Dislocation shielding and flaw tolerance in titanium nitride

Titanium nitride is a very brittle and flaw sensitive ceramic material at temperatures below 750 degrees C. In this study, we present experimental evidence of room temperature dislocation-based plasticity in the material as well as insensitivity to flaws in form of single edge notches. We performed in-situ fracture experiments inside the transmission electron microscope on 150-300 nm thick, 5 mu wide freestanding films fabricated from titanium nitride/titanium multi-layers with titanium nitride as the notched and titanium as unnotched layers. The calculated stress concentration factor for the 800 nm to 1.5 mu long notches were greater than 8, however, the terminal cracks always nucleated at the unnotched edge of the specimens and not at the notch tip. To explain such remarkable flaw tolerance, we observe motion of dislocations (pre-existing and nucleated away from the notch) towards the notch tip. We suggest that the room temperature dislocation activities are facilitated by the residual stresses in the multi-layer specimens and the thickness dependence of image forces, which reduces the effective shear modulus to promote dislocation motion. The migration of dislocations towards the notch tip shields it from stress concentration to manifest the flaw tolerance in 150 nm specimens, which is observed real time in the microscope. (C) 2010 Elsevier Ltd. All rights reserved.