MICROSTRUCTURE OF PLASMA-SPRAYED NI-AL ALLOY COATING ON MILD-STEEL

Microstructure and fracture surface morphologies were characterized fo r a cross-sectional plasma-sprayed Ni-Al alloy coating-mild steel subs trate system by scanning electron microscopy, electronic probe microan alysis (EMPA) and transmission electron microscopy. The plasma-sprayed Ni-Al coating-steel substrate interface was found to be of pure metal lurgical nature. The bonding layer consisting of Fe3Al closely relates to exothermic reactions from Ni and Al during spraying. Ni3Al, the pr oduct from the Ni-Al reaction, and Fe3Al kept a fixed crystallographic orientation relationship. From the coating-substrate interface toward the coating surface, the fracture mode changes from preferential inte rlamellar fracture, to intersplat cleavage fracture, and finally to qu asi-cleavage fracture. The changes are thought to be related to the co ntact condition between splats. Microstructural changes observed throu gh the coating thickness seem to be caused by the non-uniform cooling rate distribution.