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 μm 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 860 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 α-Cr2O3 crystal structure whereas globular oxides, up to 1 μm in size, were found with both α-Cr2O3 and NiCr2O4 structures.