Whisker-reinforced ceramic matrix composites with modified interfaces. Ph.D. Thesis
Whisker/matrix interfaces have been shown to be a controlling factor in the mechanical performance of whisker-reinforced ceramic matrix composites. This study examines the role of whisker/matrix interfaces by incorporating films of carbon at interfaces of silicon carbide whiskers and aluminum oxide matrix grains to modify the residual thermal stress and the interfacial chemistry. Measurements of mechanical properties were then made and discussed with respect to the interfacial modifications. Triaxial x-ray and pulsed neutron diffraction experiments were performed to measure the residual stresses that develop due to thermal expansion mismatch from elevated processing temperatures. Neutron diffraction experiments as a function of temperature enabled extrapolation of the strain-free temperature (temperature above which thermally-activated processes are effective at relieving residual strains) which is between 1250 and 1350 C. Interfacial structure and chemistry were assessed using high resolution and analytical electron microscopy. Composites with uncoated whiskers did not have an amorphous or crystalline interphase at the whisker/matrix interface. Graphite films were observed at some whisker/matrix interfaces in composites with 50 A coated whiskers. Other interfaces in composites with carbon-coated whiskers were indistinguishable from those in uncoated whisker composites due to the spallation of carbon-coatings from the whiskers during processing. Some carbon appeared as free-carbon in the matrix phase upon consolidation of these composites. Fracture strength-in-bending, fracture toughness and short and long crack R-curve behavior were determined experimentally as a function of carbon coating thickness. The strength degraded with increasing coating thickness consistent with weakened interfaces and the presence of free-carbon in the matrix. Uncoated whiskers optimized the fracture toughness of the composites.
- Research Organization:
- Northwestern Univ., Evanston, IL (United States)
- OSTI ID:
- 147181
- Country of Publication:
- United States
- Language:
- English
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