Fracture properties of interfacially doped Nb-Al2O3 bicrystals: II, relation of interfacial bonding, chemistry and local plasticity

Abstract As described in part I, Nb and α-Al2O3 single crystals were diffusion bonded in UHV for several different orientations of the two constituents. These were doped to give defined levels of interface coverage of Ag, Ti, Y or S. The influence of these dopants on the fracture behaviour of the several metal–ceramic interfaces was studied. Submonolayer doping causes large changes in fracture energy Jc: a strong reduction with Ag, and especially S; possibly, a small decrease with Y; and, in contrast, enhancement with Ti. Auger electron spectroscopy data indicated that the interfaces have excess oxygen and provided information on the bonding states and the local crack path. The influences of plasticity of the metal part and of interface chemistry on the bicrystal bond strength are evaluated. The change in the work of attraction, Wat, is derived by means of interface thermodynamics using measured surface and interface coverages of dopant atoms. An empirical relationship is then developed between the interfacial Jc and Wat, which directly demonstrates increasing toughness and associated crack-tip plasticity with increasing interface bonding. The toughness spans two orders of magnitude while Wat varies by 2.5 times. At a critical level of Wat, a transition causes a virtual jump in interfacial Jc; this is interpreted as an incipient blunting transition that is frustrated by the rate sensitive plasticity in the Nb.