Claudia P. Ostertag

Cathodoluminescence measurement of strained alumina single crystals

Abstract Alumina single crystals indented with a Vickers hardness indenter were investigated by cathodoluminescence (CL) imaging and spectroscopy in a scanning electron microscope. The spatial resolution of the CL images was approximately 0·4 μm. CL spectra were measured with wavelength resolution of 1·0 nm in the wavelength range of 200 to 900 nm; in the immediate region of the ruby lines (693 nm), spectra were measured with a resolution of 0·15 nm. Stress-induced frequency shifts of the ruby lines were utilized to measure the residual stresses in the region surrounding the indentation. Both hydrostatic and non-hydrostatic stresses were measured. The magnitude of the stresses was observed to vary with distance from the center of the indent and also with crystallographic orientation. After the samples were annealed at 1600°C for 10 h, no measurable stresses remained either in the center of the indent or in the surrounding area.

PROCESSING AND FRACTURE TOUGHNESS OF SINTERED FIBER REINFORCED CERAMIC MATRIX COMPOSITES

ABSTRACT The paper discusses the processing and the resulting mechanical properties of sintered fiber reinforced ceramic matrix composites. In situ observations of the sintering process revealed that stresses which develop due to the differential shrinkage between the fibers and the matrix initiate already during the heating cycle and are of sufficient magnitude and duration that crack like damage forms. Successful methods were employed for reducing and avoiding these stresses during the sintering process. Coarse grained alumina coatings deposited onto the fibers with a coating thickness of up to 10 µm delayed and reduced the stress development. Polymer coated fibers produced fully dense composites on which fracture toughness measurements were performed. Crack propagation and crack/fiber interaction was observed in situ inside a scanning electron microscope. The importance of studying both the crack front and the crack wake phenomena in fiber reinforced composites is illustrated. In specimens where the cr...