Microstructure and oxidation behavior of an Al–Ni–Cr–Cu–MoO3–SiO2 composite coating on low-carbon steel

In the present study, an Al–Ni–Cr–Cu–MoO3–SiO2 composite coating with varying elemental compositions was deposited using the mechanical alloying technique. The composite coating structure before and after an oxidation test was studied using X-ray diffraction (XRD), three-dimensional optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). A cyclic oxidation test was conducted in an 800 °C air atmosphere for 10 cycles (200 h). Based on the XRD, SEM, and EDX analyses, the coating comprised composite-refined Al, Ni, Cr, and Cu particles as well as coarse SiO2 particles. The composite coating had a thickness in the range of 36–59 μm, with good and continuous adherence to the substrate. The cyclic oxidation curve of the composite coating followed a parabolic rate law. The sample that contained 5.3 wt% MoO3 showed the best oxidation resistance after 10 oxidation cycles at 800 °C. This sample had a lower mass gain than the other samples. Moreover, this sample had a Kp of 1.2 × 10−3 mg2/cm2·s. Protective Al2O3 oxides formed on the composite coating surface with a thickness of 0.75–1.89 μm. This oxide scale was dense and fully adhered to the coating, thereby improving oxidation resistance.