How tensile tests allow a screening of the fracture toughness of hard coatings

In hard coating development there is a need for fast and efficient screening methods to assess the influence of changes in composition and structure evolution on the mechanical behavior. Nanoindentation constitutes a method that allows fast and efficient probing of mechanical properties. For another important mechanical characteristic of hard coatings, the fracture toughness, no quick benchmark test to compare coatings with various compositions and/or morphologies, exist. Therefore, the goal of this investigation was to determine if a tensile WA setup allows for fast and efficient qualitative screening of fracture toughness trends in hard coatings compared to accurate but time consuming micro-bending beam experiments. TiAlN and VAlN, each deposited on ductile Cu-substrates, were chosen for this investigation. In situ scanning electron microscopy tensile tests were performed. Here, the strain at crack initiation of the coating was utilized as the experimental parameter, which is most representative for the fracture toughness. The experiments indicate that TiAlN exhibits a higher fracture toughness than VAlN. This was confirmed using time demanding in situ micro-bending beam fracture experiments. Hence, it is established that under the given conditions macroscopic in situ SEM tensile tests can be used as a fast and efficient screening method for fracture toughness trends of hard coatings.