Interfacial fracture toughness of alumina/niobium systems

The interfacial fracture toughness of an alumina/niobium composite has been measured as a function of phase angle. The interface was formed by solid-state bonding bulk Coor's AD-999 fine-grain alumina with a commercial purity niobium at 1600°C for 0.5 hr under a pressure of 10.5 MPa. The alumina/niobium system has a number of features which makes it ideal for an investigation of interfacial fracture toughness. From HREM data we estimate that the width of the interface is no more than 10 atomic planes. Furthermore the thermal expansion coefficients of the two materials differ by less than 5% so residual stresses due to the bonding process are small. Using symmetric and asymmetric four point bend specimens we have measured the fracture toughness of homogeneous alumina and that of the alumina/niobium bimaterial in combinations of in-plane shear and tension. The fracture toughness of the homogeneous alumina is relatively insensitive to the loading phase. The measured fracture toughness K_C of the interface, however, depended strongly on phase angle. We were unable to obtain valid alumina/niobium interfacial toughness data at negative phase angle as the fracture initiates in the alumina and not at the interface. In symmetric bending at a phase angle ≈ 5°, we measured a nominal interface toughness of 4.0 MPa√m, comparable to the homogeneous alumina. We found that the toughness increased with loading phase angle to a value of K_C ≈ MPa√m at a phase between 25° and 40°. Preliminary calculations and experiments suggest that this effect is due to an asymmetric stress distribution, with respect to the interface, and plastic deformation in the niobium.