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Innovative Approaches in Design and Fabrication of 3D Braided Ceramic Matrix Composites (CMC) Fasteners


TECHNOLOGY AREA(S): Materials, Air Platform

OBJECTIVE: Develop technologies to design and fabricate 3D CMC fasteners for mechanically attaching CMC Propulsion and/or Structural components to metals.

DESCRIPTION: Ceramic Matrix Composites (CMCs) are attractive for propulsion applications due to their potential for higher temperature capability, weight reduction, and durability improvements. However, CMCs present design challenges due to their anisotropic properties, generally low interlaminar shear strength, low bearing strength, limited strain tolerance, and their reaction with alloy components at CMC/alloy interfaces [Ref 1]. Currently the prevalent mode of joining CMC to metals is to use metallic fasteners. Since the CMC component has lower Coefficient of Thermal Expansion (CTE), and bearing strength, it is desirable to use CMC fasteners for the attachments.The current baseline for a CMC fastener is the Miller fastener [Ref 2], which is a 2D, laminated CMC design with a rectangular cross-section. It is susceptible to stress concentrations at the corners and failure due to delamination. As such, it has not found wide acceptance in the industry. In this topic a 3D braided solution is sought which can be fabricated to near net-shape. This will eliminate delamination as a potential failure mode and reduce scrap during production.Proposers to this topic have to address two areas in their response. The first is to make a textile preform without over braiding over an insert as is typically done for non-uniform cross-section. Use of inserts is not desirable even if it is removed prior to consolidation as it may result in unacceptable porosity in the fastener during Pyrolysis and Infiltration Process (PIP) to consolidate the fastener.The second area that proposers must address is the interfacial coating of the 3D fastener preform. Typical fiber coating for SiC/SiC CMC is a two layer BN/Si3N4, it is usually applied through Chemical Vapor Deposition (CVD) or Atomic Layer Deposition (ALD). The coating protects the CMC fibers during high temperature CMC consolidation. Applying it to a 3D preform could pose a challenge due to the tortuous path the gases have to take through the preform and there is a risk some portions of the preform may not be coated. It is important that the proposers articulate their approach.Successful completion of the program will result in an enabling technology to join CMC components to metals. This technology is targeted for future platforms but can provide retrofit solutions for existing platforms.

PHASE I: Develop an innovative attachment concept for a realistic propulsion component that would benefit from a CMC application, but where a joining approach to an alloy is required as a critical enabler. Determine feasibility through analysis, fabrication, and testing of sub-element samples under thermal and mechanical environments.To evaluate the proposed technologies, the successful Phase I companies will fabricate SiC/SiC CMC fasteners that are 2 in. long, 0.375 in. diameter with a 45 degree countersink angle at the head. To prove feasibility the performers will provide: (1) Braid architecture design and estimate of the mechanical strengths, (2) evidence of successful coating through appropriate testing using SEM and/or FTIR, (3) evidence of matrix infiltration through porosity measurements, and (4) mechanical, wear, and recession data for fastener strengths under tension and shear which will be compared against their initial predictions. Finally, since CMC fasteners are not typically threaded, the performer will demonstrate using a flat CMC panel and metal plate the attachment scheme for securing the CMC panel to metal plate.

PHASE II: Design, fabricate, and test prototype samples to a specific component to thoroughly validate the capability of the approach. Test an engine component employing the joining methodology in a representative rig or engine environment to validate the approach.

PHASE III: Optimize the attachment/restraint methodology. Productionize and qualify the improved component.The commercial airline industry, military aircraft, aerospace industry, and high-performance automobile industry seek CMC joining as a critical enabler for reducing weight and increasing efficiencies.

KEYWORDS: Ceramic Matrix Composite (CMC), Joining, Fastening, Attachments, 3D preforming


1. Evans, Anthony G, ”Implementation Challenges for High-Temperature Composites.” International Science Lecture Series, Fifth Lecture. Washington, D.C.: National Academy Press. National Research Council (U.S.). Naval Studies Board, and United States. Office of Naval Research. 1997. 2. Miller, et al., “Composite Fastener for Use in High Temperature Environments”, US Patent 6045310, Apr 4 2000. 3. Kyosev, Y, “Advances in Braiding Technology: Specialized Techniques and Applications”, Elsevier, Cambridge, 2016

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