Nanoindentation Induced Fracture in Hard Multilayer Thin Films
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Nanoindentation induced fracture in hard multilayer thin films
A. Karimi, D. Bethmont, Y. Wang, Department of Physics, Swiss Federal Institute of Technology (EPFL) CH – 1015 Lausanne, Switzerland
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. INTRODUCTION High thermal stability and moderate friction coefficient of titanium aluminum nitrides favor them for use as thin coatings in dry cutting and high speed machining [1, 2]. This family of coatings can be deposited over a wide range of microstructures by modulating the ratio of Ti/Al and substituting a fraction of nitrogen by carbon atoms [3, 4]. Incorporation of aluminum is believed to lower thermal conductivity protecting the substrate from extensive thermal loading [5], and also favor the formation of a dense and strongly adhesive aluminum oxide on the surface which prevents further oxidation of coating under high temperature applications [6]. In addition aluminum promotes development of solid solution strengthening due to smaller atomic diameter compared to titanium. This process provides high hardness to the thin films even at elevated temperature [7], but also develops large internal stresses responsible for the formation of brittle and crack sensitive layers. To overcome this problem use of multilayers and compositionally graded films instead of monolithic coatings was suggested. Multilayers with bilayer length on the micro or nanometer scale are expected to exhibit superior mechanica
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