High-Velocity Oxyfuel Reactive Spraying of Mechanically Alloyed Ni-Ti-C Powders

Abstract

Recently, there has been considerable interest in producing cermet coatings with nanoscale carbide grains in the size range 50 to 500 nm. In this article, the production of nanoscale TiC grains in a Ni-based alloy matrix by reactive high-velocity oxyfuel (HVOF) spraying of metastable Ni-Ti-C powder is reported. Mechanical alloying of a Ni(Cr) prealloyed powder and Ti and C elemental powders was performed in a planar-type ball mill, and materials were characterized in detail using x-ray diffraction (XRD) and scanning electron micros-copy (SEM). Phase changes were correlated with milling time and other processing conditions. Results show that, by the selection of appropriate conditions, a metastable Ni-Ti-C powder could be obtained with the nominal composition 50wt.%Ni-40wt.%Ti-10wt.%C. Following sieving and classification, powder was produced with a particle size range of −38 to 8 µm, which is suitable for HVOF spraying. Coatings, approximately 250 µm thick, were deposited by HVOF spraying onto mild steel substrates, and the microstructures formed were investigated. XRD showed that a self-propagating high-temperature synthesis (SHS) reaction had occurred in the powder particles during spraying and that the principal phases present in the coating were TiC and a Ni-rich solid solution; small quantities of NiTi, TiO2, and NiTiO3 were also present. SEM revealed that the coatings had a characteristic, splatlike morphology and that TiC formed as a nanoscale dispersion, with a size range of ∼50 to 200 nm, within solidified splats. The microstructures of these reactively sprayed Ni-TiC coatings are briefly compared with those observed in HVOF-sprayed coatings deposited using prereacted SHS powder.