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 exp eriments. Powder and coating microstructures were investigated by a combina tion of X-ray diffraction, scanning electron and transmission electron micr oscopy and differential thermal analysis (DTA). The coatings had layered mo rphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metal lic matrix containing a small fraction of M3B2 particles 10-30 nm in size a nd with a tetragonal crystal structure. The Ni-rich matrix comprised both c rystalline and amorphous regions. The former was predominantly nanocrystall ine 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 th e alloy constitution. Cr2O3 and NiCr2O4 oxide phases occurred in the form o f either intersplat lamellae or globules. The thin lamellar oxides exhibite d the alpha-Cr2O3 crystal structure whereas globular oxides. up to 1 mu m i n size, were found with both alpha-Cr2O3 and NiCr2O4 structures. (C) 2000 E lsevier Science S.A. All rights reserved.