Influence of Si addition on the phase structure and oxidation behavior of PVD AlTiN and AlTiCrN coatings using high-resolution characterization techniques

The influence of Si concentrations on AlTiN and AlTiCrN coatings deposited by PVD has been investigated by using high-resolution characterization techniques: TEM, Dynamic SIMS, Atom Probe Tomography (APT) analyses and nano hardness measurements. First, investigations focus on crystallographic phase stability, microstructural observations and micromechanical studies to understand the effect of the Si addition on these two nitride coatings. Second, the oxidation mechanisms and the kinetics of oxide growth at 950 C-degrees for various durations are examined. Results indicate that the addition of Si introduces high compressive stresses in both coating groups, reaching values in the range of -6 GPa. However, the behavior of Si content differs for AlTiN coatings with and without chromium. In AlTiSiN coatings, increasing Si addition leads to reduce residual stresses, while no significant change is observed for AlTiCrSiN coatings. This stress evolution is associated with a decrease in crystallinity density of the TiAlN coatings due to Si addition, but this structural phenomenon is not observed when Si is added to the quaternary metallic coatings TiAlCrN. Si content also influences the nanohardness, but the variation among coating is not substantial, with values around 34 + /-2 GPa, and an elastic-modulus around 443 + /-40 GPa. Regarding oxidation resistance at 950 C-degrees, the addition of Si in AlTiN coating results in the formation of an external alumina oxide layer and beneath it, a nanometer sized TiO2 anatase crystallites layer. After the growth of this bi-layer oxide scale, the inward cationic diffusion of the oxygen is very significantly reduced, and it can explain its high oxidation resistance. In contrast, for AlTiCrSiN coatings, the oxide scale morphology is different, consisting of a pure TiO(2 )rutile outer layer, followed by an Al-rich oxide and a mixed oxide region of (AlCr)(2)O-3 with small islands of TiO2. The growth of this last oxide scale shows a regular increase over time, primarily driven by inward oxygen diffusion at the nitride coating interface.