Microstructure formation in high velocity oxy-fuel thermally sprayed Ni-Cr-Mo-B alloys

High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni-Cr-Mo-B alloy coatings, approximately 200 mu m thick. onto mild steel substrates. Gas atomised powders of three different compositions were used in the experiments. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy and differential thermal analysis (DTA). The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M3B2 particles 10-30 nm in size and with a tetragonal crystal structure. The Ni-rich matrix comprised both crystalline and amorphous regions. The former was predominantly nanocrystalline with a grain size of ca. 50 nm and DTA showed that the crystallization temperature of the amorphous phase varied from 800 to 800 K depending on the alloy constitution. Cr2O3 and NiCr2O4 oxide phases occurred in the form of either intersplat lamellae or globules. The thin lamellar oxides exhibited the alpha-Cr2O3 crystal structure whereas globular oxides. up to 1 mu m in size, were found with both alpha-Cr2O3 and NiCr2O4 structures. (C) 2000 Elsevier Science S.A. All rights reserved.