BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to thermal oxidative barrier coatings for organic matrix composites and to articles so coated. 
         [0002]    Organic matrix composites (OMCs) are used in the aerospace industry for the weight reductions they offer when used to replace metal components. However, exposure to high temperature environments reduces mechanical properties and causes oxidative degradation of OMCs. Thus, even currently known high temperature OMCs utilizing high temperature matrix materials, such as PMR-15 and AFR-PE-4, have limited application. 
         [0003]    One attempt to combat the problems in the art is to build thicker parts. However, the increased thickness adds weight and cost to the component as compared to what could be achieved if thermal and oxidative effects on the component were reduced. 
         [0004]    Another attempt utilizes a sacrificial layer on the component to retard material degradation. The sacrificial layer may be a thin carbon veil impregnated with the PMC resin. However, the protection provided by the sacrificial layer is lost over time. 
         [0005]    Currently, there are investigations into the use of ceramic fillers carried in polyimide matrices applied as a thermally sprayed coating for OMC components. The coating purports to improve the environmental durability and erosion resistance of the organic matrix composites. However, the thermal spraying process raises environmental, health, safety, energy, and labor issues. Additionally, it is difficult to provide a fully-cured coating system during a thermal spraying deposition process. 
         [0006]    Accordingly, it would be desirable to improve the high temperature performance of components comprising organic matrix composites by providing a coating system that improves thermal oxidative stability and mechanical performance. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The above-mentioned needs may be met by exemplary embodiments that provide a thermal oxidative barrier coating for organic matrix composite structures. Thus, coated structures formed of high temperature OMC materials could be used, for example, as replacements for metallic components in environments having temperatures greater than the maximum operating temperatures of the unmodified high temperature OMC materials. In other exemplary embodiments, coated structures formed of lower temperature OMC materials could be used in environments having temperatures greater than the maximum operating temperatures of the unmodified lower temperature OMC materials. 
         [0008]    In an exemplary embodiment there is provided a thermal oxidative barrier coating for an organic matrix composite substrate. The coating includes a bond coat disposed on at least a first surface of the substrate, and at least one thermal barrier layer substantially overlying the bond coat. The bond coat includes nano-particles carried in a polyimide matrix. The at least one thermal barrier layer comprises a silsesquioxane or an inorganic polymer. 
         [0009]    In an exemplary embodiment, an article comprising an organic matrix composite substrate and a thermal oxidative barrier coating on at least one surface of the substrate is provided. The thermal oxidative barrier coating includes a bond coat and at least one thermal barrier layer. The bond coat comprises nano-particles carried in a polyimide matrix and the at least one thermal barrier layer comprises a silsesquioxane or an inorganic polymer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0011]      FIG. 1  illustrates a portion of a component including a substrate having a thermal oxidative barrier coating thereon. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Referring to the drawing,  FIG. 1  shows a component  10  for particular use in high temperature environments such as a gas turbine engine, although other applications are contemplated within the scope of the invention. Component  10  includes a substrate  12  and a thermal oxidative barrier coating  14  on at least a first surface  16 . First surface  16  is situated on the “hot side”  18  of component  10 . The service temperature on the hot side  18  of component  10  may be up to about 725° F. (385° C.). 
         [0013]    An exemplary embodiment contemplates the use of a thermal oxidative barrier coating for high temperature OMCs in turbine engine applications. The thermal oxidative barrier coating can be applied to at least the hot side of a composite part to reduce the maximum temperature exposure of the underlying substrate and form a barrier to oxidation of the structural composite matrix. An exemplary application of a thermal oxidative barrier coating is for ducts for defining various flow paths in the engine. 
         [0014]    Thermal protection systems in the form of thermal barrier coatings (TBCs) have been used with metals for many years. In such cases, low thermal conductivity materials are coated on the surface of the part to create a thermal gradient between the service environment and the part such that the subsurface material is not exposed to a temperature above its maximum use temperature. However, OMCs present features and challenges that are separate and unique from metallic substrates. Thus, the coatings disclosed herein are termed “thermal oxidative barrier coatings” to distinguish them from thermal barrier coatings used for metallic substrates. 
         [0015]    In an exemplary embodiment, the OMC matrix material is a high temperature polyimide system such as AFR-700B, PETI-375, PMR-II-50, HFPE, AFR-PE-4, and PMR-15. However, the disclosed thermal oxidative barrier coatings may be utilized with lower temperature resin systems such as bismaleimide-based polyimide systems (BMI) (e.g., Cycom® 5250-4), which typically offer lower cost and greater ease of processing as compared to the higher temperature polyimide systems. Application of the thermal oxidative barrier coating could allow the use of lower temperature systems in higher temperature environments than previously attainable. 
         [0016]    The thermal oxidative barrier coating  14  may include an outer thermal barrier layer  22  and a bond coat  24 . In addition to bonding the outer thermal barrier layer  22 , the bond coat  24  may additionally function as an oxidation barrier. In an exemplary embodiment, because the bond coat  24  is protected by the thermal barrier layer  22 , the polymer matrix of the bond coat  24  may be the same as, or similar to, the polymer matrix of the substrate  12 . 
         [0017]    In an exemplary embodiment, the materials contemplated for use as the thermal barrier layer  22  are evaluated for thermal conductivity, coefficient of thermal expansion (CTE), thermal stability measured as a function of weight loss, specific gravity, and flexural strength and modulus. In an exemplary embodiment, it is desired to minimize the difference between the CTE of substrate  12  and the CTE of the thermal barrier layer  22 . For example, the CTE of the OMC substrate may be in the range of about 1 ppm/° F. (1.8 ppm/° C.), while the CTE of exemplary thermal barrier layers may be in the range of about 3.5 to 6 ppm/° F. (6.3 to 10.8 ppm/° C.). In exemplary embodiments, the desired density of the thermal barrier layer  22  is equal to or less than the density of the OMC substrate  12 . However, the maximum allowable density is generally dependent on the thermal conductivity of the material. The thermal conductivity of the thermal barrier layer influences the thickness necessary to realize the required thermal benefit. 
         [0018]    In an exemplary embodiment, the coating thickness is sufficient to provide a reduced temperature at the substrate/coating interface  26  of at least 100° F. (56° C.). Thus, in an exemplary embodiment, if the service temperature is approximately 725° F. (385° C.), the temperature exposure at the substrate/coating interface  26  is approximately 625° F. (329° C.), or less. In an exemplary embodiment, the coating  14  comprises a thickness of about 0.030 inches (0.76 mm) to about 0.060 inches (1.5 mm). It is believed that coated organic matrix composite substrates may be used in higher service temperatures, i.e., greater than 725° F. (385° C.), using the methods and coatings disclosed herein. 
         [0019]    An exemplary thermal barrier layer  22  comprises one or more variations of a commercially available system known as Thermablock™ coating. The coating is a two-part silsesquioxane/titanate material developed as a high temperature coating by MicroPhase Coatings, Inc. Silsesquioxanes are represented by the generic formula (RSO 1.5 ) n  wherein each silicon atom is bound to an average of one and a half (sesqui) oxygen atoms and to one hydrocarbon group (ane). Silsesquioxanes can exist in the form of polycyclic oligomers, ladder, and linear polymers. Such coatings reportedly strongly adhere to various substrates including thermoset OMCs. The two-part coating system cures at 50° F. to 100° F. (10-38° C.). The material is resistant to acids and bases, and has a maximum continuous use temperature of 2000° F. (1093° C.). The CTE of the coating variations range from about 3.5 to 5 ppm/° F. (6.3 to 9 ppm/° C.) and have a thermal conductivity of as low as 0.15 W/m·K at 560° F. (293° C.). 
         [0020]    In other embodiments, an exemplary thermal barrier layer may comprise a developmental material known as Sialyte™ poly(sialate) material which is currently under development at Cornerstone Research Group, Inc. Poly(sialates) are one general class of inorganic polymers with the base structure of (—Si—O—Al—O—). The actual structure and properties of the poly(sialate) depend on the atomic ratio of Si to Al. The CTE is typically around 5 ppm/° F. (9 ppm/° C.) for the neat resin and is tailored by the addition of fillers. A fully cured and dried cast sample is able to withstand 1650° F. (899° C.) before significant loss of strength due to phase transformation. Published data for an unfilled poly(sialate) shows a thermal conductivity ranging from 0.2 to 0.4 W/m·K. 
         [0021]    In an exemplary embodiment, the bond coat  24  may comprise a polyimide matrix containing nano-particles. Exemplary nano-particles include polyhedral oligomeric silsesquioxane, graphite flake, and clay platelets. The respective amounts of polyimide and nano-particles are determined by factors such as processability, CTE, oxygen barrier capability, and bond strength. 
         [0022]    Two exemplary polyimide resins are uncrosslinked MVK-19, a fluorinated high thermal stability resin, and Kapton® polyimide, a high T g  thermoplastic polyimide. A first MVK-19 system includes an exfoliated nano-clay filler. A second MVK-19 system includes exfoliated graphite flake. The polyimide system includes a polyhegral oligomeric silsesquioxane nanofiller. The polyhedral oligomeric silsesquioxane is available from a premixed 15 wt % solution of poly(amic acid) and the polyhedral oligomeric silsesquioxane in N-methylpyrrolidone (NMP) which is commercially available from Hybrid Plastics™. Each of the three systems is optimized as a solution, then tested as a film, and finally tested with a selected thermal barrier layer material. 
         [0023]    Processability is measured as a function of the system&#39;s viscosity and uniformity of particle distribution. Viscosity versus temperature profiles are evaluated for coating processability. Filler dispersion is measured by various diffractometry and microscopy methods. CTE is measured via dilatometry over the temperature range of −65° F. to 800° F. (−53° C. to 426° C.). 
         [0024]    Resistance to oxygen penetration is measured via oxygen diffusivity measurements on films formed from the selected formulation. Coated OMC substrate samples are exposed to thermal oxidative environments for evaluation of thermal protection. For example, a thermal oxidative stability test includes placing samples in a chamber through which a constant flow of air travels at a rate sufficient to refresh the chamber volume at a rate of 5 times/hour. The test temperature, pressure, and time is chosen to result in a measurable degradation of unprotected OMC substrate samples. Oxygen barrier capability of the coating is determined by the weight loss of protected OMC substrates relative to unprotected substrates. Although the primary role of the bond coat  24  is to adhere the thermal barrier outer layer  22 , oxygen barrier capability is a secondary benefit. 
         [0025]    Bond strength is tested at room temperature and at elevated temperature. Due to the similarities in chemistry between the polyimides of the bond coat and the OMC substrate, and the dissimilar chemistry between the polyimides of the bond coat and the thermal barrier layer, initial bond strength evaluation focuses on the adhesion at the bond coat layer/thermal barrier layer interface. Bond strength is measured via flatwise tensile tests. 
       EXAMPLES 
       [0026]    Two candidate materials for the thermal barrier layer  22  are selected to be bonded to two OMC substrates  12  with three candidate bond coat materials  24 . The OMC substrates  12  include cured panels of AFR-PE-4 prepreg and BMI (Cycom® 5250-4) prepreg. These twelve combinations are subjected to thermal cycling to evaluate the bond coat/thermal barrier layer interface. Cracking or spalling of the thermal barrier layer is also evaluated during the thermal cycling. The thermal cycling is accomplished by rapidly heating to an isothermal maximum temperature (about 750° F. (398° C.)) and then rapidly cooling to room temperature. Flatwise tensile testing at room temperature of comparable samples as formed, and after thermal testing, is performed to measure the effect on bond strength. The selected bond coats are evaluated for thermal cycling performance, oxygen diffusion to the OMC, and protection of the OMC from thermal oxidative degradation. 
         [0027]    Panels of the twelve combinations are evaluated for isothermal oxidative aging effects on select mechanical properties. Mechanical properties of flexural strength and modulus are measured per ASTM C1161. 
         [0028]    Thermodynamic calculations, measured material properties, and oxidative aging analysis are used to determine the required thickness of bond coat  24  and the thermal barrier layer  22  so that the coating  14  achieves the desired performance level for specific service conditions. 
         [0029]    In exemplary embodiments, a nano-modified bond coat precursor is applied to the selected substrate as a liquid, followed by the application of an inorganic thermal barrier layer precursor as a liquid, molding compound, prepreg, or spray, with the method determined by the specific part to be protected. The prepreg may be supported, for example, with a non-woven veil or woven material such as quartz fabric. 
         [0030]    Thus, the exemplary thermal oxidative barrier coatings provide opportunities for utilization of organic composite matrix substrates in high temperature environments. 
         [0031]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
A thermal oxidative barrier coating for organic matrix composites includes a bond coat having nano-particles dispersed in a polyimide matrix and a thermal barrier layer comprising a silsesquioxane or an inorganic polymer. The nano-particles may include clay platelets, graphite flakes or a polyhedral oligomeric silsesquioxane. The coated article may be utilized in gas turbine engine applications, particularly for a flow path duct adapted for exposure to high temperature, oxygen-containing environments.