Unravelling the potential of non-reactively sputtered (Ti,Al)N coatings

Abstract

Non-reactive magnetron sputtering was used to synthesize (Ti,Al)N coatings from a TiN/AlN (50 % AlN) composite target, with a detailed investigation into how process parameters influence phase formation, hardness, and thermal stability. Optimizing pulse parameters, such as increasing reverse pulse on-time (tₒₙ₋ᵣ) to 8016 ns or pulse frequency (fₚ) to 250 kHz, effectively suppressed the hcp phase observed under reference conditions (fₚ = 50 kHz, tₒₙ₋ᵣ = 496 ns, substrate temperatures Tₛ = 600 °C, sputter power density Pₜ = 8.6 W/cm², substrate bias Ub = –50 V). Lower Tₛ (≤ 450 °C) or Pₜ (3.4 W/cm²) also mitigate hcp phase formation, while higher Tₛ or Pₜ (up to 18.6 W/cm²) amplified it. Similarly, increasing Ub to –100 V allows to prepare single-phase fcc structured (Ti,Al)N with H up to 39.8 ± 2.1 GPa. By adding 5–10 % N₂ to the Ar working gas, the hcp phase formation can be avoided, achieving coatings with H = 38.3 ± 1.4 GPa. Detailed in-situ XRD as well as TEM studies reveal that coatings with a single-phase fcc structure provide a superior thermal stability during vacuum annealing treatments, particularly those deposited with Ub = –100 V. These retain their fcc structure up to 1100 °C and reach 44.7 ± 2.3 GPa through age hardening. This study underscores the critical role of suppressing hcp phase formation to enhance both hardness and thermal stability. Optimizing deposition parameters enables the tailoring of (Ti,Al)N coatings for demanding high-temperature applications.