Toughness measurement of thin films based on circumferential cracks induced at conical indentation

In the present study, circumferential ring cracks were produced in two types of alumina coatings under conical indentation. The alumina coatings were produced using reactive dual pulsed magnetron sputtering. The coatings were deposited at 150 and $$530\,^\circ \hbox {C}$$ which resulted in coatings with hardness values of $$9.1\pm 0.2$$ and $$20.7\pm 1.1$$ GPa, respectively. The coating fractures were studied using scanning electron microscopy and the critical parameters for fracture: load, depth and crack radius, were determined for a range of coating thicknesses for both series. The crack behavior is compared to a numerical finite element model of the system. The model assumes the coating to be linear elastic while plasticity was included in the substrate. The critical parameters for different values of fracture toughness were extracted from the FEM stress field using closed-form expressions. The behavior of the simulated data and the experimental data was found to follow similar trends for all the investigated critical parameters. Furthermore, it was found that the critical load is the fracture parameter from which a measure for the fracture toughness is most accurately obtained. The hard coatings were observed to have higher fracture toughness than the softer coatings (200 vs. $$75 \, \hbox {J/m}²$$ ).