Fracture toughness of alumina-niobium interfaces: Experiments and analyses

The mixed-mode interfacial fracture toughness of an alumina/niobium system is investigated. The interface was formed by solid-state bonding bulk Coor's AD-999 fine-grain alumina with a commercial purity niobium. The interface is very sharp; data acquired from high-resolution electron microscopy indicate that the width of the interface is no more than ten atomic planes. Furthermore the thermal expansion coefficients of the two materials differ by less than 5% so that residual stresses produced by the bonding process are minimal. The interfacial fracture toughness of the alumina/niobium system under tension-dominated load states and various combinations of in-plane shear and tension have been measured. For comparison purposes, the toughness of alumina under mixed-mode loading conditions was also measured. The fracture toughness of alumina is relatively insensitive to mode mixity. In contrast, the fracture toughness of the alumina-niobium interface depends strongly on mode mixity. At a phase angle of about 5°, we measured an average interfacial toughness of 4 MPa m^1/2 which is comparable with the toughness of homogeneous alumina. The measured toughness increases with increasing phase angle to a value of about 11 MPa m^1/2 at a phase angle between 25 and 45°. An explanation for the rapid increase in interface toughness with increasing phase angle, based on a maximum interface hoop-stress fracture criterion, is advanced.