FABRICATION OF MULTILAYER CERAMIC STRUCTURES BY CONTINUOUS FILAMENTS OF DIFFERENT COMPOSITION

Abstract

A method for constructing multiple ceramic layers by winding continuous ceramic filaments of different compositions to prepare multilayer RF-transparent structures is provided. In the method, different continuous ceramic filaments are braided to construct layers with specific dielectric constants and braiding count/thickness. Layers with same or different dielectric characteristics forms a sandwich design to fulfill the desired mechanical, thermal and electrical requirements.

Claims

1. A method for preparing a multilayer ceramic structures, comprising: winding continuous ceramic fibers of different compositions to construct layers, wherein each of the layers has a with specific dielectric constant, wherein each layer is designed to have a specific winding angle, a wound fiber thickness and space between the continuous ceramic fibers to guarantee a desired broadband RF response of an object, the continuous ceramic fibers are wound in an angular orientation between 15°-135° to optimize a structural integrity and a RF performance; forming the multilayer ceramic structure comprising multiple layers with different dielectric characteristics as a sandwich; applying a slurry infiltration over the multilayer ceramic structure constructed to fill inter-fiber gaps and obtain a finished structure, wherein the inter-fiber gaps are open spaces between the continuous ceramic fibers.

2. The method according to claim 1, further comprising: coating a support surface with a non-sticking chemical prior to the step of braiding to facilitate a removal of the layers braided.

3. The method according to claim 1, wherein the slurry infiltration is conducted under vacuum to move a slurry with optimized rheology into the inter-fiber gaps.

4. The method according to claim 3, wherein in the slurry infiltration, the multilayer ceramic structure is placed in a chamber or a container filled with the slurry or placed between and supported by a female mold and a male mold, wherein the female mold and the male mold are made of stainless steel with non-sticking surfaces, and the female mold and the male mold are fed with the slurry under pressure.

5. (canceled)

6. The method according to claim 1, wherein the multilayer ceramic structure is applicable to build broad, narrow, and single band missile radomes.

7. The method according to claim 1, wherein the finished structure is near net-shape, minimizing a post-processing time and a related product damages or a related product loss.

8. The method according to claim 1, wherein the winding step comprises wrapping and braiding techniques.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 shows sandwich structures using different fibers, where the dielectric constant ε.sub.f_h>ε.sub.f_l.

[0027] FIG. 2 shows sandwich structures using different fabrics, where the dielectric constant ε.sub.f_h>ε.sub.f_l.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The fiber-reinforced ceramic matrix composites (FR-CMC) are advanced and tailorable materials with improved toughness and damage tolerance compared to bulk ceramics. Broadly speaking, the reinforcing fibers can be classified as inorganic and organic fibers. The inorganic fibers can be further splitted to non-metallic and metallic fibers, while the organic fibers are mostly carbon and polymer fibers. The ceramic fibers belong to the non-metallic inorganic fibers family together with the glass/mineral and single crystal fibers.

[0029] The fiber material selection for CMC application is of paramount importance. It is known that the temperatures on radome material during high Mach number flights reach up to 1.000° C. limiting the choice of the fiber material. Polymer and glass fibers have 500° C. and 700° C. of degradation temperatures, respectively, which restrict their effective use in CMCs at higher temperatures. Therefore, the ceramic fibers come out as the right choice to support the high performance CMCs for airborne components exposed to elevated temperatures and mechanical/thermomechanical loads at higher speeds.

[0030] The ceramic fibers are classified as oxide or non-oxide ceramics. The ones in the former group are alumina (Al.sub.2O.sub.3) based fibers exhibiting high environmental stability but limited high temperature creep performance. The alumina composition of such fibers can be selected in a range from 10% to 100%. The non-oxide ceramic fibers are mostly SiC, which have excellent thermal creep behavior coupled with poor chemical stability. SiC portion of these fibers can vary in the 10% to 100% range depending on the operational specifications. For both fiber classes, crystallinity, morphology, uniformity along the material and the surface properties are important characteristics impacting the CMC performance in the field. Fiber coating is another critical factor determining the damage tolerance of the structure by providing a weak interface between the fiber and the matrix.

[0031] The selection between the two fiber types strongly depends on the type of the matrix or the inorganic resin filling the fiber network. The oxide fibers should ideally be used with the oxide matrix (oxide composite) and the non-oxide with the non-oxide matrix (non-oxide composite). However, intermediate mixtures are also prepared by different processing techniques, which lead to newer applications.

[0032] As for oxide composites, the fibers prepared with pure Al.sub.2O.sub.3 or Al.sub.2O.sub.3 blended with SiO.sub.2 and B.sub.2O.sub.3 at lower concentrations significantly increase the oxidation and the alkaline resistance of CMC. For non-oxide composites, SiC fibers coated with C or BN allow the SiC matrix composite resist high temperature deformation. The comparison between the fiber and the bulk forms of the Al.sub.2O.sub.3 and SiC ceramics are presented in Table 1. The significantly superior tensile strength of the fiber over the bulk is worth mentioning for consideration of these fibers under severe environmental conditions.

TABLE-US-00001 TABLE 1 Comparison of ceramic fiber vs. bulk ceramic properties Al.sub.2O.sub.3 SiC Material Properties Unit Fiber.sup.a Bulk.sup.b Fiber.sup.c Bulk.sup.d Density g/cm.sup.3 3.90 3.90 3.10 3.20 Tensile Strength MPa 2.930 400 2.600 540 Elastic Modulus GPa 373 380 420 430 CTE (40-800° C.) ppm/° C. 8.00 8.00 3.00-3.50 3.70-4.40 Continuous Use Temperature ° C. 1.000* ~1.000.sup.+ 1.150** ~1.000.sup.+ .sup.aNextel 610, .sup.bKyocera A601D (>99%) .sup.cNippon Carbon Hi-Nicalon “S” (99.8%), .sup.dKyocera SC211 *Single filament ≤ 1% strain/69 MPa/1.000 hr **Single filament 500 MPa/1.000 hr .sup.+estimated

[0033] To sum up, the ceramic fibers provide toughness while improving the damage tolerance of the bulk ceramics. The super/hypersonic missile radomes produced as bulk ceramics from materials such as fused silica, Magnesium Aluminum Silicate, Lithium Aluminum Silicate, Si.sub.3N.sub.4, SiAlON, Al.sub.2O.sub.3 run the risk of catastrophic failure under extreme conditions due to their fragile nature. The techniques used in production of these ceramics such as slip casting, glass melt casting, hot molding have low yields due to the fracture of the ceramic during consolidation, drying, firing and machining steps.

[0034] The focus of this patent is the method by which the ceramic fiber-reinforced CMCs are prepared. By following this method, the ceramic fibers and the inorganic resins compatible with these fibers can be used to prepare the airborne structures such as radomes, microwave-transparent shields, caps and noses for military and civil applications flying at subsonic, supersonic and hypersonic velocities. There is no restriction in combination of available fibers and resins as long as the materials compatibility and the RF-transparency at desired frequencies are fulfilled. Moreover, the method is applicable to build both broad, narrow and single band radomes. The type and the diameter of the fiber, braiding type, fiber aperture and thickness per layer, slurry material composition are engineered for the desired electromagnetic performance. Moreover, significant improvement can be achieved in the structural integrity if the ceramic fibers at consecutive layers are wrapped in an angular orientation between 15°-135°.

[0035] In this invention, the continuous ceramic fibers are used to form the multiple layers of the broadband radome. The dielectric constant of each layer is identified by the fiber material, its thickness and the slurry material. The broadband characteristic of the radome can be optimized by changing such individual layers varying in low and high dielectric constant. The concept and the procedure of layer build up is presented in FIG. 1 and explained as follows: [0036] For A-type sandwich: The 1st (innermost) and 3.sup.rd (outermost) layers can be constructed by ceramic fibers of higher dielectric constant with lower wrap count (ε.sub.f_h), whilst the fiber in the middle is of low dielectric constant material with higher wrap count (ε.sub.f_l). [0037] For B-type sandwich: The 1st (innermost) and 3.sup.rd (outermost) layers can be constructed by ceramic fibers of lower dielectric constant with higher wrap count (ε.sub.f_l), whilst the fiber in the middle is of high dielectric constant material with lower wrap count (ε.sub.f_h). [0038] For C-type sandwich: The 1.sup.st, 3.sup.rd and 5.sup.th layers can be constructed by ceramic fibers of higher dielectric constant with lower wrap count (ε.sub.f_h), whilst the 2nd and 4.sup.th fibers are of low dielectric constant material with higher wrap count (ε.sub.f_l). [0039] For D-sandwich: The 1.sup.st, 3.sup.rd and 5.sup.th layers can be constructed by ceramic fibers of lower dielectric constant with higher wrap count (ε.sub.f_l), whilst the 2nd and 4.sup.th fibers are of high dielectric constant material with lower wrap count (ε.sub.f_h).

[0040] Alternatively, the ceramic fabrics can also be used to construct sandwich ceramic structures as an alternative to the fibers. The fabrics are wider than fibers and hence, they accelerate the fabrication process. Should the fabrics replace the fibers, the layers can be constructed in different design alternatives as shown in FIG. 2.

[0041] Once the continuous fibers are wrapped on the mandrel and all layers of the structure satisfying the desired broadband performance are piled up, the structure is removed from the mandrel. It is basically a basket formed by an intense fiber network braided according to a specific design, which is ready for infiltration. The slurry infiltration is the process during which the slurry fills the inter-fiber gaps. This process can best be conducted under vacuum, where the fiber basket is placed in a special chamber filled with the slurry. In an embodiment, prior to braiding on it, a support surface (e.g., mandrel) is coated with a non-sticking chemical to facilitate the easy removal of the braided structure at the end of the process.

[0042] As a second alternative, the basket can be placed between and supported by female and male molds made of stainless steel with non-sticking surfaces, which are fed with the slurry. In both methods, the vacuum is applied to the closed chamber or molds, which moves the slurry with optimized rheology into the open space between the fibers.

[0043] In a different method, the basket can be dipped in a container full of thick slurry. The structure is then exposed to vacuum from the opposite side without slurry (inner side), which pulls the slurry into the apertures between the fibers.

[0044] In all of these methods, the integrity of the fiber structure must be observed carefully and preserved intact against a possible deformation caused by vacuum. As further processing, machining of the fired structures can also be considered and applied with no detrimental effects on the structure as the fibers follow the contour defined by the matrix.

[0045] The slurry infiltrated fiber network is dried and debinded cautiously. Since all thermal process have the potential to generate irreversible impacts on the structure such as crack initiation and propagation, fracture, sagging, bulging, collapsing, the debinding and sintering profiles must be carefully optimized. Therefore, the raw materials must be carefully characterized in terms of their compositions and rheological and thermo-mechanical behavior prior to processing.

[0046] The described invention is applicable for continuous oxide/non-oxide fibers and the slurries compatible with these fibers. In other words, the fiber-slurry pair has to be defined together to guarantee the materials' compatibility and the performance of the final structure. The fibers should have a sintering temperature comparable to the temperature stability range of the matrix, low CTE, low dielectric constant and loss and high thermo-stability and mechanical strength. Moreover, these characteristics are expected to be preserved/slightly deviate with temperature fluctuations. Most of these requirements can be optimized in the fused silica system, which is used in commercial missile radomes for decades. Therefore, PDC-based slurries with polysilicone, polysilozane, polycarbosilane are candidate slurries to use with selected fibers. Alternatively, slurries with materials such as alumina at varying compositions can also be used as long as the aforementioned fiber-slurry specs are matched.

[0047] The fiber selection for current radome materials such as fused silica, Magnesium Aluminum Silicate, Lithium Aluminum Silicate, Si.sub.3N.sub.4, SiAlON, Al.sub.2O.sub.3 is limited. Among all commercial products, Al.sub.2O.sub.3 and SiC are the commercially-available candidates for oxide and non-oxide fibers, respectively. The former is produced in different compositions to address the requirements in diverse applications, whilst the latter is not fully appropriate as a radome material due to its reported semi-conductive character at high temperatures.