Characterization of thermal flame sprayed coatings prepared from FeCr mechanically milled powder

This paper discusses augmenting a thermal flame spray process with a mechanically milled feedstock to deposit low porosity FeCr coatings onto a steel substrate. A variety of tests were performed to assess the quality of the coatings obtained: microstructural and chemical analyses using X-ray diffraction (XRD) and optical and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS). The microscopy studies showed that the coatings displayed a lamellar structure with two distinct darker phases and contained unmelted particles; moreover, oxides and micro-cracks were observed at the surface. A lower level of porosity was observed compared to that reported in the literature, and the chromium concentration was found to increase with decreasing coating thickness. The mechanical properties of the coatings were characterized using micro-hardness, adhesion and residual stress tests. The results show a cohesive fracture mode for all samples, whatever the coating thickness. Electrochemical tests were carried out using open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) measurements. The best electrochemical properties were observed for coatings in which the voids were sealed with a polymeric substance and those fabricated with a Ni-based bond coating. However, the thinner coatings show higher chromium concentrations, lower porosity, and better mechanical and electrochemical properties than the thicker coatings.