Influence of Zr on structure, mechanical and thermal properties of Ti-Al-N.
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Here, we study the effect of Zr addition on structure, mechanical and thermal properties of Ti(1-x)Al(x)N based coatings under the guidance of ab initio calculations.Increasing the Zr content from z = 0 to 0.17, while keeping x at ~ 0.5, results in a hardness increase from ~ 33 to 37 GPa, and a lattice parameter increase from 4.18 to 4.29 Å.Furthermore, Zr assists the formation of a dense oxide scale.
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Affiliation: Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben, 8700, Austria.
ABSTRACT
Multinary Ti-Al-N thin films are used for various applications where hard, wear and oxidation resistant materials are needed. Here, we study the effect of Zr addition on structure, mechanical and thermal properties of Ti(1-x)Al(x)N based coatings under the guidance of ab initio calculations. The preparation of Ti(1-x-z)Al(x)Zr(z)N by magnetron sputtering verifies the suggested cubic (NaCl-type) structure for x below 0.6-0.7 and z ≤ 0.4. Increasing the Zr content from z = 0 to 0.17, while keeping x at ~ 0.5, results in a hardness increase from ~ 33 to 37 GPa, and a lattice parameter increase from 4.18 to 4.29 Å. The latter are in excellent agreement with ab initio data. Alloying with Zr also promotes the formation of cubic domains but retards the formation of stable wurtzite AlN during thermal annealing. This leads to high hardness values of ~ 40 GPa over a broad temperature range of 700-1100 °C for Ti(0.40)Al(0.55)Zr(0.05)N. Furthermore, Zr assists the formation of a dense oxide scale. After 20 h exposure in air at 950 °C, where Ti(0.48)Al(0.52)N is already completely oxidized, only a ~ 1 μm thin oxide scale is formed on top of the otherwise still intact ~ 2.5 μm thin film Ti(0.40)Al(0.55)Zr(0.05)N. No MeSH data available. Related in: MedlinePlus |
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Mentions: Elemental analysis by EDX reveals that our Ti1-x-zAlxZrzN films are stoichiometric with N/metal ratios of 1 ± 0.1. As mentioned in the experimental section the composition of the metal sublattice is varied by adding Ti or Zr platelets at the race track of the Ti0.5Al0.5 compound target. Thereby the following films are obtained: Ti0.48Al0.52N with 21 Ti platelets, Ti0.40Al0.55Zr0.05N with 10 Zr platelets, Ti0.39Al0.51Zr0.10N with 20 Zr platelets, Ti0.36Al0.47Zr0.17N with 40 Zr platelets, and Ti0.34Al0.37Zr0.29N with 80 Zr platelets. XRD investigations of these as deposited films, as shown in Fig. 1, reveal a single phase cubic structure, which is in agreement with ab initio calculations. Fig. 2a presents the energy of formation, Ef, of the cubic and wurtzite Ti1-x-zAlxZrzN alloys with constant z = 0, 0.05 and 0.1 as a function of the AlN mole fraction x. The data suggest a transition from cubic to wurtzite structure Ti1-x-zAlxZrzN at x ~ 0.72, 0.70, and 0.68 for a ZrN mole fraction z of 0, 0.05, and 0.10, respectively. Since the compositional steps given by the supercell sizes are different for the cubic (1/18) and for the wurtzite (1/16) alloys, to evaluate the maximum solubility of AlN in the cubic phase we proceeded as follows. First, we fitted each set of data with constant ZrN mole fraction with a third order polynomial (a0 + a1·x + a2·x2 + a3·x3). For c-Ti1-x-zAlxZrzN, the AlN mole fraction x was varied 19 times for z = 0, 12 times for z = 0.055, and 10 times for z = 0.111. The calculations of w-Ti1-x-zAlxZrzN were obtained with 7, 6, and 5 variations in x for z = 0, 0.0625, and 0.125, respectively, in the composition range x = 0.5–1. Subsequently, for each phase (i.e. cubic or wurtzite) we fitted individually the coefficients (i.e., a0, a1, a2, and a3) of their third order polynomial for the three different ZrN mole fractions, z, with a linear expression in the ZrN contents. This way, two polynomial fits (one for the cubic and one for the wurtzite modification) as functions of x (AlN mole fraction) and z (ZrN mole fraction) were obtained. In the last step, we used these fits to estimate the cross-over between the formation energies of the cubic and wurtzite phases at fixed ZrN mole fractions (and thus to estimate the influence of Zr on the maximum AlN mole fraction in the cubic Ti1-x-zAlxZrzN). Fig. 2b shows this transition with a solid line connecting the data points (circles) as obtained for Ti1-x-zAlxZrzN with z = 0, 0.05, and 0.1 and ZrN-AlN, in the quasi-ternary TiN-AlN-ZrN diagram. It is worth noting, that the calculated transition point (maximum Al content in the cubic phase) is not too sensitive to a chosen order of the fitting polynomials: it ranges from 0.72 (z = 0) to 0.67 (z = 0.11) when using quadratic fits, from 0.705 (z = 0) to 0.68 (z = 0.11) for the third order polynomial, and from 0.70 (z = 0) to 0.68 (z = 0.11) for the fifth order polynomials. The chemical composition of our deposited films (indicated by asterisks) are deep within the single phase cubic region and hence in agreement to the XRD results. Additionally, we calculated the cubic-to-wurtzite transition for Zr1-zAlzN alloy to be ~ 0.5. |
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Affiliation: Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben, 8700, Austria.
No MeSH data available.