Fatigue at Nanoscale: An Integrated Stiffness and Depth Sensing Approach to Investigate the Mechanisms of Failure in Diamondlike Carbon Film

Nanoscale impact fatigue tests were conducted to comprehend the relative fatigue performance and failure modes of 100 nm thick diamondlike carbon (DLC) film deposited on a 4 in. diameter Si (100) wafer of 500 μm thickness. The nanofatigue tests were performed using a calibrated TriboIndenter equipped with Berkovich indenter in the load range of 300–1000 μN. Each test was conducted for a total of 999 fatigue cycles (a low cycle fatigue test). Contact depth in this load range varied from 10 to 30 nm. An integrated contact stiffness and depth sensing approach was adapted to understand the mechanisms of fatigue failure. The contact depth and stiffness data indicated some peculiar characteristics, which provided some insights into the mechanisms of cohesive and adhesive failure in thin films. Based on the contact stiffness and depth data, and surface observations of failed DLC films using atomic force microscope and scanning probe microscopy, a five-stage failure mechanism is proposed. The failure of films starts from cohesive failure via cracks perpendicular to the film/substrate interface, resulting in a decrease in contact depth with number of fatigue cycles and no appreciable change in contact stiffness. This is followed by film delamination at the film/substrate interface and release of elastic stored energy (residual stress) resulting in an increase in contact stiffness. Finally, as the film breaks apart the contact stiffness decreases with a corresponding increase in contact depth.