Microstructural Characterization of NiCrFeSiBC Coating During Long-Term Isothermal Oxidation at 850 °C

This paper reports bond coating/substrate interface evolution during isothermal oxidation tests at 850 degrees C up to 500 h in air of the Ni17Cr4Fe3.5B4SiC coating deposited on a Ti3Al substrate by high-velocity oxy-fuel (HVOF) spraying. The microstructure of the Ni17Cr4Fe3.5B4SiC/Ti3Al system was analyzed by scanning electron microscopy (SEM), Raman spectroscopy and x-ray diffraction (XRD) after isothermal oxidation. The isothermal oxidation behavior of the Ni17Cr4Fe3.5B4SiC/Ti3Al system was dominated by significant changes of interfaces. Intense interdiffusion processes took place between the Ni17Cr4Fe3.5B4SiC coating and substrate with an important change in the chemical composition of interdiffusion zones, leading to internal oxidation. Also, the oxide scale that developed on Ni17Cr4Fe3.5B4SiC coating surface was predominantly composed of Cr2O3, CrBO3, Ni-3(BO3)(2) and NiFe2-xCrxO4 spinel. After long exposure of the Ni17Cr4Fe3.5B4SiC/Ti3Al at 850 degrees C, the coating thickness decreased from 132 to 96 mu m as a consequence of internal oxidation processes and the oxide scale spallation. The chemical mismatch between thermal expansion coefficients (CTE) of Ni17Cr4Fe3.5B4SiC coating and Ti3Al substrate, as well as the intense interdiffusion processes, affected the adherence between coating and titanium substrate.