Microstructure and oxidation behavior of NiCr-chromium carbides coating prepared by powder-fed laser cladding on titanium aluminide substrate

In the present study, a NiCr–Cr₃C₂ powder mixture was prepared by mechanical alloying and then coated on titanium aluminide substrates by the powder-fed laser cladding process using a set of optimum parameters. The high temperature oxidation behavior of the substrate and coating was studied by isothermal annealing at 900 °C for 5 h. It was found that the microstructure of the coating is composed of γ solid solution with different chromium carbide phases (Cr₃C₂, Cr₇C₃ and Cr₂₃C₆). The presence of different chromium carbides in the microstructure of coating can be attributed to the partial melting of primary Cr₃C₂ and the formation of non-equilibrium carbide phases during rapid cooling of laser cladding. The NiCr-chromium carbide laser cladded coating samples showed superior oxidation resistance compared to the substrate. The oxidation mechanism of both coating and substrate follow the parabolic law, where the parabolic rate constant of the coating was 20% of that of the substrate at 900 °C. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Grazing Angle X-Ray Diffraction (GAXRD) analysis revealed that the surface of the oxide layer formed on the NiCr-chromium carbides coating and the substrate is mostly composed of Cr₂O₃ and TiO₂, respectively.