Evolution of PVD Al oxide coatings in carburizing atmospheres at high temperature

Oxide thin films have been investigated as protection against corrosion of Fe-based materials in highly carburizing atmospheres at high temperature. The present study describes the structural evolution of an Al oxide/Cr thin film coating deposited on 304 L steel after 50 h of exposure to a CH4 + H2 + residual O2 atmosphere at 1073 K. Comparisons are then made to the structural evolution observed after 20 h of exposure from a previous study [1]. The coatings were produced by reactive magnetron sputtering at low temperature. The resulting thin films had very fine columnar structure, good adhesion to the substrate, amorphous Al oxide and tensile residual stresses. The response of the films to the carburizing atmosphere was evaluated via thermogravimetric analysis (TGA). The exposed samples were analyzed by X-ray diffraction, scanning electron microscopy equipped with energy dispersive analysis, and optical microscopy. Both, the TGA and structural analyses indicate that the presence of the films prevents the massive formation of internal and external corrosion products observed in uncoated 304 L samples exposed in the same conditions. However, the films underwent some structural modifications, which resulted in abundant interdiffusion through the film changing its as-deposited characteristics.