Fracture and subcritical crack-growth behavior of Y-Si-Al-O-N glasses and Si3N4 ceramics

Fracture and environmentally assisted subcritical crack-growth processes ar e 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 t in compact tension specimens) as well as short-crack behavior (using indent ation techniques) were investigated to establish a reliable fracture toughn ess and to elucidate the anomalous densification behavior of the oxynitride glass. Environmentally assisted subcritical crack-growth processes were st udied in inert, moist, and wet environments under both cyclic and static lo ading conditions. Behavior is discussed in terms of the interaction of the environment with the crack tip, Likely mechanisms for environmentally assis ted crack growth are discussed and related to the subcritical crack-growth behavior of silicon nitride ceramics.