John Z. Gyekenyesi's research while affiliated with Cleveland State University and other places

Near stoichiometric SiC fibers, such as Sylramic and Hi-Nicalon Type S, display very good thermomechanical properties that are essential for high-temperature structural ceramic matrix composites (CMC). Recently NASA has developed a treatment for the Sylramic fiber that further improves its creep-rupture properties and environmental interactions by removing boron from the bulk and forming a thin in-situ BN coating on the fiber surface. To understand its benefits for fiber-controlled CMC properties, this treatment was performed on two-dimensional 0\90 degree woven Sylramic fabric prior to forming SiC\SiC composites with a BN interphase and a melt-infiltration SiC matrix. Tensile stress-strain behavior for the CMC showed a high modulus followed by graceful cracking at a high proportional limit, but more importantly a higher ultimate strength than for the SiC\SiC composites fabricated in the same manner, but reinforced by untreated Sylramic and Hi-Nicalon Type S fibers. It is believed that this improved ultimate strength behavior is primarily related to the in-situ BN coating that acts to protect the fibers from environmental effects introduced during the CMC fabrication process.

Composites consisting of woven Hi-Nicalon fibers, BN interphases, and different SiC matrices were studied in tension at room temperature. Composites with SiC matrices processed by CVI and melt infiltration were compared. Monotonic and load/unload/reload tensile hysteresis experiments were performed. A modal acoustic emission (AE) analyzer was used to monitor damage accumulation during the tensile test. Post test polishing of the tensile gage sections was performed to determine the extent of cracking. The occurrence and location of cracking could easily be determined using modal AE. The loss of modulus could also effectively be determined from the change in the velocity of sound across the sample. Finally, the stresses where cracks appear to intersect the load-bearing fibers correspond with high temperature low cycle fatigue run out stresses for these materials.

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.

Silicon effects on tensile and creep properties, and thermal conductivity of Hi-Nicalon SiC/SiC composites have been investigated. The composites consist of 8 layers of 5HS 2-D woven preforms of BN/SiC coated Hi-Nicalon fiber mats and a silicon matrix, or a mixture of silicon matrix and SiC particles. The Hi-Nicalon SiC/silicon and Hi-Nicalon SiC/SiC composites contained ∼24 and 13 vol% silicon, respectively. Results indicate residual silicon up to 24 vol% has no significant effect on creep and thermal conductivity, but does decrease the primary elastic modulus and stress corresponding to deviation from linear stress-strain behavior.

Ceramic matrix composites fabricated using Nicalon fiber and several pofysilsesqui-oxane-derived Si-C-O matrices were characterized by optical and scanning electron microscopy and in four-point bending. Material variables include three Nicalon sizings, poly(vinylacetate), epoxy and DCC-1, two different copofymer compositions, and several pyrolysis schedules. Shrinkage of the matrix during pyrofysis results in a 65% char yield on heating to 1500°C for both the pofyvinylmethylsils-esquioxane and polyphenylmethylsilsesquioxane copofymers studied. In the matrix material linear shrinkages of up to 19% were observed for the vinylmethyl copofymer pyrolyzed to 1200°C, and up to 13% for the phenylmethyl material pyrolyzed to the same temperature, resulting in considerable matrix cracking. Under four-point loading, most composite samples fractured in a brittle manner as the result of strong fiber-matrix bonding. Flexural strengths, moduli, and ultimate strain varied with fiber sizing and processing parameters. Testing and analysis is being applied to promote more fibrous pullout during fracture.

oxidation;isothermal;uniaxial;methodology;matrix

Gas turbine components such as combustor liners or turbine vanes are subject to regions of high stress-concentration, e.g., attachment to the frame or at cooling holes. Ceramic matrix composites (CMCs) are potential materials for high temperature applications in gas turbines. They offer some capability to relieve stress at regions of high stress-concentration via matrix damage accumulation. In this study notch sensitivity was examined for woven SiC fiber reinforced, melt-infiltrated SiC matrix composites with a BN interphase, utilizing either Hi-Nicalo(TM) fibers or the stiffer Sylramic fibers. The double-edge notched tensile test approach was used for a wide range of notch sizes and specimen widths. Both composite systems exhibited mild notch sensitivity similar to other CMC systems. Acoustic emission, detected during the tensile tests, indicated that matrix cracking occurred around notches at net-section stresses below the stress where matrix cracking first occurs in unnotched specimens. However, thermoelastic stress analysis did not show any measurable stress relief around notches after the specimens were preloaded.

Ceramic matrix composites are envisioned for use at elevated temperatures (1000 to 1400°C) in oxidizing environments. For non-oxide composites, the time-dependent failure stress is dependent on the severity of reactions between the environment and the load-bearing fibers and interphase. One of the most severe pitfalls for these types of materials occurs at intermediate temperatures, 600 to 1000°C. Environmental access to load-bearing fibers occurs through matrix cracks that are bridged by load-bearing fibers. The rate at which these composites lose the ability to carry load is partially controlled by the extent of cracking in the matrix. In this study, the damage accumulation of woven SiC/SiC composites tested in tension was quantified using unload/reload hysteresis tests and modal acoustic emission. The behavior of composites reinforced with ceramic-grade Nicalon™, Hi-Nicalon™, and Sylramic® fibers, with carbon or boron nitride interphases and chemically vapor-infiltrated or melt-infiltrated SiC matrices was investigated. The most significant finding of this study is that the formation of matrix cracks that bridge the load-bearing fibers (0° cracks) occurs at approximately the same strain for all of the woven SiC/SiC composites tested. All of these systems have at least a CVI SiC matrix layer adjacent to the interphase. In addition, the onset stress-strain condition for 0° cracks corresponds to the tensile stress-strain condition above which intermediate temperature embrittlement occurs for BN interphase SiC/SiC composites. For carbon interphase SiC/SiC composites, the critical stress-strain condition for the onset of intermediate temperature embrittlement corresponds to the first cracks in the matrix which can occur at stresses half the 0° crack onset stress.

High temperature tensile properties of unidirectional BN/SiC-coated Hi-Nicalon SiC fiber reinforced celsian matrix composites have been measured from room temperature to 1200 C (2190 F) in air. Young's modulus, the first matrix cracking stress, and the ultimate strength decreased from room temperature to 1200 C (2190 F). The applicability of various micromechanical models, in predicting room temperature values of various mechanical properties for this CMC, has also been investigated. The simple rule of mixtures produced an accurate estimate of the primary composite modulus. The first matrix cracking stress estimated from ACK theory was in good agreement with the experimental value. The modified fiber bundle failure theory of Evans gave a good estimate of the ultimate strength.

Silicon effects on tensile and creep properties, and thermal conductivity of Hi-Nicalon SiC/SiC composites have been investigated. The composites consist of 8 layers of 5HS 2-D woven preforms of BN/SiC coated Hi-Nicalon fiber mats and a silicon matrix, or a mixture of silicon matrix and SiC particles. The Hi-Nicalon SiC/silicon and Hi-Nicalon SiC/SiC composites contained about 24 and 13 vol% silicon, respectively. Results indicate residual silicon up to 24 vol% has no significant effect on creep and thermal conductivity, but does decrease the primary elastic modulus and stress corresponding to deviation from linear stress-strain behavior.