R-Curve Behavior in a Silicon Carbide Whisker/Alumina Matrix Composite

R -curve measurements were performed on a SiC whisker/Al₂O₃ matrix composite. A controlled flaw/strength technique was utilized to determine fracture resistance as a function of crack extension. Rising R -curve behavior with increasing crack extension was observed, confirming the operation of wake toughening effects on the crack growth resistance. Observations of crack/microstructure interactions revealed that bridging by intact whiskers in the crack wake was the mechanism responsible for the rising R -curve behavior.     

Interfacial Characterization of Silicon Carbide Fiber/Lithia-Alumina-Silica Glass Matrix Composites

The development of the carbon-rich interphase in Nicalon SiC fiber/Li₂O-Al₂O₃–SiO₂ glass matrix composites was examined as a function of processing parameters with the use of high-resolution scanning electron microscopy and Auger electron spetroscopy. Specifically, hot-pressing temperatures (1000°, 1100°, and 1200°C) and times (15, 30, 60, and 240 min) were systematically varied in such a manner so as to fabricate dense composites suitable for evaluation of reaction kinetics. Carbon-rich interphase thickness, which ranged from 1400 to 5400 Å (140 to 540 nm), was observed to increase with either increasing times at constant temperature or increasing temperatures at constant time. The kinetics of formation of the carbon-rich interphase followed a diffusion-controlled model, with an activation energy of 25.4 kcal/mol.     

Comparison of the Properties of Oxycarbide and Oxynitride Glasses

Oxycarbide glasses in the Mg-Al-Si-O-C system and oxynitride glasses in the Mg-Al-Si-O-N system were synthesized by conventional melting techniques, chemically analyzed, and evaluated for selected mechanical properties as a function of carbon or nitrogen content. Both glass systems exhibited increases in density, Young's elastic modulus, microhardness, and fracture toughness with increasing anionic substitutions. Carbon substitutions were found to be more effective than nitrogen additions for improving glass properties.     

Characterization of β-Silicon Nitride Whiskers

The physical, chemical, and structural properties of a commercially available β-Si₃N₄ whisker were characterized. Bulk chemical analysis indicated that the whiskers were close to stoichiometric silicon nitride, with oxygen and yttrium as the major impurities. Surface chemistry analysis by XPS analysis revealed that the surfaces consisted primarily of silicon nitride, with the oxygen and yttrium impurities concentrated at the surfaces. SEM and STEM studies indicated that the whiskers were dimensionally straight with relatively featureless surfaces, although some whiskers had Y-rich particles attached. The whiskers were also found to be of extreme crystallographic perfection, as determined by TEM analysis.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Silicon Carbide Whisker/Alumina Matrix Composites: Effect of Whisker Surface Treatment on Fracture Toughness

The fracture toughness of a 30 vol% SiC whisker/Al₂O₃ matrix composite was evaluated as a function of whisker surface chemistry. Two types of SiC whiskers (Silar-SC-9 and Tateho-SCW-1-S) were investigated. Modification of the whisker surface chemistry was achieved by subjecting the whiskers to thermal treatments under controlled atmospheres. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy, was correlated to the fracture toughness of the composites.     

Novel multianion MgSiAlOC oxycarbide glasses

New bulk oxycarbide glasses were synthesized for the first time by additions of SiC to magnesium aluminosilicate glass forming melts. Significant improvements in the glass transition temperature, microhardness, and refractoriness were achieved. The projected role of these new oxycarbide glass systems in the chemical and materials sciences and in the technology of novel glass-ceramics and composites is outlined.