Ceramic/metal interfacial crack growth: Toughening by controlled microcracks and interfacial geometries

Toughening of ceramic/metal interfaces through the use of controlled interfacial geometries and non-coplanar microcrack-like pores is examined with respect to both critical and subcritical crack growth. Patterned uniform arrays of inclined interfacial steps and of “microcracks/voids” (with width 22 μm and spacing 10 μm), out-of-plane to the main interfacial crack, were produced for glass/copper interfaces by photo-lithographic techniques combined with evaporation and diffusion bonding processes. Significant toughening and improved stress corrosion crack-growth resistance is achieved through the promotion of crack-tip shielding primarily from crack bridging. Specifically, plastic void growth within the copper is seen to generate bridged ligaments of metal film between the glass substrates; the resulting mechanical crack bridging leads to plastic stretching of the film and provides the dominant toughening mechanism, with a smaller contribution from crack deflection. Correspondingly, subcritical (pre-instability) crack-growth rates with the patterned arrays in “wet” and “dry” gaseous atmospheres are retarded by orders of magnitude compared to rates for plain interfaces. The toughness with the various patterned interfaces exhibits marked resistance-curve (R-curve) behavior with fracture toughness values increased by factors of 4–9 compared to intrinsic fracture toughness, G 0, values of ∼2 J/m2 for these plain glass/copper interfaces. Surface roughness of the glass substrate is reasoned to be a controlling parameter for the shape and magnitude of such crack-resistance curves.