Effect of Film Thickness on the Fracture Toughness of Amorphous Diamond-like Carbon

摘要

Fracture toughness, K_(IC), measurements were conducted for the first time on hydrogen-free tetrahedral amorphous Diamond-like Carbon (ta-C) MEMS-scale specimens of different thicknesses. Uniform gage microscale specimens with mathematically sharp edge pre-cracks were prepared by microindentation on the SiO_2 sacrificial layer. The radial-median crack from the indent propagated into the specimen generating a sharp pre-crack. The crack length was measured by an Atomic Force Microscope (AFM). Freestanding fracture specimens were then obtained by wet etching the SiO_2 sacrificial layer. Microtensile tests were performed on the pre-cracked specimens under mode-Ⅰ loading in fixed grip configuration. In order to investigate the specimen thickness dependence of K_(IC), fracture tests were conducted on specimens with thicknesses in the range of 0.5-3 μm. K_(IC) was 4.25 ± 0.7 MPam~(1/2) for 0.5 μm specimens, 4.4±0.4 MPam~(1/2) for 1 μm specimens, and 3.06±0.17 MPam~(1/2) for 3 μm thick specimens. The 25% lower fracture toughness of the 3 μm films points to a film thickness dependence of fracture toughness that was attributed to different through-the-thickness stresses in considerably thick ta-C films and compositional changes occurring during post-deposition processing.