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 mum thick, of a commercially available Ni-Cr based allo y 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 combinati on of X-ray diffraction, scanning electron and transmission electron micros copy. The coatings had layered morphologies due to the deposition and solid ification of successive molten or semi-molten splats. The splat microstruct ures consisted of a Ni-rich metallic matrix containing a small fraction of M23C3 particles similar to 50 nm in size. The Ni-rich matrix had a predomin antly crystalline structure with only a small fraction of amorphous phase r egions. Cr2O3, and NiCr2O4 oxide phases occurred in the form of either inte rsplat lamellae or globules, with Cr2O3 bring the predominant oxide. Coatin g microhardness values were found to be similar to 6.0 GPa which is signifi cantly higher than that of B- and C-free nickel-based alloys similarly depo sited. (C) 2001 Elsevier Science B.V. All rights reserved.