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Fracture and environmentally assisted subcritical crackgrowth processes are examined in bulk Y-Si-Al-O-N oxynitride glasses with compositions typical of the grain boundary phase of silicon nitride ceramics. Both long-crack (in compact tension specimens) as well as short-crack behavior (using indentation techniques) were investigated to establish a reliable fracture toughness and to elucidate the anomalous densification behavior of the oxynitride glass. Environmentally assisted subcritical crack-growth processes were studied in inert, moist, and wet environments under both cyclic and static loading conditions. Behavior is discussed in terms of the interaction of the environment with the crack tip. Likely mechanisms for environmentally assisted crack growth are discussed and related to the subcritical crack-growth behavior of silicon nitride ceramics.

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The optical energy gap of amorphous Se powder has been determined by a photoacoustic method and has been found to be 1.99 ? 0.02 eV. On annealing, the gap decreases linearly as a function of annealing temperature towards that of the crystalline selenium.