Oxidation resistance of Ti-Al intermetallic alloys and protection by ceramic coating

In recent years, TiAl intermetallic alloys been widely used in aircraft and automotive industry. With the aim of improving the mechanical behavior and the oxidation resistance at high temperature of the TiAl alloys firstly designed, new intermetallic alloys of second and third generation have been successively developed. In this work, the oxidation resistance in air of four intermetallic alloys of second generation (Ti-48Al-2Cr-2Nb) and third generation ( Ti-48Al-2Nb-0.7Cr-0.3Si, Ti43.5Al-4Nb-1Mo and Ti-47Al-2Cr-8Nb) was investigated by TGA equipment under isothermal conditions in the range of 800-1000 °C. These alloys were cut from the core of bars, previously processed by Electron Beam Melting and successively heat-treated. The composition of the oxide layers was investigated by XRD, SEM-EDS and XPS. Each alloy showed different oxidation behavior at high temperatures. Layer exfoliation or spallation was observed for many samples, but at very different temperatures for the different alloys. When spallation did not happen in a significant extent the oxide layers grew according to a parabolic law. The kinetic rate constants and the activation energies were calculated. These kinetic parameters allowed to assess a rank of oxidation resistance, which can be correlated with the composition of the alloys. In order to improve the oxidation resistance of Ti-48Al-2Cr-2Nb, ceramic nitride coatings were deposited by a High Power Impulse Magnetron Sputtering (HiPIMS) method. Differently engineered TiAlN and TaAlN protective films were processed and their performances compared. Samples were submitted to thermal cycling under oxidizing atmosphere up to 850° C (40 cycles) and 950°C (100 and 200 cycles), at high heating and cooling rates. For this purpose, a burner rig able to simulate the operating conditions of the different stages of turbine engines was used. The microstructure and composition of samples before and after oxidation were investigated by several techniques: microscopy (optical and SEM-EDS), X-ray photoelectron spectrometry (XPS) and X-ray diffraction (XRD). All the TiAlN coatings differently processed provided a remarkable improvement of oxidation resistance. Good adhesion properties were observed even after repeated thermal shocks. HiPIMS pretreatments of the substrate surfaces, performed before the coating deposition, significantly affected the oxidation rate, the oxide layer composition and the coating/substrate adhesion. The oxide layers formed on the sample surface showed different thickness, depending on the presence of the protective coating and the processing path adopted for its deposition. The nitride coatings appreciably enhanced the oxidation resistance and sustained repeated thermal shocks without showing damage or spallation. Differently TaAlN coating did not improve the oxidation resistance of TiAl substrate.