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Effect of Hf on structure and age hardening of Ti–Al-N thin films

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Abstract

Protective coatings for high temperature applications, as present e.g. during cutting and milling operations, require excellent mechanical and thermal properties during work load. The Ti 1 −  x Al x N system is industrially well acknowledged as it covers some of these requirements, and even exhibits increasing hardness with increasing temperature in its cubic modification, known as age hardening. The thermally activated diffusion at high temperatures however enables for the formation of wurtzite AlN, which causes a rapid reduction of mechanical properties in Ti 1 −  x Al x N coatings. The present work investigates the possibility to increase the formation temperature of w-AlN due to Hf alloying up to 10 at.% at the metal sublattice of Ti 1 −  x Al x N films. Ab initio predictions on the phase stability and decomposition products of quaternary Ti 1 −  x −  y Al x Hf y N alloys, as well as the ternary Ti 1 −  x Al x N, Hf 1 −  x Al x N and Ti 1 −  z Hf z N systems, facilitate the interpretation of the experimental findings. Vacuum annealing treatments from 600 to 1100 °C indicate that the isostructural decomposition, which is responsible for age hardening, of the Ti 1 −  x −  y Al x Hf y N films starts at lower temperatures than the ternary Ti 1 −  x Al x N coating. However, the formation of a dual phase structure of c-Ti 1 −  z Hf z N (with z =  y/(1 −  x)) and w-AlN is shifted to ~ 200 °C higher temperatures, thus retaining a film hardness of ~ 40 GPa up to ~ 1100 °C, while the Hf free films reach the respective hardness maximum of ~ 38 GPa already at ~ 900 °C. Additional annealing experiments at 850 and 950 °C for 20 h indicate a substantial improvement of the oxidation resistance with increasing amount of Hf in Ti 1 −  x −  y Al x Hf y N.

Highlights

Ab initio calculations enable for the prediction of as-deposited structures in Ti 1 −  x −  y Al x Hf y N. ► Additions of 5 mol% HfN in Ti 1 −  x −  y Al x Hf y N raise the formation temperature of w-AlN by ~ 200 °C. ► 10 at.% Hf at the metallic sublattice of Ti 1 −  x −  y Al x Hf y N protect from full oxidation at 950 °C for 20 h.

Most cited references46

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We formulate a new concept for computing with quantum cellular automata composed of arrays of nanostructured superconducting devices. The logic states are defined by the position of two trapped flux quanta (vortices) in a 2x2 blind-hole-matrix etched on a mesoscopic superconducting square. Such small computational unit-cells are well within reach of current fabrication technology. In an array of unit-cells, the vortex configuration of one cell influences the penetrating flux lines in the neighboring cell through the screening currents. Alternatively, in conjoined cells, the information transfer can be strengthened by the interactions between the supercurrents in adjacent cells. Here we present the functioning logic gates based on this fluxonic cellular automata (FCA), where the logic operations are verified through theoretical simulations performed in the framework of the time-dependent Ginzburg-Landau theory. The input signals are defined by current loops placed on top of the two diagonal blind holes of the input cell. For given current-polarization, external flux lines are attracted or repelled by the loops, forming the '0' or '1' configuration. The read-out technology may be chosen from a large variety of modern vortex imaging methods, transport and LDOS measurements.
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A periodic Energy Decomposition Analysis (pEDA) method for the Investigation of Chemical Bonding in Extended Systems

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The development and first applications of a new periodic energy decomposition analysis (pEDA) scheme for extended systems based on the Kohn-Sham approach to density functional theory are described. The pEDA decomposes the binding energy between two fragments (e.g. the adsorption energy of a molecule on a surface) into several well-defined terms: preparation, electrostatic and dispersion interaction, Pauli repulsion and orbital relaxation energies. The pEDA presented here for an AO-based implementation can handle restricted and unrestricted fragments for 0D to 3D systems considering periodic boundary conditions with and without the determination of fragment occupations. For the latter case, reciprocal space sampling is enabled. The new method gives comparable results to established schemes for molecular systems and shows good convergence with respect to the basis set (TZ2P), the integration accuracy and k-space sampling. Four typical bonding scenarios for surface adsorbate complexes were chosen to highlight the performance of the method representing insulating (CO on MgO(001)), metallic (H$$_2$$ on M(001), M = Pd, Cu) and semiconducting (CO and C$$_2$$H$$_2$$ on Si(001)c(4x2)) substrates. These examples cover the regimes of metallic, semiconducting and insulating substrates as well as bonding scenarios ranging from weakly interacting to covalent (shared electron and donor acceptor) bonding. The results presented lend confidence, that the pEDA will be a powerful tool for the analysis of surface-adsorbate binding in the future, enabling the transfer of concepts like ionic and covalent binding, donor-acceptor interaction, steric repulsion and others to extended systems.
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Understanding the nature of "superhard graphite"

(2012)
Numerous experiments showed that on cold compression graphite transforms into a new superhard and transparent allotrope. Several structures with different topologies have been proposed for this phase. While experimental data are consistent with these models, the only way to solve this puzzle is to find which structure is kinetically easiest to form. Using state-of-the-art molecular-dynamics transition path sampling simulations, we investigate kinetic pathways of the pressure-induced transformation of graphite to various superhard candidate structures. Unlike hitherto applied methods for elucidating nature of superhard graphite, transition path sampling realistically models nucleation events necessary for physically meaningful transformation kinetics. We demonstrate that nucleation mechanism and kinetics lead to $$M$$-carbon as the final product. $$W$$-carbon, initially competitor to $$M$$-carbon, is ruled out by phase growth. Bct-C$$_4$$ structure is not expected to be produced by cold compression due to less probable nucleation and higher barrier of formation.
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Author and article information

Journal
Surf Coat Technol
Surf Coat Technol
Surface & Coatings Technology
Elsevier Sequoia
0257-8972
1879-3347
25 January 2012
25 January 2012
: 206
: 10
: 2667-2672
Affiliations
[a ]Department Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben, Austria
[b ]Ulbrich of Austria GmbH, A-7052 Müllendorf, Austria
Author notes
[* ]Corresponding author. Tel.: + 43 3842 402 4229; fax: + 43 3842 402 4202. richard.rachbauer@ 123456unileoben.ac.at
Article
SCT17181
10.1016/j.surfcoat.2011.11.020
3271383
22319223

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Categories
Article

Thin films & surfaces

ti–al–hf-n, tialn, ab initio, oxidation, ti–hf-n, tialhfn