Enhanced stability of rocksalt-type AIN phase in AIN/TiN superlattices synthesized by room-temperature pulsed laser deposition

摘要

AlN/TiN superlattice coatings with bilayer thickness, Λ, of 1-20 nm were synthesized by pulsed laser deposition (PLD) at room temperature. Below a critical value of bilayer thickness, Λ_(c) ≈3 nm, the coatings formed a single-phase NaCl-type structure due to pseudomorphic stabilization of the nonequilibrium rocksalt-type (B1) AlN phase. Exceeding Λ_(c) resulted in the loss of continuous coherent growth of alternating AlN and TiN layers that is usually attributed to a rocksalt→wurtzite AlN phase transformation occurring in AlN layers when their thickness exceeds a certain critical value. However, in contrast to previous reports, a substantial amount of crystalline Bl-AlN has been detected within the volume of AlN layers in AlN/TiN superlattices with Λ>Λ_(c). The observed enhanced pseudomorphic stability of Bl-AlN in superlattices fabricated by room-temperature PLD is explained by kinetically limited conditions inherent to this deposition technique. The presence of the Bl-AlN phase in AlN/TiN multilayers is assumed to be mainly responsible for their increased hardness in the range of Λ where the Koehler hardness enhancement mechanism is inoperative.