Microstructural characterisation of a Ni-Cr-B-C based alloy coating produced by high velocity oxy-fuel thermal spraying

High velocity oxy-fuel (HVOF) thermal spraying was used to deposit coatings, approximately 200 μm thick, of a commercially available Ni-Cr based alloy containing boron and carbon [Ni-23.5Cr-3.8Cu-0.8Fe-5.9Mo-3.4W-2.8B-4.3C (at.%)]. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy. 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 M23C6 particles ∼50 nm in size. The Ni-rich matrix had a predominantly crystalline structure with only a small fraction of amorphous phase regions. Cr2O3 and NiCr2O4 oxide phases occurred in the form of either intersplat lamellae or globules, with Cr2O3 being the predominant oxide. Coating microhardness values were found to be ∼6.0 GPa which is significantly higher than that of B- and C-free nickel-based alloys similarly deposited.