Pseudomorphic stabilization on crystal structure and mechanical properties of nanocomposite Ti-Al-N thin films
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Pseudomorphic stabilization on crystal structure and mechanical properties of nanocomposite Ti-Al-N thin films A. Karimi, Th. Vasco, A. Santana IPMC - Faculty of Basic Science Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland
Abstract As the dimensions of materials are reduced to the nanometer scale, the stabilization of pseudomorphic crystal structures that differ from their bulk equilibrium phases can occur. The pseudomorphic growth could provide a substantially larger bulk modulus and greater hardness than the average of the constituent materials in nanolayered and nanocomposite thin films. To evaluate this effect in Ti1-xAlxN system, a series of nanostructured films with x up to 0.7 were deposited onto WC-Co substrates using arc PVD, and characterised in terms of structureproperty relations. Chemical composition by RBS together with HRTEM and XRD analysis showed that for the Al content below x = 0.4 a solid solution single-phase film is formed, while for x values beyond 0.5 mixed structures made of fcc-TiN and hcp-AlN, or nanocomposite films made of fcc-TiN, hcp-AlN, and fcc-AlN appeared depending on deposition conditions.
Hardness of solid solution films was found to increase almost linearly with the Al content, while two opposite behaviours were distinguished for composite structures. Hardness rapidly decreased according to the rule of mixture as soon as solid solution phase began to separate into TiN and AlN growing in their natural structures with misfit dislocation at the interface. In contrast, further hardness enhancement was measured when nanocomposites with coherent interfaces were formed due to pseudomorphic stabilization of fcc-AlN on fcc-TiN crystallites. Introduction Titanium-aluminium nitride thin films have been recognised to provide effective protection to cutting and forming tools operating under high speed cutting conditions [1, 2]. The improved performance of these coatings is believed to arise from their ability to retain higher hardness at high operating temperatures, thereby hindering abrasive and dissolution wears of cutting edges. Further improvement of the mechanical properties of TiAlN coatings could be achieved by grain size refinement down to the nanometer length scale through alloying of TiAlN coatings, or through advanced deposition techniques to favour the formation of nanocomposites and compositionally modulated nanolayers [3,4]. Several experimental works were confirmed that the nanostructured TiAlN-based nitrides exhibit better functional properties and elevated thermal and chemical stabilities compared to monolithically grown films as reviewed in references [5-7]. The effects of nanostructuring on deformation mechanisms and mechanical strengthening of monolithic materials were relatively well described by Hall-Petch relation stipulating a dependence of hardness enhancement on the square root of grain size as explained by dislocation pile-up and work hardening models [8]. In contrast, the remarkable property of nanocomposites and nanolayers is associa
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