Indentation fracture toughness and acoustic energy release in tetrahedral amorphous carbon diamond-like thin films

The fracture behavior and toughness of thin tetrahedral amorphous carbon (ta-C) films were examined using acoustic-sensing nanoindentation. Laser annealing was used to induce several levels of film stress ranging from -1.7 GPa (compressive) to +2.4 GPa (tensile). Indentations into the films generated radial cracks that extended into channel cracks with increasing indentation depth. Crack length was a function of both the indentation load and the sign and magnitude of the residual film stress, with tensile stresses promoting crack extension. Indentation fracture toughness models were used to estimate the fracture toughness of the ta-C films. Results obtained using an acoustic sensor embedded into the nanoindentation tip revealed a relationship between the released acoustic energy and the total crack length. These results were used to formulate an expression for the fracture toughness of the thin films directly from the acoustic signal, with comparable results to the standard indentation-based measurement.