Fracture and fatigue-crack growth along aluminum-alumina interfaces

The mechanical failure of metal/ceramic joints subjected to monotonic and principally cyclic loading under nominal model I (far-field) conditions was investigated for a model Al/Al₂O₃ bimaterial system using a ceramic/metal/ceramic sandwich geometry in four-point bending. Crack growth was seen to follow a path along the interface, except at very high applied driving forces (defined in terms of the range of stress intensity ΔK or the elastic strain energy release rate ΔG), where a transition to growth in the metal layer took place, often involving a change in fracture mode to microvoid coalescence. The growth of fatigue cracks proceeded over a wide range of applied ΔG sign levels, extending from values well below to values well above those required to cause fracture in the adjoining ceramic. Interfacial crack-advance mechanisms under cyclic loading were found to be similar to that in ductile metals, as evidenced by the presence of fatigue striations on the metal fracture surface. Rapid (final) failure, conversely, involved ductile fracture in the metal or activation of defects in the ceramic substrate; both scenarios occurred at similar G_c (or K_c) fracture toughness values. Quantification of the results focused attention on the extensive crack-tip blunting that occurs at high driving forces; this requires significant corrections to the usual small-scale yielding (SSY) assessments of the driving force and yields fracture energies that arc orders of magnitude above those reported for other metal/oxide systems.