Ultra high temperature environmental protection coating

Abstract

An environmental protective coating (EPC) for protecting a surface subjected to high temperature environments of more than 3000 degree F. The coating includes a dense platelet lamellar microstructure with a self-sealing, compliant binder material for holding the platelets together. The platelets may be formed from materials that are resistant to high temperatures and impermeable, such as ceramics. The lamellar microstructure creates a tortuous path for oxygen to reach the surface. The binder material may have free internal volume to increase the strain capability between the platelets and absorb increased volume during operation. The binder may be formed from a material that is softer and has a lower temperature capability than the platelets to provide the system with the required compliance and sealing capability. The binder may have sufficient glass content and glass-forming content for initial and long-term sealing purposes.

Claims

1. A method of protecting an oxidative surface of an aircraft comprising: a. forming a binder by mixing together ceramic mortar and elongate refractory oxide shells that are hollow and have ends open on opposing sides; b. forming a mixture by mixing the binder with refractory material platelets, wherein the mixture has an amount of volatile material; c. applying the mixture to the oxidative surface which is selected from the group consisting of a turbine blade surface, an exhaust washed surface, and an aircraft wing surface; d. forming a protective layer on the surface by heating the mixture to a temperature so that at least some of the volatile material volatizes to create free volume voids at random locations in the binder; and e. operating the aircraft to expose the protective layer to at least 2000 F. thereby inducing a strain in a portion of the layer that is absorbed by the free volume voids to substantially minimize stress in the layer from the induced strain.

2. The method of claim 1, wherein a spacing between adjacent platelets ranges from about 50% to about 100% of the thickness of the platelet.

3. The method of claim 1, further comprising forming the shells by coating a carbon skeleton with a layer of refractory metal, and vaporizing the skeleton thereby leaving the refractory metal.

4. The method of claim 1, wherein the platelets have a coefficient of thermal expansion that ranges from 50% to 150% of a material of the surface.

5. The method of claim 4, further comprising disposing a fluxing agent in the refractory shell that volatizes during step (d).

6. The method of claim 1, further comprising providing an additive to the binder selected from the group consisting of ceramic fibers, a sub-micron refractory metal powder, oxidative materials, fluxing agents, and combinations thereof.

7. The method of claim 6, wherein the additive has a particulate length approximately 50% to about 100% that of the width of spaces between the platelets.

8. The method of claim 1, wherein prior to step (d), the mixture is dried at a rate to minimize gas bubble formation.

9. The method of claim 1, wherein the platelets comprise a substance having a material selected from the group consisting of a refractory oxide, mixed refractory oxide, refractory ceramic, refractory metal, metal nitride, metal oxide, silicate, metal carbide, refractory alloy, intermetallic compound, MAX phase ternary carbide, and combinations thereof.

10. The method of claim 1, wherein an in-situ repair of a sealing function of the protective layer occurs as oxidizable constituents in the mixture expand and form a barrier to oxygen migration in the binder and between platelets; and fluxing constituents provide wetting to fill cracks in the protective layer.

11. The method of claim 1, wherein the binder has a modulus of elasticity that is from 0.1% to 10% of a modulus of elasticity of the platelets.

12. The method of claim 1, wherein the binder comprises a binder resin and oxidizable particulate matter.

13. The method of claim 1, wherein the binder comprises fluxing agents that are exposed to a temperature greater than an expected operating temperature.

14. The method of claim 1, further comprising adding ceramic fibers to the binder that have a length to depth ratio of less than ten.

15. The method of claim 1, wherein the shells have length to depth ratios that range from two to five.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross sectional, schematic view of an EPC applied to a surface that resists ultra high temperatures and inhibits oxidation of the surface, in accordance with embodiments of the present invention.

(2) FIG. 2 is a cross sectional, schematic view of the EPC of FIG. 1 showing an example of oxygen migration during operation, in accordance with embodiments of the present invention.

(3) FIG. 3 is a cross sectional, schematic view of the oxide formation in the EPC of FIG. 1, in accordance with embodiments of the present invention.

(4) FIG. 3A is an enlarged cross sectional, schematic view of a portion of the oxide formation shown in FIG. 3, in accordance with embodiments of the present invention.

(5) FIGS. 4-6A are schematic views of a process for manufacturing an EPC, in accordance with embodiments of the present invention.

(6) FIG. 7 is a perspective, schematic view of platelet geometry, in accordance with embodiments of the present invention.

(7) FIG. 8 is a perspective, schematic view of platelet geometry, in accordance with embodiments of the present invention.

(8) FIG. 9 is a perspective view of an alternate geometry for a scaffold used in the manufacturing of an EPC as described in FIGS. 4-6A.

(9) FIG. 10 is a perspective view of an alternate geometry for a scaffold used in the manufacturing of an EPC as described in FIGS. 4-6A.

DETAILED DESCRIPTION OF THE INVENTION

(10) The apparatus and method of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. This subject of the present disclosure may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as upper, lower, above, below, and the like are used to illustrate a relational location.

(11) It is to be understood that the subject of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the subject disclosure and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the subject disclosure is therefore to be limited only by the scope of the appended claims.

(12) Referring to FIG. 1, a sectional schematic view of an embodiment of an EPC 10 applied to a surface 12 is shown. The surface may be, any surface subjected to an ultra high temperature and can include a surface on a vehicle, an aircraft, a water craft, a space craft as well as, a turbine blade surface, exhaust washed surfaces, or an aircraft wing surface subjected to high temperature environments. The EPC 10 is capable of operating at ultra high temperatures, i.e.

(13) more than 3000 degree F.

(14) The EPC 10 includes a microstructure of platelets 14 held together by binder material 16. The platelets 14 have a thickness of from about 5 microns to 25 microns, with a length to thickness (L/T) ratio of from 5 to 20, and are resistant to temperatures above 3000 degree F. Optionally, the platelets 14 may be arranged in a lamellar or nacreous fashion, and may further optionally have a coefficient of thermal expansion that ranges from about 50% to about 150% of the thermal expansion of the material of the surface 12. Nacreous refers to the similarity in the layering of platelets to that seen in nacre or shells. In an example embodiment, staggered lamellar or nacreous layering results in spaces 13 between adjacent platelets 14 that are laterally offset from spaces 13 between adjacent platelets 14 in at least a next layer 15 of platelets 14. Optionally, the spaces 13 between adjacent platelets 14 may be offset along multiple layers of platelets 14. The platelets 14 provide structure, strength, and impermeability to the EPC 10 and may be formed from one or a combination of the following: a refractory oxide, mixed refractory oxides, refractory ceramics, refractory metals or alloys, inter-metallic compounds. Specific examples of materials for use in forming the platelets 14 include ZrB.sub.2, Ta, Cr, CrO.sub.2, CaO.sub.2, MgO.sub.2, metal nitrides, such as SiN, HfN, TaN, ZrN, ScN, YN, NB.sub.2N, NbN, Be.sub.3N.sub.2, Ta.sub.2N, Th.sub.3N.sub.2, VN, Ba.sub.3N.sub.2, AlN, UN, TlN, and BN; intermetallic compounds, such as ReW, Re.sub.24T.sub.15, OsTa.sub.3, WPl, IrTa.sub.3, PtRe, Ir.sub.3Nb, Ir.sub.3Tl, HfMo.sub.2, OsTl, RuTl, W.sub.2Zr, Nb.sub.3Sn, RhTa.sub.3, IrTl, IrNb.sub.2, YBl, Cr.sub.2Ta, Be.sub.13Zr, UBe.sub.13, Al.sub.2Mo, Rh.sub.3Ta, RuZr, IrNb.sub.3, IrTa, IrNb.sub.3, Mo.sub.3Al, GeMo.sub.5, ZrGe, Zr.sub.2Ge, Ir.sub.3Tl, Re.sub.3Mo.sub.2, OsTa.sub.3, Re.sub.3W.sub.2; silicides such as, Nb.sub.5Si.sub.3, Hf.sub.3Si.sub.2, W.sub.5Si.sub.3, Zr.sub.5Si.sub.3, TaSi2, HfSi, Mo.sub.3Si.sub.2, WSi.sub.2, Ti.sub.5Si.sub.3, Mo.sub.5Si, MoSi.sub.2, ZrSi, Zr.sub.3Si.sub.2, V.sub.5Si.sub.3, Zr.sub.2Si, Zr.sub.4Si.sub.3, Zr.sub.6Si.sub.5, Hf.sub.5Si.sub.3, Ta.sub.2Si, and Ta.sub.5Si.sub.3; silicates such as Mg.sub.2SiO.sub.4, Ca.sub.2SiO.sub.4, BaSiO.sub.4, SaSiO.sub.4, ZrSiO.sub.3, ZrSiO.sub.4, and SrSiO.sub.4; oxides, such as MgV.sub.2O.sub.4, Nd.sub.4SrO3, Ca.sub.3TtO.sub.5, MgAl.sub.2O.sub.4, MgZrO.sub.3, Be.sub.3Zr.sub.2O.sub.7, Eu.sub.2O.sub.3, CaCrO.sub.4, Gd.sub.2O.sub.3, BeZr.sub.2O.sub.3, BaThO, La.sub.2O.sub.3, Sn.sub.2O, Y.sub.2O.sub.3, Yb.sub.2O.sub.3, LaHfO.sub.3, LaCrO.sub.3, Ce.sub.2O.sub.3, BaZrO.sub.3, SrZrO.sub.3, Zr).sub.2Eu.sub.2O.sub.3, SrHfO.sub.3, SrZrO.sub.3, HfO.sub.2, ThZrO.sub.4, ThO.sub.2, UO.sub.2, MgO, ZrO.sub.2, Sr.sub.4Zr.sub.3O.sub.10, CaO, BeO, Sc2O.sub.3, CeCr.sub.2O.sub.5, SrO, DyO.sub.2, Dy.sub.2O.sub.3, CaZrO.sub.3, Cr.sub.2O.sub.3, PuO.sub.2, Pu2O3, CaCr.sub.2O.sub.4, NiAl.sub.2O.sub.4, Al.sub.2O.sub.3, La.sub.2MgO.sub.3, Al.sub.2BaO.sub.4, Al.sub.2NiO.sub.4, Cr.sub.2MgO.sub.4, Al.sub.2NiO.sub.4, SiO, HfO, SiO, TiO, and Al.sub.2SrO.sub.4; carbides, such as metal carbides, including BoC.sub.2, Ni.sub.3C, GdC.sub.2 , Be.sub.2C, YC.sub.2, Co.sub.2C, UC, BC, Ce.sub.4C, Al.sub.4C.sub.3, MoC, Mo.sub.2C, SiC, VC, WC, NB.sub.2C, TiC, W.sub.2C, THC.sub.2, THC, PrC.sub.2, U.sub.2C.sub.3, LaC.sub.2, LaC, UC.sub.2, Co.sub.3C, CaC.sub.2, SnC.sub.2, NdC.sub.2, V.sub.2C, La.sub.2C.sub.3, HfC; MAX phase type high temperature compounds, and combinations thereof.

(15) A binder 16 is shown in the spaces between adjacent platelets 14. In an example embodiment, the binder 16 (that may also be referred to as a mortar) is relatively softer than the platelets 14. Optionally, the binder 16 is made up of a binder resin and particulate matter for setting the spacing between the platelets 14. As will be discussed in more detail below, the binder 16 of the present disclosure may include additional additives. In an example embodiment, the platelets 14 can have a Young's modulus of around 510.sup.6 to 6010.sup.6 pounds per square inch; the Young's modulus of the binder 16 may range from about 0.1% to about 10% of the Young's modulus of the platelets 14. The binder 16 provides flexibility and toughness to the EPC 10. The composition of the binder 16 will be explained in more detail in a subsequent section.

(16) Referring now to FIGS. 3 and 3A, an advantage of the present disclosure is illustrated by how the staggered lamellar arrangement of platelets 14 within the EPC 10 resists oxygen O.sub.2 migration to the surface 12 by forming a tortuous path 20 for the oxygen, O.sub.2, flowing within the binder 16 and between the platelets 14. Moreover, as shown in FIGS. 3 and 3A, resistance to migration introduced by the tortuous path 20 is further enhanced by disposing oxidizable matter within the binder 16. The oxidizable matter can be made up of refractory metals, intermetallics, metals such as Al, Si, Ti, Ni, Zn, Mg, or un-oxidized constituent materials for use in forming the platelets 14 listed above. When contacted and oxidized by the migrating O.sub.2, the volume of the oxidizable matter increases and may possess fluxing abilities expanding, wetting and sealing cracks formed by thermal expansion and thermal volatization of lower temperature refractories, thereby resisting O.sub.2 flow through the binder 16. The oxidizable matter can be provided in the binder 16 as particulates, solid solutions, or as a coating on another particulate. In an example embodiment, the oxidizable matter in the binder 16 oxides to form glass oxides 22 when contacted by the migrating oxygen O.sub.2. As noted above, the oxides 22 can fill cracks, interstices, and voids in the binder 16 so that the binder 16 becomes a barrier to oxygen O.sub.2 flow; making it more difficult for the oxygen O.sub.2 to navigate through the binder 16, thereby protecting the surface 12 from oxidation.

(17) The production of oxides 22 by oxidizable elements of EPC 10, phase changes, and thermal expansion of base 12, EPC 10, platelets 14 and binder 16, at operating temperature increase their respective volumes, which can potentially lead to the EPC 10 failing due to differential volume increase and resulting strain buildup. To allow for the increase in volume and provide strain release, the binder 16 includes porous or easily cleaved particulates 42 (FIG. 4) of refractory materials. The particulates 42 (FIG. 4) provide the binder 16 of the EPC 10 with low stress, high strain free volume necessary to accommodate the increased volume due to the oxide growth 20 and thermal strains produced. In an example, high strain deflection is greater parallel to the elongated sides of the platelet 14 to provide maximum strain release in-plane. The platelets 14 together with the micro-structure of the binder 16 result in a tough EPC 10 that can protect a surface 12 from ultra high temperatures and oxidation and provide the flexibility and free volume to accommodate oxide formation and thermal expansion mismatches. In an example embodiment, the free volume locations in the binder 16 are randomly and/or irregularly spaced to thereby introduce multiple degrees of freedom within the EPC 10. The free volume can absorb strain in any direction, thereby correspondingly reducing or eliminating stress (in any direction) in the EPC 10 that might result from the strain.

(18) The EPC 10 may be manufactured using various methods. One embodiment of the manufacturing process to produce the EPC 10 is illustrated in FIGS. 4-6. As shown in FIG. 4, the binder material 16 (FIG. 1) can be formulated by utilizing a skeleton or scaffold 40 made from carbon, lower temperature materials of the same candidates for the oxidizable matter within the binder 16, or other volatizable material. The skeleton 40 may be a hollow member made up of particulate matter, preferably with a high aspect cross-section shape for low stiffness in one or more axes and with short length L to depth D (L/D) ratio that has a size in the sub-micron range to allow better processing, as shown in FIG. 4 or alternatively FIGS. 9 and 10. In an example embodiment, the L/D ratio can range from about 1 to about 10, in one embodiment the L/D ratio can range from about 2 to about 5. Once the desired skeleton 40 is provided, the skeleton 40 may be coated with a layer of refractory metal 42, refractory oxide, or their precursors, such as pre-ceramic polymer or vapor deposited components. The refractory layer 42 can then be oxidized while the skeleton 40 is vaporized slowly and allowed to permeate through the refractory layer 42 such that it is removed from the system without damage to the coating. The result is a refractory oxide shell 43 that takes on the general shape of the removed skeleton 40 and that has a hollow space 44 within, as shown in FIG. 5. The hollow space 44 in the shell 43 provides free internal volume in the binder 16 (FIG. 1) that will allow for strain release associated with oxide 20 (FIG. 3) production during operation.

(19) The high strain constituents may include one or two dimensional semi-crystalline or crystalline compounds, fractal morphologies, or constructions of stable and fugitive chemistries, and/or combinations thereof. The materials possess easily strained bonds or free volume with multi-axial or random axial orientation. The free volume is produced during a pre-firing procedure before use which produces thermal shrinkage upon cooling, cleavage of the weak bonds and the free volume desired. Other fugitive components increase the generation of free volume utilized by these cleaved constituents to produce a highly compliant micro-structure. Graphite, zirconia diboride(gr), boron nitride(gr), mica and acicular wollastonite and zirconium mullite are examples of suitable crystalline materials. High intensity ball milling of C(gr) is an example of mechanical forming of sub-micron or nano-platelet compositions with minimal bonding and fugitive character in oxidizing environment. Eutectics can produce very fine, fractal morphologies, some of these compositions have ultra high temperature ceramics and an oxidatively fugitive phase such as a MoZrC system. These constituents produce short range, strain capability as-made or as-pyrolyzed or oxidized, that can be randomly oriented as required to yield planar quasi-isotropic compliance.

(20) Referring to FIGS. 6 and 6A, the refractory oxide, high aspect ratio shells 43 can then be compounded with a mixture 46 that may include short fibers 48, ceramic mortar 50, sub-micron refractory metal powder 52, and fluxing elements 54. The short fibers 48 may be formed from ceramics or their precursors and have a length/depth ratio less than 20, and optionally less than 10. The ceramic mortar 50 can be made from ceramic binder material; examples of the sub-micron refractory metal powder 52 include Al, Hf, Si, Zr, Ta, Mg, and Ca. The sub-micron refractory metal powder 52 gives the binder 16 (FIG. 1) sealing capability against hot environmental gases via volume expanding oxide formation. The fluxing elements 54 may be anything that induces chemical activity during expected operational conditions, examples of fluxing elements 54 include boron, carbon, silicon, aluminum, titanium, tantalum, phosphorus, or others. Optionally, the fluxing element 54 may include the same candidate materials for the oxidizable matter within the binder 16. In an example embodiment, the fluxing elements 54 are disposed within one or more of the shells 43 and intermixed within the binder 16. In an example of use of this embodiment, the fluxing elements 54 are exposed to temperatures greater than at which they become active or mobile, thereby causing softening and chemical reactions through the binder 16. As such, the fluxing elements 54 are used to enhance wetting, adhesion and sealing of the EPC 10. The mixture 46 is mixed in a ball milled together with the refractory oxide shells 43 into a smooth paste that has the capability of spacing the platelets 14 (FIG. 1) from about 5% to about 100% of the platelet 14 thickness. In an example embodiment the constituent materials 48, 50, 52, 54 making up the paste have a particulate length approximately 50% to about 100% that of the platelets 14 spacing. The resulting paste forms the binder (mortar) material 16 used to hold together the platelets 14.

(21) Once the binder 16 is prepared, the platelets 14 are added in and mixed thoroughly to wet all surfaces of the platelets 14 with binder 16. Solvents or fugitive resins may be used as necessary to add processability to the fully formulated EPC 10 thereby forming a paste. The EPC 10 paste is applied to the surface 12 (FIG. 1), dried at a rate to minimize gas bubble formation, and is pre-fired to remove low temperature additives and fluxes. The EPC 10 may be fired to near anticipated operating temperature at rates that allow off-gassing of volatile compounds and impurities to proceed by molecular diffusion. The firing temperature can range from 50% to about 100% of expected operating temperature, but will depend on chemistry of the constituents. The platelets may have various geometries. For example, the platelets may be flat plates 60 arranged as shown in FIG. 7 or disks 70 as shown in FIG. 8. Each application of EPC 10 to a surface 12 may have a thickness of from about 0.05 millimeters up to about 0.5 millimeters.

(22) Applying multiple layers of the EPC 10 to the surface 12 (FIG. 1) can provide flexibility in matching performance with materials and allows a certain amount of redundancy for geometry controlled heating. Aero-surfaces geometries and impinging airflow vary significantly, as does the resultant equilibrium temperature due to aero-thermal heating. The substrate is typically a single material with a single maximum use temperature. If no appropriate substrate material is available or a lower temperature material has superior performance, extra EPC thickness can reduce the exposure of the surface 12 to allow its use, especially for sharp radii leading edges. If a single chemistry is not compatible with the increased temperatures and the surface 12 matrix, multiple EPC formulation may be applied in layers to vary the properties to the local environment and improve compatibility. Thus, a family of compatible EPCs 10 may be used to match the maximum temperature of varying geometry and airflow to allow lower temperature materials or severe geometric features. In addition, as EPC 10 recession occurs, a benign failure mode may be achieved as single EPC layer failure does not immediately expose the surface 12. As such, a color-coded system may be used to provide an early warning of degradation. For example, different regions of a blade or hypersonic leading edge on an aircraft wing can have a wide range of temperatures that increases as the radii decreases. The design of EPC system can thus be selected to match the operational temperature due to the radii and airflow and also achieve a benign and inspectable failure mode.

(23) In an example embodiment, the EPC 10 described herein has the high barrier properties of a nacreous or lamellar composite, the toughness of a soft, ceramic binder 16 reinforced with the stiff, strong platelet 14 and the sealing capabilities of a chemical composition which forms stable, viscous oxides 22, with a final volume greater than the original coating constituent to ensure crack sealing. The EPC 10 may be used to retrofit existing applications and repair damaged units. The EPC 10 may be applied to a surface in various ways. For example, the EPC 10 may be applied as a paste or diluted and sprayed onto a surface. A sheet or film of the EPC 10 may also be fabricated that can be adhered to a surface.

(24) Although the present invention has been described in detail herein with reference to the illustrative embodiments, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments of this invention and additional embodiments of this invention will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the spirit and true scope of this invention as claimed below.