Investigation of Ti0.54Al0.46/Ti0.54Al0.46N multilayer films deposited by reactive gas pulsing process by nano-indentation and electron energy-loss spectroscopy

Physical vapour deposition technology is well suited to the deposition of advanced TiAlN-based coatings. Among these thin films, multilayer systems consisting of stacked layers of metallic Ti1 − xAlx and nitride Ti1 − xAlxN with x around 0.5 are expected to have improved mechanical properties with respect to single nitride layers of the same composition. A set of Ti0.54Al0.46/Ti0.54Al0.46N multilayer films with five different periods Λ (from 4 to 50 nm) were deposited using the reactive gas pulsing process (RGPP). This RGPP approach allows the deposition of TiAl-based alloy/nitride multilayer films by radio frequency reactive magnetron sputtering with a controlled pulsing flow rate of the nitrogen reactive gas. The coherent growth of the multilayer coatings, depending on the period, is checked by X-ray diffraction and the mechanical properties are determined by Berkovich nano-indentation and friction experiments. A model to describe the dependence of the indentation modulus M and the hardness HB on the penetration depth h, the period Λ, and the film thickness ef is proposed. The indentation modulus of the multilayer films (M at h = 0 and for ef ~ 1900 nm) is found to be in the range of 340 GPa < M < 525 GPa ≈ M(Ti0.54Al0.46N). For a fixed penetration depth, M follows a Hall and Petch evolution as a function of the period (4 ≤ Λ ≤ 50 nm). The Berkovich hardness, 25 GPa < HB < 50 GPa, also presents the same kind of evolution, and for Λ < 16 nm (at h = 0), HB > HB (Ti0.54Al0.46N) = 33 GPa. Hence, a superlattice effect is clearly evidenced. Moreover, for the larger periods, the wear behaviour of these multilayered coatings seems to be dominated by the plastic deformation of the metallic layer. The multilayer coating of period Λ = 10 nm, which exhibits a diffraction pattern typical of superlattices and favourable mechanical properties, is more precisely investigated. Transmission electron microscopy confirms the main growth of the film along the [111] direction, and the evolution of the bonding of nitrogen in the direction normal to the rough interfaces between Ti0.54Al0.46 and Ti0.54Al0.46N layers is specified by electron energy-loss near-edge spectroscopy. Nitride nano-grains are included in the metallic layer, which attests to the mixing of nitrogen into the layers. The structure of these nano-grains presents a progressive evolution into the layer and gradually acquires a TiN-like structure near the interface. For this Λ = 10 nm period, the indentation modulus and hardness for different penetration depths are weakly sensitive to the multilayer film thickness.