| This study examines the mechanisms of interfacial and near interfacial crack propagation associated with the failure of sandwich specimens consisting of 99.5% pure polycrystalline alumina: (1) liquid state bonded with 99.999% pure aluminum layers and (2) partial transient liquid phase (PTLP) bonded using copper/niobium/copper interlayers. For the former system, the aluminum layer thickness was varied from 5 to 100 μm; it was found that the 3-point unnotched bending strength of the aluminum bonded joints increased, and the fracture toughness decreased, with decreasing layer thickness. Strength beams failed by ductile failure in the aluminum while fracture toughness samples failed by brittle fracture in the alumina. Under cyclic loading, crack growth occurred primarily by separating the aluminum from the alumina with evidence of ductile fatigue striations; cracks deviated into the alumina only for thin layered samples at high driving forces. Cyclic fatigue thresholds increased with decreasing layer thickness; however, the change to a brittle fatigue mechanism for thin layered samples at high driving forces was detrimental to the overall fatigue resistance. Under static loading in moist air, interfacial separation was never observed at measurable rates (≥10−9 m/sec); however, for 5 and 35 μm thick layered samples, cracks deviated off the interface and grew, sometimes stably, into the alumina resulting in time dependent failure.; For alumina PTLP bonded with copper/niobium/copper interlayers, the mean interfacial fracture toughness was found to decrease from 39 J/m2 to 21 J/m2 as temperature was increased from 25 to 1000°C. At room temperature, cyclic fatigue crack propagation occurred at both the niobium/alumina interface and in the alumina, with higher fatigue thresholds resulting from a predominantly near interfacial (alumina) crack path. During both fracture and fatigue failure, residual copper at the interface deformed and remained adhered to both sides of the fracture surface, while separation of the niobium/alumina interface appeared essentially brittle in both cases. The observed behaviors of all samples are examined in terms of modulus mismatch effects, the level of plastic constraint, the relative crack propagation resistance of each observed crack path, the loading conditions, extrinsic toughening from ductile phase and/or alumina grain bridging, and environmental influences. |
