| Four kinds of silicon nitride/silicon carbide (whisker) matrix composites synthesized from common starting materials, except the whiskers themselves, at the same processing conditions have been investigated. The interfacial structure and chemistry of and the existence of an interfacial layer at the interface will affect the mechanical properties of the composites, e.g. crack propagation under applied stress, profoundly. The chemical environment near the interface will change the local elastic moduli and flow stress and eventually the fracture toughness of the composite. High resolution electron imaging microscopy and high spatial resolution electron energy loss nanospectroscopy were used to characterize the whisker/matrix and matrix grain boundary interfaces in these composites both structurally and chemically.;The presence of a 1.5 to 2nm wide discontinuous oxygen rich amorphous layer at the four kinds of whisker/matrix interfaces and 0.5 to 2nm wide continuous oxygen rich and nitrogen depleted amorphous layer at the matrix grain boundary interfaces examined in this dissertation appeared to be general phenomena.;The wide difference between the structural widths and the chemical widths (mainly oxygen distributions) of the whisker/matrix and matrix grain boundary interfaces was attributed to oxygen dissolution in the silicon nitride matrix, from the sintering aids and surface impurities.;Sharp discontinuity in the integrated zero loss intensity observed at the interfaces in all the composites in the low loss position resolved electron energy loss spectroscopy profiles across the interfaces was observed. Greater preferential ion thinning of the interfacial regions than the whiskers and the matrix grains, chemical segregation of elements from surface impurities and sintering aids leading to local thickening of the interfacial regions or scattering of high energy electrons by heavy atoms like yttrium to angles larger than spectrometer collection angle i.e. 10 mrad may be responsible for this phenomenon.;The Fresnel fringe technique was evaluated using a... |
