Fracture toughness measurement of chromium nitride films on brass

Two methods of measuring the fracture toughness of films are critically examined using well-characterized films of CrN and Cr2N similar to6 mum in thickness bonded to brass. The first method invokes a model developed by Beuth and Klingbeil in which the film fracture stress is related to the fracture energy through an expression that accounts for work hardening of the substrate. The second method is to directly measure the displacement field around the crack tip using electron-beam moire and, subsequently, to estimate the crack-tip stress intensity factor using full-field-displacement equations. The films are prepared by magnetron sputtering on brass substrates heat-treated at various temperatures, thereby altering the microstructure and, hence, the substrate yield stress. Unexpectedly, the same films deposited on various substrates lead to very, different in-plane compressive residual stresses. The effect on crack-driving force is discussed, and a comparison between these two methods is made. Both techniques agree reasonably well and reveal that CrN exhibits a higher toughness than Cr2N.