Nanoindentation Induced Fracture in Hard Multilayer Thin Films

Abstract

Depth sensing nanoindentation and nanoscratch testing were combined with atomic force microscopy (AFM) and electron microscopy observations to study mechanical properties and fracture behavior of a number of TixAl1-xNyC1-y hard thin films. Various failure modes were activated either by normal loading-unloading or by microscratching of the samples to provide an estimation of the fracture toughness and interfacial fracture energies. By changing chemical composition and deposition conditions various nanostructured thin films including monolitically grown single layer, nanocomposite, and multilayers were coated onto the tungsten carbide-cobalt substrates. All tested films exhibit elevated mechanical properties with high hardness (38 – 45 GPa) and modulus (500 – 570 GPa). Under sufficiently high load indentation the formation of corner Palmqvist type radial cracks were usually observed because of small modulus mismatch between coating and substrate, good adhesion, and in particular high toughness of both substrate and films in spite of great difference in their respective hardness. Various failure modes were activated and the sequences of fracture events were determined using stepwise or continuously increasing load scratch tests. Some other films were found to be more sensitive to tensile stress behind the indenter which generates regularly repeated microcracks on the scratch track. Other films in particular multilayers were appeared more susceptible to compressive stress ahead of the indenter leading to local delamination at the interface between layers and the formation of irregular microcracks under the contact zone.

Microstructural aspects of fracture in nanolayered TiAlCrN thin films

“…The coatings are known to be effective in protecting the cutting edge against dissolution and high temperature abrasive wears and to provide a thermal barrier effect between chip and tool [3]. The strength enhancement gained by application of such thin films can quickly be neutralized by the formation of through thickness microcracks or failure at the interface between the film and the substrate since hard materials are unable to sustain large plastic deformation [4]. Consequently, the wear of hard films is dominated by the formation of submicron debris resulting from the nucleation and propagation of near-surface nanocracks and intercolumnar microcracks.…”