Nano-Scratching-Induced Damages and Their Effect on Fracture Properties of a Single Crystal Sapphire

Small size surface flaws due to polishing, machining and materials processing in various optical and electronic micro-devices and micro-electro-mechanical systems play an important role in system performance. In this study, nano-sized scratches were induced in a rectangular single crystal sapphire bar by the Atomic Force Microscope (AFM) diamond probe tips to investigate size dependence of such scratch-induced damages and their effect on fracture properties. It was found that internal as well as surface damages were generated as confirmed by surface microscopy observations and Tunneling Electron Microscopy (TEM). For high modulus AFM probe cantilevers, the internal damages were mainly in the form of cracks roughly 8 times of the scratch-induced shallow groove depth. When low modulus cantilevers were used, small damage zones instead of cracks were identified. 3-point bending tests were then implemented of scratched samples and final bending strength was categorized into two size regimes. When the total damage size defined as the summation of the internal crack length and surface scratch depth is above 1000nm, the bending strength roughly obeys linear fracture mechanics; for damages below this size, the strength clearly deviates and shows complex size-dependent behavior. Continuum mechanics as well as atomistic modeling was applied to provide the underlying rationale for the above experimental findings. Results obtained in this work may have potential applications in small-scale damage-related devises and systems.