Formation of amorphous and nanocrystalline phases in high velocity oxy-fuel thermally sprayed a Fe–Cr–Si–B–Mn alloy

High velocity oxy-fuel (HVOF) thermal spray was used to deposit a Fe–Cr–Si–B alloy coating onto stainless steel (1Cr18Ni9Ti) substrate. Microstructures of the powder and the coating were investigated by X-ray diffraction (XRD), scanning election microscopy (SEM), transmission election microscopy (TEM) and differential scanning calorimeter (DSC). The coating had layered morphologies due to the deposition and solidification of successive molten or half-molten splats. The microstructures of the coating consisted of a Fe–Cr-rich matrix and several kinds of borides. The Fe–Cr-rich matrix contained both amorphous phase and nanocrystalline grains with a size of 10–50 nm. The crystallization temperature of the amorphous phase was about 605 °C. The formation of the amorphous phase was attributed to the high cooling rates of molten droplets and the proper powder compositions by effective addition of Cr, Mn, Si and B. The nanocrystalline grains could result from crystallization in amorphous region or interface of the amorphous phase and borides by homogeneous and heterogeneous nucleation.