HiPIMS AlTiN Coatings

AlTiN is a promising anti-oxidation and corrosion protective material. Thanks to the formation of a coherent, compact and adherent alumina scale, it can be applied in harsh environments. In this work, the intrinsic properties of AlTiN films produced by reactive High Power Impulse Magnetron Sputtering (HiPIMS) have been investigated. Different substrates were chosen considering their technological interest: a common stainless steel (AISI 304), the T91 martensitic steel, which is conventionally used for nuclear fusion and fission plants, and, finally, a light alloy for aeronautic applications (g-TiAl), which represents a possibility to replace Ni based super-alloy in aircraft turbine engines. Working gas composition has been systematically changed (N2/(Ar+N2) atomic ratio 10, 25, 50 ,75, 100 %). Morphology, microstructure, hardness, elastic modulus, residual stress have been studied through high resolution methodologies. Microstructural analyses were carried out by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Moreover, films nano-mechanical properties, as hardness (H) and Young's modulus (E), were estimated by nano-indentation tester, while adhesion was evaluated by scratch tests. In order to investigate the average residual stress in the produced coatings, a recently proposed approach was used, which involves an incremental FIB milling of annular trenches at material surface, combined with high resolution SEM imaging . Finally, AlTiN/T91 coatings were subjected to long duration tests for the evaluation of corrosion by molten metal (Pd or Pb-Bi alloy), while all the other films were tested in air at high temperature, to investigate the behaviors of AlTiN films subjected to thermal cycles. SEM and XRD results showed a significant modification of films' microstructure due to selected deposition parameters: it ranges from quasi-amorphous structure at high N2 percentage to crystalline one at low nitrogen content. This means that the energy flux carried to the substrate considerably changes. Mechanical properties were found to be extremely promising: measured hardness reached 40 GPa. Moreover, by optimizing N2 percentage during the deposition process, it is possible to maximize the H/E ratio, which provides an indicator of the coating's elastic strain at failure. Finally, it was found how microstructure and/or mechanical properties of the coatings help to foresee the film/substrate system adhesion. The possibility to choose different experimental conditions coupled with an extensive characterization of the deposited coatings, allows to obtain useful information about structure/properties correlations and then, to optimize deposition parameters to achieve the required characteristics for specific applications.