Fracture toughness measurement of ultra-thin hard films deposited on a polymer interlayer

The fracture toughness of 100 nm-thick chromium nitride films deposited on a soft interlayer is evaluated using two different test methods. Both methods take advantage of the enhanced crack driving force resulting from the large elastic stiffness mismatch between the film and the polymer interlayer. The first method is based on the presence of channel cracks developing during the deposition as a result of large internal stress. The second method relies on nanoindentation induced cracking. The presence of a soft interlayer is shown to be essential to promote cracking otherwise impossible on a hard substrate, hence to determine the fracture toughness of very thin films. The assumptions underlying the two methods are very different, which allows critical cross comparison and assessment. A fracture toughness value between 1.5 and 2 MPa·m1/2 is obtained.