High temperature oxidation protection of ?-titanium aluminide using (Cr,Al)ON coatings deposited by high-speed physical vapor deposition

In recent years great efforts have been made in the development of y-TiAl alloys for use in aerospace applications such as turbines, where low densities and high temperature strength are required. However, y-TiAl alloys show poor oxidation resistance at temperatures T > 850 °C due to the formation of a non-protective oxide layer consisting of a mixture of TiO2 + Al2O3 in air, which could be easily spalled off, resulting in a shortened lifetime of the components. One promising way to overcome this problem is the deposition of an oxidation protective coating with low oxygen permeability at high temperatures. However, the interdiffusion between coating and substrate is still challenging even for advanced oxidation resistant coatings such as MCrAlY (M = Ni or Co) and aluminide. The present work focuses on the (Cr,Al)ON coating system, inspired from its outstanding diffusion barrier properties. Four (Cr,Al)ON coatings with different Cr:Al and N:O ratios were deposited onto y-TiAl substrate by the innovative High-Speed Physical Vapor Deposition (HS-PVD) technology, which enables the deposition of oxygen-rich coatings in a stable plasma process without target poisoning. Basing on hollow cathode discharge (HCD) and gas flow sputtering (GFS), the HS-PVD made it possible to deposit (Cr,Al)ON coatings at a deposition rate ds/dt > 16 ...m/h. The as-deposited coatings show an X-ray amorphous structure. Subsequently, the transmission electron microscopy (TEM) analysis confirmed the nanocomposite coating structure. The thermal stability and oxidation behavior of the coatings were evaluated by means of differential scanning calorimetry (DSC) and thermogravimetry (TGA) between T = 100 °C and T = 950 °C. It was confirmed that the X-ray amorphous structure even remained stable at temperatures up to T = 950 °C. Moreover, cross-sectional SEM images of the coated samples after the HT-XRD measurements showed neither the formation of oxides at the coating substrate interface nor the interdiffusion of Ti into the coating, indicating a promising performance of the diffusion barrier. The results of the conducted research reveal a high potential of the HS-PVD deposited (Cr,Al)ON coatings for the oxidation protection of y-TiAl at T > 850 °C in turbine applications.