Phase Stability and Elasticity of TiAlN

Abstract

We review results of recent combined theoretical and experimental studies of Ti_1-xAl_xN, an archetypical alloy system material for hard-coating applications. Theoretical simulations of lattice parameters, mixing enthalpies, and elastic properties are presented. Calculated phase diagrams at ambient pressure, as well as at pressure of 10 GPa, show a wide miscibility gap and broad region of compositions and temperatures where the spinodal decomposition takes place. The strong dependence of the elastic properties and sound wave anisotropy on the Al-content offers detailed understanding of the spinodal decomposition and age hardening in Ti_1-xAl_xN alloy films and multilayers. TiAlN/TiN multilayers can further improve the hardness and thermal stability compared to TiAlN since they offer means to influence the kinetics of the favorable spinodal decomposition and suppress the detrimental transformation to w-AlN. Here, we show that a 100 degree improvement in terms of w-AlN suppression can be achieved, which is of importance when the coating is used as a protective coating on metal cutting inserts.

Keywords: TiN; ab initio calculations; hard coatings; multilayer; spinodal decomposition; thermodynamics.

Figure 1

Hardness values obtained in [10] for the as-deposited and annealed monolithic TiN (black line with squares) and Ti_0.34Al_0.66N (blue, triangles), as well as multilayered Ti_0.34Al_0.66N/TiN coatings.

Figure 2

The lattice parameter of Ti_1−xAl_xN as a function of the fraction of AlN, x, obtained from EMTO-CPA calculations (filled circles) [15]. The experimental results from references [23,24,25,26] are shown for comparison.

Figure 3

(a) Isostructural cubic mixing enthalpy of Ti_1−xAl_xN at pressures P = 0 GPa and P = 10 GPa; (b) Mixing enthalpy of cubic rock salt (thick lines) and hexagonal wurtzite (thin lines) Ti_1−xAl_xN as a function of AlN fraction x at pressures P = 0 GPa and P = 10 GPa, relative to cubic TiN and hexagonal AlN.

Figure 4

Total electronic density of states (DOS) for c-Ti_1−xAl_xN calculated in [15] as a function of energy (relative to Fermi energy E_F) calculated for different fractions x of AlN (0.00, 0.50, 0.75, and 1.00) shows the presence of the metal-to-insulator transition when one goes from metalic TiN to semiconducting AlN.

Figure 5

The isostructural phase diagram of cubic Ti_1−xAl_xN as calculated with the Monte Carlo approach [28] (a) and with the mean-field approximation [15,28] (b). In the latter case we also show the phase diagram calculated at elevated pressure P = 10 GPa [27]. The binodal lines are shown with thick lines while the spinodal lines are shown by thin lines.

Figure 6

Calculated bulk modulus B and elastic stiffness constants, C₁₁, C₁₂, and C₄₄, of c-Ti_1−xAl_xN as a function of fraction x of AlN [35].

Figure 7

Calculated longitudinal sound velocity anisotropy map and Debye temperatures for c-Ti_1−xAl_xN at different functions x of AlN. The surface plots show the calculated spherical distribution (θ,φ) of the longitudinal sound velocity from top view assigned by high symmetry directions.

Figure 8

Differential scanning calorimetry measurements of the monolithic and multilayered coating [10].

Figure 9

STEM images of as-deposited sample (a) and 900°C annealed sample (d) and corresponding Al and Ti STEM-EDX elemental maps in as deposited state (b)–(c) and annealed state (e)–(f) [10].