Fracture properties of interfacially doped Nb-A12O3 bicrystals: I, fracture characteristics Original Research Article

The influence of orientation and impurities on the fracture behavior of Nb–sapphire interfaces was studied using notched bending tests. Single crystals were diffusion bonded in UHV for different interface orientations. The bicrystals were doped to produce prescribed fractional interfacial coverages of Ag. The interfacial impurity content was measured after fracture with Auger spectroscopy. The tougher bicrystals exhibit significant nonlinearity in loading. A J-integral analysis was used to account for the large plastic zones. For undoped bicrystals bonded at 1400°C, the interfacial fracture energy ranged from Jc of 77 to 2100 J/m2 depending on the interface planes of the Nb and sapphire. Greater toughnesses were derived from bonding at 1300°C, owing to less oxygen contamination of the Nb. Interfacial doping by Ag atoms leads to a strong reduction of Jc at coverages of only 0.2 to 0.5 of a monolayer. Higher fracture energy is caused by greater plastic deformation in the Nb as observed by slip lines on the metal fracture surface. Evaluation of the loading and fracture characteristics revealed that sharp precursor cracks developed initially in the ceramic. Extensive crack blunting also occurs, especially for the tougher bicrystals, but is often followed by erratic or unstable extension during which far less plasticity occurs, apparently owing to the rate sensitivity for Nb deformation.