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.
NASA Glenn Research Center (GRC) is developing a variety of advanced SiC/SiC ceramic composite (ASC) systems that allow these materials to operate for hundreds of hours under stress in air at temperatures approaching 2700 F. These SiC/SiC composite systems are lightweight (approximately 30% metal density) and, in comparison to monolithic ceramics and carbon fiber-reinforced ceramic composites, are able to reliably retain their structural properties for long times under aggressive gas-turbine engine environments. The key for the ASC systems is related first to the NASA development of the Sylramic-iBN Sic fiber, which displays higher thermal stability than any other SiC- based ceramic fibers and possesses an in-situ grown BN surface layer for higher environmental durability. This fiber is simply derived from Sylramic Sic fiber type that is currently produced at ATK COI Ceramics (COIC). Further capability is then derived by using chemical vapor infiltration (CVI) and/or polymer infiltration and pyrolysis (PIP) to form a Sic-based matrix with high creep and rupture resistance as well as high thermal conductivity. The objectives of this study were (1) to optimize the constituents and processing parameters for a Sylramic-iBN fiber reinforced ceramic composite system in which the Sic-based matrix is formed at COIC almost entirely by PIP (full PIP approach), (2) to evaluate the properties of this system in comparison to other 2700 F Sylramic-iBN systems in which the matrix is formed by full CVI and CVI + PIP, and (3) to examine the pros and cons of the full PIP approach for fabricating hot-section engine components. A key goal is the development of a composite system with low porosity, thereby providing high modulus, high matrix cracking strength, high interlaminar strength, and high thermal conductivity, a major property requirement for engine components that will experience high thermal gradients during service. Other key composite property goals are demonstration at high temperatures of high environmental resistance and high creep resistance, which in turn will result in long component life. Data are presented from a variety of laboratory tests on simple two-dimensional panels that examine these properties and compare the performance of the optimized full PIP system with those of the full CVI and CVI + PIP hybrid systems. Underlying mechanisms for performance differences in the various systems are discussed. Remaining issues for further property enhancement and for application of the full PIP approach for engine components are also discussed, as well as on-going approaches at NASA to solve these issues.
In the as-produced condition the room temperature strength (approx. 6 GPa) of Textron Specialty Materials' 50 microns CVD SiC fiber represents the highest value thus far obtained for commercially produced polycrystalline SiC fibers. To understand whether this strength can be maintained after composite processing conditions, high temperature studies were performed on the effects of time, stress, and environment on 1400 deg. C tensile creep strain and stress rupture on as-produced, chemically vapor deposited SiC fibers. Creep strain results were consistent, allowing an evaluation of time and stress effects. Test environment had no influence on creep strain but I hour annealing at 1600 deg. C in argon gas significantly reduced the total creep strain and increased the stress dependence. This is attributed to changes in the free carbon morphology and its distribution within the CVD SiC fiber. For the as-produced and annealed fibers, strength at 1400 deg. C was found to decrease from a fast fracture value of 2 GPa to a 100-hr rupture strength value of 0. 8 GPa. In addition a loss of fast fracture strength from 6 GPa is attributed to thermally induced changes in the outer carbon coating and microstructure. Scatter in rupture times made a definitive analysis of environmental and annealing effects on creep strength difficult.
The structural performance of ceramic matrix composites for both low and high-temperature applications depends strongly on key properties contained in their tensile stress-strain behavior after fabrication. These include elastic modulus, matrix cracking stress, and ultimate strength. To determine the effects of fiber architecture on these properties, melt-infiltrated SiC/SiC composite panels were fabricated using 3D orthogonal preforms and 2D fabric lay-ups with various weave patterns. To maximize composite performance, all architectures were constructed with SylramicTM SiC fibers in the in-plane directions. Where possible, the preforms and fabrics were then subjected to a treatment that in-situ formed Sylramic-iBN fibers, a fiber type which typically yields composites with the highest tensile and rupture strength. For the 3D preforms, three types of low-modulus z-fibers were used to allow high in-plane fiber fractions, equivalent to those for the 2D composites. Even though the 3D-orthogonal panels displayed well-aligned × and y fibers with low crimp and lower matrix porosity, the room-temperature elastic modulus, cracking stress, and ultimate strength of these panels were generally lower than the 2D-woven panels. It is believed that the reduced modulus and cracking stress were primarily related to fiber-rich regions, the reduced strength to matrix-rich regions.
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