M. K. Ferber

Methodology for the determination of the interfacial properties of brittle matrix composites

The interfacial properties of a glass-ceramic matrix composite (SiC/CAS) were determined from single-fibre push-out tests using the interfacial test system. The coefficient of friction, μ, the residual clamping stress, σc, and fibre axial residual stress, σz, were extracted by fitting the experimental stress versus fibre-end displacement curves using the models of Hsueh, and Kerans and Parthasarathy. Using Hsuehs model, the intrinsic interfacial frictional stress (τ=μσc) was found to be 11.1±3.2 MPa, whereas by using Kerans-Parthasarathys model it was found to be 8.2±1.5 MPa. Comparisons between these models are included, together with a discussion of data analysis techniques.

Cyclic fatigue of hot isostatically pressed silicon nitride at elevated temperatures

Cyclic fatigue properties of a hot isostatically pressed silicon nitride were investigated at 1150, 1260 and 1370 °C in ambient air. The uniaxial tensile tests were conducted under various cyclic loading wave forms and frequencies. The correlation of stress-life relations between cyclic and static fatigue results was evaluated. At 1150–1370 °C, cyclic loading caused less damage than static loading, as evidenced by the longer failure time under cyclic loading versus static loading with the same maximum applied stresses. The cyclic loading effect was more pronounced in high frequency tests at 1260 and 1370 °C and might be related to the viscoelastic behaviour of the intergranular phase. Microstructural analyses and macroscopic cyclic stress-strain and strain-time relations indicated that cyclic loading/unloading may inhibit the normal accumulation of creep damage.

Stress-displacement relation of fiber for fiber-reinforced ceramic composites during (indentation) loading and unloading

The stress-displacement relation of the fiber is analyzed for fiber-reinforced ceramic composites during axial compressive loading (indentation) and unloading on the exposed end of an embedded fiber. An unbonded fiber/matrix interface subject to Coulomb friction and residual radial clamping stresses is considered in the present study. The results show that the stress-displacement curves during loading and unloading can be used to evaluate the magnitude of the clamping stress, the coefficient of friction, and the frictional stress distribution at the interface. Specifically, in the absence of Poissons effect (i.e., when Poissons ratio of the fiber is zero), the interfacial shear stress is constant, the loading curve is parabolic, and, after complete unloading, the residual fiber displacement equals half of the maximum fiber displacement at the peak loading stress. In the presence of Poissons effect, the interfacial shear stress is not constant, and, after complete unloading, the residual fiber displacement is less than half of the maximum fiber displacement at the peak loading stress.

Characterization of fiber/matrix interfaces in composites with a boron nitride matrix

The fracture behavior of boron nitride (BN) composites reinforced with several types of carbon and ceramic fibers has been examined. Fiber properties and fiber/matrix interface characteristics were found to control the mechanical strength and toughness of the composites. Transmission electron microscopy demonstrated preferential orientation of the BN matrix parallel to the fiber surfaces. Auger electron spectroscopy was then used to examine the fiber/matrix interface and locate the path of interface debonding. Single-fiber push-out experiments were used to quantify the mechanical properties of the fiber/matrix interface. The bulk composite fracture properties were consistent with current understanding in that a lower interfacial toughness was found to enhance fiber pull-out and increase the composite toughness. Matrix abrasion during fiber sliding had a significant effect on the sliding behavior of the fibers.

Growth and properties of Ni20.3Ti2.7B6 (τ-phase) crystals☆

Single crystals of Ni20.3Ti2.7B6 1 cm in diameter were grown from the melt by the Czochralski method at 1200–1215°C. Growth parameters and crystal properties are characterized.

Evaluation of Mechanical Reliability of Silicon Nitride Vanes After Field Tests in an Industrial Gas Turbine

This paper provides a review of recent studies undertaken to examine the mechanical and thermal stability of silicon nitride ceramic vanes with and without an oxide-based environmental barrier coating (EBC) after field tests in an industrial gas turbine. Two commercially available silicon nitride vanes (i.e., AS800 and SN282) were evaluated, where the AS800 vanes had an EBC and the SN282 vanes did not. The average temperature and pressure of gas impinging upon the vanes were approximately 1066°C and 8.9 atm, respectively. Both silicon nitride vanes were subjected to exposure time up to 1818h. Scanning electron microscopy was used to provide an insight into the changes in the microstructures of silicon nitrides and EBC arising from the environmental effects. The recession of the airfoils resulting from the volatilization of the normally protective silica layer, and /or EBC, was also measured using a coordinate measuring machine. The long-term chemical as well as structural stability of the secondary phases as well as EBC were characterized using x-ray diffraction. The surface strength of exposed airfoils was evaluated using a miniature biaxial test specimen, which was prepared by a diamond core drilling.Copyright

The effects of residual α phase on the 1370 °C creep performance of yttria-doped HIPed silicon nitride

The creep behaviour at 1370°C (2500°F) of yttria-doped, hot isostatically pressed silicon nitride was examined as a function of residual α phase content. The pre-test silicon nitride materials had either 30% or 40% α phase content. The creep resistance was found to increase as the residual α phase content decreased. For equivalent times and stresses, the higher α-containing silicon nitride accumulated more creep strain and exhibited faster creep rates. The residual α phase decreased as a function of time at 1370°C and converted to β phase; it was also found that the α to β phase transformation rate was enhanced by stress. In the absence of stress, the kinetics of the α to β phase transformation at 1370°C followed a first-order reaction. If a first-order reaction was assumed for the α to β phase transformation in the presence of stress at 1370°C, then the magnitude of the reaction rate constant for this transformation was twice as large for tensile stresses equal to or greater than 130 MPa than for the reaction rate constant describing the transformation with no applied stress.

Application of Infrared Imaging to the Study of Controlled Failure of Thermal Barrier Coatings

A technique that uses high resolution infrared (IR) imaging was developed to track and analyze damage evolution of thermal barrier coatings (TBCs) during controlled mechanical testing of a TBC specimen. Coating debonding and spallation were examined during a monotonic load-to-TBC-failure test. The infrared imaging, in concert with a controlled thermal gradient in the specimen, was particularly effective in identifying and tracking localized damage evolution because the damage in the TBC was always associated with a measurable surface-temperature change. It is demonstrated that the combined use of high-resolution infrared imaging and controlled mechanical testing of TBCs is an effective method to characterize the evolution of their failure.

Evaluation of interfacial mechanical properties in SiC fiber-reinforced macro-defect-free cement composites

Abstract The application of a micro-indentation technique for the measurement of interfacial mechanical properties in a fiber-reinforced cement composite has been examined. The composite was formed by placing aligned SiC fiber tows between macro-defect-free (MDF) cement sheets after which the compact was warm pressed at 6·89 MPa (1000 psi) at 80°C. The interfacial characteristics of the composite were varied by modifying the surfaces of the fibers prior to their incorporation into the matrix. These modifications included either an application of a stearic acid film or 1000°C gas phase treatments in air, oxygen, or nitrogen for one hour. These four fiber surface treatments served as the independent parameter for the mechanical testing analysis. The interfacial characteristics, including the shear, the residual axial fiber, and debond stresses, were evaluated with a mechanical properties microprobe by measuring the force/displacement curves generated during load-unload cycling. Estimates of these three stress values were obtained by matching the experimental force/displacement curves with data predicted from an existing model. For two of the four systems investigated, relatively high residual axial fiber compressive stresses were required adequately to describe the large fiber displacement recovery obtained after complete unloading. The stresses were believed to have resulted from the large differential shrinkage between the fibers and matrix during processing. The shear and debond stress values were highest for those fibers oxidized in air and lowest for fibers coated with stearic acid. The calculated interfacial shear stress value becomes significantly overestimated when the residual axial stress has a large magnitude and is not taken into account in the calculation.

The high temperature compressive strength of non-oxide ceramic foams

Abstract The compression strengths of five coated vitreous carbon foams under development were measured at 25, 1000, 1200 and 1400°C. Only qualitative trends in the measured strengths were obtainable due to a numerical lack of specimens. One developmental foam was a pyrolytic carbon (PC)-coated reticulated vitreous carbon (RVC) foam, and it was tested in argon at the three elevated temperatures. Four developmental RVC foams had chemical vapor infiltration (CVI)-SiC coatings on them, each with different coating thicknesses and consequential different bulk densities; these SiC-RVC foams were tested in ambient air at the elevated temperatures. The strength of the PC-RVC foam was independent of temperature up to 1400°C in argon. The compressive strengths of the SiC-RVC foams having the two thinnest coatings (or the two smallest bulk densities) were equivalent in ambient air to those of the PC-RVC foam in argon up to 1400°C, while the SiC-RVC foams having the two thickest coatings (or the two greatest bulk densities) were consistently stronger. These results show thin-SiC layered SiC-RVC foams grant oxidation protection that PC-RVC foams do not possess to 1400°C, and that thicker SiC layers on SiC-RVC foams are required for added strength.