper, or zinc, which can diffuse and bond with the surface of substrate 262 and the second layer 270 deposited thereon, via any of the techniques described previously above. In one variation, the first layer 264 may comprise brass, which is a copper-zinc (Cu—Zn) alloy material. Where the substrate 262 is aluminum and the microspheres comprise nickel and/or iron, in one variation, copper and zinc can be selected for inclusion in the first layer 264. Copper and zinc both have good solid solubility in aluminum, nickel, and iron, while iron and nickel have very low solid solubility in aluminum. The first layer comprising a copper and zinc alloy can be used on aluminum substrates, such as aluminum-containing pistons. Substrates formed of steel or Inconel, such as valves, can have an optional first layer 264 for bonding to enhance bonding of the second layer 270, although such a bonding layer may not be necessary for these substrates. Thus, a first layer 264 having copper and/or zinc provides an intermediate structural layer between the substrate 262 and the second layer 270 to promote diffusion bonding between the aluminum substrate and nickel or iron microstructures. It should be appreciated, however, that the substrate 262, first layer 264, and second layer 270 are not limited to aluminum, nickel, iron and brass, but may comprise other materials.

The third layer 272 serves as the sealing layer disposed over the insulating second layer 270. The sealing third layer 272 may be a high temperature, thin film, which may be configured to withstand temperatures of at least 1,100° C. The third layer 272 may have a thickness of less than or equal to about 20 micrometers, optionally less than or equal to about 5 micrometers, optionally less than or equal to about 1 micrometer. The third layer 272 is non-permeable to gases, such as combustion gases. In this manner, the third layer 272 serves as a seal over the second layer 270. Such a seal prevents debris from combustion gases, such as unburned hydrocarbons, soot, partially reacted fuel, liquid fuel, and the like, from entering the openings and pores defined between the hollow microstructures in the second layer 270. Minimizing such debris prevents gas within the pores from being displaced by the debris that could cause the insulating properties to be reduced or eliminated. The third layer 272 may have a smooth outer surface, which can prevent the creation of turbulent airflow as the air flows. Further, a third layer 272 with a smooth surface can prevent increases in a heat transfer coefficient. In one non-limiting example, the third layer may be applied to the second layer 270 (after thermal spraying and cooling) via electroplating, vapor deposition, or other application techniques. In one variation, the third layer 272 comprises a heat resistant or corrosion resistant material. In one variation, the third layer 272 may comprise molybdenum, or vanadium. The third layer 272 is configured to be sufficiently resilient so as to resist fracturing or cracking during exposure to combustion gases, thermal fatigue, or debris. Further, the third layer 272 is configured to be sufficiently resilient so as to withstand any expansion and/or contraction of the underlying insulating second layer 270. The third layer 272 may include multiple layers.

A thermal barrier composite assembly 250 can be used in a variety of applications, such as a thermal barrier on components within an internal combustion engine. The thermal barrier composite assembly 250 may be disposed on a face or surfaces of one or more of the components of an engine, for example, on a piston, an intake valve, an exhaust valve, interior walls of an exhaust manifold, and a combustion dome, by way of non-limiting example. The thermal barrier composite assembly 250 ideally has a low thermal conductivity to reduce heat transfer losses and a low heat capacity so that the surface temperature of the thermal barrier composite assembly 250 tracks the gas temperature in the combustion chamber. Thus, the thermal barrier composite assembly 250 allows surface temperatures of the component to swing with the gas temperatures. This reduces heat transfer losses without affecting the engine's breathing capability and without causing knock. 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.

While the methods and materials described herein are particularly suitable for manufacturing components of an automobile or other vehicles, they may also be used in a variety of other industries and applications, including aerospace components, consumer goods, office equipment and furniture, construction, industrial equipment and machinery, farm equipment, or heavy machinery, by way of non-limiting example. Non-limiting examples of vehicles that can incorporate components prepared in accordance with certain aspects of the present disclosure include automobiles, trains, heavy mobile equipment, tractors, buses, motorcycles, boats, mobile homes, campers, aircraft (manned and unmanned), and tanks.

The methods and insulating coatings described herein provide low conductivity, low heat capacity thermal barrier coatings. Such thermal barrier coatings can improve fuel consumption and emissions for internal combustion engines, increasing operating temperatures, while reducing after-treatment warm up time, and improving waste heat recovery. The deposition methods provide the ability to deposit microspheres on contours and various surfaces of complex parts, which otherwise may not be possible. The insulating coatings formed by such thermal spraying methods exhibit only relatively low shrinkage as compared to insulating coatings formed of microspheres that are provided in a binder matrix that is cured or microspheres that are sintered. Further, the insulating coatings formed by the thermal spraying methods are believed to have increased adhesion levels with the underlying substrate as compared to other methods of applying microspheres.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

1. A method of forming an insulating coating comprising:

thermally spraying a jetted stream having a maximum temperature of greater than or equal to about 900° C. towards a substrate to form the insulating coating on the substrate, wherein the jetted stream comprises a plurality of hollow microspheres comprising a first metal layer that comprises a first metal selected from the group consisting of: nickel, iron, combinations, and alloys thereof and a second metal layer that comprises a second metal selected from the group consisting of: copper, zinc, tin, nickel, combinations, and alloys thereof, wherein the maximum temperature of the jetted stream during the thermal spraying is at least about 50° C. below a melting point of the first metal layer, but greater than or equal to a melting point of the second metal layer, so that the second metal layer softens and partially or fully melts to enhance adhesion and bonding with the substrate and the insulating coating has a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions.

2. The method of claim 1, wherein the insulating coating comprises a plurality of hollow microstructures having intact void regions after the thermal spraying.

3. The method of claim 1, wherein the insulating coating has a net porosity of greater than or equal to about 80 volume %.

4. The method of claim 1, wherein the insulating coating has a thickness of less than or equal to about 200 micrometers (μm).

5. The method of claim 1, wherein the maximum temperature is less than or equal to about 1,400° C.

6. (canceled)

7. The method of claim 1, wherein the substrate comprises at least one metal selected from the group consisting of: iron, copper, zinc, tin, nickel, aluminum, combinations, and alloys thereof.

8. The method of claim 1, wherein the thermal conductivity (K) is less than or equal to about 100 mW/m·K at standard temperature and pressure conditions.

9. The method of claim 1, wherein the insulating coating has a thermal capacity (cv) of less than or equal to about 100 kJ/m3·K.

10. A method of forming an insulating coating comprising:

jetting a stream comprising a plurality of hollow microspheres from a high velocity oxygen fuel (HVOF) device towards an aluminum substrate having a bonding layer comprising at least one metal selected from the group consisting of: copper, zinc, combinations, and alloys thereof, wherein each of the plurality of hollow microspheres comprises a first metal layer comprising nickel and a second metal layer comprising a second metal selected from the group consisting of: copper, zinc, combinations, and alloys thereof, wherein the stream has a maximum temperature during the jetting that is at least about 50° C. below a melting point of the first metal layer, but greater than or equal to a melting point of the second metal layer, so that the second metal layer softens and partially or fully melts to enhance adhesion and bonding with the bonding layer; and

forming the insulating coating on the bonding layer disposed on the substrate, the insulating coating having a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions.

11. The method of claim 10, wherein the insulating coating comprises a plurality of hollow microstructures having intact void regions after the thermal spraying.

12. The method of claim 10, wherein the insulating coating has a net porosity of greater than or equal to about 80 volume % and the insulating coating has a thickness of less than or equal to about 200 micrometers (μm).

13. The method of claim 10, wherein the maximum temperature is greater than or equal to about 900° C. to less than or equal to about 1,400° C.

14. (canceled)

15. The method of claim 10, wherein the first metal layer comprises nickel and the second metal layer comprises copper.

16. The method of claim 10, wherein the thermal conductivity (K) is less than or equal to about 100 mW/m·K.

17. The method of claim 10, wherein the insulating coating has a thermal capacity (cv) of less than or equal to about 100 kJ/m3˜K.

18. The method of claim 10, further comprising sintering the insulating coating after the jetting.

19. A method of forming an insulating coating comprising:

jetting a stream comprising a plurality of hollow microspheres from a high velocity oxygen fuel (HVOF) device towards a substrate to form a layer of deposited hollow microstructures, wherein the stream has a maximum temperature during the jetting that is greater than or equal to about 900° C. to less than or equal to about 1400° C. and a supersonic velocity of greater than or equal to about 343 m/s and less than or equal to about 400 m/s, wherein each of the plurality of hollow microspheres comprises a first metal layer comprising a first metal selected from the group consisting of: nickel, iron, combinations, and alloys thereof and a second metal layer comprising a second metal selected from the group consisting of: copper, zinc, tin, nickel, combinations, and alloys thereof, wherein the stream has a maximum temperature during the jetting that is at least about 50° C. below a melting point of the first metal layer, but greater than or equal to a melting point of the second metal layer, so that the second metal layer softens and partially or fully melts to enhance adhesion and bonding with the substrate; and

sintering the layer of deposited hollow microstructures to form the insulating coating on the substrate having a thermal conductivity (K) of less than or equal to about 200 mW/m·K at standard temperature and pressure conditions and a thermal capacity (cv) of less than or equal to about 100 kJ/m3·K.

20. The method of claim 19, wherein the sintering occurs by heating the deposited hollow microstructures to a temperature of greater than or equal to about 800° C. for greater than or equal to about 8 hours.