MS-11-P-5724 Production of Fe-based amorphous alloys coatings
Bulk metallic glasses present high mechanical strength and good resistance to sliding and abrasive wear and corrosion [1]. This characteristic, in association with the very high values tensile strength up to 4 GPa for Fe-based bulk glassy alloys indicating that coatings can represent good application’s opportunities for metallic glasses. In the present work, we chose the glass formers Fe60Cr8Nb8B24 , Fe72Nb4Si10B14 and Fe43.2Co28.8B19.2Nb4Si4.8 (%at) alloys to produce coatings over mild steel plate substrates using powder flame spray (PFS) and spray forming processes [2]. Nitrogen atomized powders of Fe60Cr8Nb8B24 alloy with spherical morphology in the size range < 45 µm were used for the PFS process. Fe43.2Co28.8B19.2Nb4Si4.8 glassy matrix composite coatings were produced as well by pre-placed laser cladding on AISI 1020 steel. The microstructure of the powders and coatings were characterized by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Dry sand/rubber wheel apparatus were used to evaluate wear behaviour of the amorphous coating produced by PFS process. The PFS coatings presented high fraction of amorphous phase with a layered structure, high porosity (~10%) and low oxidation level. The pre-placed coating formed micrometric-sized dendrites of the ductile α-(Fe,Co) phase homogeneously dispersed in a glassy matrix as a result of convection effects during the processing together with iron borides formed in the coating resulted in hardness of 1045 HV. Fe72Nb4Si10B14 spray formed 1mm thick coating showed porosity around 5% with almost fully amorphous structure, according to Figures 1 and 2 (SEM and TEM analysis). The low volume loss after the wear tests indicated a good wear resistance. The present results suggest that pre-placed laser cladding, spray forming and powder flame spray are promising processing routes to fabricate Fe-based glassy and nanocrystalline coatings for industrial applications.
The authors would like to thank FAPESP (São Paulo State Research Foundaton) for the finantial support thriugh the Project # 2012/18429-0, and the company Petrobras.
Fig. 1: SEM-BSE micrographs showing the microstructure of the PFS coating using atomized powder of Fe60Cr8Nb8B24 alloy showing (a) general view of the microstructure with amorphous and crystalline particles and, in detail (b) nanocrystalline FeNbB intermetallics embedded in the remaining amorphous phase. |
Fig. 2: TEM micrographs in bright field (BF) mode of coatings for Fe60Cr8Nb8B24 alloy obtained by PFS coating (C1) showing (a) general microstructure with nanocristals embedded in an amorphous matrix, (b) ring shaped selected area diffraction (SAD) patterns of typical of nanocrystalline structure. |