Influence of Cr and Si additions on the structure and oxidation resistance of TiAlN single layer HiPIMS coatings
TiAlN coatings with a single phased cubic B1 structure have been widely used in advanced machining and other high-temperature applications due to their excellent outstanding mechanical, thermal and tribological properties. We investigated the influence of different Cr and Si additions to TiAlN HiPIMS coatings on the structure, mechanical properties and oxidation resistance, including the oxide scale characterization at 950 °C for various durations.
By combining TEM and SEM observations, XRD and TGA analyses and D-SIMS characterizations of the different coatings alloys, we were able to highlight the oxidation mechanisms. At deposited temperature, SEM cross-section shows a columnar microstructure for all coatings alloys. XRD patterns displayed that all coatings alloys exhibit single-phase centred cubic structure, where a broadening in the diffraction peaks can be observed with the addition of Cr and/or Si, which indicates a reduction in grain size with the incorporation of Cr and/or Si into TiAlN coating.
The oxidation kinetics depending on the different chemical compositions was studied by TGA analysis. The study showed that the onset temperature of oxidation was increased with Cr and/or Si additions to higher than ~ 850 °C for TiAlN.
At 950 °C because of the fast Ti ions diffusion to the surface, the formation of the continuous and protective Al oxide layer was impeded, where the addition of Cr promoted the formation of a TiO2 layer over a gradient Cr(Al)2O3 layer. The oxidation resistance of TiAlSiN coatings benefits from the addition of Si content. Si delays the Ti ions diffusion to the surface, which hinders the formation of the TiO layer. Hence increase addition of Si in TiAlCrSiN coatings promotes the outward diffusion of Al and Cr that can form dense and protective Cr(Al)2O3 layer. Thanks to-depth profile analysis by SIMS after oxidation at 950 °C under an alternative atmosphere containing 16O and 18O, it has been possible to give information on the different mechanisms controlling each oxide scales.