Fig.1 exhibits the macroscopic observation results of cross-sectional microstructures of Fe based metallic glass coating layers manufactured by the thermal spray(plasma, HVOF and VPS) processes. None of the thermal-sprayed coating layers showed any macro crack inside the coating layers or large defect in the interface area(coating layers and base material). So, it was inferred that the coating layers were fabricated soundly. The thicknesses of the thermal-sprayed coating layers were 325.6 um for plasma;192.7 um for HVOF; and 52.1 um for VPS.
Fig.1. Macro images of thermal sprayed Fe based metallic glass coating layers; plasma.(b) HVOF.(c) VPS
Fig.2shows the XRD analysis results of the powders and thermal-sprayed Fe based metallic glass coating layers. The powder was foundto be metallic glass one with broad halo XRD peak. The thermal spray is a process of depositing feedstock with thermal energy and kinetic energy on the substrate. Metallic glass powder could be crystallized due to the high temperature of thermal energy in thermal spray process. However, in the XRD analysis results, the coating layers fabricated by atmospheric plasma, HVOF and VPS processes showed broad halo peaks similar to powder feedstock, and no crystalline peaks were detected.
Fig.2.XRD analysis results of metallic glass powder and coating layers.
Fig.3 exhibits the cross-sectional microstructures of Fe based metallic glass coating layers. The values of porosity measured were as follows, plasma:1.8%, HVOF:0.9% and VPS:1.2%. All of the three coating materials were found to have micro pores at the particle interfaces. In the Fe based metallic glass coating layer fabricated via the atmospheric plasma spray process, particles with unmelted powders and splat parti-cles were easily found in the coating layer. Moreover, Fe-Cr base oxide (dark gray phase) was also detected at the particle boundary. By contrast, the HVOF coating layer did not show any unmelted powder, but splat particles were present in the coating layer. Similar to the plasma material, the HVOF coating layer was found to contain O elements which were intensively distributed around particles' boundaries, but the amount of oxide was smaller than that in the plasma material. The VPS material, on the other hand, was observed to partially have unmelted powder and splat particles but no Fe-Cr base oxide at the particle boundary. The oxides were found at the particle boundary in the cases of atmospheric plasma and HVOF spray processes, which were conducted under atmospheric environment. This was because of the inflight oxidation of the powder surface at the high thermal energyduring the flight of the powder until it reached the substrate.Oxides(generated powder inflight oxidation) at the particle boundary could cause discrepancy locally in the coating layers with the initial-powder chemical composition.
Fig.3. Microstructures of thermal sprayed Fe based metallic glass coating layers;(a) plasma.(b) HVOF.(c) VPS.
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