Thermally sprayed Ni(Cr)-TiB2 coatings using powder produced by self-propagating high temperature synthesis:: microstructure and abrasive wear behaviour

High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni(Cr)-TiB2 cermet coatings onto steel substrates from a feedstock powder produced by a self-propagating high-temperature synthesis (SHS) reaction. The powder and coating microstructures were investigated by scanning electron microscopy and X-ray diffraction (XRD), whilst dry sand rubber wheel abrasive wear and microhardness tests were performed on the coatings. The SHS reacted powder was found to comprise principally 1-5 mum sized TiB2 in a crystalline, Ni-based solid solution matrix, with approximately 60 vol.% TiB2 and 40 vol.% metallic binder. XRD revealed the presence of only around 2% of the following phases: TiB, Ni2B, NiTi, TiO. The reacted compact was crushed, ground and classified into a powder of size range (+8 to -38) mum for thermal spraying. The HVOF sprayed deposits had layered, splat-like morphologies typical of thermally sprayed cermets. The coating microstructure consisted primarily of TiB2 in the Ni-rich binder phase but the boride fraction was reduced compared to that of the feedstock powder as a result of TiB2 dissolution in the molten alloy during spraying. XRD analysis of the coating indicated that the binder solidified to, in part, an amorphous/nanocrystalline structure with similar to2% of Ti-containing oxide phases also present. The TiB, Ni2B and NiTi phases were not detected in the coating. Under the wear conditions employed, angular alumina generated significantly higher wear rates than the rounded silica abradent, and the wear mechanism changed from microcutting with alumina to pull-out with silica. The wear coefficients obtained with the coating were significantly reduced compared with those of mild steel, in the case of alumina abrasive a factor of four reduction. Additionally, a significant improvement in wear resistance was also found when compared to HVOF-sprayed chromium carbide-nickel/chromium cermet coatings produced from commercially available powders. (C) 2002 Elsevier Science B.V. All rights reserved.