Fracture mechanisms in nanoscale layered hard thin films

Abstract

Depth sensing nanoindentation and nanoscratch testing were combined with atomic force microscopy and electron microscopy observations to study mechanical properties and fracture behavior of a number of TiAlN(Si, C,…) hard thin films. Various nanoscale multilayer thin films were deposited onto the cemented carbide substrates using cathodic arc PVD. Failure modes were activated either by a cycle of indentation or by microscratching of the samples to provide an estimation of the fracture toughness and interfacial fracture energies. Load–displacement curves were characterized by small discontinuities due to the formation of nano and microscale cracks through the film thickness. Under sufficiently high load the occurrence of large-scale cracks consists of the successive lateral cracks at the contact site, or corner Palmqvist type radial cracks around the contact area depending on the film structures. Such indentation behaviors can be attributed to small modulus mismatch between the film and the substrate, good adhesion of the film, and in particular high toughness of both substrate and films in spite of great differences in their respective hardness. Various modes of failure and the sequences of fracture events were determined using stepwise or continuously increasing load scratch tests. Some films were found to be more sensitive to tensile stress behind the indenter which generates through thickness vertical microcracks on the scratch track. Other films appeared to be more susceptible to compressive stress ahead of the indenter leading to local delamination at the interface between the layers and irregular microcracks under the contact area.