Effect of film thickness on the fracture toughness of amorphous diamond-like carbon

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 SiO₂ 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 SiO₂ sacrificial layer. Microtensile tests were performed on the precracked 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 MPa√m for 0.5 μm specimens, 4.4 ± 0.4 MPa√in for 1 μm specimens, and 3.06 ± 0.17 MPa√m 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.