Protective Transition Metal Nitride Coatings

Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

“…This might be due to compressive film stresses [63][64][65][66]: Based on XSA these films exhibit compressive stresses of 3 GPa while the film with x < 1 exhibits a tensile stress of 1 GPa. After annealing at temperatures < 1000°C, (Ti,Al)N x with x > 1 shows a hardness increase that is attributed to be due to age hardening [4][5][6]. In this temperature window, it is interesting to note that (Ti,Al)N x with x ≈ 1 shows a slight hardness decrease indicating that there is no age hardening effect yet.…”

“…The respective effective reduced diffusion coefficients are shown in Figure 2(d) as a function of temperature. Since spinodal decomposition is taking place at ∼ 800°C [4][5][6]9,10], the effective reduced diffusion coefficients are compared at this temperature. Decreasing the metal vacancy concentration from a constant value of 2% to the thermally induced vacancy concentration due to Schottky defects of 10 −14 decreases the diffusivity by 12 orders of magnitude, see Figure 2(d).…”

“…The here proposed existence of a paraequilibrium between the gas phase and the film during synthesis is the basis for a first theory to describe stoichiometry and hence defect formation besides growth defects of all reactively deposited thin films: Even though it is accepted that the extreme quenching rates ( ∼ 10 15 K/s: from metal vapor to solid within fs [29]) allow for growth of thermodynamically metastable phases [22,30], (Ti,Al)N being an excellent example [6,23,31], we propose that the amount of incorporated nitrogen is defined by a paraequilibrium between nitrogen in the gas phase and in the metastable film surface during growth, µ N,gasphase = µ N,film . Based on the gas-solid paraequilibrium notion we introduce a methodology utilizing defect energies from ab initio calculations for thermodynamic calculations, allowing stoichiometry and thus point defect concentrations remaining after healing out growth defects by annealing [22,28] of reactively grown films to be predicted.…”

“…The cubic AlN-rich domains then phase transform at ∼ 1000°C into thermodynamically stable hexagonal wurtzite-AlN. The concurrent volume increase leads to cracking and failure of the protective coating, thus ultimately limiting lifetime and performance of the coating [4,6,[9][10][11][12][13][14]. Defects in (Ti,Al)N have recently gained considerable interest.…”

“…Since the first report 30 years ago [1], (Ti,Al)N has replaced TiN [2,3] as industrial benchmark hard coating due to the excellent mechanical properties and an increase of ∼ 250°C in oxidation temperature [1,[4][5][6][7][8]. In the last two decades it was understood that spinodal decomposition into TiN and AlN is caused by the extreme driving force for de-mixing on the metal sublattice of the cubic NaCl structure [4][5][6][7]. The cubic AlN-rich domains then phase transform at ∼ 1000°C into thermodynamically stable hexagonal wurtzite-AlN.…”