Comparison of fracture properties of different amorphous carbon coatings using the scratch test and indentation failure method

In this work, fracture properties of different types of hydrogen-free amorphous carbon coatings were studied in detail, using ta-C, ta-C:B and a-C:Mo coatings deposited at nominal coating thickness of 1 mm and 6 mm. Two common test methods with additional advanced evaluation techniques were used. Scratch testing was conducted with load and indenter size scaled to the coating thickness, evaluating critical loads and relative area of delamination. Instrumented indentation failure testing at different final loads was carried out, analyzing the load-depth curve for pop-ins, and acoustic emission for fracture events. Afterwards, indentation sites were studied using SEM and FIB cross sections, following a new approach where crack pattern imaging was directly correlated with fracture events obtained from indentation. Observed crack modes were assigned to radial, circumferential, parallel and lateral cracks. The crack types were classified with respect to location, timing, causative stress situation, amount of plastic substrate deformation, and conclusion about the coating behavior. Furthermore, the causes for the different crack types were discussed in detail. In the case of the studied coatings, a specific fracture behavior was found for each type of carbon coating, which cannot be simply attributed to the coating hardness. Instead, it is shown that during indentation not only the first fracture events but also the entire behavior up to the maximum indentation load should be considered, allowing a more distinct differentiation of fracture behavior of different types carbon coatings.