Ceramic matrix composites (CMC) reinforced by SiC fibers, such as SiC/SiC, are being targeted for application in hot-section components of advanced propulsion and power generation engines and in first walls of advanced nuclear systems. Two "Super Sylramic" SiC fiber types, recently developed at NASA using the Sylramic fiber from COI Ceramics, are candidates for providing these components with improved thermal capability and improved performance. This paper reports on the ability of these new fiber types, Super Sylramic-iBN and Super Sylramic-SiC, to meet the key fiber requirements of these applications: high strength, high creep-rupture resistance, and high thermal conductivity. For example, creep-rupture tests performed at 1300 to 1450°C show that the creep resistance of these fibers is ∼20 and ∼7 greater than the current Sylramic and Sylramic-iBN fiber types, respectively, that have already been used to demonstrate state-of-the-art SiC/SiC composites. TEM and AES microscopic observations are presented to indicate that these improvements can be correlated with the replacement of weak grain boundary phases with stronger phases that hinder grain boundary sliding more effectively. Preliminary SiC/SiC composite results are also provided for the Super Sylramic fiber types.
In order to better understand SiC fiber behavior within CMC microstructures, mechanical tests were performed on different types of as-produced and CVI BN coated single-ply 0/90° woven fabrics. Tensile strength and creep-rupture properties were measured on single-ply fabrics at various temperatures in air for the following types of polymer-derived SiC fibers: Hi-Nicalon, Hi-Nicalon Type-S, Tyranno SA, Sylramic, and a developmental Sylramic. For each fiber type, room temperature tensile strength for resin-impregnated as-produced fabrics and for dry BN-coated fabrics were found to be in agreement with each other and with bundle theory based on previously measured results for single fibers and tows. Exposures of the fabrics to simulated CMC process conditions typically degraded fabric room-temperature strengths, particularly for those that were initially BN coated. High temperature creep properties for as-produced fabrics were also in general agreement with single fiber and single tow data, However, high-temperature fast-fracture and rupture properties were typically worse than those of single fibers and tows tested under the same conditions. The underlying mechanisms and CMC implications of the SiC-fiber fabric results are discussed, as well as the benefits of the fabric test over single fiber and tow testing.
The matrix cracking behavior of slurry cast melt‐infiltrated SiC matrix composites consisting of Sylramic‐iBN fibers with a wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in‐plane matrix cracking during unload–reload tensile tests. Two key parameters were found to control matrix‐cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed.
In the last decade, considerable progress has been made in the development and application of ceramic matrix composites consisting of silicon carbide (SiC) based matrices reinforced by small-diameter, continuous-length SiC-based fibers. For example, these SiC SiC composites are now in the early stages of implementation into hot-section components of civil aero-propulsion gas turbine engines, where in comparison to current metallic components they offer multiple advantages due to their lighter weight and higher temperature structural capability. For current production-ready SiC SiC, this temperature capability for long time structural applications is ∼1250°C, which is better than ∼1100°C for the best metallic superalloys. Foreseeing that even higher structural reliability and temperature capability would continue to increase the advantages of SiC SiC composites, progress in recent years has also been made at NASA toward improving the properties of SiC SiC composites by optimizing the various constituent materials and geometries within composite microstructures. The primary objective of this chapter is to detail this latter progress, both fundamentally and practically, with particular emphasis on recent advancements in materials and processes for the fiber, fiber coating, fiber architecture, and matrix, and in the design methods for incorporating these constituents into SiC SiC microstructures with improved thermo-structural performance.
For long term structural service, the upper temperature capability for slurry-cast melt infiltrated (MI) SiC/SiC composites is limited to approx. 1315 C because of silicon reaction with the SiC fibers. For applications requiring material temperatures in excess of 1315 C, alternate methods of manufacturing the SiC matrices without silicon are being investigated, such as a hybrid combination of CVI and PIP. In this study, stacked fabric plies of Sylramic i-BN SiC fibers were coated with a CVI BN interface layer followed by a partial CVI SiC matrix. The remaining porosity in the SiC/SiC preforms was then infiltrated with silicon carbide matrix by PIP. Thermo-mechanical property measurements indicate that these composites are stable to 1700 C in inert environments under no load conditions for 100 h and under load conditions to 1450 C in air for 300 h. The advantages, disadvantages, and potential of this composite system for high temperature applications will be discussed.
Objective: Describe and up-date progress for NASA's efforts to develop 3D architectural design tools for CMC in general and for SIC/SiC composites in particular. Describe past and current sequential work efforts aimed at: Understanding key fiber and tow physical characteristics in conventional 2D and 3D woven architectures as revealed by microstructures in the literature. Developing an Excel program for down-selecting and predicting key geometric properties and resulting key fiber-controlled properties for various conventional 3D architectures. Developing a software tool for accurately visualizing all the key geometric details of conventional 3D architectures. Validating tools by visualizing and predicting the Internal geometry and key mechanical properties of a NASA SIC/SIC panel with a 3D orthogonal architecture. Applying the predictive and visualization tools toward advanced 3D orthogonal SiC/SIC composites, and combining them into a user-friendly software program.
Evaluating the damping of reinforcement fibers is important for understanding their microstructures and the vibrational response of their structural composites. In this study the damping capacities of two types of chemically vapor deposited silicon carbide fibers were measured from -200 C to as high as 800 C. Measurements were made at frequencies in the range 50 to 15000 Hz on single cantilevered fibers. At least four sources were identified which contribute to fiber damping, the most significant being thermoelastic damping and grain boundary sliding. The mechanisms controlling all sources and their potential influence on fiber and composite performance are discussed.
