Surface Microstructure And Properties Of Arc-sprayed FeCrAl And FeCrNi Coatings Induced By High Current Pulsed Electron Beam Treatment

The arc-sprayed coatings are loose, porous and oxides-contained, and these structural defects always invoke the early corruption of whole coating system. Different kinds of post-treatment techniques have been adopted to improve the surface quality of coatings. In the present research, two types of high current pulsed electron beam (HCPEB) system, including SOLO-M and HOPE-I were used for the surface modification of arc-sprayed FeCrAl and FeCrNi coatings, respectively. The evolutions of structure-phase state and composition distribution were observed by optical microscopy, secondary electron microscopy, X-ray diffraction, X-ray fluorescence and electro-probe micro-analyzer methods. The surface properties of coating before and after HCPEB treatment were measured by microhardness, wear and corrosion resistance testing machines. The influence of HCPEB treating parameters on the surface microstructure and properties of arc-spayed coating was investigated and conclude as followings:When treating the arc-sprayed FeCrAl coating with HCPEB of long pulse duration, the pores and voids in the surface layer of coatings reduced remarkably, the modified surface became condensed and smoother. Many discrete and bulged nodules composed of Fe-Cr column grains appeared on the treated surface. XRD results showed that the Fe2O3 phase disappeared and the peaks corresponding toα-Al2O3 phase have been intensified after HCPEB treatment, which was testified by EPMA measurement. As for the formation mechanism of surface bulged nodule structure, it could be explained by the melting of coating surface under the irradiation of HCPEB, then, the melted layer would contract to fulfill the requirement of minimum liquid-solid interface energy and gave the formation of hemispherical morphology. Subsequently, the surface melting layer cooled down abruptly, the columnar grains grew fast and the growth of side arms was inhibited by the local undercooling at the fine tip of columnar grains and the narrow liquid-solid coexistence region resulting from the large temperature gradient. It was shown that the HCPEB treated FeCrAl coating exhibited good corrosion resistance in high-temperature sulfate environment, and the coatings treated with HCPEB of 50μs was improved significantly in 3.5% NaCl aqueous solution, which were attributed to the purification effect of HCPEB and the solute rejection process during Fe-Cr columnar grains solidification, the removal of Fe oxides, the partial smooth surface with seldom pores and cracks, as well as the homogenization of surface composition.The arc-sprayed FeCrNi coating treated with HCPEB of short pulse duration exhibited a condensed surface with few craters. With the increasing number of HCPEB pulses, the content of ferrite in the surface modified layer increased and the austenite decreased correspondingly. After the HCPEB treatment of 10-20 pulses, the contents of the two phases reached equal. This kind of modified structure-phase state could hinder the extension of microcrack and the growth of single phase,which offered the modified coatings with the advantages of duplex stainless steel. The HCPEB modified coatings showed higher surface microhardness, wear and corrosion resistance, which were discussed on considering the improvement in surface modified morphology, microstructure and composition distribution states.