Oxidation behaviors of Si-Cr Co-deposition coatings on a Ti-Nb-Si based ultrahigh temperature alloy at 1250°C

Si-Cr co-deposition coatings were prepared on a Ti-Nb-Si based ultrahigh temperature alloy by pack cementation processes at 1350°C for 10h. The oxidation behaviors of the coating specimens were examined isothermally at 1250°C for 5, 10, 20, 50h and 100h in static air. X-ray diffraction analysis (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) were used to identify the microstructure, compositional distribution and phase constituents of both coating and the scale developed during oxidation. The results revealed that the Si-Cr co-deposition coating was composed of 4 layers. The outermost layer was mainly composed of (Nb_1.95Cr_1.05)Cr₂Si₃ and (Ti, X)₅Si₃ phases (X represents Nb, Cr and Hf elements). Both the sublayer and transitional layer consisted of (Ti, X)₅Si₃, and (Ti, X)₅Si₄ was found as the main phase constituent in the intermediate layer. The oxidation of the coating specimens at 1250°C resulted in the formation of scales with an outer layer composed of Nb, Cr doped TiO₂ and an under layer composed of SiO₂. The oxidation kinetics of the bare alloy at 1250°C followed a linear law. However, the oxidation kinetics of the coating specimens could be described in terms of piecewise parabolic laws. The parabolic equation was y=15.81+3.92t^1/2 within 20h, and it was y=-82.71+27.12t^1/2 from 20h to 100h. The parabolic rate constant within 20h was lower than that from 20h to 100h by 1 order of magnitude. The outer layer with a higher Cr content had a better oxidation resistance than both intermediate layer and transitional layer of the coating at 1250°C.